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Electroluminescence and Photophysical Properties of Near-Infrared Luminescent Lanthanide (III) Monoporhyrinate Complexes...


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ELECTROLUMINESCENT AND PHOTOPHYS ICAL PROPERTIES OF NEARINFRARED LUMINESCENT LANT HANIDE (III) MONOPORPHYRINATE COMPLEXES AND PENDANT POLYMERS By GARRY BRIAN CUNNINGHAM A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Garry Brian Cunningham

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I dedicate this to my father.

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iv ACKNOWLEDGMENTS There are so many people that I would like to acknowledge. First and foremost is my advisor, Professor Kirk Schanze. He has been a great mentor. I would also like to acknowledge all of the peopl e that have helped me on my projects along the way including Professor John Reynolds, Prof essor Paul Holloway, Dr. Tim Foley, Dr. Mauricio Pinto, Dr. Alison Knefely, Dr. Avni Argun, Dr. T.S. Kang, Dr. Benjamin Harrison, Nisha Ananthakrishnan, Dr. Fengqi Guo, Dr. Jeremiah Mwuara, and Dr. James Boncella. I would like to thank former a nd current members of the Schanze group. I would like to thank my friends who made livi ng in Gainesville bearab le. Lastly, I would like to thank my committee.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT.....................................................................................................................xi ii CHAPTER 1 INTRODUCTION........................................................................................................1 Electrodynamics...........................................................................................................1 Absorption of Light...............................................................................................1 Emission of Light..................................................................................................3 Nonradiative Decay...............................................................................................8 Energy Transfer.....................................................................................................9 Lanthanides.................................................................................................................12 Porphyrins...................................................................................................................17 Photophysics........................................................................................................17 Redox Properties.................................................................................................20 Synthesis..............................................................................................................22 Light Emitting Diodes................................................................................................25 OLED and PLED Structure.................................................................................29 Carrier Transport.................................................................................................30 Device Efficiency................................................................................................33 Device Failure Mechanisms................................................................................34 Literature Review.......................................................................................................35 Organic Systems..................................................................................................36 Inorganic Nanoparticles.......................................................................................37 Organo-Lanthanide and OrganoTransition Metal Complexes...........................39 2 SUBSTITUTED PORPHYRIN COMPLEXES.........................................................42 Solution Photophysics................................................................................................44 Absorption...........................................................................................................44 Emission..............................................................................................................45 Light Emitting Devices...............................................................................................54

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vi Electrochemistry.........................................................................................................64 Conclusions.................................................................................................................69 Experimental...............................................................................................................70 Photophysical Measurements..............................................................................70 Device Fabrication...............................................................................................71 ITO Etching..................................................................................................71 Cleaning ITO................................................................................................72 Spin Coating.................................................................................................72 Metal Electrode Deposition..........................................................................73 Electroluminescent Device Measurements..........................................................73 Electrochemistry..................................................................................................74 3 PORPHYRIN PENDANT POLYACETYLENES.....................................................75 Introduction.................................................................................................................75 Monomer Synthesis and Polymerization....................................................................76 Photophysics...............................................................................................................84 Absorption...........................................................................................................84 Emission..............................................................................................................88 Thermally Induced Isomerization........................................................................90 Light Emitting Devices...............................................................................................92 Discussion...................................................................................................................99 Conclusions...............................................................................................................100 Experimental.............................................................................................................101 Monomer Synthesis...........................................................................................101 Polymerization...................................................................................................104 Thermogravimetric Analysis.............................................................................105 Photophysical Measurements............................................................................105 Device Fabrication and Measurement...............................................................106 4 FUTURE DIRECTIONS..........................................................................................107 Non-Radiative Decay................................................................................................107 Carrier Transport......................................................................................................109 Porphyrin Containing Polymers...............................................................................110 Conclusions...............................................................................................................112 APPENDIX ELECTRONIC PROPERTIES................................................................115 The Free-Ion Problem...............................................................................................115 The Crystal Field Problem........................................................................................119 Coupling of the Electronic and Lattice States..........................................................124 Einstein Model..........................................................................................................128 LIST OF REFERENCES.................................................................................................132 BIOGRAPHICAL SKETCH...........................................................................................148

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vii LIST OF TABLES Table page 2-1 Photophysical properties of Yb por phyrin complexes measured in CH2Cl2............45 2-2 Solvent effects on the near-infrared quantum yields of Yb porphyrin complexes...52 2-3 Electrochemical windows of solvents......................................................................65 2-4 Electrochemical properties of substituted porphyrins..............................................70 3-1 Polymerization details..............................................................................................83 3.2 Photophysical data....................................................................................................86 3-3 Electroluminescence data.........................................................................................97 4-1 Vibrational frequencies of co mmon bonds in organic molecules..........................109 A-1 Real orbitals created from linear co mbinations of orbitals labeled by their magnetic quantum number.....................................................................................123 A-2 Position and symmetry of f -orbitals in an octahe dral crystal field.........................123

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viii LIST OF FIGURES Figure page 1-1 Schematic of absorption shown as a function of thickness, concentration, and molar absorptivity......................................................................................................2 1-2 Configuration coordinate model.................................................................................3 1-3 Jablonski diagram showing the fundam ental processes of absorption, internal conversion, fluorescence, intersys tem crossing, and phosphorescence.....................4 1-4 Configuration coordinate diagram s howing the zero-phonon line and illustrating the process known as the Stokes shift.......................................................................5 1-5 Configuration coordinate model showing the process of phosphorescence...............6 1-6 Interaction of atom nuclear charge in duced angular momentum interacting with electrons spin angular momentum, resulting in spin-orbit coupling.........................7 1-7 Typical nonradiative processes throu gh (a) crossover with a ground state, (b) multiphonon emission, and (c) cross over with an excited state.................................9 1-8 Schematic showing the overlap of donor fluorescence with the acceptor absorption, fundamental for Forster energy transfer................................................11 1.9 Graphical description of f orbitals............................................................................14 1-10 Energy level diagrams for sele cted rare earth ions in LnCl3....................................15 1-11 Absorption spectra of selected lantha nide ions, showing sharp absorption with low molar absorptivity.............................................................................................16 1-12 Jablonski energy diagra m showing energy transfer from organic ligand to lanthanide metal, in this case Yb3+...........................................................................16 1-13 Structure of porphyrin macrocycle, s howing possible areas of substitution............17 1-14 Molecular orbitals for the porphyrin macrocycle for the highest occupied molecular orbital and lowest unoccupied molecular orbital calculated using the Gouterman method...................................................................................................20 1-15 Absorption spectrum of tetraphenylporphyrin.........................................................21

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ix 1-16 Fluorescence spectrum of tetraphenylporphyrin......................................................22 1-17 Schematic of Rothemund synthesis..........................................................................22 1-18 Schematic of Lindsey synthesis of porphyrins.........................................................23 1-19 Schematic of 2 + 2 condensation method for synthesis A2B2 type porphyrins........24 1-20 Schematic of lanthanide metallati on of porphyrins using the Foley method...........24 1-21 Axial substitution of lanthanide me talloporphyrins using salt metathesis...............25 1-22 Structure of aluminum tris -quinolate.......................................................................26 1-23 Photoluminescence and electroluminescence spectra of Alq3.................................26 1-24 Structure of Ir(ppy)3.................................................................................................29 1-25 Schematic of light emitting diode showing each individual layer..........................29 1-26 Structures of typical hole transport materials...........................................................30 1-27 Structure of Lanthanide tris -quinolate.....................................................................40 1-28. Structure of Lanthanide tris -DBM bathophenthroline complex...............................41 2-1 Yb porphyrin complexes used in study....................................................................43 2-2 X-ray crystal structure of Yb(TPP)Q showing coordination of a molecule of THF..........................................................................................................................44 2-3 Absorption spectra for a)Yb(TPP)Q, b) Yb(TMPP)TP, c) Yb(TPyP)L(OEt)3, d) Yb(TPP_OEH)TP in CH2Cl2 as a function of molar absorptivity...........................48 2-4 Visible emission spectra for substituted Yb3+ complexes in CH2Cl2 at room temperature...............................................................................................................49 2-5 Excitation spectra of (a) Yb(TPP)Q, (b) Yb(TMPP)TP, and (c) Yb(TPyP)L(OEt)3.....................................................................................................50 2-6 Near-infrared emission spectra for substituted Yb3+ porphyrin complexes in CH2Cl2 at room temperature....................................................................................51 2-7 Near-infrared emission spectra for subs tituted porphyrin complexes in 2Me-THF52 2-8 Visible electroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a function of increasing voltage, starting at 6V..............56

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x 2-9 NIR Electroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a function of incr easing voltage, starting at 6V.....................57 2-10 Current density Voltage (j-V) plot of Yb porphyrin devices as a function of loading wt% in PS....................................................................................................58 2-11 Near-Infrared external electroluminescent quantum efficiency for Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a function of loading in PS............................................................................................................60 2-12 Charge hopping model showing transport of charges on porphyrin molecules.......61 2-13 Current density-Voltage and NIR irradi ance-voltage plots of Yb(TPP)TP as a function of Alq3 loading: ( ) 0 wt %, ( ) 33 wt%, and ( ) 66 wt %....................62 2-14 NIR External quantum efficiency of Yb(TPP)TP as a function of Alq3 loading: ( ) 0 wt %, ( ) 33 wt%, and ( ) 66 wt %..............................................................63 2-15 Current density-voltage and NIR irradiance-voltage plots of Yb(TMPP)TP as a function of Alq3 loading: ( ) 0 wt %, ( ) 33 wt%, and ( ) 66 wt %.....................63 2-16 NIR external quantum efficiency of Yb(TMPP)TP as a function of Alq3 loading: ( ) 0 wt %, ( ) 33 wt%, and ( ) 66 wt %..............................................................64 2-17 Redox properties of free-base tetraphenyl porphyrin with respect to the saturated calomel electrode......................................................................................................65 2-18 Reduction and oxidation waves fo r free-base tetra phenylporphyrin........................66 2-19 Reduction and oxidation waves for Yb(TPP)TP......................................................67 2-20 Reduction and oxidation potentials for Yb(TPP)Q..................................................67 2-21 Reduction and oxidation waves for Yb(TMPP)TP..................................................68 2-22 Reduction and oxidation waves for Yb(TPP)L(OEt)3..............................................68 2-23 Reduction and oxidation waves for Yb(TPyP)L(OEt)3............................................69 2-24 Cartoon showing ITO substrate placed at top of beaker containing solution of aqua regia allowing vapors to etch surface..............................................................72 3-1 Synthesis of Zn(II)-5-(4-Ethynyl phenyl)-10,15,20-triphenylporphyrin..................77 3-2 Synthesis of 2-(4-methylphenyl)-5 -(4-ethnylphenyl)-1,3,4-oxadiazole..................78 3-3 Structure of (Bicyclo[2.2.1]hepta-2,5-diene)chlororhodium(I) dimer.....................79 3-4 Representative scheme of in sertion polymerization mechanism.............................81

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xi 3-5 Polyacetylene isomer structures...............................................................................81 3-6 Structure of ZnETPP ho mopolymer and copolymers..............................................82 3-7 NMR spectra of poly(ZnETPP)0.5-co-(PE)0.5...........................................................83 3-8 TGA traces of polymers of ETPP measured in air...................................................84 3-9 Absorption spectrum of (a) poly(ZnETPP) ( ), (b) poly(ZnETPP)0.14-co(PE)0.86 (----), poly(ZnETPP)0.5-co-(PE)0.5 ( ), (c) poly(ZnETPP)0.5-co(PEOXAD)0.5 ( ), and (d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 ( ) ...........87 3-10 Absorption and fluorescence spectrum of PEOXAD monomer..............................88 3-11 a)Emission ( ex = 420 nm) of poly(ZnETPP) in solution ( ), film ( ),(b) poly(ZnETPP)0.14-co-(PE)0.86 in solution ( ), film ( ) (----), poly(ZnETPP)0.5-co-(PE)0.5 in solution (---), film ( ) (c) poly(ZnETPP)0.5-co(PEOXAD)0.5 in solution ( ), and (d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 in solution ( ).............................................................................................................91 3-12 Emission of poly(ZnETPP) in film as a function of annealing time at 150 C in vacuum.....................................................................................................................92 3.13 Current density vs. voltage and irradian ce vs. voltage for a) poly(ZnETPP) b) poly(ZnETPP)0.5-co-(PE)0.5, c) poly(ZnETPP)0.5-co-(PEOXAD)0.5 d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 as a function of wt% of polystyrene.................96 3-14 Electroluminescence emission of spectrum of poly(ZnETPP) ( ), (b) poly(ZnETPP)0.14-co-(PE)0.86 ( ), poly(ZnETPP)0.5-co-(PE)0.5 ( ), (c) poly(ZnETPP)0.5-co-(PEOXAD)0.5 (), and (d) poly(ZnETPP)0.5-co(3,5CF3PE)0.5 ( ) ..............................................................................................97 3-15 TEM image of poly(ZnETPP) showi ng presence of Rh nanoparticles....................98 3.16 External quantum efficiency vs. cu rrent density for a) poly(ZnETPP) b) poly(ZnETPP)0.5-co-(PE)0.5, c) poly(ZnETPP)0.5-co-(PEOXAD)0.5 d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 as a function of wt% of polystyrene ( = 0 wt%), ( = 25 wt%), ( = 40 wt%) and ( = 50 wt%) ...........................................99 4-1 Molecule proposed to increase emission quantum yield........................................108 4-2 Conjugated polymer, PPE, containing substituted porphyrin and oxadiazole in main chain..............................................................................................................110 4-3 Scheme of a free radical chain growth mechanism................................................111 4-4 Structures of monomers.........................................................................................112

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xii 4-5 Structure of TP polymer.........................................................................................113 4-6 Synthesis of YbTPP-TP polymer...........................................................................113 4-7 Absorption spectrum of YbTPP-TP polymer.........................................................114 4-8 NIR emission spectrum of YbTPP-TP polymer.....................................................114

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xiii Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ELECTROLUMINESCENCE A ND PHOTOPHYSICAL PROP ERTIES OF NEARINFRARED LUMINESCENT LANT HANIDE (III) MONOPORHYRINATE COMPLEXES AND PENDANT POLYMERS By Garry Brian Cunningham December 2005 Chair: Kirk S. Schanze Major Department: Chemistry The photoluminescent and electroluminescent properties of substituted lanthanide monoporphyrinate complexes were investigated. The lanthanide complexes consisted of a lanthanide (Yb3+) coordinated to a substituted porphyrin, 5,10,15,20tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(3,4,5trimethoxyphenyl)porphyrin(TMPP), 5,10,15,20-tetr a(4-pyridyl)porphyrin (TPyP), or 5,10,15,20-tetra(4(2-ethylhexyloxy) porphyrin (TPP_OEH) and a capping ligand, L. The capping ligand was either an tris-pyrazoylborate (TP), (cyclopentadienyl)tris(diethoxyphos phito-P)cobaltate (L(OEt)3) or quinolinato (Q) anion. The complexes were synthesized to influence the electronic propert ies of the complex. The optical absorption and emission of thes e complexes resembled previously studied lanthanide porphyrin complexes, with phot oluminescent yields ranging from 0.01 to 0.04. Electroluminescence was observed for the porphyr in complexes blended into polystyrene. External quantum efficiencies were typically 10-4, suggesting that changes to the

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xiv porphyrin structure have little effect on the electronic nature of the complex. Blending the electron transporting AlQ3 into the device improved the ex ternal quantum efficiencies by an order of magnitude, suggesting that carr ier transport is the culprit for poor device performance. Conjugated polyacetylenes containing a Zi nc porphyrin pendant were synthesized by an insertion type polymer ization using [Rh(NBD)Cl]2. The homopolymer (poly(ZnETPP) and copolymers of ethynyl benzene (poly(ZnETPP)-co-(PE)), an oxadiazole containing group (poly(Zn ETPP)-co-(PEOXAD), or 1-ethynyl-3,5trifluoromethylbenzene (poly(ZnETPP)-co-(3,5CF3PE) were synthesized. The optical properties were studied and it was found th at the homopolymer exhibited excitonic coupling due to the overlap of the porphyrin pendants. Substitution of other comonomers reduced this coupling allowing the re turn of typical ZnTPP optical properties. Neither absorption nor emission from the polyacetylene backbone was identified. A thermally induced cis to trans isomeri zation was observed for the homopolymer, poly(ZnETPP), with the emergence of polyacetylene backbone emission. Electroluminescent devices were fabricated us ing the polymers neat or in a blend with polystyrene. The electroluminescent perfor mance of the homopolymer was poor with a maximum external quantum efficiency of 10-6. Addition of polystyrene increased the efficiency 10-fold. The copolymers with PE also showed similar characteristics. The copolymers with either PEOXAD or 3,5CF3PE further increased the external efficiency by an order of magnitude. This suggest th at adding hole blocking / electron transporting pendants to the polymer further enhances carri er transport, allowing for more efficient devices.

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1 CHAPTER 1 INTRODUCTION This chapter provides an overview of several topics. The first topic focuses on the fundamental processes that o ccur when molecules interact with light. The second part will focus on the electronic properties of lant hanides. The third part will focus on the photophysical and redox properties of porphyrin s. The fourth part will focus on the basics of electroluminescence and light emitting diodes. The final part presents the upto-date literature overview of organic and polymer complexes used in light emitting diodes, with an emphasis on near-infrared light emission. Electrodynamics Absorption of Light The perturbation of matter with light gives insight into its electronic structure. When a molecule absorbs a photon of sufficiently high energy, one of its valence (HOMO, highest occupied molecular orbital, for organic systems) electrons is promoted into an excited energetic state (LUMO + n; n = 0. ). The frequency of the light provides information about the energy differe nce between the two orbital energy levels involved in the transition. The fundamental pr ocess of the absorption of light at a given frequency is given by the formula: 0log I Abc I (0.1) which is known as the Beer-Lambert law, where A is the absorption, I is the light intensity at a given frequency measured after interaction with the sample, I0 is the light

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2 intensity at a given frequency measured before the sample, b is the pathlength of the sample measured in cm., and c is the concentr ation of the sample given in molarity. The units for the intensity of the abso rption band are given by the term, and measured in M-1cm-1. The absorptivity is proportional to th e transition dipole between the ground and final states. If the dipole moment is zero, the absorption coefficient is also zero. Figure 1-1.Schematic of absorption shown as a function of thickness, concentration, and molar absorptivity. The absorption spectra of molecules differ greatly than that of free ions, due to coupling of vibrational tran sitions (phonons) with the el ectronic transitions. The absorption is broadened and split into bands This process is known as electron-phonon coupling. According to the Franck-Condon principle, the elec tronic process of absorption takes place on a faster timescale than the nuclei can respond. The equilibrium distance between the nuclei i nvolved in a given vibrationa l transition occurs at the potential energy minima. When an electron is promoted to a higher energy level, the charge distribution of the molecule is cha nged, and the initial pos ition of nuclei is no longer the lowest in energy. The nuclei will attempt to equilibrate, but this process is slow with respect to the time scale of light absorption (10-13s: 10-15s). Therefore, the transition occurs from the lowest ground vibra tional state to multiple vibrational levels of the excited state, as shown in Figure 1-2.

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3 R E Abs. Em.ground state excited state RgRe Figure 1-2. Configura tion coordinate model. Emission of Light The photoluminescence process can best be described using the general configuration coordinate diagra m (Figure 1-2). A configurat ion coordinate diagram is a plot of the energy of electronic ground and ex cited states as a func tion of a generalized configuration coordinate R that accounts fo r the nuclear configuration of nearest neighbors about the excited nuclei. As shown by Rg and Re, the equilibrium configuration coordinate diffe rs in the ground and excited states. Wavefunctions of excited states are typically more spatially ex tended than those of th e ground state. This leads to the energy minima of the excited state, Re, to be shifted with respect to the ground state energy minima. Upon absorption of a photon, the center is promoted into an excited vibrational state of an excited electronic state. The terminal state of the absorption transition is at a hi gher-energy point than the minimum of the excited state.

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4 The lifetime of allowed excited states is ~10-8 seconds and is much longer than the lattice vibrational period. Therefore, just af ter absorption, the complex undergoes a phonon assisted relaxation to the energy minima of the excited state. This process is also known as internal conversion (IC) (see Figure 1-3) The molecule loses its excess vibrational energy by interaction with solvent or other solute molecules. Energy T1S1S2ISC IC PhosphorescenceAbsorptionFluorescenceGS Energy Energy T1S1S2ISC IC PhosphorescenceAbsorptionFluorescenceGS Figure 1-3. Jablonski diagram showing the funda mental processes of absorption, internal conversion, fluorescence, intersys tem crossing, and phosphorescence. The chromophore can radiatively return to a vibrationally excited ground state by the emission of a photon. This process is ca lled fluorescence. Fluorescence is an allowed process because it involves a tr ansition of two states with the same spin. Half of the energy difference between the emission and ab sorption energies is known as the Stokes shift. The Stokes shift depends on the relativ e position and curvature of the configuration coordinate parabolas, shown in Figure 1-4.

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5 The excited chromophore can also unde rgo a process known as inter-system crossing (ISC). This process happens when the excited state electron spin changes to produce the triplet state. Th is process involves coupling of the singlet and triplet vibrational levels of the same energy to produce a vibrationally excited triplet state. This process is followed by internal conversion to the triplet state in th e lowest vibrational level ( = 0). Once a molecule has undergone inte rsystem crossing, it can return to the ground state by emission of a photon. This process is called phosphorescence. Figure 1-4. Configuration coordinate diagram showing the zero-phonon line and illustrating the process known as the Stokes shift.[1] The ability of a molecule to undergo intersystem crossing, although quantum mechanically forbidden, is facilitated by the presence of strong spin-orbit coupling. The spin and orbital angular momentum are separa tely coupled and quantized. Therefore, the transitions between the states of opposite spin are forbidden by the conservation of

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6 momentum rule. This is the typical case for molecules with small Z numbers (e.g., organic molecules). This situation is cha nged by the presence of atoms with large Z numbers. This is called the heavy atom ef fect. With these heavy atoms, a significant mixing of the spin angular momentum and orbital angular momentum of the same electron occurs. Because of this mixing, th e two momenta are not separately conserved, but the total angular momentum is conserved. This is called spin -orbit coupling, and it leads to the increase in inter-system crossing rate s. This process is shown in Figure 1-6. Due to the large nuclear charge present, a magnetic field is produced about the electron with a directionality perpendi cular to the plane of the orbit. A magnetic field is also being produced by the electron spin motion a nd is directed along the spin axis. The interaction of these two magnetic fields is called spin-orbit coupli ng. The magnitude of this effect is controlled by the nuclear charge of the heavy atom as well as by the position of the atom in the molecule. Figure 1-5. Configuration coordinate model showing th e process of phosphorescence.

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7 Figure 1-6. Interaction of atom nuclear ch arge induced angular momentum interacting with electrons spin angular momentum resulting in spin-orbit coupling. The lifetime of an excited state and lumi nescent efficiency depend on the rates of the radiative and nonradiative transitions from (and to) the excited st ate. Highly allowed transitions have fast rates and contribute to short lifetimes and high efficiencies. A strongly competing non-radiative rate will result in both a shorter lifet ime and lower luminescent efficiency. Radiative emission can be considered to be emission of light from an excited state of a complex designated as M*: *MM (0.2) This emission is considered a random pro cess, and therefore follows first-order kinetics:[2] *[] []rdM kM dt (0.3) If it is a single process, the decay should be characterized by a single exponential, which can be expressed by a single rate constant kd. The lifetime can also be expressed in terms of the lifetime ( r) of the excited state by the equation: 1r dk (0.4)

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8 The rate constant usually has the units of s-1. Therefore the lifetime has the units of s. The decay of the excited state can occur by ma ny different possible pathways such as the emission of light, intersystem crossing, and internal conversion. The radiative rate constant is therefore the sum of all of the processes which lead to a lower concentration of the excited state. The equation below shows the relationship between the rate of fluorescence (kf), the rate of intersystem crossing (kisc), the rate of internal conversion (kic) and the rate of phosphorescence (kp). dfiscicpkkkkk (0.5) Nonradiative Decay Radiative return from the excited state is not the only possibility of completing the cycle. The alternative is nonradiative decay, which means a return without the emission of radiation.[1] Nonradiative processes compete w ith radiative processes. These nonradiative returns affect the emission efficien cy of the sample. Emission efficiency is simply derived as the number of photons emitted by the sample divided by the number of photons absorbed by the sample. There are several different ways that nonradiative decay can occur. These processes are shown in Figure 1-7. In Figur e 1-7a absorption and emission processes are possible and Stokes shifted relative to each ot her. The relaxed-excited state may reach the crossing of the two parabolas if the temperature is high enough. Via the crossing it is possible to return to the ground state in a nonr adiative manner. The excitation energy is then given up as heat to the lattice. This accounts for one of the possible thermal quenching mechanisms.

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9 In Figure 1-7b, the parabolas are parallel (S = 0) and will never cross, assuming that both states have the same force constant. It is impossi ble to reach the ground state in the way described above. However, nonradiativ e return to the ground state is possible if certain conditions are met. The energy difference between the two states, E, must be equal to or less than 4-5 times the higher vibra tional frequency of the lattice. In that case, this amount of energy can simultaneously ex cite a few high-energy phonons, and then is lost for the radiative process. The quantitative description of this process is called the energy gap law.[3-9] This process is also called multi-phonon emission. The third process consists of an electroni c crossover with another excited state. This excited state could be a higher or lo wer energy excited state. This is the fundamental process involved in intersystem crossing. ground state E excited state R E ground state excited state R (a) (b)(c) R ground state e1 e2 Figure 1-7.Typical nonradiative processes through (a) crosso ver with a ground state, (b) multiphonon emission, and (c) cross over with an excited state. [1] Energy Transfer The final process that a molecule can use to relax from the excited state is through energy transfer. The process of energy transf er occurs when an excited donor molecule D* transfers its energy to an acceptor in the groun d state A, which is then promoted to an excited state:

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10 ** NREDADAhDAh (0.6) where h E and h NR are, respectively, the emission a nd nonradiative decay created in the process. In other words, the energy of one molecule can be transferred to another molecule, which quenches the luminescence of the host and increases that of the guest. The interaction between D* and A is described by the perturbation Hamiltonian, H. The D* + A state is not considered a stationary state of the total Hamiltonian but is able to evolve into other isoe nergetic states, such as D + A*. The use of time-dependent perturbation theory assigns the pr obability of the evolution from D* + A, given by i, to the D + A* state, described by f, as 2'ifPH (0.7) where is the density of the coupled isoenerget ic donor-acceptor transitions. For typical systems, can be determined by calculating the overlap integral of donor luminescence and acceptor absorption. In general, af ter the excited stat e transfers from D* to A it rapidly relaxes nonradiatively to th e lowest vibrational level of A*. The perturbation Hamiltonian contains se veral terms; the most important are the electrostatic (Frster) and electron exchange (Dexter) interactions. Both terms are capable of inducing energy transfer. The elec trostatic interaction can be expressed as a series of multipole-multipole terms. The mo st common interaction, dipole-dipole, was described by Frster who discovered that th e rate of energy transfer depended on the distance R between the donor and acceptor molecules:[10] 6 01 ()ETR kR R (0.8)

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11 where R0 is called the Frster radius, and is the average donor emission lifetime in the absence of energy transfer corresponding to rate kD = 1/ When R is equal to R0 then kET = kD. The critical distance R0, which is the distance between the donor and acceptor where the efficiency of energy transfer is 50%, is given by the integral over all wavelengths : 6172 0 431.2510E DA R Fd nc (0.9) where E is the quantum efficiency of donor emi ssion, n is the refractive index of the host, FD is the normalized emission spectrum of the donor, and A is the molar extinction coefficient of the acceptor. It has been s hown that dipole-dipole interactions can be significant even at dist ances as large as 100.[11] Figure 1-8. Schematic showing the overl ap of donor fluorescence with the acceptor absorption, fundamental for Forster energy transfer.[1] In Frster energy transfer, the spins of bot h D and A are conserved. Therefore, the allowed transitions are 1*111*DADA (0.10) 1*313*DADA (0.11)

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12 where the superscripts indicate the spin of the molecule. The triplet-singlet transition 3*111*DADA (0.12) is forbidden, but is occasionally observe d since the triplet ex cited donor has a long lifetime and kET can be faster than phosphorescence. The process of energy transfer has also been extensively analyzed by Dexter [12] to give 2/()()()RL ETDAkReFd (0.13) where R is the distance between D* and A, and L is a constant. Because this process involves the exchange of el ectrons, it occurs only over shor t distances, ~10. Under the Dexter transfer process, it is the total spin wh ich is conserved. Therefore, triplet-triplet energy transfer is allowed. 3*113*DADA (0.14) Lanthanides The previous sections focused on the el ectronic properties and fundamental processes of any molecule. This section w ill delve deeper into the unique properties which make up the metals known as the rare earths. The most impressive feature about the spectra of the lanthanide ions is the shar pness of the lines in absorption and emission spectra. The optical properties of the la nthanides were first discovered around 1908 by Becquerel.[13] The trivalent (Ce3+ Yb3+) rare earths have an el ectronic confi guration of 4 fn5s25p6(n=1-13). The 4 f electron shell is located within the 5s5p shell, and therefore the interaction with the ligand fi eld is weak as compared to th at of transition metals. This leads to the electroni c properties of 4 f levels in the complex to be similar to that of the free ion, and changing the overall li gand field has little effect on them.[14] If the

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13 lanthanides are modeled using the configura tion-coordinate model shown in Figure 1-2, the situation corresponds to an excited state displacem ent of zero. This is stating that the 4 f wavefunctions are not influenced to any great extent by excitation. The energy structure of the lanthanides an d their many different optical transitions were precisely investigated by Dieke [15] in lanthanide fluor ide hosts, and in LaCl3 doped solids by Crosswhite.[16] Figure 1-10 shows the energy level diagrams for several selected trivalent rare earths. These diag rams are quite useful in understanding their luminescence characteristics. Energy levels prescribed by the in ner quantum number J are further split into several sublevels in the solid state by the Stark effect of the crystal field. The number of split sublevels is depe ndant on the symmetry of the crystal field, and is limited to 2J + 1 for integer values of J and J + for J of half-integer values. The absorption of lanthanides is plagued by low oscillator strengths. Transitions within the 4f manifold take place within states of the same parity, and therefore are electric dipole forbidden. Oscillator strengths of f f transitions that are made electricdipole allowed by crystal fields are 10-5 10-8, dependent on whether the site has inversion symmetry. The uneven components of the crystal field mix a small amount of opposite parity wavefunctions into the 4 f wavefunctions. This process is sometimes called intensity stealing.[17-21]" Magnetic dipole interactions within the 4 f manifold are allowed and are sometimes observed. The os cillator strengths associated with these transitions are in the range of 10-7 10-8.

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14 fz 3 fxyz fxz 2 fyz 2 fx(x 2 -3y) fy(3x 2 -y) fz(x 2 -y 2 ) fz 3 fxyz fxz 2 fyz 2 fx(x 2 -3y) fy(3x 2 -y) fz(x 2 -y 2 ) fz(x 2 -y 2 ) Figure 1.9. Graphical description of f orbitals.[22] The ability of a ligan d to sensitize the f f emission transition was discovered in 1942 by Weissman.[23] He found that the transitions of beta-diketonates could sensitize Eu3+ emission. The mechanism for this tran sition is that the molecule is excited from the singlet ground state to a singlet excited state (S0 Sn) The Sn state decays into the lowest energy singlet excited state on the ligand (Sn S1).The singlet state undergoes intersystem crossing into the ligand-based triplet state (S1 T1). The ligand then transfers its energy to the lanthanide, resu lting in a lanthanide in the excited state. The lanthanide then can undergo radiative d ecay to the ground state, releasing a photon.

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15 The importance of this discovery was soon realized, and further efforts were undertaken to attempt to improve the efficiency of lanthanide emission. Most of the work focused on use of -diketonates as the sensitizing ligands for this process, although many other organic ligands were also used. The bulk of this resear ch will be discussed later in this chapter. Figure 1-10. Energy level diagrams for selected rare earth ions in LnCl3. Unfilled circles indicate emissive states. ( )[24]

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16 Figure 1-11. Absorption spectra of selected lanthanide ions, showing sharp absorption with low molar absorptivity. [17] T1S1SnG 2F5/2 2F7/2 T1S1SnG 2F5/2 2F7/2 Figure 1-12. Jablonski energy diagram show ing energy transfer from organic ligand to lanthanide metal, in this case Yb3+. Dashed lines represent non-radiative decay.

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17 Porphyrins Porphyrins are among the most important molecules in nature. They are found in nearly every biological tissue.[25] These compounds range from chlorophyll in plants to iron porphyrin (heme) in blood.[26-28] Porphyrins are also impor tant molecules in optical materials.[29-32] Porphyrins consist of a large m acrocycle core consisting of 26 electrons. This core (see Figure 1-13) can be substituted upon by the addition of a metal and substitution along the periphery can be accomplished at e ither the meso or pyrrole position. The molecule can also be substituted upon by the introduction of an axial ligand. N N N N MR1 R2 A1 A2 Figure 1-13. Structure of porphyrin macroc ycle, showing possible areas of substitution. Photophysics The electronic structure giving rise to the absorption spec tra of porphyrins and metalloporphyrins has been de rived by a four orbital model.[33-37] Given the D4h symmetry of the porphyrin ring, four different molecular orbitals have been described, shown in Figure 1-14. The absorption bands in the visible region are described by transitions among the two highest occupied orbitals of a1u and a2u symmetry with the doubly degenerate unoccupied orbitals of eg symmetry. Transitions from the a2u state to the eg state are the lowest in energy and ca lled the Q bands. In the free base

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18 porphyrins this leads to 4 bands ranging fr om 480-650 nm. In metalloporphyrins only two peaks are seen in the Q bands due to their lower symmetry.[38] The intensity of this transition is relatively weak due to the fact th at it is symmetry forbidden. The transition from the a1u level to the eg orbital is a strong absorption called the Soret or B band. It ranges from 380 430 nm and the large intensity ( 105 M-1cm-1) is due to the fact that it is symmetry allowed. The actual peak po sitions and absorption intensity are dependent on substituents on the porphyrins ring, central metal, and axial ligands. The regular fluorescence spectra for por phyrins shows two bands which mirror the Q bands, along with the possibility of phosphorescence bands. The fluorescence bands for metalloporphyrins are typically found near 600 nm and the phosphorescence bands are found near 700 nm. In normal type porphyrins and metalloporphyrins the excited state energy diagram is fairly simplistic due to the fact that the lowest singlet and triplet states derive from the porphyrins system. Porphyrin complexes which have closed shell metals generally only show fluorescence in solution at room temperatur e, but have some phosphorescent component at low temperatures (77 K).[39] The quantum yield for fluorescence decreases with increasing atomic number of the metal, as does the phosphorescence lifetime. The phosphorescent lifetime of these complexes is usually in the range of several hundreds of s to several ms. The addition of a diamag netic metal to the porphyrin core typically quenches the room temperature fluorescence.[39] The relatively weak fluorescence and short triplet lifetimes are due to the fact that the lowest energy excited state is typically metal based and has little contribution from the porphyrin system. The exceptions are Pt2+ and Pd2+ which show both room temperature fluorescence and phosphorescence, due

PAGE 33

19 to the fact that the d d splitting of these metals is large and the porphyrin system plays the dominant role in the excited state.[40, 41] Paramagnetic metal porphyrin complexes generally only show phosphorescen ce with relatively short lifetimes. The effects of substituents on the porphyrin core have little effect on the fluorescence position and intensity. The substitution of light atoms typically leaves the fluorescence intensity and lifetime unchanged. The addition of heavy atoms at the pyrrole position has a dramatic influence on the emissive properties. For example, the addition of bromine atoms at the pyrrole position of CoTPP dramatically changed the max of the Soret absorption.[42] This led to a sharp decrease in the fluorescence intensity and a dramatic increase in the ISC yield, whic h in turn lead to a greater phosphorescence yield.[43] This effect can be explained in two different ways. One is the fact that substitution at the pyrrole w ould cause a steric effect at the porphyrin core, therefore inducing a distortion of the normal planar st ructure. The second po ssible explanation is that the a1u orbital is more sensitive to changes to the pyrrole than that of the a2u orbital. The strong electron-donating substituents, Br could induce an inversion of the ground state by destabilization of the a1u level greater than that of the a2u level.[44] Porphyrins tend to be useful ligands for the sensitization of la nthanide emission in the near-infrared due to their ease of exci tation and their low energy triplet state. Lanthanide porphyrins are less well known than that of transition metal and alkaline earth metal porphyrins. The first examples of lanthanide monoporphyrinate complexes were demonstrated in 1976 by Horrocks et al .[45] Since then, there have been several papers describing the use of porphyrins to se nsitize the near-infrared emission from lanthanides.[46-52] They showed that the coordinated lanthanide rapidl y deactivates the

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20 singlet state by inducing intersys tem crossing to the triplet state. As a result of this fast intersystem crossing, most emission fr om the singlet states is quenched.[53] Lanthanide porphyrins such as GdTPP(acac) in which the en ergy of the metal is too high to accept energy from the ligand triplet, show emission typical of heavy metal porphyrin complexes such as PtTPP. Figure 1-14. Molecular orbitals for the po rphyrin macrocycle for the highest occupied molecular orbital and lowest unoccupied molecular orbital calculated using the Gouterman method.[36] Redox Properties The use of electrochemical methods to estimate the redox properties of porphyrins is vital for understanding the photochemistry of porphyrins. In general, free-base porphyrins possess two oxidation peaks and two reduction peaks in cyclic voltammetry. These correspond to the one and two electr on oxidation and reduction of the porphyrin system. In normal metalloporphyrins a sim ilar redox pattern is observed. The central metal cation simply acts as a substituent on the porphyrin ring. The redox properties

PAGE 35

21 observed, exhibit a good correlati on with the electronegativity or inductive parameter of the central metal atom.[54, 55] Substituents on the porphyrin ring show a good correlation between the redox potentials and the Hamett values.[54, 56] The electrochemical band gap corresponds well with the optical band gap determined by the lowest energy absorption in the Q band, indicating that the central metal and substituents equally effect the HOMO and LUMO levels.[57] The number of substituents is also correlated with the shifts in the redox peak positions.[58] Distortion from planarity seems to cause a dramatic change in the oxidation potential.[54] The addition of a redox active metal complicates the overall electrochemical properties of porphy rins, due to the inte rvening oxidation and reduction potentials of the metal. The chan ge in axial ligand also seems to play an important role in the redox potentials of porphyrins.[59, 60] Wavelength / nm 400500600700 Molar Absorptivity / M-1cm-1 0 1e+5 2e+5 3e+5 4e+5 5e+5 500550600650 0 5000 10000 15000 20000 Figure 1-15. Absorption spectrum of tetrap henylporphyrin. Inset is magnification of Qbands.

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22 Wavelength / nm 620640660680700720740 Normalized Intensity / arb. units 0.0 0.2 0.4 0.6 0.8 1.0 Figure 1-16. Fluorescence spec trum of tetraphenylporphyrin. Synthesis The synthesis of symmetric porphyrins involves the condensation of aldehydes with pyrroles in dilute solutions using catalytic amounts of an organic acid, organic base (Rothemund synthesis, Figure 1-17) or str ong Lewis acid followed by oxidation by such reagents as DDQ.[61] N NH N HN Ph Ph Ph Ph CHO H N Pyridine + Figure 1-17. Schematic of Rothemund synthesis.

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23 The use of Lewis acids such as BF3 (Lindsey method, Figure 1-18) generally leads to higher yields and milder reaction conditions.[62, 63] The synthesis of asymmetric porphyrins according to the Lindsey method us ually leads to a statistical distribution of mixed porphyrins, which can be difficult to separate.[30] For porphyrins of the transA2B2 type a 2 + 2 condensation of dipyrrometh anes with aldehydes shown in Figure 1-19 is typically utilized.. N NH N HN R R R R R CHO H N 1.)TFA or BF3OEt22.) DDQ CH2Cl2+ Figure 1-18. Schematic of Lindsey synthesis of porphyrins. The metallation of porphyrins is typically simple. It involves stirring the free base porphyrin in a solution containing the metal organic complex or metal salt. This procedure works well for metals with a small i onic radius, such as Zn and Mg, but not as well for metals such as Pd or Pt. The larger metals require much higher temperatures and harsh conditions for insertion into the porphyrin core. Lanthanides, for example, requir e stirring in acetylacetone at 220C in the presence of Ln(acac)3 and the porphyrin, resulting in the lanthanide porphyrin acetylacetonate complexes in relatively low yield.[45, 64] This low yield is due to the fact that the complexes hydrolyze during the considerably long column chromatography times

PAGE 38

24 necessary to separate the product. This met hod is also limited by the fact that the axial ligand is restricted to acac, due to the fact that the axial acac ligand is not very labile. Substitution of the diketonates can be achieved, but the high temperatures and harsh conditions results in significantly low yield.[65-67] Lanthanide porphyrins have also been synthesized by amine or alkane elimination reactions of neutral lanthanide amides or alkyls with the free base porphyrin.[68, 69] NH NH R1 R2 R1 + X O H N NH N HN X X R1 R2 R1 R1 R2 R1 2 eq. 2 eq. Figure 1-19. Schematic of 2 + 2 condensation method for synthesis A2B2 type porphyrins. N N N N 2(Li DME) YbCl3 3(THF) N N N N Yb Cl O O TolueneR R R R R R R R Figure 1-20. Schematic of lanthanide metalla tion of porphyrins using the Foley method. Most recently, the Boncella group devise d a method which increased the overall metallation yield. This lanthanide porphyri n complexes were synthesized by nucleophilic displacement of the halogen (Clor I-) from anhydrous LnCl3 or LnI3 in the presence of

PAGE 39

25 dilithiated porphyrin.[70] This procedure allows for the simple substitution of the axial ligand by reaction of the Ln porphyrin halide complex with the potassium salt of the axial ligand. N N N N Yb Cl O O R R R R KTp KQ KL(OEt)3 N N N B N N N N N N N R R R R H N N N N R R R R Co P P P O EtO EtO EtO OEt O O OEt OEt N N N N R R R R N O O Yb Yb Yb Figure 1-21. Axial substitution of lanthani de metalloporphyrins using salt metathesis. Light Emitting Diodes Research into the field of organic electroluminescent materials has grown dramatically since its incep tion. Electroluminescence of organic materials was first observed in 1963 in anthracene crystals.[71, 72] However, due to the low efficiencies and high field strengths required, research into organic based electroluminescent materials was forgotten until Tang and Van Slyke prepared devices containing vapor deposited aluminum tris -(8-hydroxyquinolate)(Alq3).[73] An important discovery in the field of

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26 organic based LEDs came in 1990, when Friend produced the first polymer based device from poly(p-phenylenevinylene) (PPV).[74] N O N O N O Al Figure 1-22. Structure of aluminum tris -quinolate. Electroluminescence is the direct conversi on of electrical en ergy into light. Electroluminescence was first described in the literature by Destriau in 1936.[75] This paper showed electroluminescence from a semiconductor, ZnS, embedded into a dielectric matrix under high electric field. This led to high field electroluminescent materials. Figure 1-23. Photoluminescence and electroluminescence spectra of Alq3.[73] Inorganic light emitting diodes are simple w ith respect to their operation. Forward current is driven over a p-n junction and electron-hole recombin ation occurs through

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27 shallow states or in quantum wells. The t uning of the emission is done by choosing the correct ratio of starting materials. The ma jor problem with these devices is that only single crystalline material is capable of exercising this type of recombination radiation efficiently enough for practical use. This m eans that one must obtain epitaxial growth on cheap substrates, which is not an easy task. Because of this limitation of inorganic LED use, researchers looked for other forms of electroluminescence and materials that could work in amorphous or polycrystalline films. This mainly led to the field of organic light emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs). The term LED used for the device structure is well justified in the fact that the basic mechanism is an injection of electr ons and holes into a heterojunction, and a radiative recombination of excitons formed from the electron a nd holes. There are several points which differentiate inorga nic light emitting diodes (ILED) from OLEDs and PLEDs. The main point is the low mobility of the carriers in organic systems, approximately five or six orders of magnitude lower than that of typical III V type semiconductor systems. This is caused by the fact that the organic molecules and polymers used in these systems are amorphous and therefore the main pathway for carrier transport is a hopping mechanism. This cau ses an appreciable drop in voltage across the film thickness. The differences in spectral width are also dramatic. ILEDs have typically narrow linewidths, approximately 10 25 nm, whereas organic systems and polymers typically have linewidths typically on the order of 100 nm. Most luminescent organic molecules are considered conjugated compounds. That is, there exists an a lternating series of single and doub le (triple) bonds. Due to this overlap of the orbital wavefunctions of these adj acent atoms, the electrons occupying

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28 these orbitals are relatively delocalized.[76] In a perfect polymer the delocalized electron cloud would extend along the entire length of the chain, but impurities and defects tend to break the conj ugation. In the typical polymer film, the length of a conjugated segment rarely exceeds 15 repeat units.[77] There are two major barriers to highly e fficient displays constructed of simple organic materials or polymers. The first major drawback comes in the form of spin statistics. Spin statistics shows that 25% of all excitations created by charge recombination produce the singlet excited state, while 75% results in the triplet. Since most organic materials do not phosphoresce, this energy is lost as heat. The second major barrier is the previously discussed prob lem of broad emission. In order to create efficient displays, there must be excellent color purity and good colo r saturation. Color saturation is difficult with the broad spectral width of organic and polymer emission. In order to fix the problem created by the quantum mechanical distribution of excited states, a material which shows phosphorescence must be used. This typi cally involved using heavy metal chelate complexes, such as Ir(ppy)3. [78] While the problem of spin statistics has been taken care of, there is still the prob lem of color purity. This problem can be fixed by the use of lanthanide complexes. Si nce lanthanide emission is narrow, there can be efficient color saturation. Lanthanide s also induce the heavy atom effect, which solves the first problem as well.

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29 N N N Ir Figure 1-24. Structure of Ir(ppy)3. OLED and PLED Structure The basic structure of a typical dc-biased OLED is shown in Figure 1-25. The first layer above the glass substrate is a transpar ent conducting anode, typically indium tin oxide (ITO). The next layer is usually the hole transport layer. This layer is usually a good hole transport material, and for most devices is some type of starburst amine or poly(3,4-ethylenedioxy-2,4-thiophene)-p olystyrene sulfonate (PEDOT-PSS).[79] Figure 1-25. Schematic of light emitting diode, showing each individual layer.

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30 Figure 1-26. Structures of typical hole transport mate rials (a) NPD (b) PEDOT-PSS. The next layer is the emitting layer. This layer is thermally evaporated or spin coated onto the hole transport layer. This laye r is typically on the order of 100 nm thick. The next layer is the electron transport layer. This layer usually consists of an organic material, thermally evaporated, which possesse s great electron mobility and also works well at blocking holes from reaching the cath ode, therefore confining the carriers to the emitting layer. This material typically consists of aluminum tris -quinolate or an oxadiazole containing complex.[80-86] The next layer is the ca thode. This usually consists of a low workfunction ( ) metal such as Ca ( = 2.87 eV) deposited by thermal evaporation. A protective layer of aluminum is often deposited on top of the calcium layer to prevent oxidation. Carrier Transport The thin films used in organic or po lymer light emitting diodes are typically amorphous. The amorphous structure leads to a reduction in que nching from internal conversion processes present in crystal line materials, due to the limit of phonon interactions, and therefore lead s to a consequent increase in the radiative recombination of Frenkel type excitons.[11] The efficient generation of ex citons is strongly dependent on charge carrier injection and tr ansport through the organic layers.

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31 Due to the weak intermolecular coupli ng and the high disorder of amorphous materials, their measured carrier mobilitie s are dramatically lower than in most crystalline solids. Therefore, the conduction mechanism is usually not considered Ohmic in nature, but is often space charge limited, influenced by the presence of traps, or hopping.[87] For example, carrier trapping in di amines has a negligible effect on its conduction properties, and the hole mobilities in th ese materials are of the order of 10-3cm2/V s.[88] In electron transporting materials such as Alq3, however, the trap density is much higher, which significan tly lowers the carrier mobility.[89] The electron mobility of Alq3 is 10-4 cm2 / V s at an applied voltage of 106 V/cm, with hole mobilities at least two orders of magnitude lower.[90] The carrier mobility of most organic materials is found to be dependent on both the electri c field (E) and the temperature (T) according to[91] 1 1 2 0 2 00(,)effeffE E ETeeeE kTkT (0.15) where k is the Boltzmann constant, E0 = kT0 is the activation ener gy at zero electric field corresponding to temperature T0, 0 and 0* are the zero field carrier mobilities, 1/Teff = 1/T 1/T0, and and are constants. The difference between the cathode (anod e) Fermi level and the LUMO (HOMO) of the electron (hole) transpor t layer forms a barrier to injec tion of electrons (holes) into the active layer. For devices with a barrier larger than 0.4 eV, the current flow is primarily determined by the efficiency of carrier injection at the contacts.[91] This is typically described as the injection-limite d regime. The energy level offset at the

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32 organic-electrode interface can be tailored by choosing an el ectrode with a work function corresponding to the energy levels at each layer. In organic molecular light emitting diodes, three different conduction mechanisms are commonly observed: ohmic, space-ch arge limited (SLC) conduction, and trapped carrier limited (TCL) space charge conduction. Ohmic conduction is seen at low voltages when the density of injected carriers, ninj, is smaller than the thermally generated background free charge density, n0. In this regime, the curre nt density is given by Ohms law: 0/nJqnVd (0.16) where q is the electronic charge, n is the hole or electron mobility, V is the applied voltage, and d is the layer thickness. Space charge limited conduction is observed when ninj > n0 when charge trapping is not observed. The current density is then described by Childs law: 2 3(9/8)nV Jq d (0.17) where is the dielectric permittivity.[92] The presence of ohmic and space charge limited conductivity is observed in the low-voltage operation of devices. At higher voltages, traps located near the LUMO tend to fill. If there exists a high density of trap s, their concentration and energy distribution governs the current, resulting in the tr apped charge limited (TCL) space charge conduction regime. As the traps fill, they reduce the density of empty traps and an increase in electron mobility ensues. An anal ytical expression relating the current to the voltage in the TCL regime is given for a continuous energy distribution of traps Nt(E) below the LUMO, as given by[92]

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33 ()tLUMO t ttNEE NEe kTkT (0.18) where ELUMO is the LUMO energy, Nt is the total trap density, k is the Boltzmann constant, and Tt = Et/k, where Et is the characteristic trap energy. The current density is then given by 1 1 1 2121 (1)1m m m m TCLLUMOn m tmmV JNq Nmm d (0.19) where NLUMO is the density of states (DOS) in the LUMO band, m = Tt/T, and = (E). From this equation, it can be seen that tr ap limited conduction results in a power-law dependence of current on voltage. Device Efficiency Efficiency of devices is an important i ssue not only for energy consumption, but also for its effect on the longevity of the devi ces. This effect on longevity is due to the minimization of ohmic heating that can be achieved by operation at lower voltages. Devices with high power efficiency imply a low current-voltage product for a given luminance. Power efficiency is only one of the ways to determine device efficiency. The most commonly reported efficiency from the literature is based on external quantum efficiency. External quantum efficiency is the measure of photons produced per electrons injected.[93] One of the forms in which the ba sic equation for the external quantum efficiency ext of the device can be written as extSTPLr (0.20) where is the fraction of photons collected normal to the front surface of the device, is measure of the hole and el ectron recombination, rST is the ratio of singlet to triplet

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34 excitons formed in the material, and PL is the solid state photoluminescence quantum efficiency. It can be shown that for devices with a large refractive index:[93] 20.5 n (0.21) where n is the refractive index of the medium For devices with equal charge carrier balance the factor equals one, but this is rarely th e case with organic materials. The value for rST is, according to spin statistics, 0.25, but recently there has been some evidence that this is not always the case. [94-98] While the photoluminescence quantum efficiency can approach unity for many organic dyes, the efficiency in the solid state is typically much lower. This is due to concen tration quenching, which is an effect due to the creation of nonradiative pathways.[99-104] Given all of these conditions, devices which show only fluorescence have a upper level of efficiency approaching 15% of the solid state photoluminescence efficiency. Device Failure Mechanisms The overall stability of light emitting diodes is an important element in understanding their commercial impact. The degradation of an OLED and PLED during operation appears in four modes:[105] (1) decay in emission intensity, (2) a voltage increase in the constant current mode, (3) th e growth of nonemissive areas in the device, and (4) the eventual electr ical short circuit. One of the most evident mechanisms fo r the degradation of OLEDs is through the formation of nonemissive dark spots which in turn leads to a l ong term decrease in efficiency. These spots result from the de lamination of the metal at the metal/organic interface due to a large amount of Joule heating.[106] This tends to lead to a short-circuit

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35 condition and carbonization of the ac tive layer. This may also lead to electrode migration in these areas where the conductivity is high.[107] The most prevalent mechanism for decay in PLEDs is photooxidation during device operation.[108, 109] The extended conjugation length of polymers tends to increase the electron density at the doubl e bond making it more reactive to the electrophilic singlet oxygen. Carbonyl formation by the reaction of oxygen with alkoxy centers is also a facile method for the quenching of electrolumi nescence due to the f act that carbonyls are typically good nonradiative quenching centers. The sour ce of the singlet oxygen is energy transfer from the polymer to molecular oxygen. The primary degradation method in organic light emitting diodes is recrystallization. Excitons are rapidly quenched by defect s and charge-dipole induced fields at the surface of a grain boundary. Any given amorphous layer will recrystallize slowly as its temperature rises towa rds its glass transition temperature. The final mechanism, shared by both OLEDs and PLEDs is the electrical breakdown of the device cause d by pinhole electrical arcs.[110] These breakdowns occur usually at high voltage. The mechanism is the circuit opens around the pinhole which in turn stops the arcing. The resultant hole allo ws for moisture and air to enter the device and delaminate the material. When the num ber of burn-outs becomes too numerous, the circuit eventually shorts out and destroys the device. Literature Review The field of organic and polymer light emitting diodes has exploded in the latter part of the 20th century and has continued to expand into the present. Most of the focus has been on polymers and organic molecules which emit in the blue region of the electromagnetic spectrum. This focus on this region is due to the poor efficiency and

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36 color saturation of organic systems and molecules which emit in the blue. This is unlike molecules which emit in the green and re d, which have been designed with high efficiencies and exhibit adequate color satu ration. Recently, there are several displays based on polymer and organic systems which are commercially available.[111, 112] The research carried out here at the Univ ersity of Florida, focuses on the emission of organic and polymer systems which emit in the near-infrared. This can be accomplished by three means. The first involves organic molecules and polymers which exhibit a small HOMO-LUMO gap, which pe rmits emission in th e near-infrared. Although this is possible, the efficiency of fluorescence and device performance is severely limited by nonradiative decay pro cesses. The second means for producing emission in the near-infrared is the use of organo-transition metal or organo-lanthanide complexes. The third means for the creation of near-infrared (NIR) em ission is the use of organic functionalized semiconduc tor nanoparticles. In this section, a brief history of each of the possible directions is provided. Organic Systems There are several examples of groups obt aining near-infrared electroluminescence from organic and polymeric materials. The majority of this emission although is located in the visible, with emission carrying over into the NIR. The Holmes group, in 1995, showed near-infrared emission from a 2, 5-bis(hexyloxy)terephth alaldehyde-3-dodecyl2,5-thiophenediacetoni trile copolymer which showed broad emission extending out to 1000 nm, with an internal quantum ef ficiency of 0.2% photons/electron.[113] Fujii, in 1997, fabricated devices showing emission ta iling out to 1000 nm with phthalocyanine co-evaporated with DCM (4 -(dicyanomethylene)-2-methyl -6-(p-dimethylaminostyryl)4H-pyran), which showed effici ent emission operating at 15 V.[114] In 2000, Suzuki

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37 showed emission from an organic ioni c dye, 2-[6-(4-dimethylaminophenyl)-2,4neopentylene-1,3,5-hexatrienyl ]-3-methylbenzothiazolium perchlorate blended into poly(N-vinylcarbazole). Thes e devices showed that the emission improved over time due to the alignment of the ionic dye molecules along the bias field, with the external efficiency eventually reaching 1%.[115] Maltsev, in 2002, showed very narrow emission at 850 nm resulting from the electrolumines cence of J-aggregates of cyanine dyes blended into a polyimide matrix.[116] The Bazan group showed efficient electroluminescence ( ext = 0.1 0.3 % photons/electr on) of ethynyl linked porphyrin complexes, in 2003. These complexes show emission typical of porphyrins with a tail into the near-infrared.[117] In 2004, Suzuki again produced devices with emission into the NIR, using organic ionic dyes. He used (2-[6-(4-dimethylaminophenyl)-2,4neopentylene-1,3,5-hexatrienyl ]-3-methyl-benzothiazonium perchlorate) (LDS821) and [C41H33 Cl2N2](+).BF4(IR1051) to produce emission at 800 and 1100 nm, respectively.[118] This year, Yang created devices from alkyl-substituted fluorene, 4,7diselenophen-2'-yl-2,1,3-benzothiadiazole (SeBT), and 4,7-diselenophen-2'-yl-2,1,3benzoselenadiazole (SeBSe), which show ed emission in the 670 790 range. These devices showed external effici encies ranging from 0.3 to 1.1%.[119] Most recently, Thompson et al showed NIR emission peaking at 800 nm from a donor-acceptor copolymer of 1,4-(2,5-dihexadecyloxyphenylene) and 5,8-linked 2,3diphenylpyrido[3,4]pyrazine.[120] Inorganic Nanoparticles The use of solution processible semic onductor nanoparticles (quantum dots, QD) for electroluminescent emission in the near-in frared is a fairly recent process. These

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38 devices are based on II-VI and IV-VI se miconductor nanopartic les blended into conductive polymer matrixes. The use of QDs for electroluminescent devices is an interesting concept due to the tunability of emission created by quantum confinement. As the nanoparticle is decreased in size, th e band gap increases; therefore causing a hypsochromic shift in the emission. Th e QDs are made solution processible by the functionalization of the surf ace with organic ligands. In 2003, Steckel showed tunable elec troluminescent emission using PbS nanoparticles, with emission ranging from 1.3 1.6 m, dependent on particle size.[121] Bakeuva then increased the emission range fr om 1.1 to 1.6 m by changing the organic molecule bound to the surface from oleate lig ands to octadecylamine ligands. These devices showed an internal efficiency reaching 3%.[122] Most recently, Sargent fabricated devices containing PbSe which showed tunable emission in the near-infrared.[123] Progress in the field of nanopa rticle based light emitting diod es was further advanced in 2004 by the production of HgTe based devices which showed efficient emission and broad tunability.[124] Inorganic nanoparticles are extremely versat ile due to the large tunability of the energy gap. Efficiency is also enhan ced due to limited phonon interactions in nanoparticles, which allows for less nonrad iative decay from the excited state. Nanoparticles also have severe limitations. Th e first limitation is that nanoparticles tend to aggregate in solution and disperse poorly in thin films. The s econd major limitation is the fact that nanoparticles tend to oxidize due to the abundance of non-bonded atoms on the surface.

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39 Organo-Lanthanide and Organo-Transition Metal Complexes The use of lanthanide and transition me tal complexes is by far the most common method for emission in the near-infrared. The field of organic lanthanide metal complexes was born in the late 1950s and c ontinued throughout the 1960s with research devoted to lanthanide beta-diketonates.[125-133] Research then turned to lanthanide polypyridyl complexes.[131, 134-136] In the 1970s much of the research into lanthanide organic complexes sh ifted to porphyrins.[45, 46, 53, 64, 137-144] Since then, most of the research is into trying to improve the efficiency of the lanthanide emission.[145-154] The use of lanthanide organic complexes for electroluminescence was unknown until 1999, when Curry observed electroluminescence from tris -quinolato erbium(III) (ErQ3). This complex showed the typical 4I13/2 4I15/2 emission centered at 1.54 m.[155] Since then a great deal of research has been focused on ErQ3 due to its potential use in optical communications.[156-159] Near-infrared emission has also been seen for other lanthanide quinolate complexes. Kawamu ra in 2000, observed typical 2F5/2 2F7/2 emission from Yb3+ tris -quinolate at 985 nm operating at 15V.[160, 161] Several groups also observed near-infrared emission from Nd3+ tris -quinolate based devices, showing emission at 800, 1060, and 1300 nm.[157, 159, 162-164] Since then the focus of ra re-earth quinolate complexes has been on the effect of structure on the emi ssive properties. It was determined that functionalization of the quinolat e complex with halogens has a dramatic improvement in the emission yield.[165, 166]

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40 Figure 1-27. Struct ure of Lanthanide tris -quinolate. Lanthanide quinolate comple xes, although functional, are not the only means of sensitizing near-infrared emi ssion of lanthanide ions. N ear-infrared emission has also been observed for complexes of lanthanide di ketonates. In 2001, Hong produced devices with Pr(DBM)3Bath and Yb(DBM)3Bath (see Figure 1-28), which showed the 1D2 3F2 (890 nm), 1D2 3F3 (1015 nm), 1D2 3F4 (1065 nm), and 1D2 1G4(1550 nm) for Pr3+ and 2F5/2 2F7/2 emission from Yb3+ respectively.[167, 168] Harrison, later reported in 2003, emission from the Er3+ 4I13/2 4I15/2 (1520 nm) and Nd3+ 4F3/2 4I11/2 (880 nm), 4F3/2 4I13/2 (1060 nm), 4F3/2 4I15/2 (1330 nm) complexes of DBM.[169] Previously unseen, in organic systems, transitions for Ho3+, 5F5 5I6 (1500 nm), 5F5 5I7 (1200 nm), 5F5 5I8 (980 nm) and Tm3+, 3F4 3H4 (1400 nm) 3F4 3H6 (800 nm) were finally observed in 2004 in DBM systems.[170, 171] The use of other molecular systems to produce near-infrared emission has been achieved. For example, Sloof in 2001, produced typical Nd3+ emission from a lissamine dye functionalized terphenyl based metal complex. They determined that a larger conversion to the triplet state under electrical excitation, resulting in a more efficient Nd3+ emission.[172] Previous work in our group by Harrison et al has produced NIR emission from lanthani de porphyrin complexes.[173-177] It was determined that, the porphyrin efficiently transferred its energy to the lanthanide, produci ng the characteristic

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41 metal emission. Emission from Nd3+, Yb3+, Ho3+, and Er3+ were observed. They also determined that the change of axial c oordination ligand has little effect on the electroluminescence efficiency. Recently de ndrimer complexes with a lanthanide core were fabricated and showed efficient near-infrared emission.[178] The core of the dendrimer was an nona-coordinated Er3+ atom with keto functionalized metalloporphyrin dendrons. Excitation of the porphyrin dendr on resulted in the typical lanthanide emission. Figure 1-28. Structure of Lanthanide tris-DBM bathophenthroline complex.

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42 CHAPTER 2 SUBSTITUTED PORPHYRIN COMPLEXES This chapter discusses the results of th e study of substituted lanthanide porphyrin complexes. Previous work by Harrison et al showed that, although the porphyrin ligand is useful in sensitizing the em ission of lanthanides in light emitting diodes, its electronic properties are not ideal for highl y efficient devices. This study was undertaken to further investigate the electronic prope rties of the porphyrin macrocyc le, as well as to enhance the performance of devices created using lant hanide substituted porphyrin complexes. In order to accomplish this, several porphyrins were synt hesized by Alison Knefely, according to the procedure described in chap ter one, with substitutions of the meso phenyl group in an effort to vary the elec tronic properties of the complexes. Bulky groups were appended to the phenyl groups to un derstand the effect of aggregation in the devices. The axial ligand was also substituted. The structures of the resulting complexes are shown in Figure 2-1. The first complex studied was (quinolinato)(5,10,15,20tetraphenylporpyrinato)Yb(III) (Yb(TPP)Q). The hydroxyquinoline ligand was used as the axial ligand in an effort to increase th e electron accepting nature of the complex due to the electron poor unsaturated nitrogen. Th is complex (see Figure 2-2) was also used because it closely resembled previously studied complexes.[169] The second complex studied was (hydridotri s(1-pyrazolyl)borato)(5,1 0,15,20-tetrakis(3,4,5trimethoxyphenyl)porphyrinato)Yb(III) (Yb(TMPP)TP ). The substitution of the electron donating alkoxy groups was chosen in an effort to perturb th e overall electronic structure

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43 of the complex. It was expected that the hol e mobility for devices fabricated using this complex would be higher. The third co mplex studied was (cyclopentadienyl) tris(diethylphoshinito)cobaltate)(5,10 ,15,20-tetra(4-pyridyl)porphyrinato)Yb(III) (Yb(TPyP)L(OEt)3. The substitution of the phenyl groups for pyridyl groups was an effort to increase the electron transporting properties of the complex due to the presence of electron poor pyridine substituen ts. The use of the Klaui (L(OEt)3) ligand was motivated by the fact that the analogous complex with the TP capping ligand was insoluble in all common organi c solvents, and the Klaui ligand improved the solubility. It has been shown that the Kl aui ligand has little effect on the overall properties of the complexes.[169] The final complex studied was (hydridotris(1-pyrazolyl)borato) (5,10,15,20-tetra(4(2-ethyl hexyloxy)porphyrinato)Yb(III) (Yb(TPP_OEH)TP). It was expected that the long branched alkoxy side -chain would reduce aggregation due to the inability of complexes to approach each ot her. The photophysical, electrochemical, and device performance characteristics of these co mplexes are discussed in this chapter. N N N N Yb N O O Yb(TPP)QN N N N N N N N Yb Co P P P O EtO EtO EtO OEt O O OEt OEt Yb(TPP)L(OEt)3N N N N MeO O M e MeO MeO OMe OMe OMe OMe MeO OMe MeO MeO N N N N N N B H Yb Yb(TMPP)TPN N N N O O O O N N N N N N B H Yb Yb(TPP_OEH)TPN N N N Yb N O O Yb(TPP)QN N N N Yb N O O Yb(TPP)QN N N N N N N N Yb Co P P P O EtO EtO EtO OEt O O OEt OEt Yb(TPP)L(OEt)3N N N N N N N N Yb Co P P P O EtO EtO EtO OEt O O OEt OEt Yb(TPP)L(OEt)3N N N N MeO O M e MeO MeO OMe OMe OMe OMe MeO OMe MeO MeO N N N N N N B H Yb Yb(TMPP)TPN N N N MeO O M e MeO MeO OMe OMe OMe OMe MeO OMe MeO MeO N N N N N N B H Yb Yb(TMPP)TPN N N N O O O O N N N N N N B H Yb Yb(TPP_OEH)TPN N N N O O O O N N N N N N B H Yb Yb(TPP_OEH)TP Figure 2-1. Yb porphyrin complexes used in study.

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44 Figure 2-2. X-ray crystal st ructure of Yb(TPP)Q showing c oordination of a molecule of THF. Solution Photophysics Absorption All absorption measurements were made w ith the complexes as dilute solutions in methylene chloride (CH2Cl2) unless otherwise indicated. Figure 2-3 shows the absorption spectra for the studied ytterbium porphyrin complexes. The absorption spectra are dominated by the transitions of the porphyr in ligand, including the Soret (S0 S2) band (~ 420 nm) and the weaker Q-bands (S0 S1) (~500 600 nm). No absorption for the 4 f 4 f transitions could be seen due to their low molar absorptivities (~ 1 M-1cm-1) Furthermore, little evid ence for absorption by the capping ligand was observed in the samples studied. The Soret band showed a bathochromic shift upon coordination to the lant hanide, from 412 nm in the free base to ~ 427 nm in the complex. Overall, the absorption spectrum resembles a normal metalloporphyrin absorption sp ectrum such as Ni(TPP).[37] These results correlate with previously studied unsubstituted Ln(TPP)L complexes.[169] Although the location of the absorption peaks of each complex is slightly different, there is little evidence to suggest that changes to the porphyrin periphery has a dramatic effect on th e electronic properties

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45 of the ligand. This suggests that there is little perturbation of the -electronic system due to substitution upon the porphyrin ring. There exists some perturbation to the system by the introduction of the quinolat e capping ligand. The Soret ba nd blue-shifts with respect to other Yb(TPP) complexes, showing that th e basic ligand has some effect on the overall electronic properties of the complex. Table 2-1. Photophysical properties of Yb porphyrin complexes measured in CH2Cl2. Complex abs / nm (Log ) em / nm Yb(TPP)Q 375(4.12), 420(5.61), 516(3.48), 553(4.30), 591(3.68), 626(2.97) 600, 650, 715, 913, 927, 954, 980, 1005, 1025, 1047, Yb(TMPP)TP 378(4.03), 404s(4.72), 428(5.75), 516(3.50), 554(4.30), 592(3.57), 630(2.97) 610, 650, 715, 925, 952, 978, 1003, 1029, 1049 Yb(TPyP)L(OEt)3 403s(4.65), 427(5.76), 519(3.68), 558(4.44), 597(3.71), 626(2.92) 605, 645, 715, 923, 950, 985, 1005, 1018, 1048 Yb(TPP_OEH)TP 378(3.92), 404s(4.61), 427(5.64), 517(3.39), 553(4.18), 592(3.45), 627(2.86) 605, 650, 715, 927, 951, 974, 1003, 1020, 1050 Emission Excitation of the Yb porphyrin complexes in to the Soret or Q-bands resulted in both visible (Figure 2-4) and near-infrared emission (Figur e 2-6). In the visible region from 600 750 nm, there are three emission ba nds. The weak visible fluorescence at ~650 and ~715 nm matches the visi ble fluorescence of free-base tetraphenylporphyrin.[179] These assignments agree with previous studies of Ln(TPP)L, which came to a similar assignment partly due to the short (8 ns) lifetime of the 650 and 710 nm bands.[169] A weak emission band at ~600 nm was observed in all of the Yb porphyrin complexes. The excitation spectru m of all three emission bands helps to elucidate the origin of each emission transi tion. The excitation spectra of the three emission peaks for the Yb porphyrin complexes are shown in Figure 2-5. Comparison of the Soret bands observed in th e excitation spectra show th at the 610 nm emissions are bathochromically shifted ~15 nm from the ex citation spectra of the other emission bands.

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46 This is similar to the UV-Vis absorption ba nd shift which occurs when the porphyrin is coordinated to Ytterbium. Thus the visible emission bands at 654 and 720 nm can be attributed to free porphyrin emission whic h seems not to be influenced by the metal, while the 610 nm band corresponds to emi ssion from the lanthanide metalloporphyrin complex. All of the structures show an excitation peak around 450 nm ( em = 715), which suggest that there exists the possibility of a more co mplex structure involved in the emission, such as an aggregate. This all lead s to the conclusion that there is the presence of a free base impurity, which leads to less desirable features in the near-infrared. The near-infrared photoluminescence pr operties of the substituted ytterbium porphyrins are different and vary dramatically, in contrast to the absorption spectra, which are similar for all of the complexes. Fi gure 2-6 shows the near-infrared emission of the substituted Yb3+ porphyrin complexes. The near-inf rared region consists of a sharp peak at ~980 with additional broad bands on eac h side of the sharp peak. The structure of the spectra has been attributed to crystal fiel d splitting effects which are calculated to be on the order of hundreds of cm-1.[125] Previous work with Yb porphyrins suggests that the 2F5/2 2F7/2 emission transition is composed of eight different peaks.[169] Through curve fitting using the commercially available program Origin, the crystal field induced emission structure was estimated. For Yb(TPP)Q seven peaks could be ascertained, and for all of the other complexes there were six peaks that could be determined with confidence. All of these peaks show evidence of a crystal field splitting of ~ 500 cm-1 which agrees well with previous results.[169] Variable temperature work by ot hers has shown that higher crystal field states can be thermally populated when the excited state possesses a long

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47 lifetime.[180, 181] This agrees well with variable temperature photoluminescence studies carried out on the porphyrin complexes. As an example Figure 2-7 shows 80 K and RT emission spectra for the complexes, which clearly shows the high energy emission band. The use of different solvents can play a si gnificant role in the optical properties of organic compounds. Therefore the solven t effects on the absorption and emission properties of the Yb3+ porphyrin complexes were examined. The changes in absorption properties for all complexes were similar in various solvents. The Soret band shows a slight bathochromic shift when place d in increasing polar solvents. While the absorbance of Yb(TPP)Q varies by a few nanometers in different solvents, the solvent used has a more si gnificant effect on the NIR photoluminescence quantum yield. The effect of solvents on the quantum yields of the other Yb porphyrin complexes was less important. In general, th e quantum yields are low (<10%) due to nonradiative pathways of deactivating the excite d state. Table 2-2 shows the NIR quantum yields of the Yb porphyrin complexes in diffe rent solvents. Since the lanthanide is positioned above the plane of the porphyrin ri ng (see figure 2-2), there remain several coordination sites where a solvent molecule can access the metal ion. Yb(TPP)Q has a relatively small axial ligand which may allow for trace amounts of water present in the solvent or the solvent itself, to coordinate to the metal, which in turn lowers the radiative quantum yield. The coordination of water to lanthanide metal centers is known to play a significant role in deactivating the excited st ate of the lanthanide ion by coupling to the non-radiative O-H vibrational modes. Howeve r, in coordinating solvents, the solvent displaces the coordinated water thus removing the O-H oscillators from the proximity of the metal ion, which in turn lowers the non-radiative rate.

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48 0 1e+5 2e+5 3e+5 4e+5 5e+5 Wavelength / nm 480520560600640 0 1e+4 2e+4 Molar Absorptivity / M -1 cm -1 0 1e+5 2e+5 3e+5 4e+5 5e+5 6e+5 Wavelength / nm 480520560600640 0 1e+4 2e+4 0 2e+5 4e+5 6e+5 Wavelength / nm 480520560600640 0 1e+4 2e+4 3e+4 Wavelength / nm 300400500600700 0 1e+5 2e+5 3e+5 4e+5 5e+5 Wavelength / nm 480520560600640 0 1e+4 a) b) c) d) Figure 2-3. Absorption spectra for a)Yb(T PP)Q, b) Yb(TMPP)TP, c) Yb(TPyP)L(OEt)3, d) Yb(TPP_OEH)TP in CH2Cl2 as a function of molar absorptivity.

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49 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Wavelength / nm 500550600650700750800 0.0 0.2 0.4 0.6 0.8 1.0 Yb(TPP)Q Yb(TMPP)TP Yb(TPyP)L(OEt)3Yb(TPP_OEH)TP Figure 2-4. Visible emissi on spectra for substituted Yb3+ complexes in CH2Cl2 at room temperature ( ex = 420 nm).

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50 (a) (b)(c) (a) (b)(c) Figure 2-5. Excitation spectra of (a ) Yb(TPP)Q, (b) Yb(TMPP)TP, and (c) Yb(TPyP)L(OEt)3.

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51 Figure 2-6. Near-infrared emission spectra for substituted Yb3+ porphyrin complexes in CH2Cl2 at room temperature ( ex = 420 nm).

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52 Yb(TPP)Q Yb(TMPP)TP Yb(TPyP)L(OEt)3Wavelength / nm 90095010001050 Yb(TPP_OEH)TPWavelength / nm 900950100010501100 Yb(TPP)Q Yb(TMPP)TP Yb(TPyP)L(OEt)3Wavelength / nm 90095010001050 Yb(TPP_OEH)TPWavelength / nm 900950100010501100 Figure 2-7. Near-infrared emission spectra for substituted porphyrin complexes in 2MeTHF( ex = 420 nm) ( 80 K, --300 K). Table 2-2. Solvent effects on the near-i nfrared quantum yields of Yb porphyrin complexes. Solvent Yb(TPP)Q PL Yb(TMPP)TP PL Yb(TPyP)L(OEt)3 PL Yb(TPP_OEH)TP PL CH2Cl2 0.0091 0.041 0.031 0.034 Toluene 0.0081 0.037 0.039 0.047 THF 0.0085 0.028 0.027 0.036 DMSO 0.029 0.031 0.032 --CH3CN 0.0097 0.026 ----Hexane ------0.057 The presence of high energy oscillators with in close proximity of the lanthanide ion will increase the non-radiative decay rates of the lanthanide ex cited states. By understanding the Energy Gap Law and using Siebrands approach, th e non-radiative rate can be found using Equation 2.1.[182]

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53 22 2nr nrM kNJF hh (2.1) In Equation 2.1, nr is the density of final vibrational states, J is the electronic coupling constant due to nuclear motion, 2 M measures the coupling to the vibrational modes, N is the number of modes and F is the Franck-Condon factor. The FranckCondon factor can be described by Equation 2.2. 2211 exp()() 22 !kk F (2.2) In Equation 2.2, k is a constant, is the difference in positions of the final and initial vibrational states and is composed of the ener gy between the two states, 0E and the maximum oscillator of highest energy, max as shown in Equation 2.3. 0 max1 2E h (2.3) Using Sterlings approximation to expa nd the factorial in Equation 2.2 and the approximations for energy gaps of lanthanides corresponding to 1 to 3 vibrational quanta of the host, the non-radiative decay rate can be simplified to Equation 2.4. 0maxexp(2)nrkE h (2.4) The and terms vary very little between hosts. Therefore, as the radiative energy gap decreases, the non-radiative decay rate increases exponentially. Given the 22 5/27/2FF transition is ~ 10,000 cm-1, it is evident that the use of C-H oscillators (~3200 cm-1) plays a significant role in the deactiv ation of the lanthani de excited state.

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54 Light Emitting Devices Light emitting diodes used in this study were of the modified single emissive layer type, that is, there was no true electron tran sport layer. These devices where prepared by spin coating a thin layer of PEDOT-PSS (Bayer Baytron P VP 4083) onto cleaned ITO. The device was then dried in vacuo for 4 hours at 150 C in order to remove any residual solvent. The active layer was spin-coated and the device was place in high vacuum (10-6 torr) for several hours, again to remove residual solvent. Calcium and aluminum metal were then thermally deposited under high vacuum. The devices where then encapsulated with a commercially purchased epoxy materi al. The device architecture is shown in Figure 1-25. Devices were prepared with the Yb porphyrin complexes blended into polystyrene at varying wt% in order to determine the effects of dopant concentration on device performance. Electroluminescence spectra of the Yb porphyrin devices at 40 wt% are shown in Figures 2-8 2-9. The emission is shown as a function of voltage. At all concentrations, Yb3+ is the predominant emitter in the devices with a near-infrared emission at 977 nm. The emission observe d results from direct electron-hole recombination which occurs at the Yb porphyrin complex. As the voltage is increased, the spectral shape of the Yb3+ begins to show a defined emission band at 920 nm. This could be due to the fact that the 2F5/2 energy state of Yb3+ is in a non-symmetric environment and is split into three crystal fiel d states. The splitting of these states is on the order of a few hundred cm-1. With a sufficiently long ex cited state lifetime, thermal equilibrium is established between the lowe st and second lowest excited states which results in emission of a higher energy band at 920 nm. The Yb(TPyP)L(OEt)3 devices were less stable and failed at a lower voltage than the other devices. This can possibly be

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55 attributed to the fact that el ectrochemical experiments show that there is no reversible oxidation of the complex. The current density-voltage (j-V) profiles of the devices are shown in Figure 2-10. The turn-on voltages fo r the devices were typically 6 V for the 40 wt% devices and 5 V for the 60 wt% devices Although near-infrared emission is the predominant emission process, there exists a significant emission in the visible region which mirrors the typical porphyrin emission. This emission is roughly 1 5 % of the intensity of the near-infrared emission afte r correction for instrument response. This visible emission can be attri buted to two possible pathways Either the emission comes from a free base porphyrin impurity or it resu lts from a more complicated process of delayed fluorescence. This pr ocess of delayed fluorescence, results from a back energy transfer from the lanthanide to the porphyrin triplet state. This in turn reacts with another porphyrin in a triplet st ate, resulting in a singly excited porphyrin, which then can emit. The current densities of all of the devices were somewh at similar suggesting that change to the porphyrin structur e has little effect on charge transport properties of the complex. As the amount of Yb porphyrin complex was increased, the efficiency increased. This is indicative of better ch arge transport through the device, confirming previous studies suggesting that the por phyrin is the key to charge transport.[175, 183 ] The near-infrared external quantum effici encies, measured as photons collected per electrons injected, for 40 and 60 wt% are s how in Figure 2-11. At 40 wt% the maximum efficiency of Yb(TPP)Q is ~ 2 x 10-4 and increases to 4 x 10-5 with increase in concentration to 60 wt%. Efficiency in the other devices ranged from 1 3 x 10-4, which agrees with previous Yb(TPP)L device performance.[169] This again suggests that the changes to the porphyrin have li ttle effect on the electronic pr operties of the complexes.

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56 Figure 2-8. Visible el ectroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a func tion of increasing voltage, starting at 6V.

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57 Figure 2-9. NIR Electroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a function of increasing voltage, starting at 6V.

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58 Figure 2-10. Current density Voltage (j-V) plot of Yb porphyrin devices as a function of loading wt% in PS. The decrease in near-infrared efficiency with increasing current density is due to three possible factors. First, because the life time of the lanthanide ex cited state is long, ~ 40 sec, saturation of the emissive sites can occur at high current densities.[184] Work by others has shown that the decrease in e fficiency can be due to triplet-triplet annihilation.[98] A final possibility is due to perm anent degradation of the device through chemical reactions. Charge hopping is believ ed to be the primary charge transport mechanism in Yb porphyrin blended into the non-conducting polystyrene devices. Previous work with ZnTPP in poly(2-vinylpy ridine) showed that, when ZnTPP molecules were within 1.1 nm of each other, charge hopping occurs.[185] The porphyrin complexes in this study are expected to be within 1 nm of each other due to aggregate effects. Figure 2-12 shows the charge-hopping model. In this model holes are injected at the anode and electrons are injected at the cathode. The carriers then hop along the porphyrins until they either co mbine with the opposite carrier, and create an excited porphyrin, or are annihilated at the opposite electrode.

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59 Previous research showed that the rate of hole transport in Yb(TPP)L far exceeded the rate of electron transport.[169] In order to study this eff ect, devices were fabricated in which a known electron transporter, tr is(8-hydroxyquinolate) aluminum (Alq3), was blended into the Yb(TPP)TP/PS matrix. Fi gure 2-13 shows the current density-voltage and NIR irradiance-voltage characte ristics for 0, 33, and 50 wt% (Alq3/Yb(TPP)TP) respectively. The current density at a given voltage drops dramatically with the increased introduction of Alq3 until 66 wt% Alq3, where the devices become unstable. This decrease in current density is characteristic of an increase in charge carrier balance. The NIR irradiance increases by a factor of two from 0 to 50 wt% Alq3. This suggests that more excitons are recombining on the porphyrin s resulting in greater light output. The NIR external quantum efficiency-current de nsity characteristics are shown in Figure 214. The efficiency increases by a factor of ten and the data variance decreases with increasing Alq3 concentration again suggesting be tter charge transport and improved device performance. In order to determine if changes to th e porphyrin structure improve the charge transport capabilities, the same Alq3 blending experiment was carried out with Yb(TMPP)TP. Figure 2-15 shows the curr ent density-voltage and NIR irradiancevoltage characteristics of the devices. Again, as in the Yb(TPP)TP based devices, there is a decrease in current density and an in crease in NIR irradiance with increased concentration of Alq3. The NIR external quantum effici ency, shown in Figure 2-16, also increases by approximately a factor of te n. These results mirror those of Yb(TPP)TP, suggesting again that the change s to the porphyrin structure ha ve little effect on charge transport properties of the complexes.

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60 0.0 0.1 0.2 0.3 0.4 0.5 NIR External Quantum Efficiency / 10-4 0 1 2 3 0 1 2 3 Current Density / mAcm -2 0100200300400 0.0 0.5 1.0 Figure 2-11. Near-Infrared external electroluminescent quantum efficiency for Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a function of loading in PS. (black = 40%, red = 60%).

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61 Figure 2-12. Charge hopping model show ing transport of charges on porphyrin molecules.[177]

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62 Figure 2-13. Current density-Voltage and NI R irradiance-voltage plots of Yb(TPP)TP as a function of Alq3 loading: () 0 wt %, () 33 wt%, and () 66 wt %.

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63 Figure 2-14. NIR External quantum effici ency of Yb(TPP)TP as a function of Alq3 loading: () 0 wt %, () 33 wt%, and () 66 wt %. Figure 2-15. Current density-voltage and NI R irradiance-voltage plots of Yb(TMPP)TP as a function of Alq3 loading: () 0 wt %, () 33 wt%, and () 66 wt %.

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64 Figure 2-16. NIR external quantum efficien cy of Yb(TMPP)TP as a function of Alq3 loading: () 0 wt %, () 33 wt%, and () 66 wt %. Electrochemistry Electrochemistry provides valuable insight into the electronic properties of molecules. This technique provides informa tion on the position of the energy levels, in particular the highest occupied molecular orbital (HOMO) and th e lowest unoccupied molecular orbital (LUMO) are easily discerna ble from these measurements. The position of the HOMO of a molecule is probed by determining its anodic potential, while the position of the LUMO is determined by its ca thodic potential. Th ese positions can be referenced with respect to the vacuum leve l by adding 4.7 eV to the onset of the peak (oxidation / reduction) with respect to the ferrocene / ferrocenium (Fc / Fc+) redox couple.[186] Table 2-3 shows the electrochemical window of the solven ts used in these experiments. The electrochemical measurem ents were undertaken by Avni Argun. The measurements of the oxidation potential(s) of all samples were carried out using a Pt working electrode in CH2Cl2. The measurements of the reduction potential(s) were carried out using either a Pt or glassy carbon working electrode in tetrahydrofuran (THF).

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65 All measurements were carried out using 1 mM analyte in 0.1 M tetrabutylammonium hexafluorophospate (TBAPF6) with a platinum flag seconda ry electrode and a calibrated silver wire referen ce pseudo-electrode. Table 2-3. Electrochemi cal windows of solvents. Solvent Anodic Limit* Cathodic Limit* THF 1.8 V -3.5 V CH2Cl2 1.8 V -1.9 V MeCN 1.8 V -2.0 V potentials vs. SCE Figure 2-17. Redox properties of free-base tetraphenylporphyrin w ith respect to the saturated calomel electrode.[187] The electrochemical properties of tetraphenyl porphyrin were determined in order to verify the validity of the experiment, given th e fact that the electro chemical properties of TPP are well known. The first ox idation with respect to Fc/Fc+ was determined to be 0.54 V and was determined to be reversible. The first reduction peak was determined to be located at -1.75 V and was also reversib le. These observations correspond well with

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66 previously published results.[187] The electrochemical band-gap (HOMO LUMO gap), which was determined by the difference of the E1/2 of the anodic and cathodic waves, was determined to be ~ 2.2 eV which corresponds well with the optical gap measured by absorption. -2.3-2.2-2.1-2.0 -1.9-1.8-1.7-1.6 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 J (mA/cm2)E (V) vs. Fc/Fc+ E1/2= -1.75V R1 E1/2= -2.08V R2-0.4-0.20.00.20.40.60.81.01.21.41.6 -0.2 0.0 0.2 0.4 0.6 O1E1/2=0.89VJ (mA/cm2)E (V) vs. Fc/Fc+ E1/2=0.54V O2 -2.3-2.2-2.1-2.0 -1.9-1.8-1.7-1.6 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 J (mA/cm2)E (V) vs. Fc/Fc+ E1/2= -1.75V R1 E1/2= -2.08V R2-0.4-0.20.00.20.40.60.81.01.21.41.6 -0.2 0.0 0.2 0.4 0.6 O1E1/2=0.89VJ (mA/cm2)E (V) vs. Fc/Fc+ E1/2=0.54V O2 Figure 2-18. Reduction and oxidation waves for free-ba se tetraphenylporphyrin. In order to understand the e ffect of the lanthanide metal on the redox properties of the porphyrin system, the electrochemical pr operties of Yb(TPP)TP were studied. The first oxidation was determined to be located at 0.58 V, which is nearly identical to the non-metallated porphyrin. The first reduction pote ntial was determined to be located at 1.67 V and again corresponds well with the fr ee-base porphyrin. This suggests that the lanthanide metal has little effect on the pos itions of the HOMO and LUMO levels. Although the first oxidation wa s similar to TPP, the reduction wave for Yb(TPP)TP was significantly different with respect to TPP. Th ere exists an irreversible reduction at -2.2 V which has been attributed to Yb3+ + eYb2+. The redox properties of Yb(TPP)Q are some what similar to those of Yb(TPP)TP with the first oxidation located at 0.58 V, while the reduction has sh ifted to -1.98 V.

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67 There also exists an irreversible oxidati on at 0.35 V which is possibly due to the oxidation of the quinolate axial ligand. The reduction of th e quinolate was determined to be at 2.9 V. This evidence suggests that the quinolate ligand has only a minor effect on the electronic propertie s of the complex. -0.20.00.20.40.60.81.01.21.4 -0.2 0.0 0.2 0.4 0.6 0.8 J (mA/cm2)E (V) vs. Fc/Fc+ Eoxd 1 = 0.58V Eoxd 2 = 1.01V -3.0-2.8-2.6-2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0-0.8 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 J (mA/cm2)E (V) vs. Fc/Fc+ -1.67 V -1.99 V -2.4 V -2.63 V-0.20.00.20.40.60.81.01.21.4 -0.2 0.0 0.2 0.4 0.6 0.8 J (mA/cm2)E (V) vs. Fc/Fc+ Eoxd 1 = 0.58V Eoxd 2 = 1.01V -3.0-2.8-2.6-2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0-0.8 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 J (mA/cm2)E (V) vs. Fc/Fc+ -1.67 V -1.99 V -2.4 V -2.63 V Figure 2-19. Reduction and oxi dation waves for Yb(TPP)TP. -0.4-0.20.00.20.40.60.81.0 -0.1 0.0 0.1 0.2 0.3 0.4 J (mA/cm2)E (V) vs. Fc/Fc+ 0.58 V 0.82 V-3.0-2.8-2.6-2.4-2.2-2.0-1.8-1.6 -0.4 -0.3 -0.2 -0.1 0.0 J (mA/cm2)E (V) vs. Fc/Fc+ -1.98 V-0.4-0.20.00.20.40.60.81.0 -0.1 0.0 0.1 0.2 0.3 0.4 J (mA/cm2)E (V) vs. Fc/Fc+ 0.58 V 0.82 V-3.0-2.8-2.6-2.4-2.2-2.0-1.8-1.6 -0.4 -0.3 -0.2 -0.1 0.0 J (mA/cm2)E (V) vs. Fc/Fc+ -1.98 V Figure 2-20. Reduction and oxida tion potentials for Yb(TPP)Q. The electrochemical properties of Yb(T MPP)TP were then studied. It was expected that the electron donating alkoxy groups would make the complex easier to oxidize, but that was not the case. The firs t oxidation potential was determined to be located at 0.6 V. The first reduction potential was determined to be located at -1.97 V.

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68 This suggests that substitution on the meso phe nyl groups has little effect in influencing the redox properties of the complex. -0.4-0.20.00.20.40.60.81.01.21.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 J (mA/cm2)E (V) vs. Fc/Fc+ 1.12 V 0.95 V 0.60 V -3.0-2.8-2.6-2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 J (mA/cm2)E (V) vs Fc/Fc+ -1.97 V -2.56 V-0.4-0.20.00.20.40.60.81.01.21.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 J (mA/cm2)E (V) vs. Fc/Fc+ 1.12 V 0.95 V 0.60 V -3.0-2.8-2.6-2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 J (mA/cm2)E (V) vs Fc/Fc+ -1.97 V -2.56 V Figure 2-21. Reduction and oxida tion waves for Yb(TMPP)TP. -2.8-2.6-2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0-0.8-0.6 -0.8 -0.6 -0.4 -0.2 0.0 0.2 J (mA/cm2)E (V) vs. Fc/Fc+ R1E1/2=-1.80 V R2E1/2=-2.46 V-0.20.00.20.40.60.81.01.2 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 E1/2= 0.61V E1/2= 0.09VJ (mA/cm2)E (V) vs Fc/Fc+ -2.8-2.6-2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0-0.8-0.6 -0.8 -0.6 -0.4 -0.2 0.0 0.2 J (mA/cm2)E (V) vs. Fc/Fc+ R1E1/2=-1.80 V R2E1/2=-2.46 V-0.20.00.20.40.60.81.01.2 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 E1/2= 0.61V E1/2= 0.09VJ (mA/cm2)E (V) vs Fc/Fc+ Figure 2-22. Reduction and oxida tion waves for Yb(TPP)L(OEt)3. Next, the redox properties of Yb(TPyP)L(OEt)3 were determined. In order to understand the effect of the axial Klaui lig and, the redox properties of Yb(TPP)L(OEt)3 were first determined. It was shown that the first oxidation potential was dramatically changed from ~0.6 V for the TP complex to ~0.1 V for the Klaui complex. This lower oxidation is most likely the oxi dation of the Klaui ligand.[188] The second oxidation of the Klaui complex was similar to the first oxidation states of the other complexes, providing evidence to that sugge stion. After showing that the Klaui ligand has little

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69 effect on the overall properties, the redox pr operties due to the pyridine substitution could be determined. The complex showed no re versible oxidation, probably due to the creation of a reactive cation radical. Th is non-reversible oxida tion is most likely contributing to the poor stability of electro luminescent devices fabricated from the complex. The first reduction was similar to that of all of the other complexes, again showing that substitution at the phenyl group has little eff ect upon the over all electronic properties of the system. -2.8-2.6-2.4-2.2-2.0-1.8-1.6 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 J (mA/cm2)E (V) vs Fc/Fc+ -2.09 V -2.57 V-0.50.00.51.01.52.02.5 -2 -1 0 1 2 3 4 J (mA/cm2)E (V) vs. Fc/Fc+ Figure 2-23. Reduction and oxida tion waves for Yb(TPyP)L(OEt)3. Conclusions From these results, there come two major problems which much be addressed in order to produce more efficient near-infrare d devices. The first is that non-radiative decay is a controlling factor in the low near-infrared phot oluminescence quantum yield of lanthanide porphyrin complexes. This proce ss is facilitated by the large number of C-H oscillators which are in close proximity to the Yb3+ ion. In order to improve NIR PL quantum yield, and in turn increase the theo retical NIR EL quantum yield, substitution of these protons with heavier atoms (halogens) must occur.

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70 The second major problem is the lack of carrier balance in Yb porphyrin LEDs. Substitution on the meso-phenyl groups of TPP has little effect on the charge transport properties of these complexes. Therefore these novel complexes still facilitate hole transport and hinder electron transport. In order to correct this problem, the electron transporting ability must be increased. Th is can be accomplished by either lowering the LUMO by substitution of electron withdrawing moieties on the pyrroles of the porphyrin or by creating a molecular wire in which elec trons can flow freely, reducing the barrier for electron injection. Table 2-4. Electrochemical prope rties of substituted porphyrins. Compound Oxidation Potential (Ox1) V* Reduction Potential (Red1) V* (Red1 -Ox1) (Eg) Optical HOMOLUMO gap (from the lowest energy Q band) TPP Free base 0.54 -1.75 2.2 eV 664 nm (1.87 eV) Yb (TPP) (L(OEt)) 0.09 -1.8 1.9 eV 617 nm (2.00 eV) Yb (TPP) TP 0.58 -1.67 2.2 eV 603 nm (2.06 eV) Yb(TPP)Q 0.58 -1.98 -2.6 607 nm (2.04 eV) Yb (TMPP) TP 0.6 -1.97 2.6 eV 609 nm (2.04 eV) Yb (TPyP) (L(OEt)) -2.09 616 nm (2.01 eV) Yb (TPP_OEH) TP 0.59 -1.64 2.2 eV 604 nm (2.05 eV) Potential vs. Fc/Fc+ Experimental Photophysical Measurements All photophysical studies were conducted in 1 cm squa re quartz cuvettes unless otherwise noted. All absorption and em ission measurements were made in CH2Cl2 unless otherwise noted. Absorption spectra were obtained on a double-beam Cary-100 UV-

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71 visible spectrophotometer. Fluorescence spec tra were measured on a SPEX Fluorolog-2 equipped with a water-cooled Hamamatsu R 928 PMT for visible measurements and a liquid nitrogen cooled InGaAs diode detector for n ear-infrared measurements. All measurements were corrected for detector response. Photoluminescence quantum yields in solution ( PL) were calculated using equation 2.5, where the absorption of the reference is given by AR, and the absorption of the sample is given by As. The refractive indices of the so lvents are given by the term n. The integrated area of the emission peaks of the sample and standard are given by, FS and FR respectively. R is the quantum yield for th e standard. Yb(TPP)TP in CH2Cl2 ( = 0.033) was used as the relati ve quantum yield standard. 2 210 10R SA SS SR A RRnF nF (2.5) Device Fabrication ITO Etching Electroluminescent devices were prepar ed by masking, then etching the ITO coated glass (Delta Technologies, Rs = 8 12 / ) by exposure to aqua-regia vapor. The ITO-glass sheets were cut into 1 x 1 s quares by the use of a gl ass cutter. Packing tape was then carefully placed over the sheet of glass, ensuring that no bubbles were present. With the use of a black marker, a rectangle was drawn onto the tape to indicate where the tape would be removed. A razor knife was then used to remove the indicated areas. The ITO glass squares were then placed upon a beaker containing a freshly prepared solution of aqua-regia, where they remained for 6 minutes. After removal from the beaker, the exposed areas where wiped with a cotton swab re moving the dissolved

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72 ITO. The tape was then removed and the ITO was rinsed with isopropanol to ensure removal of any remaining acid. Aqua Regia ITO Substrate Figure 2-24. Cartoon showing ITO substrate pl aced at top of beak er containing solution of aqua regia allowing vapors to etch surface. Cleaning ITO The etched ITO squares were then pl aced into a Teflon holder. They were subsequently sonicated for 10 minutes in each of the following solutions: aqueous sodium dodecyl sulfate (SDS, Fi sher), Milli-Q water, aceton e (Fisher, ACS grade), and isopropanol (Fisher, ACS grade). The ITO-gl ass squares were dried under stream of filtered air. They were then placed into an oxygen plasma cleaner (Harrick PDC-32G) for 15 minutes. Spin Coating PEDOT-PSS (Bayer, Baytron P VP Al 4083) was used as the hole transport layer. The PEDOT-PSS suspension was first filtered through a 0.2 micron Polysulfone filter to ensure removal of particulat e matter. The PEDOT-PSS was then spin-coated (Chemat, KW-4A) onto the ITO surface at 4000 rpm for 30 seconds. The PEDOT-PSS coated ITO-glass was them dried in a vacuum over at 150C for 4 hours in order to remove residual water.

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73 Solutions of the desired wt% of porphyrin complex were created by dissolving the corresponding mass of Yb porphyrin complex into a 1 mL CHCl3 solution containing 3mg of polystyrene (PS) (Aldrich, Mn 280,0000). These solutions were then spin-coated onto the substrate at 1000 rpm. Metal Electrode Deposition The films were placed glass side down onto an inverted stage designed to fit into the chamber of the thermal evaporator (D enton Vacuum, DV502A). The ITO was then covered with a stainless stee l mask with the pattern of the electrodes desired. The masked devices are then placed into the thermal evaporator and pumped down to 10-6 torr for 12 hours. Calcium (or LiF) and alumin um layers were sequentially deposited by thermal evaporation at 2 x 10-7 torr without breaking the v acuum. The thicknesses were adjusted by the use of a calibrated oscillating quartz crystal thickness monitor. The thicknesses used for all devices were: 50 for Ca, 5 for LiF and 2000 for Al. The devices were left in the eva porator to cool for 30 minutes after deposition. The chamber was then purged with nitrogen and the de vices removed and en capsulated with epoxy (Loctite quick set epoxy) in order to minimize exposure to oxygen and moisture. Electroluminescent Device Measurements Visible and near-infrared ( < 1000 nm) el ectroluminescence spectra were recorded on a ISA-SPEX Triax 180 spectrograph fitted with a liquid nitrogen cooled CCD detector (EEV CCD, 1024 x 128 pixels, 400 1100 nm). Th e devices were placed into the device holder such that electrical contact was made between the vapor-deposited electrodes of the device and the gold pins of the device ho lder. The device holder was then mounted onto an x-y stage as close as possible to the entrance slit of the Triax 180 (4.8 cm). Using the x-y stage, the electrode to be measured is placed in the cente r of the monochromator

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74 opening. Power for electroluminescent m easurements was supplied using a Keithley 228 voltage / current source. Positive bias was applied to one of the corners of the device holder, while negative bias was applied to the electrode under inve stigation. The CCD was calibrated into energy units by the use of a primary standard quartz tungsten halogen lamp. Electrochemistry All electrochemistry was performe d using an EG&G PAR model 273A potentiostat/galvanostat in a three-electrode cell configuration c ontaining a platinum button or a glassy carbon butt on as the working electrode, a platinum flag as the counter electrode, and a silver wire as the pseudo -reference electrode calibrated with a Fc/Fc+ redox couple. Lanthanide contaning por phyrin complexes (1 mM) and 0.1M tetrabutylammonium he xafluorophospate (TBAPF6, a non-coordinating supporting electrolyte) were dissolved in non aqueous solvents such as dichloro methane (for oxidation) and THF (for reducti on). CV studies were carried out under argon blanket at a scan rate of 50 mV/s, to determine th e redox couples of the compounds. HOMO-LUMO gaps were obtained from the E1/2 difference of the first oxid ation and the first reduction couples.

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75 CHAPTER 3 PORPHYRIN PENDANT POLYACETYLENES Introduction The Nobel Prize winning discovery of dope d polyacetylene resulted in the field of conjugated polymers. Polyacetylene is th e archetypical conjugated polymer, with alternating single and double bonds, which in its doped form shows metallic behavior.[189] The polymer, however, is insoluble, infusible, an d unstable in air. This severely limits its use in optical materials. Substituted polyace tylenes are quite different from unsubstituted polyacetylenes, especially when they contain bulky substituents.[190] Backbone substitution results in an increase in photoluminescence yield and tunable emission properties dependant on the number and nature of substituents.[191-210] These polymers have been studied for possible use in li ght emitting diodes, photovoltaic materials, nonlinear optical materials, gas permeable membranes, sensors, and magnetic materials.[79, 197, 211-241] The previous chapter focused on the emissi ve properties of lanthanide porphyrin systems blended into polystyren e and their electroluminescent properties. Previous work by Harrison and coworkers demonstrated the use of conjugated polymers as hosts for blended materials for light emitting diodes.[169] This chapter discusses the combination of the two principles. The idea of the research was that the por phyrin could be appended to a conjugated polymer, resulting in more efficient carrier tran sport as well as enhanced emission through Frster energy transfer.

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76 Monomer Synthesis and Polymerization The use of porphyrin pendant polyacetyl enes was first shown in a paper by Aramata, Kajiwara, and Kamachi in 1995.[242] Their goal was to develop a polymer which had a magnetically active side ch ain. Their belief was that the electron of the unsaturated main chain would enhance the magnetic properties of the metalloporphyrin pendant. They therefore s ynthesized a porphyrin monomer bearing a terminal alkyne which could be easily polymerized using comm ercially available ca talysts. Although successful in the monomer preparation, they discovered that purification was extremely difficult and the resulting yield was extremely low (6 %). This difficulty in purification was due to the relative inability to separate the statistical mix of porphyrin products. The major problem was the choice of protecting gr oups. The use of the trimethylsilyl (TMS) group resulted in little polar ity change from the base phenyl, and therefore made column chromatography difficult. Therefore a di fferent procedure was needed in which purification could be achieve d more easily. Lindsey et al. discovered that the use of polar protecting groups ease d the purification process.[243] This was the procedure carried out to synthesize the porphyrin pendent mono mer used throughout this study, and the scheme is shown in Figure 3-1. The firs t step of the reaction was the Sonogoshira coupling of the protected acetylene to 4bromobenzaldehyde to produce 4-(3-methyl-3hydroxybut-1-yn-1-yl)benzaldehyde (1). The second step was the condensation of three equivalents of benzaldehyde with one equivalent of compound (1) with excess pyrrole in a dilute solution in CHCl3 with the lewis acid, BF3OEt2, acting as a catalyst. This was immediately followed by oxidation of the resulting porphyrinoge n to the desired porphyrins by the addition of DDQ. The resu lting product was a statistical mixture of tetraphenylporphyrin, and mono-, di-, tri-, and tetra-substitu ted porphyrins. Purification

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77 was achieved by column chromatography. The desired product, 5-[4-(3-methyl-3hydroxy-1-butyn-1-yl)phenyl]10,15,20-triphenylporphyrin (2), was then deprotected using sodium hydroxide, resulting in the product, 5-(4-e thynylphenyl)-10,15,20triphenylporphyrin (3). The porphyrin was then metallated using zinc (II) acetate to give the final product, Zn(II)-5-(4-e thynylphenyl)-10,15,20-triphenylporphyrin (4). O OH + O +N H 1) BF3OEt22) DDQ N NH N HN Ph Ph Ph OH 16% N N N N Ph Ph Ph 81 % Zn 1) NaOH / Toluene 2) Zn(OAc)2(2) (4) O Br + OH O OH Pd(PPh3)4 / TEA / CuI 40 C / 4hrs 87 %(1) O OH + O +N H 1) BF3OEt22) DDQ N NH N HN Ph Ph Ph OH 16% N N N N Ph Ph Ph 81 % Zn 1) NaOH / Toluene 2) Zn(OAc)2(2) (4) O Br + OH O OH Pd(PPh3)4 / TEA / CuI 40 C / 4hrs 87 %(1) Figure 3-1. Synthesis of Zn(II)-5-(4-Et hynylphenyl)-10,15,20-triphenylporphyrin (4). Oxadiazoles are well know for their elec tron transporting and hole blocking ability due to a high electron affinity.[228-230, 244] In an effort to increa se the electron transporting ability of the acetylene polymers, an oxadiazole containing molecule possessing a terminal acetylene was synthesized. This was accomplished by a procedure suggested by Cha and coworkers.[245] The first step involved the synthesis of 4-bromobenzhydrazide

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78 (5) by the reaction of 4-bromo-ethylbenzoate with hydrazine. The next step was the reaction of compound (5) with 4-methylbenzoyl chloride with first pyridine followed by the reaction with POCl3 to give the product 2-(4-b romophenyl)-5-(4-methyl)-1,3,4oxadiazole (6) in decent yield. The next st ep was the Sonogoshira coupling of (6) with 2methyl-3-butyn-2-ol to give the prod uct 4-2-(4-(3-methyl-3-hydroxy-1-butyn-1yl)phenyl)-5-(4-methyl)-1,3,4-oxadiazole (7). This was followed by the subsequent deprotection using NaH to give th e final product 2-(4 -methylphenyl)-5-(4ethynylphenyl)-1,3,4-oxadiazole (8). O OEt Br Br NHNH2 N2H4 / EtOH90 % O OH O Cl C2O2Cl2 Br NHNH2 1) Pyridine 2) POCl3(5) NN O Br OH Pd(PPh3)2Cl2 / DMF / TEA CuI / PPh3 50 C / 4 hrs (6) N N O HO NaH / Toluene N N O 63 %(7) (8) O OEt Br Br NHNH2 N2H4 / EtOH90 % O OH O Cl C2O2Cl2 Br NHNH2 1) Pyridine 2) POCl3(5) NN O Br OH Pd(PPh3)2Cl2 / DMF / TEA CuI / PPh3 50 C / 4 hrs (6) N N O HO NaH / Toluene N N O 63 %(7) (8) Figure 3-2. Synthesis of 2-(4-methylphenyl )-5-(4-ethnylphenyl)-1,3,4-oxadiazole (8). The use of Rhodium catalysts for the pol ymerization of acetylenes has been known for many years.[194, 246-251] The most commonly used Rh catalyst used for these

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79 polymerizations is the rhodium norbornadiene chloride dimer shown in Figure 3-3. This family of catalysts involves an insertion t ype mechanism as opposed to the metathesis mechanism known for the transiti on metal chloride polymeriza tion catalysts. This family of catalysts also results in the cis-transoid structure polymer as apposed to the transcisoid structure typically obtained by metathesis catalysts. The mechanism for the insertion type polymerization is shown in Figure 3-4. The dimeric rhodium catalyst dissociates into its monomeric form by associ ation with the polymerization solvent. The acetylene monomer then coordinates with th e rhodium metal cente r. The resulting vinylic complex is converted to the termina lly bound acetylene by the interaction with the base to remove a proton. The polymeric chain begins and grows by insertion of the coordinated acetylene monomer with the metal-carbon bond on the metal acetylide catalyst. Termination is facilitated via mono mer chain transfer. This transfer may occur when the acidic acetylenic hydrogen is transferred from the -coordinated monomer to the propagating chain. Rh Rh Cl Cl Figure 3-3. Structure of (Bicyclo[2.2.1 ]hepta-2,5-diene)chlororhodium(I) dimer. For this study, the catalyst used was th e rhodium norbornadiene chloride dimer. The reactions were all carried out in an argon atmosphere drybox. The monomer concentration was held at a constant ratio with respect to the catalyst (100:1). All reactions were performed with CHCl3 as the solvent unless otherwise noted. The polymerization of compound (4) resulted in the homopolym er, poly(ZnETPP), shown in

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80 Figure 3-5. A series of copol ymers were also created using varying monomers as well as monomer concentrations. The copolymers created are also shown in Figure 3-6. These polymers are named as poly(ZnETPP)x-co-(R)y where x + y = 1. Thus, the polymers have the name poly(ZnETPP)x-co-(PE)y when the monomer is ethynyl benzene, poly(ZnETPP)x-co-(PEOXAD)y when the monomer is compound (8), and poly(ZnETPP)x-co-(3,5CF3PE)y when the monomer is 1-ethynyl-3,5trifluoromethylbenzene. The details of the pol ymerizations are shown in Table 3-1. The isolated yield was fairly high for all of the polymers with the exception of poly(ZnETPP)0.5-co-(PEOXAD)0.5 due to the fact that the polymer was extremely soluble in most common organic solvents, resulting in the lack of ability to precipitate the polymer. The molecular weights were dete rmined by gel permeation chromatography carried out in THF with refere nce to polystyrene standards. The eluents were detected by a photodiode selectively tuned to the absorp tion of the porphyrin So ret (420 nm). The elution diagram of GPC fo r all of the polymers show ed a unimodal pattern. The poly(ZnETPP) homopolymer was insoluble in THF and therefore the molecular weight was not determined. Determination of the absolute number average molecular weight can be estimated by the formula given by[252] 1.48()nnMMGPC (3.1) The IR band present in all of the monomers at ~2110 cm-1 assigned to the stretching vibration mode of the carbon-carbon triple bond disappeared upon polymerization. The infrared absorption bands characteristic of the porphyrin ring re mained unchanged after polymerization. This confirms to the assu mption that the polymeri zation occurs through the triple bond of the ethynyl group.

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81 Figure 3-4. Representative scheme of insertion polymerization mechanism.[253] Figure 3-5. Polyacetylene isomer structures.[250]

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82 N N N N Ph Ph Ph Zn nN N N N Ph Ph Ph Zn poly(ZnETPP)poly(ZnETPP)-co-(R) R x y R = F3C CF3 NN O Figure 3-6. Structure of ZnET PP homopolymer and copolymers. The copolymer ratio was determined by 1H NMR. All NMRs were carried out in d6-DMSO at 90 C due to the poor solubility of the polymer. The NMR spectra showed broad peaks for the protons associated with the polymer (see Figure 3-7). The copolymer ratio was determined by the ratio of the inte gration of the protons on the pyrrole of the porphyrin with respect to the protons on the phenyl ring or methyl group of the other comonomer. The positions and widths of th e peaks was determined by analyzing the NMR of the homopolymers. The results of these calculations are shown in Table 3-1. The copolymers ratios with (3,5CF3PE) were unable to be determined by NMR due to the overlap in the spectrum. The copolymer ra tios determined by NMR closely (within the ability to integrate peaks in NMR) matc h the ratios determin ed by monomer feed. No glass transition was observed with DSC within the temperature range studied (80 180 C). As shown in Figure 3-8, the polym ers display good thermal stability, with a 10% weight loss occurring above 300 C with the only exception being the

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83 poly(ZnETPP) homopolymer. This stability ha s been attributed to the bulky pendant groups, which protect the polymer backbone. Table 3-1. Polymerization details. Polymer Copolymer Ratio (NMR) Mw Mn Mw/Mn Isolated Yield % Poly(ZnETPP) * 65 Poly(ZnETPP)0.15-co(PE)0.85 0.15/0.85 77500 33700 2.3 94 Poly(ZnETPP)0.3-co-(PE)0.7 0.25/0.75 74000 44000 1.7 88 Poly(ZnETPP)0.5-co-(PE)0.5 0.45/0.55 174000 78000 2.2 77 Poly(ZnETPP)0.7-co-(PE)0.3 0.65/0.35 160000 73000 2.2 82 Poly(ZnETPP)0.5-co(PEOXAD)0.5 0.5/0.5 97000 47000 2.1 13 Poly(ZnETPP)0.5-co(3,5CF3PE)0.5 ** 271000 162000 1.7 67 *Poly(ZnETPP) insoluble in THF. ** Unable to determine copolymer ratio via NMR. Figure 3-7. NMR sp ectra of poly(ZnETPP)0.5-co-(PE)0.5. Inset numbers indicate integration values.

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84 Temperature / oC 0100200300400500600700 Weight % 20 30 40 50 60 70 80 90 100 ETPP PETPP15PE85 PETPP30PE70 PETPP50PE50 PETPP67PE33 PETPP PPA Figure 3-8. TGA traces of polym ers of ETPP measured in air. Photophysics Absorption All absorption measurements were made as dilute solutions in toluene unless otherwise indicated. The monomer ZnETPP show s absorption nearly identical to ZnTPP indicating the ethynyl group has li ttle influence on the absorp tion of the porphyrin ring. Figure 3.9a shows the absorption spectrum of poly(ZnETPP). Close examination of the absorption spectrum shows that the molar exti nction coefficient of the Soret band at 408 nm is roughly an order of magnitude weaker than ZnTPP. The Soret also shows evidence of broadening with the emergence of a sec ond peak at 428 nm. These results indicate that hypochromism was due to electronic interactions occurring among porphyrin moieties in the polymer. This phenomenon has been observed for other porphyrin polymer systems and has been explained in te rms of an exciton coupling model due to the

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85 approach of two porphyrin rings.[30, 37, 254] Although absorption bands due to the conjugated main chains of poly(ZnETPP) are ex pected to be observed in the range of 250 400 nm, the bands could not be distinguish ed from the absorption due to the porphyrin ring.[255, 256] This indicates that the conjugation in the main chain is small as compared to that of unsubstituted polyacetylenes.[256] The most probable reason for this observation is due to the bulkiness of the porphyrin ring. Examples of the absorption of poly(ZnET PP)-co-(PE) copolymers are also shown in Figure 3.9b. These absorption spectra ar e nearly identical to the ZnETPP monomer and ZnTPP, suggesting that the incorporation of phenyl pendants limits the interaction of porphyrin rings reducing the exc itonic coupling. The change in the copolymer ratio has little effect on the absorption properties, with nearly identical molar absorption coefficients after adjustment for chromophor e concentration. The peak positions were also nearly identical. The absorption of the acetylenic backbone is again hidden by the absorption of the porphyrin -system. The absorption of the poly(ZnETPP)-co-(PEOXAD) copolymer is shown in Figure 3-9c. The spectrum shows the characteristic absorption of ZnTPP with a small decrease in the molar absorption coefficient of the So ret absorption transition when compared to the corresponding poly(ZnETPP)0.5-co-(PE)0.5 copolymer. The Soret shows a 5 nm shift to lower energy, indicating el ectronic interaction between the co-monomers which results in a stabilization of the excited state. Th e Q-bands also show a similar red-shift. The strong band at 300 nm is attributed to the ab sorption of the oxadiazo le moiety, shown in Figure 3-10.

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86 The absorption spectrum of the poly(ZnETPP)-co-(3,5CF3PE) is shown in figure 3.9d. The Soret shows a slight increase in intensity, when compared to the poly(ZnETPP)0.5-co-(PE)0.5 copolymer suggesting that th e chromophore concentration may be higher than expected. The position of the Soret shows a slight shift to lower energy. The Q-bands also show this ba thochromic shift, suggesting electronic interactions with the other co-monomer resulting in stabilization of the excited state. The absorption of the polymers studied show th at there is little e ffect from the main chain and that polymerization has little effect on the overa ll absorption of the system, with the exception of the homopolymer poly(ZnETPP). This polymer has strong evidence of excitonic coupling and that in corporation of any co-monomer into the polymer chain lessens this interaction. Table 3.2. Photophysical data. Polymer Quantum Yield* Absorption, nm ( M-1cm-1) Emission (solution), nm Emission (Film), nm Poly(ZnETPP) 0.010 408(54840), 428(37058), 548(4840), 588 (888) 609, 655 621, 663 Poly(ZnETPP)0.15-co(PE)0.85 0.029 420 (56250), 548 (3037), 588(608) 601, 650 613, 658 Poly(ZnETPP)0.5-co-(PE)0.5 0.032 420 (176250), 548 (9515), 586 (1900) 597, 646 617, 662 Poly(ZnETPP)0.5-co(PEOXAD)0.5 0.031 306(49280), 425 (150850), 554 (13100), 599 (6170) 611, 657 ** Poly(ZnETPP)0.5-co(3,5CF3PE)0.5 0.034 422 (222710), 551(19675), 592 (5190) 613, 659 --* Measured in reference to ZnTPP (0. 033), ** Poly(ZnETPP)0.5-co-(PEOXAD)0.5 non emissive in solid state.

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87 5e+4 0.0 5.0e+4 1.0e+5 1.5e+5 5.0e+4 1.0e+5 1.5e+5 Wavelength / nm 30040050060070 0 0 5e+4 1e+5 2e+5 2e+5 Molar Absorptivity / M-1cm-1 5e+4 0.0 5.0e+4 1.0e+5 1.5e+5 5.0e+4 1.0e+5 1.5e+5 Wavelength / nm 30040050060070 0 0 5e+4 1e+5 2e+5 2e+5 Molar Absorptivity / M-1cm-1 Figure 3-9. Absorption spect rum of (a) poly(ZnETPP) ( ) (b) poly(ZnETPP)0.14-co(PE)0.86 (----), poly(ZnETPP)0.5-co-(PE)0.5 ( ), (c) poly(ZnETPP)0.5-co(PEOXAD)0.5 ( ), and (d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 ( )

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88 Wavelength / nm 250300350400450500550 Arb. units 0.0 0.2 0.4 0.6 0.8 1.0 Figure 3-10. Absorption and fluoresce nce spectrum of PEOXAD monomer. Emission All emission experiments were carried out as dilute toluene solutions unless otherwise stated. Quantum yields were meas ured against the standard ZnTPP in toluene (0.033). The homopolymer poly(ZnETPP) showed emission, in solution, nearly identical in shape and position to that of ZnTPP. The only significant diffe rence was the drop in quantum efficiency (0.01). This can be attributed to the ex citonic coupling model suggested earlier, which increases the non-radia tive decay rate. Emission in a spin coated film shows a slight bathochromic shift with respect to solution. This observation is understandable due to increased aggregation in the solid st ate. Figure 3-11a shows the emission of poly(ZnETPP) in solution and the solid state (thin film). Figure 3-11b show the emission of the c opolymers of poly(ZnETPP)-co-(PE). The incorporation of phenyl pendants on the pol ymer drastically reduce the fluorescence quenching due to excitonic coupling. The overa ll percent of PE in corporation seems to have little effect on the overall quantum efficiency. Several different copolymers,

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89 ranging from 5% to 95% PE were prepared wi th almost identical, within experimental error, quantum efficiencies (see Table 3-2). The overall peak position and peak shapes are also nearly identical. As with the homopolymer, there ex ists a shift to lower energy of the emission peak in the solid state. The emission spectrum of the copolymer w ith the oxadiazole mo iety is shown in Figure 3.11c. The incorporation of PEOXA D has little effect on the overall quantum yield of the system. The peak shape of the porphyrin emission is sl ightly different with respect to ZnTPP, but not enough to draw any valid conclusions. The peak positions are slightly red-shifted (~ 10 nm ) with respect to ZnTPP, sugge sting electronic interaction in the excited state. The only other significant change in the overall emission properties of the system, in solution, is the emergence of a high energy emission band, centered at 350 nm (see Figure 3-10) when excited at high energy (ex = 300 nm). The system was nearly non-emissive in the solid state, suggesting a gr ound state electron transfer occurring in the complex. The G for this electron transfer can be calculated using: 0 1/200(/)(/)GEporporEoxoxE (3.2) where E1/2(pop / por+) is the oxidation potenti al of the porphyrin, E0(ox/ox-) is the reduction potential of the oxadiazole moiety, and E0-0 is the zero-phonon energy absorption of the porphyrin complex. Given the values of 0.51 eV for the oxidation potential for the porphyrin, -1.47 eV[257] as the reduction potential for the oxadiazole moiety, and 2.1 eV as the zero-phonon energy of the ZnTPP complex in the solid state, the calculation yields a G = 0.12 eV. This suggests th at the electron transfer is energetically favorable, causing the non-emissive solid state.

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90 The emission of the copolymers w ith the electron withdrawing CF3PE monomer, are shown in Figure 3-11d. The emission pr operties, in solution, are again nearly identical to that of the reference, ZnTPP. The emission of these copolymers is also redshifted, giving further evidence that electr on withdrawing groups on the polymer have an effect on the porphyrin excited state. This suggests that the polymer backbone plays a role in the overall electronic properties of the system. So me broadening of the emission peaks is also evident. This may be caused an aggregate present in solution. Thermally Induced Isomerization Thin films of poly(ZnETPP) were prepared by spin-coating a solution onto a glass substrate. The films were then placed in a vacuum oven at 150C for a fixed period of time, and then the emission properties were measured. Figure 3-12 shows the emission of thin films of poly(ZnETPP) as a functi on of annealing time. The unannealed film shows little emission and no evidence of em ission from the polyacetylene backbone. Upon heating for 90 minutes, the appearance of a new band growing in at 510 nm suggests the beginning of isomerization from the cis conformation to the trans conformation. Upon heating for 3 hours the em ission at 510 nm is significantly stronger, as is the emission of the porphyrin. The e nhanced emission of the polymer can possibly be attributed to Frster energy transfer. Th is energy transfer can occur due to the fact that the absorption of the porphyrin Q-bands have significant overlap with the emission of the polyacetylene backbone. The film shows a further enhancement of both the backbone as well as the porphyrin emi ssion after heating for 24 hours. This cis to trans isomerization has been observed for ot her singly functionalized polyacetylenes.[194, 258-264] The mechanism of this transformation involves bond scission and rotation.

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91 Figure 3-11. a)Emission (ex = 420 nm) of poly(ZnETPP) in solution ( ), film ( ),(b) poly(ZnETPP)0.14-co-(PE)0.86 in solution ( ), film ( ) (----), poly(ZnETPP)0.5-co-(PE)0.5 in solution (---), film ( ) (c) poly(ZnETPP)0.5co-(PEOXAD)0.5 in solution ( ), and (d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 in solution ( ).

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92 Wavelength / nm 500550600650700750800 Intensity 0 1e+6 2e+6 3e+6 4e+6 5e+6 Neat Film 1.5 hours 3 hours 24 hours Wavelength / nm 500550600650700750800 Intensity 0 1e+6 2e+6 3e+6 4e+6 5e+6 Neat Film 1.5 hours 3 hours 24 hours Figure 3-12. Emission of poly(ZnETPP) in f ilm as a function of annealing time at 150 C in vacuum. Intensity increases as a function of annealing time. Light Emitting Devices Light emitting diodes were fabricated with the polymers, by spin-coating the polymer solution neat or in a blend with the non-conjugated polymer polystyrene. Figure 3.13a shows the current density voltage and irradiance voltage curves for poly(ZnETPP). The neat polymer showed a tu rn on voltage of 10 V but showed almost no electroluminescence emission. Figure 3.14 shows the emissi on spectrum of the polymer in the device. The addition of 1 mg of polystyrene, (25 wt%), caused a dramatic increase in irradiance, as well as an increas e in the current density. The devices also show a turn on voltage of 4 V. This can be explained by the earlier discussed excitonic coupling model. This excitonic coupling acts as a trap causing the lowered current, as well as lowered emission. The addition of pol ystyrene acts to dissociate the excitonic state, by reducing the interaction of individua l porphyrin pendant molecules. The addition of increased amounts of polystyre ne had little additional effect on the overall properties.

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93 The overall current density of the system was extremely high with a current density of 1 A/cm2 at 12 V. The irradiance was fairly low for porphyrin based devices and results in only 1.2 W/cm2, which is over an order of magnit ude lower than devices fabricated from ZnTPP blended into polystyrene (18 W/cm2). The external quantum efficiency (photons emitted / electrons inje cted) is shown in figure 3.16a. Given the fact that the emission is based on current injected a nd photons collected, the external quantum efficiency of the neat polymer was extremely low (< 1 x 10-6). The addition of polystyrene induces an order of magnitude incr ease in the quantum efficiency. This is still approximately one order of magnitude lo wer than similar devices fabricated from porphyrin based complexes.[31, 265-268] There are several possibilities for the high current densities found in this system. The first is the high hole mobility of the polyacetylene main chain. The second major cause of the high current densities was the presence of rhodium nanoparticles formed by Joule heati ng during device operati on. This rhodium comes from the catalyst that remains attached to the polymer chain at the termination of the polymerization. TEM images confirmed th e presence of these particles (Figure 3-15). Figure 3.13b shows the current density voltage and irradiance voltage curves for the copolymer poly(ZnETPP)0.5-co-(PE)0.5. The neat polymer shows a further increase in current density, passing 1.5 A/cm2 at 12 V, and a maximum irradiance of 1 W/cm2. The addition of 1 mg of polystyrene, (25 wt%), caused a 1/3 decrease in the current density as well as a 1/3 increase in maximum irradiance. There are two possible suggestions for this finding; the first is that there was still aggregation in the solid state and that the addition of polys tyrene acted to reduce the amount of aggregation, or the polystyrene was acting as a carrier blocker (mainly holes), minimizing the amount of

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94 intra-chain carrier transport as well as re ducing the amount of holes reaching the cathode interface and the resultant annih ilation. All devices turned on at approximately 4 V. Figure 3.16b shows the external quantum effi ciency as a function of current density. These devices showed a maximum extern al quantum efficiency of 1.5 x 10-5 which was nearly identical to devices created fr om blends of PS with the poly(ZnETPP) homopolymer. The emission of the copolymer s is shown in Figure 3.14 and the peak position is slightly red-shifted with respect to the parent complex in solution. The relative intensities of the high energy peak to that of the low energy peak are also different, when compared to that of the pol ymer in solution. In electroluminescent devices, the high energy peak is significantly less intense as compared to the solvated polymer. Figure 3.13c shows the current density voltage and irradiance voltage curves for poly(ZnETPP)0.5-co-(PEOXAD)0.5. In this system the current density for the neat polymer was nearly one order of magnitude lo wer than that of the homopolymer or of copolymers with PE. This change was attributed to two factors. The first factor is the hole blocking ability of the oxadiazole moiety and the second is the electron transport capability of oxadiazole group. The former is th e most reasonable due to the fact that the current density decreased by nearly an order of magnitude but the irradiance showed very little increase. This suggests that the oxa diazole is blocking holes from reaching the electrode interface, therefore limiting non-active carriers. The addition of polystyrene further lowered the current density and increa sed the irradiance, suggesting that further blocking of holes was occurring, allowing for more efficient devices. Figure 3.16c shows the external quantum efficiency as a func tion of current density. The devices with

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95 polystyrene showed an order of magnitude in crease in efficiency, putting these devices on magnitude with ZnTPP based devices ( 3 x 10-4). The EL emission spectrum for the polymer is shown in Figure 3.14. There wa s a significant blue-shift in the emission spectrum (~ 20 nm). Figure 3.13d shows the current density voltage and irradiance voltage curves for the copolymer poly(ZnETPP)0.5-co-(3,5CF3PE). The current density was similar to devices fabricated with the homopolymer as we ll as with the copolymers containing PE. The major difference was the dramatic increase in the irradiance. The irradiance increased by an order of magnitude from devi ces fabricated with th e other copolymers. This can be explained by the presence of th e electron withdrawing trifluoromethyl groups present in the polymer. These groups helped to enhance electron transport within the polymer and therefore accounted for mo re exciton recombination on the active chromophore (the porphyrin). The addition of polystyrene had little effect on the current density or the irradiance, and therefore the data is not shown. The external quantum efficiency as a function of current density is shown in Figure 3.16d. The results were similar to the devices fabricated with th e copolymer containing the oxadiazole group. This suggests that the addition of electr on transporting / hole blocking groups further enhanced the devices properties. The EL em ission spectrum for the copolymer is shown in Figure 3.14. The emission was also blue-shi fted ~ 20 nm with respect to the solvated polymer.

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96 0.5 1.0 1.5 2.0 500 1000 1500 2000 P(ZnETPP).5-co-(PE).5 w/ 25 wt% PS 0.5 1.0 1.5 2.0 2.5 3.0 50 100 150 200 250 Poly(ZnETPP)0.5-co-(PEOXAD)0.5 w / 25 wt% PS 02468101214 0 200 400 600 800 1000 1200 1400 02468101214 0 5 10 15 20 25 0 200 400 600 800 1000 1200 1400 P(ZnETPP) 25 wt% PS 40 wt % PS 50 wt% PS 0.5 1.0 1.5 2.0 Current Density / mAcm-2Voltage / VIrradiance / Wcm-2a) b) c) d) 0.5 1.0 1.5 2.0 500 1000 1500 2000 P(ZnETPP).5-co-(PE).5 w/ 25 wt% PS 0.5 1.0 1.5 2.0 2.5 3.0 50 100 150 200 250 Poly(ZnETPP)0.5-co-(PEOXAD)0.5 w / 25 wt% PS 02468101214 0 200 400 600 800 1000 1200 1400 02468101214 0 5 10 15 20 25 0 200 400 600 800 1000 1200 1400 P(ZnETPP) 25 wt% PS 40 wt % PS 50 wt% PS 0.5 1.0 1.5 2.0 Current Density / mAcm-2Voltage / VIrradiance / Wcm-2a) b) c) d) Figure 3.13. Current density vs. voltage and irradiance vs. voltage for a) poly(ZnETPP) b) poly(ZnETPP)0.5-co-(PE)0.5, c) poly(ZnETPP)0.5-co-(PEOXAD)0.5 d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 as a function of wt% of polystyrene.

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97 Wavelength / nm 550600650700750 EL Intensity / arb. units 0.0 0.2 0.4 0.6 0.8 1.0 Figure 3-14. Electroluminescence emi ssion of spectrum of poly(ZnETPP) ( ) (b) poly(ZnETPP)0.14-co-(PE)0.86 ( ), poly(ZnETPP)0.5-co-(PE)0.5 ( ), (c) poly(ZnETPP)0.5-co-(PEOXAD)0.5 ( ), and (d) poly(ZnETPP)0.5-co(3,5CF3PE)0.5 ( ) Table 3-3. Electroluminescence data. Active Layer Turn on (V) Max EL Efficiency Peak Current Density (mAcm-2) Peak Irradiance (Wcm-2) Peak Radiance (cdm-2) Poly(ZnETPP) 10 1.6 x 10-6 962 0.06 0.2 Poly(ZnETPP)75% / PS 25% 4 1.1 x 10-5 1287 1.33 3.5 Poly(ZnETPP)60% / PS 40% 4 1.5 x 10-5 1329 1.43 3.7 Poly(ZnETPP)50% / PS 50% 4 1.8 x 10-5 900 1.15 3 Poly(ZnETPP)0.5co-(PE)0.5 3 3.3 x 10-6 2079 0.9 2.3 Poly(ZnETPP)0.5co-(PE)0.5 75%/ PS 75% 4 1.9 x 10-5 1570 1.66 4.4 Poly(ZnETPP)0.5co-(PEOXAD)0.5 4 5.1 x 10-5 286 2.5 6.5 Poly(ZnETPP)0.5co-(PEOXAD)0.5 75% / PS 25% 4 3.0 x 10-4 162 3.1 8.1 Poly(ZnETPP)0.5co-(3,5CF3PE)0.5 4 2.9 x 10-4 1414 26.7 69.4

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98 Rhodium nanoparticles Rhodium nanoparticles Figure 3-15. TEM image of poly(ZnETPP) showing presence of Rh nanoparticles

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99 0 5e-6 1e-5 2e-5 2e-5 0 5e-6 1e-5 2e-5 2e-5 0.0 5.0e-5 1.0e-4 1.5e-4 2.0e-4 02004006008001000 0 1e-4 2e-4 3e-4 Current Density / mAcm-2External Quantum Efficiencya) b) c) d) 0 5e-6 1e-5 2e-5 2e-5 0 5e-6 1e-5 2e-5 2e-5 0.0 5.0e-5 1.0e-4 1.5e-4 2.0e-4 02004006008001000 0 1e-4 2e-4 3e-4 Current Density / mAcm-2External Quantum Efficiencya) b) c) d) Figure 3.16. External quantum efficiency vs. current density for a) poly(ZnETPP) b) poly(ZnETPP)0.5-co-(PE)0.5, c) poly(ZnETPP)0.5-co-(PEOXAD)0.5 d) poly(ZnETPP)0.5-co-(3,5CF3PE)0.5 as a function of wt% of polystyrene ( = 0 wt%), ( = 25 wt%), ( = 40 wt%) and ( = 50 wt%) Discussion The photoluminescent and electrolumin escent properties of monoand disubstituted polyacetylenes typically is depe ndant on emission from the backbone of the polymer, and pendant groups act to increase the conjugation. This is not the case for the

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100 polymers fabricated using the ZnETPP monomer. This is most likely caused by the break in conjugation at the phenyl group of the porphyrin. This allows the porphyrin based polymers to act as an independent ch romophore. Therefore the optical and electroluminescent properties act more like Zn TPP than typical polyacetylenes. Several groups have produced polyacetylenes and have te sted their electroluminescent properties. The EL emission is typically in the blue or green regions of th e spectrum, depending on the substituents. The Epstein group fabricated devices using di-subs tituted polyacetylene which contained a carbazole pendant gr oup which showed emission centered around 550 nm with high current densities (1A / cm-2) for 100 nm thick films.[232] The high hole mobility and low external EL efficiency mirrors devices fabricated using poly(ZnETPP) polymers. This suggests that devices fabricated using pol yacetylenes tend to have poor device performance, mainly due to the hi gh conductivity of the main chain. Conclusions A large series of polymers containing the porphyrin pendant acetylene monomer were fabricated. All of the polymers exhibite d high molecular weights, with adequate to poor solubility in most common organic solvents. Evidence suggested that the homopolymer exhibited a grea t deal of excitonic coupli ng resulting in low emission yield, both in solution and in solid-state de vices. The incorporat ion of a phenyl pendant into the polymer reduced this coupling a nd therefore increased the photoluminescent quantum yield similar to that on ZnTPP. Th is suggested that the polyacetylene chain had only a minor effect on the photoluminescent pr operties and that the major emissive properties are due to the porphyrin pendant. De vices fabricated with these polymers were inefficient due to the high hole mobility present. This could be drastically reduced by incorporation of hole blocking or electron transporting group s appended to the polymer.

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101 These groups acted to reduce th e total number of holes or to increase the total number of electrons within the active layer. These de vices fabricated with the electron deficient groups were on par with devices fabricated with ZnTPP. As also seen in the previous chapter, the major flaw found in devices fabr icated with porphyrin pendant polymers, is the lack of carrier balance. The problem of Rhodium contam ination also contributed to poor device performance. Experimental Monomer Synthesis All reagents and solvents were obtained from Fisher, Aldrich, St rem, or GFS, were reagent grade, and were used as received unless noted othe rwise. Substituted porphyrin synthesis was achieved with signi ficant help from Fengqi Guo. 4-(3-methyl-3-hydroxybut-1-yn-1-yl)benzaldehyde (1). Following a standard procedure,[269]samples of 4-bromobenzaldehyde (17.40 g, 93 mmol), Pd(PPh3)4 (1.136 g, 2.2 mmol), and CuI (102 mg, 0.5 mmol) were placed in a Schl enk flask. The flask was then purged with argon several times. Freshl y distilled and degasse d triethylamine (50 mL) was added. After again purging with argon, 2-methylbut-3 -yn-2-ol (11 mL, 112 mmol) was added. The reaction mi xture was stirred for 6 hours at 40 C. Column chromatography (silica, CH2Cl2) of the crude reaction mixture followed by bulb-to-bulb distillation afforded a pale yellow oil (16.5 g, 93%): 1H NMR (CDCl3) 1.62 (s, 6H), 2.21 (broad s, 1H), 7.52 (2H), 7.81 (2H), 9.02 (s, 1H); 13C NMR 31.9, 66.1, 81.9, 98.8, 129.8, 130.1, 132.7, 135.9, 192.3. 5-[4-(3-methyl-3-hydroxy-1-butyn-1 -yl)phenyl]-10,15,20-triphenylporphyrin (2). Following a standard procedure,[61] a solution of 4-(3-m ethyl-3-hydroxybut-1-yn-1yl)benzaldehyde (1) (2.75 g, 14.6 mmol), be nzaldehyde (4.6 mL, 44 mmol), and pyrrole

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102 (4.0 mL, 58 mmol) in CHCl3 (800 mL) was treated with BF3OEt2 (1.8 mL, 14 mmol). The mixture was stirred at room temperatur e for 1.5 hours under argon. Then, a solution of DDQ (12 g, 53 mmol) in THF (100 mL) was added, and the mixture stirred for 1 hour in air. The mixture was concentrated to one-fourth of the original volume and passed over a silica plug (CH2Cl2). The resulting porphyrins were chromatographed [silica, CH2Cl2/hexanes (1:1)]. A second chromatography column [silica, CH2Cl2/hexanes (1:5) afforded pure compound (1.21g, 16 %): 1H NMR (CDCl3) -2.53 (s, 2H), 1.56 (s, 6 H), 2.1 (s, 1 H), 7.5 7.7 (m, 11 H), 8.1 8.3 ( m, 8 H), 8.6 8.8 (m, 8 H); abs (CH2Cl2) 419, 514, 550, 591, 646 nm. 5-(4-ethynylphenyl)-10,15,20-triphenylporphyrin (3).[243] A solution of 2 (0.6 g 0.78 mmol) in toluene (50 mL ) was treated with powdered NaOH (0.5 g, 12.5 mmol) and the reaction mixture was refluxed for 4 hours. After cooling the reaction mixture was directly poured onto a dry silica pad. The product was eluted with toluene (0.495 g, 92 %): 1H NMR (CDCl3) -2.57 (s, 2H), 3.3 (s, 1 H), 7.5 7.7 (m, 11 H), 8.1 8.3 (m, 8 H), 8.6 8.8 (m, 8 H). Zn(II)-5-(4-ethynylphenyl)-10,15,20-triphenylporphyrin (4).[243] A solution of 3 (0.25 g, 0.39 mmol) in CH2Cl2 was treated with a solution of Zn(OAc)2H2O (1.15 g, 5.24 mmol) in methanol (20 mL). The mixtur e was stirred overnight at room temperature and poured into aqueous NaHCO3. The organic layer was dried, filtered, and chromatographed [silica, CH2Cl2 / hexanes (1:1)] to afford the title product (0.25 g, 92% yield): 1H NMR (CDCl3) 3.3 (s, 1 H), 7.4 7.6 (m, 11 H), 8.0 8.2 (m, 8 H), 8.6 8.8 (m, 8 H).

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103 4-bromobenzhydrazide (5).[245] A mixture of methyl-4 -bromobenzoate (5g, 33 mmol) and hydrazine monohydrate (6 mL, 120 mmol) were re fluxed for 24 hours in 100 mL of ethanol. After the reaction was co mplete, the mixture was cooled to room temperature, and poured into ice water to pr ecipitate the white solid which was collected on a filter and washed with hexane to remove the unreacted starting materials. The resulting product was dried in a vacuum oven for 24 hours. The product yield was 90 % (4.75 g): 1H NMR (D6-DMSO) 4.53 (broad s, 2H), 7.67 (m, 2 H), 7.75 (m, 2 H), 9.86 (s, 1H). 2-(4-bromophenyl)-5-(4-methyl)-1,3,4-oxadiazole (6).[245] 1.0 g (7.3 mmol) or 4methyl benzoic acid was added to 20 mL of oxalyl chloride and refluxed for 2 hours to give 4-methylbenzoyl chloride. The exce ss oxalyl chloride was removed by vacuum distillation and the reaction mi xture was cooled to room temperature. After 30 minutes, 1.57 g (10 mmol) of 4-bromobenzylhydrazide diss olved in 20 mL of pyridine were added to the reaction flask through a dropping funnel over a period of 20 minutes. After stirring for 2 hours, the reaction mixture was poured into distilled water. The product was collected on filter paper and subsequently placed into a round bottom flask and POCl3 (20 mL) was added. The mixture was refluxed for 6 hours under an argon atmosphere. After the completion of the reaction, the reaction mi xture was slowly poured into cold water in an ice bath and 0.5 M NaOH solution was adde d to neutralize the re action mixture. The precipitate was collected on a filter, washed wi th distilled water, and recrystallized from ethanol/water (3:1 v/v). Th e product was a white solid with a yield of 65% (1.14 g) 1H NMR (d6-DMSO) 1.7 (s, 3 H), 7.7 7.8 (m, 8 H).

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104 4-2-(4-(3-methyl-3-hydroxy-1-butyn -1-yl)phenyl)-5-(4-methyl)-1,3,4oxadiazole (7).[245] 2-(4-Bromophenyl)-5-(4-methyl)-1,3,4 -oxadiazole (0.6 g, 2 mmol) was added to a Schlenk flask c ontaining 10 mL DMF, 20 mg Pd(PPh3)2Cl2, 20 mg CuI, and 20 mg PPh3. The solution was degassed with argon for 30 minutes. 10 mL of freshly distilled and degassed triethylamine was a dded and the solution was again degassed for 30 minutes. 1 mL of 2-methyl -3-butyn-2-ol was added and th e reaction was stirred at 50 for 4 hours under argon atmosphere. After comp letion of the reacti on, the mixture was cooled to room temperature. The ammonium salt and other insoluble materials were removed by filtration. The product was pur ified by column chromatography [silica, hexane/ethyl acetate (1:1)] to yield a ye llow-white solid (0.48 g, 81% yield): 1H NMR (CDCl3) 1.4 (s, 3 H), 1.6 (s, 6 H), 2.3 (broad s, 1 H), 7.6 8.1 (m, 8 H). 2-(4-methylphenyl)-5-(4-et hnylphenyl)-1,3,4-oxadiazole (8).[245] A mixture of compound 7 (0.4 g, 1.5 mmol) and NaH (0.2 g, 8.3 mmol) was refluxed for 1 hour in 20 mL of toluene. The reaction was then allo wed to cool to room temperature and the unreacted NaH was quenched by the addition of isopropanol. The product was extracted with diethyl ether several times. The pr oduct was purified by column chromatography [silica, hexane/ethyl acetate (1 :1)] and subsequently dried in a vacuum oven for 24 hours. The yield was 0.25 g (70%): 1H NMR (CDCl3) 1.4 (s, 3 H), 3.3 (s, 1 H), 7.5 8.1 (m, 8 H). Polymerization The homopolymerization of 4 as well as the copolymerization of 4 with ethynylbenzene, 8, and (5-ethynyl)-1,3-(trifluoromethyl)benzene was carried out us ing a commercially available (bicyclo[2,2,1]hepta-2,5 -diene)rhodium(I) dimer [Rh(NBD)Cl]2 (Aldrich) as a catalyst.[242] The ratio of the monomers to the catalyst was maintained at (100:1)

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105 respectively. The monomers we re dissolved in 1 mL of CHCl3 in an argon atmosphere for 30 minutes. In a separate reaction vesse l the catalyst was dissolved in 0.5 mL of CHCl3 along with 100 equivalents of triethylamine (~ 0.2 mL) also under an argon atmosphere and stirred for 30 minutes. The catalyst solution was then added at once via syringe to the monomer solution. The monomer solution was then allowed to stir at room temperature for 36 hours. The crude polymer s were then purified by precipitation from CHCl3 / acetone several times. The resulting polymers ranged in color from deep purple to light red. The molecular weight was determined via gel permeation chromatography (THF) with respect to polystyrene standards. All polymers showed a unimodal pattern in the elution diagram determined by GPC. The results of the polymerization are summarized in table 3.1. The copolymer ratios were determined by NMR. Thermogravimetric Analysis TGA thermograms were taken using a Perkin Elmer TGA-7 under an air atmosphere at a rate of 10C / minute after holding at 40 for 30 minutes. The samples all weighed between 1.5 and 2.5 mg each. Photophysical Measurements All photophysical studies were conducted in 1 cm squa re quartz cuvettes unless otherwise noted. All absorption and emission measurements were made in toluene unless otherwise noted. Absorption spectra were obtained on a double-beam Cary-100 UVvisible spectrophotometer. Fluorescence spec tra were measured on a SPEX Fluorolog-3 equipped with an air-cooled Hamamatsu R 928 PMT for visible measurements. All measurements were corrected for detector response. Quantum yields were measured against Zn(TPP) in toluene ( = 0.033).

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106 Device Fabrication and Measurement Devices were fabricated and tested as stated in the experimental section of Chapter 2.

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107 CHAPTER 4 FUTURE DIRECTIONS The previous chapters focused on the indi vidual projects ca rried out during the dissertation research. This chapter is designe d to provide insight in to the bigger picture and to provide a future directi on for this research project. Non-Radiative Decay Chapter two was devoted to functionalized ytterbium porphyrin complexes for use in light emitting diodes. It was concluded th at there were two major problems with the overall molecular design. The first majo r deficiency is the large pathway for nonradiative decay through the coupling of the la nthanide excited state to the C H bond phonon.[270] This coupling leads to a dramatic de crease in the overall population of the excited state and therefore affects bot h photoluminescent and electroluminescent efficiencies. The second major problem is th e inherently poor stab ility of the organic anion radical. This leads to the large imbala nce in the overall carrier density and mobility allowing for inefficient devices. The first correction to this problem is to lower the phonon energy of the complex and therefore lim it coupling with the excited state. As stated in Chapter 2, energy transitions wh ich can be bridged by three or less phonons have large non-radiative decay rates. This can be accomplis hed by substitution of protons on the complex with atoms with higher atomic mass, in turn reducing the energy of the oscillation. There are several different met hods to accomplish this substitution. The first is to exchange the protons for deuterium atom s. This would cause a significant shift of the vibrations to lower energy. Although th is is possible, the starting materials are

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108 prohibitively expensive, and the possibility of proton exchange w ith organic solvents exists. The substitution of the protons with halogens has been shown to increase the intersystem crossing rate, therefore increasing the triplet yield.[271, 272] Halogenation has a twofold effect. It would reduce the energy of the phonon modes of the system as well as increase the electron affinity of the complexes. This increase in electron affinity is due to the electrophilic nature of the halogens whic h in turn reduce the electron density on the porphyrin macrocycle. This causes a stabili zation of the LUMO of the complex. Halogens also cause a distortion of the plan ar nature of the complex which therefore destabilizes the HOMO. Terbium X8TPP complexes have been made and the results show that halogenation causes a decrease in HOMO-LUMO gap and a large decrease in the energies of oxidati on as well as reduction.[273] These halogenations are currently synthetically challenging using the synthetic method referred to in Chapter 1, due to the fact that lithiation of haloge nated organics has been shown to be somewhat unsafe. Alison Knefely has developed a method of la nthanide insertion into the porphyrin macrocycle using tris alkyl lanthanide complexes, wh ich has shown promise alleviating this problem. N N N N N N N N N N B H Yb X X X X X X X X F3C F3C CF3 CF3 F3C F3C R2 R1 R1 R2 R1 R1 R2 R1 R1 R2 R1 R1 Figure 4-1. Molecule proposed to increase emission quantum yield. X represents a halogen and R represents or ganic groups or hydrogens.

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109 Table 4-1. Vibrational frequencies of common bonds in organic molecules. Bond Bond Stretching Frequency (cm-1) C-H 2900 3200 C=C 1450 1680 C=N ~1800 O-H 2500 3650 C-D 2200 2400 C-F 1100 1350 C-Cl 800 700 C-Br 700 500 C-I < 500 Carrier Transport The problem of poor carrier transport can be addressed in two methods. The first method is to append electron withdrawing groups (EWG) to the positions of the porphyrin macrocycle. This was briefly t ouched upon earlier in this chapter with substitution of halogens. This process can also be carried out us ing other EWGs on the pyrroles. The most logical example would be the substitution of cyano groups. Cyano groups are known to be electron wit hdrawing and substitution at the position on the porphyrin is known to cause a dramatic infl uence on the electroni c properties of the system. This combination has been shown to cause a dramatic stabil ization of the LUMO in porphyrins, inducing the first reduction to appear at posit ive values with respect to Fc/Fc+ for tetracyano(tetraphenylporphyrin).[274] The second method was discussed in Chapter three, where the porphyrin can be a ppended to a polymer chain which contains electron transporting moieties. This met hod was shown to be effective for ZnTPP appended to polyacetylene, but not for lanthani de substitute porphyrin due to synthetic challenges posed by the terminal acetylene. The lithiation step, required for lanthanide metallation, resulted in deprotonation of the terminal acetylene followed by indeterminate side reactions.

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110 Porphyrin Containing Polymers Although the previous processes may work, th ere are several other directions which can be undertaken to further enhance the ove rall efficiencies of devices created using lanthanide porphyrin complexes. The first is a combination of a conjugated polymer with the lanthanide porphyrin, where the porphyrin is part of the polymer backbone. Figure 4-2 shows an example combining the prev iously discussed solutions, by placing a halogen or EWG substituted porphyrin into the main chain of a conjugated polymer, poly(phenylene ethynlene) (PPE), which also contains the electr on transporting / hole blocking oxadiazole moiety. N N N N N N N N N N B H Yb R2 R1 R1 R1 R2 R1 O N N y x+y xCF3 CF3 F3C F3C F3C F3C X X X X X X X R R Figure 4-2. Conjugated polym er, PPE, containing substi tuted porphyrin and oxadiazole in main chain. R represents a solublizing alkyl or alkoxy ligand. X is a halogen or EWG. A second possible future dire ction involves lanthanide porphyrins appended to nonconjugated polymers. These polymers are design ed to reduce the a ggregate properties of the complexes as well as to lim it the conductivities associated with conjugated polymers. They are also designed to reduce phase segregation observed when porphyrins are blended with non-conjugated polymers. Seve ral different polymers were synthesized

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111 using free radical induced chai n growth mechanism. A styre nyl lanthanide porphyrin was copolymerized with 4-tert-butyl styrene (TBS) and 2,2, 2-trifluoroethylmethacrylate (TFEM) using 2,2-azobisisobu tyronitrile (AIBN). The resu lting polymer contained ~ 1 mol % of the porphyrin species. The molecu lar weight was determined to be 200,000 by GPC. Absorption studies of the polymer show that the porphyrin Soret is observed but the Q-bands were too weak to detect. Fluorescence studies showed only weak fluorescence from the porphyrin with no lantha nide emission observed. Attempts to create a polymer with a higher concentration of the porphyrin species were unsuccessful. NN C C AIBN CH3 CN CH3 CN CH3 CH3 NN C C CH3 CN CH3 CN CH3 CH3 . N2C CN CH3 CH3 + 1. Initiation C CN CH3 CH3 + Styrene CH CH2 I I 2. Chain Propagation CH CH2 I R + CH CH2 I CH2 CH R 3. Termination CH CH2 CH2 CH R CH CH2 CH2 CH R + CH CH2 CH2 CH R CH CH2 CH2 CH R n nn Figure 4-3. Scheme of a free ra dical chain growth mechanism.

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112 O O F F F t -BSTFEM Figure 4-4. Struct ures of monomers. Non-conjugated polymers containing the trispyrazole borate (Tp) units were synthesized by Prof. Frieder Jaekles group in order to incorporate LnTPP complexes onto polymers. Alison Knefely appended th e lanthanide porphyrin onto the polymer using the salt metathesis reaction discussed in Chapter 1. The resulting polymers were insoluble in most organic solvents with th e exception to the slight solubility in odichlorobenzene. Molecular weights were i ndeterminate. Absorption studies performed in dichlorobenzene showed t ypical metallated porphyrin sp ectra with the Soret band ~ 423nm and three Q bands between 520 a nd 630nm. Emission studies showed Ybemission ~980 nm emission of the Yb3+ 2F5/2 2F7/2 transition with weaker emission bands at lower wavelengths, which is attributed to the crystal field splitting of the Yb3+ fstates by the axial and porphyrin ligands. De vices were not fabricated due to the poor solubility of the polymer complex. Inco rporation of alkyl or alkoxy groups onto the porphyrin may help with the solubility issues of this polymer. Conclusions Although the major deficiencies of effici ent non-radiative decay pathways and poor carrier transport in lanthanide porphyrin based near-infrared devices have been identified, the challenge still exists to help alleviate th ese problems. Progress has been made in the

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113 form of polymers which contain electron w ithdrawing groups and polymers which reduce phase separation, but there is plenty of work to be done. CH CH2 B N N N N N N CH CH2 Si 3 7 Na+Figure 4-5. Structure of TP polymer. CH CH2 B N N N N N N CH CH2 Si 3 7 Na++ YbTPPCl(DME) CH CH2 B N N N N N N CH CH2 Si 3 7 N N N N Yb DMSO 2 days Figure 4-6. Synthesis of YbTPP-TP polymer.

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114 Figure 4-7. Absorption spect rum of YbTPP-TP polymer. Figure 4-8. NIR emission spectrum of YbTP P-TP polymer. (RT, o-dichlorobenzene)

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115 APPENDIX ELECTRONIC PROPERTIES The theoretical prediction of the energy levels of a lanthanide metal complex is extremely complicated, if not impossible, because it is a many-body problem. The general approach to solving the pr oblem is to separate it into three parts. The first part is to determine the wavefunctions and energy leve ls for the ions in a spherically symmetric environment using the quantum mechanics of many electron atoms. This is known as the free-ion problem. The second part of the problem involves calcula ting the energy levels of the metal ion in the static electric field produced by the metal ion nearest neighbors. This is called the crystal field problem. The final part of the problem involves the coupling of the metals electronic states to the organic molecular states. The Free-Ion Problem Several approaches have been developed to determine the energy level structure of an isolated, multi-electron atom.[275-278] These methods are covered in many different quantum mechanics textbooks.[279-283] The following discussion is a brief review of the important concepts involve d in the calculations. The free-ion Hamiltonian, HFI, for a multi-electron atom is given by 2 22 1 00()() 244n i FIiii iij iijp Z ee Hrls mrr (A.1) where the first summation is over the N el ectrons and the second summation is over the electron pairs, ri is the position of the ith electron, e is the electron charge, and m is the rest mass of an electron. Th ere are four contributions to the full Hamiltonian. The first

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116 term is the kinetic energy of the electrons expressed in terms of the momentum operator, pi, and the electron mass. The second term is the potential energy of the electrons and a nucleus containing Z protons. The third term is the spin-orbit interaction and the last term is the inter-electron Coulomb interaction. To simplify the problem, the interelectron Coulomb interac tion is averaged over 4 radians to obtain a spherically symmetric potential that each el ectron experiences due to the other electrons. This is known as the central-field approximation. The Hamiltonian is now of the form 2 1()()() 2n i FIiiiii ip HVrrls m (A.2) Where 22 00 ,() 44ii ij iijZee Vr rr (A.3) If the spin-orbit interaction is smaller th an the inter-electronic Coulomb interaction (valid for atoms with low atomic numbers) the Russell-Saunders approximation can be applied and the spin-orbit interaction causes a splitting of the Russell-Saunders LS terms into J multiplets based on the Landr interval rule. We assume for now that the spin-orbit interaction can be ignored and we are left with the orbital Hamiltonian, Ho. 2 1() 2n i oii ip HVr m (A.4) The eigenfunctions of Ho consist of radial and angul ar components. The angular part of the eigenfunction for each electron is given by the angular components of the hydrogen eigenfunctions, but the radial compone nts differ from the radial components of the hydrogen functions. The eigenfunctions of Ho for each electron are given by

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117 ,()(,)lm lnliliinlmRrY (A.5) The one-electron eigenfunction of the Hamiltonian, Ho, are assumed to consist of independent radial, angular, and spin compone nts. The one-electron eigenfunctions of Ho are given by ,()(,)()lm inliliisi R rYSm (A.6) The multi-electron eigenfunction of Ho is given by the product of the one-electron eigenfunctions. i i (A.7) In order to ensure that the wavefunctions ar e invariant under intercha nge of two electrons and that the Pauili exclusion principle is satisfied, the multi-electron wavefunctions are written in the form of a Slater determinant. (1)...(1) 1 (1,2,3,...) ()...()lN lNN N NN (A.8) The eigenfunctions of Ho are a set of functions labeled as LSLSMM. L is the total orbital angular momentum quantum number given by i iLl (A.9) where li is the orbital angular momentum of the ith electron. S is the total spin quantum number given by i iSs (A.10) where si is the spin of an electron, which is equal to ML is the orbital magnetic quantum number, and Ms is the spin magnetic quantum number.

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118 Returning to the Hamiltonian FIoeeHHH (A.11) Where 2 04ee ij ije H r (A.12) Hee commutes with L and S so that it can only mix states with the same L and S quantum numbers. Configurational mixing of levels occurs whenever equivalent LS terms are derived for the same electronic configuration. To the extent that the configuration mixing is small, the eigenfunctions of Equation A.11 are the same as the eigenfunctions of Ho. The degeneracy of the LS terms under Ho is removed when the Coulomb interaction, Hee, is included in the Hamiltonian. Assuming neglig ible configurational mixing, the shifts of the various LS terms from their central fiel d values are calculated by solving the matrix elements LSeeLSLSMMHLSMM (A.13) These matrix elements consist of spin, a ngular, and radial over lap integrals. The spin and angular contributions can be calcula ted exactly but the radi al wavefunctions are not known and so the radial overlap integral s are expressed in te rms of Slater-Condon parameters, Fk. In this manner the energy of each LS term can be calculated and the values of the Slater-Condon paramete rs are determined empirically. The derivation of the free-energy eigenva lues and eigenfuncti ons of rare-earth ions depends upon the relative strengths of and Hee. If Hee >> then the problem proceeds as listed above. For the case where the spin-orbit coupling is not small with respect to Hee then mixing of L S L SMM functions with the same values of J and MJ

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119 requires that the eigenfunctions of the full Hamiltonian, HFI, be labeled by the functions, J LSJM, where LS J LSLSJ MM L SJMLSMMLSMMLSJM (A.14) and LSJLSMMLSJM are vector-coupling coefficients. In conclusion, the energy spectrum of a multi-electron atom is not usually determined solely from theory. If the wavefunctions of the complete Hamiltonian are taken to be Slater determinant wavefunctions, the energy levels can be expressed in terms of a small number of Slater-Condon parameters The values of these parameters are determined by measuring the emission or absorption spectrum of the atom. The Crystal Field Problem The second part of the problem is the crys tal field problem. This is the structure dependent part of the problem. The presence of a metal ion in a lattice can be modeled by assuming that the metal ion is su rrounded by a set of fixed point charges[284, 285] intended to represent neares t neighbor ligands. The poi nt charge model is an approximation because ligand covalency a nd non-spherically symmetric ligand bonding interactions are neglected. This problem can be viewed as lowering the physical symmetry of the free ion to symmetry consis tent with the bonding environment imposed by the lattice. For the sake of simplicity th e organic system will be treated similar to a lattice of similar atoms. After solving the free-ion problem the en ergies and wavefunctions of the free ion are known. These energy levels are then pert urbed by placing the free ion into the crystal field environment of the lattic e. Crystal field theory is used for the calculation of the

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120 energy levels of a metal ion in a solid stat e bonding environment. The full Hamiltonian for the metal ion in the crystal can be expressed as FICFOeeSOCFHHHHHHH (A.15) where HFI is the free-ion Hamiltonian. HCF is the crystal field Hamiltonian and can be expressed by 2 01 4CF il li Z e H R r (A.16) where the summation over l is the sum over the electrons in the unfilled shell of the metal ion and the summation over i is the sum over the neighbori ng ions (6 for octahedral) which occupy equilibrium positions, Ri. The eigenfunction and eigenvalues of H are found using one of the following methods. When the crystal field interactions is smaller than the inter-electron interaction and the spin-orbit coupling interaction, th e eigenfunctions of H are taken to be the eigenfunctions of HO and the new energy level struct ure can be determined using perturbation theory. This weak field approach works well for the shielded ions such as the trivalent rare-earth ions. The intermediate crystal field approach is used when the crystal field interaction is intermediate between that of the inter-electr on interaction and th e spin-orbit coupling interaction. Now, the spin-orbit interaction is neglected initially. The orbital angular momentum wavefunction of the free-ion are used as the starting basis functions. The matrix elements of 'CFLHL are calculated and used to generate new orbital wavefunctions. These orbital wavefunctions are multiplied by spin wavefunctions and then the effect of the spin-orbit inter action is included as a final perturbation.

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121 The last case is the strong crystal field cas e in which the crystal field interaction is stronger than the inter-electroni c interaction. In this case, the inter-elect ron and spinorbit interactions are initially neglected and the starti ng wavefunctions are products of one electron crystal field orbitals. The inter-electron interaction is taken into consideration and new orbital eigenfunctions and eigenvalues are determined. The eigenfunctions are multiplied by spin wavefunc tions and then the effect of spin-orbit interaction is calculated. A qualitative understanding of the role of the crystal field can be obtained by considering the effect of an octahedral crys tal field on a system containing f orbitals. One way of labeling these orbitals is acco rding to their orbital magnetic quantum numbers. An electron in an f orbital has orbita l angular momentum equal to 3 so the possible orbital magnetic quantum numbers ar e +3, +2, +1, 0, -1, -2, and -3. These f orbitals are labeled f+3, f+2, f+1, f0, f-1, f-2, and f-3. In a spherically symmetric environment these orbitals are degenerate. When these orbitals are placed in a non-spherical environment such as an organic lattice, these orbitals liste d above are no longer eigenstates of the system. In order to correct this problem new orbitals which are real functions are created. These symmetrical orbita ls are linear combinations of the orbitals labeled by their magnetic quantum number.[286] The crystal field Hamiltonian, HCF, for a meta l ion with a single outer electron located a distance, a, away from six lattice ions each with a charge of Ze is given by

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122 2 2222 0 2 2222 0 2 2222 011 () 4 22 11 4 22 11 4 22CFhZe HO raaxraax Ze raayraay Ze raazraaz (A.17) where r (r2 = x2 + y2 + z2) is the position of the electron of the metal ion. The Hamiltonian can be expanded assuming r < a to obtain a new expression for the Hamiltonian valid to sixth order in r. 22 4444 5 00 666242424 2 7 2424246 016353 () 4445 15 () ()( 21 4 15 42 ) 14CFhZeZe xyzr aa HO xyzxyxzyx Ze a yzzxzyr (A.18) The Hamiltonian can be further simplified by writing in terms of spherical harmonics. Inserting these spherical harm onics leads to the function 24 044 5 444 26 044 7 666495 1814 ()2 97 8322CFhZer a HO Zer aYYY YYY (A.19) The radial overlap integrals of the various ()'llmCFhmdHOd matrix elements are expressed in terms of two parameters, Dq a nd Fr, for f-electron systems. These matrix elements are necessary to determine the secular determinant, which ultimately provides the energy levels.

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123 Table A-1. Real orbitals created from linear combinations of orb itals labeled by their magnetic quantum number. f-orbitals 30z f f 2331 () 2xz f ff 233() 2yzi f ff ()221 () 2zxy f ff 2222 ()() 2zxyi f ff 2211 (3)1 () 2xxy f ff 2211 (3)() 2yxyi f ff 4 52 165 er Dq a (A.20) 6 75 572 Z er Fr a (A.21) Solving for the eigenvalues of the matrix for the f-electron systems results in two separate three-fold degenerate states (6Dq+20 Fr and -2Dq -36Fr) and a singlydegenerate state at -12Dq+48Fr. The a2u orbital is lowest in energy followed by the three-fold degenerate t2u orbitals and the three-fold t1u orbitals at highest energy. Table A-2. Position and symmetry of f -orbitals in an octahedral crystal field. Orbitals Position & Symmetry fxyz -12Dq+ 48 Fr, a2u fx(5x 2 r 2 ), fz(5z 2 -r 2 ), fy(5y 2 -r 2 ) -2Dq 36 Fr, t2u fz(x 2 -y 2 ), fx(y 2 -z 2 ), fy(z 2 -x 2 ) 6Dq + 20 Fr, t1u

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124 Coupling of the Electronic and Lattice States The energy levels of a metal ion in a so lid can be described using a Hamiltonian that includes contributions from the free ion and crystal field Ham iltonians and potential and kinetic energy contributions from the l attice. The free-ion and crystal field contributions have been discussed in the prev ious two sections. The total Hamiltonian is given by 2()(,)() 2l FIiCFilIl l l p HHrHrRVR m (A.22) where HFI is the free-ion Hamiltonian, HCF is the crystal-field Hamiltonian, VI(Rl) is the inter-ion potential energy and the last term is the sum of the kinetic energy of the lattice ions. Rl is used to denote la ttice ion coordinates and ri is used to denote the positions of the metal ions electrons. The Hamiltonian is complicated because of the coupling of the electronic and ionic systems that occurs through the crystal field Hamiltonian. The electronic and ionic coordinates are decoupled using the Born-Oppenheimer approximation.[17, 287] The electronic contribution of the static lattice to the Hamiltonian is given by ()(,)()OFIiCFilIlHHrHrRVR (A.23) where Rl is considered a parameter, not a variable That is, the static lattice problem can be treated by solving over a desi red range of fixed values of Rl. The static Hamiltonian can be further separated into elect ronic and inter-ionic contributions. (,)()OeilIlHHrRVR (A.24) The eigenfunctions of Ho are given by (,)ailrR The subscript, a, is necessary to label the electronic state b ecause the value of Rl depends upon the electronic state of the

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125 system. The eigenvalues of the static Hamiltonian are labeled E(a)(Rl) and for each Rl can be obtained from ()(,)()(,)a OaillailHrRERrR (A.25) ()()(,)() ()()a laeilaaIla elIlERHrRVR HRVR (A.26) The full Hamiltonian for the dynamic lattice is given by 2 02l l l p HH m (A.27) For a lattice comprised of a si ngle type of ion with a mass, ml, the Hamiltonian can be expressed as 2 22Ol l lHH m (A.28) The eigenfunctions of the full Hamiltoni an are assumed to be of the form (,)()ailalrRR (A.29) These eigenfunctions are known as Born-Oppe nheimer functions. Applying the lattice kinetic energy operator to wavefunctions of this form yields 22 222(,)(,)2()() 22lailailalalalaala llrRrR mm (A.30) In the Born-Oppenheimer approximation the firs t two terms are assumed to be negligible compared to the last term. This assumes th at the electronic stat e does not have a strong dependence on the value of Rl. Physically this describes a situation in which the metal ion does not change electronic states due to mo tion of the lattice atoms. The Schrdinger equation for the metal ion in the lattice can be written as

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126 2 2 0(,)()()(,)(,)() 2ailallalailailal l lHrRRRrRErRR m (A.31) Simplifying gives 2 ()2()(,)()()(,)(,)() 2a lailallalailailal l lERrRRRrRErRR m (A.32) 2 ()2()()() 2a llalal l lERRER m (A.33) The lattice atoms are assumed to oscillate about an equilibrium position so that the position of the latti ce atoms is given by Rl = Rl (a)(O) + q1 (a) where Rl (a)(0) is the equilibrium coordinate of ion l in the electronic state a and ql (a) is the displacement of the ion form its equilibrium position. The electr onic energy can be expressed in terms of contributions from the static lattice and cont ributions from the dynamic lattice. The static contribution is E0 (a) where E0 (a) is given by ()()()()() 0((0))((0))aaaaa elIlEHRVR (A.34) The dynamic contribution is ED (a) which is given by ()()()()()()()aaaaa DelIlEVqVq (A.35) Ve(a) is the contribution if the dynamic lattice to the electro nic Hamiltonian, He(a). The total energy is given by ()()()()()() 0()()()aaaaaa lelIl E REVqVq (A.36) ()()()() 0()()aaaa llEREVq (A.37) The Schrdinger equation can be rewritten as 2 2()()() 0()()() 2aaa llalal l lVqREER m (A.38)

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127 where V(a)(ql(a)) is the potential en ergy of a 3-dimensional network of coupled oscillators. The standard appr oach to this problem is to describe the problem in terms of collective motions of the lattice atoms known as normal modes of oscillation.[288] The normal modes are a function of a new set of coordinates, Qk, rather than the individual lattice atom coordinates, ql. This is a transformation from the lattice coordinates to normal coordinates. If V(a)(Qk) has a quadratic dependence on Qk, then Equation A.38 can be solved by analogy to a system of c oupled harmonic oscillat ors. The lattice eigenfunctions, a(Q(a)), can be expressed as the produc t of linear harmonic oscillator eigenfunctions, kn, where k labels the normal modes. ()()()aa ka kQn (A.39) Equation A.38 becomes 2 2()()1 () 22aa lkakka lk lVQn m (A.40) The energy of the metal ion that is in the electronic state, a, can be written as a summation of contributions from the static and dynamic lattices. ()() 01 2aa kk kEEn (A.41) The lattice can support many different nor mal vibrational mode s. In order to simplify the analysis, a single normal mode is assumed to be the dominant lattice contribution to the energy levels of the metal ion. This mode is generally taken to be the totally symmetric breathing mode. In the breathing mode the lattice atoms near the metal ion oscillate in phase towards and aw ay from the metal ion. The equilibrium separation between the metal ion and its sh ell of nearest neig hbors represents the

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128 equilibrium position of the normal coordinate, Q0 (a). Q0 (a) is called the configuration coordinate. Recall that th e electronic wavefunction, a, depends upon the lattice coordinates. The electronic wa vefunction is calculated using the equilibrium value of Q. The full wavefunction can be written as () 0(,)()a aiarQn (A.42) Einstein Model To understand the electronic pr operties of a system, it is important to understand the processes involved. In sp ectroscopic experiments the natu re of matter is probed by measuring the absorption or emission of elect romagnetic waves. A useful model of the interaction between light and matter was developed by Albert Einstein.[289-291] The Einstein model assumes that atoms exist in a se ries of discrete energy levels. Transitions between the various energy levels are acco mpanied by the emission or absorption of photons. The transition must conserve energy, so the energy of the photon, must equal the difference in energy between the two levels that participate in the transition. The energy density of a radiation field in th ermal equilibrium with a collection of atoms at temperature, T, is given by 3 23() exp1bc kT (A.43) where is the angular frequency of the electromagnetic radiation, c is the velocity of light, and kb is the Boltzman constant. Consider the simple case of a radiation field interacting with a collection of two level atoms. There are N1 atoms in the lower energy level which has energy, E1, and a degeneracy of g1. Similarly, there are N2 atoms in the

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129 upper energy level which has energy, E2, and a degeneracy of g2. Atoms in the upper level can spontaneously transfer to the lowe r level with the simu ltaneous emission of a photon. The energy of the photon emitted is given by 21EE (A.44) The rate at which a single atom spontane ously decays from the upper level is called the Einstein A coefficient. Two other pro cesses can occur if the atoms are placed in a radiation field. These two processes are know n as absorption and stimulated emission. Absorption of a photon transfers an atom fr om the lower level to the upper level. Stimulated emission transfers an atom from the upper level to the lower level with the simultaneous emission of a photon. Stimulated emission is the pro cess involved in the operation of lasers. The rate of absorption is proportional to the Einstein coefficient, B12, and the rate of stimulated emission is pr oportional to the Eins tein coefficient, B21. The rates for all three processes are given below. Spontaneous emission rate = N2A Absorption rate = N1B12 ( ) Stimulated emission rate = N2B21 ( ) Under steady state conditions, the rate of cha nge in the number of atoms in each level is equal to zero. The rate of change of the num ber of atoms in the lower level is given by 1 1122212()()dN NBNBNA dt (A.45) Setting the rate equal to zer o yields the relationship 1122212()()NBNBNA (A.46) The relative sizes of the N1 and N2 populations are given by

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130 22 11expbNg NgkT (A.47) if the two energy levels are in thermal equilibrium at temperature, T. Combining equation (A.45) and equati on (A.46) and solving for () yields 2 1 2 1221 1exp () expb bg A gkT g B B gkT (A.48) The relationships between A, B12, and B21 are determined by simultaneously satisfying equation (A.46) and equation (A.48). 2 1221 1g B B g (A.49) 3 212 22 B g A c (A.50) Another important quantity in spectr oscopy is the spontaneous lifetime, of an excited state. The spontaneous lifetime of an ex cited state is determined by the equation 2 2dN NA dt (A.51) which when solved yields 22(0)exp()NNttA (A.52) 221 (0)expNNt (A.53) where 1A. The Einstein coefficients are related to many other quantities commonly used in spectroscopic measurements. These rela tionships are derived in many books on

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131 spectroscopy and thus will not be derived here.[292] Briefly, the electric dipole strength, S, is defined as the square of th e electric dipole transition moment, 21er 2 21Ser (A.54) The absorption coefficient, of a homogeneous sample is defined by the following equation. ()(0)exp()vvv I xIx (A.55) Iv (0) is the intensity of light with frequency, incident upon the sample. Iv(x) is the intensity of the light after it has passed a distance x through the sample. Absorption will occur over a range of frequencies determined by the lineshape of the transition. The absorption coefficient integrated over the wi dth of the transition is proportional to the Einstein coefficient, B12. 0 00 112vv v vvhv dvnB c (A.56) where n1 is the concentration of atoms in the lo wer level. The absorption coefficient can be converted into absorption per atom if the concentration in the lo wer level is known. The absorption per atom is called th e radiative atomic cross-section, v, and it is equal to v divided by the concentration, n1. 0 00 12vv v vvhv dvB c (A.57) The relationship between the osc illator strength, f, and the integrated intensity of the absorption band is given by the equation below. 0 02 1 04vv v vve dvNf mc (A.58)

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148 BIOGRAPHICAL SKETCH Garry Brian Cunningham was born D ecember 20, 1972, in New Castle, Pennsylvania. After graduation from high sc hool, he underwent a four year stint in the U.S. Army as a fire direction control sp ecialist. He then attended Lock Haven University, where he planned on getting a degr ee in pharmacy. It was then, during all of the prerequisite science classes, that he deci ded pharmacy was not for him. He realized his passion was in chemistry a nd subsequently changed his majo r. He graduated in 1999, with his B.S. in chemistry. After receivi ng his bachelors degree, Garry continued his studies at Washington State University (WSU ). After some unforeseen and unfortunate events, he decided to leave WSU with his M. S. degree in physical i norganic chemistry. He then moved on to the University of Fl orida, where he pursued a Ph.D. degree in inorganic chemistry in 2002. Upon graduati on, Garry will be moving to North Carolina to work at International Tec hnology Center (ITC), wh ere he will be a research associate. Beyond then, the future has not been written.


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Permanent Link: http://ufdc.ufl.edu/UFE0012302/00001

Material Information

Title: Electroluminescence and Photophysical Properties of Near-Infrared Luminescent Lanthanide (III) Monoporhyrinate Complexes and Pendant Polymers
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012302:00001

Permanent Link: http://ufdc.ufl.edu/UFE0012302/00001

Material Information

Title: Electroluminescence and Photophysical Properties of Near-Infrared Luminescent Lanthanide (III) Monoporhyrinate Complexes and Pendant Polymers
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012302:00001


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Full Text












ELECTROLUMINESCENT AND PHOTOPHYSICAL PROPERTIES OF NEAR-
INFRARED LUMINESCENT LANTHANIDE (III) MONOPORPHYRINATE
COMPLEXES AND PENDANT POLYMERS












By

GARRY BRIAN CUNNINGHAM


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Garry Brian Cunningham




























I dedicate this to my father.















ACKNOWLEDGMENTS

There are so many people that I would like to acknowledge. First and foremost is

my advisor, Professor Kirk Schanze. He has been a great mentor. I would also like to

acknowledge all of the people that have helped me on my projects along the way

including Professor John Reynolds, Professor Paul Holloway, Dr. Tim Foley, Dr.

Mauricio Pinto, Dr. Alison Knefely, Dr. Avni Argun, Dr. T.S. Kang, Dr. Benjamin

Harrison, Nisha Ananthakrishnan, Dr. Fengqi Guo, Dr. Jeremiah Mwuara, and Dr. James

Boncella. I would like to thank former and current members of the Schanze group. I

would like to thank my friends who made living in Gainesville bearable. Lastly, I would

like to thank my committee.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ....................................................... ............ .............. .. vii

L IST O F FIG U R E S .............. ............................ ............. ........... ... ........ viii

A B S T R A C T .............................................. ..........................................x iii

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

E lectrodynam ics ...................................................................... 1
A absorption of Light .................................................... .... ............. ..
E m mission of L eight ....................................................... .......... ...... .. 3
N onradiative D ecay ............................................ .... .... ................ .8
Energy Transfer .................................................................. .......... 9
L a n th a n id e s ........................................................................................................... 12
P o rp h y rin s ...............................17.............................
Photophysics ............................................ 17
R e d o x P ro p ertie s ........................................................................................... 2 0
Synthesis .............................................. 22
L eight E m hitting D iodes ............................................................25
OLED and PLED Structure ............................................................ .....29
Carrier Transport ................................. ........................... .... ...... 30
D evice E efficiency ............................................................33
D evice Failure M echanism s ........................................................ 34
L iteratu re R ev iew ................................................................................................. 3 5
O rg an ic Sy stem s .............................................................................. 3 6
Inorganic Nanoparticles................................................37
Organo-Lanthanide and Organo-Transition Metal Complexes ...........................39

2 SUBSTITUTED PORPHYRIN COMPLEXES ........................................ ...42

Solution Photophysics ................................................ ................. 44
Absorption ....................................................... ..................... 44
E m mission .......... ......................................................................... 45
L eight E m hitting D evices.. ........................................................................ ...............54


v










Electrochemistry ........................................... .................. .... ........ 64
C o n c lu sio n s........................................................................................................... 6 9
Experimental ................................................................ .... ...................70
Photophysical M easurem ents ........................................ ......................... 70
D evice F abrication .......... ............................................................ ...... .... ... .7 1
IT O E tch in g .............................................................................. 7 1
C leaning IT O ................................................................... ............... 72
S p in C o atin g ........................................................................................... 7 2
M etal Electrode D eposition...................................... ........................ 73
Electroluminescent Device Measurements..................................... ..................73
Electrochem istry .................. .......................... .. ....... ................. 74

3 PORPHYRIN PENDANT POLYACETYLENES................................ ................75

In tro d u ctio n ........................................................................................................... 7 5
Monomer Synthesis and Polymerization................................................................76
Photophysics ....................................................................................................................84
Absorption ................................. .......................... ... .... ........ 84
E m mission .........................................88
Thermally Induced Isomerization.............................. ......... 90
L ight E m hitting D ev ices......................................................................................... 92
D isc u ssio n ............................................................................................................. 9 9
C o n c lu sio n s......................................................................................................... 1 0 0
E xperim mental ...........................................10............................1
M on om er Sy nth esis ..................................................................................... 10 1
Polymerization......................................... 104
Therm ogravim etric A analysis ...................................................................105
Photophysical M easurem ents ..................................................................... 105
Device Fabrication and Measurement .........................................................106

4 FUTURE DIRECTIONS .......... ......... ................. ......... 107

N on -R adiativ e D ecay .......................................................................................... 107
C carrier T ran sp o rt ................................................................................................ 10 9
Porphyrin Containing Polym ers ............................................................. ........ 110
C o n c lu sio n s......................................................................................................... 1 12

APPENDIX ELECTRONIC PROPERTIES ..........................................................115

T h e F ree-Ion P rob lem ......................................................................................... 1 15
The Crystal Field Problem ................................. .......................... ........ 119
Coupling of the Electronic and Lattice States ........................................................ 124
E in stein M o d el .................................................................................................12 8

L IST O F R E F E R E N C E S ........................................................................................... 132

BIOGRAPHICAL SKETCH .............................................................. ...............148
















LIST OF TABLES


Table pge

2-1 Photophysical properties of Yb porphyrin complexes measured in CH2C12............45

2-2 Solvent effects on the near-infrared quantum yields of Yb porphyrin complexes...52

2-3 Electrochemical windows of solvents....................................................................65

2-4 Electrochemical properties of substituted porphyrins...........................................70

3-1 P olym erization details. ............................ ........................ ........ .. ...... ............83

3.2 Photophysical data .................. ...................................... ... ............ 86

3-3 E lectrolum inescence data. ........................................ .........................................97

4-1 Vibrational frequencies of common bonds in organic molecules........................ 109

A-1 Real orbitals created from linear combinations of orbitals labeled by their
m agnetic quantum num ber. ............................................. ........................... 123

A-2 Position and symmetry off-orbitals in an octahedral crystal field.........................123















LIST OF FIGURES


Figure pge

1-1 Schematic of absorption shown as a function of thickness, concentration, and
m olar absorptivity. ..............................................................2

1-2 Configuration coordinate model.............................................................. 3

1-3 Jablonski diagram showing the fundamental processes of absorption, internal
conversion, fluorescence, intersystem crossing, and phosphorescence .................4

1-4 Configuration coordinate diagram showing the zero-phonon line and illustrating
the process known as the Stoke's shift .......... .... .................................. 5

1-5 Configuration coordinate model showing the process of phosphorescence .............6

1-6 Interaction of atom nuclear charge induced angular momentum interacting with
electron's spin angular momentum, resulting in spin-orbit coupling........................7

1-7 Typical nonradiative processes through (a) crossover with a ground state, (b)
multiphonon emission, and (c) crossover with an excited state................................9

1-8 Schematic showing the overlap of donor fluorescence with the acceptor
absorption, fundamental for Forster energy transfer ............................................11

1.9 Graphical description off orbitals. ........................................ ........................ 14

1-10 Energy level diagrams for selected rare earth ions in LnC13............................... 15

1-11 Absorption spectra of selected lanthanide ions, showing sharp absorption with
low m olar absorptivity. ........................... ......... .. .. ..... ............ 16

1-12 Jablonski energy diagram showing energy transfer from organic ligand to
lanthanide m etal, in this case Yb3 ............................................... ........ ....... 16

1-13 Structure of porphyrin macrocycle, showing possible areas of substitution............17

1-14 Molecular orbitals for the porphyrin macrocycle for the highest occupied
molecular orbital and lowest unoccupied molecular orbital calculated using the
G outerm an m ethod. ............................... ......................... ......... .. ................20

1-15 Absorption spectrum of tetraphenylporphyrin. ........................................21









1-16 Fluorescence spectrum of tetraphenylporphyrin. ........ .....................................22

1-17 Schematic of Rothemund synthesis.......................... ..................... ..............22

1-18 Schem atic of Lindsey synthesis of porphyrins .................................... .................23

1-19 Schematic of 2 + 2 condensation method for synthesis A2B2 type porphyrins........24

1-20 Schematic of lanthanide metallation of porphyrins using the Foley method...........24

1-21 Axial substitution of lanthanide metalloporphyrins using salt metathesis ..............25

1-22 Structure of aluminum tris-quinolate. ........................................... ............... 26

1-23 Photoluminescence and electroluminescence spectra of Alq3. .............................26

1-24 Structure of Ir(ppy)3. ............................................................. .................. .... 29

1-25 Schematic of light emitting diode, showing each individual layer. .......................29

1-26 Structures of typical hole transport materials .............. ........................................30

1-27 Structure of Lanthanide tris-quinolate. ....................................... ............... 40

1-28. Structure of Lanthanide tris-DBM bathophenthroline complex....................... 41

2-1 Yb porphyrin complexes used in study. ......................................... ...............43

2-2 X-ray crystal structure of Yb(TPP)Q showing coordination of a molecule of
T H F ................................................................................44

2-3 Absorption spectra for a)Yb(TPP)Q, b) Yb(TMPP)TP, c) Yb(TPyP)L(OEt)3, d)
Yb(TPP_OEH)TP in CH2C2 as a function of molar absorptivity........................48

2-4 Visible emission spectra for substituted Yb3+ complexes in CH2C12 at room
tem perature ............... ......... ............................................ ................ ........49

2-5 Excitation spectra of (a) Yb(TPP)Q, (b) Yb(TMPP)TP, and (c)
Yb(TPyP)L(OEt)3 .................. ....................................... ... ........ .... 50

2-6 Near-infrared emission spectra for substituted Yb3+ porphyrin complexes in
CH 2C12 at room tem perature ............................................................................. 51

2-7 Near-infrared emission spectra for substituted porphyrin complexes in 2Me-THF 52

2-8 Visible electroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3,
and Yb(TPP_OEH)TP as a function of increasing voltage, starting at 6V..............56









2-9 NIR Electroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and
Yb(TPP_OEH)TP as a function of increasing voltage, starting at 6V.................57

2-10 Current density Voltage (j-V) plot ofYb porphyrin devices as a function of
loading w t% in P S. ...................... .................... ................. ...........58

2-11 Near-Infrared external electroluminescent quantum efficiency for Yb(TPP)Q,
Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a function of
loading in P S ...................................................... ................. 60

2-12 Charge hopping model showing transport of charges on porphyrin molecules.......61

2-13 Current density-Voltage and NIR irradiance-voltage plots of Yb(TPP)TP as a
function of Alq3 loading: (*) 0 wt %, (0) 33 wt%, and (A) 66 wt % ....................62

2-14 NIR External quantum efficiency of Yb(TPP)TP as a function of Alq3 loading:
(*) 0 wt %, ( ) 33 wt%, and (A) 66 wt %. .............. .............. ............... 63

2-15 Current density-voltage and NIR irradiance-voltage plots of Yb(TMPP)TP as a
function of Alq3 loading: (*) 0 wt %, (0) 33 wt%, and (-) 66 wt %.....................63

2-16 NIR external quantum efficiency of Yb(TMPP)TP as a function of Alq3 loading:
(*) 0 wt %, () 33 wt%, and (-) 66 wt %. ............................................. 64

2-17 Redox properties of free-base tetraphenylporphyrin with respect to the saturated
calom el electrode............................................................................ ............... 6 5

2-18 Reduction and oxidation waves for free-base tetraphenylporphyrin......................66

2-19 Reduction and oxidation waves for Yb(TPP)TP............................................. 67

2-20 Reduction and oxidation potentials for Yb(TPP)Q. ............................. ............... 67

2-21 Reduction and oxidation waves for Yb(TMPP)TP. ............................................68

2-22 Reduction and oxidation waves for Yb(TPP)L(OEt)3................... ......... .......68

2-23 Reduction and oxidation waves for Yb(TPyP)L(OEt)3............. .................69

2-24 Cartoon showing ITO substrate placed at top of beaker containing solution of
aqua regia allowing vapors to etch surface. .................................. ..................72

3-1 Synthesis of Zn(II)-5-(4-Ethynylphenyl)-10,15,20-triphenylporphyrin. ...............77

3-2 Synthesis of 2-(4-methylphenyl)-5-(4-ethnylphenyl)-1,3,4-oxadiazole. ................78

3-3 Structure of(Bicyclo[2.2.1]hepta-2,5-diene)chlororhodium(I) dimer ...................79

3-4 Representative scheme of insertion polymerization mechanism. ............................81









3-5 Polyacetylene isom er structures ................................... .............................. ........ 81

3-6 Structure of ZnETPP homopolymer and copolymers. .............................................82

3-7 NMR spectra of poly(ZnETPP)o.5-co-(PE)o.5........................................................83

3-8 TGA traces of polymers of ETPP measured in air ......... ...................................84

3-9 Absorption spectrum of (a) poly(ZnETPP) (-), (b) poly(ZnETPP)o.14-co-
(PE)o.86 (----), poly(ZnETPP)o.5-co-(PE)o.5 (- --), (c) poly(ZnETPP)o.5-co-
(PEOXAD)o.5 (- -), and (d) poly(ZnETPP)o.5-co-(3,5CF3PE)o.5 (- **-). .........87

3-10 Absorption and fluorescence spectrum of PEOXAD monomer. .............................88

3-11 a)Emission (Xex = 420 nm) of poly(ZnETPP) in solution (-), film ( **),(b)
poly(ZnETPP)o.14-co-(PE)0.86 in solution (-), film (***) (----),
poly(ZnETPP)o.5-co-(PE)o.5 in solution (---), film (-), (c) poly(ZnETPP)o.5-co-
(PEOXAD)o.5 in solution (-), and (d) poly(ZnETPP)o.5-co-(3,5CF3PE)o.5 in
solution (- ). .........................................................................91

3-12 Emission of poly(ZnETPP) in film as a function of annealing time at 150 C in
vacuum .............................................................. .. .... ..... ......... 92

3.13 Current density vs. voltage and irradiance vs. voltage for a) poly(ZnETPP) b)
poly(ZnETPP)o.5-co-(PE)o.5, c) poly(ZnETPP)o.5-co-(PEOXAD)o.5 d)
poly(ZnETPP)o.5-co-(3,5CF3PE)o.5 as a function of wt% of polystyrene................96

3-14 Electroluminescence emission of spectrum of poly(ZnETPP) (-), (b)
poly(ZnETPP)o.14-co-(PE)0.86 (- -), poly(ZnETPP)o.5-co-(PE)o.5 ( -), (c)
poly(ZnETPP)o.5-co-(PEOXAD)o.5 (***), and (d) poly(ZnETPP)o.5-co-
(3,5CF3PE)o.5 (- *" ) ................................................... ................ ............... 97

3-15 TEM image of poly(ZnETPP) showing presence of Rh nanoparticles....................98

3.16 External quantum efficiency vs. current density for a) poly(ZnETPP) b)
poly(ZnETPP)o.5-co-(PE)o.5, c) poly(ZnETPP)o.5-co-(PEOXAD)o.5 d)
poly(ZnETPP)o.5-co-(3,5CF3PE)o.5 as a function of wt% of polystyrene (* = 0
wt%), (m = 25 wt%), (A= 40 wt%) and (+ = 50 wt%) ......................................99

4-1 Molecule proposed to increase emission quantum yield.............. ............... 108

4-2 Conjugated polymer, PPE, containing substituted porphyrin and oxadiazole in
m a in c h a in ...........................................................................................................1 1 0

4-3 Scheme of a free radical chain growth mechanism........... ......... .................111

4-4 Structures of m onom ers. .......................................................... .............. 112









4-5 Structure of TP polymer. ............ ........ .....................113

4-6 Synthesis of YbTPP-TP polymer............. ........................... .....................113

4-7 Absorption spectrum of YbTPP-TP polymer.................................... ............... 114

4-8 NIR emission spectrum of YbTPP-TP polymer .............................................. 114















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

ELECTROLUMINESCENCE AND PHOTOPHYSICAL PROPERTIES OF NEAR-
INFRARED LUMINESCENT LANTHANIDE (III) MONOPORHYRINATE
COMPLEXES AND PENDANT POLYMERS
By

Garry Brian Cunningham

December 2005

Chair: Kirk S. Schanze
Major Department: Chemistry

The photoluminescent and electroluminescent properties of substituted lanthanide

monoporphyrinate complexes were investigated. The lanthanide complexes consisted of

a lanthanide (Yb3+) coordinated to a substituted porphyrin, 5,10,15,20-

tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(3,4,5-

trimethoxyphenyl)porphyrin(TMPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP), or

5,10,15,20-tetra(4(2'-ethylhexyloxy)porphyrin (TPP_OEH) and a capping ligand, L. The

capping ligand was either an tris-pyrazoylborate (TP),

(cyclopentadienyl)tris(diethoxyphosphito-P)cobaltate (L(OEt)3) or quinolinato (Q) anion.

The complexes were synthesized to influence the electronic properties of the complex.

The optical absorption and emission of these complexes resembled previously studied

lanthanide porphyrin complexes, with photoluminescent yields ranging from 0.01 to 0.04.

Electroluminescence was observed for the porphyrin complexes blended into polystyrene.

External quantum efficiencies were typically 10-4, suggesting that changes to the









porphyrin structure have little effect on the electronic nature of the complex. Blending

the electron transporting AlQ3 into the device improved the external quantum efficiencies

by an order of magnitude, suggesting that carrier transport is the culprit for poor device

performance.

Conjugated polyacetylenes containing a Zinc porphyrin pendant were synthesized

by an insertion type polymerization using [Rh(NBD)Cl]2. The homopolymer

(poly(ZnETPP) and copolymers of ethynyl benzene (poly(ZnETPP)-co-(PE)), an

oxadiazole containing group (poly(ZnETPP)-co-(PEOXAD), or 1-ethynyl-3,5-

trifluoromethylbenzene (poly(ZnETPP)-co-(3,5CF3PE) were synthesized. The optical

properties were studied and it was found that the homopolymer exhibited excitonic

coupling due to the overlap of the porphyrin pendants. Substitution of other co-

monomers reduced this coupling allowing the return of typical ZnTPP optical properties.

Neither absorption nor emission from the polyacetylene backbone was identified. A

thermally induced cis to trans isomerization was observed for the homopolymer,

poly(ZnETPP), with the emergence of polyacetylene backbone emission.

Electroluminescent devices were fabricated using the polymers neat or in a blend with

polystyrene. The electroluminescent performance of the homopolymer was poor with a

maximum external quantum efficiency of 10-6. Addition of polystyrene increased the

efficiency 10-fold. The copolymers with PE also showed similar characteristics. The

copolymers with either PEOXAD or 3,5CF3PE further increased the external efficiency

by an order of magnitude. This suggest that adding hole blocking / electron transporting

pendants to the polymer further enhances carrier transport, allowing for more efficient

devices.














CHAPTER 1
INTRODUCTION

This chapter provides an overview of several topics. The first topic focuses on the

fundamental processes that occur when molecules interact with light. The second part

will focus on the electronic properties of lanthanides. The third part will focus on the

photophysical and redox properties of porphyrins. The fourth part will focus on the

basics of electroluminescence and light emitting diodes. The final part presents the up-

to-date literature overview of organic and polymer complexes used in light emitting

diodes, with an emphasis on near-infrared light emission.

Electrodynamics

Absorption of Light

The perturbation of matter with light gives insight into its electronic structure.

When a molecule absorbs a photon of sufficiently high energy, one of its valence

(HOMO, highest occupied molecular orbital, for organic systems) electrons is promoted

into an excited energetic state (LUMO + n; n = 0....00). The frequency of the light

provides information about the energy difference between the two orbital energy levels

involved in the transition. The fundamental process of the absorption of light at a given

frequency is given by the formula:

I
A = -log- = bc (0.1)
10

which is known as the Beer-Lambert law, where A is the absorption, I is the light

intensity at a given frequency measured after interaction with the sample, Io is the light









intensity at a given frequency measured before the sample, b is the pathlength of the

sample measured in cm., and c is the concentration of the sample given in molarity. The

units for the intensity of the absorption band are given by the term, F, and measured in

M^cm-1. The absorptivity is proportional to the transition dipole between the ground and

final states. If the dipole moment is zero, the absorption coefficient is also zero.



0o (k) 0)


U(kh), C


d

Figure 1-1.Schematic of absorption shown as a function of thickness, concentration, and
molar absorptivity.

The absorption spectra of molecules differ greatly than that of free ions, due to

coupling of vibrational transitions (phonons) with the electronic transitions. The

absorption is broadened and split into bands. This process is known as electron-phonon

coupling. According to the Franck-Condon principle, the electronic process of

absorption takes place on a faster timescale than the nuclei can respond. The equilibrium

distance between the nuclei involved in a given vibrational transition occurs at the

potential energy minima. When an electron is promoted to a higher energy level, the

charge distribution of the molecule is changed, and the initial position of nuclei is no

longer the lowest in energy. The nuclei will attempt to equilibrate, but this process is

slow with respect to the time scale of light absorption (10-13s: 10-1s). Therefore, the

transition occurs from the lowest ground vibrational state to multiple vibrational levels of

the excited state, as shown in Figure 1-2.

















E
Em.

Abs.





-R
R, Re

Figure 1-2. Configuration coordinate model.

Emission of Light

The photoluminescence process can best be described using the general

configuration coordinate diagram (Figure 1-2). A configuration coordinate diagram is a

plot of the energy of electronic ground and excited states as a function of a generalized

configuration coordinate R that accounts for the nuclear configuration of nearest

neighbors about the excited nuclei. As shown by Rg and Re, the equilibrium

configuration coordinate differs in the ground and excited states. Wavefunctions of

excited states are typically more spatially extended than those of the ground state. This

leads to the energy minima of the excited state, Re, to be shifted with respect to the

ground state energy minima. Upon absorption of a photon, the center is promoted into an

excited vibrational state of an excited electronic state. The terminal state of the

absorption transition is at a higher-energy point than the minimum of the excited state.








The lifetime of allowed excited states is -10.8 seconds and is much longer than the lattice

vibrational period. Therefore, just after absorption, the complex undergoes a phonon

assisted relaxation to the energy minima of the excited state. This process is also known

as internal conversion (IC) (see Figure 1-3). The molecule loses its excess vibrational

energy by interaction with solvent or other solute molecules.

S2


IC




0D T


Z- ;Phosphorescence


GS

Figure 1-3. Jablonski diagram showing the fundamental processes of absorption, internal
conversion, fluorescence, intersystem crossing, and phosphorescence.

The chromophore can radiatively return to a vibrationally excited ground state by

the emission of a photon. This process is called fluorescence. Fluorescence is an allowed

process because it involves a transition of two states with the same spin. Half of the

energy difference between the emission and absorption energies is known as the Stokes

shift. The Stokes shift depends on the relative position and curvature of the configuration

coordinate parabolas, shown in Figure 1-4.









The excited chromophore can also undergo a process known as inter-system

crossing (ISC). This process happens when the excited state electron spin changes to

produce the triplet state. This process involves coupling of the singlet and triplet

vibrational levels of the same energy to produce a vibrationally excited triplet state. This

process is followed by internal conversion to the triplet state in the lowest vibrational

level (v = 0). Once a molecule has undergone intersystem crossing, it can return to the

ground state by emission of a photon. This process is called phosphorescence.

absorption




- -o


.Stokes shift


Figure 1-4. Configuration coordinate diagram showing the zero-phonon line and
illustrating the process known as the Stoke's shift.[1l

The ability of a molecule to undergo intersystem crossing, although quantum

mechanically forbidden, is facilitated by the presence of strong spin-orbit coupling. The

spin and orbital angular momentum are separately coupled and quantized. Therefore, the

transitions between the states of opposite spin are forbidden by the conservation of









momentum rule. This is the typical case for molecules with small Z numbers (e.g.,

organic molecules). This situation is changed by the presence of atoms with large Z

numbers. This is called the heavy atom effect. With these heavy atoms, a significant

mixing of the spin angular momentum and orbital angular momentum of the same

electron occurs. Because of this mixing, the two moment are not separately conserved,

but the total angular momentum is conserved. This is called spin-orbit coupling, and it

leads to the increase in inter-system crossing rates. This process is shown in Figure 1-6.

Due to the large nuclear charge present, a magnetic field is produced about the electron

with a directionality perpendicular to the plane of the orbit. A magnetic field is also

being produced by the electron spin motion and is directed along the spin axis. The

interaction of these two magnetic fields is called spin-orbit coupling. The magnitude of

this effect is controlled by the nuclear charge of the heavy atom as well as by the position

of the atom in the molecule.





ISC T,







Phosphorescence


Figure 1-5. Configuration coordinate model showing the process of phosphorescence.













Spinning Electron
(Spin Angular Momentum)




Nuclear Charge
(Orbital Angular Momentum)

Figure 1-6. Interaction of atom nuclear charge induced angular momentum interacting
with electron's spin angular momentum, resulting in spin-orbit coupling.

The lifetime of an excited state and luminescent efficiency depend on the rates of

the radiative and nonradiative transitions from (and to) the excited state. Highly allowed

transitions have fast rates and contribute to short lifetimes and high efficiencies. A

strongly competing non-radiative rate will result in both a shorter lifetime and lower

luminescent efficiency. Radiative emission can be considered to be emission of light

from an excited state of a complex designated as M*:

M' =M + ho (0.2)

This emission is considered a random process, and therefore follows first-order

kinetics: [2]

-d[M*]
= k,[M*] (0.3)
dt

If it is a single process, the decay should be characterized by a single exponential, which

can be expressed by a single rate constant kd. The lifetime can also be expressed in terms

of the lifetime (Tr) of the excited state by the equation:

1
Tr = (0.4)
kd









The rate constant usually has the units of s-1. Therefore the lifetime has the units of s.

The decay of the excited state can occur by many different possible pathways such as the

emission of light, intersystem crossing, and internal conversion. The radiative rate

constant is therefore the sum of all of the processes which lead to a lower concentration

of the excited state. The equation below shows the relationship between the rate of

fluorescence (kf), the rate of intersystem crossing (kisc), the rate of internal conversion

(kic) and the rate of phosphorescence (kp).

kd = kf + k + k + k (0.5)

Nonradiative Decay

Radiative return from the excited state is not the only possibility of completing the

cycle. The alternative is nonradiative decay, which means a return without the emission

of radiation.[1] Nonradiative processes compete with radiative processes. These

nonradiative returns affect the emission efficiency of the sample. Emission efficiency is

simply derived as the number of photons emitted by the sample divided by the number of

photons absorbed by the sample.

There are several different ways that nonradiative decay can occur. These

processes are shown in Figure 1-7. In Figure 1-7a absorption and emission processes are

possible and Stokes shifted relative to each other. The relaxed-excited state may reach

the crossing of the two parabolas if the temperature is high enough. Via the crossing it is

possible to return to the ground state in a nonradiative manner. The excitation energy is

then given up as heat to the lattice. This accounts for one of the possible thermal

quenching mechanisms.









In Figure 1-7b, the parabolas are parallel (S = 0) and will never cross, assuming

that both states have the same force constant. It is impossible to reach the ground state in

the way described above. However, nonradiative return to the ground state is possible if

certain conditions are met. The energy difference between the two states, AE, must be

equal to or less than 4-5 times the higher vibrational frequency of the lattice. In that case,

this amount of energy can simultaneously excite a few high-energy phonons, and then is

lost for the radiative process. The quantitative description of this process is called the

energy gap law.[13-9 This process is also called multi-phonon emission.

The third process consists of an electronic crossover with another excited state.

This excited state could be a higher or lower energy excited state. This is the

fundamental process involved in intersystem crossing.

excited .
state excited
state el

E a
ground ----
-r-

s AE / ground /ground
state state

R R R

(a) (b) (c)

Figure 1-7.Typical nonradiative processes through (a) crossover with a ground state, (b)
multiphonon emission, and (c) crossover with an excited state. [1]

Energy Transfer

The final process that a molecule can use to relax from the excited state is through

energy transfer. The process of energy transfer occurs when an excited donor molecule

D* transfers its energy to an acceptor in the ground state A, which is then promoted to an

excited state:









D* +A D+A* +hv, > D+A+hvE (0.6)

where hvE and hVNR are, respectively, the emission and nonradiative decay created in the

process. In other words, the energy of one molecule can be transferred to another

molecule, which quenches the luminescence of the host and increases that of the guest.

The interaction between D* and A is described by the perturbation Hamiltonian,

H'. The D* + A state is not considered a stationary state of the total Hamiltonian but is

able to evolve into other isoenergetic states, such as D + A*. The use of time-dependent

perturbation theory assigns the probability of the evolution from D + A, given by 'i, to

the D + A* state, described by If, as

Poc p( H' WfY (0.7)

where p is the density of the coupled isoenergetic donor-acceptor transitions. For typical

systems, p can be determined by calculating the overlap integral of donor luminescence

and acceptor absorption. In general, after the excited state transfers from D* to A it

rapidly relaxes nonradiatively to the lowest vibrational level of A*.

The perturbation Hamiltonian contains several terms; the most important are the

electrostatic (Forster) and electron exchange (Dexter) interactions. Both terms are

capable of inducing energy transfer. The electrostatic interaction can be expressed as a

series of multipole-multipole terms. The most common interaction, dipole-dipole, was

described by Forster who discovered that the rate of energy transfer depended on the

distance R between the donor and acceptor molecules:[10]


kly(R') K= jJ (0.8)









where Ro is called the Forster radius, and T is the average donor emission lifetime in the

absence of energy transfer corresponding to rate kD = 1/T. When R is equal to Ro then kET

= kD. The critical distance Ro, which is the distance between the donor and acceptor

where the efficiency of energy transfer is 50%, is given by the integral over all

wavelengths K:

-=1.25 x1017 tJD (A) a (A) 2dA (0.9)


where pE is the quantum efficiency of donor emission, n is the refractive index of the

host, FD is the normalized emission spectrum of the donor, and aA is the molar extinction

coefficient of the acceptor. It has been shown that dipole-dipole interactions can be

significant even at distances as large as 100A.111


Acceptor

abs. fl.
Donor
I I
abs. fl. I








Figure 1-8. Schematic showing the overlap of donor fluorescence with the acceptor
absorption, fundamental for Forster energy transfer.[1l

In Forster energy transfer, the spins of both D and A are conserved. Therefore, the

allowed transitions are

D + A 'D1+'A* (0.10)

'D + 3A 'D+ 3A* (0.11)









where the superscripts indicate the spin of the molecule. The triplet-singlet transition

3D* + A '1D + 1A* (0.12)

is forbidden, but is occasionally observed since the triplet excited donor has a long

lifetime and kET can be faster than phosphorescence.

The process of energy transfer has also been extensively analyzed by Dexter [12] to

give

k,(R)ce Rc e- LIFD ( A (A)dA (0.13)

where R is the distance between D* and A, and L is a constant. Because this process

involves the exchange of electrons, it occurs only over short distances, -10A. Under the

Dexter transfer process, it is the total spin which is conserved. Therefore, triplet-triplet

energy transfer is allowed.

D* + 1A 'D+ 3A (0.14)

Lanthanides

The previous sections focused on the electronic properties and fundamental

processes of any molecule. This section will delve deeper into the unique properties

which make up the metals known as the rare earths. The most impressive feature about

the spectra of the lanthanide ions is the sharpness of the lines in absorption and emission

spectra. The optical properties of the lanthanides were first discovered around 1908 by

Becquerel.1131 The trivalent (Ce3+ Yb3) rare earths have an electronic configuration of

4f5s25p6(n=1-13). The 4felectron shell is located within the 5s5p shell, and therefore

the interaction with the ligand field is weak as compared to that of transition metals. This

leads to the electronic properties of 4flevels in the complex to be similar to that of the

free ion, and changing the overall ligand field has little effect on them.[14] If the









lanthanides are modeled using the configuration-coordinate model shown in Figure 1-2,

the situation corresponds to an excited state displacement of zero. This is stating that the

4fwavefunctions are not influenced to any great extent by excitation.

The energy structure of the lanthanides and their many different optical transitions

were precisely investigated by Dieke [15] in lanthanide fluoride hosts, and in LaC13 doped

solids by Crosswhite.1161 Figure 1-10 shows the energy level diagrams for several

selected trivalent rare earths. These diagrams are quite useful in understanding their

luminescence characteristics. Energy levels prescribed by the inner quantum number J

are further split into several sublevels in the solid state by the Stark effect of the crystal

field. The number of split sublevels is dependant on the symmetry of the crystal field,

and is limited to 2J + 1 for integer values of J and J + /2 for J of half-integer values.

The absorption of lanthanides is plagued by low oscillator strengths. Transitions

within the 4f manifold take place within states of the same parity, and therefore are

electric dipole forbidden. Oscillator strengths of f-f transitions that are made electric-

dipole allowed by crystal fields are 10-5 10-8, dependent on whether the site has

inversion symmetry. The uneven components of the crystal field mix a small amount of

opposite parity wavefunctions into the 4fwavefunctions. This process is sometimes

called "intensity stealing.[17-21]" Magnetic dipole interactions within the 4fmanifold are

allowed and are sometimes observed. The oscillator strengths associated with these

transitions are in the range of 10-7 108.



















f3 fxyz fx2










fyz2 fx(x2 -3y) fy(3x2-y)







f 2 2
z(x -y )


Figure 1.9. Graphical description off orbitals.[22]

The ability of a ligand to sensitize thef -femission transition was discovered in

1942 by Weissman.[231 He found that the 7t -* 7t* transitions of beta-diketonates could

sensitize Eu3 emission. The mechanism for this transition is that the molecule is excited

from the singlet ground state to a singlet excited state (So -* Sn) The Sn state decays

into the lowest energy singlet excited state on the ligand (Sn -- Si).The singlet state

undergoes intersystem crossing into the ligand-based triplet state (Si--Ti). The ligand

then transfers its energy to the lanthanide, resulting in a lanthanide in the excited state.

The lanthanide then can undergo radiative decay to the ground state, releasing a photon.






15


The importance of this discovery was soon realized, and further efforts were

undertaken to attempt to improve the efficiency of lanthanide emission. Most of the

work focused on use of P-diketonates as the sensitizing ligands for this process, although

many other organic ligands were also used. The bulk of this research will be discussed

later in this chapter.


3.5 r


7F


S8F7

Gd3"-


9 5D4o, Fo i










Y6F 6 H512





Tb3- D, +


- 419/2



--- 411\/2


-r4I 152

Er3 1


--o- 2F52


Yb3+


Figure 1-10. Energy level diagrams for selected rare earth ions in LnCl3. Unfilled circles
indicate emissive states. (o -> )[24]


- 4G7/2
-a-m
___ 6^


-- 5L6
-- 5D3

-- 5D2I
--SD

SD7


2.0


1.5


0.0 L


S 132




-- SH52

Smn3+


6F52
--o-6F5.










4
3
2



3

2



S0
g 8

I f


Figure 1-11. Absorption spectra of selected lanthanide ions,
with low molar absorptivity. [17]


showing sharp absorption


- -


Figure 1-12. Jablonski energy diagram showing energy transfer from organic ligand to
lanthanide metal, in this case Yb3+. Dashed lines represent non-radiative
decay.


Ho3I

L


400 500 600 700






400 500 600 700






400 500 600 700

Sm3




400 500 600 700
Wavelength, nm









Porphyrins

Porphyrins are among the most important molecules in nature. They are found in

nearly every biological tissue.[25] These compounds range from chlorophyll in plants to

iron porphyrin (heme) in blood.[26-28] Porphyrins are also important molecules in optical

materials. [29-32]

Porphyrins consist of a large macrocycle core consisting of 267t electrons. This

core (see Figure 1-13) can be substituted upon by the addition of a metal and substitution

along the periphery can be accomplished at either the meso or pyrrole position. The

molecule can also be substituted upon by the introduction of an axial ligand.




N\ N^

Al M--M.,, IiA2 //- RI

N N

R2


Figure 1-13. Structure of porphyrin macrocycle, showing possible areas of substitution.

Photophysics

The electronic structure giving rise to the absorption spectra of porphyrins and

metalloporphyrins has been derived by a four orbital model.[33-37] Given the D4h

symmetry of the porphyrin ring, four different molecular orbitals have been described,

shown in Figure 1-14. The absorption bands in the visible region are described by

transitions among the two highest occupied 7t orbitals of alu and a2u symmetry with the

doubly degenerate unoccupied 7t* orbitals of eg symmetry. Transitions from the a2u state

to the eg7t* state are the lowest in energy and called the Q bands. In the free base









porphyrins this leads to 4 bands ranging from 480-650 nm. In metalloporphyrins only two

peaks are seen in the Q bands due to their lower symmetry.[38] The intensity of this

transition is relatively weak due to the fact that it is symmetry forbidden. The transition

from the alu level to the eg7t* orbital is a strong absorption called the Soret or B band. It

ranges from 380 430 nm and the large intensity (E z 105 M-^cm-1) is due to the fact that

it is symmetry allowed. The actual peak positions and absorption intensity are dependent

on substituents on the porphyrins ring, central metal, and axial ligands.

The regular fluorescence spectra for porphyrins shows two bands which mirror the

Q bands, along with the possibility of phosphorescence bands. The fluorescence bands

for metalloporphyrins are typically found near 600 nm and the phosphorescence bands

are found near 700 nm. In normal type porphyrins and metalloporphyrins the excited

state energy diagram is fairly simplistic due to the fact that the lowest singlet and triplet

states derive from the porphyrins 7t system.

Porphyrin complexes which have closed shell metals generally only show

fluorescence in solution at room temperature, but have some phosphorescent component

at low temperatures (77 K).[39] The quantum yield for fluorescence decreases with

increasing atomic number of the metal, as does the phosphorescence lifetime. The

phosphorescent lifetime of these complexes is usually in the range of several hundreds of

ts to several ms. The addition of a diamagnetic metal to the porphyrin core typically

quenches the room temperature fluorescence.[39] The relatively weak fluorescence and

short triplet lifetimes are due to the fact that the lowest energy excited state is typically

metal based and has little contribution from the porphyrin t system. The exceptions are

Pt2+ and Pd2+ which show both room temperature fluorescence and phosphorescence, due









to the fact that the d -* d splitting of these metals is large and the porphyrin n system

plays the dominant role in the excited state. 40, 41] Paramagnetic metal porphyrin

complexes generally only show phosphorescence with relatively short lifetimes.

The effects of substituents on the porphyrin core have little effect on the

fluorescence position and intensity. The substitution of "light" atoms typically leaves the

fluorescence intensity and lifetime unchanged. The addition of heavy atoms at the

pyrrole position has a dramatic influence on the emissive properties. For example, the

addition of bromine atoms at the pyrrole position of CoTPP dramatically changed the

Xmax of the Soret absorption.[421 This led to a sharp decrease in the fluorescence intensity

and a dramatic increase in the ISC yield, which in turn lead to a greater phosphorescence

yield.[43] This effect can be explained in two different ways. One is the fact that

substitution at the pyrrole would cause a steric effect at the porphyrin core, therefore

inducing a distortion of the normal planar structure. The second possible explanation is

that the alu orbital is more sensitive to changes to the pyrrole than that of the a2u orbital.

The strong electron-donating substituents, Br, could induce an inversion of the ground

state by destabilization of the alu level greater than that of the a2u level.[44]

Porphyrins tend to be useful ligands for the sensitization of lanthanide emission in

the near-infrared due to their ease of excitation and their low energy triplet state.

Lanthanide porphyrins are less well known than that of transition metal and alkaline earth

metal porphyrins. The first examples of lanthanide monoporphyrinate complexes were

demonstrated in 1976 by Horrocks et al.[45] Since then, there have been several papers

describing the use of porphyrins to sensitize the near-infrared emission from

lanthanides.146-521 They showed that the coordinated lanthanide rapidly deactivates the









singlet state by inducing intersystem crossing to the triplet state. As a result of this fast

intersystem crossing, most emission from the singlet states is quenched.[53] Lanthanide

porphyrins such as GdTPP(acac) in which the energy of the metal is too high to accept

energy from the ligand triplet, show emission typical of heavy metal porphyrin

complexes such as PtTPP.




-I",.., I I ..I LIMO




clWd) C2(g)



i :HOMO




bl(a2u) b2(alu)

Figure 1-14. Molecular orbitals for the porphyrin macrocycle for the highest occupied
molecular orbital and lowest unoccupied molecular orbital calculated using
the Gouterman method.[36]

Redox Properties

The use of electrochemical methods to estimate the redox properties of porphyrins

is vital for understanding the photochemistry of porphyrins. In general, free-base

porphyrins possess two oxidation peaks and two reduction peaks in cyclic voltammetry.

These correspond to the one and two electron oxidation and reduction of the porphyrin 7t

system. In "normal" metalloporphyrins a similar redox pattern is observed. The central

metal cation simply acts as a substituent on the porphyrin ring. The redox properties










observed, exhibit a good correlation with the electronegativity or inductive parameter of

the central metal atom.[54' 55] Substituents on the porphyrin ring show a good correlation

between the redox potentials and the Hamett a values.154, 56] The electrochemical band

gap corresponds well with the optical band gap determined by the lowest energy

absorption in the Q band, indicating that the central metal and substituents equally effect

the HOMO and LUMO levels.[571 The number of substituents is also correlated with the

shifts in the redox peak positions.[58] Distortion from planarity seems to cause a dramatic

change in the oxidation potential.[54] The addition of a redox active metal complicates the

overall electrochemical properties of porphyrins, due to the intervening oxidation and

reduction potentials of the metal. The change in axial ligand also seems to play an

important role in the redox potentials of porphyrins.[59, 60]




5e+5
20000



4e+5 15000 -
-Y

10000 -
3e+5


S55000
0
2e+5 -

00 -
Se+5 500 550 600 650
le+5



0
400 500 600 700

Wavelength / nm

Figure 1-15. Absorption spectrum of tetraphenylporphyrin. Inset is magnification of Q-
bands.















1.0 -



0.8

0 .




S0.4
N

E
Z 0.2



0.0
620 640 660 680 700 720 740

Wavelength / nm


Figure 1-16. Fluorescence spectrum of tetraphenylporphyrin.

Synthesis

The synthesis of symmetric porphyrins involves the condensation of aldehydes


with pyrroles in dilute solutions using catalytic amounts of an organic acid, organic base


(Rothemund synthesis, Figure 1-17) or strong Lewis acid followed by oxidation by such


reagents as DDQ.161]

Ph




\ NH N;=a-/
H
OHN + Pyridine
CHO + Ph Ph


Figure 1-17. Schematic of Rothemund synthesis.










The use of Lewis acids such as BF3 (Lindsey method, Figure 1-18) generally leads

to higher yields and milder reaction conditions.[62' 63] The synthesis of asymmetric

porphyrins according to the Lindsey method usually leads to a statistical distribution of

mixed porphyrins, which can be difficult to separate.[30] For porphyrins of the trans-

A2B2 type a 2 + 2 condensation of dipyrromethanes with aldehydes shown in Figure 1-19

is typically utilized..

R




H 1.)TFA or BF30Et2 NH N
N 2.) DDQ
R-CHO CH2C2 R

N HN




R

Figure 1-18. Schematic of Lindsey synthesis of porphyrins.

The metallation of porphyrins is typically simple. It involves stirring the free base

porphyrin in a solution containing the metal organic complex or metal salt. This

procedure works well for metals with a small ionic radius, such as Zn and Mg, but not as

well for metals such as Pd or Pt. The larger metals require much higher temperatures and

harsh conditions for insertion into the porphyrin core.

Lanthanides, for example, require stirring in acetylacetone at 220'C in the presence

of Ln(acac)3 and the porphyrin, resulting in the lanthanide porphyrin acetylacetonate

complexes in relatively low yield.[45' 64] This low yield is due to the fact that the

complexes hydrolyze during the considerably long column chromatography times










necessary to separate the product. This method is also limited by the fact that the axial

ligand is restricted to acac, due to the fact that the axial acac ligand is not very labile.

Substitution of the diketonates can be achieved, but the high temperatures and harsh

conditions results in significantly low yield.[65-67] Lanthanide porphyrins have also been

synthesized by amine or alkane elimination reactions of neutral lanthanide amides or

alkyls with the free base porphyrin.[68' 69]

x



0 H
R1 \-NH R, NNH N R,
R2 R2 2
/ NH

2 eq. 2 eq.


X

Figure 1-19. Schematic of 2 + 2 condensation method for synthesis A2B2 type
porphyrins.

R R



N N 2(L1 DMEJ
R -- R +YbCl3*3(THF) R-Y R
e Toluene O c
N NN



R R

Figure 1-20. Schematic of lanthanide metallation of porphyrins using the Foley method.

Most recently, the Boncella group devised a method which increased the overall

metallation yield. This lanthanide porphyrin complexes were synthesized by nucleophilic

displacement of the halogen (Cl1 or F) from anhydrous LnC13 or LnI3 in the presence of









dilithiated porphyrin.[70] This procedure allows for the simple substitution of the axial

ligand by reaction of the Ln porphyrin halide complex with the potassium salt of the axial

ligand.


OEt
B EtO Co /OEt

OPv O
\KTp 0N, V
b KTp KL(OEt)3


R- R> R R
N RRR
R RKQ R R







N


R N


Figure 1-21. Axial substitution of lanthanide metalloporphyrins using salt metathesis.

Light Emitting Diodes

Research into the field of organic electroluminescent materials has grown

dramatically since its inception. Electroluminescence of organic materials was first

observed in 1963 in anthracene crystals.171' 72] However, due to the low efficiencies and

high field strengths required, research into organic based electroluminescent materials

was forgotten until Tang and Van Slyke prepared devices containing vapor deposited

aluminum tris-(8-hydroxyquinolate)(Alq3).[731 An important discovery in the field of










organic based LED's came in 1990, when Friend produced the first polymer based device

from poly(p-phenylenevinylene) (PPV).[74]


Figure 1-22. Structure of aluminum tris-quinolate.

Electroluminescence is the direct conversion of electrical energy into light.

Electroluminescence was first described in the literature by Destriau in 1936.[751 This

paper showed electroluminescence from a semiconductor, ZnS, embedded into a

dielectric matrix under high electric field. This led to high field electroluminescent

materials.




SPL

SI EL

c 0.4


ID 0.2

0.0


400 500 600 700 800
Wavelength [nm]


Figure 1-23. Photoluminescence and electroluminescence spectra of Alq3.[73]

Inorganic light emitting diodes are simple with respect to their operation. Forward

current is driven over a p-n junction and electron-hole recombination occurs through









shallow states or in quantum wells. The tuning of the emission is done by choosing the

correct ratio of starting materials. The major problem with these devices is that only

single crystalline material is capable of exercising this type of recombination radiation

efficiently enough for practical use. This means that one must obtain epitaxial growth on

cheap substrates, which is not an easy task. Because of this limitation of inorganic LED

use, researchers looked for other forms of electroluminescence and materials that could

work in amorphous or polycrystalline films. This mainly led to the field of organic light

emitting diodes (OLEDs) and polymer light emitting diodes (PLEDs).

The term LED used for the device structure is well justified in the fact that the

basic mechanism is an injection of electrons and holes into a heterojunction, and a

radiative recombination of excitons formed from the electron and holes. There are

several points which differentiate inorganic light emitting diodes (ILED) from OLEDs

and PLEDs. The main point is the low mobility of the carriers in organic systems,

approximately five or six orders of magnitude lower than that of typical III V type

semiconductor systems. This is caused by the fact that the organic molecules and

polymers used in these systems are amorphous and therefore the main pathway for carrier

transport is a hopping mechanism. This causes an appreciable drop in voltage across the

film thickness. The differences in spectral width are also dramatic. ILEDs have typically

narrow linewidths, approximately 10 25 nm, whereas organic systems and polymers

typically have linewidths typically on the order of 100 nm.

Most luminescent organic molecules are considered 7n conjugated compounds. That

is, there exists an alternating series of single and double (triple) bonds. Due to this

overlap of the 7n orbital wavefunctions of these adjacent atoms, the electrons occupying









these orbitals are relatively delocalized.[761 In a perfect polymer the delocalized n

electron cloud would extend along the entire length of the chain, but impurities and

defects tend to break the conjugation. In the typical polymer film, the length of a

conjugated segment rarely exceeds 15 repeat units.[77]

There are two major barriers to highly efficient displays constructed of simple

organic materials or polymers. The first major drawback comes in the form of spin

statistics. Spin statistics shows that 25% of all excitations created by charge

recombination produce the singlet excited state, while 75% results in the triplet. Since

most organic materials do not phosphoresce, this energy is lost as heat. The second

major barrier is the previously discussed problem of broad emission. In order to create

efficient displays, there must be excellent color purity and good color saturation. Color

saturation is difficult with the broad spectral width of organic and polymer emission. In

order to fix the problem created by the quantum mechanical distribution of excited states,

a material which shows phosphorescence must be used. This typically involved using

heavy metal chelate complexes, such as Ir(ppy)3[78] While the problem of spin statistics

has been taken care of, there is still the problem of color purity. This problem can be

fixed by the use of lanthanide complexes. Since lanthanide emission is narrow, there can

be efficient color saturation. Lanthanides also induce the heavy atom effect, which

solves the first problem as well.
























Figure 1-24. Structure of Ir(ppy)3.

OLED and PLED Structure

The basic structure of a typical dc-biased OLED is shown in Figure 1-25. The first

layer above the glass substrate is a transparent conducting anode, typically indium tin

oxide (ITO). The next layer is usually the hole transport layer. This layer is usually a

good hole transport material, and for most devices is some type of starburst amine or

poly(3,4-ethylenedioxy-2,4-thiophene)-polystyrene sulfonate (PEDOT-PSS).[79]





Cathode
ETL

Emissive Laye

HTL


Figure 1-25. Schematic of light emitting diode, showing each individual layer.





















Figure 1-26. Structures of typical hole transport materials (a) NPD (b) PEDOT-PSS.

The next layer is the emitting layer. This layer is thermally evaporated or spin

coated onto the hole transport layer. This layer is typically on the order of 100 nm thick.

The next layer is the electron transport layer. This layer usually consists of an organic

material, thermally evaporated, which possesses great electron mobility and also works

well at blocking holes from reaching the cathode, therefore confining the carriers to the

emitting layer. This material typically consists of aluminum tris-quinolate or an

oxadiazole containing complex.[80-86] The next layer is the cathode. This usually consists

of a low workfunction (yp) metal such as Ca (p = 2.87 eV) deposited by thermal

evaporation. A protective layer of aluminum is often deposited on top of the calcium

layer to prevent oxidation.

Carrier Transport

The thin films used in organic or polymer light emitting diodes are typically

amorphous. The amorphous structure leads to a reduction in quenching from internal

conversion processes present in crystalline materials, due to the limit of phonon

interactions, and therefore leads to a consequent increase in the radiative recombination

of Frenkel type excitons.111] The efficient generation of excitons is strongly dependent

on charge carrier injection and transport through the organic layers.









Due to the weak intermolecular coupling and the high disorder of amorphous

materials, their measured carrier mobilities are dramatically lower than in most

crystalline solids. Therefore, the conduction mechanism is usually not considered Ohmic

in nature, but is often space charge limited, influenced by the presence of traps, or

hopping. 871 For example, carrier trapping in diamines has a negligible effect on its

conduction properties, and the hole mobilities in these materials are of the order of

10-3cm2/V s.1881 In electron transporting materials such as Alq3, however, the trap density

is much higher, which significantly lowers the carrier mobility.[89] The electron mobility

of Alq3 is 10-4 cm2 / V s at an applied voltage of 106 V/cm, with hole mobilities at least

two orders of magnitude lower.190] The carrier mobility of most organic materials is

found to be dependent on both the electric field (E) and the temperature (T) according

to[91]


E /E2 8E1>
u(E, T)=oe e oe eaE 2 (0.15)
kT kff kT )


where k is the Boltzmann constant, Eo = kTo is the activation energy at zero electric field

corresponding to temperature To, po and [to* are the zero field carrier mobilities, 1/Teff=

1/T I/To, and 0 and a are constants.

The difference between the cathode (anode) Fermi level and the LUMO (HOMO)

of the electron (hole) transport layer forms a barrier to injection of electrons (holes) into

the active layer. For devices with a barrier larger than 0.4 eV, the current flow is

primarily determined by the efficiency of carrier injection at the contacts.[91] This is

typically described as the injection-limited regime. The energy level offset at the









organic-electrode interface can be tailored by choosing an electrode with a work function

corresponding to the energy levels at each layer.

In organic molecular light emitting diodes, three different conduction mechanisms

are commonly observed: ohmic, space-charge limited (SLC) conduction, and trapped

carrier limited (TCL) space charge conduction. Ohmic conduction is seen at low voltages

when the density of injected carriers, ninj, is smaller than the thermally generated

background free charge density, no. In this regime, the current density is given by Ohm's

law:

J= q,noV /d (0.16)

where q is the electronic charge, [n is the hole or electron mobility, V is the applied

voltage, and d is the layer thickness. Space charge limited conduction is observed when

ninj > no when charge trapping is not observed. The current density is then described by

Child's law:

V2
J=(9 /8)qu 3 (0.17)


where E is the dielectric permittivity.[92]

The presence of ohmic and space charge limited conductivity is observed in the

low-voltage operation of devices. At higher voltages, traps located near the LUMO tend

to fill. If there exists a high density of traps, their concentration and energy distribution

governs the current, resulting in the trapped charge limited (TCL) space charge

conduction regime. As the traps fill, they reduce the density of empty traps and an

increase in electron mobility ensues. An analytical expression relating the current to the

voltage in the TCL regime is given for a continuous energy distribution of traps Nt(E)

below the LUMO, as given by[92]









NE EI
N,(E) = eKELUM (0.18)


where ELUMO is the LUMO energy, Nt is the total trap density, k is the Boltzmann

constant, and Tt = Et/k, where Et is the characteristic trap energy. The current density is

then given by


I [/M^ N,(m + 1) mm+l) d(2d+l)
LCL o dnq (0.19)


where NLUMO is the density of states (DOS) in the LUMO band, m = Tt/T, and t = p(E).

From this equation, it can be seen that trap limited conduction results in a power-law

dependence of current on voltage.

Device Efficiency

Efficiency of devices is an important issue not only for energy consumption, but

also for its effect on the longevity of the devices. This effect on longevity is due to the

minimization of ohmic heating that can be achieved by operation at lower voltages.

Devices with high power efficiency imply a low current-voltage product for a given

luminance. Power efficiency is only one of the ways to determine device efficiency. The

most commonly reported efficiency from the literature is based on external quantum

efficiency. External quantum efficiency is the measure of photons produced per electrons

injected.[93] One of the forms in which the basic equation for the external quantum

efficiency rext of the device can be written as

7ext :, = ^srsT L (0.20)

where is the fraction of photons collected normal to the front surface of the device, y is

measure of the hole and electron recombination, rsT is the ratio of singlet to triplet









excitons formed in the material, and 4PL is the solid state photoluminescence quantum

efficiency. It can be shown that for devices with a large refractive index:193]

0.5 (0.21)
n 2

where n is the refractive index of the medium. For devices with equal charge carrier

balance the factor y equals one, but this is rarely the case with organic materials. The

value for rsT is, according to spin statistics, 0.25, but recently there has been some

evidence that this is not always the case.[94-98] While the photoluminescence quantum

efficiency can approach unity for many organic dyes, the efficiency in the solid state is

typically much lower. This is due to "concentration quenching", which is an effect due to

the creation of nonradiative pathways.[99-104] Given all of these conditions, devices which

show only fluorescence have a upper level of efficiency approaching 15% of the solid

state photoluminescence efficiency.

Device Failure Mechanisms

The overall stability of light emitting diodes is an important element in

understanding their commercial impact. The degradation of an OLED and PLED during

operation appears in four modes:1105] (1) decay in emission intensity, (2) a voltage

increase in the constant current mode, (3) the growth of nonemissive areas in the device,

and (4) the eventual electrical short circuit.

One of the most evident mechanisms for the degradation of OLEDs is through the

formation of nonemissive "dark" spots which in turn leads to a long term decrease in

efficiency. These spots result from the delamination of the metal at the metal/organic

interface due to a large amount of Joule heating.[106] This tends to lead to a short-circuit









condition and carbonization of the active layer. This may also lead to electrode migration

in these areas where the conductivity is high.11071

The most prevalent mechanism for decay in PLEDs is photooxidation during

device operation.[108' 109] The extended conjugation length of polymers tends to increase

the electron density at the double bond making it more reactive to the electrophilic singlet

oxygen. Carbonyl formation by the reaction of oxygen with alkoxy centers is also a

facile method for the quenching of electroluminescence due to the fact that carbonyls are

typically good nonradiative quenching centers. The source of the singlet oxygen is

energy transfer from the polymer to molecular oxygen.

The primary degradation method in organic light emitting diodes is

recrystallization. Excitons are rapidly quenched by defects and charge-dipole induced

fields at the surface of a grain boundary. Any given amorphous layer will recrystallize

slowly as its temperature rises towards its glass transition temperature.

The final mechanism, shared by both OLEDs and PLEDs is the electrical

breakdown of the device caused by pinhole electrical arcs.11101 These breakdowns occur

usually at high voltage. The mechanism is the circuit opens around the pinhole which in

turn stops the arcing. The resultant hole allows for moisture and air to enter the device

and delaminate the material. When the number of burn-outs becomes too numerous, the

circuit eventually shorts out and destroys the device.

Literature Review

The field of organic and polymer light emitting diodes has exploded in the latter

part of the 20th century and has continued to expand into the present. Most of the focus

has been on polymers and organic molecules which emit in the blue region of the

electromagnetic spectrum. This focus on this region is due to the poor efficiency and









color saturation of organic systems and molecules which emit in the blue. This is unlike

molecules which emit in the green and red, which have been designed with high

efficiencies and exhibit adequate color saturation. Recently, there are several displays

based on polymer and organic systems which are commercially available.111' 112]

The research carried out here at the University of Florida, focuses on the emission

of organic and polymer systems which emit in the near-infrared. This can be

accomplished by three means. The first involves organic molecules and polymers which

exhibit a small HOMO-LUMO gap, which permits emission in the near-infrared.

Although this is possible, the efficiency of fluorescence and device performance is

severely limited by nonradiative decay processes. The second means for producing

emission in the near-infrared is the use of organo-transition metal or organo-lanthanide

complexes. The third means for the creation of near-infrared (NIR) emission is the use of

organic functionalized semiconductor nanoparticles. In this section, a brief history of

each of the possible directions is provided.

Organic Systems

There are several examples of groups obtaining near-infrared electroluminescence

from organic and polymeric materials. The majority of this emission although is located

in the visible, with emission carrying over into the NIR. The Holmes group, in 1995,

showed near-infrared emission from a 2,5-bis(hexyloxy)terephthalaldehyde-3-dodecyl-

2,5-thiophenediacetonitrile copolymer which showed broad emission extending out to

1000 nm, with an internal quantum efficiency of 0.2% photons/electron.[113] Fujii, in

1997, fabricated devices showing emission tailing out to 1000 nm with phthalocyanine

co-evaporated with DCM (4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-

4H-pyran), which showed efficient emission operating at 15 V.114] In 2000, Suzuki









showed emission from an organic ionic dye, 2-[6-(4-dimethylaminophenyl)-2,4-

neopentylene-1,3,5-hexatrienyl]-3-methylbenzothiazolium perchlorate blended into

poly(N-vinylcarbazole). These devices showed that the emission improved over time due

to the alignment of the ionic dye molecules along the bias field, with the external

efficiency eventually reaching 1%.[115] Maltsev, in 2002, showed very narrow emission

at 850 nm resulting from the electroluminescence of J-aggregates of cyanine dyes

blended into a polyimide matrix.[116] The Bazan group showed efficient

electroluminescence (,ext = 0.1 0.3 % photons/electron) of ethynyl linked porphyrin

complexes, in 2003. These complexes show emission typical of porphyrins with a tail

into the near-infrared. 117] In 2004, Suzuki again produced devices with emission into

the NIR, using organic ionic dyes. He used (2-[6-(4-dimethylaminophenyl)-2,4-

neopentylene-1,3,5-hexatrienyl]-3-methyl-benzothiazonium perchlorate) (LDS821) and

[C41H33 C12N2](+).BF4- (IR1051) to produce emission at 800 and 1100 nm,

respectively.[1181 This year, Yang created devices from alkyl-substituted fluorene, 4,7-

diselenophen-2'-yl-2,1,3-benzothiadiazole (SeBT), and 4,7-diselenophen-2'-yl-2,1,3-

benzoselenadiazole (SeBSe), which showed emission in the 670 790 range. These

devices showed external efficiencies ranging from 0.3 to 1.1%.[119] Most recently,

Thompson et al showed NIR emission peaking at 800 nm from a donor-acceptor

copolymer of 1,4-(2,5-dihexadecyloxyphenylene) and 5,8-linked 2,3-

diphenylpyrido[3,4]pyrazine. 120]

Inorganic Nanoparticles

The use of solution processible semiconductor nanoparticles (quantum dots, QD)

for electroluminescent emission in the near-infrared is a fairly recent process. These









devices are based on II-VI and IV-VI semiconductor nanoparticles blended into

conductive polymer matrixes. The use of QDs for electroluminescent devices is an

interesting concept due to the tunability of emission created by quantum confinement. As

the nanoparticle is decreased in size, the band gap increases; therefore causing a

hypsochromic shift in the emission. The QDs are made solution processible by the

functionalization of the surface with organic ligands.

In 2003, Steckel showed tunable electroluminescent emission using PbS

nanoparticles, with emission ranging from 1.3 1.6 atm, dependent on particle size.1121]

Bakeuva then increased the emission range from 1.1 to 1.6 [tm by changing the organic

molecule bound to the surface from oleate ligands to octadecylamine ligands. These

devices showed an internal efficiency reaching 3%.[122] Most recently, Sargent fabricated

devices containing PbSe which showed tunable emission in the near-infrared.[123]

Progress in the field of nanoparticle based light emitting diodes was further advanced in

2004 by the production of HgTe based devices, which showed efficient emission and

broad tunability.[124]

Inorganic nanoparticles are extremely versatile due to the large tunability of the

energy gap. Efficiency is also enhanced due to limited phonon interactions in

nanoparticles, which allows for less nonradiative decay from the excited state.

Nanoparticles also have severe limitations. The first limitation is that nanoparticles tend

to aggregate in solution and disperse poorly in thin films. The second major limitation is

the fact that nanoparticles tend to oxidize due to the abundance of non-bonded atoms on

the surface.









Organo-Lanthanide and Organo-Transition Metal Complexes

The use of lanthanide and transition metal complexes is by far the most common

method for emission in the near-infrared. The field of organic lanthanide metal

complexes was born in the late 1950s and continued throughout the 1960s with research

devoted to lanthanide beta-diketonates.1125-133] Research then turned to lanthanide

polypyridyl complexes.1131' 134-136] In the 1970s much of the research into lanthanide

organic complexes shifted to porphyrins.[45' 46, 53, 64, 137-144] Since then, most of the

research is into trying to improve the efficiency of the lanthanide emission.[145-154]

The use of lanthanide organic complexes for electroluminescence was unknown until

1999, when Curry observed electroluminescence from tris-quinolato erbium(III) (ErQ3).

This complex showed the typical 4113/2 4115/2 emission centered at 1.54 am.155] Since

then a great deal of research has been focused on ErQ3 due to its potential use in optical

communications.[156-159] Near-infrared emission has also been seen for other lanthanide

quinolate complexes. Kawamura in 2000, observed typical 2F5/2 2F7/2 emission from

Yb3+ tris-quinolate at 985 nm operating at 15V.1160, 161] Several groups also observed

near-infrared emission from Nd3+ tris-quinolate based devices, showing emission at 800,

1060, and 1300 nm.[157, 159, 162-164] Since then the focus of rare-earth quinolate complexes

has been on the effect of structure on the emissive properties. It was determined that

functionalization of the quinolate complex with halogens has a dramatic improvement in

the emission yield.[165' 166]



















Figure 1-27. Structure of Lanthanide tris-quinolate.

Lanthanide quinolate complexes, although functional, are not the only means of

sensitizing near-infrared emission of lanthanide ions. Near-infrared emission has also

been observed for complexes of lanthanide diketonates. In 2001, Hong produced devices

with Pr(DBM)3Bath and Yb(DBM)3Bath (see Figure 1-28), which showed the 1D2 -- 3F2

(890 nm), 1D2 3F3 (1015 nm), 1D2 -_ 3F4 (1065 nm), and 1D2 -- 1G4(1550 nm) for Pr3+

and 2F5/2 2F7/2 emission from Yb3+ respectively.1167' 168] Harrison, later reported in

2003, emission from the Er3+ 413/2 4115/2 (1520 nm) and Nd3+ 4F3/2 4111/2 (880 nm),

4F3/2 4113/2 (1060 nm), 4F3/2 4 15/2 (1330 nm) complexes of DBM.11691 Previously

unseen, in organic systems, transitions for Ho3+ 5F5 -* 56 (1500 nm), 5F5 -* 57 (1200

nm), 5F5 51s (980 nm) and Tm3+, 3F4 3H4 (1400 nm) 3F4 3H6 (800 nm) were

finally observed in 2004 in DBM systems.[170, 171]

The use of other molecular systems to produce near-infrared emission has been

achieved. For example, Sloof in 2001, produced typical Nd3+ emission from a lissamine

dye functionalized terphenyl based metal complex. They determined that a larger

conversion to the triplet state under electrical excitation, resulting in a more efficient

Nd3+ emission.[172] Previous work in our group by Harrison et al. has produced NIR

emission from lanthanide porphyrin complexes.[173-177] It was determined that, the

porphyrin efficiently transferred its energy to the lanthanide, producing the characteristic










metal emission. Emission from Nd3+, Yb3+, Ho3+, and Er3+ were observed. They also

determined that the change of axial coordination ligand has little effect on the

electroluminescence efficiency. Recently dendrimer complexes with a lanthanide core

were fabricated and showed efficient near-infrared emission.[1781 The core of the

dendrimer was an nona-coordinated Er3+ atom with keto functionalized metalloporphyrin

dendrons. Excitation of the porphyrin dendron resulted in the typical lanthanide

emission.















l. E OBMI.aI Il

Figure 1-28. Structure of Lanthanide tris-DBM bathophenthroline complex.














CHAPTER 2
SUBSTITUTED PORPHYRIN COMPLEXES

This chapter discusses the results of the study of substituted lanthanide porphyrin

complexes. Previous work by Harrison et al showed that, although the porphyrin ligand

is useful in sensitizing the emission of lanthanides in light emitting diodes, its electronic

properties are not ideal for highly efficient devices. This study was undertaken to further

investigate the electronic properties of the porphyrin macrocycle, as well as to enhance

the performance of devices created using lanthanide substituted porphyrin complexes. In

order to accomplish this, several porphyrins were synthesized by Alison Knefely,

according to the procedure described in chapter one, with substitutions of the meso

phenyl group in an effort to vary the electronic properties of the complexes. Bulky

groups were appended to the phenyl groups to understand the effect of aggregation in the

devices. The axial ligand was also substituted. The structures of the resulting

complexes are shown in Figure 2-1.

The first complex studied was (quinolinato)(5,10,15,20-

tetraphenylporpyrinato)Yb(III) (Yb(TPP)Q). The hydroxyquinoline ligand was used as

the axial ligand in an effort to increase the electron accepting nature of the complex due

to the electron poor unsaturated nitrogen. This complex (see Figure 2-2) was also used

because it closely resembled previously studied complexes.[169] The second complex

studied was (hydridotris(1-pyrazolyl)borato)(5,10,15,20-tetrakis(3,4,5-

trimethoxyphenyl)porphyrinato)Yb(III) (Yb(TMPP)TP). The substitution of the electron

donating alkoxy groups was chosen in an effort to perturb the overall electronic structure










of the complex. It was expected that the hole mobility for devices fabricated using this

complex would be higher. The third complex studied was (cyclopentadienyl)

tris(diethylphoshinito)cobaltate)(5,10,15,20-tetra(4-pyridyl)porphyrinato)Yb(III)

(Yb(TPyP)L(OEt)3. The substitution of the phenyl groups for pyridyl groups was an

effort to increase the electron transporting properties of the complex due to the presence

of electron poor pyridine substituents. The use of the Klaui (L(OEt)3) ligand was

motivated by the fact that the analogous complex with the TP capping ligand was

insoluble in all common organic solvents, and the Klaui ligand improved the solubility.

It has been shown that the Klaui ligand has little effect on the overall properties of the

complexes.[169] The final complex studied was (hydridotris(1-pyrazolyl)borato)

(5,10,15,20-tetra(4(2'-ethylhexyloxy)porphyrinato)Yb(III) (Yb(TPP_OEH)TP). It was

expected that the long branched alkoxy side-chain would reduce aggregation due to the

inability of complexes to approach each other. The photophysical, electrochemical, and

device performance characteristics of these complexes are discussed in this chapter.


0


mN 0-

\/ !_/ MeO O

Yb(TPP)Q Yb(TMPP)TP

S OEt
EtO Co--p Et
EtOP Et OEt


NN

N N


Yb(TPP)L(OEt)3 Yb(TPP_OEH)TP

Figure 2-1. Yb porphyrin complexes used in study.























Figure 2-2. X-ray crystal structure of Yb(TPP)Q showing coordination of a molecule of
THF.

Solution Photophysics

Absorption

All absorption measurements were made with the complexes as dilute solutions in

methylene chloride (CH2C2) unless otherwise indicated. Figure 2-3 shows the

absorption spectra for the studied ytterbium porphyrin complexes. The absorption

spectra are dominated by the 7n -* 7n* transitions of the porphyrin ligand, including the

Soret (So -- S2) band (- 420 nm) and the weaker Q-bands (So -- Si) (-500 600 nm).

No absorption for the 4f-- 4ftransitions could be seen due to their low molar

absorptivities (- 1-10 M-cm-1) Furthermore, little evidence for absorption by the

capping ligand was observed in the samples studied. The Soret band showed a

bathochromic shift upon coordination to the lanthanide, from 412 nm in the free base to -

427 nm in the complex. Overall, the absorption spectrum resembles a "normal"

metalloporphyrin absorption spectrum such as Ni(TPP).1371 These results correlate with

previously studied unsubstituted Ln(TPP)L complexes.[169] Although the location of the

absorption peaks of each complex is slightly different, there is little evidence to suggest

that changes to the porphyrin periphery has a dramatic effect on the electronic properties









of the ligand. This suggests that there is little perturbation of the 7t-electronic system due

to substitution upon the porphyrin ring. There exists some perturbation to the system by

the introduction of the quinolate capping ligand. The Soret band blue-shifts with respect

to other Yb(TPP) complexes, showing that the basic ligand has some effect on the overall

electronic properties of the complex.

Table 2-1. Photophysical properties of Yb porphyrin complexes measured in CH2C12.
Complex habs / nm (Log e) ,em / nm
Yb(TPP)Q 375(4.12), 420(5.61), 516(3.48), 553(4.30), 600, 650, 715, 913, 927, 954,
591(3.68), 626(2.97) 980, 1005, 1025, 1047,
Yb(TMPP)TP 378(4.03), 404s(4.72), 428(5.75), 516(3.50), 610, 650, 715, 925, 952, 978,
554(4.30), 592(3.57), 630(2.97) 1003, 1029, 1049
Yb(TPyP)L(OEt)3 403s(4.65), 427(5.76), 519(3.68), 558(4.44), 605, 645, 715, 923, 950, 985,
597(3.71), 626(2.92) 1005, 1018, 1048
Yb(TPP_OEH)TP 378(3.92), 404s(4.61), 427(5.64), 517(3.39), 605,650, 715, 927, 951,974,
553(4.18), 592(3.45), 627(2.86) 1003, 1020, 1050


Emission

Excitation of the Yb porphyrin complexes into the Soret or Q-bands resulted in

both visible (Figure 2-4) and near-infrared emission (Figure 2-6). In the visible region

from 600 750 nm, there are three emission bands. The weak visible fluorescence at

-650 and -715 nm matches the visible fluorescence of free-base

tetraphenylporphyrin.[1791 These assignments agree with previous studies of Ln(TPP)L,

which came to a similar assignment partly due to the short (8 ns) lifetime of the 650 and

710 nm bands.[169] A weak emission band at -600 nm was observed in all of the Yb

porphyrin complexes. The excitation spectrum of all three emission bands helps to

elucidate the origin of each emission transition. The excitation spectra of the three

emission peaks for the Yb porphyrin complexes are shown in Figure 2-5. Comparison of

the Soret bands observed in the excitation spectra show that the 610 nm emissions are

bathochromically shifted -15 nm from the excitation spectra of the other emission bands.









This is similar to the UV-Vis absorption band shift which occurs when the porphyrin is

coordinated to Ytterbium. Thus the visible emission bands at 654 and 720 nm can be

attributed to "free" porphyrin emission which seems not to be influenced by the metal,

while the 610 nm band corresponds to emission from the lanthanide metalloporphyrin

complex. All of the structures show an excitation peak around 450 nm (Xem = 715),

which suggest that there exists the possibility of a more complex structure involved in the

emission, such as an aggregate. This all leads to the conclusion that there is the presence

of a free base impurity, which leads to less desirable features in the near-infrared.

The near-infrared photoluminescence properties of the substituted ytterbium

porphyrins are different and vary dramatically, in contrast to the absorption spectra,

which are similar for all of the complexes. Figure 2-6 shows the near-infrared emission of

the substituted Yb3 porphyrin complexes. The near-infrared region consists of a sharp

peak at -980 with additional broad bands on each side of the sharp peak. The structure of

the spectra has been attributed to crystal field splitting effects which are calculated to be

on the order of hundreds of cm-1.[125]

Previous work with Yb porphyrins suggests that the 2F5/2 2F7/2 emission

transition is composed of eight different peaks.[169] Through curve fitting using the

commercially available program Originc, the crystal field induced emission structure was

estimated. For Yb(TPP)Q seven peaks could be ascertained, and for all of the other

complexes there were six peaks that could be determined with confidence. All of these

peaks show evidence of a crystal field splitting of- 500 cm-1 which agrees well with

previous results.[169] Variable temperature work by others has shown that higher crystal

field states can be thermally populated when the excited state possesses a long









lifetime.1180, 181] This agrees well with variable temperature photoluminescence studies

carried out on the porphyrin complexes. As an example Figure 2-7 shows 80 K and RT

emission spectra for the complexes, which clearly shows the high energy emission band.

The use of different solvents can play a significant role in the optical properties of

organic compounds. Therefore the solvent effects on the absorption and emission

properties of the Yb3+ porphyrin complexes were examined. The changes in absorption

properties for all complexes were similar in various solvents. The Soret band shows a

slight bathochromic shift when placed in increasing polar solvents.

While the absorbance of Yb(TPP)Q varies by a few nanometers in different

solvents, the solvent used has a more significant effect on the NIR photoluminescence

quantum yield. The effect of solvents on the quantum yields of the other Yb porphyrin

complexes was less important. In general, the quantum yields are low (<10%) due to non-

radiative pathways of deactivating the excited state. Table 2-2 shows the NIR quantum

yields of the Yb porphyrin complexes in different solvents. Since the lanthanide is

positioned above the plane of the porphyrin ring (see figure 2-2), there remain several

coordination sites where a solvent molecule can access the metal ion. Yb(TPP)Q has a

relatively small axial ligand which may allow for trace amounts of water present in the

solvent or the solvent itself, to coordinate to the metal, which in turn lowers the radiative

quantum yield. The coordination of water to lanthanide metal centers is known to play a

significant role in deactivating the excited state of the lanthanide ion by coupling to the

non-radiative O-H vibrational modes. However, in coordinating solvents, the solvent

displaces the coordinated water thus removing the O-H oscillators from the proximity of

the metal ion, which in turn lowers the non-radiative rate.







48




5e+5
a) 2e+4
4e+5
le+4
3e+5
0
2e+5 480 520 560 600 640
Wavelength / nm
le+5

0
6e+5 b) 2e+4
2e+4
5e+5
le+4
4e+5

3e+5 0
E 480 520 560 600 640
2e+5 e Wavelength / nm

l e+5

Q- c) 3e+4
6 6e+5
-l 2e+4

CU le+4
S4e+5

480 520 560 600 640
2e+5 Wavelength / nm


0
5e+5 d)
le+4
4e+5

3e+5 0
480 520 560 600 640
2e+5 Wavelength / nm

le+5

0 ^ --,
300 400 500 600 700

Wavelength / nm

Figure 2-3. Absorption spectra for a)Yb(TPP)Q, b) Yb(TMPP)TP, c) Yb(TPyP)L(OEt)3,
d) Yb(TPP_OEH)TP in CH2C12 as a function of molar absorptivity.














1.0

0.8

0.6

0.4

0.2

0.0
Yb(TMPP)TP
1.0

0.8

0.6

0.4

0.2

0.0 Yb(TPyP)L(OEt)3
1.0

0.8

0.6

0.4

0.2

0.0
SYb(TPP_OEH)TP
1.0

0.8

0.6

0.4

0.2

0.0
500 550 600 650 700 750 800

Wavelength / nm


Figure 2-4. Visible emission spectra for substituted Yb3+ complexes in CH2C2 at room
temperature (Xex = 420 nm).







50






(a) (b) (c)
SRen Le



= 650650nm m z645rm







715nm= 715m
[E I 15 75nm







wla.. glli / nm W.a egti nm W.engt / nm

Figure 2-5. Excitation spectra of (a) Yb(TPP)Q, (b) Yb(TMPP)TP, and (c)
Yb(TPyP)L(OEt)3.














1.0

0.8

0.6

0.4

0.2

0.0
Y
1.0

0.8

0.6

0.4

0.2

0.0
1.0

0.8

06

0.4

0.2

0.0
Yk
1.0

0.8

0.6

0.4

0.2

0.0
900


1050 1100


Figure 2-6. Near-infrared emission spectra for substituted Yb3+ porphyrin complexes in
CH2C12 at room temperature (kex = 420 nm).


3 1000

Wavelength / nr


































900 950 1000 1050 900 950 1000 1050 1100
Wavelength / nm Wavelength / nm

Figure 2-7. Near-infrared emission spectra for substituted porphyrin complexes in 2Me-
THF(Xex = 420 nm) (- 80 K, --- 300 K).

Table 2-2. Solvent effects on the near-infrared quantum yields of Yb porphyrin
complexes.
Solvent Yb(TPP)Q Yb(TMPP)TP Yb(TPyP)L(OEt)3 Yb(TPPOEH)TP
OPL OPL OPL OPL
CH2C12 0.0091 0.041 0.031 0.034
Toluene 0.0081 0.037 0.039 0.047
THF 0.0085 0.028 0.027 0.036
DMSO 0.029 0.031 0.032 ---
CH3CN 0.0097 0.026 --- ---
Hexane --- --- --- 0.057


The presence of high energy oscillators within close proximity of the lanthanide ion

will increase the non-radiative decay rates of the lanthanide excited states. By

understanding the Energy Gap Law and using Siebrand's approach, the non-radiative rate

can be found using Equation 2.1.[182]










k 2= NrJp,,, 2 P 2F (2.1)


In Equation 2.1, p,, is the density of final vibrational states, J is the electronic

McOe
coupling constant due to nuclear motion, p measures the coupling to the vibrational
2h

modes, N. is the number of modes and F is the Franck-Condon factor. The Franck-

Condon factor can be described by Equation 2.2.


exp(- kA2)( kA2)v
F = 2 2 (2.2)


In Equation 2.2, k is a constant, A is the difference in positions of the final and initial

vibrational states and v is composed of the energy between the two states, AE0 and the

maximum oscillator of highest energy, Omax as shown in Equation 2.3.


v= E -1 (2.3)
2h max

Using Sterling's approximation to expand the factorial in Equation 2.2 and the

approximations for energy gaps of lanthanides corresponding to 1 to 3 vibrational quanta

of the host, the non-radiative decay rate can be simplified to Equation 2.4.

k, = P exp [-(AE 2h oma)a] (2.4)

The a and / terms vary very little between hosts. Therefore, as the radiative

energy gap decreases, the non-radiative decay rate increases exponentially. Given the

2F5/ 1 2F7/2 transition is 10,000 cm-1, it is evident that the use of C-H oscillators

(-3200 cm-1) plays a significant role in the deactivation of the lanthanide excited state.









Light Emitting Devices

Light emitting diodes used in this study were of the modified single emissive layer

type, that is, there was no true electron transport layer. These devices where prepared by

spin coating a thin layer of PEDOT-PSS (Bayer Baytron P VP 4083) onto cleaned ITO.

The device was then dried in vacuo for 4 hours at 150 C in order to remove any residual

solvent. The active layer was spin-coated and the device was place in high vacuum (10-6

torr) for several hours, again to remove residual solvent. Calcium and aluminum metal

were then thermally deposited under high vacuum. The devices where then encapsulated

with a commercially purchased epoxy material. The device architecture is shown in

Figure 1-25.

Devices were prepared with the Yb porphyrin complexes blended into polystyrene

at varying wt% in order to determine the effects of dopant concentration on device

performance. Electroluminescence spectra of the Yb porphyrin devices at 40 wt% are

shown in Figures 2-8 2-9. The emission is shown as a function of voltage. At all

concentrations, Yb3+ is the predominant emitter in the devices with a near-infrared

emission at 977 nm. The emission observed results from direct electron-hole

recombination which occurs at the Yb porphyrin complex. As the voltage is increased,

the spectral shape of the Yb3+ begins to show a defined emission band at 920 nm. This

could be due to the fact that the 2F5/2 energy state of Yb3+ is in a non-symmetric

environment and is split into three crystal field states. The splitting of these states is on

the order of a few hundred cm-1. With a sufficiently long excited state lifetime, thermal

equilibrium is established between the lowest and second lowest excited states which

results in emission of a higher energy band at 920 nm. The Yb(TPyP)L(OEt)3 devices

were less stable and failed at a lower voltage than the other devices. This can possibly be









attributed to the fact that electrochemical experiments show that there is no reversible

oxidation of the complex. The current density-voltage (j-V) profiles of the devices are

shown in Figure 2-10. The turn-on voltages for the devices were typically 6 V for the 40

wt% devices and 5 V for the 60 wt% devices. Although near-infrared emission is the

predominant emission process, there exists a significant emission in the visible region

which mirrors the typical porphyrin emission. This emission is roughly 1 5 % of the

intensity of the near-infrared emission after correction for instrument response. This

visible emission can be attributed to two possible pathways. Either the emission comes

from a free base porphyrin impurity or it results from a more complicated process of

delayed fluorescence. This process of delayed fluorescence, results from a back energy

transfer from the lanthanide to the porphyrin triplet state. This in turn reacts with another

porphyrin in a triplet state, resulting in a singly excited porphyrin, which then can emit.

The current densities of all of the devices were somewhat similar suggesting that

change to the porphyrin structure has little effect on charge transport properties of the

complex. As the amount of Yb porphyrin complex was increased, the efficiency

increased. This is indicative of better charge transport through the device, confirming

previous studies suggesting that the porphyrin is the key to charge transport.[175, 183 ]

The near-infrared external quantum efficiencies, measured as photons collected per

electrons injected, for 40 and 60 wt% are show in Figure 2-11. At 40 wt% the maximum

efficiency of Yb(TPP)Q is 2 x 10-4 and increases to 4 x 10-5 with increase in

concentration to 60 wt%. Efficiency in the other devices ranged from 1 3 x 10-4, which

agrees with previous Yb(TPP)L device performance.[169] This again suggests that the

changes to the porphyrin have little effect on the electronic properties of the complexes.







56





400







2D


400


400





100




























400 0 600 700 B.o.
10Wavelength m


Figure 2-8. Visible electroluminescence of Yb(TPP)Q, Yb(TMPP)TP,





400 500 600 700 00.
Wavelength / nm


Figure 2-8. Visible electroluminescence of Yb(TPP)Q, Yb(TMPP)TP,
Yb(TPyP)L(OEt)3, and Yb(TPPOEH)TP as a function of increasing voltage,
starting at 6V.







57





















300l


200


U), 100
fo ),-,---- --- ----- .-- ,.,----------_---_ ,.















150



100









I t








2w-" ,.. ,-.



850 900 950 1000 1050

Wavelength nm


Figure 2-9. NIR Electroluminescence of Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3,
and Yb(TPPOEH)TP as a function of increasing voltage, starting at 6V.







58





300 Yb('mPP 40O%
SYb(TPP)06Q%
S YbI(TrPP)L(OB) 40%
E Yb(TPyP)L(OB), 6%
v Yh(TMPP)TP40%
E v YhbTMPP)TPo M
S200 Yb(TPPOEH)TP 40%
+* Yb(TPPOEH)TP% *%
*
S* *
2100



0 sii i '
0 2 4 6 8 10 12 14
Voltage I V


Figure 2-10. Current density Voltage (j-V) plot of Yb porphyrin devices as a function
of loading wt% in PS.

The decrease in near-infrared efficiency with increasing current density is due to

three possible factors. First, because the lifetime of the lanthanide excited state is long, ~

40 [tsec, saturation of the emissive sites can occur at high current densities.[184] Work by

others has shown that the decrease in efficiency can be due to triplet-triplet

annihilation.[98] A final possibility is due to permanent degradation of the device through

chemical reactions. Charge hopping is believed to be the primary charge transport

mechanism in Yb porphyrin blended into the non-conducting polystyrene devices.

Previous work with ZnTPP in poly(2-vinylpyridine) showed that, when ZnTPP molecules

were within 1.1 nm of each other, charge hopping occurs.[185] The porphyrin complexes

in this study are expected to be within 1 nm of each other due to aggregate effects.

Figure 2-12 shows the charge-hopping model. In this model holes are injected at

the anode and electrons are injected at the cathode. The carriers then hop along the

porphyrins until they either combine with the opposite carrier, and create an excited

porphyrin, or are annihilated at the opposite electrode.









Previous research showed that the rate of hole transport in Yb(TPP)L far exceeded

the rate of electron transport.[169] In order to study this effect, devices were fabricated in

which a known electron transporter, tris(8-hydroxyquinolate) aluminum (Alq3), was

blended into the Yb(TPP)TP/PS matrix. Figure 2-13 shows the current density-voltage

and NIR irradiance-voltage characteristics for 0, 33, and 50 wt% (Alq3/Yb(TPP)TP)

respectively. The current density at a given voltage drops dramatically with the increased

introduction of Alq3 until 66 wt% Alq3, where the devices become unstable. This

decrease in current density is characteristic of an increase in charge carrier balance. The

NIR irradiance increases by a factor of two from 0 to 50 wt% Alq3. This suggests that

more excitons are recombining on the porphyrins resulting in greater light output. The

NIR external quantum efficiency-current density characteristics are shown in Figure 2-

14. The efficiency increases by a factor often and the data variance decreases with

increasing Alq3 concentration again suggesting better charge transport and improved

device performance.

In order to determine if changes to the porphyrin structure improve the charge

transport capabilities, the same Alq3 blending experiment was carried out with

Yb(TMPP)TP. Figure 2-15 shows the current density-voltage and NIR irradiance-

voltage characteristics of the devices. Again, as in the Yb(TPP)TP based devices, there is

a decrease in current density and an increase in NIR irradiance with increased

concentration of Alq3. The NIR external quantum efficiency, shown in Figure 2-16, also

increases by approximately a factor of ten. These results mirror those of Yb(TPP)TP,

suggesting again that the changes to the porphyrin structure have little effect on charge

transport properties of the complexes.







60


0.5

0.4 I I

0.3 o

0.2
*

0.1 0 S

0.0

." V
vy
3 -v v


2 v v


v V
Y V v


4- V v

















1 0 ._ 5 + T*
*
0.0




U 1 m0 30

m,"' .




1.0 0 *



0.5 *, 0 4
-- ,... *: :::::

*
0.0 I------------
0 100 200 300 400
Current Density / mAcm-2


Figure 2-11. Near-Infrared external electroluminescent quantum efficiency for
Yb(TPP)Q, Yb(TMPP)TP, Yb(TPyP)L(OEt)3, and Yb(TPP_OEH)TP as a
function of loading in PS. (black = 40%, red = 60%).











0
~L'-tH


1
2>


Figure 2-12. Charge hopping model showing transport of charges on porphyrin
molecules.[177]













300


200


100


E 0
,300

E
,200
(-I-'
C(
0100
-I-'
a)
0
300


200


100


0


0 2 4 6 8 10 12 4


6 8 10 12


Voltage / V


Figure 2-13. Current density-Voltage and NIR irradiance-voltage plots of Yb(TPP)TP as
a function of Alq3 loading: (*) 0 wt %, (0) 33 wt%, and (A) 66 wt %.


8

*
*
** *
ga : *
** .
***




*



** *




U
a A
A a
A U
A




& A A


aAAAAAA A~


U (c4
E
15


10

r-
5 (a

0


15 z


10


5


0
















A


I A
.4


o 1.5e-3

LU


E 1.0e-3
--i
r

0
"-I-


cc 5.0e-4
l-4-

LU

So.o0
Z


A
A
A


* OmgAIQ3
* 1mg AIQ3
A 2mgAIQ3



A


* *0


20 40 60 80 100


Current Density / mA cm-2


Figure 2-14. NIR External quantum efficiency of Yb(TPP)TP as a function of Alq3
loading: (*) 0 wt %, (0) 33 wt%, and (A) 66 wt %.


150



75



E o

150
E


S75
a,



- o
150


Voltage I V


Figure 2-15. Current density-voltage and NIR irradiance-voltage plots of Yb(TMPP)TP
as a function of Alq3 loading: (*) 0 wt %, (U) 33 wt%, and (A) 66 wt %.


W
*







64




3.0e-3
A Omg Aq3
2 5L-3 a 2rmgAlq3
U A
A
S20e-3





ILl
Z 5.0e-4
o
**. ,i *

0 20 40 60 80 100
Current Density / mAcm2


Figure 2-16. NIR external quantum efficiency of Yb(TMPP)TP as a function of Alq3
loading: (*) 0 wt %, (0) 33 wt%, and (A) 66 wt %.

Electrochemistry

Electrochemistry provides valuable insight into the electronic properties of

molecules. This technique provides information on the position of the energy levels, in

particular the highest occupied molecular orbital (HOMO) and the lowest unoccupied

molecular orbital (LUMO) are easily discernable from these measurements. The position

of the HOMO of a molecule is probed by determining its anodic potential, while the

position of the LUMO is determined by its cathodic potential. These positions can be

referenced with respect to the vacuum level by adding 4.7 eV to the onset of the peak

(oxidation / reduction) with respect to the ferrocene / ferrocenium (Fc / Fc ) redox

couple.[186] Table 2-3 shows the electrochemical window of the solvents used in these

experiments. The electrochemical measurements were undertaken by Avni Argun. The

measurements of the oxidation potential(s) of all samples were carried out using a Pt

working electrode in CH2C2. The measurements of the reduction potential(s) were

carried out using either a Pt or glassy carbon working electrode in tetrahydrofuran (THF).









All measurements were carried out using 1 mM analyte in 0.1 M tetrabutylammonium

hexafluorophospate (TBAPF6) with a platinum flag secondary electrode and a calibrated

silver wire reference pseudo-electrode.

Table 2-3. Electrochemical windows of solvents.
Solvent Anodic Cathodic
Limit* Limit*
THF 1.8 V -3.5 V
CH2C12 1.8V -1.9V
MeCN 1.8 V -2.0 V
* potentials vs. SCE


HOMO-LUMO

4-------AReidL.OXl ------.



-A-
K-- 1ASRed-Red1[
A x-0DxO I




2.00 1.20 0.40 -040 -120 .200

Potential(V vsSCE)

Figure 2-17. Redox properties of free-base tetraphenylporphyrin with respect to the
saturated calomel electrode.[187]

The electrochemical properties of tetraphenylporphyrin were determined in order to

verify the validity of the experiment, given the fact that the electrochemical properties of

TPP are well known. The first oxidation with respect to Fc/Fc+ was determined to be

0.54 V and was determined to be reversible. The first reduction peak was determined to

be located at -1.75 V and was also reversible. These observations correspond well with











previously published results.[187] The electrochemical "band-gap" (HOMO LUMO


gap), which was determined by the difference of the E1/2 of the anodic and cathodic

waves, was determined to be 2.2 eV which corresponds well with the optical gap

measured by absorption.


0.6
12
10 -
R 0.4 -

06 0 75V
,E oE E112=0 54V
04 0.2
02 -
00 02
0.0 E/2=0 89V
-0 2
-0 4 -E = -2 08V
-0 6 -0.2
-23 -22 -21 -20 -1 9 -1 8 -1 7 -1 6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
E (V) vs. Fc/Fc+ E (V) vs. Fc/Fc



Figure 2-18. Reduction and oxidation waves for free-base tetraphenylporphyrin.

In order to understand the effect of the lanthanide metal on the redox properties of

the porphyrin system, the electrochemical properties of Yb(TPP)TP were studied. The

first oxidation was determined to be located at 0.58 V, which is nearly identical to the

non-metallated porphyrin. The first reduction potential was determined to be located at -

1.67 V and again corresponds well with the free-base porphyrin. This suggests that the

lanthanide metal has little effect on the positions of the HOMO and LUMO levels.

Although the first oxidation was similar to TPP, the reduction wave for Yb(TPP)TP was

significantly different with respect to TPP. There exists an irreversible reduction at -2.2


V which has been attributed to Yb3++ e- Yb2+


The redox properties of Yb(TPP)Q are somewhat similar to those of Yb(TPP)TP

with the first oxidation located at 0.58 V, while the reduction has shifted to -1.98 V.








67



There also exists an irreversible oxidation at 0.35 V which is possibly due to the


oxidation of the quinolate axial ligand. The reduction of the quinolate was determined to


be at 2.9 V. This evidence suggests that the quinolate ligand has only a minor effect on


the electronic properties of the complex.


/ \i -1 67 V
167V
r

-199V



SJ -24V

-2 63 V
28-26 -2 4 -22 -20 -1 81 -16 -14 -12 0 -0
E (V) vs. Fc/Fc+


08

06

1 04 Eoxd 1 = 0 58V
Eoxd 2= 1 01V
02

00

-02 -

8 -02 00 02 04 06 08 10 12 14
E (V) vs. Fc/Fc+


Figure 2-19. Reduction and oxidation waves for Yb(TPP)TP.


1 98 V


-28 -26 -24 -22 -20 -18 -16
E (V) vs. Fc/Fc+


058V.


'0 82 V


-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E (V) vs. Fc/Fc+


Figure 2-20. Reduction and oxidation potentials for Yb(TPP)Q.


The electrochemical properties of Yb(TMPP)TP were then studied. It was


expected that the electron donating alkoxy groups would make the complex easier to


oxidize, but that was not the case. The first oxidation potential was determined to be


located at 0.6 V. The first reduction potential was determined to be located at -1.97 V.


005

000

-005

-0 10

-0 15

-020

-025

-030

-035
-30


00


-01


-02


-03


-04
30







68



This suggests that substitution on the meso phenyl groups has little effect in influencing


the redox properties of the complex.


1 97 V


-2 56 V


-30 -28 -26 -24 -22 -20 -18 -1 6 -14 -12 -1 0
E (V) vs Fc/Fc+


1 12V

095V /


0 60 V


-04 -02 00 02 04 06 08 10 12 14
E (V) vs. Fc/Fc+


Figure 2-21. Reduction and oxidation waves for Yb(TMPP)TP.


E =-2 46 V


E =-1 80 V


E (V) vs Fc/Fc+


E (V) vs Fc/Fc+


Figure 2-22. Reduction and oxidation waves for Yb(TPP)L(OEt)3.


Next, the redox properties of Yb(TPyP)L(OEt)3 were determined. In order to


understand the effect of the axial Klaui ligand, the redox properties of Yb(TPP)L(OEt)3


were first determined. It was shown that the first oxidation potential was dramatically


changed from -0.6 V for the TP complex to -0.1 V for the Klaui complex. This lower


oxidation is most likely the oxidation of the Klaui ligand.[188] The second oxidation of


the Klaui complex was similar to the first oxidation states of the other complexes,


providing evidence to that suggestion. After showing that the Klaui ligand has little







69


effect on the overall properties, the redox properties due to the pyridine substitution could

be determined. The complex showed no reversible oxidation, probably due to the

creation of a reactive cation radical. This non-reversible oxidation is most likely

contributing to the poor stability of electroluminescent devices fabricated from the

complex. The first reduction was similar to that of all of the other complexes, again

showing that substitution at the phenyl group has little effect upon the overall electronic

properties of the system.


01 4
00 3
-01 2
2 09 V
E -02 1
< -2 57 V
-03
-04 -
-1
-05
-2
-06 '
-28 -26 -24 -22 -20 -18 -16 -05 00 05 10 15 20 25
E (V) vs Fc/Fc+ E (V) vs. Fc/Fc+



Figure 2-23. Reduction and oxidation waves for Yb(TPyP)L(OEt)3.

Conclusions

From these results, there come two major problems which much be addressed in

order to produce more efficient near-infrared devices. The first is that non-radiative

decay is a controlling factor in the low near-infrared photoluminescence quantum yield of

lanthanide porphyrin complexes. This process is facilitated by the large number of C-H

oscillators which are in close proximity to the Yb3 ion. In order to improve NIR PL

quantum yield, and in turn increase the theoretical NIR EL quantum yield, substitution of

these protons with heavier atoms halogenss) must occur.









The second major problem is the lack of carrier balance in Yb porphyrin LEDs.

Substitution on the meso-phenyl groups of TPP has little effect on the charge transport

properties of these complexes. Therefore these novel complexes still facilitate hole

transport and hinder electron transport. In order to correct this problem, the electron

transporting ability must be increased. This can be accomplished by either lowering the

LUMO by substitution of electron withdrawing moieties on the pyrroles of the porphyrin

or by creating a "molecular wire" in which electrons can flow freely, reducing the barrier

for electron injection.

Table 2-4. Electrochemical properties of substituted porphyrins.
Compound Oxidation Reduction (Redl -Oxl) Optical HOMO-
Potential Potential (Eg) LUMO gap
(Ox1) (Redl) (from the lowest
V* V* energy Q band)
TPP 0.54 -1.75 2.2 eV 664 nm (1.87 eV)
Free base
Yb (TPP) 0.09 -1.8 1.9 eV 617 nm (2.00 eV)
(L(OEt))
Yb (TPP) 0.58 -1.67 2.2 eV 603 nm (2.06 eV)
TP
Yb(TPP)Q 0.58 -1.98 -2.6 607 nm (2.04 eV)

Yb (TMPP) 0.6 -1.97 2.6 eV 609 nm (2.04 eV)
TP
Yb (TPyP) -2.09 616 nm (2.01 eV)
(L(OEt))
Yb 0.59 -1.64 2.2 eV 604 nm (2.05 eV)
(TPP OEH)
TP
* Potential vs. Fc/Fc
Experimental

Photophysical Measurements

All photophysical studies were conducted in 1 cm square quartz cuvettes unless

otherwise noted. All absorption and emission measurements were made in CH2C12 unless

otherwise noted. Absorption spectra were obtained on a double-beam Cary-100 UV-









visible spectrophotometer. Fluorescence spectra were measured on a SPEX Fluorolog-2

equipped with a water-cooled Hamamatsu R928 PMT for visible measurements and a

liquid nitrogen cooled InGaAs diode detector for near-infrared measurements. All

measurements were corrected for detector response.

Photoluminescence quantum yields in solution (OPL) were calculated using

equation 2.5, where the absorption of the reference is given by AR, and the absorption of

the sample is given by As. The refractive indices of the solvents are given by the term n.

The integrated area of the emission peaks of the sample and standard are given by, Fs and

FR respectively. OR is the quantum yield for the standard. Yb(TPP)TP in CH2C12 ( =

0.033) was used as the relative quantum yield standard.

10 AR n2 F
1s =- I 2 F R (2.5)
Rs 10- R

Device Fabrication

ITO Etching

Electroluminescent devices were prepared by masking, then etching the ITO

coated glass (Delta Technologies, Rs = 8 12 Q / o) by exposure to aqua-regia vapor.

The ITO-glass sheets were cut into 1" x 1" squares by the use of a glass cutter. Packing

tape was then carefully placed over the sheet of glass, ensuring that no bubbles were

present. With the use of a black marker, a rectangle was drawn onto the tape to indicate

where the tape would be removed. A razor knife was then used to remove the indicated

areas. The ITO glass squares were then placed upon a beaker containing a freshly

prepared solution of aqua-regia, where they remained for 6 minutes. After removal from

the beaker, the exposed areas where wiped with a cotton swab removing the dissolved









ITO. The tape was then removed and the ITO was rinsed with isopropanol to ensure

removal of any remaining acid.

ITO Substrate







Aqua Regia




Figure 2-24. Cartoon showing ITO substrate placed at top of beaker containing solution
of aqua regia allowing vapors to etch surface.

Cleaning ITO

The etched ITO squares were then placed into a Teflon holder. They were

subsequently sonicated for 10 minutes in each of the following solutions: aqueous

sodium dodecyl sulfate (SDS, Fisher), Milli-Q water, acetone (Fisher, ACS grade), and

isopropanol (Fisher, ACS grade). The ITO-glass squares were dried under stream of

filtered air. They were then placed into an oxygen plasma cleaner (Harrick PDC-32G)

for 15 minutes.

Spin Coating

PEDOT-PSS (Bayer, Baytron P VP Al 4083) was used as the hole transport layer.

The PEDOT-PSS suspension was first filtered through a 0.2 micron Polysulfone filter to

ensure removal of particulate matter. The PEDOT-PSS was then spin-coated (Chemat,

KW-4A) onto the ITO surface at 4000 rpm for 30 seconds. The PEDOT-PSS coated

ITO-glass was them dried in a vacuum over at 150C for 4 hours in order to remove

residual water.









Solutions of the desired wt% of porphyrin complex were created by dissolving the

corresponding mass of Yb porphyrin complex into a 1 mL CHC13 solution containing

3mg of polystyrene (PS) (Aldrich, Mn 280,0000). These solutions were then spin-coated

onto the substrate at 1000 rpm.

Metal Electrode Deposition

The films were placed glass side down onto an inverted stage designed to fit into

the chamber of the thermal evaporator (Denton Vacuum, DV502A). The ITO was then

covered with a stainless steel mask with the pattern of the electrodes desired. The

masked devices are then placed into the thermal evaporator and pumped down to 10-6 torr

for 12 hours. Calcium (or LiF) and aluminum layers were sequentially deposited by

thermal evaporation at 2 x 10-7 torr without breaking the vacuum. The thicknesses were

adjusted by the use of a calibrated oscillating quartz crystal thickness monitor. The

thicknesses used for all devices were: 50 A for Ca, 5 A for LiF and 2000 A for Al. The

devices were left in the evaporator to cool for 30 minutes after deposition. The chamber

was then purged with nitrogen and the devices removed and encapsulated with epoxy

(Loctite quick set epoxy) in order to minimize exposure to oxygen and moisture.

Electroluminescent Device Measurements

Visible and near-infrared (< 1000 nm) electroluminescence spectra were recorded

on a ISA-SPEX Triax 180 spectrograph fitted with a liquid nitrogen cooled CCD detector

(EEV CCD, 1024 x 128 pixels, 400 1100 nm). The devices were placed into the device

holder such that electrical contact was made between the vapor-deposited electrodes of

the device and the gold pins of the device holder. The device holder was then mounted

onto an x-y stage as close as possible to the entrance slit of the Triax 180 (4.8 cm). Using

the x-y stage, the electrode to be measured is placed in the center of the monochromator









opening. Power for electroluminescent measurements was supplied using a Keithley 228

voltage / current source. Positive bias was applied to one of the corners of the device

holder, while negative bias was applied to the electrode under investigation. The CCD

was calibrated into energy units by the use of a primary standard quartz tungsten halogen

lamp.

Electrochemistry

All electrochemistry was performed using an EG&G PAR model 273A

potentiostat/galvanostat in a three-electrode cell configuration containing a platinum

button or a glassy carbon button as the working electrode, a platinum flag as the counter

electrode, and a silver wire as the pseudo-reference electrode calibrated with a Fc/Fc

redox couple. Lanthanide containing porphyrin complexes (1 mM) and 0.1M

tetrabutylammonium hexafluorophospate (TBAPF6, a non-coordinating supporting

electrolyte) were dissolved in non aqueous solvents such as dichloro methane (for

oxidation) and THF (for reduction). CV studies were carried out under argon blanket at a

scan rate of 50 mV/s, to determine the redox couples of the compounds. HOMO-LUMO

gaps were obtained from the E1/2 difference of the first oxidation and the first reduction

couples.














CHAPTER 3
PORPHYRIN PENDANT POLYACETYLENES

Introduction

The Nobel Prize winning discovery of doped polyacetylene resulted in the field of

conjugated polymers. Polyacetylene is the archetypical conjugated polymer, with

alternating single and double bonds, which in its doped form shows metallic behavior.1189]

The polymer, however, is insoluble, infusible, and unstable in air. This severely limits its

use in optical materials. Substituted polyacetylenes are quite different from unsubstituted

polyacetylenes, especially when they contain bulky substituents.[190] Backbone

substitution results in an increase in photoluminescence yield and tunable emission

properties dependant on the number and nature of substituents.[191-210] These polymers

have been studied for possible use in light emitting diodes, photovoltaic materials,

nonlinear optical materials, gas permeable membranes, sensors, and magnetic

materials.179, 197, 211-241]

The previous chapter focused on the emissive properties of lanthanide porphyrin

systems blended into polystyrene and their electroluminescent properties. Previous work

by Harrison and coworkers demonstrated the use of conjugated polymers as hosts for

blended materials for light emitting diodes.[169] This chapter discusses the combination of

the two principles. The idea of the research was that the porphyrin could be appended to

a conjugated polymer, resulting in more efficient carrier transport as well as enhanced

emission through Forster energy transfer.









Monomer Synthesis and Polymerization

The use of porphyrin pendant polyacetylenes was first shown in a paper by

Aramata, Kajiwara, and Kamachi in 1995.1242] Their goal was to develop a polymer

which had a magnetically active side chain. Their belief was that the 7n electron of the

unsaturated main chain would enhance the magnetic properties of the metalloporphyrin

pendant. They therefore synthesized a porphyrin monomer bearing a terminal alkyne

which could be easily polymerized using commercially available catalysts. Although

successful in the monomer preparation, they discovered that purification was extremely

difficult and the resulting yield was extremely low (6 %). This difficulty in purification

was due to the relative inability to separate the statistical mix of porphyrin products. The

major problem was the choice of protecting groups. The use of the trimethylsilyl (TMS)

group resulted in little polarity change from the base phenyl, and therefore made column

chromatography difficult. Therefore a different procedure was needed in which

purification could be achieved more easily. Lindsey et al. discovered that the use of

polar protecting groups eased the purification process.[243] This was the procedure carried

out to synthesize the porphyrin pendent monomer used throughout this study, and the

scheme is shown in Figure 3-1. The first step of the reaction was the Sonogoshira

coupling of the protected acetylene to 4-bromobenzaldehyde to produce 4-(3-methyl-3-

hydroxybut-l-yn-1-yl)benzaldehyde (1). The second step was the condensation of three

equivalents of benzaldehyde with one equivalent of compound (1) with excess pyrrole in

a dilute solution in CHC13 with the lewis acid, BF3-OEt2, acting as a catalyst. This was

immediately followed by oxidation of the resulting porphyrinogen to the desired

porphyrins by the addition of DDQ. The resulting product was a statistical mixture of

tetraphenylporphyrin, and mono-, di-, tri-, and tetra-substituted porphyrins. Purification










was achieved by column chromatography. The desired product, 5-[4-(3-methyl-3-

hydroxy-1-butyn-1-yl)phenyl]-10,15,20-triphenylporphyrin (2), was then deprotected

using sodium hydroxide, resulting in the product, 5-(4-ethynylphenyl)- 10,15,20-

triphenylporphyrin (3). The porphyrin was then metallated using zinc (II) acetate to give

the final product, Zn(II)-5-(4-ethynylphenyl)-10,15,20-triphenylporphyrin (4).

Br
OH Pd(PPh3)4 /TEA Cul
40 C 4hrs OH


87 % (1)
0o





H


Ph Ph


NH N 1) NaOH Toluene N N
2) Zn(OAc)2
Ph = OH "Ph ZnP
IN HN N INN NNI


Ph Ph
16% (2) 81s (4)






Figure 3-1. Synthesis of Zn(II)-5-(4-Ethynylphenyl)-10,15,20-triphenylporphyrin (4).

Oxadiazoles are well know for their electron transporting and hole blocking ability

due to a high electron affinity.1228-230' 244] In an effort to increase the electron transporting

ability of the acetylene polymers, an oxadiazole containing molecule possessing a

terminal acetylene was synthesized. This was accomplished by a procedure suggested by

Cha and coworkers.12451 The first step involved the synthesis of 4-bromobenzhydrazide










(5) by the reaction of 4-bromo-ethylbenzoate with hydrazine. The next step was the

reaction of compound (5) with 4-methylbenzoyl chloride with first pyridine followed by

the reaction with POC13 to give the product 2-(4-bromophenyl)-5-(4-methyl)-1,3,4-

oxadiazole (6) in decent yield. The next step was the Sonogoshira coupling of (6) with 2-

methyl-3-butyn-2-ol to give the product 4-2-(4-(3-methyl-3-hydroxy-l-butyn-1-

yl)phenyl)-5-(4-methyl)-1,3,4-oxadiazole (7). This was followed by the subsequent

deprotection using NaH to give the final product 2-(4-methylphenyl)-5-(4-

ethynylphenyl)-1,3,4-oxadiazole (8).





Br N2H4 / EtOH BNHNH2
Br
OEt ^_/
90 %
O OH 0 CI
1) Pyridine
S C202C12 + Br NHNH2 2)POC13

(5)


OH

Br O/ \ Pd(PPh3)2C12 / DMF / TEA
N-N Cu/ PPh3
(6) 50 C / 4 hrs


HO NaH Toluene
N-N
(7)

0

N-N
63 % (8)

Figure 3-2. Synthesis of 2-(4-methylphenyl)-5-(4-ethnylphenyl)-1,3,4-oxadiazole (8).

The use of Rhodium catalysts for the polymerization of acetylenes has been known

for many years.1194' 246-251] The most commonly used Rh catalyst used for these









polymerizations is the rhodium norbomadiene chloride dimer shown in Figure 3-3. This

family of catalysts involves an insertion type mechanism as opposed to the metathesis

mechanism known for the transition metal chloride polymerization catalysts. This family

of catalysts also results in the cis-transoid structure polymer as apposed to the trans-

cisoid structure typically obtained by metathesis catalysts. The mechanism for the

insertion type polymerization is shown in Figure 3-4. The dimeric rhodium catalyst

dissociates into its monomeric form by association with the polymerization solvent. The

acetylene monomer then coordinates with the rhodium metal center. The resulting

vinylic complex is converted to the terminally bound acetylene by the interaction with the

base to remove a proton. The polymeric chain begins and grows by insertion of the 7t-

coordinated acetylene monomer with the metal-carbon a bond on the metal acetylide

catalyst. Termination is facilitated via monomer chain transfer. This transfer may occur

when the acidic acetylenic hydrogen is transferred from the 7t-coordinated monomer to

the propagating chain.

CI


Rh Rh'


Cl

Figure 3-3. Structure of (Bicyclo[2.2.1 ]hepta-2,5-diene)chlororhodium(I) dimer.

For this study, the catalyst used was the rhodium norbornadiene chloride dimer.

The reactions were all carried out in an argon atmosphere drybox. The monomer

concentration was held at a constant ratio with respect to the catalyst (100:1). All

reactions were performed with CHC13 as the solvent unless otherwise noted. The

polymerization of compound (4) resulted in the homopolymer, poly(ZnETPP), shown in









Figure 3-5. A series of copolymers were also created using varying monomers as well as

monomer concentrations. The copolymers created are also shown in Figure 3-6. These

polymers are named as poly(ZnETPP)x-co-(R)y where x + y = 1. Thus, the polymers

have the name poly(ZnETPP)x-co-(PE)y when the monomer is ethynyl benzene,

poly(ZnETPP)x-co-(PEOXAD)y when the monomer is compound (8), and

poly(ZnETPP)x-co-(3,5CF3PE)y when the monomer is 1-ethynyl-3,5-

trifluoromethylbenzene. The details of the polymerizations are shown in Table 3-1. The

isolated yield was fairly high for all of the polymers with the exception of

poly(ZnETPP)o.5-co-(PEOXAD)o.5 due to the fact that the polymer was extremely soluble

in most common organic solvents, resulting in the lack of ability to precipitate the

polymer. The molecular weights were determined by gel permeation chromatography

carried out in THF with reference to polystyrene standards. The eluents were detected by

a photodiode selectively tuned to the absorption of the porphyrin Soret (420 nm). The

elution diagram of GPC for all of the polymers showed a unimodal pattern. The

poly(ZnETPP) homopolymer was insoluble in THF and therefore the molecular weight

was not determined. Determination of the absolute number average molecular weight

can be estimated by the formula given by[252]

Mn =1.48Mn(GPC) (3.1)

The IR band present in all of the monomers at -2110 cm-1 assigned to the stretching

vibration mode of the carbon-carbon triple bond disappeared upon polymerization. The

infrared absorption bands characteristic of the porphyrin ring remained unchanged after

polymerization. This confirms to the assumption that the polymerization occurs through

the triple bond of the ethynyl group.











^w^^-c'Nw


t Cu
'I

H -~-R


R R
H R


aR R H R H R
H RH R
R R


Figure 3-4. Representative scheme of insertion polymerization mechanism.[253]




(I) CT (cis-transoid)





(II) TC (trans-cisoid)





(IU) TT (trans- transoid)


Figure 3-5. Polyacetylene isomer structures.[250]


- a


~CI



















Ph- Zn 'PhPh Zn 'Ph
N N- N N-


Ph Ph
poly(ZnETPP) poly(ZnETPP)-co-(R)


R=
F3C CF3 N-N

Figure 3-6. Structure of ZnETPP homopolymer and copolymers.

The copolymer ratio was determined by H NMR. All NMRs were carried out in

d6-DMSO at 90 C due to the poor solubility of the polymer. The NMR spectra showed

broad peaks for the protons associated with the polymer (see Figure 3-7). The copolymer

ratio was determined by the ratio of the integration of the protons on the pyrrole of the

porphyrin with respect to the protons on the phenyl ring or methyl group of the other co-

monomer. The positions and widths of the peaks was determined by analyzing the NMR

of the homopolymers. The results of these calculations are shown in Table 3-1. The

copolymers ratios with (3,5CF3PE) were unable to be determined by NMR due to the

overlap in the spectrum. The copolymer ratios determined by NMR closely (within the

ability to integrate peaks in NMR) match the ratios determined by monomer feed.

No glass transition was observed with DSC within the temperature range studied (-

80 180o C). As shown in Figure 3-8, the polymers display good thermal stability, with

a 10% weight loss occurring above 300oC with the only exception being the









poly(ZnETPP) homopolymer. This stability has been attributed to the bulky pendant

groups, which protect the polymer backbone.

Table 3-1. Polymerization details.
Polymer Copolymer Mw Mn Mw/Mn Isolated
Ratio Yield %
(NMR)
Poly(ZnETPP) 65

Poly(ZnETPP)o.15-co- 0.15/0.85 77500 33700 2.3 94
(PE)o.85
Poly(ZnETPP)o.3-co-(PE)o.7 0.25/0.75 74000 44000 1.7 88

Poly(ZnETPP)o.5-co-(PE)o.5 0.45/0.55 174000 78000 2.2 77

Poly(ZnETPP)o.7-co-(PE)o.3 0.65/0.35 160000 73000 2.2 82


Poly(ZnETPP)o.5-co- 0.5/0.5 97000 47000 2.1 13
(PEOXAD)o.5
Poly(ZnETPP)o.5-co- ** 271000 162000 1.7 67
(3,5CF3PE)o.5
*Poly(ZnETPP) insoluble in THF. ** Unable to determine copolymer ratio via NMR.


Figure 3-7. NMR spectra of poly(ZnETPP)o.5-co-(PE)o.5. Inset numbers indicate
integration values.














100 -

90

80

7 70

60- ETPP
-- PETPP15PE85
PETPP30PE70
50 --- PETPP50PE50
PETPP67PE33
40 PETPP
PPA
30

20 '
0 100 200 300 400 500 600 700

Temperature / OC


Figure 3-8. TGA traces of polymers of ETPP measured in air.

Photophysics

Absorption

All absorption measurements were made as dilute solutions in toluene unless

otherwise indicated. The monomer ZnETPP shows absorption nearly identical to ZnTPP

indicating the ethynyl group has little influence on the absorption of the porphyrin ring.

Figure 3.9a shows the absorption spectrum of poly(ZnETPP). Close examination of the

absorption spectrum shows that the molar extinction coefficient of the Soret band at 408

nm is roughly an order of magnitude weaker than ZnTPP. The Soret also shows evidence

of broadening with the emergence of a second peak at 428 nm. These results indicate

that hypochromism was due to electronic interactions occurring among porphyrin

moieties in the polymer. This phenomenon has been observed for other porphyrin

polymer systems and has been explained in terms of an exciton coupling model due to the









approach of two porphyrin rings.[30, 37, 254] Although absorption bands due to the

conjugated main chains of poly(ZnETPP) are expected to be observed in the range of 250

- 400 nm, the bands could not be distinguished from the absorption due to the porphyrin

ring.[255,256] This indicates that the conjugation in the main chain is small as compared to

that of unsubstituted polyacetylenes.12561 The most probable reason for this observation is

due to the bulkiness of the porphyrin ring.

Examples of the absorption of poly(ZnETPP)-co-(PE) copolymers are also shown

in Figure 3.9b. These absorption spectra are nearly identical to the ZnETPP monomer

and ZnTPP, suggesting that the incorporation of phenyl pendants limits the interaction of

porphyrin rings reducing the excitonic coupling. The change in the copolymer ratio has

little effect on the absorption properties, with nearly identical molar absorption

coefficients after adjustment for chromophore concentration. The peak positions were

also nearly identical. The absorption of the acetylenic backbone is again hidden by the

absorption of the porphyrin 7t-system.

The absorption of the poly(ZnETPP)-co-(PEOXAD) copolymer is shown in Figure

3-9c. The spectrum shows the characteristic absorption of ZnTPP with a small decrease

in the molar absorption coefficient of the Soret absorption transition when compared to

the corresponding poly(ZnETPP)o.5-co-(PE)o.5 copolymer. The Soret shows a 5 nm shift

to lower energy, indicating electronic interaction between the co-monomers which results

in a stabilization of the excited state. The Q-bands also show a similar red-shift. The

strong band at 300 nm is attributed to the absorption of the oxadiazole moiety, shown in

Figure 3-10.









The absorption spectrum of the poly(ZnETPP)-co-(3,5CF3PE) is shown in figure

3.9d. The Soret shows a slight increase in intensity, when compared to the

poly(ZnETPP)o.5-co-(PE)o.5 copolymer suggesting that the chromophore concentration

may be higher than expected. The position of the Soret shows a slight shift to lower

energy. The Q-bands also show this bathochromic shift, suggesting electronic

interactions with the other co-monomer resulting in stabilization of the excited state.

The absorption of the polymers studied show that there is little effect from the main

chain and that polymerization has little effect on the overall absorption of the system,

with the exception of the homopolymer poly(ZnETPP). This polymer has strong

evidence of excitonic coupling and that incorporation of any co-monomer into the

polymer chain lessens this interaction.

Table 3.2. Photophysical data.
Polymer Quantum Absorption, nm (E, Emission Emission
Yield* M-^cm-1) (solution), (Film),
nm nm
Poly(ZnETPP) 0.010 408(54840), 609, 655 621,663
428(37058),
548(4840), 588 (888)
Poly(ZnETPP)o.15-co- 0.029 420 (56250), 548 601,650 613,658
(PE)o.s5 (3037), 588(608)
Poly(ZnETPP)o.5-co-(PE)o.5 0.032 420 (176250), 548 597, 646 617, 662
(9515), 586 (1900)
Poly(ZnETPP)o.5-co- 0.031 306(49280), 425 611,657 **
(PEOXAD)o.5 (150850), 554
(13100), 599 (6170)
Poly(ZnETPP)o.5-co- 0.034 422 (222710), 613,659
(3,5CF3PE)o.5 551(19675), 592
(5190)
* Measured in reference to ZnTPP (0.033), ** Poly(ZnETPP)0.5-co-(PEOXAD)0.5 non
emissive in solid state.