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This is to certify that the
dissertation entitled

INVESTIGATION OF SELECTED PHOTOCHROMIC
COMPOUNDS FOR OPTICAL DATA STORAGE

presented by

ALEXIS A BLEVINS

has been accepted towards fulfillment
of the requirements for the

Ph.D. degree in Chemistry

 

 

MA

” ’Major Piofessor’s Signature

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DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6/07 p:/CIRC/DateDue.indd-p.1

 

INVESTIGATION OF SELECTED PHOTOCHROMIC COMPOUNDS FOR
OPTICAL DATA STORAGE

BY

Alexis A Blevins

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

2007

ABSTRACT

INVESTIGATION OF SELECTED PHOTOCHROMIC COMPOUNDS FOR
OPTICAL DATA STORAGE

By

Alexis A Blevins

We report on the steady state and transient spectroscopy and
characterization of a series p-diamido azobenzene and p-hydroxyspiropyran
compounds. Of particular interest in these systems is the mechanism and
energetics of the isomerization dynamics. Steady state spectroscopic results in
conjunction with semi empirical modeling of isomerization in these systems
reveals that the dominant ground state isomerization mechanism is transient
rehybridization with the activation energy depending sensitively on the identity of
the substituents for the family of azobenzene compounds. Spiropyrans
incorporate the added mechanism of the formation and fission of a 0-0 bond,
however with the added mechanism the complexity of the system increases.
These results place limits on the utility of these compounds, in particular the

mechanism of isomerization, for optical information storage applications.

In loving memory of Michael David Evans
and our attempt to be simple men

ACKNOWLEDGMENTS

I would like to thank first and foremost Gary Blanchard and the entire
Blanchard group without whom none of this research would have been possible.
Of the many members several require additional acknowledgment; John Roberts
for assistance in surface experiments and purification of grams of spiropyran, Jay
Major for enforcing work ethics and still having fun, and finally Michelle Rini who
is my comrade in bad luck. I would like to thank the faculty in entirety for support
and insight during my tenure at Michigan State University, especially my
committee members, Greg Swain, Dana Spence, and Greg Baker. I will also
always be thankful for the time that l have been granted with Dr. Kathy Severin,
Dr. Tom Carter, and Dr. Merlin Bruening because I have learned so much about
science and life from you.

I have to thank my parents for giving me the support and encouragement
that I needed to be successful in graduate school and hopefully for life. I know
that I have learned my work ethics and desire for knowledge from your examples.
My family has been a huge support network that I have relied on heavily during
this process and I would like to say thank you publicly. So many things could
have happened when dad died but we stuck together and that means everything
to me, and has allowed me to finish this journey that I began with his support.
Lastly I would like to thank my wonderful wife Sandra. If I gained nothing further
from graduate school than her hand in marriage I would still consider it to be a
success. I appreciate your support and understanding and I will always treasure

it. For those whom I missed despite my best intent, thank you.

TABLE OF CONTENTS

Chapter 1: Introduction ........................................................................... 1
Chapter 2: Response of Azobenzene to Irradiation ...................................... 21
Chapter 3: Synthesis of Azobenzene Derivatives ........................................ 29
Chapter 4: Azobenzene Calculations ....................................................... 49
Chapter 5: Time Resolved Spectroscopy ................................................... 61
Chapter 6: Spiropyran Introduction .......................................................... 74
Chapter 7: Spiropyran Synthesis .............................................................. 81
Chapter 8: Spiropyran Spectroscopy ....................................................... 115
Chapter 9: Conclusions ........................................................................ 147
Appendix A: Mass Spectra of Azobenzene Compounds .............................. 153
Appendix B: Published Azobenzene Calculated Energy Surfaces ................. 159
Appendix C: Mass Spectra of Spiropyran Compounds ................................ 166

LIST OF TABLES

Table 3.1. Reaction conditions and yields for synthesized azobenzene
compounds ......................................................................................... 40

Table 3.2. Absorption maxima and molar absorptivities of azobenzene, p-
diaminoazobenzene, and the p-diamidoazobenzenes 1-5 ............................. 43

Table 7.1. Extinction coefficient of p-hydoxyspiropyran, extinction coefficients
are reported in units of L / mol*cm .......................................................... 112

Table 8.1. Optical response of p-hydroxyspiropyran ordered by time constant of
optical opening .................................................................................. 123

Table 8.2. Optical response of p-hydroxyspiropyran ordered by time constant of
optical opening ................................................................................... 124

Table 8.3. Optical response of p-hydroxyspiropyran ordered by absolute
increase in absorbance of wavelength of interest ...................................... 125

Table 8.4. Optical response of p-hydroxyspiropyran ordered by wavelength of
maximum absorbance in the visible region .............................................. 126

Table 8.5. Time constant for thermal closing of optically opened p-
hyd roxyspiropyran .............................................................................. 1 32

Table 8.6. Calculated equilibrium constants for thermal relaxation of p-
hydroxyspiropyran correlated to solvent properties .................................... 144

vi

LIST OF FIGURES

Figure 1.1. Binary logic recording versus n-bit logic recording, in this example n
= 8 ...................................................................................................... 4

Figure 1.2. Optical storage device based on current generations technology.
Adapted from reference 1 ........................................................................ 6

Figure 1.3. 30 arrangement for optical storage device based on stacks of 20
recording layers adapted from reference 2 .................................................. 7

Figure 1.4. General photochromic molecule behavior .................................... 9
Figure 1.5. 2D representation of the energy diagram for general isomerization
process. The lower the isomerization barrier the shorter the

time constant 1 ..................................................................................... 12

Figure 1.6. Examples of photochromic molecules and the geometric
rearrangements that occur ..................................................................... 13

Figure 1.7. Example of side reactions and thermal back reactions on a fulgide
system. An elimination reaction that takes place is the elimination of an ethyl
group by extracting the proton boxed. Adapted from reference 1 ................... 14

Figure 2.1: Schematic of one possible layered polymer system for separations
based on change in free volume .............................................................. 24

Figure 2.2: lsomerization mechanisms for azobenzene. Adapted from reference
1 3 ..................................................................................................... 25

Figure 3.1. Calculation of change in position and spacing of azobenzene
terminal groups, R, upon isomerization for both meta and para disubstituted
azobenzene ....................................................................................... 30

Scheme 3.1: Synthetic route to disubstituted azobenzene ............................ 34

Figure 3.2 NMR spectrum of p—diamidoazobenzene before and after

recrystallization ................................................................................... 36
Figure 3.3: NMR spectrum of azobenzene in deuterated methanol ................. 37
Figure 3.4: Proton NMR spectrum of p-diaminoazobenzene .......................... 38
Figure 3.5: Proton NMR spectrum of p—diamidoazobenzene .......................... 38

vii

Figure 3.6: NMR spectra of a series of p-diamidoazobenzenes ...................... 39
Figure 3.7: Absorbance spectrum of azobenzene ....................................... 46

Figure 3.8: Absorbance spectrum of p-diaminoazobenzene .......................... 46

Figure 3.9 : Absorbance spectra ‘of azobenzene, p—diaminoazobenzene, and p-
diamidoazobenzene .............................................................................. 47

Figure 4.1: Calculated energy levels and oscillator strengths for azobenzene,
diaminoazobenzene, and diamidoazobenzene ........................................... 52

Figure 4.2 Three dimensional isomerization energy diagram for rehybridization
mechanism for azobenzene .................................................................... 53

Figure 4.3 Three dimensional isomerization energy diagram for rotation
mechanism for azobenzene .................................................................... 54

Figure 4.4 Three dimensional isomerization energy diagram for rehybridization
mechanism for diaminoazobenzene ......................................................... 55

Figure 4.5 Three dimensional isomerization energy diagram for rotation
mechanism for diaminoazobenzene ......................................................... 56

Figure 4.6 Three dimensional isomerization energy diagram for rehybridization
mechanism for diamidoazobenzene ......................................................... 57

Figure 4.7 Three dimensional isomerization energy diagram for rotation
mechanism for diamidoazobenzene ......................................................... 58

Figure 5.1. Absorbance spectra of 5 upon varying durations of ultraviolet
irradiation ........................................................................................... 64

Figure 5.2 . Block diagram of UVNis instrument modification ........................ 65

Figure 5.3. Example of an absorbance versus concentration graph used to
determine molar absorptivity ................................................................... 65

Figure 5.4. Recovery of the absorption due to the trans band in azobenzene at
318 nm .............................................................................................. 68

Figure 5.5. Recovery of the absorption due to the trans band in p-
diaminoazobenzene at 370 nm ............................................................... 69

Figure 5.6. Recovery of the absorption due to the trans band in p-
diamidoazobenzene at 370 nm ................................................................ 7O

viii

Figure 6.1. Photoreactivity of a spiropyran. .............................................. 76

Figure 6.2. Absorbance of p-nitrospiropyran. In the absorbance spectrum A is

the closed form of the molecule and B is the open form ............................... 77
Figure 7.1. Terpolymer system for substrate resurfacing .............................. 82
Figure 7.2. Locations for chemical attachment points on spiropyrans ............ 84

Figure 7.3. Calculated isomerization barrier of p-nitrospiropyran. This
calculation only incorporates the isomerization and not the bond formation ...... 85

Figure 7.4. Calculated isomerization barrier of p-hydroxyspiropyran. This
calculation only incorporates the isomerization and not the bond formation ...... 86

Scheme 7.1. Original synthesis adapted from reference 23 .......................... 88
Scheme 7.2. Synthetic route used to synthesize hydroxyl substituted

spiropyran .......................................................................................... 90
Figure 7.5. NMR proton spectrum of Fischer’s Base starting material ............. 92
Figure 7.6. Proton NMR of 2,5—dihydroxybenzaldehyde ............................... 94
Figure 7.7. Predicted proton NMR shifts ................................................... 95
Figure 7.8. Proton NMR spectrum of p-HBSP in acetonitrile .......................... 96
Figure 7.9. Effect of temperature on NMR spectrum of closed p-HBSP............97

Figure 7.10. NMR spectrum of optically opened and closed forms of p-HBSP at
33°C .................................................................................................. 98

Figure 7.11. NMR spectrum of optically opened and closed forms of p-HBSP at
33°C ................................................................................................... 99

Figure 7.12. NMR spectrum of optically opened and closed forms of p-HBSP at -
60°C ................................................................................................ 100

Figure 7.13. NMR spectrum of optically opened and closed forms of p-HBSP at -
60°C ................................................................................................ 101

Figure 7.14. Cosy spectrum of p-HBSP in acetonitrile and the predicted and
observed proton shifts ......................................................................... 103

Figure 7.15. Expanded view of Cosy spectrum of p-HBSP .......................... 104

Figure 7.16. Absorbance of p-nitrospiropyran in methanol. The black line is the
absorbance of the closed form and red line is the open form of the
ch romophore ..................................................................................... 1 05

Figure 7.17. Comparison of absorbance spectra of p-nitrospiropyran and p-
hyd roxylspiropyran .............................................................................. 1 06

Figure 7.18. Absorbance of p-hydroxyspiropyran in methanol. The black line is
the absorbance of the closed form and red line is the open form of the
chromophore ..................................................................................... 107

Figure 7.19. Absorbance of benzaldehyde in methanol. The black line is the
absorbance of the closed form and red line is the open form of the
chromophore ..................................................................................... 108

Figure 7.20. Absorbance of Fischer’s Base in methanol. The black line is the
absorbance of the closed form and red line is the open form of the
chromophore ..................................................................................... 1 09

Figure 7.21. Determining the extinction coefficient from the concentration versus
absorbance plot where the slope is the extinction coefficient ........................ 110

Figure 8.1. Formation of open form of p-hydroxyspiropyran upon increasing
ultraviolet irradiation time ..................................................................... 1 17

Figure 8.2. Formation of open form of p-hydroxyspiropyran upon increasing
ultraviolet irradiation time ..................................................................... 1 18

Figure 8.3. Formation of open form of p—hydroxyspiropyran upon increasing
ultraviolet irradiation time ..................................................................... 1 19

Figure 8.4. Formation of open form of p-hydroxyspiropyran upon increasing
ultraviolet irradiation time ..................................................................... 120

Figure 8.5. Relaxation of open form of p-hydroxyspiropyran to closed form with
increasing amounts of dark time ............................................................ 128

Figure 8.6. Relaxation of open form of p-hydroxyspiropyran to closed form with
increasing amounts of dark time ............................................................ 129

Figure 8.7. Relaxation of open form of p-hydroxyspiropyran to closed form with
increasing amounts of dark time ............................................................ 130

Figure 8.8. Relaxation of open form to closed form of p-hydroxyspiropyran with
increasing amounts of dark time ............................................................ 131

Figure 8.9. Closing of p-hydroxyspiropyran after initial relaxation time by visible
light ................................................................................................. 1 34

Figure 8.10. Closing of p-hydroxyspiropyran after initial relaxation time by visible
light ................................................................................................. 135

Figure 8.11. Possible equilibrium reactions for spiropyrans. Of these the photo
closing has the fewest variables to consider ............................................. 137

Figure 8.12. Plots of absorbance versus time to determine the order of the
kinetics of the closing reaction of p-hydroxyspiropyran. Plots should generate a
linear line .......................................................................................... 138

Figure 8.13. Plot of difference of the natural log of absorbance versus time to
determine the kinetic order of the closing reaction of p-hydroxyspiropyran ...... 139

Figure 8.14. Reversible isomerization equilibrium expressions to evaluate the
kinetic process of spiropyran isomerization .............................................. 140

xi

LIST OF ABBREVIATIONS

Nuclear magnetic resonance is abbreviated as NMR.
Highest occupied molecular orbital is abbreviated as HOMO.
Lowest unoccupied molecular orbital is abbreviated as LUMO.
Dimethylsulfoxide is abbreviated as DMSO.
Tetrahydrofuran is abbreviated as THF.
Dimethylformamide is abbreviated as DMF.
p-nitrospiropyran is abbreviated as BPS.

p-hydroxyspiropyran is abbreviated as p-HBSP.

xii

CHAPTER 1

INTRODUCTION

As technology advances, the amount of information obtained per
experiment increases greatly. For example, as the resolution of images
increases, the storage space required for each image increases dramatically. At
some point in the future there may come a time where the current capabilities of
data storage will fail to meet the needs of future techniques Like high resolution
imaging.

Magnetic storage devices are inexpensive to manufacture and the
technology to record and recover information has been fully developed.
However, these devices rely on the magnetic properties of the recording material
to store information. As floppy disks and similar materials, such as magnetic
tapes, age the matrix or support material becomes brittle and begins to lose the
structural integrity resulting in loss of data. In the case of magnetic tapes, this
aging of the material can be accelerated by the fact that the reading head of the
drive comes into contact with the recording material thus placing pressure on any
weak spot, leading to breakages. A more attractive method of data storage is to
record and read information in such a way that the material is physically
contacted as little as possible and where the integrity of the stored information
does not rely on the matrix material directly. Use of a reflected laser beam is an
obvious choice for read/write/erase operations as this is the basis for the

compact disk or CD. CD and digital versatile disk (DVD) technology has become

the standard in external storage devices due to the great portability and the low
cost. With an estimated lifespan of 50 years, longevity is no longer a primary
concern.

One issue with the current generation of optical data storage device is that
the capacity can be increased in only two ways. The first method to increase
capacity is to add additional storage layers to the disk. Moving from a single to
multiple layers linearly increases the device capacity but also adds additional
cost to the material due to the requirement of layers that are partially reflective.
These partially reflective layers incorporate an angular dependence to allow for
proper layer selection by the recording and reading lasers. The angular
dependence ultimately limits the number of layers that can be deposited and
used without cross-talk between the layers. It has been demonstrated that with
the high definition DVD (HD-DVD) technology, up to three layers can be used
with no observed cross talk during reading; however, three layers only allow for
the capacity to be increased by three times.

The second method to increase capacity of a disk is to reduce the
wavelength of the laser used to record and read information from the disk. This
benefit derives from the fact that the diffraction limited focal spot size for a beam
of light scales as N2. We have seen this benefit with the emergence of HD-DVD
and Blu-ray disks which employ a laser of 405nm. Standard DVDs use a laser of
650nm to record and read information which leads to a capacity of 4768 while
Blu-ray has a capacity of 2568. This increase in capacity is greater than the 4

times expected due to the implementation of a different lens system than DVD

systems use. To obtain optical recording devices that have 100 times the
capacity of DVDs, the recording laser would have to have a wavelength of 65 nm
and be in the format of a solid state device that could be manufactured cheaply.
This is not feasible on fundamental physical grounds, owing to the short
wavelength required. Ultimately, the amount of information that can be stored on
a disk is limited by the diffraction limit. The current generation of optical storage
devices should be called opto-thennal storage devices because the information
is recorded to the disk through the burning of pits, either into the dye material or
the plastic layer. To reach beyond the diffraction limit of the system it is
necessary to move beyond pit burning as a storage mechanism, and into
accessing a chemical property of the recording material while maintaing the
benefits of optical recording methodology. Accessing the chemical properties of
the dye molecule may allow for information to be recorded in a grayscale
manner, allowing for greater storage density than the current binary logic model.
In a binary logic system there are only two defined states, on and off, and
information is recorded based on a series of “1”s and “0”s. Moving to a grayscale
allows for more than two logical states to exist, allowing for greater information
content within a physical resolution element. The precise enhancement in
storage capacity achievable through such alternative approaches depends, of
course, on the details of the information storage methodology. This type of
recording system is often described as n-bit logic and is schematized in Figure
1.1. In this example information is recorded as one of 8 logic levels, and to

decode the information the reflected laser beam’s intensity is measured.

 

 

 

 

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Figure 1.1. Binary logic recording versus n-bit logic

recording, in this example n = 8.

 

 

Implication of this approach would require a relatively simple modification to the
current technology, moving from a digital (1/0) detector to an analog detector
such as a power meter.

The ideal device should employ many of the aspects of the current
generation while adding the benefit of allowing for the increase in capacity that is
needed for future applications. The resulting device could look like Figure 1.2
where a plastic substrate is used for support, then coated with a reflective layer,
on this a layer of the recording material can be deposited by either vapor
deposition or spin coating, and finally a cover layer placed over the top.1 From
this orientation the same reflected laser technology can be applied to the new
storage device. In this device the recording medium is comprised of a material
that can be used for n-bit recording. An alternative to this approach is to
construct a 3D recording device to allow for greater data capacity per unit area.
This can orientation be achieved by several methods, the simplest is to stack
several ZD layers of recording material as in Figure 1.3,2 or to construct a solid
block of recording material such as a solid crystal.3 In the case of the solid
crystals the photorefractive properties could be accessed to store information.
Both of these approaches dispense with device rotation and replace the
hardware used for the reflection of a laser beam with a microscope based
reading and writing system. This allows for variables such as numerical
apertures and intensity to be adjusted to give the greatest efficiency. It has been
demonstrated that great improvements can be made with the introduction of a

confocal microscope.4 The inclusion of a microscope for detection will introduce

Pre-groove
Substrate (plastic)

Reflective Layer (Au)
Recording layer (PMMA)
Air gap

Cover plate (quartz)

 

 

 

Laser Beam

Figure 1.2. Optical storage device based on current
generations technology. Adapted from reference 1.

 
   
    
  

Beam splitter

  

Frequency
doubler

.5...-.::

 

Memory medium ' i

Figure 1.3. 3D arrangement for an optical storage device
based on stacks of ZD recording layers adapted from
reference 2.

additional cost that the rotating 2D approach will not incur, so the logical first step
toward developing a n-bit optical storage device is to utilize the mechanical
aspects of the current technology and select a molecular system that will allow
the demonstration of n-bit logical storage. It is this selection process that is the
focus of this dissertation.

A logical molecular property to access for writing and reading information
is the absorbance of the dye molecule. We can easily determine the difference
in absorbance intensity in hundredths of an absorbance unit and the wavelength
of maximum absorbance to nanometers. Several classes of chromophores exist
that exhibit absorption spectra that depend on their conformation. This type of
chromophore is labeled photochromic. Photochromic molecules are defined as
molecules that undergo a reversible change in conformation with at least one
pathway depending on irradiation, with the conformational changes leading to
corresponding changes in spectroscopic properties. These molecules are often
characterized by having one form that absorbs strongly in the ultraviolet region
and one that absorbs strongly in the visible region. Figure 1.4 demonstrates the
principle associated with photochromic information storage. Through this shift,
we should be able to monitor a wavelength in the region of choice, either visible
or ultraviolet, and determine which state the molecule is in; With the wavelengths
being determined by the chromophore and matrix selected. The change in
wavelength of maximum absorbance is typically determined by either the shift
from 1741* interactions to n-1T* and o-rr*, or from new steric interactions of the

molecule, with both types of change being mediated by changes in the

 

660ff39

 

Abs.

 

 

 

7i. (nm)

 

 

 

Figure 1.4. General photochromic molecule behavior.

conformation of the molecule. The simplest example of a conformational change
can be seen in Figure 1.5 for which stilbene, a widely studied molecule,
undergoes an isomerization.5 Figure 1.5 demonstrates the approximation of a 1-
D isomerization coordinate, and shows that isomerization depends critically on
the electronic state of the isomerizing molecule. In the ground state the trans
conformation is lower in energy than the cis conformation. For stilbene, the trans
and cis absorption bands are spectrally shifted from one another because of the
change in the extent of conjugation in going from trans to cis conformations. To
isomerize to the cis form of the molecule, the trans conformer must overcome an
energetic barrier labeled Ea. In the ground state (So) this activation energy is
large. However, when the molecule is excited to the first excited singlet state
(S1) there is a minimum on the S1 surface located above the So isomerization
barrier maximum. From the S1 state the molecule can relax back into the ground
state and there is a finite probability that either the cis or the trans conformer will
be formed in the ground state. If we evaluate the ratio of the two isomers at a
steady state, we can determine a branching ratio for the relaxation process to the
ground state. Once a population of the cis conformer is created it can be
monitored as it relaxes to the lower energy trans form. The time constant 1'
associated with the relaxation process can be related to the energetic height of

the isomerization barrier through the Arrhenius equation. Typically, the prefactor

 

"Ea
k=%=Aexp T (1.1)

k

10

—60000 i

k =10'25'lexp mol =357hz=—1—=3ms (1.2)

8.314 J *300K T

 

 

mol *
term A is taken to be 1012, meaning a 60 KJ/mol barrier (typical of polyenes) will
take about 3 ms to relax from cis to trans. This is not a usefully long lifetime for
information storage. Thus, the chromophore selected should have the highest
isomerization barrier available to prevent thermal loss of fidelity.

Many photochromic molecules have been reported in the literature,
(Figure 1.6) and isomerization, while an essential component to photochromic
behavior, is not always the only chemical step involved.”29 It can be seen in
Figure 1.6 that all of the molecules undergo an isomerization process as stilbene
does; and some also undergo a covalent bond formation once the molecule is in
the correct conformation and the nucleophilic atom is close enough.
Photochromic molecules are often difficult to control due to the variety of thermal
reactions and side reactions that can take place." 7'10' 2‘ One of the first
photochromic molecules discovered and the first that we considered was the
class of fulgides, which were discovered in 1905. These molecules received little
attention for the purpose of optical data storage until the early 1960s, after some
of the undesired side reactions were eliminated.‘ Figure 1.7 shows the chemical
reactions that the fulgide can undergo, illustrating the problems often associated
with this class of molecules. Both conformations 1 and 2 can form molecule 3
either through optical irradiation (1-)3) or thermally (293). Continued heating or
irradiation of 3 leads to two molecules 4 and 6 where the difference is the

absence or presence of a proton at the carbon to which R1 is

11

 

 

 

 

 

 

 

lsomerization Coordinate
trans cis

 

Figure 1.5. 2D representation of the energy
diagram for general isomerization process. The
lower the isomerization barrier the shorter the time

constant T.

12

 

2M Ojl

R2 Azobenzene Stilbene

 

Diarylethene
ma: ‘5; 9 p
Spiropyran

Figure 1.6. Examples of photochromic molecules
and the geometric rearrangements that occur.

13

 

 

 

 

 

0 n3 at” 0
o A
R3 m 1. 5H M llol
o
R‘ k R‘ o - n' o
1 [0]
“1° mm H QQ ° 5
A RSR‘ O 4 0
R1 O ,//A' 3 hv 1 [O]

n‘ o [1,3H-lshlft “5' o
R3 0 “3:06
2 n5n‘ o

/ a . «0
-EIH
H o o

 

Figure 1.7. Example of side reactions and thermal
back reactions on a fulgide system. An elimination
reaction that takes place is the elimination of an
ethyl group by extracting the proton boxed.
Adapted from reference 1.

14

attached. It is also seen that 3 can be oxidized to form an entirely different
molecule, 5 where the stereo chemistry is determined by the substituents
present. One such reaction is displayed at the bottom of Figure 1.7 where it is
shown that the boxed proton can be eliminated, leading to the loss of an ethyl
group upon heating. The removal of the proton at the boxed location imparts
greater stability allowing the evaluation of fulgides for use in optical storage
devices.1 This is only one example, but it illustrates a point that must be
considered when selecting which photochromic molecule to use. Based on the
propensity of the fulgides to undergo side reactions, this class of photochromic
molecules are not the ideal molecule to study for reversible information storage.

Other photochromic molecules have varying properties that can be
beneficial if used in the appropriate system. Diarylethenes are potentially useful
as data storage molecules because they are known to convert quantitatively and
reversibly between the two conformers. One problem that diarylethenes face is
the absence of thermal stability of the conformers.8

Spiropyrans were the last class photochromic molecules that we
evaluated for potential use in optical information storage. Spiropyrans undergo
an isomerization which places the reactive oxygen in close proximity to the
carbon at to the nitrogen in the heterocyclic ring, allowing for the formation of a C-
0 bond. The “open” form of spiropyrans have two resonance structures; one that
is fully conjugated and one that is zwitterionic. Spiropyrans are typically reported
with 3 nitro group at the para position, and behave as a “push-pull” system. We

are interested in replacing the nitro group with a hydroxyl group to make the

15

incorporation of spiropyrans into maleimide-vinyl ether alternating copolymers
more facile.‘5'7

Among photochromic molecules there has not emerged a single best
choice for optical data storage. We selected initially the azobenzenes because
of the wealth of literature available, and the absence of information on the effects
of structural modifications that we were interested in making. The azobenzenes
ultimately proved to be limited in their utility by the sensitivity of the thermal
isomerization barrier height to chemical modification on the rings. We then
examined spiropyrans due to their ability to exhibit reversible bond formation with
relatively few environmental requirements and the apparent ease of synthesis.

Another consideration that must be evaluated is the incorporation of the
chromophore into an appropriate matrix. Current examples of working devices
incorporate the chromophore through simply dissolving the molecule into a
polymer mixture and then spin coating the recording later on a substrate. This
approach has been utilized for both 20 and 3D recording systems.6'"'15 For this
technology to be successful, the recording material should have as much order
as possible. The random distribution of the chromophore throughout the
recording layer limits the storage capacity and reversibility. There have been
many examples of a crystalline approach to 3D optical storage, however these
crystals are difficult to construct and are sensitive to the local environment.3'9'3O
Based on these two structural extremes, we believe that chromophores could be
synthesized with attachment points to allow direct chemical attachment to either

the substrate or to a polymer layer. With the azobenzene chromophore we

16

selected to begin with a diamido functionality with plans to further functionalize to
a dihydroxyl functionality. We ultimately abandoned this family of molecules
because of our experimental findings. For the spiropyrans, the inclusion of the
hydroxyl functionality was the logical choice to allow facile incorporation in
polymer systems. The compounds evaluated here were synthesized in house
because they are not commercially available.

We first evaluated the effect of functionalization on the isomerization
behavior of azobenzenes. We performed semi-empirical calculations to probe
the isomerization mechanism of azobenzene. From these calculations we
predicted the isomerization barrier would change little with the functionalization.
Once synthesized, the isomerization behavior of the molecules was evaluated
through the use of both the branching ratio and the measurement of the time
constant for thermal back conversion from cis to trans.(FIgure 1.6) Once the time
constant is found for the thermal reaction, the isomerization barrier can be
calculated through the Arrhenius equation. From these findings, it was
determined that lsomerization barrier alone is insufficient in “looking” the
conformation of the molecule, and an additional chemical process needs to
operate to allow for persistence of the conformer. Spiropyrans exhibit an
isomerization coupled with a covalent bond formation and cleavage. Again, we
can use the time constant to determine the effect of functionalization on looking
in the conformation. With the spiropyrans, we wanted also to know the effect of
the solvent used to determine the rate constants characteristic of the system.

The kinetics are important if the grayscale recording method is to be

17

implemented in the final design. We need to be able to predict the percentage of
the molecules converted from an irradiation for an amount of time.

The purpose of this work is evaluate the usefulness of substituted
azobenzenes and spiropyrans as optical data storage chromophores. In chapter
4 we evaluate semi-empirical calculations to estimate the effect of substitution on
the isomerization surface. We also evaluate the energetic differences between
the rotation and inversion mechanism for the isomerization of the azobenzene
molecule. In chapter 5 we evaluate the absorbance spectra of the family of
substituted azobenzene to evaluate the isomerization barrier. This body of work
established a methodology that can be used to evaluate spiropyrans for the
purpose of optical data storage. Chapters 7 and 8 use this methodology to
evaluate the synthesized family of spiropyrans. The overall goal of this research
is to determine the feasibility of these compounds as photochromic optical data

storage chromophores.

18

Literature Cited

1.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

Yokoyama, Y. Chem. Rev., 2000, 100, 1717

Parthenopoulos, D.; Rentzepis, P. Science, 1989, 245, 843
Kawata, Y.; lshitobi, H.; Kawata, S. Optics Letters, 1998, 23, 756
Toriumi, A.; Kawata, S.; Gu, M. Optics Letters, 1998, 23, 1924
Prak, N.S.; Waldeck, D.H. J. Chem. Phys., 1989, 91, 943

Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev., 2000, 100, 1741
Minikin, V. Chem. Rev., 2004, 104, 2751

lrie, M. Chem. Rev., 2000, 100, 1685

Kawata, 8.; Kawata, Y. Chem. Rev., 2000, 100, 1777

Tamai, N.; Miyasaka, H. Chem. Rev., 2000, 100, 1875

Tork, A.; Boudreault, F.; Roberge, M.; Ritcey, A.; Lessard, R.; Galstian, T.
Applied Optics, 2001, 40, 1180.

Toriumi, A.; Herrmann, J. M.; Kawata, S. Optics Lett., 1997, 22, 555
Raymo, F. M.; Tomasulo, M. J. Phys. Chem. A, 2005, 109, 7343

Tamaoki, N.; Keuren, E.V.; Matsuda, H.; Hasegawa, K.; Yamoka, T.
Appl. Phys. Lett., 1996, 69, 1188

Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Phys. Chem. B, 2001,
105, 10282

Suzuki, T.; Kawata, Y.; Kahata, S.; Kato, T. Chem. Commun., 2003, 2004

Rosario, R.; Gust, D.; Hayes, M.; Springer, J.; Garcia, A. A. Langmuir,
2003, 19, 8801

Choi, D. H.; Ban, S. Y.; Kim, J. H. Bull. Korean Chem. Soc., 2003, 24, 441

Mrish'ima, Y.; Kobayashi, T.; Nozakura, S. Macromolecules, 1988, 21, 101

19

20.
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22.

23.
24.

25.

26.

27.

28.

29.

30.

lrie, M.; Menju, A.; Hayashi, K. Macromolecules, 1979, 12, 1176
Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev., 2000, 100, 1741

Wojtyk, J.T.C.; Wasey, A.; Kazmaier, P.M.; H02, 8.; Buncel E. J. Phys.
Chem. A, 2000, 104, 9046

Suzuki, T.; Kawata, Y.; Kahata, S.; Kato, T. Chem. Commun., 2003, 2004
lpe, B. l.; Mahima, S.; Thomas, K.G. J. Am. Chem. Soc., 2003, 125, 7174

lnouye, M.; Ueno, M.; Tsuchiya, K.; Nakayama, N.; Konishi, T.; Kitao, T. J.
Org. Chem, 1992, 57, 5377

Gorner, H. Phys. Chem. Chem. Phys, 2001, 3, 416

Kamada, M; Sumaru, K.; Kanamori, T.; Shinbo, T. Langmuir, 2004, 20,
9315

Venturini, C.G; Andreaus, J.; Machado, G.; Machado, C. Org. Biomol.
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Bee, S.Y.; Arnold, B.R. J. Phys. Org. Chem, 2004, 17, 187

Glezer, E.N.; Milosavljevic, M.; Huang, L.; Finlay, R.J.; Her, T.-H.; Callan,
J.P.; Mazur, E. Optics Letters, 1996, 21, 2023

20

Chapter 2

RESPONSE OF AZOBENZENE TO IRRADIATION

Azobenzenes are a family of compounds that has found wide use because
of their facile photoisomerization and relatively high barrier to thermal (ground
state) isomerization. These properties are of potential utility in the development
of molecular-scale information storage and optical switching strategies."11
Among the key issues under investigation for the azobenzenes are the ability of
substitution about their aromatic rings to mediate their optical properties and
isomerization behavior. While it is typically assumed that the linear optical
response and isomerization behavior of these chromophores is linked, this
relationship has not been established. The purpose of this work is to examine
how the presence of substituents on the azobenzene phenyl rings influences the
spectroscopic and isomerization behavior of these compounds. We have
synthesized a family of symmetrically para-disubstituted azobenzenes and have
studied their steady state and time-resolved optical properties. We have also
studied the branching ratio for isomerization and the rate of So trans isomer
recovery following photoisomerization for these species. Our findings indicate
that the addition of substituents to the azobenzene chromophore can influence
the steady state and time-resolved optical properties of the chromophore
significantly, and the isomerization surfaces for these molecules are affected as
well. This is not a surprising result because of the relationship between

electronic structure and state ordering, and the electron density distribution for

21

the bond(s) that dominate the isomerization coordinate. We understand this
finding in the context of the N=N bond dominating the isomerization behavior of
these molecules, and the effective bond order of this moiety is influenced by the
presence of electron-donating or electron-withdrawing substituents on the phenyl
rings. Our finding that the isomerization barrier is reduced when electron-
donating substituents are placed on the rings indicates that it is the 1r* state that
is influenced most strongly, and these findings are supported by the steady state
spectra. Our data also point to the potential importance of asymmetric
substitution of the azobenzene rings in structurally mediating isomerization.12

A secondary goal of this research was to evaluate substituted
azobenzenes as interlayer polymer linkages. Using an isomerizable linkage,
several applications may become feasible. In a layered polymer system it may
be possible to both lock in the desired isomer and preserve the ability to switch to
the alternative upon irradiation depending on the structure of the linkage and its
concentration. Careful control of structure and concentration can lead to the
eventual development of an optical storage device capable of both writing and
erasing on a short time scale, where the “on” and “off” states have different
absorbances and possibly different topography. The spectroscopic and physical
changes of the polymer system allow for multiple methods of nondestructive
reading of the information from the media. Another application that can be
realized is the formation of a stacked polymer capable of separations based on
size exclusion. Separations would work on the premise of a change in free

volume of the polymer matrix due to the inter-layer linkage isomerization. (Figure

22

2.1) This scheme allows pore size and identity of polymer layers to be altered for
chemical specificity independent of the separation based exclusively on size.
These two applications rely on the isomerization properties of an interlayer
linkage, and we evaluate structured azobenzene derivatives with those goals in
mind.

Until recently, the accepted model for the isomerization of azobenzenes
was that there is a significant barrier to isomerization in the So state, with the
magnitude of the barrier being a consequence of the azo (N=N) moiety. We are
not aware of an accurate determination of the thermal isomerization barrier for
azobenzenes, either experimental or computational, but it has been thought to be
on the order of ~50 kcal/mol and, because of this substantial energy, the
modeling of this barrier cannot be done reliably using the usual assumption of a
one-dimensional isomerization coordinate, i.e. both isomerization and ring
rotation must be coordinated in some unresolved manner. When excited to the
S1 state, the azobenzene chromophore is thought to isomerize by an inversion,
or rehybridization mechanism, and excitation to the 82 state is thought to cause
isomerization to proceed by rotation about the azo bond, which is taken to be
substantially single-bond in character.12 The two proposed mechanisms for
azobenzene isomerization are illustrated in Figure 2.2. This widely accepted
picture has continued to stir debate, owing to the relative timescales of large
amplitude molecular motion and internal conversion from the $2 to the S1

manifold.

23

   

l1>I2

Figure 2.1: Schematic of one possible layered polymer
system for separations based on change in free volume.

24

flkMN—©

Inversion Inversional
Transition State

Q N.

..©/
6

Rotational Transition State

Rotation

Figure 2.2: lsomerization mechanisms for azobenzene. Adapted
from reference 13

25

Recent elegant work by both the Tahara12 and Kobayashi13 groups has
cast doubt on the “standard” picture. Using femtosecond optical techniques,
Tahara’s group has shown that excitation to the 82 state of azobenzene
undergoes rapid relaxation to the S1 state, where isomerization proceeds.12
When viewed in the context of the characteristically short lifetime of highly
excited electronic states in organic molecules, and the time required for inertial
motion of the phenyl ring(s), these results are fully consistent with the behavior of
most organic chromophores. The Kobayashi group, using chirped femtosecond
pulses has demonstrated clearly that vibronic coupling is substantial in an excited
azobenzene derivative and that the redistribution of energy within the vibrational
manifold of the S1 state serves to mediate the isomerization behavior of that
species.13 These findings on the earliest stages of relaxation within
azobenzenes not only shed significant light on the factors mediating
isomerization, they underscore the fact that a full understanding of isomerization
in azobenzenes remains to be achieved.

With this background, we consider whether or not it is possible to mediate
the isomerization behavior of azobenzenes by synthetic means. We are
interested in being able to incorporate p-disubstituted azobenzenes into layered
assemblies for the purposes of probing local structure using isomerization and for
determining whether or not a layered structural motif is capable of storing
information via optical read/write means. In an attempt to address these issues,
we have focused on the synthesis and characterization of a family of simple p-

disubstituted azobenzenes, with particular interest in the influence of the para

26

disubstitution on the spectroscopic and isomerization behavior of the resulting
chromophores. The compounds studied are structurally simplified models for
disubstituted species capable of layer incorporation. We consider the

experimental spectroscopic and calculated isomerization results separately.

27

Literature Cited

1. Saremi, F.; Tieke, B. Adv. Mater., 1998, 10, 388.

2. Wang, C.; Fei, H.; Xia, J.; Yang, Y.; Wei, 2.; Yang, 0.; Sun, G. Appl. Phys. B.,
1999, 68, 1117.

3. Howe, L. A.; Jaycox, G. D. J. Poly. Sci. A. Poly. Chem, 1998, 36, 2827.
4. Yoshii, K.; Machida, S.; Horie, K. J. Poly. Sci. B. Poly. Phys., 2000, 38, 3098.

5. Aoshima, Y.; Egami, C.; Kawata, Y.; Sugihara, O.; Tsuchimori, M.; Watanabe,
O.; Fugimara, H.; Okamoto, N. Polym. Adv. Tech., 2000, 11, 575.

6. Zilker, S. J.; Bieringer, T.; Haarer, D.; Stein, R. S.; van Egmond, J. W.;
Kostromine, S. G. Adv. Mater, 1998, 10, 855.

7. Eicher, J. J.; Orlic, S.; Schulz, R.; Rubner, J. Opt. Lett., 2001, 26, 581.
8. Hagen, R.; Bieringer, T. Adv. Mater, 2001, 13, 1805.

9. Pedersen, T. G.; Johansen, P. M.; Pedersen, H. C. J. Opt. A: Pure Appl. Opt,
2000, 2, 272.

10. Patane, S.; Arena, A.; Allegrini, M.; Andreozzi, L.; Faetti, M.; Giordano, M.
Opt. Commun., 2002, 210, 37.

11. Astrand, P.-O.; Ramanujam, P. S.; Hvilsted, S.; Bak, K. L.; Suauer, S. P. A.
J. Am. Chem. Soc., 2000, 122, 3482.

12. Tamada, K.; Akiyama, H.; Wei, T.-X.; Kim, S.-A. Langmiur, 2003, 19,2306.

13. Fujino, T.; Arzhantsev, S. Y.; Tahara, T. J. Phys. Chem. A, 2001, 105, 8123.

14. Saito, T.; Kobayashi, T. J. Phys. Chem. A, 2002, 106, 9436.

28

Chapter 3

SYNTHESIS OF AZOBENZENE DERIVIATIVES

A switching application in a polymer matrix requires that the molecule
undergoing the switching operation needs to have significant local freedom. This
freedom can be achieved many ways; through careful selection of a molecule’s
synthetic substitution and control on concentration in the matrix, most of the
freedom can be achieved. We are exploring the use of azobenzene in a layered
polymer matrix. Photoisomerization of the azobenzene moiety gives rise to
spectral shifts, and this family of molecules has been evaluated for photochromic
optical switching by other groups.HS The meta and para-disubstituted varieties of
azobenzene are the most useful to evaluate due to the feasibility of their
incorporation in polymer side groups through functionalized substituents. The
position of the functionalized substituents on the azobenzene ring greatly affects
the overall molecular length and thus the degree of change in free volume of the
polymer matrix. Calculations conducted in Hyperchem® v. 6.0 show that the
distance differential (Figure 3.1) for the meta-disubstituted species upon
isomerization is approximately 1.1 A. The same process for the para-
disubstituted species yields a change of 5.5 A. This leads to the incorporation of
the meta—disubstituted azobenzene as the switching molecule due to reduced

strain on the molecule.(Figure 3.1)

29

 

1)

 

R
N\ hV N
11.9A \N ___, H

U
:3 NO. I

R R
Q hv > Nb
E ,1} 9.6 A
R

 

V

 

—L_

 

 

10.7A

 

._ R

4L.

Figure 3.1: Calculation of change in position and spacing of
azobenzene terminal groups, R, upon isomerization for both
meta and para disubstituted azobenzene.

3O

The synthesis of substituted azobenzenes, of necessity, requires reactions
targeted to specific ring positions. Any of the ring substituents used in the
reactions act as an ortho, para-director and a meta deactivator. There are a
number of possible ways to overcome this issue. The first is to place a
substituent on the ortho position relative to the azo nitrogen that directs further
addition to the meta position. This however limits the molecular variety that can
be employed and may have significant steric consequences on the efficiency and
rate of photoisomerization. Another possibility is to start with substituted anilines
and form the azo bond as the final step. Forming the azo bond as the last step
becomes a major problem as the connectivity to the polymer can trigger the
meta-deactivation behavior of the azo moiety, subsequently the loss of meta
substituents is a common result. Most of the connections to polymer sheets
considered for the azobenzenes were of the covalent type that the Blanchard
group has extensive experience with. The overall difficulty of meta-disubstituted
azobenzene synthesis lead to the selection of the para disubstituted molecules
for the isomerization studies.

As mentioned previously the para-disubstituted azobenzenes lack the
structural freedom that is desired for facile isomerization in a lamellar
environment. To a limited degree the lack of freedom can be overcome by only
connecting one side of the molecule to the polymer sheets. Singular connectivity
has been evaluated by other groups by incorporating functionalized azobenzenes
in poly(methyl methacralate) matrices in a “doping” procedure.5'8 While doping

makes optical storage possible and simultaneously provides a probe into local

31

chemical environment, it limits the overall usefulness of the devices for reasons
of orientational distributions of the chromophores and confining effects of the
polymer matrix. Providing connectivity at each end of the molecule affords
higher durability and increased degree of order, which nearly directly correlates
to overall increased switching cycles before degradation. With that, the para-
disubstituted azobenzene needed to be functionalized with a moiety that has
enough structural freedom to accommodate the steric limitations of p-
disubstituted azobenzene. Alkane chains have many degrees of freedom, which
can be used to accommodate structural changes. In addition alkane chains are
relatively inert and allow for simple polarity modulated processes to be employed
in the purification of the synthesized molecule. We decided that starting with a
diaminoazobenzene the alkane chain could be attached through an amide
functionality. We anticipated that the attachment of anything to the azobenzene
ring(s) would influence Its optical response, but theoretical predictions of these
changes are not reliable. The ability to connect to the interlayer moieties will
eventually need to be introduced on the ring of the azobenzene para to the azo
bond but a model study of this family of azobenzenes was conducted first to
evaluate the effect of ring substitution on the optical and isomerization properties
of this family of chromophores (Chapter 4).

The series of p~diamidoazobenzenes we report on here were synthesized
using a modification of a standard polyamido polymerization route.9 We have
replaced the dicarboxylic acid with a monofunctional acid chloride to prevent

polymerization. The reaction is illustrated in Scheme 3.1. 4,4’-Azodianiline was

32

purchased from Sigma-Aldrich (CAS 538-41-0). The reactions were carried out
under an inert atmosphere to limit hydrolysis of the acid chloride. Methylene
chloride, acetone and tetrahydrofuran were purchased from Sigma-Aldrich and
used as received. N-methylmorpholine was used to scavenge hydrochloric acid
produced during the reaction. Reaction times of thirty minutes at room
temperature were typical, with little or no increase in yield for longer reaction
times. Typical reaction conditions were for 2 mmol of 4,4’-azodianiline to be
combined with 8 mmol of N-methylmorpholine and 8 mmol of the appropriate
acid chloride in 150 mL of CH2Cl2. Following completion of the reaction, the
solution was washed with 150 mL of 2M HCI. The diamidoazobenzene
precipitated from solution along with the appropriate acid byproduct and was
collected by filtration. The resulting solid was dissolved in acetone and washed
with an equal volume of brine solution (saturated NaCI) to separate the
derivatized azobenzene from the acid reaction byproducts, and the crude product
was then separated from the acetone. The solid was purified by recrystallization
from methanol/water (80:20). As the alkyl substituent of the acid chloride
increased in length, the purity of the p-diamidoazobenzene product could be
increased significantly by additional washing with brine solution. The reaction to
synthesize 5 produced lower yields due to the nonpolar nature of the product.
For this reaction THF was used in place of CHzClz as the solvent due to the
limited solubility of nonanoyl chloride in CH2C|2. The use of THF as the solvent
required a methanol washing of the recovered solid to remove unreacted starting

materials, but eliminated the need for recrystallization. Purity of the

33

H2”H~\NNH2
0 (TD)

 

R Cl
Room Temperature
~30 minutes
CHZCIZ
Argon Blanket
H v
RTN
O /N
N / i
N R
1 R = CH3 H
2 R = CH2CH3

3 R = CH2CH2CH3
4 R = CH2CH2CH2CH3
5 R = CH20H2CH20H20H2CH2CH20H3

Scheme 3.1: Synthetic route to disubstituted azobenzene.

34

compounds synthesized was determined by proton NMR and UVNisibIe
spectroscopy. Figure 3.2 shows the progression from starting material (top) to
final product (bottom) showing an impure substance (middle). Mass spectra
were taken for each of the products and can be found in Appendix A.

The NMR spectra reported here were obtained on a Varian 300 MHz
instrument using a standard proton collection method. The proton NMR
spectrum of azobenzene demonstrates two groupings of protons at about 7.5,
and 7.9 ppm. These groupings represent all the protons on the phenyl rings of
the azobenzene and are split according to Figure 3.3. The addition of the amine
groups further splits the phenyl protons to 6.5 and 7.9 ppm and should add an
additional peak to represent the amine protons; however this has never been
experimentally seen.(Figure 3.4) The conversion to the amide removes one of
the amine protons and replaces it with alkyl protons which are seen in the
spectrum. As the alkyl chain is extended the shift of the amide proton is
unaffected and additional peaks appear in the aliphatic region (6 0.6 to 1.6 ppm)
of the spectrum to correspond with the additional methylenes.(Figure 3.5 and
Figure 3.6) The associated proton shifts, synthetic yields, and general
descriptions for the compounds are presented below. The reaction conditions

and yields are presented in Table 3.1.

35

 

 

 

8. b a
”ASL gt JL L
Ld iv
d1 OCH
ab 0 C d“: dwc O a

 

 

 

 

 

 

 

_'T’T—T‘I l I I' T-I' T TT‘I T T_'[’ITI'AI*Tkl I'TTTT‘TTfi I ‘I T'TWTI—‘I‘ T_T T‘T T‘rT—T—TTI T“T"I 7*”

9 8 7 6 3 2 I

6 (fapm) ‘

Figure 3.2 NMR spectrum of p-diamidoazobenzene before
and after recrystallization.

36

Phenyl Protons

 

Figure 3.3: NMR spectrum of azobenzene in deuterated

methanol.

 

37

Solvent

 

L

Solvent

Phenyl Protons

 

 

 

 

 

Solvent
1L JL 24L
I T T T T I T T T I I T fij—I T T I T T T T T— T I T T T T I T T T T I T T T T T r r T T I Tfir T T
10 9 8 7 6 5 4 3 2 1

8 ppm
Figure 3.4: Proton NMR spectrum of p-diaminoazobenzene.

Solvent

 

Methylene Protons

Phenyl Protons

 

 

 

 

 

 

I

5 ppm
Figure 3.5: Proton NMR spectrum of p-diacetamidoazobenzene.

38

Diaminoazobenze

 

 

 

- IL 419 z a
R: .1 IL - .. -
2 JLLL

 

 

.R': 9M... .. JM

MEL.
8: “all ,JLJLJLdLKL

IIIIIIIIVIIIIIIIIIIIIEIITTIIIWIITTI‘IIIIIIIIIIIIIIIIIIIIIIITWII IIIIIIII II IITIIITIIIII I

9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 201510 0.5

 

 

 

 

Figure 3.6: NMR spectra of a series of p-diamidoazobenzenes.

39

 

Reaction Conditions I Results

 

Compoun Time Solvent Crude Recrystallized

 

 

d Stirred Yield Yield
(min)

1 28 CH2Cl2 80% 53%

2 28 CHzClz 73% 54%

3 30 CHzClz 46% 44%

4 28 CHzClz 34% 30%

5 40 THF 69% No

Recrystallizatio
n

 

Table 3.1 : Reaction conditions and yield for synthesized
compounds.

40

 

Compound 1. (N-[4-(4-Acetylamino-phenylazo)-phenylj-acetamide) This
compound was produced with a crude yield of 80% and recrystallized yield of
53% using CHZCIZ as reaction solvent. The resulting brown flakes showed
proton NMR peaks (300 MHz, d-methanol) at 5: 2.14 ppm (s, 6H alkyl protons),
7.70 ppm (m, 4H on benzene ring), and 7.90 ppm (m, 4H on benzene ring). The
mass spectrum (GC/MS) of the compound showed peaks at m/z 43, 65, 92, 134,
162, and 296.

Compound 2. (N-[4-(4-Propionylamino-phenylazo)-phenylj-propionamide)
This compound was produced with a crude yield of 73% and recrystallized yield
of 54% using CHZCIZ as reaction solvent. The resulting brown flakes showed
proton NMR peaks (300 MHz, d-methanol) at 8: 1.22 ppm (m, 6H [3 to carbonyl),
2.44ppm (m, 4H a to carbonyl), 7.77 ppm (m, 4H on benzene ring), and 7.87
ppm (m, 4H on benzene ring). The mass spectrum (GC/MS) of the compound
showed peaks at m/z 57, 92, 119, 148, 149, 176, 324, 325.

Compound 3. (N—[4-(4-Butyrylamino-phenylazo)-phenyl]-butyramide) This
compound was produced with a crude yield of 46% and recrystallized yield of
44% using CH2CI2 as reaction solvent. The resulting orange needles showed
proton NMR peaks (300 MHz, d-methanol) at 6: 0.92 ppm (m, 6H y to carbonyl),
1.63 ppm (m, 4H 8 to carbonyl), 2.29 ppm (m, 4H a to carbonyl), 7.70 ppm (m,
4H on benzene ring), and 7.90 ppm (m, 4H on benzene ring). The mass
spectrum (GC/MS) of the compound showed peaks at m/z 43, 71, 92, 120, 162,

190, 211, 282, 352, and 353.

41

Compound 4. (Pentanoic acid [4-(4-pentanoylamino-phenylazo)-phenyl]-
amide) This compound was produced with a crude yield of 34% and
recrystallized yield of 30% using CH20l2 as reaction solvent. The resulting
orange powder showed proton NMR peaks (300 MHz, d-methanol) at 8: 0.89
ppm (m, 6H 8 to carbonyl), 1.32 ppm (m, 4H y to carbonyl), 1.61 ppm (m, 4H [3 to
carbonyl), 2.33 ppm (m, 4H d to carbonyl), 7.67 ppm (m, 4H on benzene ring),
and 7.78 ppm (m, 4H on benzene ring). The mass spectrum (GC/MS) of the
compound showed peaks at mlz 57, 92, 120, 176, 211, 296, 380, and 381.

Compound 5. (Nonanoic acid [4-(4-nonanoylamino-phenylazo)-phenyl]-
amide) This compound was produced with a crude yield of 69% using THF
solvent and no recrystallization was performed. The resulting yellow powder
showed proton NMR peaks (300 MHz, d-dimethylsulfoxide) at 8: 0.85 ppm (d, 6H
1 to carbonyl), 1.25 ppm (m; 20H 7], (I), a, 5, and y to carbonyl), 1.55 ppm (m, 4H 8
to carbonyl), 2.33 ppm (m, 4H (1 to carbonyl), 6.81 ppm (d, 4H on benzene ring),
7.52 ppm (d, 4H on benzene ring), and 7.81 ppm (s, amide proton). The mass
spectrum (GC/MS) of the compound showed peaks at mlz 43, 108, 120, 211,
232, 352, 490, and 492.

The absorbance spectra of the various azobenzenes reported here were
obtained on a Varian/Cary model 300 UVNisible absorption spectrometer.
Spectral resolution for all measurements was 1 nm. The molar absorptivities and

absorption maxima of the compounds are given in Table 3.2.

42

 

Absorption
Compound smax (leol-cm) £450 (Umol-cm)
maximum (nm)

 

 

 

 

 

 

 

 

Azobenzene 31 3 22400 500

p-

394 27800 1630
diaminoazobenzene

1 364 17900 1 300
2 364 23750 2400
3 364 24200 2530
4 364 24900 2500
5 364 25300 2450

 

 

 

 

Table 3.2. Absorption maxima and molar absorptivities of azobenzene, p-

diaminoazobenzene and the p-diamidoazobenzenes 1-5.

43

 

The steady state absorption spectroscopy of azobenzene is well
understood. The S1 <— So transitions for both the trans and cis forms are
characterized by weak absorption bands ( s~ 500 leol-cm for cis, 3 ~ 2,000
Umol-cm for trans), and these bands are centered at ~ 434 nm. The S2 <— So
absorption bands for both isomers are stronger, with the trans band at 313 nm (8
~ 22,400 Umol-cm) and the cis band at 254 nm (8 ~ 12,000 Umol-cm).1°"2 The
addition of amino groups at the para positions causes a substantial change in the
absorption spectrum. For p—diaminoazobenzene, the dominant absorption band
is at ~ 420 nm (e = 22,800 Umol-cm), with weaker bands seen at 310 nm and
250 nm. The spectra can be seen in Figures 3.7, 3.8, and 3.9. For the solution
phase spectra we recorded, we did not attempt to isolate cis and trans
conformers, so it is not clear based on the experimental data if the bands at 310
nm and 250 nm correspond to trans and cis forms, respectively, or if these bands
represent two different excited electronic states of the same (presumably trans)
conformer. This remarkable change in the absorption spectra resulting from para
disubstitution reflects a change in the electronic excited state(s), and this is seen
in the results of semi-empirical calculations for both cis and trans conformers
(Figure 4.1), where the dominant absorption transition (82 (— So) is seen to red-
shift by ~ 7000 cm'1 upon addition of the p-amino functionalities. Reaction of the
diamino functionalities to produce the p—diamido azobenzene derivatives
produces a blue-shifted absorption band relative to the p-diamino compound.
The experimental spectra can be reconciled with the results of semi-empirical

calculations for both conformers (Figure 4.1), and for all of the azobenzenes, it

44

appears that multiple SIn <— So transitions contribute significantly to the observed
linear optical response. While the spectral red shift seen for
diamidoazobenzenes relative to diaminoazobenzenes may appear to violate
one’s intuition, we believe that these'findings are the result of changes in
transition oscillator strength of low lying transitions with the addition of amido
substituents rather than actual band shifts. Both experimental data and semi-
empirical calculations point to the significant substituent-dependence of the
electronic state energies of the azobenzenes.

A series of p—diamidoazobenzenes was synthesized according to Scheme
3.1 using a classic polyamido polymerization route with relatively high yields and
purity. The para position was selected for substitution due to ease of synthesis.
The purity of the synthesis was checked by proton NMR and it was found that
with increased alkyl chain length of the amide substituent additional peaks
appeared in the aliphatic region of the spectrum. It was found that all of the p-
diamidoazobenzenes have identical absorbance maxima suggesting that the
electronic states of the compounds are identical. This absorbance maximum
differs from that for p—diaminoazobenzene and that of azobenzene, suggesting
that electronic states of the three compounds are quite different. This difference
was the motivation for a series of calculations to model the isomerization of the

molecules.

45

 

 

 

 

.UWZHS

 

 

 

 

 

 

 

 

a)
8
m 0.8 -
E .
C) Sg‘é'ISO
U)
‘0 0.6 ‘
<
8
.5 0.4 ‘
g l
5 0.2 - cis + trans
I:
81 (- S0
01)-
I r I r I ' I
200 300 400 500 600
Wavelength (nm)
Figure 3.7: Absorbance spectrum of azobenzene.
1.0 -
8 l
c 0.8 -
<0
.0
5
m 06-
.0
<
8
.5 011—
To
g 0.2 -
I!
0.0 -
I ' I ' I r I
200 300 400 500 600

Wavelength (nm)

 

Figure 3.8: Absorbance spectrum of p-diaminoazobenzene.

46

 

 

absorbance (a.u.)

azobenzene diaminoazobenzene
1 - 5
1.0 r

.o
co
I

 

.0
O\
r

.O
4:.
r

 

 

0.2 F

 

 

0.0 A I l I 1 l L I l bk ~-_..
200 250 300 350 400 450 500 550

wavelength (nm)

 

Figure 3.9 : Absorbance spectra of azobenzene, p-
diaminoazobenzene, and p-diamidoazobenzene.

47

Literature Cited

1. Howe, L. A.; Jaycox, G. D. J. Poly. Sci. A. Poly. Chem, 1998, 36, 2827.
2. Saremi, F.; Tieke, B. Adv. Mater., 1998, 10, 388.

3. Ruslim, C.; lchimura, K. J. Mater. Chem, 1999, 9, 673.

4. Delaire, J. A.; Nakatani, K. Chem. Rev., 2000, 100, 1817.

5. Badjic, J. D.; Kostic, N. M. J. Mater. Chem, 2001, 11, 408.

6. Seki, T.; Fukuchi, T.; lchimura, K. Langmuir, 2002, 18, 5462.

7. Patane, S.; Arena, A.; Allegrini, M.; Andreozzi, L.; Faetti, M.; Giordano, M.
Opt. Commun., 2002, 210, 37.

8. Hagen, R.; Bieringer, T. Adv. Mater., 2001, 13, 1805.

9. Misra, G. S. Introductory Polymer Chemistry, Wiley Eastern Limited: New
Delhi, India, 1993.

10. Lednev, l. K.; Ye, T.-Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V.
R.; Umapathy, S.; Hester, R. E.; Moore, J. N. Chem. Phys. Lett., 1998, 290,
68.

11. Lednev, l. K.; Ye, T.-Q.; Abbott, L. C.; Hester, R. E.; Moore, J. N. J. Phys.
Chem. A, 1998, 102, 9161.

12. Zimmerman, G.; Chow, L. Y.; Paik, U. J. J. Am. Chem. Soc., 1958, 80, 3528.

48

Chapter 4

AZOBENZENE CALCULATIONS

Modeling azobenzenes at the Semi-empirical level is a difficult undertaking
due primarily to the presence of the poorly parameterized —N=N- functionality.
Ideally, the perfect model of a compound would take into account every aspect of
said molecule and the interactions with the surroundings. The closest to this
ideal at the current time is to perform a series of ab initio calculations. These
calculations model the Tr-electrons and all of the bonds. The problem with ab
initio calculations is that they require large amounts of processing power and
time. In the case of azobenzene, the conjugated Tr-system actually lends itself
well to parameterization and timescale of the ab initio calculation lengthens
unacceptably. One alternative to ab initio studies is to perform semi-empirical“2
calculations. These calculations only take into account the n-electrons and, as
such, require that many assumptions be made. The n—system and bond motions
are both parameterized and inserted into the equation in a semi-empirical
calculation, where as they would be generated by the calculation itself in an ab
initio experiment. Both methods require that the lowest energy conformer of the
molecule be determined and used as the starting point. Semi-empirical
calculations were performed on a Windows-based PC using Hyperchem® v. 6.0.
Dihedral angles were incremented for isomerization barrier calculations using
macros written in Microsoft Excel®. For these calculations, initial geometry

optimization was performed using PM3 parameterization, then single point

49

calculations were made for the molecule at each incremented angle without
additional geometry optimization. For calculations at the optimized So geometry
we report energy levels and oscillator strengths for cis and trans conformers
(Figure 4.1). Based on these stepped geometry calculations, the isomerization
surfaces for azobenzene have been calculated and are presented in Figures 4.2
thru 4.7.

We have attempted the calculation of the isomerization surfaces for the
substituted azobenzenes, and we find that semi-empirical calculations do not
reflect the experimental substituent-dependence of the back-isomerization barrier
height. This is not surprising given the parameterization used in semi-empirical
calculations and the multidimensional nature of the isomerization coordinate for
azobenzenes. Even with the limitations of such calculations, we can extract
useful information from them. Many groups have evaluated the isomerization
process for azobenzene. Most groups have primarily concentrated on the
conversion on the 82 state surfacef”4 One issue that has not been addressed is
the mechanism of ground state (thermal) isomerization for this class of
molecules. Our calculations for azobenzene, presented in Figures 4.2 and 4.3,
show the calculated energy surfaces for the So (a) and S1 (b) states, for
isomerization by rehybridization of one N (Figure 4.2) or rotation about the N=N
bond (Figure 4.3). These calculated results predict that (thermal) isomerization
in the ground state proceeds by rehybridization and not by -N=N- bond rotation.
We calculate a cis —> trans barrier height of 33 kcal/mol for So rehybridization,

compared to a 48 kcal/mol barrier for So ring rotation. When comparing the

50

calculated results for isomerization on the So and S1 surfaces, a relatively small
S1 barrier is indicated for rehybridization (~ 12 kcal/mol), and the S1 surface is
calculated to be barrierless for ring rotation. While it is tempting to use this
information to infer the dominance of N=N rotation as the primary isomerization
coordinate in the S1, we do not feel justified in making this conclusion because
an almost barrierless isomerization pathway can be traced out on either surface
provided the ring rotation coordinate has sufficient time to locate the minimum on
the S1 surface prior to relaxation back to the 80 surface. These calculated results
should be taken as an indication of the substantially more facile nature of
isomerization on the S1 surface than is possible on the So surface for
azobenzene.

Corresponding calculations were conducted on both the p-
diaminoazobenzene and p-diamidoazobenzene and are presented in Figures
4.4, 4.5, 4.6, and 4.7. In Figure 4.5 the region of high energy is due to a
calculational error and is not representative of the real molecule. It is remarkable
that both the inversion and rotation mechanism calculations yield nearly identical
results for azobenzene, p—diaminoazobenzene, and p—diamidoazobenzene. This
similarity suggests that substitution at the para position on the molecule has little
to no effect on the isomerization. It is believed that similar calculations on a
series of meta substituted azobenzenes would yield differentiated surfaces due
to the interference of the substituent with the rotation mechanism. This

interaction is removed by para substitution.

51

Energy (cm'l)

Energy (cm'l)

Energy (cm'1)

40000 .:.___:

30000 ——-

20000 L

 

40000

20000

 

40000

30000

4L

20000

508T

 

l'
L__
30000 “—

tran

£ft|31947

T2 azobenzen

S1 f = 0.001
T1
So

tran

T5
S3 f = 0.007
4
diaminoazobenzen

82 f = 1.61
:1 f = 0.01
3
2
T1
So
tran
g3 f: 0.21
82 f = 1.29

I diamidoazobenzen

S1 f= 0.03
T

S

Ci.

I! ll ”"1

 

I“

ci

 

 

 

 

 

 

 

 

 

ci

 

 

 

S4f= 0.39

1813
§f= 0.11

81 f = 0.35
T.

So

Figure 4.1: Calculated energy levels and oscillator strengths for
azobenzene, diaminoazobenzene, and diamidoazobenzene.

 

 

     

[200
I
I -—~-175
' ' '7' "I l
Iz/lf/xf’éfaiffl/AEKJ’” .-
‘ 4‘ N450
L B
' E
Q
N125
A. ,,,,,,,,,,
439 4100
’6 1%
I
943 1(2)
0 ~75
<9
’89/ 200 220 240
O 0 150 180
(.6 120 140 . n ang‘e (degree)
0% N=N rehybndrzatro
o
9)

Figure 4.2 Three dimensional isomerization
energy diagram for rehybridization mechanism for

azobenzene.

53

 

 

 

     

3;:[ 200
”-7“- 180
I!" lii'liilim‘i r 7‘
”fin“ I i“ 160
------- ' h E:
““-“ 14o \
«I. Q
5‘ 120
¢u
1’9 ‘ 100
a 1?
’6 3
a; A
"7 ~1§8
o 160
”3% 0 40 so 80 100 120 140
20
f 0 . de reeS)
0% N=N rotation ang\e( g
A
0G
“‘1

Figure 4.3 Three dimensional isomerization

energy diagram for rotation mechanism for
azobenzene.

l l
______ __c_i__.

ililill/I/I/llllll

 

A.
’?o
” 1??
o
6% 130
o
‘3;
06f 0 140 16° 1-
0% 120 N=N rehybridizatlo
3’0
d)

 

 

220

225

— 200

~175

7 1 so

--~125

 

200
80“ angle (degreeS)

Figure 4.4 Three dimensional isomerization
energy diagram for rehybridization mechanism

for diaminoazobenzene.

55

KJ/mol

 

 

 

 

[200
—175
fl ’1- 150
T ~125

o-
’39 ~ ~1oo

, . 7
° 1?? “
I
9%- 13
°% ”7:80
0/ 140 160
O 0 40 60 80 100 120

gle (degreeS)

N=N rotation an

Figure 4.5 Three dimensional isomerization
energy diagram for rotation mechanism for
diaminoazobenzene.

56

KJ/mol

150

KJ/mol

 

 

      

""""" 5 {i125
.. lll’l'l"""rw
I'llll‘ ‘ . ‘ Il'l'llwl 100
h 75
sighted it“ , \-
" 7 ~ 50
cc ‘5’ ‘ ‘
o
0” 25
O N
’5’;- 1%
09 120
a 9 ~o
‘96 240
r o l80 20°
(6 120 140 160 d reeS)
0» . - le( 99
- on anQ
0°“) N=N rehybr‘d‘za“

Figure 4.6 Three dimensional isomerization
energy diagram for rehybridization mechanism for
diamidoazobenzene.

57

 

 

 

150
mu cl
___________ Ely -125
+... lllllll llll"
Mimi l l“ I - “ -1oo
Q
50
N ‘, -...~ 25
0 ~ 0
$6 80 100 120 140 160180
66 0 0 20 40 60 reeS)
98> N=N rotation ang‘e (deg
“6)

Figure 4.7 Three dimensional isomerization
energy diagram for rotation mechanism for
diamidoazobenzene.

58

Semi-empirical calculations were conducted on the model series of p-
disubstituted azobenzenes using Hyperchem® v. 6.0 and Microsoft Excel®. From
these calculations energy levels and oscillator strengths can be calculated as
presented in Figure 4.1. These calculated energy levels qualitatively agree with
steady state spectroscopy but the oscillator strengths appear to deviate from
those observed experimentally. The calculated isomerization surfaces suggest
that, at the para position, substitution has little effect on the isomerization barrier
and mechanism. It would appear, based solely on these calculations, that the
isomerization rates should be the same for azobenzene, p-diaminoazobenzene,
and p—diamidoazobenzene. We find these calculated results to be at odds with
the experimental data, and take this finding to underscore the limitations inherent

to parameterized calculations.

59

Literature Cited

A

Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc., 1977, 99, 4899.

Dewar, M. J. S.; Theil, W. J. Am. Chem. Soc., 1977, 99, 4907.

Fujino, T.; Arzhantsev, S. Y.; Tahara, T. J. Phys. Chem. A, 2001, 105, 8123.
Saito, T.; Kobayashi, T. J. Phys. Chem. A, 2002, 106, 9436.

Astrand, P.-O.; Ramanujam, P. S.; Hvilsted, S.; Bak, K. L.; Suauer, S. P. A.
J. Am. Chem. Soc., 2000, 122, 3482.

lshikawa, T.; Noro, T.; Shoda, T. J. Chem. Phys, 2001, 115, 7503.

Fliegl, H.; Kohn, A.; Hattig, C.; Ahlrichs, R. J. Am. Chem. Soc., 2003, 125,
9821.

Zimmerman, G.; Chow, L. Y.; Paik, U. J. J. Am. Chem. Soc., 1958, 80, 3528.
Shultz, T.; Quenneville, J.; Levine, B.; Toniolo, A.; Martinez, T. J.;

Lochbrunner, S.; Schmitt, M.; Shaffer, J. P.; Zgierski, M. Z.; Stolow, A. J.
Am. Chem. Soc., 2003, 125, 8098.

60

Chapter 5

TIME RESOLVED SPECTROSCOPY OF AZOBENZENE

The azobenzene derivatives studied exhibit remarkable substituent-
dependent variations in their steady state and time-resolved optical responses.
These trends are attributed to substantial changes in the electronic state
energies and singlet transition oscillator strengths calculated at the semi-
empirical level. It appears, based on a comparison of experimental and
calculated results for these compounds, that the transition energies are predicted
relatively well, but the oscillator strengths for these transitions are not. Given
these findings, intuition may suggest that the isomerization behavior of these
molecules would likewise vary with substitution, and we examine this issue next.

To understand the isomerization behavior of azobenzene and the
symmetrically disubstituted azobenzenes, we have evaluated the branching ratio
for isomer formation and the recovery time required for the samples to return to
an equilibrium ground state population distribution. The branching ratio data is
indicative of the location of the ground state surface maximum and the excited
electronic state surface minimum. In the limit that these two electronic state
features exist at the same point in conformational space, we would expect a
branching ratio of 1 (i. e. 50% of the excited molecules relax to form a trans
conformer and 50% relax to form a cis conformer. Deviations from this ideal

behavior are indicative of either a non-correspondence between the electronic

61

state surface minima and maxima or a ground state potential energy surface that
is not well characterized in the context of Rulliere’s model.8

If the azobenzene sample begins as a ratio near 50% of either isomer
upon irradiation this ratio can be shifted to promote one isomer over the other.
This can be seen in Figure 5.1 where 5 is promoted to primarily the cis form upon
90 minutes of irradiation under an ultraviolet lamp. These spectra were taken in
1-octanol using the spectrometer described in Chapter 4. Upon varying reaction
conditions and noting nearly no spectral alteration, it was determined that the
instrument could be affecting the experiment. The spectrometer used was a dual
beam instrument where the sample beam constantly illuminates the sample.

This was found to be a problem for both the branching ratio and the time
correlated relaxation experiment. To counteract this problem, the instrument was
modified by placing a neutral density filter in the path in front of the sample holder
as pictured in Figure 5.2.

To make a meaningful analysis of the branching ratio it is necessary to
quantitatively analyze the steady state peaks corresponding to both cis and trans
isomers. Concentrations work well for this application where each peak gives the
concentration of that isomer. In the case of azobenzene the peak at 318 nm can
be used to calculate the concentration of trans azobenzene at that moment.
Likewise the peak at 254 nm can be used to calculate the concentration of cis
azobenzene at that moment. The branching ratio can then be obtained by
evaluating the ratio of one of the concentrations versus the total concentration of

isomers. To calculate the concentration of each peak the molar extinction

62

coefficient was needed at any wavelength that proved to identify an isomer. The
molar extinction coefficients were calculated by evaluating the change in
absorbance for a dilution series of the compounds in methanol. These yielded

graphs as in Figure 5.3 where the slope is the extinction coefficient and the line

 

Absorption
Compound smax (L/mol-cm) £450 (L/mol-cm)
maximum (nm)

 

 

 

 

 

 

 

 

Azobenzene 31 3 22400 500

p-

394 27800 1630

diaminoazobenzene

1 364 17900 1 300
2 364 23750 2400
3 364 24200 2530
4 364 24900 2500
5 364 25300 2450

 

 

 

 

 

Table 5.2. Absorption maxima and molar absorptivities of azobenzene, p-

diaminoazobenzene and the p—diamidoazobenzenes 1-5.

63

1.0-

 

 

 

 

 

 

 

 

 

 

— Room
— W30
0.8 _ —— UV60
— UV90
—-—— UV1140
0.6- A
as
.0
<
0.4 -
0.2 -
0.0 I I I I I m l ' 1 1 1 i I
200 250 300 350 400 450 500
Wavelength (nm)

Figure 5.1: Absorbance spectra of 5 upon varying durations of

ultraviolet irradiation.

64

Neutral Density Filter

 

 

 

 

 

 

 

Source

 

 

 

 

 

...... /-----

Sample 5 Detector

 

 

Reference Cell

Figure 5.2 : Block diagram of UVNis instrument modification.

 

1.6!
1.4-
1.2-
1.03
0.83

0.6 -

Absorbance

0.4 d

0.2 -

0.0 -

 

 

I V l ' l ' I T l ‘ l ' I ' I

Concentration (Molar X10'5)

 

 

 

Figure 5.3 : An absorbance versus concentration graph used to
determine molar absorptivity of azobenzene monitoring 318nm.

65

should go through the origin since at zero concentration there should be no
absorbance. Subtracting the x-intercept from the concentration and then
reevaluating the absorbance versus concentration graph can correct any
deviation from the origin observed. Methanol was used as the solvent due to the
high degree of solubility of all the molecules of interest. Solvent selection was
not crucial since the molar extinction coefficient is essentially solvent
independent. The coefficients along with the corresponding wavelengths are
presented in Table 5.1.

To characterize the branching ratio, we irradiate a sample until the ratio of
trans to cis remains constant, as measured by absorption spectroscopy.
Because the extinction coefficients of the cis and trans bands will, in general,
differ, we need to correct the data either through band ratio measurements or by
direct conversion of the absorbance data to concentration data using Beer’s law.
Upon UV irradiation, the ratio of trans to cis conformers reaches a steady state,
which recovers to the equilibrium ratio once UV irradiation of the sample is
stopped. The steady state [trans]/[cis] ratio, prior to recovery, is 0.19 for
azobenzene, 0.51 for diamidoazobenzene and 4.5 for diaminoazobenzene. We
note the similarity of azobenzene and diamidoazobenzene, and the contrasting
behavior or diaminoazobenzene. These data point to the barrier height for cis —+
trans isomerization being largest for azobenzene and smallest for p-
diaminoazobenzene, in agreement with the direct measurements we report

below.

66

Of perhaps greater significance than the branching ratio data are those for
the isomerization recovery. We have measured the isomerization recovery time
constants for azobenzene, p-diaminoazobenzene and p-diamidoazobenzene.
The time constants are for cis —-> trans conversion and vary enormously with
substitution and scale qualitatively with the energy of the dominant electronic
transition of the chromophores. For azobenzene, we find that the recovery time
after photoisomerization is 10,900 minutes (Figure 5.4), for the p-
diamidoazobenzenes, the recovery time is 317 minutes (Figure 5.5), and for p-
diaminoazobenzene, the recovery time is 4.7 minutes (Figure 5.6). These
remarkably different time constants for trans recovery are due to variations in the
isomerization barrier height, which are related electron density distribution about
the azo bonds in these compounds. While the back isomerization time constants
are remarkably different for these compounds, it is important to keep in mind that
there is a logarithmic relationship between the measured time constant and the
barrier height. Since the process of interest is monomolecular we have used the

Arrhenius equation to calculate isomerization barrier heights. Where A is a
1 33";
k = - = Ae "7'
T
prefactor, r is measured from experimental data, and AE is the variable of
interest. We have calculated the barrier heights consistent with the experimental
back-isomerization data for the substituted azobenzenes, assuming a prefactor
for the activated process of 1013 Hz. For p-diaminoazobenzene, with a

characteristic recovery time constant of 2.4 minutes, we calculate a barrier height

of 20.8 kcal/mol, for p-diamidoazobenzene, with a recovery time constant of 317

67

Absorbance (a.u.)

1.4-

    

 

 

1.2a
1.0%
0.8-
0.6- < > I
0.4-
trans recovery, 1: = 10904 i 1 min
0.2-
l ' l 1 l ' l ' l ' l
0.0 socio’ 1.o<10‘ 1.5m“ zoao‘ 25x10‘
Trrre(rrin)

Figure 5.4 : Recovery of the absorption due to the trans band in
azobenzene at 318 nm.

68

N
.1
C?

N
5?

 

trans recovery, I = 2.4 i 0.1 min

Absorbance (a.u.)
ii 7’

“5:5

5?

 

 

1m I n I I f I I I I I I

Tirre(m'n)

Figure 5.5 : Recovery of the absorption due to the trans
band in p-diaminoazobenzene at 370 nm.

69

Absorbance (a.u.)

1.01

0.9 -

.0
U1
1

 

N—.-'N}-I—‘

O

 

 

I I

I l I I II I I I l I
ED1QD1ZD14CD1ED1KD

Tirre (rn'n)

ill lfi
ZD4CDGtD

Figure 5.6 : Recovery of the absorption due to the trans band
in p-diamidoazobenzene at 370 nm.

70

minutes, we calculate a barrier height of 23.7 kcal/mol and for azobenzene, with
a recovery time constant of 10,900 minutes, we calculate a barrier height of 25.8
kcal/mol. These barrier heights, for So cis —> trans conversion vary substantially
less than the recovery time data would seem to imply, owing to the logarithmic
relationship between the recovery rate constant and the barrier height. The
semi—empirical calculations for azobenzene are in qualitative agreement with the
experimental data.

We note that there is an inverse correlation between barrier height and
electron donor strength of the para substituents. This correlation is the result of
an effective increase in the average electron density of the 11* (anti-bonding)
state, reducing the effective bond order of the N=N bond and thereby reducing
the isomerization barrier height. We seek to understand the detailed basis for
this relationship and find that the linear optical response of these compounds is
useful for this purpose. The ground state (thermal) barrier for isomerization is
correlated inversely with the transition cross section of the first allowed electronic
transition for these azobenzenes. All of the azobenzenes studied here have
transitions in the 400 - 450 nm region (Figure 4.7), so it is not the energy of the
$1 «— So transition that is related to the thermal back isomerization barrier height.
Rather, it is the transition cross section of the lowest energy transition that is
related to the barrier height. We rationalize this relationship by noting that a
large transition cross section is reflective of large overlap integrals for the
transitions. The isomerization barrier height is related to the bond order of the

N=N bond, and the Sr *— So transition coordinate is thought to lie along the long

71

axis of the azobenzene molecule. We postulate that the isomerization and
electronic transition coordinates are nearly the same. The ground electronic
states (HOMOs) of the azobenzenes are characterized by a relatively high bond
order of ~ 2 for the N=N bond. The first excited singlet states (HOMOs) of these
compounds are characterized by a substantial reduction in electron density and
thus bond order on excitation. Because the activation barrier for ground state
back isomerization of the azobenzenes involves a high energy intermediate
state, mixing with states in the $1 manifold is possible, and the strength of this
coupling is reflected in the transition cross sections of the Sr <— 80 transitions for
both the cis and trans forms. Thus, the larger the transition cross section for the
Sr <— 80 transition (Table 3.2), the more single-bond character there will be for
the transition state on the So isomerization surface, giving rise to a lower barrier
height. This explanation provides qualitative agreement with the experimental
data and does not invoke coupling of higher excited electronic states.

It was determined through time resolved experiments that the identity of
the para substituent alters the overall time of isomerization. It was also seen that
the p-diaminoazobenzene seems to be different than the other p-disubstituted

azobenzenes. This result can be seen in Figures 5.4 to 5.6.

72

Literature Cited

—8

Jiang, Y.; Hambir, S. A.; Blanchard, G. J. Opt. Commun., 1993, 99, 216.

Bado, P.; Wilson, S. B.; Wilson, K. R. Rev. Sci. Instrum, 1982, 53, 706.

. Andor, L.; Lorincz, A.; Siemion, J.; Smith, D. 0.; Rice, S. A. Rev. Sci. Instrum,

1984, 55, 64.

Blanchard, G. J.; Wirth, M. J. Anal. Chem, 1986, 58, 532.

Shultz, T.; Quenneville, J.; Levine, B.; Toniolo, A.; Martinez, T. J.;
Lochbrunner, S.; Schmitt, M.; Shaffer, J. P.; Zgierski, M. Z.; Stolow, A. J. Am.
Chem. Soc., 2003, 125, 8098.

Lednev, l. K.; Ye, T.-Q.; Matousek, P.; Towrie, M.; Foggi, P.; Neuwahl, F. V.
R.; Umapathy, ,S.; Hester, R. E.; Moore, J. N. Chem. Phys. Lett., 1998, 290,
68.

Lednev, l. K.; Ye, T.-Q.; Abbott, L. C.; Hester, R. E.; Moore, J. N. J. Phys.
Chem. A, 1998, 102, 9161.

Rulliere, C. Chem. Phys. Lett., 1976, 43, 303.

73

Chapter 6

SPIROPYRAN INTRODUCTION

The azobenzene data it was shown that the isomerization mechanism
alone is too easily reversible to support optical data storage through locking in
the conformation of the molecule. The next logical step is to incorporate an
additional step in the switching process to facilitate the persistence of the desired
conformation. From the literature there is a wealth of compounds that have both
an isomerization mechanism and a covalent bond formation.M From these
compounds spiropyrans were selected due to the apparent ease of synthesis and
the comparably limited number of side reactions this family of molecules is
known to undergo.M Spiropyrans have been incorporated into a wide variety of
systems, with applications ranging from metal chelation to optical data storage.
While spiropyran has been used for chelating heavy metals in solution,‘W the
primary focus has been on optical data storage. Since the initial reports on the
spiropyrans, the property of most interest was the photochromic behavior of
spiropyrans. As with all photochromic molecules, spiropyrans undergo a
geometric change that alters the optical absorption response of the molecule.
Upon irradiation with ultraviolet light, the molecule undergoes a bond scission
and subsequent isomerization to a merocyamine structure. With irradiation by
visible light, the molecule undergoes trans -) cis isomerization followed by the
formation of a carbon oxygen bond at the same point of bond breakage to reform

the closed form of the molecule. It is the reversible nature of this covalently

74

bonded structural change that makes the spiropyrans attractive candidates for
optical information storage. This equilibrium can be seen in Figure 6.1. There
have been efforts to determine the solvochromatic effects on the nitro substituted
spiropyran, and the effects of substitution.‘Ma The importance of the substituent-
related effects is that predictability is vital and all interactions must be fully
understood to give the optical storage system a chance at success. The
absorbance spectrum of the nitro substituted spiropyran can be seen in Figure
6.2 where both the closed and opened forms of the molecule are present. This
molecule has been investigated by many spectroscopic techniques, with the
absorbance spectrum being the primary focus. The reason for the attention to
the absorbance spectrum can be seen in Figure 6.2, where there is a large
difference between the spectra of the open and closed forms primarily in the
visible region.

The use of spiropyrans as Optical data storage chromophores requires
that the device be constructed in a manner to allow the molecule to be accessed
by light. The least complicated structure is a two dimensional arrangement in
which the recording material can be deposited much like that of the current
generation of data storage devices. As can be seen in Figure 1.1, the
chromophore is accessed by a laser to write the information to the disk. This
laser would be in the UV region to access the closed form of the molecule and
excite the molecule to allow the carbon oxygen bond to break and subsequent
isomerization from the cis to the trans form of the opened molecule. The

information could then be read using a visible laser reflected from the reflective

75

 

 

 

 

 

Figure 6.1. Photoreactivity of a spiropyran.

76

 

(15-

(L4-

 

 

o.3- B
a, A
.o
< /
(12-
01-
01) . r . r v u
300 400 500 600

Wavelength (nm)

Figure 6.2. Absorbance of p-nitrospiropyran. In the
absorbance spectrum A is the closed form of the molecule
and B is the open form.

77

layer to a detector. While the current technology is limited to the diffraction limit
of the laser used, this system can go beyond this limitation by monitoring the
intensity of the reflected beam. Monitoring the intensity allows the overall system
to move from binary logic to n bit logic, allowing for more information to be stored
in a given amount of space. Since recordable space is almost always the limiting
factor to the capacity of a storage device, a second geometric arrangement can
be developed in which a 3D array can be constructed. This 30 approach can be
achieved most easily through the stacking of several quasi ZD layers of recording
material. (Figure 1.2)14 This arrangement requires that the individual layers can
be accessed independently and with little or no cross-talk. A microscope of
some sort is required for reliable access to individual layers. Unfortunately, this
means additional cost of production for commercial devices using this type of
storage device. While increased capacity is needed for the future, this
advancement can not succeed with a media system that is significantly more
expensive than the current generation.

Incorporation of the chromophore into the system is another concern that
must be addressed. With the use of the nitro substituted spiropyrans, there is not
a chemical attachment point that one can access readily, therefore the molecule
is simply dissolved in a hot polymer solution and spin coated on a substrate.
While this procedure does incorporate the chromophore into the polymer layer
there is no real control over distribution of the molecule across the surface of the
substrate. For the storage device to be successful there needs to be enough

organization that the performance is reproducible. A logical solution to this

78

problem is to synthesize into the chromophore the chemical functionality that
allows for bonding to a polymer layer and through the use of the polymer layer
limited organizational control can be imparted into the solid system. Some of the
features that the polymer layer can control are aspects such as chromophore
density loading, number of recording layers in layered systems, and optimum
free space to allow for chromophore operation. The optimum free space can be
determined through the analysis of multiple samples with differing linking
chemistry while maintaining the same loading density of the chromophore.
Through the polymer each aspect should be capable of being optimized for the
final solid sample.

In this research we investigate the appropriate location for a chemical
attachment point on the spiropyran chromophore and what the spectroscopic
consequences are. We also attempt to characterize the molecule and determine
the equilibrium expressions for a hydroxyl terminated spiropyran. To determine
the response to irradiation absorbance spectroscopy was used based on the
pronounced difference in the absorbance spectrum. We can then treat the time
resolved absorbance spectroscopy with the same methodology used in the
azobenzene system to determine the isomerization barrier height, time constants
for relaxation, and photo initiated opening. We also evaluated the effect of
solvent on the chromophore response in an attempt to evaluate the synthesized

spiropyran in the historical methodology.

79

Literature Cited

10.

11.

12.

13.

14.

Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev., 2000, 100, 1741

Wojtyk, J.T.C.; Wasey, A.; Kazmaier, P.M.; H02, 8.; Buncel E. J. Phys.
Chem. A, 2000, 104, 9046

Yolyama, T. Chem. Rev., 2000, 100, 1717

lrie, M. Chem. Rev., 2000, 100, 1685

Suzuki, T.; Kawata, Y.; Kahata, S.; Kato, T. Chem. Commun., 2003, 2004
lpe, B. |.; Mahima, S.; Thomas, K.G. J. Am. Chem. Soc., 2003, 125, 7174

lnouye, M.; Ueno, M.; Tsuchiya, K.; Nakayama, N.; Konishi, T.; Kitao, T. J.
Org. Chem, 1992, 57, 5377

Wojtyk, J.T.C.; Wasey, A.; Kazmaier, P.M.; H02, 8.; Buncel, E. J. Phys.
Chem. A, 2000, 104, 9046

Gorner, l-l. Phys. Chem. Chem. Phys, 2001, 3, 416

Kamada, M.; Sumaru, K.; Kanamori, T.; Shinbo, T. Langmuir, 2004, 20,
9315

Venturini, C.G; Andreaus, J.; Machado, G.; Machado, C. Org. Biomol.
Chem, 2005, 3, 1751

Bee, S.Y.; Arnold, BR. J. Phys. Org. Chem, 2004, 17, 187
Minkin, V. Chem. Rev., 2004, 104, 2751

Parthenopoulos, D.; Rentzepis, P. Science, 1989, 245, 843

80

Chapter 7

SPIROPYRAN SYNTHESIS

Functionalized spiropyrans have been incorporated into polymer systems
for various different recording systems by simply dissolving the chromophore in a
polymer solution and then casting that solution onto a substrate.16 To construct
a functional optical storage device, there needs to be some level of organization
to optimize efficiency and uniformity. Spin coating does not produce films with a
high degree of order and therefore an alternative method of chromophore
deposition needs to be established. By chemically attaching the chromophore to
a polymer, the density of the chromophore can be controlled through the
synthesis of the polymer. Using polymer chemistry a high degree of order can
be achieved. There has been only a limited effort aimed at chemically attaching
a spiropyran to a polymer in any system previously evaluated.7'11 The identity of
the polymer system used in the construction of the recording device determines
which functionality needs to be incorporated into the spiropyran chromophore. In
the research presented here the polymer system to be used is a maleimide vinyl
ether terpolymer system where some fraction of the maleimide monomer is
terminated with hydroxyl functional groups.(Figure 7.1) The full spectrum of
linking chemistries that can be utilized has been described previously.”21 The
simplest attachment chemistry utilizes either a diisocyanate or a diacid chloride
to attach either the polymer to a substrate or a chromophore to the polymer layer

through the hydroxyl group, thus a hydroxyl group must be synthesized into the

81

 

 

 

 

0&0 Al
O O

+ 1. Stir 12hrs N
NH2 RT, CHCI3 _

2. Acetic Anhydride

70°C, 3-4hrs
OH

OH

 

In It
:5 + <5 + “7
OH 1.AIBN,THF
70°C 6-l8hr

2. Ether

on
o N 00 N O

OH

Figure 7.1. Terpolymer system for substrate
resurfacing.

82

spiropyran chromophore. The selectivity of the linking chemistry with the
hydroxyl groups on the terpolymer is unresolved at this time but there would
remain hydroxyl groups available for reactions. There are several locations
available for functionalization on the-spiropyran molecule. Figure 7.2 illustrates
positions that need to be considered and the resulting spiropyrans that are
accessible for incorporation into the polymer as a side group.

Before any molecules were synthesized, semi-empirical calculations were
performed to estimate what the isomerization barrier should be for the
functionalized spiropyrans shown in Figure 7.2. The semi-empirical modeling
was used instead of an ab initio calculation to reduce the amount of time required
for the computations. As with azobenzenes conjugated 11 systems have been
parameterized effectively for semi-empirical calculations. These calculations
were performed on a Windows-based PC using Hyperchem® v. 6.0. Dihedral
angles were incremented for isomerization barrier calculations using macros
written in Microsoft Excel®. For these calculations, initial geometry optimization
was performed using PM3 parameterization, then single point calculations were
made for the molecule at each incremented angle without additional geometry
optimization. Based on these stepped geometry calculations, the isomerization
surfaces for p-nitro spiropyran and p-hydroxyl spiropyran have been calculated
and are presented in Figures 7.3 and Figure 7.4. From these calculations we
estimate an isomerization barrier of ~140 KJ/mol, which is substantially larger
than the 108 KJ/mol observed for azobenzene.22 These calculations only

account for the isomerization process and do not account for the formation of the

83

 

Figure 7.2. Locations for chemical attachment points on
spiropyrans.

84

 

 

.-.,.......
lI
hith—
s
A
A
O

 

 

 

 

a
05’

FT
i i ii?“
E

O)
O

 

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r
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' I

I.
.4
l

 

 

O)
O

 

 

l
Energy (KJ/m

 

 

A
O

 

 

N
N
O

 

Figure 7.3. Calculated isomerization barrier of
p-nitrospiropyran. This calculation only incorporates
the isomerization and not the bond formation

85

120

100

80

60

4o

20

-20

 

(\09

C:
C rotation 0H

Figure 7.4. Calculated isomerization barrier of
p-hydroxyspiropyran. This calculation only
incorporates the isomerization and not the bond

formation

86

Energy (KJ/mol)

C-O bond that we hope to use as a chemical “latch”. Based on the correlation
between the calculated isomerization barriers and the experimental data, these
calculations for the spiropyrans are taken only as estimates to determine whether
further investigation is warranted. ltis worth noting that the calculated
isomerization barrier heights are on the same order of magnitude as that of the
azobenzenes, underscoring the fact that the functionalization may lead to the
overall success of this optical data storage system. These calculated results do
warrant further investigation into the optical response of a functionalized
spiropyran.

The first spiropyrans synthesized were described in detail by Koelsh and
Workman in 1952.23 (Scheme 7.1) The spiropyrans were condensed from freshly
prepared indoline and a functionalized salicylaldehyde in boiling alcohol. Then
the solid was collected, washed with hot alcohol, and recrystallized from acetone.
From this beginning there have been many suggested modifications to the
synthesis to improve yield or allow for selected functionalization of the spiropyran
molecule.”32 Of particular debate is which is the best synthetic route for
obtaining hydroxyl functionalized spiropyrans since the solubility of the
substituted molecules is substantially different than that of the starting materials.
To a great extent, the synthetic route used will be determined by the point on the
spiropyran molecule where the attachment is to be made. To functionalize from
the nitrogen at the x position the appropriate indoline needs to be synthesized

and purified. We conducted a preliminary investigation into attaching an alkyl

87

37.5 mmol 29.7 mmol

20 mL Ethanol
Boiling
2 hours

 

LX=Y=H
lll,X=OCH3,Y=H
V,X=H,Y=Br

vu,x=H,\r=No2
Vlll,X=Y= No2

 

Scheme 7.1. Original synthesis adapted from reference 23.

88

chain terminated in an alcoholic functionality. With appropriate indoline in hand
the condensation reaction with a nitrosalicyladehyde can be conducted to
produce the desired spiropyran.33 While we found that the alcoholic alkyl chain
containing spiropyran could be synthesized, it was difficult to isolate only the
desired product and abandoned. A more accessible synthetic route was to
attach the hydroxyl group to the benzene ring on either the indoline or the
aldehyde and then conduct the condensation reaction. The choice to attach the
alcohol group on the aldehyde portion of the spiropyran was decided because
multiple hydroxyl terminated benzaldehydes are available from Aldrich while the
catalog only contains four functionalized indolines. Of the functionalized
indolines, only one contains a reactive group from which a chemical hook could
be incorporated. From the reactant considerations it was clear that the chemistry
that needed to be developed was the reaction with a stock indoline, in this case
Fischer’s base, and an appropriately functionalized benzaldehyde.

The reaction that we completed is described in Scheme 7.2 and is a
derivative of the original synthesis. Fischer’s base (6.41 mmol) purchased from
Aldrich and used as received was reacted with 2,5-dihydroxybenzaldehyde (2.19
mmol) in refluxing ethanol (60 mL) for 3 hr. The solvent was removed under
reduced pressure, leaving a tar containing the product and unreacted starting
materials. The tar was dissolved in the minimal amount of chloroform,
approximately 3mL, and precipitated in hexanes, approximately 10mL. This step
removes most of the reactants, however the application mandates the highest

purity of chromophore, so further purification is required. From thin layer

89

OH
0
e + \
N
I HO

64mmm 219mmm

60 mL Ethanol
90°C
3 hours

 

 

 

Scheme 7.2. Synthetic route used to synthesize hydroxy
substituted spiropyran.

90

chromatography it was found that the best solvent combination for column
chromatography was 2:1 hexanes: ether.

The loading of the column determines the quality of the separation and
therefore attention was paid to how to properly load the spiropyran reaction
mixture onto the column. The column was prepared from a silica gel slurry in the
hexanes: ether solvent system using at least 100 grams of silica to every 1 gram
of chromophore. The previously obtained solid was dissolved in the minimal
amount of diethyl ether and carefully loaded onto the column with the flow
stopped. Pure hexanes were then added to the column to get the solvent ratio to
the desired 2:1 ratio. The column was then operated in either gravity or flash
mode. The second phase was collected and found to contain the spiropyran.
The solvent was then removed under reduced pressure and the solid collected.
The solid was then dissolved in the minimal amount of chloroform (~10 mL) and
precipitated in hexanes (~200 mL). The resulting brown to light brown solid was
evaluated with GC/MS which can be found in Appendix C and 1HNMR. The
mass spectrum showed mlz of 294.3 (parent ion), 278, 263, 159 (most intense),
1 15, 77.

The NMR spectra reported here were obtained on a Varian 500 MHz
instrument using a standard proton collection method. The spectra were
collected at room temperature, -10°C, and -60°C to allow for the visualization of
any isomers that are present. The proton spectrum of Fischer’s Base shows
proton peaks at 1.33, 3.01, 3.83, 6.53, 6.74, 7.07, and 7.12 ppm. The proton

spectrum of the Fischer’s base can be seen in Figure 7.5. The benzaldehyde

91

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shows proton peaks at 4.53, 6.88, 6.98, 7.04, 7.06, 9.82, 10.59 ppm and can be
seen in Figure 7.6. So it is expected that the spiropyran should have peaks at
similar shifts if the molecule is in the closed form. The open form of the molecule
should be similar but with differentiation due to the carbon carbon double bond
having altered geometry. This change of geometry and calculated proton shifts
can be seen in Figure 7.7. In our research we found peaks at 1.02 (s), 1.20 (s),
2.61 (s), 5.74 (d), 6.43 (d), 6.54 (d), 6.58 (s), 6.78 (t), 6.85 (d), 7.06 (d), 7.12 (t)
ppm.(Figure 7.8) When we reduce the temperature from 33°C to -60°C we see
the temperature dependent shifts of peaks which can be seen in Figure 7.9. We
also note the loss of resolution on the splitting of the peaks, this is thought to be
the result of an increase of viscosity as the temperature approaches the freezing
point. Upon irradiation we see essentially the same spectrum despite the
predicted differences. We know that the irradiation is opening the some
percentage of the molecule due to changes in the absorbance spectrum acquired
of the sample prior to the measurement by the NMR instrumentation. it is know
that the open form persist the duration of the experiment due to the persistence
of the deep red color of the solution observed before and after acquiring the
proton spectrum. When we compare the 33°C or the -60°C spectrum of the
open and closed forms of the spiropyran we see very little differences and
nothing as substantial as predicted.(Figure 7.10 through 7.13) Despite the lack
of the shifts in the proton peak positions there is strong evidence that the
molecule is behaving in a photochromic fashion through the UVNis spectrum.

Peak assignments were conducted through a Correlation SpectroscopY (COSY)

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experiment. The COSY experiment gives correlations between J-coupled signals
by watching the interactions of two pulses at right angles to each other. From the
contour map in Figure 7.14 the assignments of the protons were made through
the typical analysis on the provided software. As seen in Figure 7.7 the peak
assignments were close to the predicted values.

The absorbance spectra reported here were obtained on a Varian/Cary
model 300 UVMsible absorption spectrometer. The absorbance of the nitro
substituted spiropyran has been studied extensively and can be seen in Figure
7.16. Upon ultraviolet irradiation the molecule opens, and causing a change in
the absorbance spectrum, primarily in the visible region. (Figure 7.16) The
sample was irradiated in a quartz cuvette by an 8 Watt mercury lamp purchased
from Fischer Scientific. The cuvette was positioned 22 cm away from the source.
This arrangement was imposed to attempt to limit the amount of heating
associated with the irradiation experiment. Changing the nitro functionality to a
hydroxyl group has minor effects on the absorbance spectrum. Figure 7.17
shows the difference between the two spectra. Again upon ultraviolet irradiation
the molecule opens at the C-O bond and a visible absorbance band appears in
the spectrum. (Figure 7.18) It is worth noting that neither the starting Fischer’s
base nor the benzaldehyde show any bands in the visible region upon UV
irradiation. (Figure 7.19 and 7.20) Figure 7.21 shows the extinction coefficient of
these absorbance bands have been determined in through a standard dilution
series. When the absorbance of the band is plotted against the concentration the

slope of the line is taken as the extinction coefficient, these values can be seen in

102

 

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1.39 1.39 5-98 1,021.20 5.74
H H688 H Ha

    

 

 

2_85 6.59 6.48 2.61 543 654

Predicted Proton Shifts Observed Proton Shifts

Figure 7.14. COSY spectrum of p-HBSP in acetonitrile
and the predicted and observed proton shifts.

103

 

 

 

 

 

 

 

 

 

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104

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0.5 -

 

0.0

 

l ' l ' I ' l ' l ' l
200 300 400 500 600 700
Wavelength (nm)

Figure 7.16. Absorbance of p-nitrospiropyran in
methanol. The black line is the absorbance of the closed
form and red line is the open form of the chromophore.

105

 

p-nitrospiropyran UV Opening in Methanol

2.5 -

 

—— Closed
2.0 - —— Opened

 

 

 

1.5-1

Abs.

1.0~

 

0.5 a

 

 

0-0 I ' I ' 1 ' T ' I ' 1
200 300 400 500 600 700
Wavelength (nm)

p—hydroxyspiropyran UV Opening in Methanol

1.8-

 

1.6 - — Closed
—— Opened

 

 

 

1.4-1

I

1.2-

1.0-

Abs.

0.8 -

0.6 -

0.4 1'

1
0.2 -

0-0 r 1 T I ' r ' FT ‘ I
200 300 400 500 600 700

Wavelength (nm)

 

Figure 7.17. Comparison of absorbance spectra of
p—nitrospiropyran and p—hydroxylspiropyran.

106

p-hydroxyspiropyran UV Opening in Methanol

1.8-

 

1.6 - — Closed
—— Opened

 

 

 

1.4-1
1.2-

1.0-1

Abs.

0.8 .
0.6 -
0.4 5

0.2 -

 

0.0

 

' r ' r ' T ' r ' 1
200 300 400 500 600 700
Wavelength (nm)

Figure 7.18. Absorbance of p-hydroxyspiropyran

in methanol. The black line is the absorbance of the
closed form and red line is the open form of the
chromophore.

107

 

Benzaldehyde Ultraviolet Irradiation Test
115-

1:4-

 

1.2 - Room Conditions

—- UV Irradiated

 

 

 

 

1.0-l

013-

Abs.

0131

0A1-

 

012-

 

01)

 

l ' I ' l ' l T l ' l
200 300 400 500 600 700
Wavelength (nm)

Figure 7.19. Absorbance of benzaldehyde in methanol.
The black line is the absorbance of the closed form and
red line is the open form of the chromophore.

108

F ischer’s Base Ultraviolet Irradiation Test
1

0.7 a

0.6 -

 

— Room Conditions
0.5 -1 —— UV Irradiated

fi

0.4 -

 

 

 

Abs.

 

0.3 -

 

 

0.2 -

 

0.1-

 

 

 

0.0 —|—-r-—T—1

l ' l ' l ' I
200 300 400 500 600 700
Wavelength (nm)

Figure 7.20. Absorbance of Fischer’s Base in methanol.
The black line is the absorbance of the closed form and
red line is the open form of the chromophore.

109

Absorbance at 297nm with Varying Concentration of p-HBSP

1.4-

1.2-

   

y = 5353.3x

 

     

 

   

. 2 _
. 0.8_ R -o.9999
8
< 0.6-
0.4-
0.2-
0.0 . , . . . . . .
0.0 5.0x10'5 1.0x10“ 1.5x104 2.0x104 2.5x10“
Concentration moIIL
Absorbance at 501nm with Varying Concentration
°-9j of uv Irradiated p-HBSP
0.8-
0.7-
0.6-
05‘ y = 12371x
05 . R2 = 0.9921
:2 04-

0.3 -l
0.2 -

0.1-

 

 

0.0 . ,

0.0 1.5x10‘5 3.0x10'5 4.5x10'5 6.0x10'5
Concentration mol/L

Figure 7.21. Determining the extinction coefficient from the
concentration versus absorbance plot where the slope is the extinction
coefficient.

110

Table 7.1. The resulting increase in the visible band shows that the molecule is
opening, however the lack of proton shifts in the NMR spectrum leads to
questioning what percent of the closed population is forced open. The spectral
response can be attributed to the open form having a drastically larger extinction
coefficient. While the exact value would be quite difficult to determine we can
approximate the value by completing the opening experiment across a range of
concentrations. When the absorbance versus concentration plot is formed again
we can take the slope to approximate the extinction coefficient. Naturally this
value can be skewed by the exact concentration of the open form generated from
the irradiation experiment. However, this value does imply that the irradiation
experiment only opens a small percentage of the total concentration of the

chromophore.

111

 

 

 

 

 

 

 

Closed UV Irradiated
Wavelength extinction extinction
297 5353.3
335 3551 .8
393 3533.7
485 12409

 

 

 

 

Table 7.1. Extinction coefficient of p-hydoxyspiropyran,
extinction coefficients are reported in units of L I mol*cm.

112

Literature Cited

10.
11.
12.
13.
14.
15.
16.

17.

18.

19.

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Suzuki, T.; Kawata, Y.; Kahata, S.; Kato, T. Chem. Commun., 2003, 2004

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113

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Org. Chem, 1992, 57, 5377

114

Chapter 8

SPIROPYRAN SPECTROSCOPY

Before hydroxyl terminated spiropyrans can be useful in a storage device
the ring opening and closing reaction must be fully characterized. The central
issues are the reversibility, lifetime and branching ratio for the process. One of
the keys to successful optical data storage is chromophore response
predictability in the system. We attempt to understand the dynamics and
equilibrium of the optical “toggling” of the spiropyran chromophore, first in
solution and then bound to a substrate. The molecule has two basic forms, open
and closed, with the open form having two resonance structures as presented in
Figure 6.1. As with most photochromic molecules, the spiropyran compounds
suffer from thermal back reaction that must be taken into consideration before
the overall equilibrium can be understood. The approach that we took toward
examining this chromophore was to first analyze the response to ultraviolet light,
then the thermal response, and finally the response to visible irradiation. Each of
the processes provides insight into the operation of this family of molecules.

lrradiating a sample containing the chromophore in solution with ultraviolet
light gives rise to cleavage of a carbon oxygen bond (Figure 6.1) and then an
isomerization to the trans form of the “open” molecule. The lowest energy
conformer of this molecule is a trans conformation about the newly formed
polyene chain. To evaluate just the UV—induced opening of the spiropyran, the

following experiment was conducted. The sample was prepared by dissolving

115

the spiropyran in uM concentrations in a solvent of choice. This concentration
range was used to minimize contributions from aggregation phenomena. The
solution was allowed to sit at room temperature for 30 minutes. There were no
steps taken to remove dissolved oxygen from the samples. The samples were,
however, stoppered to prevent a change in concentrations due to solvent
evaporation. The sample was then irradiated by an 8 Watt mercury lamp at a
distance of 22 cm. The absorbance spectrum of the irradiated sample was then
taken. The sample was then discarded and a fresh aliquot was taken to be
irradiated for a different duration. Irradiation times were varied from 30 seconds
to 90 minutes. All measurements for a given solvent were taken from the same
stock solution to eliminate total spiropyran concentration as a variable. The
resulting spectra can be seen in Figures 8.1 and 8.2. There is a substantial
response in the visible region while there is only a limited response in the UV
region. Due to the observed response to irradiation and the standard treatment
in the literature, only the visible region is to be considered from this point.“9
There is an enormous amount of information in the full spectrum, however a
more useful analysis is to plot the absorbance of the visible band versus
irradiation time. When this plot is constructed the data can be fit to an
exponential growth as is seen in Figure 8.3 and 8.4 to find the time constant of
the opening of the molecule. It was found that for each solvent used there was a
time at which the absorbance ceased to increase with increased irradiation time.

At this time it was assumed that a near steady state of the open form of the

116

p-HBSP in Methanol with Varying UV Irradiation Time

 

 

 

 

 

 

 

 

2.0 -
1.8 - .
1 6 ' UV Irradiation Time
. ‘ V _ 0
1_4 J —— 30 seconds
, . —1 minutes
12 _ —-—— 2 minutes
. j . —-— 3 minutes
«a .
.o 1.0 d . —-—- 5 minutes
< q . — 10 minutes
—-—- 30 minues
0'8 ‘ — 60 minutes
—— 90 minutes
0'6“ —— 120 minutes
0.4 - ~ 165 minutes
0.2 ~
0.0 I I I l I I I I I
200 300 400 500 600 700

Wavelength (nm)

Figure 8.1. Formation of open form of p—hydroxyspiropyran
upon increasing ultraviolet irradiation time.

117

p-HBSP in Chloroform with Varying UV Irradiation Time
0.7

UV Irradiation Time

—- 0

—— 30 seconds
—— 1 minutes
—— 2 minutes
—— 3 minutes
—— 5 minutes
— 10 minutes
— 30 minutes

 

Abs.

 

 

 

 

 

 

0.0

 

T . r ._
400 500 600 700
Wavelength (nm)

Figure 8.2. Formation of open form of p-hydroxyspiropyran
upon increasing ultraviolet irradiation time.

118

p-HBSP in Methanol Observing 471nm

  
  

 

 

 

0_35_ UV Opening .
0.30-
0.25- y = y0 + A1*(1-exp(-(x-x0)/t))
020‘ R2 =0 .978
.8- u
< 0-15' yo = -0.069 1 0.000
010: x0 = O
' ‘ A1 = 0.298 1 0.019
0.05_ 1: = 50.650 '1'. 8.747
‘ I'-
0.00 -—l—l i ' ' I
fl l ! I l r I ' T ' I ' l ' l ' l
20 0 2o 40 60 80 100 120 140 160 180

Time (min)

Figure 8.3. Formation of open form of p-hydroxyspiropyran
upon increasing ultraviolet irradiation time.

119

Abs.

 

‘p-HBSP in Chloroform Observing 485nm

UV Opening

    

 

 

 

0.30-
I
I
0.25-
“” ‘ y = yo + A1*<1-exp(-<x-xo)/r»
015- R2 =0 .994
0.10- y0 = 0.009 1 0.001
x0 = 0
0.0% A1 = 0.321 1 0.028
T = 1.42 i 0.304
0.00 I I u ' .
010 ' 015 V 110 ' 115 fi 210 f 215 I 310
Time (min)

Figure 8.4. Formation of open form of p-hydroxyspiropyran
upon increasing ultraviolet irradiation time.

120

 

chromophore was established.

Multiple solvents were used to see the solvochromatic shifts and the
overall influence that the specific properties of the varying solvents would have
on the chromophore response. It has been noted in the literature for the nitro
substituted spiropyrans, the dielectric constant of the solvent has large influence
on both the wavelength of maximum absorbance and the overall response of the
molecule in the visible region.2'3'5""15 This is not a surprising results since one of
the resonance structures of the open form of the molecule is zwitterionic and can
be stabilized in polar solvent. When we attempt to fit the time constants that we
obtained to the dielectric constant of the solvent we see no correlation. Looking
more at properties of the solvent, we attempted to fit the time constant to a
solvent property. We evaluated the FY, scale which is an empirical scale that
monitors the fluorescence of pyrene in varying solvents. We also evaluated the
ET(30) scale which is based on the solvochromism of betaine-30 dye. Finally, we
evaluated the Hildebrand Solubility Parameters, which attempt to take into
account all intermolecular forces such as hydrogen bonding. Table 8.1 shows
that none of these parameters are linearly proportional to the time constant of the
opening process. Since the solvents used are either protic or aprotic, we
separated the classes and still there is no correlation. (Table 8.2) To evaluate
the possibility of a solvochromatic shift the wavelength of maximum visible
absorbance is tabulated with the solvent parameters, and again there is no
correlation. (Table 8.3) We also compared the absolute absorbance, taken as

highest absorbance achieved minus the absorbance at room conditions, versus

121

 

the solvent parameters again we fail to find any correlation. (Table 8.4) This L
lack of correlation demonstrates two things. First that changing an electron I
donating group (-NOz) for an electron withdrawing group (-OH) does influence
the optical response of the molecule. Second, this is a complex system involving
both an isomerization and bond formation/cleavage. Based on these findings it is
clear that this experiment is underdetermined and thus the correlation will not be
established cleanly. lf accepted as a phenomenological study then we have a
basic understanding of how the spiropyran chromophore that we synthesized will
react to UV irradiation. We will return to a discussion of the kinetics of ring
opening and closure after a considering the other contributions here.

The next molecular consideration to be evaluated is the thermal back
reaction, which is characteristic of most photochromic molecules. While it is
possible to open the molecule thermally, relatively little has been done to utilize
this means of ring opening. The thermal component of the overall equilibrium
that has been of concern is the molecule converting from the open form to the
closed form based solely on thermal energy. To evaluate the extent to which the
p-hydroxyl spiropyran molecule undergoes thermal back reaction, the following
series of experiments were conducted. A solution of the chromophore was made
on the [M concentration scale. The sample was irradiated with ultraviolet light
for 30 seconds to 90 minutes, based on which solvent was used, to achieve the
highest absorbance in the visible region and reach the near steady state

concentration of the open form of the spiropyran. This solution was then allowed

122

 

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126

to sit in a dark environment (the absorbance spectrometer) and the absorbance

 

spectrum was taken every 30 minutes to evaluate the change in the visible band
due to the molecule thermally closing. The resulting absorbance spectra can be
seen in Figures 8.5 and 8.6. To eliminate concerns that the measurement was
influencing the results, a series of experiments were conducted to determine the
interaction of the instrument on the sample. A sample of the closed spiropyran
was allowed to sit in a beam of 254nm light from the instrument and the
absorbance in the visible region was monitored to observe the generation of the
optically opened species. It was found from these experiments that only if the
instrument was fixed at the wavelength of maximum absorbance in either the
visible region, for the thermal relaxation experiment, or ultraviolet region, for the
opening experiments, that there was any interaction. To create a measurable
effect the sample had to be irradiated for at least 30 minutes at one of the
absorbance bands. Again, the more useful representation of the data is to form a
time versus absorbance plot, as is shown in Figures 8.7 and 8.8. It is worth
noting that the chromophore does not relax at the same rate in each of the
solvents. The relaxation times are presented in Table 8.5. In each of the
solvents, the maximum closing was found to take place in the first 90 minutes.
After this initial time in most solvents the molecule continues to relax to the
closed form gradually over time. The experiments were conducted for 8 hours, at
which point none of the solutions reached the absorbance of the solution before
UV irradiation. In methanol, dimethylfonnamide, ethyl acetate, and

tetrahydrofuran it was seen that there was little to no thermal relaxation of the

127

 

Abs.

p—HBSP in Chloroform Relaxation

0.5 .

    
  
  
 
 
 
 
 
 
 
 
 
 
 
   
 
   

Time After Irradiation

— 0
—— 30 minutes
— 1 hours
— 1.5 hours
—— 2 hours
—— 2.5 hours
— 3 hours
—-—- 3.5 hours
—— 4 hours
—— 4.5 hours
---- 5 hours
‘-—— 5.5 hours
— 6 hours
—— 6.5 hours
—— 7 hours
—— 7.5 hours
—- 8 hours

 

0.4 -

 

0.3 -

 

 

0.2 -

 

0.1

 

 

 

0.0

 

r

- T

I l '
400 500 600 700
Wavelength (um)

I
300

Figure 8.5. Relaxation of the open form of
p-hydroxyspiropyran to the closed form with increasing
amounts of dark time.

128

p-HB SP in Cyclohexanes Relaxation

  
  
 
 
  
 
 
 
 
 
 
 
 
 
 

2.0
Time After Irradiation

— 0
—— 30 minutes
—— 1 hours
——- 1.5 hours
—— 2 hours
—— 2.5 hours
— 3 hours
-— 3.5 hours
—— 4 hours
—— 4.5 hours
— 5 hours
5.5 hours
—- 6 hours

—— 6.5 hours
1 —— 7 hours

 

1.5-

Abs.

1.0-

0.5 -

—— 7.5 hours
—- 8 hours

 

 

 
 

 

 

0.0

 

V T U I V l' U l I I
200 300 400 500 600 700
Wavelength (nm)

Figure 8.6. Relaxation of the open form of
p-hydroxyspiropyran to the closed form with increasing
dark time.

129

p-HBSP in Chloroform Observing 485nm

 

 

 

 

 

 

030-: Thermal Closing
0.28-
026-: y = y0 + A1*exp(-(x-xo)/t)
0.24- R2 =0 .973
0.22; y0 = 0.136 1 0.002
0.20: X0 = O
3- 0.18: A1 = 0.156 1 0.007
< . T = 0.535 1 0.056
0.16-
0.14;
0.12;; I
0.00 ! 'II'IIIl—H-u.
6 5 3: «'3 ' {a
Time (Hr)

Figure 8.7. Relaxation of the open form of
p-hydroxyspiropyran to the closed form with increasing
dark time.

130

Abs.

_ p-HBSP in Methanol Observing 466nm

0'32 Thermal Closing

.
0.31 -
‘ Y = Yo + A1*eXP('(X'Xo)/T)
0.30- R2 =0 .988

- y0 = 0.270 1 0.0004
0.29- x0 = 0

- A1 = 0.045 1 0.001
0-28- ‘r = 0.333 1 0.028

 

 

 

 

Time (Hr)

Figure 8.8. Relaxation of the open form to the
closed form of p-hydroxyspiropyran with increasing
amounts of dark time.

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molecule indicating that with proper solvent selection the thermal component of
the equilibrium can be minimized. This lack of thermal back conversion is most
likely due to hydrogen bonding and stabilization of the zwitterionic conformation
of the optically opened spiropyran.

The final component of the solution based experiments that needed to be
evaluated was the ability to optically close a population of the open chromophore.
Based on the thermal relaxation experiments it is necessary to allow the UV
irradiated samples to relax for 90 minutes before any optical closing experiments
can be conducted. Again, samples were prepared in the pi! concentration range
to prevent unwanted intermolecular processes from taking place. The samples
were irradiated with 254 nm light to produce the near steady state population of
the open form of the molecule and then allowed to relax in the instrument for 90
minutes. After the relaxation time the samples were then irradiated by an 8 Watt
cool white fluorescent lamp at a distance of 22 cm, for times ranging from 30
seconds to 90 minutes. Again, an irradiation time versus absorbance plot can be
constructed to analyze the data obtained. As can be seen in Figures 8.9 and
8.10 we generated the open concentration and then the concentration decreased
over the first 90 minutes. After 90 minutes, the sample was then irradiated with
light, which is seen as a change in the slope of the absorbance. This change in
slope indicates that we can, in fact, close the molecule optically.

With the experimental data in hand the task of determining the kinetics of
the system was completed from a phenomenological standpoint. The initial

attempt to determine the rate constant and thus describe the kinetics of the

133

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0.050 -
0.045 -

0.040 -

Abs

0.035 -
0.030 -
0.025 -

0.020 ‘1

 

0.015

p-I-IBSP in Hexanes Observing 480 nm
Closing of the Chromophore by Visible Light

Visible
Irradiation ' 420 450 480 . 510 T 540
Begins ' Wavelength (nm)

    

 

 

 

 

V T I l V I
50 100 150 200
Time (min)

Figure 8.9. Closing of p—hydroxyspiropyran after initial
relaxation time by visible light.

134

p-HBSP in Chloroform Observing 485 nm
Closing of the Chromophore by Visible Light

 

 

 

 

 

 

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0.14- 0-15‘
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I . I
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. I I I .
0.08d
l l I ' l
50 100 150 200
Time (min)

Figure 8.10. Closing of p-hydroxyspiropyran after initial
relaxation time by visible light.

135

system can be thought to be outlined by Figure 8.11.16 From this approximation
the thermal process should be have the fewest variables and give the initial
insight into the kinetics of the system. Monitoring the concentration of the open
form of the molecule relax back into the closed form of the molecule should allow
for the rate to be determined. With this process the photo-closing of the
molecule is limited, therefore we can omit it from the equation. We are also
eliminating any possible thermal opening since that would have also already
taken place. To determine the rate of the thermal back conversion the
absorbance, natural log of absorbance, and 1 over the absorbance versus time
were plotted.(Figure 8.12) Unfortunately when the plots are evaluated it is clear
that there is no linear region and thus based on this treatment the reaction is
neither first or second order, unlike the authors found in their system despite the
similarities found in comparison of both of the absorbance versus time plots. An
Arrhenius plot could give some insight into what the order of the rate constant as
presented in literature.” ‘8 However, such plots were not possible because
measurements were made at only one temperature. Based on reference 18, we
can evaluate the rate of the thermal closing by plotting the natural log of the
difference of initial and equilibrium absorbance and the natural log of the
difference between absorbance at time t and equilibrium absorbance.(Figure
8.13) As can be seen, this approximates a straight line but the fit is not as precise

as we would like. Going to an earlier reference“"20

we can treat the system as in
Figure 8.14. We can evaluate the system in terms of the equilibrium of the

closed form with the cis conformer with the trans conformer.

136

 

A ——' B (hv; (DAB) (I) Photo Opening
B —+ A (A; k l3A) (ll) Thermal Closing
B —-> A (hv; (DEA) (lll) Photo Closing
A —'* B (A; k AB) (IV) Thermal Opening

Figure 8.11. Possible equilibrium reactions for

spiropyrans. Of these the photo closing has the fewest
variables to consider.

137

 

 

 

 

 

 

 

 

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8 a o o
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3 6* °
5 o
1- 4 q
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2 _
0 T l fl l l
0 2 4 6 8 10
Time (hr)

 

 

 

Figure 8.12. Plots of absorbance versus time to determine
the order of the kinetics of the closing reaction of
p-hydroxyspiropyran. Plots should generate a linear line.

138

 

 

 

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the closing reaction of p—hydroxyspiropyran.

139

 

 

 

dlA1l/dt = 'k1lA1]+k2[A2]

d[A2]/dt = k1[A1]-k2[A2]-k3[A2]+k4[A3]
d[A3]/dt = k3[A2]-k4[A3]

Figure 8.14. Reversible isomerization equilibrium
expressions to evaluate the kinetic process of spiropyran
isomerization

140

In this system the concentration of the trans form is approximated by the
absorbance band in the visible region. We can calculate the concentration of
trans through the extinction coefficient of the visible band. The steady state
concentration of the cis conformer is small based on the short lifetime (0-10ps)
found in the literature.21 It is possible that the cis conformer should absorb
slightly blue shifted from the observed visible band. We have not seen the cis
band, but the lack of presence is likely due to the very low concentration of the
conformer. We can take the concentration of the closed form of the spiropyran
from the UV band in the absorbance spectrum. However, the UV region of the
absorbance spectrum is overlapped with other features. Once the system is set
up we can describe the system mathematically as presented in Figure 8.14 and
arrive at a set of differential equations. We are left with the approximate
concentration of the trans conformer based on approximate extinction coefficient.
We know that the total concentration of spiropyran is the sum of concentration of
the closed spiropyran, cis conformer, and the trans conformer. Our goal is to
understand qualitatively how the equilibrium constants change with the solvent
identity. To reach this goal we conducted the following experiment. A sample of
spiropyran was irradiated with UV light which forms the population of the open
conformer. After irradiation the trans concentration is approximated by 1-x, the
closed concentration is approximated by x, and the cis concentration is
approximately zero. From this state we can monitor the change in concentration
of the trans through the visible band in the absorbance spectrum. Using these

equations we can model the shape of the system. This can be further modified

141

by substituting experimentally determined rate constants for the conversion of the
cis conformer to the trans conformer found to be approximately 1011 Hz.21 There
are some caviots that need to be considered. The first is that this is an
undetermined problem because we don’t have a good gauge on the k1/k2
processes. The second is that k3/k4 is known to be fast on the order of ps from
literature. Since we know k31k4 within a factor of 10, which assumes that the
barrier is solvent independent to first approximation. This statement is based on
prior work that shows solution phase isomerization measurements showing a
small solvent contribution to the isomerization processes. From these

assumptions we can say that any variation is dominated by the k1lk2 of which the

k1 . k3
CIS

2 4

 

 

Closed Trans

 

 

solvent can mediate the reaction barrier. We can approximate the total

equilibrium constant through the slow step which is the conversion of the cis

_ [Cis] k _ [Trans]
0 [Closed] "W" [Cis]

 

 

__ [Trans] _ [Cis] *[Trans] _ k _
'”"" [Closed] [Closed] [Cis] °

 

conformer to the closed form of the molecule. We can approximate this through

the ratio of the time constant of the opening over the time constant of the closing.

k _ kopening _ 1:closing
(0’0! _ k — T

closing

 

opening

142

With the approximate equilibrium constants are calculated the values were then
compared to the parameters of the solvents. These processes should be
mediated by the medium that they are taking place in. From Table 8.6 we can
see that there is no correlation between any of the solvent parameters and the
calculated equilibrium values. This discovery brings into light a possible error in
the assumptions of this model. This model requires that the equilibrium constant
of the isomerization process is both fast and constant. If the solvent mediates
the isomerization rate then the problem becomes underdetermined and lost. At
this point it is not clear what the exact effects of the solvent are but these
problems suggest that the solvent is mediating both the isomerization and the
opening and closing rate. It is unclear as to how exactly the interactions are
taking place but these findings suggest that the directions of the interactions of
the solvent with the isomerization and rate of opening and closing are in opposite
directions. This problem may ultimately be solved with the introduction of
temperature control on the system. With temperature controlled time resolved
absorbance spectroscopy the variables could be isolated and the overall
equilibrium expression could be solved. As the system is evaluated currently
there lacks the degree of control required for a successful optical storage system
in that the equilibrium can not be expressed thus there lacks the predictability

required for the application.

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144

Literature Cited

10.
11.

12.

13.

14.

15.

16.

Yoshida, T.; Morinaka, A. J. Photochem. Photobiol. A: Chem, 1992, 63,
227

Kameda, M.; Sumaru, K.; Kanamori, T.; Shinbo, T. Langmuir, 2004, 20,
9315

Fissi, A.; Pieroni, O.; Angelini, N.; Lenci, F. Macromolecules, 1999, 32,
71 16

Pfeifer, U.; Fukumura, H.; Misawa, H.; Kitamura, N.; Masuhra, H. J. Am.
Chem. Soc., 1992, 114, 4417

Zhang, J.Z.; Schwartz, B.J.; King, J.C.; Harris, C.B. J. Am. Chem. Soc.,
1992, 114, 10921

Kawanishi, Y.; Seki, K.; Tamaki, T.; Sakuragi, M.; Suzuki, Y. J. Photochem.
Photobiol. A: Chem, 1997, 109, 237

Sumaru, K.; Kameda, M.; Kanamori, T.; Shinbo, T. Macromolecules, 2004,
37, 4949

Song, L.; Jares-Erijman, E.A.; Jovin, T.M. J. Photochem. Photobiol. A:
Chem, 2002, 150, 177

Pfeifer-Fukumura, U. J. Photochem. Photobiol. A: Chem, 1997, 111, 145
Kerkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev., 2000, 100, 1741
Minkin, V. Chem. Rev., 2004, 104, 2751

Wojtyk, J.T.C.; Wasey, A.; Kazmaier, P.M.; H02, 8.; Buncel, E. J. Phys.
Chem. A, 2000, 104, 9046

Gorner, H. Phys. Chem. Chem. Phys, 2001, 3, 416

Venturini, C.; Andreaus, J.; Machado, V.; Machado, C. Org. Biomol.
Chem, 2005, 3, 1751

Bae, S.Y.; Arnold, B.R. J. Phys. Org. Chem, 2004, 17, 187

Metelitsa, A.V.; Micheau, J.C.; Voloshin, N.A.; Voloshina, E.N.; Minikin,
W. J. Phys. Chem. A, 2001, 105, 8417

145

 

17.

18.

19.

20.

21.

Hobley, J.; Malatesta, V. Phys. Chem. Chem. Phys., 2000, 2, 57

Song, X.; Zhou, J.; Li, Y.; Tang, Y. J. Photochem. Photobiol. A: Chem,
1995, 92, 99

Bamford, C.H.; Tipper, C.F.H. Chemical Kinetics, Elsevier Publishing
Company: Amsterdam, The Netherlands 1969, Vol. 2, Chapter 1.

Compton, R.G.; Hancock, G. C.F.H. Chemical Kinetics, Elsevier
Publishing Company: Amsterdam, The Netherlands 1998, Vol. 36,
Chapter 2.

Hobley, J.; Pfeifer—Fukumura, U.; Bletz, M.; Asahi, T.; Masuhara, H.;
Fukumura, H. J. Phys. Chem. A, 2002, 106, 2265

146

 

Chapter 9

CONCLUSIONS

The goal of this research was to evaluate azobenzene derivatives and
spiropyrans as possible probes and interlayer linkages in stacked polymer
systems. The use of an isomerizable interlayer linkages enables the
development of optical storage devices based on the change of conformation
and polymers for separations based on change in free volume associated with
isomerization. The isomerization and spectroscopy of the chromophores must
fully be understood before any development can be achieved. Selection of the
proper chromophores to synthesize was determined based on the position of
chemical attachment point and ability to be purified. The steady state
spectroscopy gave little insight into the mechanism and overall behavior of the
isomerization process. To achieve this goal, a series of calculations were
performed on a series of p-disubstituted azobenzenes and p-substituted
spiropyrans. These calculations indicated that the isomerization mechanism was
independent of substitution in the para positions. To validate the calculations a
series of time resolved spectroscopy measurements were made on the systems.
Time correlated UVNisible spectroscopy was conducted to determine the time
constant for isomerization in the case of azobenzene, and the open and closure
mechanism in the case of spiropyran. Upon analyzing the data, it was

determined that further calculations were needed to fully understand the results.

147

A series of p-diamidoazobenzenes was synthesized from p-
diaminoazobenzene. The synthesis selected was based on a classic polyamido
polymerization route using only monofucntional monomers to reach the desired
compounds. The yields obtained for each compound are presented in Table 3.1.
Careful purification greatly improved purity and only mildly affected yield. The
UVNis spectrum shifted with substitution, as expected. All of the p-
diamidoazobenzenes absorb at the same wavelength, which is shifted from pure
azobenzene and p—diaminoazobenzene. NMR spectroscopy revealed the shifts
expected and the technique was used as a tool to analyze the purity of the
synthesized compounds.

Semi-empirical calculations were performed on a series of modified
azobenzenes, and p-diaminoazobenzene. For these calculations the lowest
energy conformer was determined by a PM3 basis set. Energy levels and
oscillator strengths were calculated yielding Figure 4.1. From steady-state and
time-correlated spectroscopy, it was determined that the energy levels were
accurate but the oscillator strengths were incorrect. A set of isomerization
surfaces was calculated for each one of the molecules based both on the
inversion and rotation isomerization mechanisms. From these surfaces it was
determined that substitution at the para position does not affect the isomerization
mechanism.

It was found through time correlated UVNis spectroscopy that the
substitution also affects the time constant of the isomerization process. This is at

odds with [the calculations performed. The explanation is that the alterations in

148

time constant observed are actually due to very slight changes in energy level.

 

The order of time constants is distributed as diamino, diamido, and then
azobenzene having the longest time constant. These time constants are
separated by an order of magnitude.

Based on the azobenzene results, adoption of a different molecule for use
as a probe or in storage applications is needed. This decision was based on the
limited control on the isomerization process that a scientist has on the
azobenzene moiety based solely on substitution. Two classes of molecules
present possible routes of investigation, fulgides and spiropyrans. Azobenzene
compounds will alter the free volume of a stacked polymer system upon
isomerization. The next step in realizing the separation membrane is to
functionalize the ends of the alkane chains on the diamidoazobenzene species.

Spiropyrans were selected for investigation based on the detrimental side
reactions found in the fulgide family of chromophores. The p-hydroxyspiropyran
was synthesized through a modification of the initial synthetic route discovered in
1952 by Koelsch and Workman. It was determined that the p-hydroxyl moiety
was the proper chemical attachment point due to the overall ease of synthesis
and purification which leads to reduced costs for production. It was determined
that through this synthetic route yields of 60% were easily obtainable. It was
found that the UV/ViS spectroscopy varied from the p-nitrospiropyran as was
expected. It was found that the wavelength of maximum absorbance for the
optically opened conformer varied with solvent but not in a predictable manner.

An attempt to correlate this solvochromatic shift to solvent parameters was

149

attempted and failed. The spectroscopic response to ultraviolet irradiation was
observed through absorbance spectroscopy and NMR. In the absorbance
spectrum it was found that upon irradiation a peak appeared in the visible region
which corresponded to the formation of the open conformer of the spiropyran.
(Figure 7.18.) The NMR spectrum showed little difference between the closed
conformer and the optically opened conformer leading to the belief that the
irradiation process was inefficient yielding only a small percentage of conversion
with the parameters selected for this study. The extinction coefficient data
collected supports this finding in that the coefficient is an order of magnitude
larger for the opened conformer than the closed.

Semi-empirical calculations were performed for the p-nitrospiropyran and
the p-hydroxyspiropyran to evaluate the effect of the substitution of an electron
withdrawing group for an electron donating group. These calculations also gave
an initial determination of the isomerization barrier for spiropyrans, this value was
on the same order of magnitude as that of the azobenzenes previously
mentioned.

In conclusion, it was determined that isomerization mechanisms, in
general, are much too facile to support long-term information storage based on
spectroscopic differences between conformers. It is believed that the additional
bond fission and formation would act as a chemical “latch” to increase the
persistence of a selected conformer, however this was not found to be the case.

With the introduction of the chemical latch the complexity of the kinetic processes

150

taking place introduce uncertainty into the system which makes the response

less predictable and therefore difficult to use as a basis for optical data storage.

151

APPENDICES

152

APPENDIX A

MASS SPECTRA OF SYNTHESIZED AZOBENZENE COMPOUNDS

This appendix presents the collected mass spectra of the synthesized p-

diamidoazobenzenes.

153

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APPENDIX B

PUBLISHED CALCULATED ENERGY SURFACES OF AZOBENZENES

This appendix presents the published calculated energy surfaces of

azobenzene, p—diaminoazobenzene, and odiamidoazobenzene.

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APPENDIX C

MASS SPECTRA OF SPIROPYRAN MATERIALS

This appendix presents the collected mass spectra of the starting

materials for the synthesis of p-hydroxyspiropyran.

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”Ian” '8."

 

 

 

Gum-.3 Buns 8:6: .0. 86M

169

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