Metallo-porphyrin containing zirconium phosphonate thin films

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Title:
Metallo-porphyrin containing zirconium phosphonate thin films structure and catalysis
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xiv, 171 leaves : ill. ; 29 cm.
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Lee, Christine Marie Nixon, 1973-
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Chemistry thesis, Ph. D   ( lcsh )
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Thesis:
Thesis (Ph. D.)--University of Florida, 2000.
Bibliography:
Includes bibliographical references (leaves 164-170).
Statement of Responsibility:
by Christine Marie Nixon Lee.
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Printout.
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Vita.

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METALLO-PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILMS: STRUCTURE AND CATALYSIS













By

CHRISTINE MARIE NIXON LEE


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

UNIVERSITY OF FLORIDA


2000














ACKNOWLEDGMENTS


First, I would like to thank some of the teachers who have pointed me toward

chemistry and supported me in this long educational adventure. I thank Mr. Roger

Craig from Lexington High School, for being so excited about chemistry, and Dr. Jim

McCargar, for being a tireless ambassador of general and physical chemistry, and for

encouraging (pushing?) me to pursue research opportunities outside of Baldwin-

Wallace College. I would also like to thank Dr. Gary Kosloski, who cared about my

music even after I defected to the other half of the liberal arts and sciences. And to

Dr. John West and Dr. William Samuels, I extend many thanks for accepting me into

their research programs and giving me two unique and valuable perspectives on

research outside of the academic environment.

I must thank Dr. Dan Talham for his patience, the limits of which, I have

surely tested. I would also like to thank him for accepting a physical polymer

chemistry convert and for teaching me to appreciate materials and surface chemistry.

I would also like to thank him for his time and effort in encouraging all of us to be

organized and effective speakers and writers. For that, I will be eternally grateful.

I could not have completed this document or the research that it describes

without the collaborative assistance from the Bujoli group in Nantes, France.

Especially to Fabrice Odobel, who has contributed significant time and energy to








making and teaching others to make the porphyrins and ligands on which this

dissertation is focused, Merci!

Quick notes of thanks are also due to the Butler Polymer Research

Laboratories for sharing their instrumentation and to Eric Lambers and the Major

Analytical Instrumentation Center for allowing me to use the XPS. Also, thanks to

Joe and Raymond in the chemistry department machine shop for working with me on

designing and building the catalysis flow cells.

Without many wonderful friends, my time at University of Florida would have

been much less enjoyable. So, I thank Louann Troutman, Tracey Hawkins, Dean and

Annie Welsh, Denise Main, and Debby Tindall, and of course, Jen Batten and Leroy

Kloeppner (and Alex) the greatest of friends. I owe Jen and Leroy special thanks,

not only for their efforts in editing this dissertation, but also for their support and love

along the way.

And lastly, my family. All the gratitude in the world goes to my mom and dad,

Pat and Ron, and to my brothers Joe and John and my soon-to-be sister, Jullia. And to

my grandmothers, Evelyn Nixon and Margaret Loeber, and in memory of my

Grandpas Bill and Lee. I have been truly blessed. Thanks, too, to my new family,

George and Agnes, Brian and Greg Lee who are the best second family I could

imagine.

To my husband, Larry, my best friend and biggest fan, I owe more thanks than

can be expressed.















TABLE OF CONTENTS

page

ACKN OW LEDGM EN TS ................................................................................................. ii

LIST O F TA BLES ....................................................................................... ............... vii

LIST O F FIG U RES ................................................................................................... viii

A BSTRA CT.......................................................................................... ........................... iii

CHAPTERS

1 INTRODUCTION .................................... ...................................... 1

1.1 U ltrathin Film s........................................................................................ 1
1.1.1 Langmuir-Blodgett Films and Characterization ..........................3
1.1.2 Self-assembled Films ................................................................15
1.2 Hybrid Organic/Inorganic Ultrathin Films Based on Layered Solids .......16
1.2.1 Background.................................................... ... ......... 16
1.2.2 Self-assembled Films Incorporating Metal Phosphonate
B inding..................................................... ................... 19
1.2.3 Metal Phosphonate Langmuir-Blodgett Films............. ........20
1.2.4 Dual-Function Langmuir-Blodgett Films ......................................23
1.3 Background on Porphyrins ...................................................................24
1.3.1 Optical Behavior of Porphyrins ................................. ...............24
1.3.2 Background on Manganese Porphyrins ...................................33
1.3.3 Immobilization of Porphyrins .....................................................35
1.3.4 Heterocyclic Ligand Cocatalysts ........................................ ...36
1.4 Dissertation Overview ............................................................... .......... 38

2 EXPERIMENTAL ...............................................................................................42

2.1 Langmuir-Blodgett and Self-Assembled Film Preparation Procedure......42
2.1.1 General Langmuir-Blodgett and Self-Assembly Procedures.........42
2.1.1 Characterization ................................................................. 46
2.2 Porphyrin Films .................................................................................48
2.2.1 Palladium Porphyrin Films...................................... ............48
2.2.2 Manganese Porphyrin Films ........................................................49









2.2.3 Manganese Porphyrin/Imidazole Mixed Films..............................51
2.3 C atalysis..................................................................................................... 53
2.3.1 Catalysis using PhIO as an oxidant........................................53
2.3.2 Catalysis using Peroxides as oxidants....................................57


3 PALLADIUM PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILM S.. ................................................................................................ 58

3.1 Background on Palladium Porphyrin Films...............................................58
3.2 R esults............................................................................ ........................61
3.2.1 UV-vis of Palladium Porphyrin Solutions...................................61
3.2.2 Langmuir Monolayers of Palldium Porphyrins ...........................63
3.2.3 Langmuir-Blodgett Films .........................................................69
2.3.3 Conclusions.................................................................... ..............79


4 MANGANESE PORPHYRIN CONTAINING ZIRCONIUM
PHOSPHONATE THIN FILMS ...................................................................... 83

4.1 Background ........................................................ ............. ...................... 83
4.2 UV-vis Behavior of MnTPPs...................................................................85
4.2.1 Solution studies.............................................. ..................85
4.2.2 Langmuir Monolayers..................................................................93
4.2.3 Langmuir-Blodgett Films of pure MnP4 ......................................95
4.2.4 Self-assembled films of MnP4...................................................100
4.3 C onclusions..............................................................................................106


5 INCORPORATION OF AN IMIDAZOLE LIGAND INTO MANGANESE
PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
TH IN FILM S.................................................................................................... 109

5.1 Background.... ...... .. ........ ....................................................................... 109
5.2 Solution Studies .................................................................................. 113
5.2.1 MnPO and MnP4 with ImH.......................................... ......... 113
5.2.2 MnP4 and MnPO with ImODPA................................................ 115
5.3 Film Studies .......................................................... .............................. 117
5.3.1 Langmuir-Blodgett Films containing substituted MnP4 .............117
5.3.2 Mn-porphyrins substituted into self-assembled films of
Im O D PA ................................................................................. 124
5.3.3 Self-assembling the MnP4 and ImODPA from a
mixed solution.........................................................................130
5.3.4 Other methods for preparing ImODPA and MnP4 containing
film s ....................................................................................... 131








5.3.5 Characterization of films containing MnP4 and ImODPA
by XPS and ATR-IR..............................................................134
5.4 Conclusions............................................................................................ 141

6 MANGANESE PORPHYRIN AND IMIDAZOLE CONTAINING
ZIRCONIUM PHOSPHONATE THIN FILMS AS CATALYSTS ....................144

6.1 Background ..............................................................................................144
6.2 R esults................................................................................................ 147
6.2.1 Catalysis With PhIO as the Oxidant ............................................147
6.2.2 Catalysis Using H202 as the Oxidant......................................... 159
6.3 Conclusions......................................................................................163

REFEREN CES.......... ......................................... ..................................................164

BIOGRAPHICAL SKETCH ...................................................................................... 171












LIST OF TABLES


Table page

3.1 UV-vis data from symmetric and alternating films of PdP4. Xm is given for
monolayers, and interlayer thickness is given for multilayers of films
transferred under a variety of transfer conditions.......................................... 71

3.2 UV-vis data from symmetric and alternating films ofPdP1. max is given for
monolayers, and interlayer thickness is given for multilayers of films
transferred under a variety of transfer conditions........................................77

6.1 Time dependence of epoxidation of cyclooctene using 40 umol cyclooctene
and 5 jmol PhIO in lmL of solution. To the homogeneous reaction was
added inmol ofM nP ........................................................ ....................151

6.2 Conversion of cyclooctene to cycloctene oxide with 40 pmol cyclooctene
and 5 pmol PhIO in ImL of solution using MnP4 LB film........... ..........154

6.3 Conversion of cyclooctene to cyclooctene oxide with varying cyclooctene
to PhIO ratios in ImL of solution using MnP4 SA films over 24 hr.............155

6.4 Conversion of cyclooctene to cyclooctene oxide with 40 upmol cyclooctene
and 5 pmol PhIO in ImL of solution and in films containing imidazole......157

6.5 Comparison of blanks and homogeneous epoxidation yields in vials vs. in
the reaction cells ....................................... .............. .............................158

6.6 Conversion of cyclooctene to cyclooctene oxide with 400 pmol cyclooctene
and 80 pmol H202 in ImL of solution using imidazole and porphyrin ........ 161














LIST OF FIGURES


Figure page

1.1 Schematic of an isotherm and corresponding monolayer behavior.......................

1.2 Schematic of X-, Y-, and Z-type Langmuir-Blodgett multilayers........................7

1.3 X-ray diffraction diagram ................................................................................... 9

1.4 Illustration of ATR-IR Experiment...................................................... ........ 10

1.5 Schem atic of XPS experiment ............................................................................. 11

1.6 Schematic of polarized UV-vis experimental beam directions............................13

1.7 Behavior of the oblique dichroic ratio versus an orientation parameter (P)..........14

1.8 Crystal structure of zirconium phosphate ......................................................... 17

1.9 Comparison between tradition LB films and metal-phosphonate LB films ..........20

1.10 Schematic of formation of divalent or trivalent metal phosphonate films.............22

1.11 Structures of porphyrin-type molecules A) porphine B) free base
porphyrin, and C) pthalocyanine.................................................................25

1.12 UV-vis spectrum of a metallo-porphyrin (PdTPP)..............................................25

1.13 Outline of 16-member principle resonance structure of metallo-porphyrin..........26

1.14 Gouterman's four-orbital model .......................................................................27

1.15 Transition dipole moments in metallo-porphyrin .............................................30

1.16 Porphyrin chromophore interactions: The square represents the
chromophore and its disecting axes. A) H-type or face-to-face
aggregates; B) edge-to-edge aggregates; C) J-type or head-to-tail
aggregates .............................................. ................................................... 32
1.17 Suggested mechanism of olefin epoxidation catalyzed by MnTPP.....................35










2.1 Schematic of Langmuir-Blodgett trough and monolayer ......................................42

2.2 Schematic of the three-step deposition process used for zirconium
phosphonate film s..................................................................... .....................45

2.3 Schematic of catalysis cell, side view............................................................54

2.4 Schematic of catalysis cell, top view .................................................................55

3.1 Structures of A) PdP4 and B) PdP .......................................................................59

3.2 Schematic of Pd-porphyrin films formed: a) alternating ODPA/Zr/PdP,
b) alternating ODPA/Zr/PdP:ODPA mixed film, c) symmetric
PdP/Zr/PdP, d) symmetric PdP:ODPA/Zr/PdP:ODPA .................................60

3.3 Solution UV-vis of Pd-porphyrins in CHC13: A) PdP4, B) PdP .......................62

3.4 Solution UV-vis of PdP4 in EtOH and water compared to CHC13......................63

3.5 Isotherms of PdP4, pure and mixed with ODPA (PdP4:ODPA), on a
w ater subphase.................. .......................................................... ...........64

3.6 Reflectance UV-vis of PdP4 on water subphase..............................................65

3.7 Isotherms of PdP1, pure and mixed with ODPA (PdP1:ODPA), on a
w after subphase.................................................................................................66

3.8 Reflectance UV-vis of PdPI on water subphase....................................................67

3.9 Reflectance UV-vis of 10% PdP4: 90% ODPA on a water subphase ...................68

3.10 Mean molecular area vs. ratio of ODPA/Porphyrin: A) PdP4, B) PdPI ..............69

3.11 Transmission UV-vis of PdP4 films transferred at high and low MMA.
Absorbance scale corresonds to the film transferred at 300 A2 molecule'1 .....72

3.12 UV-vis of SA PdP4 films rinsed in hot CHC3 ............................................ ....75

3.13 Transmission UV-vis of films of PdP 1 transferred at high and low MMA...........76

3.14 Absorbance of Soret vs. time rinsed in hot CHC13: A) PdP4, B) PdPl ................78

3.15 Illustration of orientation and packing of PdP 1 films transferred at high and
low M M A ................................................................................................. 79








3.16 Illustration of orientation and packing of PdP4 films transferred at high and
low M M A ................................................................................................... 80

4.1 Structures of A) MnP4 and B) MnPO .............................................................84

4.2 UV-vis of M nPO in CHC13................................................................................... 87

4.3 Solvent behavior of MnP4 in water, EtOH and CHC3 ........................................88

4.4 UV-vis concentration study of MnP4 in CHCl3: a) 10-6 M, b) 10-5 M ................88

4.5 MnPO in CHCI3 (1 x 10"7 M) with ethylphosphonic acid: a) pure MnPO,
b) 1 x 104 M ethylphosphonic acid, c) 2 x 104 M ethylphosphonic acid,
d) 3 x 104 M ethylphosphonic acid, e) pure MnP4 .......................................90

4.6 Solution UV-vis investigation of MnP4's sensitivity to displacement of
R-PO(OH)2 by chloride at 1 x 10-5 M. The arrows indicate the changes
in the intensity of the peaks as the chloride concentration changes from
0.0 M to 0.1 M while the concentration of MnP4 stay constant in CHC13 ......93

4.7 Isotherm of MnP4 on water subphase............. .......................................................94

4.8 Reflectance UV-vis of MnP4 on water subphase ................................................95

4.9 UV-vis of MnP4 capping layers transferred onto ODPA/Zr at different
surface pressures (indicated by the arrows)...........................................96

4.10 LB films of MnP4 transferred at A) 15 mN/m and B) 5 mN/m rinsed in
CH C13....... .............. ........................................................ .......................98

4.11 MnP4 transferred by LB at 0.7 mN m'' and rinsed in CH3CN: A)
transferred from a 0.5 mg mL-' solution...................................................99

4.12 MnP4 transferred from 0.1 M [CI-] aqueous subphase at 4 mN m' ..................100

4.13 MnP4 self-assembled from EtOH/H20 and rinsed in CHC13. The legend
indicates the spectra after rinsing, after being left overnight and the
rinsed again over a three day period ....................................................101

4.14 SA MnP4 films with rinsing in hot CH3CN ....................................................102

4.15 UV-vis response of a SA MnP4 film during rinsing with hot CH3CN.............. 103

4.16 UV-vis of MnP4 self-assembled films before and after rinsing in hot EtOH......104

4.17 MnP4 self-assembled from a 0.1 M chloride solution....................................... 105









4.18 XPS of MnP4 SA film. The insert is an enlarged view of the same spectrum
between 200 and 80 eV .............................. .............................................106

5.1 Structures of A) MnP4, B) MnPO, C) ImODPA and D) ImH............................110

5.2 Simplified Schematic of MnP4 and ImODPA incorporation in films................. 11

5.3 Solvent response of A) MnPO and B) MnP4 to ImH...........................................114

5.4 Solvent response of A) MnPO and B) MnP4 to ImODPA. Legends indicate
the molar ratio of MnP to ImODPA ............................................................ 116

5.5 UV-vis of ODPA/Zr/HDPA, SA MnP4 film rinsed in hot CHCl3 .................119

5.6 MnP4 substituted onto ImODPA:HDPA LB films after CHCl3 rinsing .............121

5.7 MnP4 substituted onto a 25% ImODPA/HDPA film, rinsed in room
temperature and hot CHCl3....................................................................... 122

5.8 UV-vis of an ImODPA/ MnP4 film after drying ............................................... 123

5.9 MnPO attached to a 25% ImODPA/HDPA LB film and rinsed in hot CHC1......124

5.10 MnP4 substituted onto a pure ImODPA SA film ..............................................126

5.11 Reversibility of the chloride/phosphonic acid binding ...................................... 127

5.12 MnP4 substituted film rinsed in chloride and t-butylamine solutions...............28

5.13 MnP4 substituted from a 0.1 M Cl- solution onto an ImODPA layer, and
compared to an MnPO solution with ImH binding......................................129

5.14 ImODPA/MnP4 self-assembled from 70/30 mixture and rinsed in hot
C H C 13............................................... ..................................................... 131

5.15 ImODPA substituted into a MnP4 LB film transferred at 10 mN m'' ..............132

5.16 LB film of MnP4/ImODPA transferred from a 25/75 mixture on an aqueous
subphase, pH 11.3...... ............................................................................ ...133

5.17 XPS multiplex scan over the Nis region of A) ImODPA, and B) MnP4 self-
assembled films. The dashed line represents the Gaussian peak fit .............134








5.18 XPS multiplex scan of Nls region of ImODPA/MnP4 film self-assembled
out of 70/30 CH2C12 solution. The dashed lines represent the Gaussian
peak fits.................................................................................................... 136

5.19 XPS multiplex scan of mODPA/MnP4 film self-assembled from a 70/30
m ixture in EtOH/H20 ................................... ............................................136

5.20 ATR-IR of ImODPA SA film............................................................................... 138

5.21 Increase in absorbance intensity of 2918 cm'" peak in ImODPA with
SA tim e ..........................................................................................................139

5.22 ATR-IR ofalkyl region of: A) MnP4 substituted on a 100% ImODPA base
capping layer, B) MnP4 substituted on a 25% ImODPA base
capping layer............................................................................................ 141

6.1 SA MnP4 film before and after 24 hr. catalysis run with 40:5:20
cyclooctene: PhIO:decane in CH2C2 ....................................................148

6.2 SA MnP4 film before and after 2 hr. catalysis run with 40:5:20
cyclooctene: PhIO:decane in CH2C12 ..........................................................149

6.3 UV-vis of MnP4 film SA from chloride containing solution used in
catalysis with PhIO after 6 hr.......................................................................150

6.4 Bleaching of MnPO in homogeneous catalysis reaction with PhIO.....................152

6.5 MnP4 LB film before and after 24 hr catalysis reaction....................................153

6.6 SA ImODPA/SA MnP4 studied with PhIO for epoxidation of cyclooctene .......156

6.7 SA ImODPA/SA MnP4 studied in the epoxidation of cyclooctene
using H 20 2 ................................................................................................... 159

6.8 SA ImODPA/SA MnP4 after rinsing and after 24 hr in catalysis reaction
w ith excess H 20 2........................................................................................ 161

6.9 SA ImODPA/SA MnP4 film with catalysis using 8 .mol cyclooctene
to 0.2 m ol H 20 2 ........................................................................................... 162














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


METALLO-PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
THIN FILMS: STRUCTURE AND CATALYSIS

By

Christine Marie Nixon Lee

August 2000

Chairperson: Daniel R. Talham
Major Department: Chemistry

Thin films containing mono- and tetra-phosphonic acid palladium tetraphenyl

porphyrins and tetra-phosphonic acid manganese tetraphenyl porphyrins, PdP1, PdP4,

and MnP4, were prepared by both Langmuir-Blodgett (LB) and self-assembly (SA)

techniques. Within the hydrophilic regions of these films was incorporated a

zirconium phosphonate network which lent significant stability and flexibility of

preparation to these films.

In the LB films of the palladium porphyrins, it was found that the mono-

phosphonic acid porphyrins aggregated under all film preparation conditions and in

solutions at high concentrations. However, the chromophore aggregation could be

controlled in the tetra-phosphonic acid porphyrins when the films were transferred at

mean molecular areas greater or near the mean molecular area of the chromophore

itself. Self-assembling the PdP4 was another means of controlling the chromophore








interaction in the films. Because chromophore aggregation was expected to inhibit

catalysis, film preparation conditions were sought in order to avoid this.

LB and SA films of pure manganese porphyrins were successfully prepared by

a number of different methods. Aggregation appeared insignificant when transferred

at high mean molecular areas and when the films were self-assembled. The pure

manganese porphyrin films were successful at catalyzing the epoxidation of

cyclooctene using iodosylbenzene as the oxidant.

To activate the porphyrin for catalysis with peroxide oxidants, a heterocyclic

ligand was also incorporated into the manganese porphyrin containing films. The

heterocyclic ligand used was an imidazole substituted with an octadecylphosphonic

acid chain. Both the manganese porphyrin and the imidazole amphiphile were

tethered to a zirconium phosphonate network, first for ease of film synthesis and

second, to stabilize the film for use in the catalysis reactions.

Though significant catalyst degradation has been reported in homogeneous and

alternative heterogeneous catalysis studies, the manganese porphyrin and imidazole

containing zirconium phosphonate films were generally more resistant to degradation

under catalysis conditions. The stability of the films toward epoxidation conditions

has led to easily recyclable catalysts.













CHAPTER 1
INTRODUCTION


1.1 Ultrathin Films



The study of ultrathin films, especially monomolecular thick films, enables the

study of two-dimensional systems and allows the simplification of complicated

thermodynamic behaviors. Recent interest in monolayers and multilayers focuses on

the many potential applications of organized and functional thin films, which include

optoelectronics,1-3 coatings,4-7 chemical sensors,1,6,8-10 and heterogeneous

catalysts.ll-13 In order to prepare these organized and essentially two-dimensional

structures, the Langmuir-Blodgett (LB) and self-assembly (SA) techniques have been

developed.

The study of monolayer thick films began long before the early twentieth

century investigations of Irving Langmuir and Katharine Blodgett. It is believed that

centuries ago drops of oil were used to calm waves in ponds and other small bodies of

water.14,15 Benjamin Franklin studied monolayers of oil on the surface of a pond.

Agnes Pockels pioneered the study of a monolayer on the water surface in the

laboratory environment, and is credited with building the first trough.15 However, the

first systematic study of monolayers of amphiphilic molecules on aqueous surfaces

began with Irving Langmuir's studies at GE Laboratories and, hence his name is

associated with a fundamental method of preparing organized, monomolecular thick





2


films.16 His associate, Katherine Blodgett first transferred these films from the water

surface onto solid supports.17

The SA technique was first described in scientific reports in the late 1940's and

mid 1950's.18,19 This technique relies on surface-active molecules and the appropriate

surface being placed in contact with one another through a solvent medium. The films

prepared by this method tend to be more stable than traditional LB films because of

the types of surface interactions that drive their formation.

The traditional LB technique employs different surface interactions depending

on the method of transfer. First, as in the case of hydrophilic-hydrophilic transfers of

neutral amphiphiles from a pure aqueous subphase, the interactions are primarily

hydrogen bonding in nature. Ionic bonding is commonly observed in the case of

hydrophilic-hydrophilic transfers from a metal ion-containing subphase. In the case of

hydrophobic-hydrophobic transfers, van der Waals interactions are involved. During

LB depositions, the film is usually physisorbed to the surface implying some through-

space interaction, while in SA films, the molecules are adsorbed to the surface through

a chemical bond. The SA method relies on the formation of covalent bonds between

the surface-active component of the molecule and the solid substrate often resulting in

more stable films.20

In LB films, the hydrophilic head group typically dictates the area the molecule

fills in the interfacial region. The alkyl groups, therefore, adjust to maximize the van

der Waals contacts leading to organized packing within this region.21 Unfortunately,

control over the film packing and organization achieved by the LB technique is absent

in the SA method. The molecular organization in SAMs (self-assembled monolayers)

is dictated solely by the geometry of the active sites on the surface. However, in LB

films, the pressure and area of deposition can be selected to deposit a particular phase

of the monolayer and to influence the transferred film organization. An understanding









of the mechanics of both the LB and SA processes is important in appreciating thin

film research and its application to many areas of science.20


1.1.1 Langmuir-Blodgett Films and Characterization

.1.1.11 Langmuir monolayer formation: the isotherm. The Langmuir

monolayer is achieved by placing droplets of an amphiphile solution, in a volatile

solvent such as CHC13, on the aqueous subphase in such a way as to uniformly spread

the compound on the surface. Typically, the presence of an amphiphile works to

decrease the surface tension, and the difference is defined as the surface pressure. In

Equation 1.1, H is the two-dimensional surface pressure typically measured in mN m1,

Yo is the initial surface tension of the subphase and yf is the surface tension of the

subphase and film.22,23



n = o Y(1.1)


The record of the monolayer formation on the water surface, the I vs. area (A)

isotherm, depends on the change in the surface pressure with a change in the mean

area per molecule (MMA) on the surface.15,22 The surface pressure is measured using

a sensitive microbalance, such as a Wilhelmy balance, while the area is computer

controlled using movable barriers, which define the monolayer boundaries.

Upon room temperature spreading, the monolayer is at a very low density and

behaves like a two-dimensional gaseous phase (Figure 1.1). In this phase, the

molecules are theoretically not in constant contact with one another, although

aggregation may occur depending on the affinity of the amphiphilic molecules for one
another. There are random collisions, but there is no appreciable increase in surface









pressure because, in effect, there is not a true monolayer, and the presence of the

amphiphile has virtually no effect on the surface tension of the subphase.20 There is

some debate on the existence of a two-dimensional gaseous state; some researchers

claim that there is always aggregates formed, which interact within the gaseous

phase.23

As the monolayer is compressed, the molecules will come in contact with one

another, and an observable rise in H occurs. In the initial region of noticeable pressure

increase, the molecules are colliding, but the film is in a fluid-like state. The

molecules have no long-range orientational order and they are not close-packed or

organized. This region is often called the liquid-expanded state (LE) (Figure 1.1).

Within the LE phase, the alkyl chains have many degrees of freedom, and gauche

conformations are observed within the chains.

The further compression of long chain fatty acid amphiphiles leads to a more

crystalline and organized monolayer. In this phase, the hydrophobic tails of the

amphiphiles adopt an overall orientational order that is maintained within the film

domains. This orientational order seeks to balance the van der Waals interactions

between the alkyl chains with the pressure applied by the barriers. Within this region,

referred to as the liquid-condensed phase (LC) (Figure 1.1), the slope of the isotherm

is very steep, meaning that the pressure goes up rapidly without much change in the

MMA. In some cases, the structure of the amphiphiles may prohibit the formation of a

close-packed monolayer regardless of the pressure, and therefore, the monolayer enters

and remains in the LE phase.15

When the monolayer cannot compress any further, additional applied pressure
from the barriers causes the monolayer to fold either over or under itself forming

collapsed regions. The collapse point is identified as the first point of deviation from

the linear slope of the LC region of the isotherm. 15,22















C. Liquid Condensed State



E

E- *** *.

B. Liquid Expanded State


\ F L-* -
A. Gaseous State




MMA (A2 molecule-1)



Figure 1.1: Schematic of an isotherm and corresponding monolayer behavior.


1.1.1.2 Langmuir monolaver characterization: creep and hysterisis tests and
reflectance spectroscopy. Although the isotherm is very important in identifying the

general behavior of a monolayer film, it does not give specific information about such
things as the monolayer stability, alkyl chain orientation, or aggregation of the
amphiphiles. These more specific ideas of monolayer behavior can be discerned from
experiments such as creep and hysterisis tests, reflectance UV-vis, fluorescence

microscopy, Brewster angle microscopy, and surface potential measurements,23 to









name a few.15,22 The methods applying to the films discussed in this dissertation will

be described here.

Creep tests provide information on monolayer quality and can be recorded in

two ways. First, the monolayer is compressed to a certain pressure and then that

pressure is maintained by the barriers expanding or compressing as necessary while

recording the change in area with time. Second, the monolayer is compressed to a

defined area, which is maintained by stationary barriers, and the pressure is monitored

over time. Stable monolayers will show a static surface pressure or little movement in

the barriers after the desired pressure is reached. Unstable monolayers go through

constant and sometimes drastic rearrangements, which force contraction or expansion

of the barriers. For example, the hydrophobic nature of the amphiphile may be

insufficient and the amphiphile may dissolve into the subphase forcing the barriers to

compress to maintain nI. Also, the vapor pressure of the amphiphile may lead to their

evaporation, causing the surface pressure at constant area to decrease or the barriers to

move forward to hold constant pressure in proportion to the instability of the film.

Alternatively, the amphiphiles may have a strong affinity for one another, leading to

agglomeration and either causing an anomalous change in the surface pressure at

constant area or forcing the barriers to work to maintain the surface pressure.22

Hysterisis studies monitor the effect of the monolayer stability on the

reproducibility of the isotherm upon compression and decompression. If the

amphiphiles tend to aggregate, the isotherm will not retrace its compression curve in

its decompression cycle. From creep tests and hysterisis experiments, the ability of

the monolayer to hold its form and to possibly be transferred can be ascertained.24

Reflectance UV-vis spectroscopy is used to understand the optical behavior of
monolayers of chromophore-containing amphiphiles on the water surface. One

method for studying the Langmuir monolayer by reflectance UV-vis involves placing









a mirror on the base of the trough and reflecting a beam through the monolayer onto

this mirror, and back into a detector. These studies can help determine the onset of

aggregation in films with such a tendency.25,26

1.1.3 Langmuir-Blodgett film formation. LB films are formed by vertically

transferring Langmuir monolayers from the water surface onto a solid support. There

are three common vertical dipping techniques, which form X, Y, and Z-type films

(Figure 1.2). 15,23 In X-type LB films, a Langmuir monolayer is transferred onto a

hydrophobic substrate in such a way as to maintain head to tail type interactions. In Z-

type LB films, the monolayer is transferred onto a hydrophilic substrate also forming

head to tail interactions. X- and Z-type films can be prepared on a specially designed

trough, which allows one stroke to be made through a monolayer and the next to be

made through a clean subphase. However, some amphiphiles have a preference for

this type of interaction and upon regular dipping, these structures form spontaneously.

Y-type multilayers are most common and can be prepared on either hydrophilic or

hydrophobic substrates. Y-type multilayers are typically the most stable due to the

strength of the head-head, tail-tail interactions.15


t --- --- --]-- -tp
0 ----[. ---
----- -- X-type

---0---0
-0 e----- --O
_---- ---* Z-- Z-type
S- ~- --

---- -- --O

SFigure .2: h YoX-type

Figure 1.2: Schematic of X-, Y-, and Z-type Langmuir-Blodgett multilayers.











1.1.1.4 Langmuir-Blodgett film characterization. The quality of the transferred

film is first indicated by the transfer ratio, which is a measure of the change in the area

of the monolayer versus the area of the substrate coated by the monolayer. A transfer

ratio of unity indicates that the monolayer is transferred with the same area per

molecule that it had on the water surface. This "perfect" transfer ratio assumes that the

monolayer on the water surface was stable and was not reorganizing significantly

during transfer. A consistent deviation from unity could imply a change in

organization upon transfer; however, if the transfer ratio is irregular, the transferred

film is probably poor quality.15

There are many analytical techniques used to study transferred films. Film

characteristics typically of interest are thickness, interlayer spacing, molecular

orientation and packing, film coverage, surface topology, chemical composition, and

optical and magnetic properties. The techniques used to study these parameters are

well described in the literature.20

X-ray diffraction is a reliable technique to probe interlayer spacing, and from

interlayer spacing, film thickness can be inferred. The X-rays are essentially reflected

from planes of higher electron-density. The interaction of X-rays with the crystalline

planes can be described by Bragg's law (Equation 1.2):



nA = 2dsin0 (1.2)


where n is an integer, X is the wavelength of the radiation, d is the interlayer spacing,

and 8 is the angle of incidence and reflection of the beam.27

Ideally, there should be a significant difference between the electron density of

the head group and that of the hydrophobic region allowing X-ray diffraction peaks to







be observed in LB films. The d-spacing, which quantifies the periodicity between
planes of high electron density, therefore, is the measure of the distance between head
groups. This technique is very sensitive to long-range periodicity, and many narrow
001 peaks are typically indicative of a well-defined layered architecture (Figure 1.3).23





S I 1O
A- .. .. ... f


${UTTT1TTTTW
L1


d-spacing


Figure 1.3 X-ray diffraction diagram.

To study the chemical make-up of the surface, attenuated total reflectance
FTIR (ATR) and X-ray photoelectron spectroscopy (XPS) are employed. ATR studies
involve transferring a film onto a crystal, such as parallelogram-shaped germanium or
silicon crystal with ends cut at 450 angles. This is an ideal method for recording the IR
spectra of films because it allows the film to be sampled many times due to the
internal reflection of the IR beam through the crystal. ATR also provides information
about the surface coverage and packing modes. Further, polarized studies using this
technique can give information about the film organization and orientation (Figure
1.4).15 Background information pertaining to films studied by ATR-IR, allows


^








elucidation of information about films containing new amphiphiles. For example, by
comparing the areas of the alkyl stretches of a new amphiphile to that of well-
understood fatty-acid films, an idea of the transfer quality can be obtained. This tool
is particularly helpful in studying monolayers with transfer ratios varying from unity.
ATR-FTIR can also indicate the packing nature of the alkyl chains. If the
chains are in a mostly trans configuration and close-packed, the asymmetric CH2
stretch (vCH2) will occur at 2918 cm-1 and have a full width at its half maximum
(FWHM) of ca. 20 cm-1. The symmetric CH2 stretch (v,CH2) will occur at ca. 2852
cm-1. If there are a significant number of gauche interactions in the chains, the v,CH2
and vCH, stretches will shift to higher energies (ca. 2924 cm-1 for the vCH2 and ca.
2856 cm-1 for the v.CH, stretches).28,29





^11L1 LMM


11111 111111111 I beam

Figure 1.4: Schematic of the ATR-IR Experiment.


XPS is a method used to determine the elemental make-up, and possibly,
atomic proportions within a film based on the photoelectron effect. XPS measures the
energy of an expelled electron as the surface is bombarded with a monochromatic X-
radiation source (Figure 1.5). When a high-energy source is applied, the kinetic energy


WTW- "
A A








of the emitted electrons can be related to the excess energy and to the strength of the

electron's binding by Equation 1.3, which is the Einstein photoelectric law:27



Ek, = hv e- E, (1.3)


where Eb is the binding energy, e is the charge of the electron, h is Planck's constant, v
is the frequency of the radiation, and <) is the work function corresponding to the
minimum energy required for ejection of an electron.

4 Detector
Ek = hv- Eb
Monochromatic X-ray
Beam
e






Figure 1.5: Schematic of XPS experiment.

The XPS spectrum plots electron counts versus binding energy. Binding
energies are unique to each element present in the film, as well as to that element's
chemical environment and oxidation state. Therefore, from a scan over a wide range
of binding energies, called a survey scan, XPS results can be used to define the
elements present in the sample. A more extensive scan, or a multiplex scan, over a
narrow range of binding energies can clarify peak splitting, which can be assigned to a
change in the chemical environment or oxidation state. Offord, et. al., in their study of
a Ru-porphyrin linked to a thiol on gold SA film containing a percentage ofimidazole
terminated thiol surfactants, observed two peaks within the N,, region of the XPS









spectrum. These two peaks were associated with the different nitrogen environments

in the imidazole and porphyrin, clearly indicating that both species were present in the

mixed film.30

The intensities of the XPS peaks can be used to determine the relative ratios of
the elements present, and can indicate the type of crystalline lattice formed. However,

the intensities of the XPS peaks are sensitive to many parameters such as the element's

electron escape depth, which can complicate the determination of the elemental ratios.
In a given sample, the observed relative peak intensities are compared to a calculated

value based on Equation 1.4:




-dP ^
I;fexp d.
I1 = (si (1.4)
IA / exp + /;I exp[ -- +...
,,A(sin) A,,(sin 0)


where IA is the relative intensity of element A, IA' is the atomic sensitivity factor, d. is

the overlayer thickness, 0 is the incident angle of the X-ray beam, and %, is the

inelastic mean free path. The inelastic mean free path represents the distance over

which 60% of the electrons can travel before inter-electron collisions lead to a loss of

energy31 and is defined by:32



,e = 10[49/(Ekin 2)+0.11(E o)05] (A) (1.5)



UV-vis spectroscopy reveals a film's optical behavior. Typically, films are
transferred onto glass or quartz substrates and transmittance studies are performed.









Using polarizers and stages that allow careful placement of the substrates at different

angles of incidence to the beam, the orientation of the chromophores within the films

can be determined. By comparing the absorbance intensity in the s- and p-

polarization, a dichroic ratio can be calculated using Equation 1.6:




A
P


Orientational order within the plane of the film and substrate is obtained by looking at

results from 0" and 90 (s and p) polarization as the beam is normal to the surface (0

angle of incidence). Studying the polarization at higher angles of incidence (typically

450) enables the determination of the orientation out of the plane (Figure 1.6).



o0 polarization


> 9 polarization






450 orientation
Qo orientation


Figure 1.6: Schematic of polarized UV-vis experimental beam directions.










The dichroic ratio can be used to determine the chromophore orientation in a

film. In the case of films containing porphyrin chromophores, Orrit et al. determined

that the dichroic ratio can be translated into an orientation parameter (P) using the

graph shown in Figure 1.7,25 and hence, an orientation angle (0) can be established

using Equation 1.7:


P=(cos2 0)


(1.7)


0 represents the angel between the surface normal and the chromophore's molecular

plane. As is often observed within the plane of a film containing porphyrins, if D is

unity, there is no preferred orientation. If D = 1.5, P = 0 and therefore, 0 = 90 as

measured from the surface normal.




I I I I I

1.5 1: A,= 0.2%
S3 2: A= 5%
3: A,= 10%
4: A, = 20%
5: A = 50%
1.0 5



0.5

S= 450


0.0 0.2 0.4 0.6 0.8 1.0
Orientation Parameter (P)

Figure 1.7: Behavior of the oblique dichroic ratio versus an orientation parameter
(P).25









Porphyrin containing films often have oblique dichroic ratios of approximately

1.5, corresponding to the chromophores lying parallel to the substrate. For example,

Zhang et al., obtained such a result in LB films containing a free base tetraphenyl

porphyrin either pure or mixed with stearic acid, where the porphyrin was the

hydrophilic head group with long alkyl substituents.33


1.1.2. Self-Assembled Films

Zisman introduced SAMs to the literature little more than 50 years ago. His

studies involved self-assembling long-chained alcohols onto a glass surface using

hexadecane as the inert solvent.18 This study showed that films prepared in this

manner had wetting properties similar to those seen in films prepared by the LB

technique.

Sagiv et al. studied octadecyltrichlorosilane (OTS) on hydroxylated surfaces,

such as glass, to form a siloxane polymer. Multilayers were produced if the

amphiphile was terminated at both a and o positions by surface-active groups.15,34

When a and co positions were two different functionalities, subsequent dipping in self-

assembly solutions produced non-centrosymmetric films. Unfortunately, studies have

shown that small defects in the early layers can magnify upon multilayer formation

such that nearly all order breaks down by the tenth layer.20 The SA of OTS is now

commonly used for hydrophobisizing glass slides for LB substrates.

The chemisorption ofthiols on gold was initiated by Nuzzo and Allara35 and

continued by Porter,36 Whitesides,37 and others. Alkyl thiols, in which the chain

lengths range from one carbon to over twenty, have been studied. Closely packed

layers were observed when the chain length exceeded eleven carbons.36 The gold

surface used in these studies was formed by vacuum evaporation onto cleaved alkali-









metal halide surfaces.37 Many of the surfactants studied by self-assembly did not form

stable monolayers on aqueous subphases, and therefore, could not be studied by the

LB method. However, these surfactants were easily studied by the SA method.


1.2. Hybrid Organic/Inorganic Ultrathin Films Based on Layered Solids


1.2.1. Background

A new class of LB films has been developed, which incorporate an inorganic

metal phosphonate network into the polar region.28,38-41 These films, which can

contain a variety of organic groups and metals, were inspired by and show analogous

behavior to their solid-state metal phosphonate analogues. In addition, these films are

more stable than typical fatty-acid based LB films, and the inorganic lattice provides

potential function.42

The metal phosphonate solid-state materials are attractive because they can be

prepared at low temperatures from aqueous solutions. Further, the structures can

provide a model system to which the film properties can be compared Due to the

structure of the metal lattices, which directs the film formation, the orientation and

packing of the alkyl region is predictable.

Metal phosphonate materials are especially interesting due to their potential

applications in the areas of sorbents43 and catalysts,44 and because of their layered

structures, these materials can be used as intercalation compounds.45-49 The interest in

the metal phosphonates was sparked by their potential as inorganic ion exchange

materials;50-52 however, the organic region can also be modified and functionalized,

providing a straightforward method for preparing a wide variety of materials.









1.2.1.1. Zirconium Phosphonate solids. Clearfield published early work on

metal phosphonates and phosphates in the 1960's.53 The focus at this time was on the

zirconium solids which form two preferred phases, a and y, which have the

compositions Zr(HPO4)2-H20 and Zr(P04)(H2P04)-2H20, respectively. In these

layered materials, a two-dimensional metal lattice is formed and separated from an

adjacent metal lattice by the organic layer in the phosphonates or by the hydrogen

bonds from the fourth hydroxy site in the phosphates. The interlayer area in these

solids forms a possible domain for intercalation of inorganic materials such as

amines.53


OH OH OH OH






OH OH OH OH
OH OH OH OH





OH OH OH OH
0 Z,

0 P
O o

Figure 1.8: Crystal structure of zirconium phosphate.53



The crystal structure of the Zr-phenylphosphonate solid was determined in the

1980's and found to form a structure similar to the a-phase. Subsequent studies

revealed that any alkyl or aryl group whose area was under 24 A2 would form an

identical metal lattice structure while only the interlayer distance changed. In the n-









alkyl phosphonates, it was found that there is a tilt angle in the chains of between 550

and 600. This tilt allows for maximization of the van der Waals forces between the

chains even within the solid-state materials. Though the structures formed are dictated

by the geometry of the inorganic lattice, the hydrophobic region does rearrange to

balance these strong forces with the maximization of their overlap.53 A change in the

organic group can lead to very different structures and properties of the solids. Bulkier

groups may form new crystal structures or have three-dimensional metal lattice

formation. Also, the different organic groups can impart different function to the

films.

1.2.1.2. Divalent and trivalent metal phosphonate solids. After extensive

research on the zirconium phosphonate solids, interest branched to divalent and

trivalent metal phosphonates. The poor solubility of the zirconium phosphonate solids

in most standard solvents meant achieving single crystals was difficult; therefore, most

of the crystal structure data was achieved from powder X-ray diffraction patterns.

However, di- and trivalent metals tend to be soluble in acidic solutions, allowing

single crystals to be obtained by slowly changing the solvent pH or the metal ion

concentration.

Metal phosphonate materials have been prepared with a variety of different

divalent metals such as Mg2+, Mn2, Zn2+, Ca2, and Cd2+.54-56 From the crystal

structures, it was determined that the composition of the divalent series metal

phosphonates is M"(O3PR)'H20 for Mg, Mn, Zn, Ca and Cd. In these materials,

layers of the metal atoms are octahedrally coordinated by five phosphonate oxygens

and one water molecule, with each phosphonate group coordinating four metal atoms

making a cross-linked M-O network. A second structure, the orthorhombic

MII(HO3PR)2, was observed for the Ca phosphonates. A structural exception to the









above divalent series is Cu(II).48,57 The Cu atoms in these phosphonate materials are

five coordinate and form a distorted tetragonal pyramidal geometry.

Mallouk has prepared a series of lanthanide phosphonates. The structure of

these materials is given as Ln(III)H(O3PR)2, where Ln represents La, Sm, or Ce. The

lanthanide-series phosphonates are more soluble than the zirconium solids, but less

soluble than the divalent materials. Therefore, single crystal data was not easily

obtained.58,59

ATR-IR provides a facile method for characterization of the metal phosphonate

lattice formation. Vibrational modes assigned to the phosphonate are extremely

sensitive to the mode of metal binding. Thomas et al. have assigned the va(CH2)

vibrational frequencies for the divalent metal-phosphonates to be in the range of ca.

1050 1100 cmn' where the v,(CH2) stretches occur ca. 970 990 cmn'.60 Each metal

phosphonate material has characteristic stretches in these regions.


1.2.2. Self-Assembled Films Incorporating Metal Phosphonate Binding

After Sagiv's work with self-assembling films of octadecyltrichlorosilane

(OTS), it seemed a natural step to translate the formation of the thermodynamically

stable and insoluble layered metal phosphonate solids into ultrathin films. Self-

assembly was the first technique employed to produce metal phosphonate thin films.

Mallouk and coworkers formed the metal phosphonate self-assembled films by first,

exposing a silicon or gold surface to an appropriate template forming alkyl-mercaptan

that was substituted with a terminal phosphonic acid.61,62 This phosphonic acid was

active toward the metal salt solution in which the substrate was then dipped. After

metallating the surface, the substrate was dipped in a solution of an a, co-

bisphosphonic acid, which left another phosphonic acid on the surface to be









subsequently metallated, and the cycle continued until multilayered films were

fabricated.

The self-assembly of metal phosphonate films is made possible by a very
strong attraction between certain metal ions, particularly the tetravalent metals such as

Zr4, for the phosphonate groups of alkyl phosphonic acids.61-63 However, the
individual metal salts and the phosphonate are themselves soluble. This particular

affinity between metal and phosphonate, makes possible the formation of

monomolecular layers during each step of the cycle and the ability to assemble

controlled multilayers.


1.2.3. Metal Phosphonate Langmuir-Blodgett Films

Two methods of film formation have been employed to incorporate the metal
phosphonate lattice into the polar region of LB films, one for the divalent and trivalent
metals which are soluble in acidic media, and one for the tetravalent and some

trivalent metals which are insoluble even at low pH. A schematic comparing

traditional LB films to metal-phosphonate LB films is shown in Figure 1.9.



Hydrophobic
A., region

Polar
region MM
Metal
phosphonate
region
A B

Figure 1.9: Comparison between A) traditional LB films and B) metal-phosphonate
LB films.









The zirconium metals have such a high oxophilicity for the phosphonate

oxygens, that when a phosphonic acid amphiphile is spread on the surface of a

zirconium cation containing aqueous subphase, the monolayer crystallizes before it

can be transferred. Therefore, a three-step deposition procedure has been developed.

A phosphonic acid containing monolayer is formed on the surface of a pure water

subphase and transferred onto a hydrophobic substrate. Onto this phosphonic acid

surface, a layer of zirconium is assembled, followed by the transfer of a second LB

monolayer containing a phosphonic acid. This three-step deposition technique will be

described in detail in Chapter 2.28,38

Advantages of this three-step technique include the fact that at the hydrophilic

stage, the monolayer is stable and can be independently characterized by ATR-FTIR

or XPS, etc. Second, this method allows the formation of alternating films in which

the template and capping layers do not have to be formed of the same amphiphile. The

option of forming alternating films is important because some amphiphiles do not

transfer on the down stroke but will transfer onto an ODPA template layer. A

disadvantage of the three-step deposition of the phosphonate films occurs at the self-

assembly of the zirconium lattice. The self-assembly of the zirconium onto the ODPA

template causes the metal phosphonate lattice to be amorphous, whereas in a one-step

deposition, the metal lattice is crystalline.

An alternative technique of film formation is employed for the divalent and

trivalent metals.41 In these cases, the metal salts are dissolved in the aqueous

subphase, the phosphonic acid monolayer is formed on the surface, and the

hydrophobic substrate is dipped down and then up through the same compressed

monolayer forming a complete metal-phosphonate layer or an LB bilayer. The

crystallization of the metal-phosphonate lattice occurs on the slow upstroke of the film

through the monolayer (Figure 1.10).








In the one-step method, the pH of the subphase is as crucial to successful
lattice formation as it is in the formation of the solids in aqueous solutions. If the pH
is too high, the affinity of the metals for the deprotonated phosphonates will be too
high and crystallization of the lattice will occur in the Langmuir monolayer rather than
upon transfer. As with the zirconium films, these films will be too rigid to be
successfully transferred.



'M=f

i 11..4 11iiui







--oO e--- --Oo*-








Figure 1.10: Schematic of formation of divalent or trivalent metal phosphonate films.


If the pH is too low, the phosphonate will remain completely protonated, and the films
will transfer without metal binding. Fortunately, the pH effects on the crystallization
of the monolayer have signatures in the isotherm behavior.64 When the pH is too high,
the rigid monolayer gives an erroneous but characteristic isotherm that has a much









higher onset and shallower incline. We believe this isotherm behavior is due to the

rigid films causing deflections in the Wilhelmy balance rather than showing an

increase in surface pressure.


1.2.4. Dual-Function Langmuir-Blodgett Films

After the extensive characterization of simple alkyl metal phosphonate LB

films, there was interest in incorporating function into the organic region that might be

paired with properties in the inorganic lattice to form a "dual function" LB film. As

models, phenoxy and biphenoxy alkyl phosphonic acids were prepared, and divalent,

trivalent, and tetravalent metal phosphonate films were studied. 65,66 Additionally,

films containing azobenzene-derivatized phosphonic acid amphiphiles were

synthesized, and metal phosphonate films were formed also with divalent, trivalent,

and tetravalent metals.67 These results prove that larger organic groups can be

incorporated into the metal phosphonate LB films while maintaining the integrity of

the inorganic lattice structure.

Potential applications of dual functional metal-phosphonate thin films include

magnetic switches, in which the magnetic behavior of the inorganic lattice can be

altered by a structural change in the organic region. Also, films containing a

conductive or non-linear optic organic region as well as a magnetic inorganic lattice

could act as a sensor. This dissertation will focus on the preparation ofporphyrin

containing zirconium phosphonate films where the metal phosphonate lattice acts to

stabilize the films toward potential catalytic reaction conditions.











1.3 Background on Porphyrins


1.3.1 Optical Behavior of Porphyrins

Porphyrins are a common research focus in physics, chemistry, and biology.

Physical and chemical interest in porphyrins stems, for example, from their highly

conjugated structure that allows facile electron-transfer,68-72 and from their chemical

activity at an exposed metal that may be active toward catalysis or chemical

sensing.1,8,73-75 Biologists and biochemists are interested in the common biological

building blocks that are based on the porphyrin structure.76-78

The core structure of the porphyrin is the completely saturated porphine

macrocycle (Figure 1.11).79 Upon reducing this macrocycle to the unsaturated form,

the porphyrin chromophore is achieved. By hydrolyzing one of the pyrrole units, the

chlorin compound is prepared. Another important structure based on the porphine

core is the phthalocyanine or the tetraazatetrabenzporphyrin. Each of these structures

includes either two protons, as in the free base porphyrin, or a coordinated metal

within the center of the porphyrin, called the metallo-porphyrin. Examples of

biologically active porphyrins include chlorophyll, which is a manganese-coordinated

chlorin molecule, and heme, which is an iron-substituted porphyrin.













NH HN N, N N,
HH / N HH N
NH HN N N N 'N



A B C
Figure 1.11: Structures of porphyrin-type molecules A) porphine B) free base
porphyrin, and C) pthalocyanine.


Porphyrins have characteristic and strong optical transitions by which they can

be identified. The bands often observed in visible spectra ofporphyrins include the B

or Soret Band and Q Bands, as seen in Figure 1.12 for a palladium

tetraphenylporphyrin (PdTPP). The Soret Band is associated with the allowed n n*

transition and is typically seen between 380 and 420 nm.80



2.0

Soret (B) band
1.5


1.0 -
A


Wavelength (nm)


Figure 1.12: UV-vis spectrum of a metallo-porphyrin (PdTPP).











The Q-Bands are observed between 500 and 600 nm. The lower energy Q-

Band (Qn) is associated with the electronic origin, Q(0,0) of the lower energy singlet

excited state. The higher energy Q-Band (Qp) has a contribution from a vibration

mode and is denoted Q(1,0). Both Q(0,0) and Q(1,0) are quasi-allowed transitions

with relatively low absorbance intentisties. The Q-Bands are highly sensitive to the

symmetry of the molecule. In porphyrins of D4, symmetry such as metalloporphyrins,

or the diacidic or dibasic forms of the porphyrin, two Q Bands are observed as

pictured in Figure 1.12. The free-base porphyrin is of D2, symmetry and the

degeneracy of the Q-Bands is disrupted, splitting the Q-Bands into four peaks.80

The above described transitions are due to the porphyrin i-electrons and are

n-n* in nature. If these transitions are unperturbed by the central substituent, the




N N
M
N N Q B




Figure 1.13: Outline of 16-member principal resonance structure ofmetallo-
porphyrin.

porphyrin is classified as "regular". Similarly, the emission spectra of regular

porphyrins are determined solely by the chromophore itself. The above explanation of
the UV-visible behavior of porphyrins is based on the free-electron model, in which

the core of the porphyrin, the 16-member heterocyclic, conjugated ring behaves like a

free-electron wire (Figure 1.13). Another popular theory is Gouterman's four-orbital









model (Figure 1.14), which combines the Htickel-MO theory with the free-electron
model. In this model, Gouterman describes four orbitals, two LUMOs, c,(eg) and c2

(eg) each with five nodes, which are degenerate in energy, and two HOMOs, b,(a2.)
and b2(a,), each with four nodes, which are not degenerate. According to the four
orbital model, the Soret Band corresponds to the transition from the lower energy a,.
orbital to the eg orbital, giving a higher energy transition. The Q-Bands arise from the
transition from the a2 orbital, which is higher in energy giving a lower energy

transition.25,80



Cl(eg) 2(eg)













bi(a2u)
....., 2(ale


^ / ^---- -^ j -- -- ---------
% ,

,i "----------. "''-:----...--(--u'---

!'

I? --


Figure 1.14: Gouterman's four-orbital model.









However, there are also "irregular" porphyrins. Irregular porphyrins, typically
metalloporphyrins, are broken down into categories called hypso- and hyper-

porphyrins. In the case of irregular porphyrins, the central metal contains partially
filled shells, which introduce a possibility of metal electrons mixing with porphyrin 7-

electrons. This mixing is caused by the possibility of metal to porphyrin back-binding

due to similar energies of the metals d-orbitals and the porphyrin's n-orbitals. The

central metal ion can lead to significant changes in the optical and emission spectra.

The metal and its oxidation state determine which category the porphyrin's optical

behavior will fall into. Also, the release of electron density from the metal to the

porphyrin enables the metal to stay co-planar with the chromophore as the effective

size of the metal is reduced.79

Hypsoporphyrins have central metals of groups eight through eleven with
configurations d" where m = 6 9 and have filled eg(dn) orbitals. The inclusion of

these metal ions is often associated with a bathochromic or blue shift relative to the

corresponding free base porphyrin. Common hypsoporphyrins include Ni(II), Pd(II),

and Pt(II)-porphyrins. The Ni(II) porphyrins are easily affected by basic axial ligands,

whereas Pd(II) and Pt(II) are typically four coordinate and appear insensitive to the

potential ligand environment 80,81

The second class of irregular porphyrins is called the hyperporphyrins, which
is further broken down into subclasses called p-type, d-type, and pseudonormal

hyperporphyrins. Most metallo-porphyrins classified as hyperporphyrins have central
metals with easily accessible lower oxidation states. Of these, Mn(III) and Fe(III) are
the most well studied due to their biological implications. The spectra of

hyperporphyrins exhibit the Soret and Q-Bands as before with some possible shifting.
Additional prominent absorption bands may be seen typically at higher energies
relative to the Soret Band. The hyperporphyrin spectra demonstrate the effects due to









metal-ligand charge transfer (MLCT) mixed with the porphyrin r-nt* transitions even

within the Soret Band. The MLCT Bands can be porphyrin to metal, metal to

porphyrin, or even axial ligand to metal. Due to the spectral sensitivity to
chromophore substituents, to the metal its oxidation state, and to the nature of the axial

ligand, and the additional UV-vis Bands, hyperporphyrin spectra are much more

difficult to analyze.80,81

The d-type hyperporphyrins include metals of groups six through eight.

Mn(III)-porphyrin, for example, is d4 and S = 2, or high-spin, and is a characteristic d-

type porphyrin. In chloroform, Mn(III) tetraphenylporphyrin shows six peaks. An
early researcher of the Mn-porphyrins, Boucher, termed these peaks by Roman

numerals going from low to high energy. The first two peaks are in the far-red region

between 800 and 650 nm. Bands III and IV absorb in a region similar to the Q-Bands

in regular porphyrins, between 500 and 650 nm. Band V is similar to and often called

the Soret Band though this band now includes contributions from metal to ligand

mixing. Band VI is typically around 350 nm. The ratio of Bands V and VI is very

sensitive to axial ligands and ring substituents. These bands are due to porphyrin to

metal charge transfer a,,(7t), a2z,() to e,(dn), which implies a necessity for one or more

vacancies in the eg(dn) orbital of the metal and reduction potentials which are not too

negative.80,81

Finally, pseudonormal hyperporphyrins include VO(IV), Cr(II), Mn(II),

Mo(IV), La and Ac where S # 0. These metals show normal absorption spectra with a

weak extra absorption possible in the far-red region. All of these metals have a

partially filled or empty e,(dt) orbital, but charge-transfers from the porphyrin to the

metal are too high in energy to be observed in the UV-vis region. In addition, further

reduction takes these metals to unstable oxidation states which makes this an even

higher energy transition and highly unlikely 80,81









In addition to intramolecular effects such as the metal, substituents, and axial

ligands, intermolecular effects such as aggregation can significantly alter the electronic

behavior of porphyrins. Aggregation in these chromophores has been described

thoroughly by Kasha's exciton theory. This theory looks at aggregation only from the

point of view of overlapping transition dipole moments (Figure 1.15) and not as

interacting x-systems. In metallo-porphyrins, the transition dipole moments are

equivalent due to the symmetry of the chromophore.76,82,83


Figure 1.15: Transition dipole moments in metallo-porphyrin.


Aggregation, according to Kasha's model, splits the excitation energy of the

monomer (E) into high and low energy components. Equation 1.8 describes the

energy dependence on aggregation by:


Et =EO +DV


(1.8)


where D is the dispersion energy which is highly dependent on the change in

environment upon aggregation, and V is the exciton splitting energy. 76,83,84









When the chromophores are interacting with the transition dipole moments
parallel, the exciton energy can be described by Equation 1.9:



V -= M 1, (-cos a)) (1.9)



where M is the transition dipole moment, R is the center-to-center distance, N is the
number of chromophores, and a is the angle between R and M (Figure 1.16). So, if a
< 54.7, V will be positive, and the exciton splitting will be greater and a red shift will

be observed as the transition shifts to lower energy. A red shift is observed in what are
called J-aggregates where both M, and My make angles less than 54.7 with the R

vector. If a > 54.7, V will be negative, and the exciton splitting energy will be lower

leading to a blue shift in the spectrum. When M, and My are both greater than 54.7
from R, the aggregates are termed H-type. If ac < 54.7 and ay > 54.7 the spectral
components will split and part of the band will shift red and part will shift blue; this

spectral behavior is seen in edge-to-edge type aggregates. There can be combinations
and varying degrees of these types of interactions within an aggregated domain

possibly leading to complicated spectra, but in general, the optical spectra ease

identification of electronic behavior of porphyrin chromophores (Figure 1.16). 76,83,84













A.R.




A B C


Figure 1.16: Porphyrin chromophore interactions: The square represents the
chromophore and its disecting axes. A) H-type or face-to-face aggregates; B) edge-to-
edge aggregates; C) J-type or head-to-tail aggregates.

The transition dipoles Mx and My are typically parallel to the plane of the
chromophore except when the nature of the metal in the chromophore center causes a
puckering of the ring. Therefore, polarized UV-vis experiments can easily indicate
orientational changes of the chromophore within a film (Figure 1.16).76,83,84
Incorporation of porphyrins into LB films is currently of interest in scientific

literature. These films are designed in order to prepare selective gas-sensors,1,74,75
photovoltaic devices,85 electron-transfer materials,72,86 molecular wires,87 and novel

heterogeneous catalyst systems.88 However, a difficulty arises in the stability of the
samples using the "typical" LB methods of purely hydrophilic/hydrophobic
interactions or by self-assembly involving tethering through a ligand. Including a
metal-phosphonate lattice into these films should significantly improve the stability
and the applicability of these materials in ultra-thin, organized films.
Typically, LB films containing porphyrin constituents have been studied in
which the chromophore itself is the polar head group. These porphyrin films have
been successfully prepared with either the molecule sufficiently diluted with a film
stabilizing amphiphile,33,88-90 such as stearic acid, or with long hydrophobic chains









attached to the chromophore to stabilize the monolayer on the water surface.84,91

There are significant disadvantages to this method of film preparation. First, the

chromophore is buried in the film interior on a transfer onto a hydrophilic substrate,

and commonly, the hydrophobic interactions necessary to deposit onto a hydrophobic

substrate are too weak for successful transfer. Also, the hydrophilic interactions are

typically of a hydrogen-binding nature making this a relatively weak interaction

destabilizing the film. Finally, the conditions necessary for transferring the traditional

porphyrin LB films facilitate aggregate formation, which can be detrimental in certain
applications, such as catalysis.

The aggregation, or chromophore n-n interactions, is often a consequence of

the film forming procedures. First, compression of the film on the water surface

forces the eventual overlap or tilting of the chromophores.84,88,90 Also, the decreased

affinity of the derivatized chromophores for water tends to force the chromophores to

aggregate rather than to spread on the water surface.92 Understanding the molecular
orientation, aggregation, and morphology ofporphyrin LB films is critical because

each is intimately linked to chromophore behavior. For example, aggregation can

significantly reduce or eliminate the efficiency of the porphyrin in catalysis76 or the

ability of the porphyrin to bind probe molecules in a sensor.92 Therefore, it is

desirable to find methods for forming porphyrin LB films with no aggregation.


1.3.2. Background on Manganese Porphyrins

Biomimetic systems involving porphyrin catalysts have often been discussed in
scientific literature over the past 20 years. Manganese and iron porphyrins are

commonly studied oxidation catalysts and are prevalent elements in biological

processes.93-96 Biochemical oxidation reactions employing metallo-porphyrins

involve reversible site-specific binding of the substrate such that the substrate is within









reach of the oxygen atom on the metal. After the oxygen has been successfully

transferred to the substrate, the product is released and the catalyst is regenerated.93

Manganese porphyrins are probably most well known as epoxidation and

hydroxylation catalysts whether under heterogeneous or homogeneous conditions.

The manganese porphyrin catalysts can utilize a number of different oxidants such as

iodosylarenes, alkylhydroperoxides, hydrogen peroxides, and perchlorates among

others, in order to accomplish the facile oxidation of deactivated olefins, alkanes,

alcohols, ethers, and amines.97-100 A hyper-valent metal-oxo species is believed to be

the active intermediate in the oxidation process in cases such as dioxygen activation of

Cytochrome P-450, or in oxygen transfer from iodosylbenzene, peracids, or

hypochlorite oxidants.101 Though there is some debate on the actual mechanism of the

epoxidation, there are a few possible routes (Figure 1.17). The suggested first and

rate-determining step is the formation of a charge-transfer complex. Whether the

reaction then proceeds through epoxidation or rearrangement is dependent on the

oxidation potentials of the alkenes and the oxidants, steric and electronic structures of

the reactants, and the ability of the substrates to undergo rearrangement.97

Porphyrins have also been studied in chiral catalysis. Lai and co-workers studied

the asymmetric aziridation of alkenes using a chiral manganese porphyrin catalyst.102

They found that with bulky chiral substituents on the porphyrin, successful nitrene

transfer to alkenes was achieved. Enantiomeric excess ranging from 43 to 68% and

product yields greater than 70% were obtained. In these catalysis studies, the reactive

intermediate was a Mn(IV) complex.












oxidant



0

concerted /
oxene insertion .. -



I < / \



possible rate-limiting
formation of a charge-
transfer complex
Figure 1.17: Suggested mechanism ofolefi epoxidation catalyzed by MnTPP.


1.3.3. Immobilization of Porphyrins

The ability of porphyrins to efficiently catalyze both the epoxidation of olefins

and the hydroxylation of alkanes unfortunately leaves the porphyrin and its

superstructure vulnerable as potential substrates. However, nature has developed

mechanisms to eliminate these unwanted complications. For example, an enzyme and

its cofactors may form metal-oxo complexes only when the substrate molecule is

confined within an enzymatic cavity. Also, the tertiary protein structure prevents the
active porphyrin catalyst from approaching other potentially oxidizable
metalloporphyrins, and it makes the structure rigid, protecting the amino acid
backbone and the side-chains from intermolecular oxidation by contacting the active
site. These biosystems are difficult to mimic in the laboratory; however, successful









biomimetic catalysts have been prepared with bulky, rigid groups substituted on the

porphyrin chromophore.l10

One alternative solution to the problem of internal oxidation or intermolecular

oxidative destruction of the porphyrin catalyst is immobilization of the chromophore.

Immobilization involves tethering the porphyrin to a surface such as a film,88 an

inorganic solid particle,103-106 a polymer,107,108 a membrane,109 or a resin.110

Immobilized porphyrins as biomimetics and as heterogeneous catalysts have been well

explored in the past several years.104-106,110 Tethering of porphyrins to a solid support

can not only reduce or eliminate oxidative destruction of the active catalyst, but can

also aid in the catalyst recovery after the reaction is completed.

Heterocyclic ligands are commonly used as the link between the porphyrin and

surface in many immobilized porphyrin systems.94,98,111 Unfortunately, binding the

metallo-porphyrin to the imidazole allows little control over the porphyrin orientation

in the films. Additionally, in these circumstances, there is no chemical connection

between the porphyrin and the surface other than the ligand, which leaves the

porphyrin vulnerable to removal from the surface by ligand displacement, changing

the reaction conditions.30,94,98,'11 An alternative method for tethering the porphyrins

to surfaces has been established, which uses four alkyl phosphonic acid substituents

that can be attached to a zirconium phosphonate network making a very stable

catalytic film.


1.3.4 Heterocyclic Ligand Cocatalvsts

1.3.4.1. Ligand activation of the porphyrin catalyst. Heterocyclic ligands are
well documented in the literature as activating Fe(III) and Mn(III) porphyrins for

catalysis with oxidants such as alkyl or hydrogen peroxides.94,98,99 Porphyrins









immobilized on an ion-exchange resin support showed significant increases in

catalytic activity in the presence of either imidazole or 4-methylimidazole. With the

heterocyclic ligand present, nearly quantitative conversion of cyclooctene to

cyclooctene oxide was achieved, relative to only 5% conversion in the absence of

imidazole over the same time period.112 Likewise, Arasasingham et al. found a 4 to

10 fold increase in the rate of the reaction between a manganese porphyrin and an

oxygen source commonly used in olefin epoxidation reactions, t-BuOOH, in the

presence of imidazole. Since the oxidation of the porphyrin accelerates, a rate increase

should also be observed in the overall epoxidation reaction.98

According to Yuan and Bruice, the reaction of the Mn(III)TPP Cl complexes

with peroxide oxidants only proceeds in the presence of a heterocyclic nitrogen base

ligand such as imidazole or pyridine. The imidazole ligation was pH dependent and

was evident only above pH 5. Consequently, the enhanced oxidation rate was also pH

dependent. Further, with common oxidants, nitrogen base ligation led to a significant

increase in the oxygen transfer rate. 11

The rate increase could be due to a general-base catalysis and/or ligation of the

imidazole (ImH) to the manganese ion.98 Activation by ligation ofImH is supported

by the fact that when 2,4,6-trimethyl-pyridine is used as the base, which is sterically

forbidden from porphyrin ligation, no increase in the reaction rate was observed.

However, when the ImH concentration was below a saturation level, the rate increase

was linear with ImH concentration up to a saturation level. An increase in the

oxidation rate with a basic ligand is likely due to the increase in the electron density at

the metal center arising from donation of the lone pair of electrons from the ImH.98

The presence of the ImH as an axial ligand has been shown also to stabilize the metal-

oxo compound.99









1.3.4.2. Spectral evidence of axial ligand. ImH to porphyrin binding should be

apparent in the UV-vis spectra. The formation of a bis-imidazole Mn(III)TPP Cl

complex was demonstrated by a broadening shift in the Soret Band from 478 nm for

the pure porphyrin to 472 nm. ll Interestingly, the equilibrium constants for the

formation of the mono- and bis-ligated imidazole-porphyrin complexes are similar and

their absorption spectra are nearly identical. Therefore, at high concentrations of

imidazole in solution, it is possible that the observed spectra arise from the formation

of bis-imidazole complexes. 11,1 13 However, the preferred formation of the mono- vs.

bis-imidazole complexes has caused some disagreement, and some authors claim that

even at saturated concentrations of imidazole, the principal component is the mono-

imidazole only.114

In the UV-vis of the Mn-porphyrin, the Soret Band of the Mn-porphyrins is the

most sensitive to the axial ligand. The change in the Soret energy is due to the charge

induced on the porphyrin chromophore through the metal. Electron-donating axial

ligands induce negative charge on the macrocycle, separating the bonding and anti-

bonding orbitals of the porphyrin, and increasing the transition energy.115 Hard

anions, whose binding is strengthened by increased ionic character of the central

metal, prefer localization of positive charge on the metal, which leaves the

chromophore with more negative charge.15 Similarly, as a basic ligand takes on more

hard base character, the X, will shift to higher energies.


1.4 Dissertation Overview


The overall goal of this dissertation was to prepare zirconium phosphonate thin
films by both the SA and LB technique that contained catalytic Mn-porphyrins. The

purposes of the zirconium phosphonate network were to stablize the manganese









containing films to reaction conditions and to allow these films to be recycled in a

number of catalytic studies. Chapter 2 is an overview of the experimental techniques

used to prepare and characterize the films described in this dissertation, and materials

and instrumentation used in this pursuit are also presented.

Films containing a Pd-tetraphenyl porphyrin were prepared to develop film

preparation procedures and to better analyze the UV-vis properties of porphyrin

containing films. Substituted tetraphenyl porphyrins, palladium 5,10,15,20-

tetrakis(2,3,5,6-tetrafluorophenyl-4-octadecyloxyphosphonic acid)porphyrin (PdP4)

and palladium 5,10,15-tris(2,6-dichlorophenyl)-20- (2,3,5,6-tetrafluorophenyl-4-

octadecyloxyphosphonic acid)porphyrin (PdP ), have been studied as Langmuir

monolayers and as zirconium phosphonate LB and SA films.

Films were prepared incorporating the pure porphyrins and the porphyrins

mixed with octadecylphosphonic acid (ODPA). The Langmuir monolayers were

characterized with pressure vs. area isotherms and reflectance UV-vis spectroscopy.

Using a three-step deposition technique, symmetric and alternating zirconium

phosphonate bilayers and multilayers were prepared by the LB technique. PdP4

containing films were also prepared by the SA technique. In all PdPI and PdP4 films,

the porphyrin constituent resided in the hydrophobic region of the monolayer and the

phosphonate substituents bound zirconium ions in the hydrophilic region.

LB and SA films were studied with transmittance UV-vis and the LB films

were further investigated using X-ray diffraction. Control over chromophore

interaction was achieved by chemical modification of the amphiphiles and by selection

of appropriate transfer conditions. For example, reduced aggregation was seen in LB

films of the tetraphosphonic acid substituted porphyrin PdP4 transferred at mean

molecular areas (MMA) larger than the area per molecule of the substituted porphyrin

and in SA films. In these films, the porphyrin macrocycles are non-aggregated and









oriented parallel to the surface. In contrast, the monophosphonic acid substituted

PdPI aggregates under all of the deposition conditions studied.

Stability of the Pd-porphyrin LB and SA films was examined by exposing the
films to refluxing chloroform. UV-vis absorbance after immersion in chloroform

confirmed conclusions that in films of PdP 1, many of the chromophores are not

tethered to the inorganic network and are easily removed, whereas in films of PdP4, all

molecules bind to the zirconium phosphonate extended network making these films

very resilient.

Study of the Pd-porphyrins led to significant understanding of the behavior of
tetra- and mono-phosphonic acid porphyrins in LB films. Chapter 3 describes the

results of these studies, which were the first to show incorporation of porphyrins at the

exterior of metal-lattice containing films.

Manganese tetraphenyl porphyrins are well known epoxidation

catalysts,97,99,116 and the incorporation of these catalysts into zirconium phosphonate

films should improve their catalytic efficiency as well as their stability and

recoverability. Films containing manganese 5,10,15,20-tetrakis(2,3,5,6-

tetrafluorophenyl-4-octadecyloxyphosphonic acid)porphyrin (MnP4) have been

prepared using the LB and SA techniques. The formation of these films involved

modifying traditional LB procedures with SA techniques, which is possible with the

use of zirconium phosphonate networks. From Langmuir monolayer and LB studies

of the pure tetraphosphonic acid porphyrin, it appears that the MnP4 amphiphiles tend

to form face-to-face aggregates, or H-aggregates, when assembled at the air-water

interface, and this aggregation is translated into the transferred films. Attenuated total

reflectance (ATR) IR, UV-vis, XPS and stability studies confirm the presence of the

porphyrin. Thorough characterization of the MnP4 containing films is described in
Chapter 4.









The heterocyclic imidazole ligand has been shown to improve the catalytic

efficiency of Mn-porphyrins, and MnP4 films containing the imidazole ligand have

been successfully developed. These films were prepared by a variety of methods

involving a combination of LB, SA and substitution procedures. In solution, it is seen

that binding of a non-amphiphilic imidazole causes a small blue shift of the Mn-

porphyrin Soret band; however, a dominant influence on the Soret band in the films
and in solutions containing the ImODPA ligand comes from the metals axial

environment -- especially halide binding. Mixed films containing both the imidazole

phosphonic acid (ImODPA) and the MnP4 molecules have been prepared and

characterized by ATR-IR, UV-vis, and XPS. The preparation and characterization of

imidazole and MnP4 containing films is presented in Chapter 5.

The epoxidation of cyclooctene using iodosylbenzene was catalyzed by the

pure MnP4 containing films with substrate to oxidant ratios of 20:5, 40:5, and 60:5

over a variety of reaction times. The self-assembled MnP4 films proved to have

slightly improved catalytic efficiency relative to the analogous LB films likely due to

the increased aggregation observed in LB deposited films. The mixed

ImODPA/MnP4 films showed catalytic activity in the presence of the peroxide

oxidants. These films were examined with different substrate to oxidant ratios. The

porphyrin containing films, both with and without ImODPA were resistant to

degradation under most examined reaction conditions. The catalysis results involving

both PhlO and H202 oxidants with pure porphyrin and mixed porphyrin/imidazole

films are described in Chapter 6.









CHAPTER 2
EXPERIMENTAL



2.1 Langmuir-Blodgett and Self-Assembled Films



2.1.1. General Langmuir-Blodgett and Self-Assembly Procedures

2.1.1.1. Film Formation. The general procedure for forming LB films starts

with the Langmuir monolayer, which are prepared on a Langmuir trough. The trough

consists of a rectangular piece of Teflon, typically 1 cm deep, supported on a metal

base with Teflon barriers, shown as black rectangles in Figure 2.1. A Teflon well is

carved in the center of a double barrier trough for transferring monolayers. The

spreading solution is prepared by dissolving the amphiphile of interest in a volatile

solvent, such as CHC13. The solution is carefully applied to the subphase surface,

ideally spreading the molecules uniformly over the surface.




T Syringe

Wilhelmy
Balance
Barrier Surfactant
i Bmolecule
Aqueous
Subphase


Teflon Trough Wilhelmy
Plate

Figure 2.1: Schematic of Langmuir-Blodgett trough and monolayer.









The amphiphiles are shown in Figure 2.1 as gray circles, representing the

hydrophilic head group, and black lines, representing the hydrophobic tails. The

subphase, which is usually aqueous, must be nanopure. Because there is such a small

amount of amphiphile present, the monolayer is extremely sensitive to contaminants -

especially lipids and other surfactants and ions found in soaps and tap water.

The barriers compress the amphiphiles at a constant speed. In studying the H-

A isotherm, the film is compressed until it collapses. For LB transfers, the film is

compressed until the desired transfer pressure is achieved. At this point, the

monolayer is held at constant pressure for approximately two minutes until the

monolayer is stabilized, then the solid substrate is dipped vertically down through this

monolayer.

The monolayers are first characterized with II-A isotherms. In modem

computer operated systems, the concentration (mg mL"') and the molecular weight (g

mol-1), or the concentration in mol L', of the compound being spread is entered into

the program along with the spreading surface area in mm2. From this information, the

program can calculate the MMA in A2 molecule '. As the barriers move together and

the surface is compressed, the effective MMA is decreased and the surface pressure

increases.

The preparation of the zirconium phosphonate porphyrin films took place by a

three-step deposition procedure (Figure 2.2).28,29,38 A glass sample vial was placed in

the subphase in the well of the trough. Octadecylphosphonic acid was spread from 0.3

mg mL-1 CHC13 solutions and compressed at 15 20 mm min-1 on the water surface.

At 20 mN m-', the substrate was dipped down through the monolayer surface and into

the sample vial at 8 mm min-1, transferring the ODPA template layer. The substrate

and the vial were then removed from the trough and an amount ofzirconyl chloride

was added to the vial to make the solution ca. 4 x 10-5 M in Zr4+. After 20 min in the









zirconium solution, the substrate was removed from the vial and rinsed with water.

After the template layer was successfully prepared, it was dried and characterized

independently by ATR-IR, XPS, and UV-vis if needed.

Capping layers were prepared by a variety of methods, which will be described

for each different film type in Chapters 3, 4 and 5. LB or SA methods could be used

to form the capping layers. To form the capping layer and complete the zirconium

phosphonate bilayer by the LB technique, the now hydrophilic substrate was lowered

into the trough, a monolayer was spread on the surface and compressed to the desired

pressure, and the substrate was raised through the monolayer at 5 mm min7'. To form

the capping layer by self-assembly, the hydrophilic surface was submerged in a

solution of the desired molecules at about 10-5 M in an appropriate solvent, usually

EtOH/H20 (9/1). The capping layer was then allowed to self-assemble.

2.1.1.2. Materials and Methods. Materials used to prepare the porphyrin

containing films included octadecylphosphonic acid (ODPA), zirconyl chloride

(ZnOCI-8H20), and the porphyrins themselves. The porphyrins were provided by

Bruno Bujoli, Fabrice Odobel, Karine LeClair, and Laurent Camus from the

Laboratoire de Synthese Organique, at the Faculte des Sciences et des Techniques de

Nantes in Nantes, France. ODPA was used as purchased from Alfa Aesar (Ward Hill,

MA). Zirconyl chloride, 98% was used as supplied from Aldrich (Milwaukee, WI).

Octadecyltrichlorosilane (OTS) 95%, used to silanize and hence hydrophobicize the

substrates, was also used as purchased from Aldrich. Amylene stabilized HPLC grade

CHC13 was used as a spreading solvent, and was used as received from Acros

(Pittsburgh, PA) and Fisher Scientific (Pittsburgh, PA).

A KSV 2000 system (Stratford, CT) was used in combination with a

homemade, double barrier Teflon trough for the Langmuir monolayer studies and LB

film preparation.









STEP 1


1 01 Oi*


Transfer template
from water surface
STrnf tmle


' !i


SSample vial in trough


STEP 2


SA Porphyrin
(}^


V --*_-- *--1
I-- .--I

I |: I


Figure 2.2: Schematic of the three-step deposition process used for zirconium
phosphonate films.


STEP 3


Zr4+(


0 -









The surface area of the 2000 trough was 343 cm2 (36.5 cm x 9.4 cm). A

platinum or filter paper Wilhelmy plate, suspended from a KSV microbalance,

measured the surface pressure. Subphases were usually pure water with a resistivity of

17-18 MC cm-1 produced from a Barnstead NANOpure (Boston, MA) purification

system.

The films were transferred from the aqueous surface onto solid supports. Glass

microscope slides and glass coverslips were purchased from Fischer (Pittsburgh, PA)

and were used as substrates for UV-vis and catalysis studies. Single crystal silicon

wafers (1 00) were purchased from Semiconductor Processing Company (Boston,

MA), and cut using a diamond glass cutter to 25 mm x 15 mm x 0.8 mm for XPS

studies. These substrates were cleaned using piranha etch, which is 1:4 H2S04: 30%

H202, a new hydrophilic surface was prepared by the RCA procedure,117 which

involved first, heating in a 5:1:1 solution of water, 30% H202, and NH40H, and

second, heating in a 6:1:1 solution of water, 30% H202 and HC1. Then the substrates

were sonicated for 15 minutes each in methanol, 50/50 by volume methanol/

chloroform, and chloroform. The substrates were then sonicated in a 2%

octadecyltrichlorosilane (OTS) solution in hexadecane and CHC1, for two hours.

Finally, the substrates were sonicated for 15 minutes each in CHC13, 50/50 by volume

CH3OH/ CHC13, and CH3OH.118


2.1.2. Characterization

2.1.2.1. UV-vis. Transmittance UV-visible experiments were performed on a

Cary 50 spectrophotometer by Varian with an average resolution of 2 nm. Porphyrin

solutions were studied by UV-vis in EtOH, H20, CHC13, and CHCl2 solvents. The

behavior of the porphyrin with different potential ligands was investigated by mixing

the porphyrin solution with ethylphosphonic acid, t-butyl ammonium halides (chloride









and bromide) (Aldrich), and imidazole with no alkyl substituents (ImH) (Kodak). A 1

cm x 1 cm x 3 cm quartz cuvette held the sample, and the background using the

corresponding pure solvent was subtracted.

A Teflon substrate holder with grooves cut at 450 to one another was used to
obtain sampling at 0 (beam normal to the substrate) and 45* incidence to the substrate

surface. A plane, visible polarizer was used to select s- and p-polarized light.

Reflectance UV-vis experiments were performed on a KSV 2000 mini-trough using an

Oriel spectrophotometer and a 77410 filter with a range from 200 600 nm.

2.1.2.2. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy
(XPS) was performed on a Perkin-Elmer PHI 5000 Series spectrometer using the Mg

Ka line source at 1253.6 eV. The instrumental resolution was 2.0 eV, with anode

voltage and power settings of 15 kV and 300 W, respectively. The operating pressure

was around 5 x 10-9 atm. Survey scans were performed at a 450 takeoff angle with a

pass energy of 89.45 eV. During multiplex scans, 80 100 scans were run at each

peak over a 20-40 eV range with a pass energy of 37.35 eV.

2.1.2.3 X-ray Diffraction. In order to obtain low angle X-ray diffraction
(XRD) patterns, multilayer films were transferred onto a hydrophobic glass slide. The

diffraction patterns were obtained using a Phillips APD 3720 X-ray powder

diffractometer with the CuKa line, X = 1.54 A, as the source for films ranging from 10

to 15 bilayers.

2.1.2.4. Attenuated Total Reflectance Infrared. Attenuated total reflectance

infrared spectroscopy was performed on a Mattson Instruments (Madison, WI)
Research Series-1 FTIR spectrometer equipped with a deuterated triglyceride sulfide

detector and a Harrick (Ossining, NY) TMP stage which held the Ge crystal substrate.

ATR-FTIR spectra consisted of 500 scans at 2 cm-1 resolution and were referenced to
the silanized crystal or previous bilayers.











2.2 Porphyrin Films


2.2.1. Palladium Porphvrin Films

The palladium porphyrins studied were palladium 5,10,15,20-tetrakis(2,3,5,6-

tetrafluorophenyl-4-octadecyloxyphosphonic acid)porphyrin (PdP4) and palladium

5,10,15-tris(2,6-dichlorophenyl)-20- (2,3,5,6-tetrafluorophenyl-4-

octadecyloxyphosphonic acid)porphyrin (PdPI). The Bujoli group provided us with

these porphyrin amphiphiles.

The Pd-porphyrins made well-behaved monolayers; therefore, these

amphiphiles were often transferred by the LB technique. Additionally, the Pd-

porphyrins were studied in diluted mixtures with ODPA in an attempt to disrupt the

aggregate formation in the films. For mixed, Pd-porphyrin/ODPA films, the two

materials were simultaneously dissolved in a CHC13 solution. The weighted average

concentration and molecular weight were calculated and used in the KSV software to

monitor the MMA with compression. Ratios of PdP (1 and 4) to ODPA studied

included 1:0, 1:1, 1:4, 1:9 and 0:1, respectively.

The creep of the pure palladium-porphyrin Langmuir monolayers was studied

at high and low pressures over 30 min, or slightly longer than the time of one

deposition. At a constant pressure of 12 15 mN m-1, the area changed by 6% and

12% for PdP4 and PdPI, respectively. At low pressure (3 5 mN m-1), the change in

area was 3% and 7% for PdP4 and PdP1, respectively. The instability in the

monolayers led to a necessary correction in the transfer ratios. The corrected transfer

ratios for the pure PdP4 were 1.0 1.4 at high pressures and 1.0 1.1 at low pressures.

For the pure PdPl, the corrected transfer ratios were 0.8 1.0 and 0.9 1.0 for high









pressure and low pressure transfers, respectively. The transfer ratios of the porphyrin

films mixed with ODPA consistently showed uncorrected transfer ratios near unity.

Monolayers of PdP1 and PdP4 were also studied on both heated and basic

subphases. To heat the trough, an Isotemp Refrigerating Circulating Model 900

(Fisher Scientific) pump with a water/ethylene glycol bath was used. The temperature

of the subphase was monitored using a KSV thermosensor. A 0.01 M KOH solution

was added to the subphase to adjust the pH to the desired value.

PdP4 films were also studied by self-assembly. The SA solution was prepared

by diluting 1 mL of a 0.5 mg mL"' solution of PdP4 in CHC13 to 30 mL with a 9/1

EtOH/H20 mixture. The film was allowed to SA for approximately 2 hours before

studying by UV-vis.


2.2.2. Manganese Porphyrin Films

Manganese 5,10,15,20-tetrakis(2,3,5,6-tetrafluorophenyl-4-

octadecyloxyphosphonic acid)porphyrin (MnP4) and a model porphyrin, manganese

5,10,15,20-tetrakis (pentafluorophenyl)porphyrin (MnPO) were prepared by the Bujoli

group. Again, ODPA was used for the template layers, zirconyl chloride was used to

prepare the zirconium network, and CHC13 was used as the spreading solvent. Also, t-

butylammonium chloride (t-BuNH2, CI) (Aldrich) was used as a chloride source for

SA deposited films. NaCI (Acros) was used as the chloride source for LB transferred

films.

To form the MnP4 monolayers, a 0.4 mg mL'1 solution was prepared in CHC13

(often, in order to dissolve the porphyrins, up to 5% ethanol was added and the

solution was sonicated for about an hour). An appropriate volume of solution was

spread on the aqueous surface in order to reach and hold the desired transfer pressure









throughout the deposition. A variety of surface pressures were used for transfer and

will be described in more detail in Chapter 4.

To prepare SA Mn-porphyrin films, 1 mL of a 0.5 mg mL"' MnP4 solution in

EtOH was diluted to 30 mL with a 9/1 EtOH/H20 mixture in a 50 mL vial.

Alternatively, a 0.5 mg mL'" MnP4 solution in CHCIl was diluted with pure CHCl1 or

CH2Cl2. The zirconated ODPA surface was exposed to the SA solution for 2 hr unless

otherwise specified. After 2 hr, there were typically some physisorbed chromophores,

which were rinsed off the surface using a hot solvent such as CHCl3 or CH3CN.

As is discussed in Chapter 4, halogenated solvents were originally chosen as

self-assembly solvents to eliminate possible ethoxide or water binding. However, the

oxide coating on the exposed zirconated ODPA template is soluble in EtOI/H20

solvents making the zirconium available for binding the phosphonic acids of the

porphyrin capping layer. From UV-vis studies, the films formed from these different

solvent systems appeared to behave similarly. Therefore, because the film formation

mechanism from EtOH/H20 was better understood, this solvent mixture was normally

used.

To induce chloride binding at the Mn-porphyrin's axial position, a SA solution

containing approximately 0.01 0.1 M t-BuNH3I ClI along with the porphyrin in

EtOH/H2O was prepared. The zirconated ODPA template was submerged in this

solution for 2 hr. Alternatively, chloride ions were incorporated into the aqueous

subphase used for LB transfer of the MnP4 monolayers using NaCI at 0.1 M and

greater concentrations.

From UV-vis results of the MnP4 and MnPO with ethylphosphonic acid, it

appeared that the phosphonic acid might cause the Mn(III)-porphyrin to go through a

spin state crossover from high-spin to low-spin Mn(III). In order to examine this

magnetic change, the Evan's NMR method was used.119,120 A solution of 10% t-









butanol in CDCl3 was injected into a small capillary tube using a long syringe needle.
The depth of the solution in the capillary tube reached approximately 2". Two
standard NMR tubes were then filled to approximately 1" with sample solution. The
first was filled with pure MnPO (0.0106 g, 100 pmol) dissolved in the 10% t-

butanol/CDCl3 solution. The second was filled with MnPO (0.0106 g, 100 pmol) and

ethylphosphonic acid (0.220 g, 2000 pmol). The reference solution in the capillary
was inserted into the porphyrin containing NMR sample, and the magnetic

susceptibility of the solute, Xg (cm3 g'-) induced by the magnetic porphyrin was

approximated by:



-3Af
f (2.1)



where Af is the diamagnetic frequency shift, f is the spectrometer frequency, and m is

the mass of substance per mL of solution. Compared to literature values, the
differences in the X8 of MnPO with and without ethylphosphonic acid did not
correspond to a spin state change in the Mn(III).


2.2.3. Manganese Porphyrin/Imidazole Mixed Films

The general method used to prepare MnP4/imidazole films in this study
involved the initial formation of a zirconated octadecylphosphonic acid (ODPA)

template, as before. Onto this template, a film of either pure imidazole
octadecylphosphonic acid (ImODPA), which was prepared by the Bujoli group, or a
mixture of ImODPA and MnP4 could be formed. The zirconium phosphonate
network provided a means for locking the porphyrin and the imidazole into the films,
resulting in films that were stable toward the conditions used for the catalysis









reactions. Even under relatively harsh conditions such as elevated temperatures or

rapid solvent flow, the porphyrins films appeared to be stable. Additionally, the

zirconium phosphonate network made preparation of the MnP4-imidazole films

possible by a wide variety of mechanisms.

MnP4 and ImODPA could be incorporated into both LB and SA films. In
order to accommodate ImODPA into LB films, two types of spreading solutions were

used: 1) ImODPA mixed with a stabilizing agent such as hexadecylphosphonic acid
(HDPA), or 2) ImODPA mixed with the MnP4 amphiphile. Alone, the ImODPA did

not form sufficiently stable monolayers for transfer. The porphyrin, MnP4 was

substituted into the films of pure ImODPA or ImODPA/HDPA from an EtOH/H20

solution. Alternatively, the MnP4 film was transferred by the LB technique and then

the ImODPA was substituted into these films.

First, in the case of the mixed 25% ImODPA/75% HDPA films, 200 gL of a
solution with a weighted average concentration of 0.17 mg/mL and a weighted average

molecular weight of 329.86 mg mmol"' was spread on the water surface and

compressed to 12 mN m-'. The film was transferred at 5 mm min' on the upstroke

completing the zirconium network. Films made in this way were abbreviated

ODPA/Zr/25% ImODPA. These films were then placed in a solution of the MnP4 at

ca.10-5 M in 9/1 EtOH/H20 and the porphyrin phosphonic acids were allowed to

substitute into the defect or vacant sites in the film for 2 hr. Transmission UV-vis of

these films confirms the ability to include the porphyrins by this method, and the

resulting films were called ODPA/Zr/25% ImODPA, SA MnP4.

Films containing both the MnP4 and the ImODPA transferred by the LB
technique from a mixed monolayer were also prepared. A 70/30 mixture of
MnP4/ImODPA, respectively, was dissolved in CHC13 with a weighted concentration

and MW of 0.289 mg/mL and 992.70, respectively. 175 pL of this solution was









spread and transferred at 3 mm min"' on the upstroke forming the ODPA/Zr/30:70

MnP4:ImODPA films.

For the self-assembly of the imidazole onto a zirconated ODPA template, 2 mL
of a 0.5 mg mL-' solution of ImODPA in EtOH was dissolved in a 9/1 EtOH/H20

mixture in a 50 mL vial. The substrate containing the zirconated ODPA template was

placed in the vial and the film was allowed to self-assemble for 2 hr. When the self-

assembly procedure was complete, the film was rinsed with nanopure water and dried

with forced air. The MnP4 molecules were allowed to substitute into these films as

described in section 2.3.2. Similarly, mixed MnP4 and ImODPA solutions were

prepared at a variety of ratios in EtOH/H20, and the mixed monolayer was allowed to

self-assemble for 2 hr.

The ImODPA was deprotonated as mentioned in Chapter 5 by soaking the
mixed film in an EtOH solution containing t-butyl amine. The t-butyl amine was used

to deprotonate the imidazole within the films so the ligand would be available for

binding the central manganese. The concentration of this solution was not precise but

was consistently ca. 0.1 M and rinsing times were ca. 15 min.


2.3 Catalysis


2.3.1. Catalysis using PhIO as an oxidant

Pure porphyrin films behaved as catalysts for the epoxidation of cyclooctene
using iodosylbenzene (PhIO) as the oxidant. PhIO was synthesized from the diacetate

precursor using NaOH.121 Iodobenzene diacetate (3.0 g, 9.3 mmol) was placed in an

Erlenmeyer flask. 30 mL of 3 N NaOH was added with stirring over 5 min. The

mixture was stirred for 15 min and left to sit uncovered 45 min. 100 mL deionized

H20 was added with stirring and the yellow solid was filtered using a Buchner funnel.









The solid was collected and washed with another 100 mL aliquot of H20. The solid

was filtered again, and washed with CHC13 2 times in a beaker and filtered. The solid

was dried in a vacuum desiccator. The product's melting point corresponded well to

the literature value of 210 C. Iodometric titration, involving converting the PhIO

product to Phi and I2 with HI and titrating with sodium thiosulfate, gave a purity of
99%.122

PhIO (27.5 mg, 125 pmol) was diluted in 25 mL CH2Cl2. The PhIO compound
was only slightly soluble in CH2C12, so it was diluted and then sonicated for at least 30

min. After sonicating, an amount of the cyclooctene was added and the mixture was

stirred for about 1 min. Decane (97 gtL, 500 imol or 24 jL, 125 Pmol), the internal

standard, was also added with stirring. ImL samples of this mixture were used for

both a blank run and a homogeneous run. In all homogeneous experiments, 1 jL of a

1 mM solution of MnPO was added to the blank solution to investigate the epoxide

yields with the porphyrin in solution.


Outlet port Inlet port


I L



Screws to hold
Su cells together


Figure 2.3: Schematic of catalysis cell, side view.






55


Approximately 5 mL of the above oxidant/substrate solution was transferred
into a small Erlenmeyer flask and from there loaded into the flow cells used for

studying catalysis with the films. Also, blanks and homogeneous solutions were

loaded into cells containing blank films (no catalyst) for studying product yields

affected by the flow cell.


Groove with
Viton cell: roove with Viton cell:
Bottom plate Von Top plate




0.06" wide
ledge



Inlet/Outlet ,
hole

Figure 2.4: Schematic of catalysis cell, top view.


The flow cells were built by the University of Florida machine shop. The cell
was made from two blocks of Delrin into which was carved a cell with dimensions:

0.96" x 1.38" x 0.039". Inlet and outlet tubes were place at the cell edges. One end

of a 1' length of 1/16" ID Viton tubing (Cole-Parmer, Vernon Hills, IL) was

connected to the inlet port and the other end was submerged in the reaction solution.

A Cole-Parmer Masterflex peristaltic pump (model 7553-70) (6-600 rpm) with an
easy-load head was used to introduce the solution into the cell, then the open end of

the Viton tubing was removed from the solution and connected to the outlet port of the

cell. The solution was circulated around the film using the peristaltic pump.









The reaction products were studied by GC. The GC instrument was a
Shimadzu GC-17A (Columbia, MD) with a hydrogen flame ionization detector. A 1

p.L portion of the reaction solution was injected onto the 25 m, 0.025 mm ID RTX-5

column (Crossbond, 5% diphenyl-95% dimethyl polysiloxane). The column was held

at 500 C for 3 min and then ramped at 100 C minm' for 15 min.

Sensitivity factors (k) were determined using decane and o-dichlorobenzene as
the internal standards. A series of runs were performed for both the cyclooctene

(CyO) and the cyclooctene oxide (CyOO) standards. Equation 2.2 was used to
calculate 'k' from the GC trace areas of the standard (AJ and the product (Ao) and

the known sample weights (wcoo and w,).




kco = WcO(2.2)



After an average k value was obtained for the CyO and CyOO, the catalysis yields

were determined from the reaction mixture using Equation 2.3:




WCyOO = ----koosA (2.3)



Because the PhIO oxidant was rather insoluble, a series of 1 mL aliquots of a
1.1 g L'' solution of PhIO in CH2C12 were dried and weighed. The final weights were

1.1 mg 9%, assuring us that the amount of oxidant in each reaction was

approximately the same.

In order to compare nearly the same concentrations of catalyst in both the
homogeneous and heterogeneous cases, the concentration ofporphyrins in the films









was calculated to be, at most, 1 nmol. In was, however, difficult to keep this

concentration constant. In the homogeneous reactions, 1 nmol of the corresponding

non-amphiphilic MnPO was used.

Imidazole is reported to improve the catalytic efficiency of the Mn-porphyrins

in the presence of a peroxide oxidant, but for comparison, we also tried to see if

imidazole would improve the catalytic efficiency in the presence of PhIO. For this

experiment, 0.1 tmol of ImH was added to the blank and homogeneous solutions

using a PhIO solution with 40 pmol CO, 5 pmol PhIO and 20 umol decane. Also,

films prepared by SA ImODPA and SA MnP4 were used in the flow cells with this

same PhIO solution. The reactions were, again, stirred for 24 hr.


2.3.2 Catalysis using peroxide oxidants

In studying the catalysis of the epoxidation of cyclooctene (CyO) with H202,

many different substrate to oxidant ratios were studied. The epoxide yields were

greatest, however, when very dilute solutions of reactants were used to keep the

proportion of reactants to catalyst near a factor of 10 and the substrate was used in

excess. The starting materials, CyO and H202, were dissolved in 250 mL of CH2C12

with o-dichlorobenzene as the internal standard. ImL aliquots of this solution were

used for the reaction blank and homogeneous reactions and contained 8 umol CyO,

0.2 pLmol H202, and 0.2 p.mol of o-dichlorobenzene. To the homogeneous reaction

were added 0.4 imol ImH and 0.001 upmol of MnPO. The original solution was

pumped into the flow cells using the peristaltic pump, and these reactions were

allowed to stir for 24 hr at room temperature.













CHAPTER 3
PALLADIUM PORPHYRIN CONTAINING ZIRCONIUM PHOSPHONATE
LANGMUIR-BLODGETT FILMS


3.1. Background on Palladium Porphyrin Films


Langmuir monolayers and LB films of the derivatized palladium tetraphenyl

porphyrin molecules, palladium 5,10,15,20-tetrakis(2,3,5,6-tetrafluorophenyl-4-

octadecyloxyphosphonic acid)porphyrin and palladium 5,10,15-tris(2,6-

dichlorophenyl)-20- (2,3,5,6-tetrafluorophenyl-4-octadecyloxyphosphonic

acid)porphyrin, referred to as PdP4 and PdP1, respectively, are described in this

chapter. The central Pd metal is a four-coordinate, diamagnetic ds metal, and is co-

planar with the porphyrin ligand. Being four-coordinate, there are no complicating

axial ligands to consider.

Porphyrins PdP4 and PdPI are substituted with four and one

octadecylphosphonic acid groups, respectively. These molecules differ from many

other porphyrin amphiphiles in that there is a hydrophilic group at the end of the alkyl

chain substituent. In many literature reports, the porphyrin group is often the

hydrophilic part of LB film forming amphiphiles.91 Molecules PdP4 and PdPI

(Figure 3.1) were designed to investigate whether porphyrins can be incorporated into

metal phosphonate LB films. Also, because of their well-understood spectroscopic

behavior, the PdP molecules were used to study how the orientation and aggregation

of the porphyrin can be controlled in the deposited films.











8P03H2 (R)


R O(CH2)18P03H2


A B
Figure 3.1: Structures of A) PdP4 and B) PdP1.


Palladium polyhalogenated porphyrins were chosen based on the following
considerations. First, palladium porphyrins are not demetallated in acidic conditions,

and their diamagnetic character makes following the synthesis with NMR

spectroscopy possible. Second, manganese and iron polyhalogenated porphyrins are

well-known catalysts for the oxidation of hydrocarbons;123 therefore, if palladium is

replaced by one of these metals, the films can be used for catalytic applications.

Third, pentafluorophenyl substituents on porphyrins allow straightforward

functionalization of this ligand by aromatic nucleophilic displacement with an alcohol.
In addition, the ether bonds are more stable toward hydrolysis and less hydrophilic
than the ester or amide linkages usually used to tether alkyl chains on porphyrins.

High hydrophobicity of the linkage is a requirement because competition with the

polar phosphonic acid head-group should be avoided during the LB film preparation.








LB films of PdP4 and PdPI were formed incorporating a zirconium
phosphonate network. The strong tendency of zirconium ions to crosslink the
phosphonate groups precludes the normal deposition of organophosphonate

monolayers with the metal in the subphase.28 Therefore, a previously developed

three-step deposition procedure was used (Figure 2.2) as described in Chapter 2.28,38
Both symmetric (PdP/Zr/PdP) and alternating (ODPA/Zr/PdP) films have been
prepared in this way (Figure 3.2).



a b c d





Zr Zr Zr 'Zr






Hydrophobic Substrate

= PdP1 and PdP4 = OPA





Figure 3.2: Schematic of Pd-porphyrin films formed: a) alternating ODPA/Zr/PdP, b)
alternating ODPA/Zr/PdP:ODPA mixed film, c) symmetric PdP/Zr/PdP, d) symmetric
PdP:ODPA/Zr/PdP:ODPA.

Control over aggregation of the porphyrin chromophores is achieved through a
combination of molecular design and the careful choice of the conditions for transfer









of the films. Aggregation is decreased or eliminated in the films of the tetra-

substituted PdP4 when transferred at very high mean molecular area MMA and at high

subphase pH. Mixtures of this amphiphile with ODPA transferred at high

temperatures (400C) and high MMA showed a similar decrease in inter-chromophore

interaction. The four long-chain phosphonic acid substituents significantly aid the

spreading of monomeric porphyrin species and the strength of the zirconium

phosphonate interaction assures their isolation in the transferred films. In similar

studies of the mono-substituted PdP 1, aggregation was observed under all of the

transfer conditions explored, indicating that none of the deposition procedures

overcome the tendency of the molecules to aggregate.


3.2. Results


3.2.1. UV-vis of Palladium Porphvrin Solutions

The palladium porphyrins show spectral responses in the UV-vis consistent

with hypso-type metallo-porphyrins. For each porphyrin, a strong Soret Band (or B

Band) is present above 400 nm and two Q Bands are centered around 550 nm.80,124

Solution studies of the porphyrins PdP4 and PdP1 were performed in ethanol and

chloroform, and the absorbance dependence on concentration was investigated.

Solutions ranging from 10-11 M to 10-6 M were studied (Figure 3.3). In CHC13, the

Soret Band was consistently at 410 to 411 nm for the porphyrin PdP4. In CHC13,

therefore, PdP4 shows no sign of solution aggregation. For porphyrin PdP1 at 10-11

M, the Soret Band absorbed at 411 nm; however, as the concentration was raised, the

Band shifted to 414 nm. Because PdPI has only one long chain substituent, the

likelihood of aggregation is increased; therefore, in CHCl3, the PdP1 chromophores J-

aggregate at high concentrations.76,92 Interestingly, the Soret Bands for both PdP4










and PdPI absorb at 411 nm at 10-11 M, so the long chains have no effect on the Soret

Band of the non-aggregated chromophore.


0.10-


0.05-


0.00-

0.15-


0.10-


0.05-


0.00-


Wavelength (nm)


Figure 3.3: Solution UV-vis of Pd-porphyrins in CHCl3: A) PdP4, B) PdPl. The
absorbance scale refers to the 106 M curve. The 10"" M curve has been enlarged for
Band comparison.


The studies of the same molecules at identical concentrations in EtOH and

water showed very different behavior (Figure 3.4). In EtOH at 10-11 M, both PdP4 and

PdP1 show Soret Bands at 414 415 nm. This peak is significantly to the red of the

Soret Bands in CHCI3; however, it is known that more polar solvents tend to stabilize









the excited states in n-nt* transitions, shifting this Band to lower energies.25,26 As the

concentrations of both PdP4 and PdPI were raised, the Soret Band systematically

shifted blue to 407 and 411 nm for the porphyrins PdP4 and PdP1, respectively,

implying H-aggregation of the chromophores in EtOH.76,92 Going from EtOH to

CHC13 to H20, at 106 M, there is an obvious red shift in the X,. This red shift does

not correspond to a solvent polarity shift, but it does correspond to a shift in the

solubility of PdP4. PdP4 is very soluble in EtOH and only slightly soluble in water.


0.10

0.08

0.06

0.04

0.02

0.00


Wavelength (nm)


Figure 3.4 Solution UV-vis of PdP4 in EtOH and water compared to CHC13.



3.2.2 Langmuir Monolavers of Palladium Porphyrins

The room temperature pressure (H) vs. area (MMA) isotherm of PdP4 on water

at pH 5.5 is shown in Figure 3.5. There is a measurable onset of surface pressure near

220 A2 molecule-1, followed by a gradual increase in pressure as the film is

compressed, with an apparent phase transition giving a steeper rise in pressure near









115 A2 molecule-'. The MMA of the tetraphenyl porphyrin is 200 A2 molecule-'

implying that at the onset, the tetrasubstituted porphyrin molecules are not aggregated

or stacked.91 However, this arrangement is not stable to pressure, and as the film is

compressed, the molecules are forced to rearrange.


50 100 150 200
Mean molecular area (A2 molecule1)


Figure 3.5: Isotherms of PdP4, pure and mixed with ODPA (PdP4:ODPA), on a water
subphase.

The change in the aggregation of PdP4 during compression can be observed

with reflectance UV-vis spectroscopy of the Langmuir monolayer (Figure 3.6). As the

film is compressed from a MMA of 370 A2 molecule-' through 220 A2 molecule-', the

Xmax remains between 416 and 417 nm, similar to the ,max observed for the non-

aggregated porphyrin in EtOH. At areas between 220 and 100 A2 molecule-', the Soret

Band shifts to 418 419 nm, and below 100 A2 molecule-' the Soret Band shifts

further to near 421 nm. The shift in the Soret Band suggests a change in the









interaction of the chromophores at different pressures. At MMA larger than and

comparable to the size of the chromophore itself, the porphyrin rings cannot be

aggregating to any significant extent or the onset of surface pressure would occur at

lower areas. The red shift of the Soret Band as the area is decreased indicates

enhanced chromophore aggregation at lower MMA.




0.10-

45 d d
00 I
0.08




0.04 0 (A1re1 nale") I

0.02 a

0.00

360 380 400 420 440
Wavelength (nm)




Figure 3.6: Reflectance UV-vis of PdP4 on water subphase.



In contrast to PdP4, the H-A isotherm ofPdPI (Figure 3.7) indicates that these

molecules aggregate even in the absence of applied pressure. No significant increase

in surface pressure is seen until areas below 120 A2 molecule-1. The pressure rises to

only 5 mN m-i at 60 A2 molecule-1, below which the pressure increases until the film

collapses below 36 A2 molecule-1. The isotherm cannot reflect a true molecular

monolayer, but rather results from the compression of aggregates at the water surface.









Evidence of aggregation at all MMA is seen in the reflectance UV-vis spectra. Figure

3.8 shows the Soret Band as a function of MMA from greater than 120 A2 molecule-'

to film collapse at 36 A2 molecule-1. The Soret Band does not shift during

compression, and the nmax of 426 nm indicates that the porphyrins are aggregated at

each stage of the isotherm.


Mean molecular area (A2 molecule1)


Figure 3.7: Isotherms ofPdPI, pure and mixed with ODPA (PdP :ODPA), on a water
subphase.

A common procedure for enhancing the stability and processibility of unstable

Langmuir monolayers, and to reduce aggregation, is to mix the amphiphile of interest

with a good film-forming amphiphile.33,88-90 In this pursuit, both of the porphyrins

were mixed with ODPA, which is a well-studied amphiphile that forms a liquid-

condensed phase on the water surface and easily binds to an exposed Zr-phosphonate

surface. As the percentage of ODPA is increased, the isotherms increasingly take on









characteristics of the liquid-condensed phase of ODPA, although features present in

the isotherms of the pure porphyrins are also present in the isotherms of the mixed

films (Figure 3.5 and 3.7).





0.12 -
50 .-'
0.10 -_40 f
E: 30
0.08 20 e
8 I1 C d
U 0.06- 0
f' 25 50 75 100125150
M MA (A' molecule')
00 b
0.02- a -

0.00

-0.02
360 380 400 420 440
Wavelength (nm)



Figure 3.8: Reflectance UV-vis of PdP1 on water subphase.



In addition, the collapse pressure increases with the concentration of ODPA indicating

that the films become more stable as ODPA is added. However, diluting the porphyrin

film with ODPA does not appear to greatly affect the aggregation. Reflectance UV-

vis of a Langmuir monolayer of a 1:9 mixture of PdP4 with ODPA is shown in Figure

3.9. The Xmax shifts from 415.5 nm at high MMA to 420 nm as the film is

compressed, just as it does in the films of pure PdP4 (Figure 3.9). However, the

porphyrins do not appear to be aggregated in the mixed film at high MMA.












0.03- 60 e e

45
d

C

0.01 2 30 40 50 b
I a
oMoW (A9 rr2eaie)


0.00


-0.01I
360 380 400 420 440
Wavelength (nm)



Figure 3.9: Reflectance UV-vis of 10% PdP4: 90% ODPA on a water subphase.



The molecular areas in Figures 3.5 and 3.7 are weighted averages of the

porphyrin and ODPA molecules. The MMA of the porphyrin molecules in the mixed

films can be calculated using Equation 3.1: 89



S (SPOR + NSOPA) (3.1)
(N+1)



where Smix is the MMA of the mixture determined from the isotherm, SPOR is the

MMA of the porphyrin within the mixed films, SODPA is the MMA of the ODPA

amphiphile in pure ODPA films, and N is the molar ratio of ODPA to porphyrin.

SPOR was calculated in the ODPA mixtures of each porphyrin at pressures of 5 mN m-1

and 15 mN m-1 and the results are plotted in Figure 3.10.






69







300
A 5 mN m1 75 B m 5mN m"
225- 15mN m-1 '-~ 15mN m

150- -

75. 25

01 0
0 2 4 6 81 10 0 2 46 81 0
N (OPA/POR) N (OPA/POR)



Figure 3.10: Mean molecular area vs. ratio of ODPA/Porphyrin: A) PdP4, B) PdPl.



If the ODPA diluent were breaking apart the preferred organization of the

porphyrins in the films, SPOR would increase as the aggregates separate. The decrease

in SPOR in the mixed films suggests that either the porphyrin chromophores are

reorienting in the mixed films or aggregation actually increases in the mixed films.

However, it does not appear that porphyrin aggregation decreases in the mixed

monolayers.



3.2.3 Langmuir-Blodgett Films

LB films of PdP4 and PdPI were prepared using the deposition procedure

described in Figure 2.2. The stepwise deposition allows fabrication of both symmetric

films, where the template and capping monolayers are the same, and alternating films,

where the two monolayers in the bilayer are different. Both types of films were

prepared for each porphyrin (Figure 3.2). It has been shown that a zirconated ODPA

template layer frequently provides the best substrate for transferring a capping layer.65


1









The extremely well organized and oxophilic surface allows deposition of almost any

phosphonic acid monolayer, including those that are not stable monolayers and would

normally not transfer. Monolayers of PdP4 and PdPI were transferred at a range of

temperatures, pressures and subphase pHs (Tables 1 and 2). Films of the porphyrins

mixed with ODPA were also transferred under a variety of conditions. Under some

conditions, perfect, organized monolayers were obviously not formed, but the films

could be transferred onto solid supports and studied.

3.2.3.1 Films of compound PdP4. To form alternating films of PdP4, the

Langmuir monolayers were transferred as capping layers onto zirconated ODPA

template layers. Films were transferred at different surface pressures and the Soret

Band of the transferred films was used to monitor differences in chromophore

aggregation in the deposited films. The UV-vis spectrum of a film transferred at 130

A2 molecule-1 (15 mN m-1) is shown in Figure 3.11, where the kmax of the Soret Band

appears at 420 nm, significantly red-shifted from any of the solution spectra of PdP4.

The red-shift suggests increased aggregation, which is expected because at such a

small MMA, the chromophores must be either tilting perpendicular to the surface and

organizing side-by-side, or sliding over one another to form bilayers or multilayers.

Polarized UV-vis spectroscopy indicates the porphyrins are oriented parallel to the

surface, implying the latter arrangement.

Layers of PdP4 were also transferred at 190 (12mN m-1) and

300 A2 molecule-' (Figure 3.11). The Soret Band shifts to 418 nm for the film

transferred at 190 A2 molecule-1 and to 416 nm for the film transferred at 300 A2

molecule-', indicating less aggregation in films transferred at high MMA. At these

larger MMA, the porphyrin chromophores should be lying flat at the air-water

interface with little aggregation and they appear to remain non-interacting when

transferred.









Table 3.1: UV-vis data from symmetric and alternating films of PdP4. L, is given
for monolayers, and interlayer thickness is given for multilayers of films transferred
under a variety of transfer conditions.

Film Transfer FI of pH*** Temp ,, thickness
Area Transfer (C) (nm) (A)
(A2 mol.')* (mN/m)


OPA/Zr/PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/10% PdP4
OPA/Zr/10% PdP4
OPA/Zr/25% PdP4
OPA/Zr/50% PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/ PdP4
OPA/Zr/25% PdP4
OPA/Zr/25% PdP4
OPA/Zr/10% PdP4
OPA/Zr/10% PdP4
PdP4/Zr/ PdP4
PdP4/Zr/ PdP4
10% PdP4/Zr/ 10%
PdP4


300
190
180
130
100
90
37
33
50
74
300
190
300
85
160
50
60
35
50
190
130
37


- 5.5
4 5.5
5 5.5
15 5.5
25 5.5
35 5.5
5 5.5
15 5.5
15 5.5
15 5.5
- 9.4
4 9.4
- 11.1
15 5.5
4 5.5
15 5.5
4 5.5
15 5.5
4 5.5
4 5.5
15 5.5
5 5.5


isothermss)


23-25
23-25
23 25
23-25
23-25
23 25
23-25
23-25
23 25
23-25
23-25
23-25
23-25
40
40
40
40
40
40
23 25
23 25
23-25


416
418
418
420
420
420
415
418
420
420
416
416
414
419
417
415
415
415
415
416
418
418


* Area of the chromophore and diluent as determined from Figure 3.5
** Corresponding pressure from Figure 3.5 isothermm)
*** pH of nano-pure water from filtration system is about 5.5













0.04- -- -300 A molecule"






0.002 \\ -

350 400 450 500 550 600
Wavelength (nm)


Figure 3.11: Transmission UV-vis of PdP4 films transferred at high and low MMA.
Absorbance scale corresponds to the film transferred at 300 A2 molecule'. The
absorbance for the film transferred at 130 A2 molecule-' has been divided by 10.

As the pH was raised, the amphiphiles became slightly more water-soluble and

the monolayer was increasingly susceptible to creep. However, films ofPdP4

compressed to 300 A2 molecule-1 were deposited onto zirconated ODPA templates

from subphases of pH 9.4 and 11.1. As the pH increased, Xmax of the Soret Band of

the transferred film decreased to 414 nm for the film deposited at pH 11.1. This was

the lowest value of Xmax, and therefore, the least aggregated LB transferred film of

PdP4. The Xmax of this Soret Band corresponds to that ofchromophore PdP4 in EtOH

at 10-12 M which is believed to be non-aggregated.

Consistently, D = 1 0.02 when measured at 0 incidence, indicating no

preferred in-plane orientation of the chromophore in the PdP4 and PdPI films.

However, in all films, D # 1 when measured at 45 o incidence. For films transferred at

high surface area, it is expected that the porphyrins should lie flat with all four

phosphonates tethered to the surface. Indeed, this is observed for films transferred at









190 A2 molecule-1 and 300 A2 molecule-' where the tilt angle, 0, with respect to the

surface normal is observed to be 90. Interestingly, the porphyrins also appear to lie

parallel to the surface in the films transferred at 130 A2 molecule-' where 0 is also

measured as approximately 90 o. This result implies that in films transferred at areas

smaller than the MMA of the flat porphyrin macrocycle, the molecules overlap,

stacking in bilayers or multilayers but with very little change in the tilt angle. There is

a larger uncertainty, possibly 10 in the measurement as the tilt angles near 90 0;26

however, these results confirm that the chromophores are lying approximately flat in

all of the films in this study.

Multilayers of the alternating ODPA/Zr/PdP4 films can be deposited and X-ray

diffraction confirms the layered nature of the films. Two or three orders of the (001)

Bragg peaks can be observed in each case. Films transferred at 190 A2 molecule-'

have a bilayer thickness of 42 A, which is smaller than the 48 A thickness seen in pure

ODPA/Zr/ODPA bilayers,28 suggesting that the 18-carbon tethers of PdP4 are not

fully extended in the alternating films. For the film transferred at 130 A2 molecule-,

the bilayer thickness increases to 47 A as the tetrasubstituted chromophores begin to

overlap.

Symmetric bilayers of PdP4/Zr/PdP4 fabricated according to Figure 2.2, were

also studied. Porphyrin PdP4 could be transferred on the down stroke onto a

hydrophobic substrate under a variety of conditions. After zirconation, deposition of a

capping layer of PdP4 results in a symmetric bilayer. The Soret Band is very similar

to that from alternating films deposited at the same area per molecule, and polarized

UV-vis indicates the porphyrins are also lying parallel to the surface. However, the

layers are poorly organized, as (001) Bragg peaks could not be seen in diffraction from









9-bilayer films. It is probably poor organization in the template layer ofporphyrin

PdP4 that is responsible for the lack of a well-defined layered structure.38

Mixed monolayers of PdP4 with ODPA were transferred onto ODPA templates

at different points along the surface pressure vs. area isotherms as shown in Table 1,

and the aggregation of the porphyrin in the transferred film parallels that seen in the

films of the pure porphyrins. For films transferred at pressures of 15 mN m-1, the

Soret Band appeared at 420 nm, shifting to 415 nm when transferred at pressures less

than 5 mN m-1 which, again, corresponds to the non-aggregated form seen in EtOH.

In all cases, polarized UV-vis indicates that the porphyrins orient parallel to the

surface.

The mixed monolayers show an interesting effect with increased temperature.

A mixed monolayer of 10% PdP4 with ODPA transferred at 15 mN m-' on a subphase

heated to 40" C shows a Soret Band max of 415 nm, shifted from 420 nm for the same

film deposited at room temperature. As the subphase is heated, the aggregates appear

to break-up in the mixed film. A similar effect is not seen on the pure films of PdP4.

It appears that the ODPA plays a role in breaking up the aggregated domains at higher

temperatures.

Films of PdP4 were also prepared by the SA technique. After the zirconated

ODPA template had been exposed to a PdP4 solution in EtOH/H20 for 2 hr, the

porphyrins were successfully incorporated into these films. The Soret Band appeared

at 414 nm, which then shifted to 411 nm after 60 min rinsing in hot CHC1. The Xmx

in the SA film was the closest of any of the PdP films to that seen in the dilute

solution. Therefore, it appears that non-aggregated assemblies of PdP4 are easily

obtained by self-assembly (Figure 3.12). However, the overall absorbance intensity of

these non-aggregated films is lower than observed in the films transferred by the LB

technique at high MMA.
















0.010


B 0.005-


0.000

-0.005
350 375 400 425 450 475
Wavelength (nm)


Figure 3.12: UV-vis of SA PdP4 films rinsed in hot CHCl3.



3.2.3.2. Films of compound PdP1. The H-A isotherms and the reflectance

UV-vis experiments described above indicate that the molecules of PdP1 aggregate

upon spreading, and this aggregation is preserved in the transferred films. In contrast

to PdP4, the monophosphonate PdPI is only slightly influenced by attempts to break

up the aggregates by changing the deposition conditions. The UV-vis spectrum of a

capping layer of PdP1 transferred at 52 A2 molecule-1 (12 mN m-1) is shown in Figure

3.13, where the Soret Band appears at 426 nm, consistent with the value observed in

the reflectance spectrum taken from the water interface. The shape of the Soret Band

does not change for films deposited at higher MMA, higher temperatures, or in

mixtures with ODPA. The peak position shifts only slightly (Table 2). The

orientation of the chromophores were also unaffected by the deposition conditions.

Polarized spectra consistently give tilt angles of 900, corresponding to the porphyrins

lying flat.


















0.04


0.02 -


0.00 l%

400 500 600
Wavelength (nm)



Figure 3.13: Transmission UV-vis of films of PdP 1 transferred at high and low
MMA.


X-ray diffraction from alternating films ofPdPI transferred at 52 A2

molecule-1 onto a zirconated ODPA template gives a layer thickness of 61 A (Table

3.2). This thickness is larger than that of the alternating films of ODPA/Zr/PdP4 or

ODPA/Zr/ODPA bilayers.28 Since optical spectroscopy indicates the molecules lie

flat, the enhanced thickness of the layer suggests they transfer as stacked bilayers or

multilayers. Further evidence for this arrangement comes from the film stability

studies, described below, which indicate that part of the transferred film of porphyrin

PdPI is physisorbed to the surface.









Table 3.2: UV-vis data from symmetric and alternating films ofPdPl. X, is given
for monolayers, and interlayer thickness is given for multilayers of films transferred
under a variety of transfer conditions.

Film Area of HI of Temp X. (nm) thickness
Transfer Transfer (C) (A)
(A2/molecule)* (mN/m)


OPA/Zr/PdPI
OPA/Zr/PdPI
OPA/Zr/ PdP1
OPA/Zr/ PdP 1
OPA/Zr/ PdP1
OPA/Zr/10%
PdPI
OPA/Zr/25%
PdP1
OPA/Zr/50%
PdPI
OPA/Zr/ PdP 1
OPA/Zr/ PdP1
OPA/Zr/ PdP 1
OPA/Zr/ PdPI
OPA/Zr/10%
PdP1
OPA/Zr/10%
PdPI
OPA/Zr/25%
PdPI
PdP1/Zr/ PdPI
10% PdP1/Zr/
10% PdPI


73
52
41
38
36
27


30


38


300
190
300
73
30


26


30


52
26


23-25
23 25
23-25
23 25
23 25
23-25


12 23-25


12 23-25


23-25
23-25
23-25
40
40


40


40


23-25
23-25


426
426
428
428
428
426


426


426


416
416
414
424
424


425


424


426
426


* Area of the chromophore and diluent as determined from Figure 3.7 isothermss)
** Corresponding pressure from Figure 3.7 isothermm)
*** pH ofnano-pure water from filtration system is about 5.5











3.2.3.3. Film stability. Zirconium phosphonate LB films are insoluble in

organic solvents due to the cross-linking within the zirconium-phosphonate extended

network.28 In order to monitor how well the porphyrin layers bind to the zirconated

template, transferred films of PdP4 and PdPI were rinsed with chloroform in a Soxhlet

extractor. Figure 3.19 shows the Soret Band absorbances as a function of washing

time in hot CHCl3. Figure 3.14 shows that none of the film of PdP4 is washed away

after 1 hr in chloroform, suggesting that all of the molecules are tethered to the

zirconated ODPA template. The same result was obtained for films of PdP4 deposited

at both higher and lower pressures or in mixtures with ODPA.

In contrast, the absorbance of the LB films of PdP1 exposed to CHC13

decreased significantly due to the desorption ofchromophores. The absorbance

leveled off after 20 min, to a value corresponding to the truly surface confined

chromophores (Figure 3.14). This result suggests that the stacked layers of

chromophores in the porphyrin PdPI films were partially physisorbed on the surface.



0.125-

0.100

0.075 a
-o
^ 0.050
< ~xS>


Time in Soxhlet (min)


Figure 3.14: Absorbance of Soret vs. time rinsed in hot CHCl3: a) PdP4, b) PdP1.











3.3. Conclusions


The behavior of the monosubstituted and tetrasubstituted porphyrins is very

different on the water surface, and the differences are carried over to the transferred

films. Porphyrin PdPI spreads on the water surface to a limited extent. Our proposal

for how the molecules behave on the water surface is shown schematically in Figure

3.15. Optical spectroscopy indicates that PdPI aggregates, but the nr-A isotherm and

X-ray diffraction from the transferred layers suggest that the aggregates are only a few

molecules thick. The aggregates are present at both high and low MMA and can be

transferred, as aggregates, onto the zirconated ODPA templates. Some molecules

from each aggregate chemisorb to the zirconated surface through zirconium

phosphonate linkages, but some are physisorbed as part of the preformed aggregates.

When exposed to hot chloroform, the physisorbed part of the film is dissolved away.



50 I



S30

120 'T .

10 "1--

0
20 40 60 80 100 120 140
MMA (A2 molecule)



Figure 3.15: Illustration of orientation and packing of PdP films transferred at high
and low MMA.









Chemical modification with four alkylphosphonic acid sidegroups allows the

porphyrin to spread completely. Porphyrin PdP4 spreads to a monolayer thick film at

high MMA, and as the film is compressed, an increase in surface pressure is registered

near 200 A2 molecule-1, corresponding to the area of the flat porphyrin macrocycle.

However, the side-by-side arrangement of the porphyrin chromophores is not stable as

the pressure is increased and the film rearranges, with the molecules sliding over one

another to form multiple chromophore layers. This behavior is illustrated in Figure

3.16.




50

40



S20

10-

0-
50 100 150 200 250
MMA (A2 molecule)



Figure 3.16: Illustration of orientation and packing of PdP4 films transferred
at high and low MMA.


The films of PdP4 can be transferred at high MMA onto zirconated ODPA

templates to form monolayer or submonolayer films of the porphyrin chemisorbed to

the surface with the chromophore ring oriented parallel to the surface. Films of PdP4

can also be transferred at lower MMA, where the reflectance UV-vis indicates the









porphyrins are interacting. Analysis of the transferred films suggests that the

porphyrin chromophores are lying flat and overlapping each other to form layers that

are a few molecules thick. In contrast to the films of PdPI, all of the molecules in the

aggregated films ofPdP4 appear to be chemisorbed to the surface. None of the film is

lost during rinsing with hot chloroform. The different behavior probably results from

the fact that four phosphonic acid groups increase the chance of each molecule

bonding onto the zirconium phosphonate network. Any orientation of the porphyrin

macrocycle will direct at least one alkylphosphonic acid side chain toward the surface.

Also, all of the phosphonic acids have the potential to reach the water surface at high

MMA. Forcing a strongly hydrophilic group off of the water surface requires more

energy than reorganization of the alkyl chains or shifting the chromophore

interactions.

Whether the Langmuir monolayers are transferred intact or reorganized during

film transfer is not yet completely clear. At high MMA, molecules of PdP4 lie flat on

the water surface and this arrangement appears to be preserved in the transferred films

based on the similar m~,. However, when the films are transferred at lower MMA,

where the films are clearly aggregated, there could be some rearrangement. There is a

significant driving force for forming zirconium phosphonate linkages, and the

aggregates could rearrange during transfer to further maximize interactions with the

zirconated surface. While the porphyrins are clearly oriented parallel to the surface in

the transferred film, providing each porphyrin the chance to form multiple zirconium

phosphonate bonds, it is not known if the molecules aggregate the same way on the

water surface.

The LB procedure used in these studies takes advantage of the binding energy

of the zirconium phosphonate continuous network and is shown to be quite versatile.

Both symmetric and alternating layer films can be prepared. Use of the zirconated









ODPA template layer allows almost any phosphonic acid derivatized amphiphile to

transfer in a capping layer.28,38,65,125 Unlike conventional LB depositions, films of

PdP4 and PdPI can be transferred onto the zirconated template layers at any surface

pressure, allowing, in the case of PdP4, the arrangement of the molecules in the

transferred films to be tuned by choice of the area-per-molecule at deposition. The

films do not need to be stable Langmuir monolayers in order to transfer, as the driving

force is formation of the zirconium phosphonate bonds. It is the strength of the

zirconium phosphonate interaction, in particular the lattice energy associated with the

zirconium phosphonate extended network, that is responsible for the exceptional

stability of these non-traditional LB films.28,38

Zirconium phosphonate LB films, like solid-state zirconium phosphonates, are

insoluble in organic solvents and under most aqueous conditions. The inorganic

network has also been shown to enhance the thermal stability of LB films.126 The

zirconium phosphonate inorganic extended network adds substantial stability to the

films, which are insoluble under most organic and aqueous conditions. The methods

developed here with the palladium tetraphenyl porphyrins can also be applied to other

porphyrin systems, and in this way the vast array of physical and chemical
characteristics of porphyrins, including catalytic activity, should be able to be

incorporated in stable LB films.












CHAPTER 4
MANGANESE PORPHYRIN CONTAINING
ZIRCONIUM PHOSPHONATE THIN FILMS


4.1 Background


Monolayer and film work using the molecule manganese 5,10,15,20-

tetrakis(2,3,5,6-tetrafluorophenyl-4-octadecyloxyphosphonic acid)porphyrin, or

MnP4, will be discussed in Chapter 4 (Figure 4.1A). For comparison, work done

using a similar molecule without the four alkylphosphonic acid chains, manganese

5,10,15,20-tetrakis(penta-fluorophenyl)porphyrin, or MnPO, will also be discussed

(Figure 4.1B). The manganese porphyrins are structurally and chemically more

complex than the palladium porphyrins. The Mn(III) central metal is a 5- or 6-

coordinate d4 metal.80,127,128 Depending on the ligand character, Mn(III) is either S =

2 (high spin) or S = 1 (low spin).129 Also, depending on the axial ligand or ligands,

the Mn(III) may or may not be co-planar with the porphyrin ligand. Mn(III) also has

an easily accessible lower oxidation state, which leads to significant metal/porphyrin

electronic interactions,80 the Mn(III)-porphyrins have a tendency to form face-to-face

dimers bridged through an axial ligand,130 and the Mn(III)-porphyrins are vulnerable

to demetallation under certain conditions. Therefore, film characterization using these

molecules was much more complicated than with the Pd-porphyrins.

To investigate the catalytic properties of manganese-porphyrin films, film

preparation procedures involving the tethering of Mn-porphyrins to a metal








phosphonate network were developed. This method involves the initial formation of a
zirconated octadecylphosphonic acid (ODPA) template onto which a film of pure
MnP4 can be SA or transferred via the LB technique (Figure 2.2). Including MnP4 in
a zirconium phosphonate network provided films that were stable toward harsh
organic conditions. Also, the strong oxophilicity of the zirconium for the phosphonate
oxygens enabled the film preparation procedure to be easily altered and fine tuned and
complete film characterization to be carried out.



A O(CH 2)18PO 3H2 (R) B F
F F F F
I c II
F F F F



F /)<.^ F F F F F



R F


Figure 4.1: Structures of A) MnP4 and B) MnPO.


The MnP4 molecule is similar to the PdP4 molecule described in Chapter 3, in
which the manganese tetraphenylporphyrin (MnTPP) chromophore and the strongly
hydrophilic phosphonate groups are separated by 18-carbon chains (Figure 4.1 A). This
geometry allowed for the porphyrin to be sitting at the exterior of the film and
available for catalysis while the phosphonates were buried in the hydrophilic region
and available for binding to the stabilizing inorganic network. The incorporation of
this network significantly improves the resistance of the film to typically destructive









forces such as solvent, heat, or time.126 In addition, having four amphiphilic chains on

the porphyrin permits the formation of Langmuir monolayers of these materials

without diluting the amphiphiles with a good film forming amphiphile such as stearic

acid.33,88-90

The MnP4 films were first investigated on the water surface in a Langmuir

monolayer. An isotherm of this material showed significant film compressibility

(Figure 4.2). Reflectance UV-vis showed that the porphyrins formed face-to-face

dimers above ca. 10 mN m"', which were maintained upon transfer onto glass

substrates.

The procedure for MnP4 film formation was directed by the film

characterization results. In most cases, evidence suggests that the phosphonic acid

tethers on the porphyrins were able to bind to the metal phosphonate lattice. Film

stability was monitored by UV-vis, which displayed no significant chromophore loss

after 5 minutes in hot CHCl3 or CH2Cl2. Although the zirconium phosphonate lattice

contributes no interesting physical phenomena to the final film, the strong oxophilicity

of the phosphonate oxygens for the zirconium lattice allow for a wide variety of stable

films to be formed.


4.2 UV-vis Behavior of MnTPPs


4.2.1. Solution Studies

Both MnTPPs displayed electronic behavior in solution consistent with d-type

hyperporphyrins. Mn(II)-porphyrins have absorption spectra similar to free-base

porphyrins due to a lack of metal-porphyrin interaction; however, the absorption

spectrum of the Mn(III)-porphyrin is quite different. According to Gouterman,80

Mn(III)TPP is a classic d-type hyperporphyrin with extra absorption bands at higher









energies relative to max. The MnP4 and MnPO with chloride axial ligands are

spectrally consistent with d4 porphyrins in high-spin configurations.131 The strong

absorption near 450-485 nm is often called Band V, due to the fact that this transition

is not pure 7t-n* in nature but includes metal-porphyrin orbital mixing. However,

traditionally, it is still often called the Soret Band.80 One strong band commonly seen

to the blue of the Soret Band is called Band VI. A prominent peak, often observed

specifically in the MnP4 UV-vis spectrum ca. 410 nm, is referred to as Band Va.

Ligand effects and orientation or aggregation effects are reflected primarily in

the shape or shift of the Soret Band and in the extinction coefficient (e).132 Therefore,

the behavior of this band was carefully monitored. Also, different solvents used for

porphyrin investigations caused changes in the absorption spectra. If a coordinating

solvent was used, the axial ligand was displaced by a solvent molecule causing a shift

in the Soret Band and in the V/VI intensity ratio.80,132

4.2.1.1. UV-vis of MnPO in solution. A concentration study of MnPO in

CH2C12 showed that the Xmax was consistently at 475 477 nm between 10-5 and 10-8

M (8 = 1.25 x 107 M-' m-'). These results suggest that the MnPO chromophore had

little tendency to aggregate in these dilute solutions, and that the axial ligand was

probably chloride. 132 In EtOH, the max of the MnPO solution was also constant over

the same concentration range; however, the Soret Band was blue shifted to 454 456

nm. According to Mu, the coordination of two axial methanol ligands to a MnTPP

caused a 10 nm blue shift relative to the chloride bound moieties; therefore, it is

believed that this blue shift is due to bis-EtOH binding.133 Figure 4.2 shows the

solution UV-vis of MnPO in EtOH and CHC13 at 10" M. Not only is the Soret Band

shifted, but the ratio of Band V to Band VI has also changed, indicating a change in

the metallo-porphyrin ligand environment.