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Ultrafast Studies of a Photochromic Oxazine in Solution

Permanent Link: http://ufdc.ufl.edu/UFE0024994/00001

Material Information

Title: Ultrafast Studies of a Photochromic Oxazine in Solution
Physical Description: 1 online resource (132 p.)
Language: english
Creator: Altan, Aysun
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: ansiotropy, dendrimers, oxazine, photochromism, ultrafast
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: This dissertation is composed of two studies. Ultrafast studies of a photochromic oxazine are the main project of the dissertation. An independent study that describes the anisotropic behavior of the phenylene ethynylene dendrimers is also presented. Photochromic molecules alter their structure and electronic properties upon excitation by optical light sources. The photogenerated form of the photochromic molecule can return to its original form either thermally or photodynamically. If the reverse reaction is thermal, the photochromic cycles of the molecules can be achieved simply turning on and off the light source. Photochromic molecules with the ability to restore the output level in each cycle are good candidates for molecular switches. The rate of switching cycles is an important property of the photochromic molecules that needs to be known for the implementation of the molecular switches. In this study, we worked with a novel photochromic oxazine which exhibits photochromism by a ring opening reaction. The C-O bond cleavage in the ring opening reaction occurs very fast and has to be investigated by ultrafast spectroscopy. The ultrafast measurements are complemented with preliminary steady state experiments. To explore the electronic properties of the states involved in the ring opening mechanism, the experiments are performed in the presence and the absence of benzophenone, a good triplet sensitizer, and also they are measured in solutions of different solvents (acetonitrile and hexane). Two distinctive transient absorption bands are observed in the experiments. One of the bands, which is formed at around 505 nm, raises within 250 femtosecond (fs) and decays with 2.2 ps and 3.7 ps time constants. This band is assigned to a charge separated state. The second band, which is absorbing at around 440 nm, is attributed to the open form of the photochromic molecule. The open form of the molecule is produced indirectly from the charge separated state with 12.5 ps time constant and directly from first excited with 6.3 ps time constant and remains unchanged at least 650 ps. Although, the band belonging to a charge separated state appears and disappears within the rise time of the ring opened form, it was proved that this state does not make a significant contribution to the production of the open form of the photochromic oxazine. The minor study that completes the dissertation is on the anisotropic behaviors of unsymmetrical phenylene ethyneylene dendrimers. The unsymmetrical Phenylne Ethynylene denderimers we studied exhibited low excitation anisotropy values along the excitation spectra at room temperature and at 77K confirming energy transfer from the longer conjugated segments to shorter ones with different transition moments. The excitation anisotropy of these molecules at77 K exhibited a complex behavior suggesting the presence of more than one electronic state contributing to the anisotropic behavior.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Aysun Altan.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Kleiman, Valeria D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024994:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024994/00001

Material Information

Title: Ultrafast Studies of a Photochromic Oxazine in Solution
Physical Description: 1 online resource (132 p.)
Language: english
Creator: Altan, Aysun
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: ansiotropy, dendrimers, oxazine, photochromism, ultrafast
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: This dissertation is composed of two studies. Ultrafast studies of a photochromic oxazine are the main project of the dissertation. An independent study that describes the anisotropic behavior of the phenylene ethynylene dendrimers is also presented. Photochromic molecules alter their structure and electronic properties upon excitation by optical light sources. The photogenerated form of the photochromic molecule can return to its original form either thermally or photodynamically. If the reverse reaction is thermal, the photochromic cycles of the molecules can be achieved simply turning on and off the light source. Photochromic molecules with the ability to restore the output level in each cycle are good candidates for molecular switches. The rate of switching cycles is an important property of the photochromic molecules that needs to be known for the implementation of the molecular switches. In this study, we worked with a novel photochromic oxazine which exhibits photochromism by a ring opening reaction. The C-O bond cleavage in the ring opening reaction occurs very fast and has to be investigated by ultrafast spectroscopy. The ultrafast measurements are complemented with preliminary steady state experiments. To explore the electronic properties of the states involved in the ring opening mechanism, the experiments are performed in the presence and the absence of benzophenone, a good triplet sensitizer, and also they are measured in solutions of different solvents (acetonitrile and hexane). Two distinctive transient absorption bands are observed in the experiments. One of the bands, which is formed at around 505 nm, raises within 250 femtosecond (fs) and decays with 2.2 ps and 3.7 ps time constants. This band is assigned to a charge separated state. The second band, which is absorbing at around 440 nm, is attributed to the open form of the photochromic molecule. The open form of the molecule is produced indirectly from the charge separated state with 12.5 ps time constant and directly from first excited with 6.3 ps time constant and remains unchanged at least 650 ps. Although, the band belonging to a charge separated state appears and disappears within the rise time of the ring opened form, it was proved that this state does not make a significant contribution to the production of the open form of the photochromic oxazine. The minor study that completes the dissertation is on the anisotropic behaviors of unsymmetrical phenylene ethyneylene dendrimers. The unsymmetrical Phenylne Ethynylene denderimers we studied exhibited low excitation anisotropy values along the excitation spectra at room temperature and at 77K confirming energy transfer from the longer conjugated segments to shorter ones with different transition moments. The excitation anisotropy of these molecules at77 K exhibited a complex behavior suggesting the presence of more than one electronic state contributing to the anisotropic behavior.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Aysun Altan.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Kleiman, Valeria D.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024994:00001


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1 ULTRAFAST STUDIES OF A PHOTOCHROMIC OXAZINE IN SOLUTION By AYSUN ALTAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF P HILOSOPHY UNIVERSITY OF FLORIDA 2009

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2 2009 Aysun Altan

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3 To My Family

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4 ACKNOWLEDGMENTS Even though with this Ph D dissertation only one pers on can earn the title, it would not be possible without th e support of numerous people in my life. First of all, I would like to express my thanks to my a dvisor Professor Valeria D. Kleiman for her guidance, enthusiasm and encouragement during my Ph.D years. Along with the countless things related to the field that I have learnt from her, she is the person who taught looking for perfection, which will be useful in every stage of my life. I wish to thank my supervisory committ ee members, Professor Nicolo Omenetto for teaching me the basics of lasers, and Professors David Micha, Russ Bowers and David Hahn for accompanying me in the final stage of my graduate career. I thank Professor Franisco Raymo fr om University of Miami fo r providing the photochromic oxazines for this project. I also thank gra duate coordinator Dr. Ben Smith and graduate program assistant Lori Clark for their smiley f aces all the time and their help and support during my gradute career. I want to express my thankfulness to the former and present members of Kleiman group for providing me an invaluable working environment. I want to thank Dr. Evrim Ata for her friendship and help for understand ing the photophysical properties of the dendrimers. I thank Dr. Lindsey Hardison for me ntoring in basics of transient absorption set up and to Dr. Gustavo Moriena for all of his labview and matlab support. My special thanks goes to Dr. Daniel Kuroda for his help in all subjects related to laboratory work, his scientific and personal advices, support and encouragement, and being a good friend and office mate. I thank Dr. Chandra Pal Singh for sharing his optics and lasers knowledge, academic and life experiences. For last two years, I always enjoyed his friendship as well as discussing about my project with him. I want to present my special

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5 gratitude to Sevnur Kmrl not only for being a colleague with whom I shared the working times, coffee breaks and the lunches but also one of the very special people in my life to have a lifelong friendship with. I also thank Shiori Yamazaki for being a nice office mate, Derek LaMo ntagne and the newest group members Jared Tate and Allison LaFramboise for their patience in editing my written materials. I thank Ahu Demir, Fatma Bilge Tutak, Sevnur -Murat Keeli, zlem Demir, Tezcan Sinan Cem triplet for their invaluable friendship. They colored my life in Gainesville and I enjoyed their unforgettable friendship a lot in my last five years. alil unconditional love and endless support in every moment of my life. I also send my special thanks to my dear uncle Halilibrahim Altan for always believing in my abilitie s and my grandpa Ahmet Altan for motivating me to be a science literate woman since my early childhood.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................................... 4 LIST OF TABLES ............................................................................................................................... 8 LIST OF FIGURES .............................................................................................................................. 9 ABSTRACT ........................................................................................................................................ 14 CHAPTER 1 INTRODUCTION ...................................................................................................................... 16 Outline of th e Dissertation ......................................................................................................... 16 History of Photochromism ......................................................................................................... 17 Definition of Photochromism ..................................................................................................... 18 Photochemical and Photophysical Processes Related to Photochromism ............................... 21 Solvent Effects on Absorption and Emission Spectra .............................................................. 23 Energy transfer ............................................................................................................................ 24 Radiative Energy Transfer .................................................................................................. 25 Non -Radiative Energy Transf er .......................................................................................... 26 Selection Rules .................................................................................................................... 28 Singlet -singlet energy transfer .................................................................................... 28 Triplet triplet energy transfer ...................................................................................... 29 Triplet -singlet energ y transfer ..................................................................................... 29 Singlet -triplet energy transfer ...................................................................................... 29 Fluorescence Anisotropy ............................................................................................................ 30 2 EXPERIMENTAL METHODS ................................................................................................. 32 Steady State Measurements ........................................................................................................ 32 Steady State Anisotropy Experiments ................................................................................ 32 Time -Resolved Measurements ................................................................................................... 35 Pump -probe Spectroscopy .......................................................................................................... 35 Ultrafast Transient Absorptio n Spectroscopy .................................................................... 36 Femtosecond Laser Source ................................................................................................. 41 Experimental Set up ............................................................................................................ 43 Signal Detection System ..................................................................................................... 45 Time Resolution of the Experi ment ................................................................................... 47 Data Analysis Methods ............................................................................................................... 48 3 RING OPENING MECHANISM OF A NOVEL PHOTOCHROMIC OXAZINE ................................................................................................................................... 54 Materials and Experimental Methods ........................................................................................ 56

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7 Steady State Spectr oscopy.......................................................................................................... 57 Transient Absorption Spectroscopy ........................................................................................... 62 Sensitization Experiments ................................................................................................... 68 Decomposition of Transient Absorption Spectra .............................................................. 70 Data Analysis Results ................................................................................................................. 71 4 ANISOTROPY OF PHENYLENE ETHYNYLENE DENDRIMERS ................................... 87 2G1-m OH ................................................................................................................................... 93 2G2-m OH ................................................................................................................................... 97 2G2-m -per .................................................................................................................................. 100 Conclusion ................................................................................................................................. 103 5 CONCLUSION AND PERSPECTIVE ................................................................................... 105 APPENDIX A ................................................................................................................................... 109 APPENDIX B ................................................................................................................................... 115 APPENDIX C ................................................................................................................................... 121 LIST OF REFERENCES ................................................................................................................. 124 BIOGRAPHICAL SKETCH ........................................................................................................... 132

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8 LIST OF TABLES Table page 1 1 Main photochromic fam ilies ................................................................................................. 20 3 1 Absorption wavelengths ( ) and molar extinction coefficients ( ) and energy difference between S0 and S1 in acetonitrile. ........................................................................ 58 3.2 Singular values of the electronic states of benzophenone in pure benzophenone and benzophenone and oxazine mixture solution. ...................................... 81 3 3 Rate constants provided by the kinetic model. ..................................................................... 85

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9 LIST OF FIGURES Figure page 1 1 Typical curve analysis for a photochromic system. ............................................................. 18 1 2 Jablonski diagram .................................................................................................................. 22 1 3 Cou lombic and exchange mechanism in non -radiative energy trans fer. ............................ 26 1 4 Representation of the electronic energy difference between D* and A* the absorption bandwidth, and the vibronic bandwidt h ......................................................... 27 2 1 Schematic diagram for measurement of anisotropy experiments.62 ................................... 33 2 2 Schematic di agram for the measurement of IVV, IVH ,IHH, and IHV.62 .................................. 33 2 3 Basic principle of t ransient absorption experiment .63.......................................................... 36 2 4 Scheme of certain signals in transient absorption measurement. ........................................ 37 2 5 Chirp per mm with respect to the energy (wavelength) of the light for water. .................. 40 2 6 Schematic rep resentation of transient absorption set up. .................................................... 44 2 7 The image of probe and reference beams on CCD chip. ..................................................... 45 2 8 Coherent artifact of hexane excited at 320 nm, probed at 355 nm. .................................... 47 2 9 Results of for ward EFA, and backw ard EFA. ...................................................................... 50 2 10 Scheme of ALS algorithm.98 ................................................................................................. 52 3 1 Chemical structure of model indoline 4 -nitroanisole, and photochromic oxazine .................................................................................................................................... 57 3 2 Normalized absor ption spectra of oxazine, and 4 -nitroanisole ........................................... 57 3 3 Normali zed absorption andemission spaectra of oxazine ................................................. 59 3 4 Emission spectra of pure oxa zine, and oxazine benzophenone mixtures ....................... 60 3 5 Emission spectra of oxazine in hexane, and in acetonitrile ................................................ 61 3 6 Absorption s pec tra of oxazine in hexane, and in acetonitrile ............................................ 62 3 7 Stucture of the open form of the photochromic oxazine. .................................................... 63

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10 3 8 Transient absortion spectra of photochromic oxazine after excitation at =320 nm. with 2 ps time steps. ....................................................................................................... 64 3 9 Transient absortion spectra of photochromic oxazine after excitation at =320 nm. .......................................................................................................................................... 65 3 10 Chemical structure of neutral, and radical cation form of the model indoline. ................. 66 3 11 Transient absorption spectrum of oxazine r ecorded at 800 fs in acetonitrile after excitation at =320 nm. Steady state absorption of oxidized indoline in acetonitile. .............................................................................................................................. 67 3 12 Transient absorption spectrum of oxazine recorded at 800 ps in acetonitrile, and in hexan e. ......................................................................................................................... 67 3 13 Transient absorption spectra of pure benzophenone, and benzophenone oxazine mixtures with 1 ps time steps. ................................................................................. 69 3 14 Transient absorption spect ra of pure oxazine, and oxazine benzophenone mixtures ................................................................................................................................ 70 3 15 Transient absorption of photochromic oxazine at 436 nm. ................................................. 71 3 16 Singular values, result of SVD analysis. .............................................................................. 72 3 17 Spectral, and temporal components of oxazine in acetonitrile, result of SVD analysis. .................................................................................................................................. 72 3 18 Results of forward EFA, Results of backward EFA of oxazine in acetonitrile ................. 73 3 19 The result from EFA used as initial guess for temporal profiles of oxazine in acetonitrile. ............................................................................................................................. 74 3 20 Spectral and temporal components of oxazine in acetonitrile, result of MCR ALS analysis. ......................................................................................................................... 74 3 21 Experimental, reconstru cted transient absorption data, and difference between experimental and reconstructed transient absorption data of oxazine in acetonitrile. ............................................................................................................................. 75 3 22 S pectral, and temporal componets of pure oxa zine, oxazine benzophenone mixtures. ................................................................................................................................ 76 3 23 Experimental, reconstructed transient absorption data, and difference between experimental and reconstructed transient abso rption data of oxazine and benzophenone with the ratio of 2/100 in acetonitrile. ......................................................... 77

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11 3 24 Experimental, reconstructed transient absorption data, and difference between experimental and reconstructed tra nsient absorption data of oxazine and benzophenone with the ratio of 4/100 in acetonitrile. ......................................................... 78 3 25 Experim ental, reconstructed transient absorption data, and difference between experimental and reconstruc ted tra nsient absorption data of oxazine and benzophenone with the ratio of 6/100 in acetonitrile. ......................................................... 79 3 26 Spectral, and temporal components from transient absorption data of benzophenone in a cetonitrile. ................................................................................................ 80 3 27 Increase of the temporal component appeared upon the addition of oxazine into benzophenone. ............................................................................................................... 81 3 28 Compar ison of spectral and temporal component for the charge separated state ab sorption band in pure oxazine, and oxazine -benzophenone mixture ..................... 82 3 29 Energy level scheme of the photoisomerization of oxazine. ............................................... 84 3 30 Comparison between temporal components of the oxazine to and prediction of kinetic model for temporal components ........................................................................... 85 4 1 Model st ructures of monodendrons with symmetrical branches, and unsymmetrical branches ........................................................................................................ 88 4 2 Structures of PE dendrimers. ................................................................................................. 92 4 3 Excitation and em ission spectra of 2G1 -m OH at 298 K, and 77 K. ................................. 93 4 4 Excitation spectrum and excitation anisotropy of 2G1-m OH at 298 K. ........................... 94 4 5 Excitation spectrum and excitation anisotr opy of 2G1-m OH at 77 K. ............................. 95 4 6 Excitation and emission spectra of 2G2-m OH at 298 K, and 77 K. .................................. 97 4 7 Excitation spectrum and excitation anisotropy of 2G2-m OH at 298 K. ........................... 99 4 8 Excitation spectrum a nd excitation anisotropy of 2G2-m OH at 77 K ........................... 100 4 9 Excitation and em ission spectra of 2G2 -m OH at 298 K, and 77 K. ............................... 101 4 10 Excitation spectrum an d excitation anisotrop y of 2G2-m -per at 298 K. ......................... 102 4 11 Excitation spectrum and excitation anisotropy of 2G2-m -per at 77 K. ........................... 103 A 1 Front panel of th e chirp correction program. ..................................................................... 109 A 2 A part of the block diagram of the chirp correction program ........................................... 110

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12 A 3 Linear and quadratic fac tors for air, BK7, CaF2, fused silica, MeOH, and water ..................................................................................................................................... 111 A 4 Chirp per mm with respect to the energy (wavelength) of the light for air. ..................... 112 A 5 Chirp per mm with respect to the energy (wavelength) of the light for BK7. ................. 112 A 6 Chirp per mm with respect to the energy (wavelength) of the light for CaF2. ................ 113 A 7 Chirp per mm with respect to the energy (wavelength) of the light for fused silica. ..................................................................................................................................... 113 A 8 Chirp per mm with respect to the energy (w avelength) of the light for MeOH. .............. 114 B1 Spectral, and temporal components of oxazine and benzophenone with the ratio of benzophenone/oxazine= 2/100 in acetonitrile, result of SVD analysis. .............. 115 B2 Spectral, and temporal components of oxazine and benzophenone with the ratio of benzophenone/oxazine= 4/100in acetonitrile, result of SVD analysis. ............... 115 B3 Spectral, and tem poral components of oxazine and benzophenone with the ratio of benzophenone/oxazine= 6/100 in acetonitrile, result of SVD analysis. .............. 116 B4 Results of forward, and backward EFA of oxazine and benzophenone with the ratio of benzophenone/oxazine= 2/100in acetonitrile. ................................................ 116 B5 Results of forward and backward EFA of oxazine and benzophenone with the ratio of benzophenone/oxazine= 4/100 in acetonitrile. ............................................... 117 B6 Results of forward, and backward EFA of oxazine and benzophenone with the ratio of benzophenone/oxazine= 6/100 in acetonitrile. ............................................... 117 B7 Spect ral, and temporal components of benzophenone and oxazine with the ratio of oxazine/benzophenone= 2.6/1000, result of SVD analysis. ................................. 118 B8 Spectral, and tem poral components of benzophenone and oxazine with the ratio of oxazine/benzophenone= 4/1000, result of SVD analysis. .................................... 119 B9 Spectral, a nd temporal components of benzophenone and oxazine with the ratio of oxazine/benzophenone= 5.2/1000, result of SVD analysis. ................................. 120 C1 Energy level scheme of the photoisomerization of oxazine. ............................................. 121 C2 Comparison between temporal components of the oxazine and prediction of kinetic model for charge separated state, and the open form. ........................................... 121 C3 Energy level scheme of the photoisomerization of oxazine. ............................................. 122

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13 C4 Comparison between temporal components o f the oxazine and prediction of kinetic m odel for charge separated state, and the open form ........................................... 122 C5 Energy level scheme of the photoisomerization of oxazine. ............................................. 123 C6 Comparison between temporal components of the oxazine and prediction of kinetic model for charge separated state, and the open form ........................................... 123

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14 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 ULTRAFAST STUDIES OF A PHOTOCHROMIC OXAZINE IN SOLUTION By Aysun Altan December 2009 Chair: Valeria D. Kleiman Major: Chemistry This dissertation is composed of two st udies. Ultrafast studies of a photochromic oxazine are the main project of the dissertat ion. An independent study that describes the anisotropic behavior of the phenylene et hynylene dendrimers is also presented. Photochromic molecules alter their st ructure and electr onic properties upon excitation by optical light sources. The photogenerated form of the photochromic molecule can return to its original form either thermally or photodynamically. If the reverse reaction is thermal, the photochromic cycles of the molecules can be achieved simply turning on and off the light source. P hotochromic molecules with the ability to restore the output level in each cycle ar e good candidates for molecular switches. The rate of switching cycles is an important property of the photochromic molecules that needs to be known for the implementa tion of the molecular switches. In this study, we worked with a nove l photochromic oxazine which exhibits photochromism by a ring opening reaction. The C-O bond cleavage in the ring opening reaction occurs very fast and has to be investigated by ultrafast spectroscopy. The ultrafast measurements are complemented w ith preliminary steady state experiments.

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15 To explore the electronic properties of the states involved in the ring opening mechanism, the expe riments are performed in the presence and the absence of benzophenone, a good triplet sensitizer and also they are measured in solutions of different solvents ( acetonitrile and hexane ). Two distinctive transient absorption bands are observed in the experi ments. One of the bands, which is formed at around 505 nm, raises within 2 50 femtosecond (fs) and decays with 2.2 ps and 3.7 ps time constants. This band is assigned to a charge separated state The second band which is absorbing at around 440 nm is attr ibuted to the open form of the photochromic molecule. The open form of the molecule is produced indirectly from the charge separated state with 12.5 ps time constant and directly from first excited with 6.3 ps time cons tant and remains unchanged at least 6 50 ps. Although, the band belonging to a charge separated state appears and disappears within the rise time of the ring opened form it was proved that this state does not make a significant contributi on to the production of the open form of the photochrom ic oxazine. The minor study that completes the dissertation is on the anisotropic behaviors of unsymmetrical phenyle ne ethyneylene dendrimers The unsymmetrical Phenylne Ethynylene denderimers we studied exhibited low excitation anisotropy values along th e excitation spectra at r oom temperature and at 77K confirming energy transfer from the longer conjugated segments to shorter ones with different transition moments. The excitation anisotropy of these molecules at77 K exhibited a complex behavior suggesting the presence of more than one electronic state contributing to the anisotropic behavior.

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16 CHAPTER 1 INTRODUCTION Outline of the Dissertation The main scope of this work is to investigate the ring opening mechanism of a novel photochromic oxazine. We ha ve conducted experimental studies to understand the electronic properties of the states involved in this mechanism. For this purpose the nature of the energy transfer mechanism between photochromic oxazine and benzopheneone is examined. Chapter 1 is an introduction to photochromism and photochemical and photophysical processes that can be observed in a photochromic system. This chapter also includes a brief introduction to energy transfer mechanisms and the solvent -solute interactions. Additionally, the an isotropy phenomenon, that is used to characterize the absorbing and emitting state of the unsymmetrical phenylene ethynylene dendrimers is introduced. Chapter 2 focuses on the experimental methods which were used to study the dynamics of ring opening mec hanism of the photochromic oxazine. It also explains the anisotropy measurement performed to elucidate the presence of multiple electro nic states in unsymmetrical phenylene ethynylene (PE) dendrimers. In Chapter 3, steady state and transient absorption dat a for the photochromic oxazine, and the data analysis methods and the results are presented. This chapter is complemented with supplementary dat a analysis results in Appendix B The analysis results allowed us to build a kinetic model to explain the ring o pening dynamics of the photochromic molecule. Chapter 4 describes the steady state anisotropy of unsymmetrical phenylene ethynylene dendrimers at room and low temperature. The anisotropy experiments are supported with the steady state excitation and emiss ion experiments which are performed at 298 K and 77 K. Finally, Chapter 5 describes the conclusion and provides a perspective into future look.

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17 History of Photochromism Before 1900, few significant examples of photochromism were reported. The earliest cont ribution to the field was done by Fritzche in 1867.1 He observed that the orange color of the tetracene solution vanished in the daylight and it was regenerated in the dark. Later, ter Meer2 found that potassium salt of dinitroethane in the solid state changed color from yellow to red when it was moved from dark t o daylight. Following these examples Phipson3 reported that a painted gate post had a different color from day to night due to a zinc pigment in the paint. Marckwal d4 was the first scientist who recognized photochromism as a phenomenon and described it as a reversible color changing process in 1899. He called this light driven process phototropy. Although the term phototropy was used for a while, it was not a prope r name since it had already been used by the biological sciences to describe the movement of the plant toward the light.4 The real development in the area occurred after 1940. In the period from 1940 to 1960 many studies were done in the synthesis of new organic and i norganic molecules with photochromic properties.5 In this period Hirsberg and Fisher were the pioneers of the field.68 In 1950, Hirsberg introduced the term photochromism, which is derived from two Greek words photo (light) and chroma (color) to describe the phenomenon of changing color as a response to light. Then, in a few years, he proposed a chemical memory model based on a reversible change of photochromic coumpounds as a fir st study related t o the potential applications of photochromism.8 After 1960 the field has shown a steady growth. Photochromic glasses became available in that era. Since then the research studies rela ted to the photophysical and photochemical properties of photoch romic molecules became very pop ular.5

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18 Definition of Photochromism Photochromism is defined as the reversible transformation of a molecule induced by light in one or both di rections between two forms of a molecule each absorbs in different spec tral regions.9 B Ah or h 1 2 (1 1) Figure 1 1 describes a typical photochromic system. Before t1, the molecules in the system are in the form of A. At t1, the system is not perturbed yet by a light source. Then the system responds to the perturbation by changing from form A to form B of the photochromic molecules. During the time between t1 and t2 the concentration of molecules in B form increases. Then at t=t2 the perturbation source is removed and the molecules in B form go back to the original form either thermally or photodynamically. Figure 1 1. Typical curve analysis for a photochromic system. Adapted from reference.9 U pon the excitation of the system, p hotochromism can be observed in specific molecular systems with a mechanism of heterolytic and homolytic bond cleavage5, cis -trans isomerism5 10, tautomerism5, 11, hydrogen transfer5 12, and cyclization of conjugated chromophores5, 13. The kinetics of these mechanisms provides useful information for implementation of the applications of that system. Absorbance of the colored species Time t 1 t 2 Steady State Concentration

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19 In an i deal photochromic system, the color less form has as much as possible absorption in the ultraviolet (UV) spectrum while the colored form has a wide range of absorption across the visible spectrum. Additionally, the response of the mate rial to the UV irradiation should be quick, the fading rate of the colored form must be controllable and the material should be photostable for many coloration cycles. Spiropyrans, spir ooxazines, chromenes, fulgides and diarylethenes are five main classes of photochromic compounds .5,9,14 They show photochromism with a bond cleavage mechanism and they are widely studied because they can approach those ideal requirements (Table 1 1) Closed (colorless) and open (colored) forms of Spiropyrans14 34, Spiro oxazines5,9,14,15,31,32,35 37, and Chromenes5,9,38 42 absorb in UV and visible region of the spectrum, respectively. The c olored form of these molecules goes back to the colorless form either thermally or by the irradiation of light. On the other hand, the open form of Fulgides5,9,40,4345 and Diaryleth enes5,9,40,4649 are colorless and absorb in UV region while the closed forms are colored and absorb s in visible region of the spectrum. For these molecules, the fading reaction occurs only in case of irradiation with another source of light. In a p hotochromic reaction, in addition to change s in absorption spectra various physical properties of the photochromic material such as refractive index, dielectric constant, oxidation reduction potential, and geometrical structure can change. These changes allow the photochromic materials to have applications in various areas.50 The application of photochromic materials in ophthalmics, plastic lenses for sunglasses is common alt hough the fading rates of these materials are not as high as desired.50 Therefore, studies are still going on in order to develop better materials for these applications. Along with the practical applications of the photochromic compounds, there are many potential application

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20 areas such as optic memories for data storage ,45,48,49,51 53optical switches ,15,45,48 nonlinear device components ,54 etc. Additionally, incorporation of the photochrom ic compounds into a polymer chain and their possible application areas are the focus of many research groups54,55. Table 1 1. Main photochromic families Photochromic family Closed form Open form Spiropyrans Spirooxazines Chromenes Fulgides Diarylethenes Photochromic compounds are used also in novelty printing successfully. Fulgides are the photochromic family which is used mostly in novelty printing due to the low temperature dependency of the fading rates.9,50 The items produced in this area are mostly toys and T -shirts. N O N+O-O N O N O O R O O S S F F F F F F NH+N+O-O ONH+N N+O-O OO R O O O S S F F F F F F

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21 Photochromic compounds can be used as molecular switches in data stor age due to their ability of interchanging betw een two or more states. F or a successful molecular switch, the compounds s hould meet the requirements such as thermal stability, fatigue resistance, high efficiency of photoreactions, high reaction rates in bot h direction of photochromic reaction, and solubility in polymer matrix. The potential of spiropyrans ,9,17,51,56 sp iro oxazines ,9,17,51,56 diarylethenes ,9,15,52,56 fulgides9,45,56 for meeting these requirements is the subject of an active research area. Photochemical and Photophysical Processes Related to Photochromism The successful applications of the photochromic materials require the eliminations of the limitations of these materials. U nderstanding the reaction rate s and the mechanisms of photochromic compounds is important for the innovation of the materials with desired properties for different applications. The knowledge of basics of photophysical and photochemical process es in general is helpful in characterization of the photochromic systems. For an electronic transition to occur, a photon needs to be absorbed by the material that promotes the removal of an electron from an orbital of the ground s t ate to an unoccupied orbital of the excited state. The excited state can go through various processes such as internal conversion, intersystem crossing, fluorescence, and phosphorescence. The electronic states and the possible transitions between these states are shown on a Jablo nski diagram in Figure 1 2. In most molecules, with a singlet ground state, absorption occurs from ground state to the available singlet states (S1or Sn) approximately in 1015s. Internal conversion (IC), and intersystem crossing (ISC) are nonradiative transitions between either states with the same spin This is the maximum interaction time available for absorption of a photon by a mol ecule. A blue light has a wavelength of the order of 4000 A The speed of light is 3x1018 A sec-1. So, the time required for this photon to pass a point is t= 4000 A/3x1018 A sec-1 (speed of light) and it is equal to time of order of 10-15.

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22 multiplicity, or between two isoenergetic vibrational levels of electronic states with different spin multiplicity, respectively. Internal conversion occurs in 1011109 s while intersystem crossing tak es 1010108 s. Fluorescence is a radiative transition usually from S1 state to S0 state (Kashas rule57) occurring in most cases in a time scale of 1010107 s. The triplet (T1) state which is populated via intersystem crossing is de -excited by a radiative process that is called phosphorescence in 1061 s. Figure 1 2. Jablonski diagram Not all the compounds absorb at every wavelength with the same efficiency. For a given compound, absorbance A( and transmittance T( ) describe the efficiency of the light absorption at a specific wavelength (Equation 1 2). ) ( log ) ( T A (1 2) where 0) ( I I T 0I and I are the intensities of the light beams entering and leaving the absorbing medium, respectively. Internal Conve rsion Intersystem Crossing S 2 S 1 S 0 T 1 Phosphorescence Fluorescence Absorption

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23 Under low intensity conditions, absorbance of a sample follows the Beer -La mbert Law58 where is the molar absorption coefficient (L mol1 cm1) c is the concentration of the sample (mol L1) and l is the absorption path length (namely the thickness of absorbing medium ) (cm) cl I I A) ( log ) (0 (1 3) The electronic transition upon the absorption of a photon is an instantaneous process. Due to the fact that atomic nuclei in molecules do not have time to change positions in this process, they ar e vertical transitions. This explanation is called the Franck Condon58 principle. Due to the vertical transitions, the same transitions are involved in both absorption and emission which creates a symmetric absorpti on and emission spectra. Generally, the emission can occur at longer wavelengths than the wavelengths at which the molecule absorbs. The molecule can be excited to higher vibrational levels within the S1 state or to the higher electronic states. If the molecule is excited to the higher electronic states, it relaxes to the S1 state via internal conversion. Thus, in most cases emission occurs from S1 irrespective of excitation wavelengths. Therefore, the emission spectrum is the mirror image of S0 S1 absorption. This is known as Kashas Rule.57 Additionally, emission wavelengths can be shifted to the longer wavelengths due to the stabilization of the excited state by the solvent. Solvent Effects on Absorption and Emission Spectra In literature, it is reported that solvent polarity has a strong effect in the photochromic reactions of the spiropyrans and the related molecules .28,31,58 73 In our study we also observed spectral shifts on the absorption and the emission spectra of the photochromic oxazine in different solvents (Chapter 3) Depending on the solven t properties, shifts in the absorption and emission spectra due to the stabilization of the excited state of the flourophor es are very easy to observe. These shifts are

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24 called solvatochromic shifts since they are originated from the solvent -solute interact ions, which are commonly described with the Van der Waals interactions or hydrogen bondings. Molecules which show solvatochromic shifts are called solvatochromic molecules. With the increasing solvent polarity positive solvatochromism is bathochromic (red) shift while negative solvatochromism is hyp sochromic (blue) shift.58,6 3 Polar molecules are more sensitive to changes on the polarity of the solvents, as compared to the nonpolar molecules. Following the excitation of polar molecules, the charge distribution within the molecule is reorganized. In polar solvents, the dipole moments in excited state ( e) are larger than the dipole moments they have in ground state ( g). After the reorientation of the solvent, this results in lowering the energy of excited state and emitting at longer wavelengths. Increasing solvent polarity ma kes this effect larger. In polar solvents, charge separated state has the lowest energy and emission mostly comes form that state. On the other hand, in nonpolar solvents, charge separation is not favorable and most of the emission comes from the locally e xcited, Franck Condon state.58 The solvent polarity can lower the energy of the excited state and determine which state has the lowest energy. Energy transfer To characterize the electronic states which are involved in the ring opening mechanism of the photochromic oxazine, we conducted sensitization experiments using benzophenone (Chapter 3) Identifying the properties of the energy transfer between the photochromic oxazine and benzophenone helps describing the prop erties of the electronic states of the photochromic oxazine. In sensitization experiments, the energy transfer occurs from excited sensitizer (donor) to quencher (acceptor). The energy transfer can occur between those species if the energy of the

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25 excited sensitizer is higher than the energy of the excited quencher. Also, the energy transfer process should occur within the natural life time of the excited sensitizer. Energy transfer can be classified in two main categories: radiative and non radiative energ y transfer. In radiative energy transfer, a photon emitted by a molecule, called the donor (D), is absorbed by another molecule, called the acceptor (A). Spectral overlap between the emission of the donor and the absorption of acceptor is the requirement f or this sort of energy transfer. On the other hand, besides the spectral overlap nonradiative energy transfer depends on the short or long -range interactions between the molecules. Radiative Energy Transfer For the radiative energy transfer to occur the d onor needs to be excited by absorption of a photon with a frequency that the acceptor molecule can not absorb. Then it emits another photon with a frequency that the acceptor can absorb. In this way, the excitation energy is transferred to the acceptor mol ecule by the absorption of the photon emitted by the donor as it is s ummarized at Equation 1 4 1* DhD (1 4 a) 2* DDh (1 -4 b ) 2* hAA (1 4 c ) The efficiency of the radiative energy transfer depends on the quantum yield of the donor, concentration of the acceptor, extinction coefficient of the acceptor at the emission wavelengths of the donor, and the overlap between the emission spectru m and the absorption spectrum of the acceptor. Radiative energy transfer is more probable when these factors have the highest value.

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26 Non -Radiative Energy Transfer In non radiative energy transfer the donor molecule interacts with the acceptor molecule, wh ich occurs in the presence coupling between the electronic states of the donor and the acceptor. Non radiative energy transfer can be explained with Fermis Golden rule65 which is formulated as Probability 2 2 (* *)() if DADA H h (1 5 ) where i and fare the wave function of initial and final states respectively, i s the measure of density of the final states and it related to the overlap integral, and H defines the specific interaction related to the coupling of initial and final states. The interactions between the initial and the final states can be either a Coulombic or exchange interaction (Figure 1 3). Therefore H can be rewritten as, H=Hcoulombic + Hexchange. Figure 1 3.Coulombic and exchange mechanism in nonradiative energy transfer. Adapted from the reference.65

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27 In the Coulombic inter action mechanism which is shown with dotted arrows in Figure 1 3, the energy transfer occurred at a distance in presence of coupling between the states of A* and D*. In order to determine the magnitude of the interaction, Frster introduced levels of coupling between the states of D* and A* as strong, weak, and ver y weak coupling based on the relative values of interaction energy (U), the electronic energy difference between D* and A* ( E the absorption bandwidth ( and the vibronic bandwidth ( (Figure 1 4).58,63,65 Figure 1 4. Representation of the electronic energy difference between D* and A* ( E the absorption bandwidth ( and the vibronic bandwidth ( The Coulombic interaction can be described as strong coupling if the condition U>> E, and U>> holds. Under this condition, all of the vibronic levels can be involved in the energy transfer process. In other words, the molecules are at resonance with each other. In the case of weak coupling, the interaction energy is l arger than the vibronic band width but smaller than the absorption band width (U>> U>> Therefore the excitation energy is not delocalized; it is more localized as compared to the strong coupling. The coupling is called very weak if the interacti on energy is lower than the absorption band width as well as the vibronic band width (U<< In very weak coupling, the resonance between the states is restricted.58 For the Coloumbic interaction Frster61 showed that D* A* E

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28 ) ( ) (6 0 2 A DA D ETJ R k k Coulombic k (1 6 ) where k is a constant related to the refractive index and concentration, accounts for the orientation factor of the transition dipoles. J(is the integrated overlap of the emission curve of the donor (ID) and the absorption curve of the acceptor ( In contrast to Coloumbic interaction, the exchange requires the physic al contact of the interacting pairs by having the electron clouds of the interacting species overlapped in space. Dexter61 has worked on energy transfer by electron exchange and showed that ) / 2 exp( ) ( L R KJ exchange kDA ET (1 7 ) where K takes the orbital interactions into account, J is the spectral overlap with the normalized extinction coefficient of acceptor. In this way the energy transfer rate constant is independent of absorption characte ristics of the acceptor. RDA and L are the donor acceptor separation and van der Waals radii, respectively. Selection Rules In the sensitization experiments using oxazine -benzophenone mixtures, determining the electronic properties of the quenched state of the benzophenone, and the dynamics of that state helps understanding the electronic properties of the sensitized state of the oxazine. In this sense, basic selection rules in energy transfer mechanisms give an idea about the possible states of oxazine tha t can be sensitized by the singlet or triplet state of the benzophenone. Singlet -singlet energy transfer Singlet -singlet energy transfer is the interaction between a singlet state of the donor and a singlet state of the acceptor as it is shown in Equatio n 1 8 These interactions can be either

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29 Coulombic or exchange interactions since both are spin allowed in singlet -singlet energy transfer. ) ( ) ( ) ( ) ( *1 0 0 1S A S D S A S D (1 8 ) Triplet -trip let energy transfer Triplet triplet energy transfer occurs via the interaction of the excited donor in its triplet state and acceptor in its ground state and producing excited acceptor in its triplet state (Equation 1 9 ). ) ( ) ( ) ( ) ( *1 0 0 1T A S D S A T D (1 9 ) Triplet triplet energy transfer generally occurs by exchange mechanism. Therefore, the close interaction (10 15 A) of donor and the acceptor is needed for efficient energy transfer. Triplet -singlet energy transfer In triplet -singlet energy transfer, the Coulombic and exchange mechanisms are both spin forbidden. Therefore, this type of energy transfer is slow. The relatively long lifetime of the triplet states of the donors compensate the low rate constant and make the energy transfer from the triplet state of the donor to the singlet state of the acceptor possible (Equation 1 10). ) ( ) ( ) ( ) ( *1 0 0 1S A S D S A T D (1 10) Singlet -triplet energy transfer Singlet -triplet energy transfer occurs rarely compared to the others. It is shown in Equation 1 11 as the excited singlet donor interacts with the ground-state singlet acceptor to produce an excited triplet acceptor. ) ( ) ( ) ( ) ( *1 0 0 1T A S D S A S D (1 11) The energy transfer that causes the spin flipping can be enhanced by the spin orbit coupling processes.

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30 Fluorescence Anisotropy The anisotropic behavior of the unsymmetrical phenylene ethynylene dendrimers are investigated at room temperature and 77 K as an independent project. Before the experimental results are discussed in Chapter 4, we introduce the basics of the anisotropy concept in detail In homoge nous solutions, molecules are randomly oriented. When these molecules are exposed to linearly polarized light, the molecules with the transition moment oriented in the same direction with the polarization of light are more likely to be excited. Then, the m olecules in the excited state will have a specific orientation and the light emitted is polarized to some extent. The extent of polarization of the emitted light is described by the term anisotropy (r) which is equal to I I I I rII II2 (1 12) where III and I are the intensities of the detected light with the polarization parallel and perpendicular to the polarization of the excitation source, respectively. The molecules with zero anisotropy display nonpolarized emission. Anisotropy values provide information about the angle between the absorption and emission transition moments since the fundamental anisotropy, the anisotropy measured in the absence of any molecular motion; (r0) can be c alculated by using Equation 1 13. 2 1 cos 3 5 22 0r (1 13) where is the angle between the absorption and emission transition moments. Theoretically, for a randomly oriented samples in 3 -dimension, r0 is equal to 0.2 (minimum) and 0.4 (maximum) when and 0 respectively.

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31 For the molecules follow ing the Kashas rule,57 emission is independent of the excitation wavelength and it comes from the lowest excited s tate regardless of the exci tation wavelength For this reason, the anisotropy does not change with the emission wavelength. However, the fundamental anisotropy changes with excitation wavelength since the absorption transition moment is oriented differentl y at each excitation wavelength that reach different electronic states causing the change in and consequently in anisotropy values. Sometimes, more than one state can contribute to the observed anisotropy. In this case, the total anisotropy is calculated as 01 1 0 0) ( ) ( ) ( r f r f rn n (1 14) where fn( is the fractional contribution of the nth state to the total absorption at the wavelength and r0n is the experimental (limiting) anisotropy of the mentioned state. For absorption from more than one state, 1 ) ( )(1 f fn ) ( ) ( ) ( A f An n ) ( ) ( ) (1 1 A f A where A( is the total absorption and the An( and A1( are individual absorptions of the nth and 1st states, respectively. Excitation anisotropy could be used to resolve the overlapping e lectronic transitions. Constant and monotonic increase in excitation anisotropy values across an absorption band corresponds to a single transition under this band. On the other hand, complex behavior of the anisotropy across an absorption band results from the overlapping transitions. For large molecules in which rotational depolarization is slow, the decrease in excitat ion anisotropy in a system containing donor acceptor pairs can be evaluated as an evidence of energy transfer due to the loss of orientati on during the transfer process.

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32 CHAPTER 2 EXPERIMENTAL METHODS This chapter describes the experimental methods employed to uncover the ring opening mechanism of a photochromic oxazine (Chapter 3) and steady state anisotropic behaviors of the pheylene e thylene dendrimers at room and low temperatures (Chapter 4). In this study, steady state absorption, emission and excitation spectroscopy as well as transient absorption spectroscopy were used. Steady State Measurements Absorption spectra were recorded on a Hewlett Packard diode array spectrophotometer (8452A). Emission and excitation spectra were measured with a JobinYvon instrument (Fluorolog 3). Steady State Anisotropy Experiments The anisotropy (r) is defined as the normalization of the difference bet ween the parallel and perpendicular polarized light intensities to the total intensity (Equation 2 1 ). I I I I rII II2 (2 1 ) Intensitie s used to calculate the anisotropy are measured using an instrument with the configuration illustrated in Figure 2 1 The polarizations are determined with respect to the polarization of the excitation light. The parallel polarized light has the same orien tation as the excitation source while the perpendicular polarization oriented at a 90 angle with respect to excitation source The fluorimeter used for the steady state experiments has an L -shape configuration (Figure 2 1 ) with a single detection channel This single detection channel has different sensitivities for parallel and vertical polarizations of the emitted light. Consequently, the measured intensities of

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33 the polarized light are not the true intensities of parallel and vertical intensities. In or der to measure the actual intensities we need to take into account the sensitivity of the detection channel. Figure 2 1 Schematic diagram for measurement of anisotropy experiments.63 In our measurements, four combinations of excitation and emission polarizations (IVV, IVH, IHH, IHV) are used to eliminate the error whi ch comes from the inst ruments sensitivity (Figure 2 2 ). The subscripts of the lights indicate the orientation of the excitation and emission polarizers. For example, IHV represents the intensity measured when the excitation and emission are polarized hori zontally and vertically, respectively. Figure 2 2 Schematic diagram for the measurement of A) IVV and IVH B)IHH and IHV.63 Detector Light Source Polarizer Polarizer y z x II II sample Polarizer y z x III I sample VVI VHI Vertical Excitation H orizontal Excitation Polarizer y z x I I sample HVI HHI B A

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34 The sensitivity factors of the emission channel for the vertically and horizontally polarized components, SV and SH respectively, can be obtained from the measured intensities with different polari zations since the intensities, IVV, IVH, IHH, and IHV are defined as: II V VVI kS I (2 2 a) I kS IH VH (2 2 b) I kS IV HV (2 2 c) I kS IH HH (2 2 d) where k is the proportionality factor to compensate for the instrumental factors other than polarization sensitivities and quantum yield of the molecule under study. By combining the Equations from 2 2a to 22 d, one can get I I G I I S S I III II H V VH VV (2 3 ) G I I S S I IH V HH HV (2 4 ) where G is called a grating factor and is used to correct the variations due to the polarizations in anisotropy experiments. T he rearrangement of Equation 2.1 makes it clear how to involve the G factor in the anisotropy calculations (Equation 25 ). VH VV VH VV II IIGI I GI I I I I I r 2 2 1 (2 5 ) The anisotropy measurements of phe ylene ethylene dendrimers were performed at 298 K and 77 K. 77 K is obtained by using a liquid nitrogen flow cryostat (Oxford Instruments) and a ll

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35 of the samples for low temperature measurements are prepared in 2 Methyl tetrahyrofuran (2 MeTHF) which forms a clear glass at 77 K. Time -Resolved Measurements Compared to time resolved measurements, steady state measurements are easier to perform and the instrumentation is simpler and less expensive. However, during the steady state measurements, the time and i ntensity averaging processes result in the lost of dynamic information of the system under investigation. T ime resolved measurements provide information to understand the excited state dynamics of photophysical, photochemical and photobiological processes.63 Pump probe Spectroscopy Pump -probe spectroscopy is a time -resolved spe ctroscopy to probe and characterize that electronic and structural properties of the transient states formed during the light initiated processes Processes occurring in time scales as fast as femtosecond can be probed by these techniques. In a typical pum p probe experiment in the femtosecond (fs) picosecond (ps) regime, two ultrashort pulses are overlapped spatially and temporally on a sample. The pump pulse excites the sample at t=0. The optical changes induced in the sample upon the excitation are probe d by the probe pulse which is delayed with respect to the pump pulse. The effect of the pump pulse on the sample may be ana lyzed in two different ways: For example, Raman scattering spectroscopy ,64 laser in duced fluorescence62 and coherent an ti -S tokes Raman spectroscopy (CARS)74 techniques can be used to detect new effects created in the sample by the probe itsel f before and after the action of pump pul se. T ransient absorption technique allows one to compare the changes in probe pulse characteristics such as intensity, phase and wave vector after passing through the sample before and after the perturbation of the sample by the pump pulse .64

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36 Ultrafast T r ansient Absorption Spectroscopy By employing the transient absorption method, the changes in the absorption spectru m of a sample upon excitation by an ultrashort pulse can be observed and measured with respect to time. The general principle of this technique is shown in Figure 2 3 At time t=0, the pump beam excites the sample. The probe beam crosses the sample at a t ime t= t+ t where t is prov ided by an optical delay line. The detector which can be a photodi ode or a CCD measures I0 ( ) and I ( t) The relation between I0 ( ) and I ( t) can be explained by the B eer -Lambert Law58 in Equation 2 6 l t N I t I ) ( 10 ) ( 0 ) ( where is the extinction coefficient of the sample at wavelength N( t) is the population absorbing at time t at wavelength and in l the length of the sam ple excited. Figure 2 3 Basic principle of transient absorption experiment. Figure is adapted from reference.64 The major goal of the transient absorption experiment is to measure the optical de nsity of transient absorption, so called change in absorption, defined as: l t N t I I t A ) ( ) ( ) ( log ) (0 (2 7 )

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37 In a transient absorption experiment, we observe signals as positive or ne gative changes in absorption. A positive A displays a photoinduced absorption process, while a negative A can be observed due to either ground state bleach or a stimulated emission process. All the possible transitions and the related changes in absorpti on spectra are shown in Figure 24 When the sample is exposed to the excitation pulse, a certain number of molecules will be excited. In other words, the number of the molecules at the ground state will decrease. The ground state absorption in the presen ce of the pump beam will be lower than the absorption of the state in the absence of the pump beam. In this situation, a negative transient absorption signal called ground state bleach will be observed. The bleach signal should be expected instantaneously after the excitation at the steady state absorption wavelengths. Figure 2 4 Scheme of certain signals in transient absorption measurement. Photoinduced absorption (also called excited state absorption) will be observed if the molecules at the ex cited state are excited to a higher state by the probe pulse. This type of process will lead a positive change in absorption ( A). S 0 S 1 t excitation pump probe bleach photoinduced abso rption probe probe stimulated emission A 0

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38 Another change of absorption can be observed at the steady state emission wavelengths. This signal will be detected in case of stimulated emission, which occurs if the probe pulse stimulates the molecules at the excited state to return back to the gr ound state. In this case, the detector is exposed to relatively more photons, which makes A negative. Besides the general considerations of the technique, some practical aspects such as probe characteristics, detection systems and experimental tricks to a void artifacts75 should be taken into consideration for an effective technique. The probe can either be a monochromatic beam, or it can have a broad spectrum. In the former case, the researcher has to know where to dete ct the species created upon excitation in the sample and choose a probe wavelength accordingly. This kind of study leans on preliminary data which helps to estimate the active spectral domain of the species to be probed. On the other hand, in the latter ca se it is possible to detect the expected species in the sample after the excitation as well as unexpected ones since it allows recording transient absorption at different wavelengths simultaneously. If the spectral domain where the excited species are acti ve is not known it is better to use a broad band probe where continuum generation is a possibility.64 Continuum generation arises from the propagation of intense picosecond or femtosecond pulses through a condensed or gaseous media. The origin of this process is mainly governed by Self Phase Modulation (SPM).76,77 SPM appears when the strong laser beam produces a refractive index change in the medium and then the medium causes a phase change on the incoming beam as a response. I n the case of a pulsed laser input, laser intensity varies in time that leads to a SPM in time. Since the derivative of the phase with respect to time gives the angular frequency of a wave, SPM also occurs as a modulation in the frequency domain. Thus, the process ends up with a self -induced spectral broadening.76

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39 The probe in a transient absorption experiment can be detected by a Charge Coupled Device (CCD) camera or photodiode, depending on the spectral properties of the pr obe beam. When simultaneous measurements at different wavelengths are necessary, the detector should be a CCD and the optical density is measured directly as a function of wavelength. In a transient absorption experiment variables such pump and probe relat ive pola rizations group velocity dispersion (GVD) along the optical pathway, various nois e sources such as ambient light or electronic noise need to be considered in order to avoid experime ntal artifacts in the data.78,79 If the pump beam is polarized in a particular direction, the spatial distribution of the excited molecules will be anisotropic (See Introduction, Chapter 1). Spatial randomization can occur via reorientation of the molecules within the reorientation time. Thus, the dynamics observed are not only the reflections of lifetimes of the excited molecules but also their reorientation time. If the aim of the experiment does not include measuring the anisotropic behavior of the molecules, the angle between p ump and probe should be a set to 54.7 the magic angle where fundamental anisotropy is equal to zero. Generally, electromagnetic waves propagate at different group velocities in different media that creating group velocity dispersion (GVD). Accordingly, the probe frequencies traveling through the different optical components are not at the same speed due to GVD.7880 In other words, in a supercontinuum probe, the blue wavelengths arrive at the sample later than the red wavelengths since for the media with normal dispersion, the index of refraction for blue wavelengths is higher than that for the red wavelengths. It is possible to get rid of this effect in the measured spectrum by correcting the collected data numeri cally. This method is practical in case of well known continuum dispersion.64,76

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40 In our experiments, the biggest chirp we observed between 343 nm and 500 nm was about 800 fs. Thus, chirp correction is not necessary fo r the data which has time steps larger than 800 fs. Only the data with time steps smaller than 800 fs were corrected numerically by using a home-made Labview program. This program calcula tes the dispersion of light in every medium which the light interact with. The velocity of the light in different medi ums at different wavelengths depends on the refractive index of the medium at that wavelength since The chirp correction program uses the Sellmeier equation (Equation 2-8) which gives an empirical relation between the refractive index and the wavelength. 222 2 123 222 123()1 BBB n CCC (2-8) where B and C are the Sellmeier coefficients determined experimentally.81,82 The chirp at different wavelengths per mm is calculated by the program for diffe rent media. Figure 2-5 shows the chirp per mm with respect to the energy (wavelength) of the light for water (See Appendix A for air, BK7, CaF2, fused silica, and methyl alcohol). Figure 2-5. Chirp per mm with respect to the energy (wavelength) of the light for water. () () c v n 1.52.02.53.03.54.0 0 50 100 150 200 250 300 Relative time (fs)Energy (eV)

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41 The ultrafast transient absorption apparatus used in this study consists of three major components: femtoseco nd laser source, pump -probe set up and detection system. Femtosecond Laser S ource A commercially available ultrafast laser system composed of Millenia, Tusunami, Evolution, Spitfire and OPA from Spectra Physics () is used as the excitation source in expe riments in this thesis work. The function of these individual elements is explained briefly. Millen n ia Vs: It is a high power, visible cw solid state laser providing 532 nm output with power range 210 W. It uses neodymium yttrium vanadate (Nd:YVO4) as th e solid state laser media. The diode pump light is absorbed by the Nd:YVO4 crystals and emitted as the output at 1064 nm. 1064 nm goes through a frequency doubling process on a phase matched, temperaturetuned LBO crystal and 532 nm becomes the output of t he laser. Tsunami: Tsunami is a solid state laser which uses Titanium -doped sapphire as a lasing media. It is pumped by the Mille n nia output. Tsunami delivers ~35 fs pulses with 82 MHz repetition rate. The output of the Tsunami is centered at around 7 90 nm with approximately 35 nm bandwidth (FWHM) and used as the seed of regenerative amplifier. Evolution X: It is a diode -pumped laser which designed around a neodymium: yttrium lithium fluoride (Nd:YLF) laser head pumped by four AlGaAs laser diodes. It provides Q switched pulses with average powers greater than 6W at 527 nm at repetition rates of 1 kHz. Regenerative Amplifier, Spitfire: It is an optical am plifier which has Ti:Sa crystalvf as the active laser medium and is pumped by Evolution X. It us es a Chirped Pulse Amplification technique72 to amplify the ~35 fs pulses at 82MHz repetition rate leaving the Tsunami. The pulses entering the amplifier undergo first the process of stretching, then amplification and finally compression before they are re leased from the system. The system

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42 produces pulses with energy in a single pulse reaching up to 1 mJ centered at 790 nm with pulse widths around 50 fs at 1 kHz repetition rate. Optical parametric amplifier (OPA) : It provides a wide range of wavelengt h opt ions for the pump pulse. Man y complex systems absorb at wavelengths that can not be delivered by the fundamental Ti: Sapphire regenerative amplifier or through its direct frequency conversion with harmonic generation. These required wavelengths are supplie d by the OPAs. The OPA does not operate with the same principle of a laser. While a conventional laser operates on population inversion OPAs gain is driven by nonlinear process as second harmonic generation (SHG), fourth harmonic generation (FHG), sum fre quency mixing (SFM), difference frequency mixing (DFM) in which white light continuum is used as a seed and beta -barium borate (BBO) crystal as nonlinear medium. Pump ( Spitfire output) and seed beams are overlapped spatially and in time on BBO crystal and generate two types output beams: signal and idler. Energy conservation determines the frequency of the signal and idler as pumpsignalidler or 111 pumpsignalidler The wavelength for signal and idler outputs are in range of 1.1 1.6 m a nd 1.6 3.0 m respectively. Either signal or the idler outputs of the OPA can be used depending on the purpose of usage in the experiments. The signal and idler can be separated easily by taking the advantage of that they have the different polarizations (Signal is horizontally and idler is vertically polarized). In our experiments we used 320 nm as the excitation source in our transient absorption experiments since photochromic oxazine has a high extinction coefficient at this wavelength. The beam which has a spectral peak at 320 nm is generated as the FHG of the signal output of the OPA. The FHG of the signal is achieved using two BBO crystals. After the FHG crystal,

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43 TLM 1 mirrors, which have a high reflectivity at 320 nm, were used for a high power outp ut. The power of the generated beam at 320 nm was around 2.5 J after the generation and it decreases to ~600 nJ at the sample position. Experimental Set up The schematic representation of the experimental transient absorption s et up is presented in Figur e 2 6 ~ 400 J of the output of amplified laser is used by an optical parametric amplifier (OPA) and followed by forth harmonic generation to produce 2.5 J pump beam at 320 nm with 1 kHz repetition rate. The pump beam generated in the OPA follows an opti cal path containing a telescope. The t elescope is composed of two quartz (a lens concave with f= 50 mm and a convex lens with f=150 mm) lenses and it is used to decrease the spot size of the pump beam and collimate it. The decrease of the spot size is determined by the focal lengths of the used lenses. In our set up, after the telescope, the beam size is decreased to 1/3 of the original size since the focal lengths of the lenses used to build the telescope is has a ratio of 1/3 (50/150) After the telescope t he collimated beam goes through an optical delay line consisting of two perpendicularly mounted mirrors on a computer controlled motorized translation stage (Model No: M 415 D6, Physik Instrumente) which is used to change the time delay between pump and probe. After the delay stage the pump beam passes through a chopper wheel in order to compare the signal with and without the pump. Then, a parabolic mirror (Janos Technology, A8037207, off axis mirror with reflected focal length of f=152.4 mm) focuses it into the sample where it is overlapped with the probe. A small fraction of the amplified laser output (~4 J) is focused onto a 1.5 mm thick CaF2 window (1 diameter PW 1006CFUV from CVI laser) with a lens of 100 mm focal length in

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44 order to generate t he white light continuum. The CaF2 is held in a homemade rotating cell to eliminate the intensity fluctuations. After generation, the white light continuum is recollimated by an off axis parabolic mirror (Janos Technology, A8037175, reflected effective fo cal length of f =25.40 mm ) and split into two beams via a beam splitter ( n eutral density filter with optical density of 0.3) in order to compensate fluctuations of laser power. One of these beams ( probe ) is used for probing the perturbation in the sampl e by the pump beam. Thus it is overlapped with the pump beam at the sample position The other beam is used as a reference and it passes through an unperturbed area of the sample. Measurements are performed at magic angle where pump and probe are linearl y polarized at 54.7 with respect to each other. Figure 2 6 Schematic representation of transient absorption set up. The perturbation created by the pump in the sample will be completely measured as long as the size of the pr obe beam is equal to or smaller than the pump be am size Otherwise, the probe beam detection would reflect perturbed as well as some unperturbed volumes of the spectrograph polarizer sample CCD 50 50% CaF 2 plate probe femtosecond laser syste 790 nm OPA d elay stage chopper pump reference SHG& FHG

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45 sample which will result in an insignificant change in absorption spectrum. Accordingly, during the measurements the probe and the pump beams have the diameters of 124 m and 140 m, respectively. The beam sizes are measured by using the knife -edge scanning method.8385 After the sample, probe and the reference beams are directed and focused on to the slit of the spectrograph at t wo different heights and read by the charge coupled device (CCD) camera. Signal Detection S ystem The CCD camera is attached to a spectrograph (Shamrock SR 303i) that separates light into its component wavelengths combined with Andor iStar that is an opt ical spectral analyzer. The spectrograph can be used for the wavelength range from 190 nm to 10 m. The grating used in our experiments had a line density 300 l/mm. CCD is basically a silicon based semiconductor chip containing 256 rows and 1024 columns for spectrographic applications. These rows and columns compose a two dimensional imaging area with 256x1024 pixels. In our experiments, t he reference and probe beams approximately had a width of 10 rows on CCD chip in a 280 nm window (from 350 nm to 630 nm) The images of probe and reference were apart from each other (~ 50 rows) (Figure 2 7 ). Figure 2 7 The image of probe and reference beams on CCD chip. Reference Probe 250 200 150 100 50 400 500 600 450 550 350 630 Rows Wavelength (nm)

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46 If an image is projected onto the array, light falling onto the pixels produces the corresponding charge pattern on the arrays of the CCD by producing electrons on the pixels of the arra y. The created charge pattern is transferred from the chip to the shift register by a series of horizontal transparent electrodes that cover the array. The shifts register runs below the imaging area. It is parallel to the light collecting rows and it has the same number of pixels as rows of imaging area. However, it is masked to avoid light to fall on it. The shift register also has a series of electrodes which are parallel to the columns. They are used to transfer the produced charge patterns into the am plifier pixel by pixel. Then the output of the amplifier is converted into a binary number via an analog to digital converter (A/D). The change in transmission read by CCD is recorded by the computer conne cted to the CCD as a function of wavelength at diff erent time points by using a homemade labview program. Afterwards during the data processing, the change in t ransmission is converted to change s in absorption. In steady state measurements, transmission (T), absorption (A) and the relationship between tran smission and absorption are defined as: ) ( ) ( ) (0 I I T (2 9 ) ) ( log ) ( 1 log ) ( ) ( log ) (0 T T I I A (2 10) The data collected in our system is recorded as absolute values of change in transmission since the change in the probe beam spectral intensity is normalized to the reference beam intensity in order to compensate for the fluctuations of laser power as it is expressed in Equation 2 11.

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47 00 01tt refref pumpnopumppumpnopumptpump nopump tnopump t ref nopumpII II TTI T TTI I I (2-11) Then, by using the Equation 2.11, the change in absorption is defined as 1log T T A (2-12) The time resolved data presented in Chapter 3 are average of around 30-40 experiments. I collected the average of 30 laser shot at a single time point in each experiment that allows me to have data average of ~1000 lase r shots at a single time step. Time Resolution of the Experiment The instrument response functi on (IRF) of the system is m easured by taking advantage of coherent artifacts in th e pure solvent due to th e cross phase modulation86-88 of pump and probe beams. These coherent effects are observed in acetonitrile and hexane Figure 2-8 shows the Figure 2-8. Coherent arti fact of hexane excited at 320 nm, probed at 355 nm. -0.3-0.2-0.10.00.10.20.3 -0.005 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 T/TTime (ps)

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48 coherent artifact si gnal in hexane when the solvent is excited at 320 nm and probed at 355 nm. T he temporal width of IRF at FWHM is measured as ~250 fs (Figure 2 8 ). Data Analysis Methods Transient Absorption data is an n x m matrix where the size of the matrix shows that the w hole data is the composition of a set of m spectra each containing n successive delay times. Each spectrum can contain contributions from p components. Since each one of the p components has a specific extinction coefficient at different wavelengths, t hey exhibit characteristic spectral responses at those wavelengths. The spectral responses of all of the components can be represented by an n x p matrix which is called S here. Because of the excited state dynamics of the system, concentrations of the components can change from one spectrum to another. The concentration profiles of p components in each spectrum can be put in the form of a p x m matrix, C. Since absorbance of a species at a specific wavelength can be written as the multiplication of the conce ntration and extinction coefficient of that species, transient absorption data can be explained as product of S and C ma t rices similarly in Equation 2 13 where D matrix represents the whole transient absorption data.89 91 SC D (2 13) In the analysis of transient absorption data it is critical to estimate the num ber of components in the system which are spectrally distinguishable species. This task can be achieved by employing the Singular Value Decomposition (SVD) method in determining the rank of matrix D. The rank of the data matrix provides a lower limit for the number of the components of the system. In this way, application of SVD gives the opportunity to determine the representative and opt imum data for further analysis.89,90 SVD is a mathematical method which decomposes the n x m mat rices into three ma t rices according to Equation 2 14

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49 TDUWV (2 14) None of the matrices provided by SVD have a chemical or ph ysical meaning. U and V represent orthogonal matrices with size of n x m and m x m respectively. W is an m x m square diagonal matrix and it contains the singular values in decreasing order of magnitudes. The magnitude of these values is a measure of the contrib ution of the corresponding columns of matrices U and V to the reconstruction of original matrix D. The components have values close to zero do not make significant contribution to the total system. The components with the relatively higher singular values can be used to explain the system. Therefore, the minimum number of components which are required to explain the system can be determined according to the ma gnitude of the singular values. However, in the presence of noise it may not be easy to set a thres hold that dissects the contribution of the components from the contribution of noise. In this case, evolving factor analysis methods (EFA) can be employed to find out the significant number of the components present in the system.89 The basic idea of EFA is to follow the evolution of the singular values in time. It evaluates the rank of the gradually increasing submatrices formed by the addition of one row at a ti me. If the rows are added from the top of the matrix to the bottom, it is called forward EFA and elicits the appearance of the singular values with increasing time. What is more, the disappearance of a singular value with increasing time can be detected wi th backward EFA where the rows are added from bottom to the top.89,92 The significant singular values determined by the EFA are those singular values above the noise level defined by the pool of nonsignificant singular values.93,94 Significant singular values, which correspond to the components of the system, can be clearly seen by plotting the results of forward and backward EFA as it is shown in Figure 2.9.

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50 The system in Fi g ure 2 9 consists of three components which are represented by the singular values above the noise level, the shaded area of the plot in figure. Figure 2 9 ( ) Results of forward EFA ( ) Results of backward EFA. Adapted from refer ence.89 Upon the application of EFA to the transient absorption data, size of the S ( n x p ) and C (p x m ) matrices in Equation 2 13 is discovered. The number of signi ficant values which are exhibited by the EFA is equal to the number of components (p) in the system. The number of components provided by the EFA is equal to the number of SVD components which have significant singular values. Consequently, U and V matrice s will have the same dimensions as S and C, respectively. Also, they contain the same information wit h S and C. Therefore, Equation 2 1 3 and 2 14 ca n be combined as in Equation 2 15. TSCUWV (2 15) If one examines Equation 2 15 closely, one finds that the columns of U and V matrices contain spectral and concentration information, respectively. These columns are mathematically in dependent vectors and they might not have a physical meaning. However, they are linear

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51 combinations of the physically meaningful vectors that fit a certain kinetic model. In order to find out these vectors a rotation can be performed on U or V matrix by us ing a pxp rotation matrix. In this thesis work, the rotation is performed by employing an algorithm called Multivariate Curve Resolution Alternating Least Squares (MCR -ALS) instead of finding a rotation matrix. MCR -ALS is an iterative self -modeling method relying on the alternating least squares (ALS) algorithm. It provides information about the kinetic and spectral profiles of the system components. The resolution of the method depends on the correct estimation of number of the components contributing to the system and application of chemical and mathematical constrains.95,96 Therefore the output of the EFA can be a proper input for the MCR -ALS method as the initial estimation. MCR -ALS method decomposes a data matri x D ( n x m ) containing the combined information about an evolving system, into the matrices which contain the pure spectral and kinetic profiles of the system components. The decomposit i on is described in Equation 2 16 where the columns of C ( n x p ) represent kinetic profiles and rows of ST (p x m ) describe the related spectral profiles of p species. E ( n x m ) is the random perturbation matrix containing the residuals.9597 E CS DT (2 16) Despite all the advantages, curve resolution methods including MCR -ALS do not deliver a single solution for the set of components whose linear combinations descri be the original data. One can claim that the biggest drawback of using these methods is rotational ambiguities; however, this drawback can be eliminated by applying some constraints in spectral and concentration domain.96

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52 Applying the various constrains during the execution of a Matlab routine in which the MCR -ALS algorithms are written is not an easy task. Fortunately, a user -friendly graphical interface98 developed by Chemometrics Group, University of Barcelona is available online. The algorithm used during the process is summarized in the scheme i n Figure 2 -10. In this process, Figure 2 10. Scheme of ALS algorithm.98 first the number of the components in the experimental data matrix D is determined by means of SVD. Then, initial estimates of concentration or spectral profiles by using EFA method. After choosing suitable constrains, least -squares calculation is performed until convergence is

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53 achieved. In other words, the process is repeated until one can get the lack of fit (LOF ) under a selected threshold value. LOF is a measurement of goodness of the optimization process. It represents the difference between the original data D and the data created by the reconstruction from the CST product obtained by MCR -ALS. The value of LOF is calculat e d according to the Equation 2 17 where dij is an element of the input data and eij is the difference between the input and the MCR -ALS reproduction. j i ij j i ijd e LOF, 2 2100 (%) (2 17)

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54 CHAPTER 3 RING OPENING MECHANI SM OF A NOVEL PHOTOC HROMIC OXAZINE Photochromic compounds respond to optical stimulations by changing their structural and electrical properties. In a photochromic system, a new state is for med by light excitation. and this photogenerated state can return to the original state thermally or with exposure of another light source. Therefore, by simply turning optical input on and off, photochromic compounds can complete many successive switching cycles by altering and resetting an output property Since they interchange between two states, these materials appear as possible constituents of the molecular switches. The design of molecular switches is a challenge of miniaturization in future nanotec hnology. Design and synthesis of new photoresponsive materials with improved properties such as colorobility, fatigue resistance, and photostability is the primary purpose of the recent studies in field.5,9,35,56,99, 100 Spiropyrans and spirooxazines are the most studied photochromic families as molecular switch candidates because of their reversible optical activity.5,9,27,101 These molecules present two heterocyclic parts li nked together by a common Carbon atom with sp3 hybridization. They absorb at the UV region. The excitation with UV light leads to the changes in the molecular structure and absorption spectra. Removal of the excitation source results in returning to the or iginal structure ( Chapter 1).9 The photochromic reaction mechanism and the reaction dynamics of spiropyrans and spirooxazines were studied with time resolved resonance Raman Spectroscopy ,14,34,102 laser flash photolysis ,20,23,71,73,103105 quenching experiments ,106 and picosecond and femtosecond absorption spectroscopies21,36,37,107110 by various research groups. These studies uncovered important facts about the mechanism and the dynamics o f photochromic reaction of the spiropyrans and spirooxazines.

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55 In spiropyrans and spirooxazines, the first step of ring opening mechanism, the C O bond cleavage, occurs in picoseconds20,37,104,111 while the isomeriz ation of the molecule from cis to trans form around the C=C double bond takes place in microseconds.17,18,2224,112 For the nitrosubstituted spiropyrans17,18,23,24,112 and spirooxazines ,40 the triplet states play an important role in the ring opening mechanism of the molecules. On the other hand, triplet states do not participate in the ring opening mechanism of spiropyrans16,19 and spiroxazines without nitro groups.26,36,113 The merocyanines (open form of the spiropyrans and spirooxazines) of these mol ecules are directly formed from the singlet excited state. In the mentioned studies it is reported that the C O bond cleavage occurs in 150700 fs114 time period while the relaxation of cis -cisoid isomers to various merocyanine forms takes place in the time range from 50 100 ps to 1.3 1.7 ns.109 In recent years, there have been many trials of implementation of molecular switches115 123 by taking the advantage of reversible isomerization properties of spiropyrans and the rel ated molecules. However, in these trials researchers were facing two major problems. The thermal back reaction takes several minutes after the optical input is turned off. Additionally, the number of switching cycles that can be performed by the compound i s limited.99 Improved materials with reversible optical isomerization properties similar to spiropyrans but free from the limitations of spiropyrans are needed for the realization of molecular switch es in future technology. Raymo and co -workers reported a newly synthesized molecular switch which displays photochromic properties based on photoinduced ring opening and thermal closing of an oxazine ring .99,100 Upo n ultraviolet excitation, the oxazine ring opens and the photogenerated product is a chromophore which absorbs at around 440 nm. The original form of the molecule is fully recovered in 50 ns after the removal of excitation source. Furthermore, it is stated that this

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56 photochromic oxazine can survive more than 3000 switching cycles without any evidence of decomposition.99,100 Although the absorption spectrum and the lifetime of the chromophore which is generated during the coloration process are well defined, still there are unanswered questions about the coloring mechanism; What is the rise time of the chromophore absorbing at 440 nm? Are there any intermediate states involved in the ring opening mechanism? The goal o f this study is to uncover the ring opening mechanism of the photochromic oxazine by employing Ultrafast Transient Absorption (TA) and Steady State techniques. Applying the same techniques, complementary quenching experiments are performed by using benzoph enone which is known as a very good triplet sensitizer.65 The experimental results are analyzed using the methods Singular Value Decomposition (SVD), Evolving Factor Analysis (EFA), and Multivariate Curve Resolution-Alternating Least Squares (MCR -ALS) methods. A kinetic model is proposed to explain the ring opening mechanism of the novel photochromic oxazine. Materials and Experimental Methods The novel photochromic oxazine is provided by Prof. Raymo (Universiy of Miami).100 4 nitroanisole and the model indoline in Figure 3.1 are two chromophoric fragments of the novel photochr omic oxazine. T heir properties are i ndependent and the sum of their individual steady state absorption spectrum resembles to the steady state absorption spectra of the photochromic oxazine.100 Sample solutions are prepared in HPLC grade acetonitrile and hexane from Fisher Scientific without further purification. Steady state measurements and transient absorption experiments are performed in 1 cm and 5 mm optical path length quartz cuvettes, respectively. The laser system, and transient absorption set up are described in detail in Chapter 2.

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57 A B C Figure 3-1. Chemical structure of A) model indoline B) 4-nitroanisole C) photochromic oxazine All of solutions used in sensitization e xperiments with benzophenone are deoxygenated since oxygen is a quencher of triplet state. To degas the solutions, argon is purged in to the solutions for 20 minutes. This procedure appl ied to the solutions of pure oxazine, pure benzophenone, and oxazine and benzophenone mixtures which are used both in steady state and time resolved experiments. Steady State Spectroscopy The steady state absorption spectrum of photochr omic oxazine is shown in Figure 3-2. The first absorption band of the oxazine has a wide ov erlapping region with th e first absorption band Figure 3-2. Normalized absorption sp ectra of oxazine ( ), a nd 4-nitroanisole ( ) in acetonitrile 250275300325350375400 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Normalized AbsorbanceWavelength (nm)

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58 of the 4 -ni troanisole (Figure 3 2). A pproximate values for the energy differences between ground state and the first excited state of oxazine, 4 nitroanisole and the model indoline and the extinction coefficients of those compounds (at the absorption maximum) are listed in Table 3 1. Table 3 1. Absorption wavelengths ( ) and molar extinction coefficients ( ) and energy difference between S0 and S1 in acetonitrile. Compound Energy difference S 1 S 0 (kcal/mol)* (nm) m cm1 model indoline 95 110 281 3.90.2 4 nitroanisole 75 115 307 11.10.6 photochromic oxazine 70 110 316 110.6 *Calculated from the absorption spectra in reference.100 Oxazine in acetonitrile shows very weak emission betw een 330 and 600 nm (Figure 3 3) upon the excitation at =320 nm. It is not easy to compare the steady state absorption and emission spectra of the oxazine in acetonitrile since the emission spectrum has a low S/N ratio because of very low counts in emission (Figure 3 3). In this case, we can not determine whet her absorption and emission presents the similar transitions or not. From the values given in Table 3.1 and Tomasulo et al. ,100 we conclude that when we excite the oxazine at =320 nm, the nitroanisole part of the molecule will be excited since t he indoline part does not absorb at this wavelength. On the other hand, there was no significant emission from 4 -nitroanisole in acetonitrile after the excitation at = 320 nm. These observations suggest that the emission of the oxazine in acetonitrile is not originated from the nitroanisole part of the molecule and there is an interaction between indoline and the niroanisole parts in excited state. In the literature, it is stated that n itroaromatic molecules exhibit neither fluorescence nor phosphorescence at an y temperatures in any solvents.124126 Take zaki and coworkers determined the triplet life time of nitrobenzene in alkane, benzene, ethanol and water between 400 and 900 ps using picosecond time resolved transient grating method.126 Yip et. al reported that the nitrobenzene in THF has a triplet state with a lifetime of 800 ps.127 Triplet lifetime of the

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59 nitrophenyl esters was inve stigated by Mir et al. In their study, they determined that 4 nitroanisole forms a triplet state in water. However, they could not detect any triplet state of the 4 nitroanisole in acetonitrile and by considering the studies about the nitrobenzene they con cluded that they could not observe the triplet state of 4 nitroanisole in acetonitrile with nanosecond spectroscopy because 4 nitroanisole has a triplet state with lifetime in picosecond time domain.124 Figure 3 3. Normalized absorpt ion spectrum ( ), emission spectrum ( ) of oxazine in acetonitrile( exc= 320 nm). Since it is known that 4 -nitroanisole possesses a triplet state, it is possible that oxazine also has a similar type of triplet state. Sensitization experiments using benzophenone were performed in order to detect whether the triplet state o f 4 -nitroanisole and oxazine has any role in the ring opening mechanism. In these experiments, solutions of benzophenone and oxazine at various concentrations were investigated using steady state emission and transient absorption spectroscopy. In steady st ate, addition of benzophenone to an oxazine solution in acetonitrile enhanced the emission intensity of the oxazine after excitation at =320 nm. Figure 3-4 shows the 250 300 350 400 450 500 550 600 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Wavelength (nm)

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60 comparison of emission spectrum of oxazine with and without benzophenone in solution. Additionally, we observed that the phosphorescence of benzophenone is quenched in the presence of oxazine. It seems that energy transfer occurs from the triplet state of the benzophenone to the oxazine. The energy of the triplet state of the benzophenone is reported as 69 kcal/mol65 and the emission of the oxazine enhanced in the presence of the benzophenone has a peak around 360 nm which means that the emissive state has energy around 79 kcal/mol. Since the emissive state is energetically higher than the triplet state of the benzophenone an energy transfer from triplet state of the benzophenone to oxazine is not possible. The quenching of the benzophenone phosphorescence is observed because of not the direct quenching of triplet state but the precursor of it. Figure 3 4. Emission spectra of oxazine 4.6x105 M ( ), and oxazine with benzophenone 3.26x107M ( ), 6.52x107M ( ) in acetonitrile, exc= 320 nm. Inset shows the same data in a expanded scale. Hupl and coworkers studied two types of spiro[cyclohexadieneindoline] and observed an emissive state. They discovered that this emissive state exhibits a significant solvent effect since the pe ak of the emission band shifts to red with increasing polarity of the solvent. They concluded that this emission belongs to a charge separated state which is created in the excited 350 400 450 500 550 6000 20000 40000 60000 80000 100000 120000 140000 350 400 450 500 550 600 0 1000 2000 3000 4000 5000 EmissionWavelength (nm) EmissionWavelength (nm)

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61 state of the photochromic molecule s between the indoline, where th e nitrogen act as the electron donor, and the aroylcyclohexadiene moiety, where the keto group is the acceptor.69,70 In order to check the solvent sensitivity, we measure emission spectra of oxazine after the addition of benzophenone in acetonitrile and hexa ne. We collected the em ission spectra of the mixtures upon the excitation at 320 nm and observe d a red shift of about 10 nm in acetonitrile compared to the emission spectrum in hexane (Figure 3-5). The spectral shifts can be due to specific fluorophore-solvent effects63 and charge separation in the excited state as Hupl and coworkers reported.63,69,70 If the shift in emission spectrum is observed due to the general fluorophor e-solvent effects, the same amount of shift should be seen in the absorption spectrum of the molecule. However, in case of charge separation, the molecules have larger dipole moments in the excited state ( e) than in the ground state (g). The more polar environment results in lowering the energy of excited state and emitting at longer wavelengths. In this case, th e shift observed in emission spectrum would be more significant than the spectral shift occurred in absorption spectrum. Figure 3-5. Emission spectra of oxazine in hexane ( ) and in acetonitrile ( ). 340360380400420440460 0.0 0.2 0.4 0.6 0.8 1.0 Normalized EmissionWavelength (nm)

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62 In order to characterize the emissive state of the photochromic ox azine, the absorption spectra of the molecule in both solvents are also compared (Figure 3-6). The absorption spectrum was red shifted about 4nm in acetonitrile compared to the absorption spectrum in hexane. In this case, the shift in absorption spectra is less th an the one observed in emission spectra implying that e is larger than g. Figure 3-6. Absorption spectra of oxazine in hexane ( ) and in acetonitrile ( ). According to steady state spectroscopy results, we suspect that there is a charge separated state that forms following th e excitation of oxazine at =320 nm. The emission of that state is enhanced in the presence of benzophenone. Since energetically, the energy transfer can not be from the triplet state, we propose that energy transfer occurs from singlet state of the benzophenone either to the singlet state state or to the charge se parated state of the oxazine. The time resolved data will provide better unde rstanding of ring opening mechanism by giving detailed information about the energy transfer from benzophenone to oxazine. Transient Absorption Spectroscopy Raymo et al. reported that the UV excitati on of the photochromic oxazine induces the cleavage of C-O bond, and this bond cleavage produces the open form of the molecule (Figure 3260280300320340360 0.2 0.4 0.6 0.8 1.0 Normalized AbsorptionWavelength (nm)

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63 7). They observed a strong absorption band at 440 nm in acetonitrile when the sample is excited at = 355 nm. This absorption band is very similar to the steady state absorption spectrum of pnitrophenolate which is also a part of the open form of the oxazine (in the circle of red dots in Figure 3 7). Under these conditions, they concluded that upon the excitation at =355 nm, the closed form of the oxazine goes to the open form, which contains p -nitrophenolate chromophore as an absorbing unit. Therefore the absorption band observed at 440 nm is attributed t o the open form of the photochromic oxazine.99,100 What is more, they observed that the species absorbing at 440 nm is generated within the 6 ns excitation pulse and goes back to the original form with a lifetime of 22 ns.99,100 In these conditions, the formation of the photogenerated state is not resolved. In order to monitor the ring opening mechanism a better time resolution is needed. In this thesis, the photogeneration pr ocess (ring opening mechanism) is examined via picosecond transient absorption spectroscopy which is the technique described in detail in Chapter 2. Figure 3 7.Stucture of the open form of the photochromic oxazine. Picosecond transient absorption spectroscopy data of the photchromic oxazine in acetonitrile between 12 and 50 ps with 2 ps resolution collected after the excitation at = 320 nm is presented in a wavelength vs. time graph in Figure 3 8. Two regions are present with N+C H3C H3O-N+O-O

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64 positive changes in absorption spectrum which corresponds to photoinduced absorption. The color coding of the plot gives an idea about the intensity of the si gnal observed. The darkest red color corresponds to the strong photoinduced absorption, while the darkest blue color shows a lower negative signal value. One of the absorption bands raises within10 ps with a spectral peak around 440 nm. Changes in the dyna mics were not detected until ~600 ps which was the farthest time delay that can be reached with our instrument. The shape of the photoinduced absorption band and the relative long lifetime leads to conclude that the absorbing species is the final product o f ring opening process. Figure 3 8. Transient absortion spectra of photochromic oxazine after excitation at =320 nm. with 2 ps time steps. Upon excitation at = 320 nm another absorption band, which was not deteceted via na nose c ond transient abs orption spectroscopy, is observed. This band has a spectral maximum around 505 nm, and it rises and decays within 4 ps. Under these circumtances, 2 ps time Wavelength (nm) Time (ps) 350 380 410 440 470 505 540 570 -12 8 28 48 68 88 108 128 148 -0.01 0 0.01 0.02 0.03 OD PIA I PIA II

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65 resolution is not good enough to characterize the band in detail. Therefore, transient absorption sp ectra with higher time step resolution is collected and presented in Figure 3 9. This previously unknown band rises within the IRF (250 fs, Chapter 2) and decays within 2 ps. Note that this time period is shorter than the rise time of the absortion band assigned to the open form of the oxazine. Accordingly, we suggest that the absorption band, peaking around 500 nm, belongs to an intermidiate species in the ring opening mechanism of the oxazine. Time resolved sensitization experiments with benzophenone in acetonitrile provide a better understanding of the mechanism and the electronic properties of the species involeved in the mechanism. Figure 3 9. Transient absortion spectra of photochromic oxazine after excitation at =320 nm Polar molecules which contain both electron donating and electron withdrawing groups suffer from charge localizations on specific subunits that constitute the molecule. These charge localizations occur according to the ionization potentials (IP) of the subunits. In Poisson et al.s study, the ionization potentials of p-nitronitroanisole, and indoline, which are the subunits of the photochromic oxazine studied in this thesis, are given as 8.9 eV, and 7.0 eV, respectively.31 Due Wavelength (nm) Time (ps) 350 380 410 440 470 505 540 570 -1 0 0.1 0.2 0.6 2 3 4 5 6 7 10 -0.01 -0.005 0 0.005 0.01 0.015 0.02 0.025 OD

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66 to the difference in IPs of the subunits, charge separation on the excited state can occur throughout the molecule. In this case, the localization of positive charge on indoline subunit, and the negative charge would reside on anisole part of the molecule. Raymo and coworkers collected the electrochemical absorption spectrum of a model indoline after oxidation to form the radical cation (Figure 3 10).128 Transient absorption spectrum of oxazine recorded at 800 fs shows the photoinduced absorption band 505 nm and matches w ell with the steady state absorption of the radical cation (Figure 3 -11). This matching can be evaluated as the formation of the charge separated species in a few hundred femtoseconds after the excitation. Figure 3 10. Chemical structure of A) neutr al B) radical cation form of the model indoline. Additionally, the transient absorption spectra of the oxazine exhibit a solvent effect which supports the idea of the formation of a charge separated state. Transient absorption spectra of the oxazine in ace tonitrile and hexane each recorded at 800 fs and compared in Figure 3 12. In hexane, the peak of the absorption band has shifted to 515 nm while it was observed at 505 nm in acetonitrile. Hupl et al. reported that very polar excited species are stabilized in polar solvents due to positive solvatochromism and observed red shifts in fluorescence spectra of spiroindolines and their merocyanine with inc reasing solvent polarity.69,70 As we mentioned before, this red shif t follows the stabilization of the emissive excited state. This shift to an excited state with lower energy is responsible for a blue shift on excited state excitation energies. Therefore, it is reasonable to observe a red shift in transient absorption spe ctra with decreasing solvent polarity if the absorption band obsreved at 800 fs belongs to a charge separated state. N O C H3C H3C H3 N+ O C H3C H3C H3 A B

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67 Figure 3 11.( ) Transient absorption spectrum of oxazine recorded at 800 fs in acetonitrile after excitation at =320 nm. ( ) Steady state absorption of oxidized indoline in acetonitile. Figure 3 12.Transient absorption spectrum of oxazine recorded at 800 ps in acetonitrile ( ) and in hexane ( ) after excitation at =320 nm. It was mention ed before that transient absorption experiments are performed in order to figure out if there is a triplet state involved in ring opening mechanism of the oxazine or not. For this purpose, the solutions with different ratios of benzophenone to oxazine in a cetonitrile were 0.00 0.01 0.02 0.03 350 400 450 500 550 600 0.1 0.2 0.3 Wavelength (nm)Steady State Absorption A 450 460 470 480 490 500 510 520 530 540 550 0.2 0.4 0.6 0.8 1.0 Normalized AWavelength (n m)

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68 created. In the same way, oxazine with various concentrations has been added to the benzophenone in acetonitrile. Sensitization Experiments Transient absorption spectrum of 0.05 M benzopheneone is presented in Figure 3 13 part A. In the d ata, a strong photoinduced absorption (PIA) band is observed with a spectral peak around 523 nm. Close scrutiny of Figure 3 13 shows that between 015 ps there is a small blue shift of this PIA signal. Aloise et al investigated the triplet formation of ben zophenone and showed that at early times PIA from a singlet state of benzophenone is observed, followed by the creation of the triplet state. This process occurs within c.a. 15 ps and leads to the strong triplet state absorption at 523 nm. The data present ed in Figure 3 13 part A does not contain any oxazine in it. The amount of oxazine in 0.05 M benzophenone in part C is higher than in part B P art D shows the transient absorption data for 0.05 M benzophenone with highest amount of oxazine Upon the additi on of oxazine to the benzophenone solution, a PIA band with a spectral peak around 505 nm appears. As the amount of oxazine increasing in solution, the intensity of newly appeared peak increases while the intensity of PIA that belongs to the triplet state of the benzophenone decreases. On the other hand, once the triplet state of benzophenone is formed its intensity does not change anymore, which means that there is no interaction between the triplet state of the benzophenone and oxazine in solution. In ess ence, even though most of the absorption is from the BP the energy is quickly transferred to the oxazine creating the fast 505 nm band and decreasing the formation of be n zophenone triplet state. Figure 3 14 A shows the transient absorption spectrum of 1.95x104 M oxazine in acetonitrile. In the data, we observed two PIA bands. One of the bands has a spectral peak around 500 nm and it is detectable in early times. Following the decay of this band, another PIA

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69 band peaking around 440 nm appears. This band was assigned to the open form of the oxazine in Tomasulo et. al.99,100 Figure 3 13. Transient absorption spectra of A) 0.05 M benzophenone, 0.05 M benzophenone with oxazine with the ratio of oxazine/ben zophenone B) 2.6/1000, C) 4/1000, D)5.2/1000 in a cetonitrile with 1 ps time steps The change in transient absorption spectra of 1.95x104 M oxazine upon the addition of small amount of benzophenone is presented in Figure 3 14 in parts B, C, and D. Transie nt absorption intensity of PIA around 500 nm increases with the increasing amount of benzopheneone in the solution. However, a detectable change was not observed at the other PIA band of oxazine at 440 nm. Overall, from steady state emission experiments an d time resolved experiments we conclude that an energy transfer occurs from benzophenone to oxazine. Steady state and time resolved experiments show that the emission and photoinduced absortion intensity of newly formed state increases upon the addition of benzophenone in the oxazine solutions and this state Wavelength(nm) Time(ps) 400 450 500 550 600 0 10 20 30 40 50 Wavelength(nm) Time(ps) 400 450 500 550 600 0 10 20 30 40 50 Wavelength(nm) Time(ps) 400 450 500 550 600 0 10 20 30 40 50 Wavelength(nm) Time(ps) 400 450 500 550 600 0 10 20 30 40 50 0 0.005 0.01 0.015 0.02 PIA PIA I PIA II PIA I PIA II PIA I PIA II OD B D A C

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70 is the one primarily formed when oxazine is added to benzophenone. Due to the energetic of electronic states in each molecule, we inferred that the triplet state of benzophenone was not involved in the energy transfer mechanism and that the energy transfer occurred from the precursor singlet state of the benzophenone to either singlet state or directly to the charged separated state of the oxazine. Figure 3 14. T ransient absorption spectr a of A ) 1.95x104 M oxazine, 1.95x104 M oxazine with benzophenone with the ratio of benzophenone/oxazine B) 2/100 C) 4/100 D) 6/100 in acetonitrile with 1 ps resolution. Decomposition of Transient Absorption Spectra In time resolved ex periments, the charg e separated state and the open form of the oxazine have overlapping regions in their absorption spectra although the bands have different kinetics. This can be appreciated in Figure 3 15. It shows the transient absorption at 436 nm where both states contr ibute to the signal. An initial peak forms and decays within a few picoseconds and a second component rises after the decay of that peak. T he total transient absorption data needs to Wavelength(nm) Time(ps) 400 450 500 550 600 -1 10 20 30 40 50 Wavelength(nm) Time(ps) 400 450 500 550 600 0 10 20 30 40 50 Wavelength(nm) Time(ps) 400 450 500 550 600 -1 10 20 30 40 50 Wavelength(nm) Time(ps) 400 450 500 550 600 0 10 20 30 40 50 0 0.005 0.01 0.015 0.02 0.025 0.03 PIA I PIA I OD PIA II PIA I PIA II PIA II PIA II PIA I B D A C

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71 be analyzed in a way that allows uncovering the individual dynamics of each state present in the system. Figure 3-15. Transient absorption of photochromic oxazine at 436 nm. It is not obvious in the total transient absorption spectrum th at the charge separated state is a precursor of the open form or whether it is i nvolved in a mechanism that competes with the formation of open form of the oxazine. Additionall y, some other states which are not detectable in the total transient absorption spectrum might be formed during the ring opening process of the oxazine. Analysis methods which can provide inform ation about kinetics of individual states and allow the construction of a kinetic model are need. Singular Value Decomposition (SVD), Evolving Factor Analysis (EFA), and Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) were used as analysis methods in order to get the require d information for the construction of a kinetic model. Data Analysis Results The SVD analysis of the transient absorp tion data collected between 380 and 630 nm within 50 ps after the excitation of oxazine in acetonitrile at = 320 nm (Figure 3-14 A) gives two significant components that can explain the complete set of whole transient absorption data -1001020304050 0.000 0.005 0.010 0.015 ATime (ps)

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72 400 450 500 550 600 0 Wavelength (nm) 400 450 500 550 600 0 Wavelength (nm) 0 10 20 30 40 0 Time (ps) 0 10 20 30 40 0 Time (ps) (Figure 3 16). The magnitude of first two singular values is distinguished from the third an d following singular values whose magnitudes are close to zero. Figure 3 16. Singular values, result of SVD analysis. Figure 3 17 presents the first two spectral and temporal components of the oxazi ne in acetonitrile. Although these two SVD comp onents reconstruct the data very well they do not Figure 3 17.Spectral (A1, and A2) and temporal (B1, and B2) components of 1.95x10 4 M oxazine in acetonitrile, result of SVD analysis. 0 10 20 30 40 50 60 0 0.5 1 1.5 2 Number of the Singular Values Amplitude of the Singular Values 1.7292 0.1858 A1 A2 B1 B2

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73 have a chemical or p hysical meaning. C omponents of the sy stem with physical meaning (population, absorption states) need to be determined by using MCR -ALS which allows the rotation of these components, generating a new set of spectral and temporal components. Before applying the MCR -ALS algorithm, EFA can show how the components of the system evolve in time. This method provides an initial guess for the MCR -ALS. Results of EFA method confirm that two significant components contribute to the transient absorption spectra (Figure 3 18). T he doted line on the Figure 3 18 is plac ed to set the noise level. Components contributing above that noise level are considered as the principle components. The temporal response of these two principle components a re provided in Figure 3 19. They are used as initial guesses to dete rmine the temporal and spectral profiles of the components using MCR -ALS. Figure 3 18. ( ) Results of forward EFA, ( ) Results of backward EFA of 1.95x10 4 M oxazine in acetonitrile arising from a charge separated state (Figure 3 20 A1). The temporal component corresponding to the 440 nm absorption band rises during 50 ps (Figure 3 20 B2), while the other component with absorption band around 500 nm rises within the instrument response function of ~250 fs and decays very fast and goes t o zero in 50 ps (Figure 3 20 B1). 5 10 15 20 25 30 35 40 45 50 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 log(eigenvalues) Time (ps)

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74 0 0.01 0.02 400 450 500 550 600 0 0.01 0.02 Wavelength (nm) 0 0.5 1 -10 0 10 20 30 40 50 0 0.5 1 Time (ps) Figure 3 19. The result from EFA used as initial guess for temporal profiles of 1.95x104 M oxazine in acetonitrile The MCR -ALS method gives two spe ctral components: one has an absorption band peaked around 440 nm which is assigned to the absorption of the open f orm of the oxazine ( Figure 3 20 A2) ,the other one has an absorption peak around 500 nm which we assigned as the absorption Figure 3 20. Spectral (A1 -A2), and temporal components (B1 B2) of 1.95x104 M oxazine in acetonitrile, result of MCR -ALS analysis. A1 A2 B1 B2 10 20 30 40 50 0 0.05 0.1 0.15 0.2 Time (ps)

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75 We re constructed the transient absorption data evaluating TCS D (SeeChapter 2) from the MCR -ALS com ponents presented in Figure 3 20. Comparison of the experimental and reconstruc ted data is shown in Figure 3 21. The goodness of the optimization process is ev aluated using the percent of lack of fit (LOF). The difference between the Figure 3 21.Experimental ( ), reconstructed ( ) transient absorp tion data, and difference between experimental and reconstructed transient absorption data ( ) of 1.95x10 4 M oxazine in acetonitrile. experimental data and the reconstructed data is also a measure of how well the experimental data is expla ined by t he MCR -ALS analysis Data collected after the addition of benzophenone at various concentrations to a solution of oxazine in acetonitrile was also analyzed (SVD and EFA results can be seen in Appendix B ). For 400 450 500 550 600 0 0.01 0.02 0.03 at 1 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 15 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 35 ps 400 450 500 550 600 0 0.01 0.02 0.03 Wavelength(nm) at 50 ps 0 10 20 30 40 0 0.01 0.02 0.03 at 400 nm 0 10 20 30 40 0 0.01 0.02 0.03 at 440 nm 0 10 20 30 40 0 0.01 0.02 0.03 at 505 nm -10 0 10 20 30 40 50 0 0.01 0.02 0.03 Time(ps) at 550 nm

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76 0 10 20 30 40 0 0.2 0.4 0.6 0.8 1 -10 0 10 20 30 40 50 0 0.2 0.4 0.6 0.8 1 Time (ps) 400 450 500 550 600 0 0.01 0.02 0.03 0.04 400 450 500 550 600 0 0.005 0.01 0.015 Wavelength (nm) each ratio of concentrations, the data can be explained with two spectral/temporal components similar to the components found for the solution of pure oxazine in acetonitrile. The main difference observed in the analysis of each mixture is that the absorption band at around 500 nm increases with increased amount of benzophenone in the mixture. There is no change observed in the temporal components ass ociated with each spectral band (Figure 3 22). Figure 3 22. A1 -A2) Spectral componets of 1.95x10 4 M oxazine ( ), 1.95x104 M oxazine a nd benzophenone with the ratio of benzophenone/oxazine 2/100 ( ), 4/100 ( ) 6/100 ( ) and B1 B2) corresponding temporal components for each solution The spectral and temporal components obtained from the analysis of each data solution are used to reconstruct the data in each case. (See Figure 3 23, Figure 324 and Figure 3 25) The residuals show no structure, an indication of good match between experiment and analysis. Additionally, the LOF values are calculated as 1.21 %, for 1.95x104 M oxazine in acetonitrile and1.85 %, 1.60%, 2.74% for the solutions of 1.95x104 M oxazine and benzophenone with the ratio of benzophenone/oxazine 2/100, 4/100, 6/100 respectively. Even though benzophenone is

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77 400 450 500 550 600 0 0.01 0.02 0.03 at 1 ps 0 10 20 30 40 0 0.01 0.02 0.03 at 400 nm 400 450 500 550 600 0 0.01 0.02 0.03 at 15 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 35 ps 400 450 500 550 600 0 0.01 0.02 0.03 Wavelength(nm) at 50 ps 0 10 20 30 40 0 0.01 0.02 0.03 at 440 nm 0 10 20 30 40 0 0.01 0.02 0.03 at 505 nm -10 0 10 20 30 40 50 0 0.01 0.02 0.03 Time(ps) at 550 nm added to the oxa zine solution, the observed compo nents belong to the oxazine, and none of the benzophenone components are observed in the analysis of data. Figure 3 23.Experimental ( ), reconstructed ( ) transient absorption data, and difference between experimental and recons tructed transient absorption data ( ) of 1.95x104 M oxazine and benzophenone with the ratio of 2/100 in acetonitrile. As benzophenone is added, this relative contribution from the band peaked around 500 nm increases meaning that sensitization from benzophenone occurs either directly to the charge separated state or to a state that is the precursor of this charge separated state. On the other hand, since the absorption band centered at 440 nm is not affected by the presence of benzophenone, we conclu de that the charge separated state (or the precursor of the charge separated state) is not a precursor in the mechanism of ring opening in this oxazine.

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78 400 450 500 550 600 0 0.01 0.02 0.03 at 1 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 15 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 35 ps 400 450 500 550 600 0 0.01 0.02 0.03 Wavelength(nm) at 50 ps 0 10 20 30 40 0 0.01 0.02 0.03 at 400 nm 0 10 20 30 40 0 0.01 0.02 0.03 at 440 nm 0 10 20 30 40 0 0.01 0.02 0.03 at 505 nm -10 0 10 20 30 40 50 0 0.01 0.02 0.03 Time(ps) at 550 nm Figure 3 24.Experimental ( ), reconstructed ( ) transient absorption data, and difference between experimental and reconstructed transient absorption data ( ) of 1.95x104 M oxazine and benzophenone with the ratio of 4/100 in acetonitrile. In order to understand which state of benzophenone is being quenched in the presenc e of the oxazine, we measured the transient absorption of mixtures of various concentrations of oxazine and 0.05 M of benzophenone in acetonitrile. The experi mental data is analyzed using the methods mentioned before and results are compared to the pure ben zophenone data which is collected under the same experimental conditions. While I was working on transient absorption experiments of benzophenone, Aloise et. al.89 published a report investigating the intersystem crossing in benzophenone. In the agreement with their results, after the excitation at 320 nm we observe three active electronic states (See

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79 400 450 500 550 600 0 0.02 0.04 at 1 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 15 ps 400 450 500 550 600 0 0.01 0.02 0.03 at 35 ps 400 450 500 550 600 0 0.01 0.02 0.03 Wavelength(nm) at 50 ps 0 10 20 30 40 0 0.02 0.04 at 400 nm 0 10 20 30 40 0 0.01 0.02 0.03 at 440 nm 0 10 20 30 40 0 0.02 0.04 at 505 nm -10 0 10 20 30 40 50 0 0.01 0.02 0.03 Time(ps) at 550 nm Figure 3 13 A).The first singlet state (S1) of benzophenone is populated right after the excitation. Decay of S1 state (~6.5 ps) leads the formation of an intermediate state (IS). This process is followed by the formation of the triplet state of the molecule in ~10 ps. Spectral and temporal signatures of these states (p rovided by the SVD, EFA, and MCR -ALS analysis) are presented in Figure 3 26. The SVD components before the application of MCR -ALS can be seen in Appendix B. Figure 3 25.Experimental ( ), reconstructed ( ) transient absorption dat a, and difference between experimental and reconstructed transient absorption data ( ) of 1.95x104 M oxazine and benzophenone with the ratio of 6/100 in acetonitrile. After oxazine is added to the benzophenone solution an additional absorption band appears around 500 nm (See Figure 3 13 B, C, D), similar to the absorption band of the charge separated state of the oxazine. However, the absorption band around 440 nm corresponding to the open

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80 400 450 500 550 600 0 0.005 0.01 -10 0 10 20 30 40 50 0 0.5 1 400 450 500 550 600 0 0.005 0.01 0.015 -10 0 10 20 30 40 50 0 0.5 1 400 450 500 550 600 0 0.01 0.02 Wavelength (nm) -10 0 10 20 30 40 50 0 0.5 1 Time (ps) form of the oxazine was not detected. Additionally, the first excited singlet state of the benzophenone was not detectable in the transient absorption of the mixtures since it was quenched in the presence oxazine. Moreover, absorptio n intensity of triplet state of benzophenone decreased with the increasing amount of oxazine in solution, while its kin etic curve is not changed. ( rise time of the triplet state is ~10 ps) (Figure 3 26). Figure 3 26. A1 -A3 ) Spectral B1 B3 ) temporal components from transient absorption data of benzophenone in acetoni trile. A1and B1 belong to S1, A2 and B2 belong to an intermediate, and A3 and B3 belong to T1. When the SVD, EFA, MCR -ALS methods are used for the analysis of the transient absorpti on data of the benzopheone / oxazine mixtures, a component with an absorpti on peak around 500 nm appears. This component rises and decays within 2 ps. Since this component is not present in the benzophenone data we assign it to the oxazine. When the concentration of the 400 450 500 550 600 0 0.005 0.01 0.015 0.02 Wavelength (nm) A1 A2 A3 B1 B2 B3

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81 oxazine is increased in the mixture solution, the contributi on comes from this new band increases (Figure 3 27) and the contributions of benzophenone components decrease The relative contributions of each component (singular values) are presented in Table 3 -2 Figure 3 27. Increase of the temporal component appeared upon the addition of oxazine into benzophenone, 0.05 M benzophenone and oxazine with the ratio of oxazine/benzophenone = 2.6/1000 ( ), 4/1000 ( ), and 5.2/1000 ( ) in acetonitrile. Table 3.2 Singular values of the electr onic states of benzophenone in pure benzophenone and benzophenone and oxazine mixture solution. This new band is similar in spectral and temporal response to the ba nd observed for the charge separated state in oxazine. However, that new band is slightly red shifted and narrower com pared to the component attributed to the charge separat ed state of oxazine (Figure 3 28). Mixtures Singular Values of T 1 S 1 IS 0.05 M benzophenone 1.2882 0.1045 0.0429 0.05 M benzophenone and oxazine with oxazine/benzophenone=2.6/1000 0.9911 0.0435 0.0341 0.05 M benzophenone and oxazine with oxazine/benzophenone=4/1000 0.8753 0.0391 0.0339 0.05 M benzophenone and oxazine with oxazine/benzophenone=5.2/1000 0.5268 0.0367 0.0010 400 450 500 550 600 0 0.005 0.01 0.015 0.02 Wavelength (nm)

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82 -10 0 10 20 30 40 50 0 0.2 0.4 0.6 0.8 1 Time (ps) 400 450 500 550 600 0 0.2 0.4 0.6 0.8 1 Wavelength (nm) These differences can be explained with the loca lization of the transferred energy on the nitroanisole part of the oxazine. Figure 3 28.Comparis on of spectral and temporal component for the charge separated state absorption band in pure oxazine ( ) and mixture of 0.05 M benzophenone wit h 2.6x104M oxazine ( ) in acetonitrile. Kinetic Model Following excitation at nm, the steady state spectra of oxazine shows small emission from an excited state sensitive to solvent polarity. The shift observed in the emission spectra in solvents with different polarities lead us to propose that the emission arises from a cha rge separated state. What is more, comparison of absorption spectrum of the o xidized model indoline with the transient absorption of the oxazine at 800 fs supports the idea about the formation of a charge separated state, with excited state absorption maxi mum at ca. 500 nm. In the transient absorption experiments, in addition to the band assigned to the charge se parated state, the band attributed to the open form of the oxazine is also observed. According to the results of time resolved experiments, the ch arge separated state forms within the instrument response function (~250 fs) and initially it decays fast ( decays with 2.2 ps and 3.7 ps

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83 time constants) and then it goes to zero within 50 ps. Open form of the molecule rises during 50 ps. What is more, in t he sensitization experiments with benzophenone we observed that the change in absorption of the charge separated incre ases with the increasing concentration of the benzophenone while the absorption of the open form of the molecule is not affected by the p resence of benzophenone Finally, the transient absorption data shows that the triplet state is not involved in the ring opening mechanism. From t he experimental results we conclude that the charge separation occurs in the excited state and it competes wi th the ring opening channel In order to better understand the mechanism, we build a model based on the experimental results T he validity of the model is checked by fitting the temporal components of the transient absorption data of oxazine in ace tonitril e For this purpose, the number of the points in the temporal components is increased by interpolation. Then the interpolated data is fitted by using a home -made Matlab fitting program which calculates the po pulations of the state by numerically integratin g the differential form of the kinetic equations. Whil e building the model (Figure 3 2 9 ), and considering the results of the sensitization experiments we assume that the charge separated state and the open form are produced directly from the first excite d singlet state (S1) in addition we assume that charge separated does not contribute to the ring opening mechanism. Although looking at the temporal components it might suggest a 2 state model, the decay of the 1st component does not correspond to the ris e of the 2nd component. T he fitting trials showed that the charge separated state contributes indirectly to the ring opening. This process involves a n intermediate state which is not observable in the transient

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84 absorption experiments, but who influences th e dynamics. Figure 3 29. Energy level scheme of the photoisomerization of oxazine. T he suggested kinetic model and the temporal components of oxazine ma t ch very well (Figure 3 30). T he rate constants provided by the fitting pro cedure are presented in Table 3 3. According to the fitting results one can conclude that that from the excited state there are two relaxation channels. The first one is the charge transfer from within the closed form of the oxazine from indoline part to t he nitroanisole part of the molecule, which occurs in the excited state as it is known for the lactone form of rhodamines.129 S econd channel is the C O bond cleavage and isomerisation to the open form of the oxazine. What is more, the ring opening process occurs mostly from the S1 state. The charge separated state contributes indirectly to the N O N O2 S CS S 0 S 0 S 1 1 X 6

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85 -10 0 10 20 30 40 50 0 0.2 0.4 0.6 0.8 1 Time (ps) ring opening mechanism, but this contribution is small compared to the r eaction with rate constant of k6. Figure 3 30. Interpolated forms of temporal components belong to the charge separated state ( o ) and open form of the o xazine ( o ) and prediction of kinetic model for charge separated state ( ), and the open form ( ). Table 3 3. Rate constants provided by the kinetic model. n k n (ps 1 ) n(ps) 1 < 0.25 2 0.27 3.7 3 0.46 2.2 4 0.18 5.5 5 0.08 12.5 6 0.16 6.3

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86 What is more, there are other kinetic models that we have tried in the process of finding the appropriate kinetic model for our system. These kinetic models and the fittings according to the m are presented in Appendix C.

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87 CHAPTER 4 ANISOTROPY OF PHENYL E NE ETHYNYLENE DENDRI MERS Lately, researchers have an interest in developing artificial photosynthetic mimics to be used as the components of the photonic devices. In the natural photosynthetic process, the solar energy is absorbed by the chlorophyll molecules at all over the organism and transferred to the reaction center. In the reaction center, light energy is converted to the chemi cal energy. In this sense, dend rimers receive the attention as prospective photosynthetic mimics due to their structural, phy sical and energetic properties.130135 Dendrimers are highly branched treelike macromolecules.130 Dendrimers are characterized by their generation (branching points) number and the branching of the end groups136 and they can be divided into three structural units as core or focal moiety, branches, and the end groups placed on the periphery. S ymmetrical dendrimers with equivalent branches and good light harvesting properties have been reported by many groups.137 141 Peng et al.142 introduced unsymmetrical dendrimers with structurally unequivalent branches. The structural differences between the dendrimers with symmet rical and unsymmetrical branching are presented in Figure 4 1. Structural properties of the unsymmetrical dendrimers (Figure 4 1) allows shortcuts in the communication between the end groups and the core while in the symmetrical dendrimers, each sub branch is involved in the communication of the periphery and the core. Addi tionally, in unsymmetrical dend rimers the nu mber of absorbing units grows faster with inc reasing generation. For these reasons, it is claimed that the dendrimers with unsymetrical branchi ng can be better candidates to serve as light harvesting antennae and energy -transferring funnels.142 In dendrimers, the efficient energy transfer from periphery to the core requires an energy gradient between the outside branches and the branches close to the center of the molecule. That

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88 energy gradient can be created with branches with increasing length from periphery to the center of the molecule.142 A B Figure 4 1. Model structures of monodendrons with A) symmetrical branches, and B) unsymmetrical branches The positions of branching affect the nature of the excitation energy transfer mechanism. When the branching occurs at meta positions it breaks the conjugation between the segments of molecule causing the localization of the excitation energy on individual segments .136,138,140,143 When the dendrimers include ortho and para branching there are more the delocalized excitations throughout the conjugated segments. It is not easy to estimate the type of energy transfer mechanism in a conjugated molecule where the excitations are delocalized133 although it is known th at the interactions between well separated segments, where the excitations localized, can occur through the Frster m echanism.139,144147 In unsymmetrical P henylethynylene (PE) dendrimers, which are introduced by Peng and coworkers, the energy gradient is achieved by the ortho and para linkages of the phenylethynylenes, leading to segments of different conjugation lengths and broad absorption spectrum.142

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89 Frchet and coworkers introduced a flexible dye sensitized dendrimer which contains poly(aryl ether) groups The dyes in the scaffold are electronically separated from each other T he absorption spe ctrum of the dendrimer shows the characteristic absorption bands of the individual dyes. They claimed that the energy transfer from periphery to the center of the dendrimer occurs via stepwise Frster mechanism with efficiency higher than 97%.139 Moore group prepared a series o f rigid phenyl ethynylene (PE ) dendrimers. In these extended dendrimers the energy gradient was created with the increasing conjugation length from the periphery to the center of the dendrimer. This energy gradient leads to a unidirectional energy transfe r toward the center of the dendrimer with the efficiency close to unity.136,138,140,148 Additionally, the energy transfer mechanisms of PE dendrimers are studied by other groups theoretically143,149,150 and experimentally.151,152 Mukamel and coworkers reported that the electron -hole pairs created during the excitation of PE dendrimers were localized within the segments connected by benz ene rings substituted at the meta position.143 The energy funneling properties of PE dendrimer called nanostar was investigated by Kleiman and coworkers with femtosecond degenerate pump -probe spectroscopy.151 In that study, they determined that the excitation energy is localized on subunits and energy transfer occurs stepwise in subpicosecond time sc ale. They claimed that Frster t heory can be used to explain the experimental energy trans fer rates qualitatively. What is more, the combined experimental and theoretical studies performed in Martinez group showed that the meta substitution in symmetrical dendrimers breaks the conjugation at ground state but not at the excited state.149,153 The report from the Goodson group supported the existence of delocalized excited states in symmetrical compact PE dendrimers .152

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90 Melinger et al.154 investigated the photophysica l properties of unsymmetrical PE monodendrons142 in solution by employing the techniques of steady state absorption and fluorescence spectroscopy, time dependent fluorescence, and ultrafast degenerate pump-probe spectroscopy. The unsymmetrical dendiritic st ructures examined exhibit broad absorption spectral range due to the different conjugation lengths of the different bra nches. They showed that the energy transfer from the PE backbone to a perylene tra p attached to the center of the dendrimer is highly e fficient (~90 %). They showed that the energy transfer can occur via a coupling through space mechanism (Frster energy t ransfer). The uns ymmetrical PE dendrimers, constituted from the ortho and para substituted PE units in various lengths, were synthesized by Peng and coworkers.142,155,156 The intramolecular interactions and the ene rgy transfer mechanism in these unsymmetrical PE dendrimers have been investigated by the Kleiman Group.157 They employed steady state, ultrafast fluorescence, an d transient absorption spectroscopy in order to the determine extent of delocalization within the dendrimer. They observed that in unsymmetrical PE dendrimers the initial excitation is delocalized through the molecule Within~400 fs after the excitation, s ome degree of localization is observed. When an ethynylene perylene (EPer) trap is added to the system, the excitation energy is transferred to the trap by a direct and a stepwise mechanism in a subpicosecond time scale. The conjugation length of the segm ents determines the extent of localization on the segments and thus the number states involved in the energy transfer mechanism. Fluorescence anisotropy is a powerful method to investigate the interactions between segments and excitation energy transfer me chanism in dendritic structures.

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91 Anisotropy measures the relative changes in the orientation of the absorption and the emission transition moments with respect to each other (See Chapter 2 for a detailed description) The transition moments for absorption and emission lie along the specific directions within a fluorophore. Fluorophores absorb the photons which has an electric vector that is oriented parallel to the transition moments. Emitted light also has an orientation axis in fluorophore. T he maximum measured anisotropy depends on t he relative angle between the absorption and emission transition moments.63 The measured anisotropies may be lower than the maximum theoretical values. Energy transfer between the fluorophores is a factor that causes the decrease in anisotropy values. Thus, fluorescence anisotropy contributes to di scover the excitation energy transfer. For the mole cules which follow the Kashas r ule,57 the aniso tropy does not change with the emission wavelength since they emit from the first excited state regardless of the excitation wavelength. On the other hand, the fundamental anisotropy changes with the excitation wavelength since the absorption dipole moment might be oriented differently at particular excitation wavelength s causing a change in the relative orientations of the transition moments .58,63 In this work we try to elucidate the presence of multiple electro nic states in unsymmetrical phenylene ethynylene (PE) dendrimers called 2G1-m OH, 2G2-m OH, and 2G2-m -per by inve stigating the different orientation of the absorption and emission transition dipole moments via anisotropy experiments at 298 K and 77 K. We characterize the absorbing and emitting state of the unsymmetrical PE dendrimers according to changes observed at the anisotropy values and differences in the excitation, and emission spectra when temperature is decreased from 298 K to 77K. The details of the anisotropy measurements are given in Chapter2.

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92 The dendrimers used here, 2G1-m OH, 2G2m -OH, and 2G2-m per (Fi gure 4 2 ), were synthesized by Peng and coworkers.142,155,156 In the notation which is used to name the dendrimers, the letter G means generation, the number before the letter G displays the number of the arms a round the focal point and the subscript shows the number of the generations T he letter m represents the position where the functional group is substituted to the phenyl ring at the focal point of the dendrons. For example, 2G1-m -OH mean s that two of the first generation phenylethynelene dendritic structures are attached to a phenyl ring which has a meta OH substitution. Figure 4 2 Structures of PE dendrimers. A) 2G1-m OH, B) 2G2-m OH, C) 2G2-m -per. In all experiments, samples were dissolv ed in 2 -CH3THF and the optical densities were kept below 0.1 in order to prevent any aggregation158 or excimer formation.159,160 Absorption spectra of the sa mples were recorded on a Varian Cary 100 spectrophotometer while the emission and excitation spectra were measured with a Jobin Yvon instrument (Fluorolog3). A O H H3CO OCH3 O H H3CO OCH3OCH3H3CO OCH3H3CO H3CO OCH3OCH3H3CO OCH3H3CO A B C

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93 liquid nitrogen flow cryostat (Oxford instruments) was used in low temperature experiments (77K ). 2G1m -OH Steady state excitation spectra of 2G1-m OH at 298 K and 77 K were collected between 300 nm and 400 nm after setting the detection wavelength to 420 nm. Emission spectrum was recorded in the spectral region from 350 nm to 550 nm upon the excit ation at 302 nm and 310 nm at 298 K and 77 K, respectively. The spectra collected at 77 K have sharper bands instead of broad shoulders compared to the spectra colle cted at 298 K Additionally, hidden bands under the broad excitation and emission bands col lected at room temperature were uncovered with the low temp erature measurements (Figure 4 3 ). Figure 4 3 Excitation and emission spectra of 2G1 -m OH at A) 298 K, B) 77 K. 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emission Normalized Excitation 300 350 400 450 500 550 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Excitation Normalized Emission Wavelength (nm) A B

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94 At room temperature, phenyl rings are free to rotate. The rotation of the phenyl rings leads to formation of the segments with different c onjugated lengths. The presence of segments with different conjugation lengths cause the inhomogenous broadeni ng in the excitation and the emission spectra. On the other hand, when the temperature decreases from 298 K to 77 K the rotation of the phenyl rings are limited. Thus, the segments probably become flatter at 77 K decrease the inhomogenity in the excitation a nd the emission spectra at that temperature. The fluorescence bands of 2G1-m-OH observed at 77K resembles to the main excitation bands at longer wavelengths where the branches with longer conjugation lengths absorb. This means that the emission comes mainly from the segments with the longest conjugation length. This observation suggests that if the molecule is excited even with s horter wavelengths, where the segments with shorter conj ugation lengths absor b, the excitation is transferred to the segments with longer conjugation lengths. The excitation anisotropy of 2G1-m-OH detected at 420 nm at room temperature is measured between 300 nm and 405 nm. The overa ll anisotropy values are under 0.1 and they monotonically increase from 0.01 to 0.07 throughout the excitation bandwidth (Figure 4-4). Figure 4-4. Excitation spectrum ( ), and excitation anisotropy ( ) of 2G1-m-OH at 298 K. 0.0 0.5 1.0 300320340360380400 0.0 0.2 0.4 Excitation AnsiotropyWavelength (nm)

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95 At room temperature, thermal energy allows for the formation of the segments with a broad distribution of the conjugati on lengths and these segments w ill have different orientations. Anisotropy values close to zero confirm that th e excitation and the emi ssion transition moments have different orientations. Anisotropy values at longer excitation wave lengths are relatively higher than at shorter wavelength s suggesting energy tran sfer from short conjugated segments to longer conjugated segments. The excitation anisotropy of 2G1-m-OH at 77 K is presented in Figure 4-5. In contrast to room temperature results, at low temperature th ere are four distinct regions with different anisotropy values. Anisotropy values increase from 0.02 to 0.1 between 300 nm and 340 nm. In the region from 340 nm to 375 nm a relatively constant anisotropy values around 0.1 were observed. These values are incr eased to 0.25 in the spectral region between 375 nm and 400 nm while there is a sharp decrease after 400 nm wh ere the excitation dimi nishes. The excitation spectrum at low temperature is also shaded with different colors corr esponding to the spectral regions with different anisotropy values. Figure 4-5. Excitation spectrum ( ), and excitation anisotro py ( ) of 2G1-m-OH at 77 K. 300320340360380400 0.0 0.2 0.4 0.0 0.5 1.0 Anisotropy Wavelength (nm) Excitation

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96 This complex behavior of the excitation anisotropy shows that there is more than one electronic state contributing to the total anisotropy. At room temperature, the phenyl rings can rotate freely aroun d ethynylene bonds causing the delocalization of the excitations th roughout the longer segments On the other hand, these rotations are limited at low temperature that results in the localization of excitations on specific segments. At low temperature, the anisotropy values in the region from 300 nm to 340 nm increases monotonically showing that there are at least two states contributing to the anisotropy in that region ( shaded with two colors). The constant anisotropy values in th e spectral region from 340 nm and 375 nm is because the anisotropy comes from a single state (the spectral region shaded with single color). The spectral region with increasing anisotropy values from 375 nm to 390 nm shows again the contribution from diffe rent states (shaded as intersection of two states). In the region 390 nm the molecule shows a constant anisotropy value of 0.25 since it is the contribution of only one state (blue shaded area). The gradually increased anisotropies, from 0.02 to 0.25 between 300 nm 400 nm, show that the energy is transferred from the conjugated segments that absorb at shorter wavelengths to the ones with longer conjugation length and absorbing at longer wavelengths. The overall anisotropy (0.02 0.25) is smaller than the fundamental anisotropy value of 0.4, what is observed when the exc itation and emission transition moments have the same orientations, suggesting that the excitation and emission transition moments of 2G1-m OH have different orientations. What is more, the sharp increase after 400 nm is observed due to the emission which comes from the segments with longest conjugation lengths.

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97 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emission Normalized Excitation 300 350 400 450 500 5500.0 0.2 0.4 0.6 0.8 1.0 Wavelength (nm) Normalized Emission Normalized Excitation 2G2m -OH The room temperature and low temperature excitation spectra of 2G2-m OH were colle cted after setting the detection wavelength to 450 nm. Emission spectrum was recorded upon the excitation at 320 nm, and at 375 nm at room temperature, and at 77 K respectively. The emission and excitation spe ctra are presented in Figure 4 6 In the low temperature excitation spectrum of 2G2-m OH, we observe new sharp bands hidden under the broad excitation ba nd at room temperature Additionally, t he emission peaks beco mes sharper and present better defined vibronic structure at 77 K compared to the spectral peaks observed at room temperature. Figure 4 6 Excitation and emission spectra of 2G2-m O H at A) 298 K, B) 77 K. 2G2-m OH has a broader excitation spectrum than 2G1-m OH since in unsymmetrical dendritic structures the ortho linkages prevent the phenyl rings to have a planar geometry. In this way, segments with various conju gation lengths are c reated causing the inhomogenous B A

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98 broadening in excitation spectra. Additionally, the conjugation lengths increase with the increasing generation due to the para linkages which result in long conjugated segments Longer conjugated segments shift the spectrum of 2G2-m OH co mpared to the red compared to 2G1-m OH. On the other hand, at low temperature the featured bands suggest localization of excitation to some extent on different conjugated segments. What is more, the shape of the emission spectrum at low tem perature resembles the excitation spectrum at longer wavelengths supporting that the emission comes from the segments with longer conjugation lengths which absorb at longer wavelengths suggesting the energy transfer from shorter conjugated segments. Thes e observations are consistent with the results of experiments performed with 2G1-m OH A red shift in the emission spectrum of 2G2 -m OH at 77 K compare d to the emis sion spectrum at 298 K was observed. This red shift shows that a more planar geometry was a d o pted by the phenyl rings at low temperature. The anisotropy values of 2G2-m OH at room temperature, collected setting the detection wavelength to 4 50 nm is presented in Figure 4 7 It shows anisotropy values very close to zero (0.010.07) throughout the e xcitation wavelengths. We can evaluate the low anisotropy values as a result of multiple energy transfer steps between segments with different orientations. At low temperature (77 K), the anisotropy vs. wavelength plot (Figure 4 8 ) exhibits three different spectral regions (roughly defined) with different anisotropy values. The region between 300 nm and 380 nm has constant anisotropy value around 0.01. The constant anisotropy values suggest that these anisotropy values are only from one state (shaded in magenta). The monotonic increase of the anisotropy from 0.01 to 0.04 in the spectral region between 380 and 420 nm shows that there are at least two electronic states contributing to the anisotropy of the system.

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99 The spectral region corresponding to this anisotropic beha vior marked as the intersection of two regions belongs to different electronic states which are represented with different colors (magenta and blue). After 420 nm, the anisotropy values show a sharp increase to 0.1 within the following 10 nm. Additionally, a sharp decrease wa s observed in the anis otropy values after 430 nm. Therefore we concluded that the anisotropy value at that region comes from single state (spectral region shaded in blue). Figure 4-7. Excitation spectrum ( ), and excitation anisotropy ( ) of 2G2-m-OH at 298 K. At room temperature the thermal energy allows the free rotation of the phenyl rings leading to the conjugated segments in various lengths. At low temperature, free rotation of the phenyl rings is limited causing the formation of longe r conjugated segments. The shorter wavelengths show lower anisotropy values compared to the ones at longer wavelengt hs supporting the idea of the energy transfer from the segments with s horter conjugation lengths to the segments which shows longer conjugations. The overa ll anisotropy values are lowe r than 0.4 which is supposed to be observed when the excitation and the emissi on transition moments are oriented in parallel. In this case, lower anisotropy values shows that the excitation and emission transition moments have different orientations. 300320340360380400420440 0.0 0.2 0.4 0.0 0.5 1.0 AnisotropyWavelength (nm) Excitation

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100 Figure 4-8. Excitation spectrum ( ), and excitation anisotropy ( ) of 2G2-m-OH at 77 K. 2G2-m-per The excitation (detected at 550 nm) and emission spectra (after excitation at 325 nm) of 2G2-m-per are presented in Figure 4-9. Comparing the Figure 4-6 and Figure 4-9, I can conclude that 2G2-m-OH and 2G2-m-per have similar excitation spectra except the spectral region where wavelengths are greater than 425 nm. The excitation spectrum of 2G2-m-per from =300 nm to =425 nm is assigned to the backbone of the molecu le. The rest of the spectrum belongs to the ethynelene perylene (EPer) part of the molecu le, which is used as an energy acceptor here.157 Thus, the excitati on spectrum of 2G2-m-per which contains the characteristics of both 2G2-m-OH and EPer suggest weak coupling in ground st ate between dendritic backbone and EPer.157 At 77 K (Figure 4-9 B), the sp ectral region assigned to the excitation spectrum of the backbone presents additional, sharper transitions compared to the same spectral region at room temperature. The excitation spectrum in the region where ethynylene peryle ne absorbs is similar to the spectrum at room temperat ure in terms of number of the peaks although the vibronic bands become sharper at low temperature. Overall, as temperature is lowered the excitation and the emission spectra are red shifted. 300320340360380400420440 0.0 0.2 0.4 0.0 0.5 1.0 Anisotropy Wavelength (nm) Excitation

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101 Figure 4 9 Excitation and emission spectra of 2G2 -m OH at A) 298 K, B) 77 K. The differences between the excitation spectrum of 2G2-m -per at 77 K and 298 K originates from different lengths of the conjugated segments at those temperatures. The results are similar to ones observed for 2G1-m OH and 2G2-m OH. At room temperature, phenyl rings are oriented in any direction while the EPer trap is almost flat. Thus, the orientation of the phenyl rings at various directions varies the conjugation lengths In this case, a broad featureless excitation is observed instead of having sharp and featured peaks due to the longer and more homogenously distributed conjugation lengths as in case of 77 K. Figure 4 10 shows the excitation anisotropy of 2G2-m -per detected at 550 nm (EPer emission) at room temperature. Two regions with distinc tive anisotropies are observed in the anisotropy vs. wavelength plot. The wavelengths corresponding to excitation of the backbo ne ( nm) exhibit anisotropy values around zero while the excitation in the region where EPer 0.0 0.2 0.4 0.6 0.8 1.0 Normalized Emission Normalized Excitation 300 350 400 450 500 550 0.0 0.2 0.4 0.6 0.8 1.0 Wavelength (nm) Normalized Emission Normalized Excitation A B

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102 can be excited directly ( nm) yields an anisotropy value of ~ 0.1. Excitations of perylene at wavelengths above 360 nm cause S0 S1 transition. In this spectra l region it shows constant and positive anisotropy.161 The low anisotropy values at short wavelength s (298 K) suggest that the excitations are depolarized due to the energy tr ansfer process between the segm ents with different conjugation lengths which have orientati on of the excitation and emissi on transition dipoles. The higher anisotropy values at longer wavelengths were obs erved due to the direct excitation of the EPer having relatively similarly oriented exc itation and emission transition moments. Figure 4-10. Excitation spectrum ( ), and excitation anisotropy ( ) of 2G2-m-per at 298 K. The results for low temperature (77 K) are presented in Figure 4-11. The low temperature excitation anisotropy has also tw o distinctive spectral regions. In the spectral region from 300 nm to 425nm lower anisotropy values (0.06) were observed compared to the anisotropies (0.25) at wavelengths longer than 425 nm. The lower anisotropies were observed due to the energy transfer from backbone to the EPer. Since there is an ethynyl group between ba ckbone and perylene, th e relative orientation between the two moieties can have different values and following energy transfer the 300325350375400425450475500 0.0 0.2 0.4 0.0 0.5 1.0 AnisotropyWavelength (nm) Excitation

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103 polarization is decreased The higher anisotropy values at the wavelengths longer than =425 nm is because at these wavelengths the perylene can be excited directly and there is no energy transfer from EPer suggesting that the excitation energy transfer is unidirectional and it occurs only from backbone to EPer. Figure 4 11. Exci tation spectrum ( ), and excitation anisotropy ( ) of 2G2-m -per at 77 K. Conclusion The phenyl rings which are free to rotate around the ethynylne bonds create the segments with various conjugation lengths. The variability in the conjugation leng ths of the segments makes the excitation spectra of 2G1-m OH, 2G2-m OH, and 2G2-m -per broad at room temperature E xcitations are delocalized along these long conjugated segments at that temperature On the other hand, the bands hidden under the broad excita tion bands at room tem perature were uncovered at 77 K. In other words, the inhomogeneity of the spectrum is broken since conjugated segments with similar lengths are produced due to the limted rotation of the phenyl rings at that temperature. The results of the anisotropy experiments support the conclusions we reached with the steady state excitation and the emission experiment s. The anisotropy measurements for 2G1-m 300 325 350 375 400 425 450 475 500 0.0 0.2 0.4 Anisotropy Wavelength (nm)0.0 0.5 1.0 Excitation

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104 OH, 2G2-m OH at room temperature confirm the delocalization of the excitations along the c onjugated segments. At 298 K, 2G2-m -per show very low anisotropy values at the excitation wavelengths where the backbone has absorption and higher anisotropies at the wavelengths where EPer can be directly excited. These two spectral regions with different anisotropy values suggest that the backbone and the EPer are weakly coupled and there are two states that contribute the total anisotropy of the system. Additionall y, very low anisotropy values obtained after backbone excitation are due to strong coupling between the branches within the backbone and energy transfer from backbone to the EPer acceptor. At 77 K, the complex anisotropic behavior of the molecules, 2G1-m -OH, 2G2-m OH, and 2G2-m -per, shows that the excitations are lo calized on segments with longe r conjugation lengths. It also suggests that there is more than one state involved in the energy transfer mechanism for each molecule. Overall, the low anisotropy values at shorter wavelengths and relatively higher anisotropies at longer wavelengths show that an energy transfer occurs from the segments with shorter conjugation lengths to the segments with longer conjugations with excitation and emission transition moments have different orientations.

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105 CHAPTER 5 CONCLUSION AND PERSP ECTIVE Since Hirsberg8 proposed a chemical memory model based on photochromic properties of a spiropyran in 19 56, a vast number of studies were performed searching the photochromic properties of the organic compounds. As a r esult of these studies the number of the potential applications of photochromic molecules has extended. For successful implementation of most of applications, properties such as achieving high number of fast switching cycles and recovering the output are r equired. These requirements directed the researchers into a search of materials possessing the required properties. For this purpose, design and synthesis of materials with desired properties has been developed. In this dissertation, a novel photochromic oxazine with faster switching cycle and higher fatigue resistance was studied in detail. The photophysical characterization of the molecule was obtained by steady state and ultrafast transient absorption spectroscopy. The goal of this study was to answer so me fundamental questions related to the ring opening mechanism of the photochromic oxazine, which represents the coloring reaction of the molecule. For instance, what is the rise time of the state that corresponds to the open form of the molecule? Are ther e any intermediate states involved in the ring opening mechanism? If any intermediates are observed, what are the electronic properties of the states corresponding to these intermediates? Initial experimental results at steady state showed that a state wi th very weak emissive properties appears when the molecule is excited at the wavelength at which the molecule has the highest extinction coefficient. The emissive property of the state is enhanced upon the addition of benzophenone, a good triplet sensitize r.65,162165 W hen the emission experiments were repeated in another solvent with different polarity, the peak of the emission band shifted These preliminary observations lead us to ask the questions: Does the emiss ion belongs to a triplet

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106 state? Does this state have a charge separation property similar to the results presented in literature ? Time resolved transient abso rption experiments provided experimental results that helped us answer these questions. In transie nt absorption e xperiments, we observed two bands one absorbing at around 500 nm rises within the instrument response function (250 fs) and decays with 2.2 ps and 3.7 ps time constants. The intensity of this band increased in the presence of benzophenone. However, the lifetime of the band was short for a state with triplet properties. Therefore, we decided that the emission band at steady state is not a triplet state. Additionally, this band exhibits a solvent effect similar to the emission band observed at steady state. Moreover, the transient absorption band was very similar to the absorption of the radical cation form of the model indole for the photochromic oxazine. When the results are evaluated cumulatively, it is concluded that the state which shows e mission at steady state and transient absorption at around 500 nm is a charge separated state. A second transient absorption band is ob served at around 440 nm which is indirectly from the charge separated state with 12.5 ps time constant and directly from first excited with 6.3 ps time constants. This band is attributed to the open form of the molecule since the shape and maximum of the band coincides with the absorption of open form of the molecule reported in literature. No changes were observed in t he intensity and dynamics of this band upon the addition of the benzophenone. W e concluded that the charge separated s tate does not make a significant contribution to the production of the open form of the photochromic oxazine. In order to decide which states a re involved in the energy transfer from benzophenone to the charge separated state of the oxazine, the transient absorption of benzophenone was compared to the transient absorption of mixtures of benzophenone and oxazine to it. The

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107 comparison showed that o nce it is formed, there is no change in the intensity and the dynamics of the triplet state of th e benzophenone which means that the triplet state of benzophenone does not involve in the energy transfer mechanism. The energy transfer occurs from the single t state of the benzophenone to the charge separated st ate of the photochromic oxazine before the triplet state state of benzophenone is formed. The experimental results showed that the photochromic oxazine has very fast ring opening dynamics possessing on e of the desired properties of photochromic molecules in solution. In future, these dynamics can be investigated in solid media since the applications can be implemented in solid medium. Additionally, the effect of the substituted groups in switching dynam ics of the photochromic oxazine can be investigated in both solution and solid phase for the purpose of developing the rate of switching cycles. We hope that the discoveries we presented in this study will bring the attempts of finding the right photochrom ic material for the successful implementation of the photochromic molecules one step further. In chapter 4, an independent project is presented. We investigated the presence of multiple electronic states and orientation of transition diploes of these state s in unsymmetrical PE dendrimers. For this purpose, we performed steady state anisotr opy experiments at 298K and 77K. At room temperature all of the unsymmetrical Phenylne Ethynylene denderimers we studied exhibited low excitation anisotropy values along the excitation spectra. It is concluded that the low anisotropy values at room temperature were observed due to energy transfer form the longer conjugated segments to shorter ones wit h different transition moments. The anisotropy vales at 77 K of the unsym metrical PE dendrimers exhibited a complex behavior along with the excitation spectra of the molecules. This complex behavior led us to

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108 conclude that there was more than one state contributing to the anisotropic behavior of the molecules Due to their struc tural, physical and energetic properties, dendrimers are proposed as light harvesting component of the solar energy converters in addition to their potential applications in photonic devices. We hope that the observations related to anisotropic behavior of the unsymmetrical PE dendrimers open new possibi lities of design and synthesis of new dendritic structures with good light harvesting properties.

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109 APPENDIX A The chirp correction is done af ter the data i s collected. The front panel of the Labv iew progra m is given in Figure A 1 Before running the program the thickness of each medium that light interacts with should be known. These are needed to be into the boxes under the names of each material. If any of the materials are not present in your set up, put 0.00 mm as the thickness of that material. Figure A 1. Front panel of the chirp correction program.

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110 A part of the block diagram of the chirp correction program is given in Figure A 2. In that part of the program it can be seen what are the parameters needed to calculate the chirp and correct the data. There are two characteristic numbers which are called linear and quadratic factors for each material to be entered into the program. Linear and quadratic factors for air, BK7, CaF2, fused silica, MeOH an d water are already present in the program (Figure A 3) Figure A 2. A part of the block diagram of the chirp correction program

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111 Figure A 3 Linear and quadratic factors for air, BK7, CaF2, fused silica, MeOH and water In order to get the linear and the quadratic factors we used Sellmeier equation and the chirp per mm with respect to energy (wavelength) for air, BK7, CaF2, fused silica, M eOH is calculated. Then this calculated data is fitted by using a quadratic equation. The resultant equations are g iven on the plots in Figure s A 4, A 5, A 6, A 7, and A 8. The multiplier of x is the linear term and the multiplier of x2 is the quadratic term.

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112 Figure A-4. Chirp per mm with respect to the energy (wavelength) of the light for air. Figure A-5. Chirp per mm with respect to the energy (wavelength) of the light for BK7. 1.52.02.53.03.54.0 0 30 60 90 120 150 180 y = -23.2x + 16x 2 relative time (fs)energy (eV)1.52.02.53.03.54.0 0 100 200 300 400 y = -60.7x + 40.1x2 relative time (fs)energy (eV)

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113 Figure A-6. Chirp per mm with respect to the energy (wavelength) of the light for CaF2. Figure A-7. Chirp per mm with resp ect to the energy (wavelength) of the light for fused silica. 1.52.02.53.03.54.0 0 100 200 300 400 500 600 700 y = -92x + 61.4x 2 relative time (fs)energy (eV)1.52.02.53.03.54.0 0 50 100 150 200 250 300 350 y = -47.5x + 31.9x2 relative time (fs)energy (eV)

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114 Figure A-8.Chirp per mm with respect to the energy (wavelength) of the light for MeOH. 1.52.02.53.03.54.0 0 50 100 150 200 250 y = -33x + 23.4x 2 relative time (fs)energy (eV)

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115 400 450 500 550 600 0 Wavelength (nm) 400 450 500 550 600 0 Wavelength (nm) 0 10 20 30 40 0 Time (ps) 0 10 20 30 40 0 Time (ps) 400 500 600 0 400 500 600 0 Wavelength (nm) 0 10 20 30 40 0 0 10 20 30 40 0 Time (ps) APPENDIX B The SVD components presented in this section does not have any physical meaning. They are linear c ombination of physically meaningful components. Figure B 1. A1 -A2) Spectral, and B1 -B2) temporal components of 1.95x104 M oxazine and benzophenone with the ratio of benzophenone/oxazine= 2/100 in acetonitrile, result of SVD analysis. Figure B 2. A1 -A2) Spectral and B1 -B2 ) temporal components of 1.95x104 M oxazine and benzophenone with the ratio of benzophenone/oxazine= 4/100in acetonitrile, result of SVD analysis. B1 B2 A1 A2 B1 B2 A1 A2

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116 400 500 600 0 400 500 600 0 Wavelength (nm) 0 10 20 30 40 0 0 10 20 30 40 0 Time (ps) Figure B 3. A1 -A2) Spectral, and B1 -B2) temporal components of 1.95x10-4 M oxazine and benzophenone with the ratio of benzophenone/oxazine= 6/100 in acetonitrile, result of SVD analysis. Figure B 4. ( ) Results of forward EFA, ( ) Results of backward EFA of 1.95x 10 4 M oxazine and benzophenone with the ratio of benzophenone/oxazine= 2/100in acetonitrile. 10 20 30 40 50 -3 -2 -1 0 1 time (ps) log(eigenvalues) B2 B1 A2 A 1

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117 Figure B 5. ( ) Results of forward EFA, ( ) Results of backward EFA of 1.95x10 4 M oxazine and benzophenone with the ratio of benzophenone/oxazine= 4/100 in acetonitrile. Figure B 6. ( ) Results of forward EFA, ( ) Results of backward EFA of 1.95x10 4 M oxazine and benzophenone with the ratio of benzophenone/oxazine= 6/100 in acetonitrile. 10 20 30 40 50 -3 -2 -1 0 1 time (ps) log(eigenvalues) 10 20 30 40 50 -3 -2 -1 0 1 time (ps) log(eigenvalues)

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118 0 0 0 400 500 600 0 wavelength(nm) 0 0 0 0 0 20 40 0 time (ps) Figure B 7 A1 -A4) Spectra l, and B1 -B4) temporal components of 0.05 M benzophenone and oxazine with the ratio of oxazine/benzophenone= 2.6/1000, result of SVD analysis. B1 B2 B3 B4 A1 A2 A3 A4

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119 0 0 0 400 500 600 0 wavelength(nm) 0 0 0 0 20 40 0 time (ps) Figure B 8. A1 -A4) Spectral, and B1 -B4) temporal components of 0.05 M benzophenone and oxazine with the ratio of oxazine/benzophenone= 4/1000, result of SVD analysis. A1 A2 A3 A4 B1 B2 B3 B4

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120 Figure B 9. A1 -A4) Spectral, and B1 -B4) temporal components of 0.05 M benzophenone and oxazine with the ratio of oxazine/benzophenone= 5.2/1000, result of SVD analysis. 0 0 0 400 500 600 0 wavelength(nm) 0 0 0 0 20 40 0 time (ps) A1 A2 A3 A4 B1 B2 B3 B4

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121 -10 0 10 20 30 40 50 0 0.2 0.4 0.6 0.8 1 Time (ps) APPENDIX C This section presents the tried kinetic models and the fitting trials according these models. Figure C 1. Energy level scheme of the photoisomerization of oxazine. Figu re C 2 Interpolated forms of temporal components belong to the charge separated state (o) and open form of the oxazine (o) and prediction of kinetic model for charge separated state ( ), and the open form ( ). N O N O2 S CS S 0 S 0 S 1 1

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122 -10 0 10 20 30 40 50 0 0.2 0.4 0.6 0.8 1 Time (ps) Fig ure C 3. Energy level scheme of the photoisomerization of oxazine. Figure C 4 Interpolated forms of temporal components belong to the charge separated state ( o ) and open form of the oxazine ( o ) and prediction of kinetic model for charge separated state ( ), and the open form ( ). N O N O2 S CS S 0 S 0 S 1 1

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123 -10 0 10 20 30 40 50 0 0.2 0.4 0.6 0.8 1 Time (ps) Figure C 5. Energy level scheme of the photoisomerization of oxazine. Figure C 6 Interpolated forms of temporal com ponents belong to the charge separated state ( o ) and open form of the oxazine ( o ) and prediction of kinetic model for charge separated state ( ), and the open form ( ). N O N O2 S CS S 0 S 0 S 1 1 4

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132 BIOGRAPHICAL SKETCH Ays un Altan was born in 1981 in Kesan, Turkey. She lived there with her family until she finished elementary school. Afterwards, she moved to Edirne to attend Edirne Anatolian Teacher High School which is a boarding s chool for four years. Following those years, she began her studies at Bogazici University in Istanbul for six years which included the undergraduate and M.Sc. studies in teaching chemistry. After graduating from Bogazici University in 2004, she came to Un iversity of Florida and began her doctoral studies under the supervision of Dr. Valeria D. Kleiman in the area of ultrafast laser spectroscopy of photochromic oxazine.