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THE EFFECT OF POSITIONAL SUBSTITUTION ON THE
OPTICAL RESPONSE OF SYMMETRICALLY
DISUBSTITUTED AZOBENZENE DERIVATIVES

presented by
Alexis Amon Blevins

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Master of Science

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THE EFFECT OF POSITIONAL SUBSTITUTION ON THE OPTICAL RESPONSE
OF SYMMETRICALLY DISUBSTITUTED AZOBENZENE DERIVATIVES

By

Alexis Amon Blevins

A THESIS

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

Master of Science

Department of Chemistry

2003

ABSTRACT

THE EFFECT OF POSITIONAL SUBSTITUTION ON THE OPTICAL
RESPONSE OF SYMMETRICALLY DISUBSTITUTED AZOBENZENE
DERIVATIVES

By

Alexis Amon Blevins

We report on the steady state and transient spectroscopy and
characterization of a series p-diamido azobenzene compounds where the length
of the amido substituents are varied in a regular manner. 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. These results place
limits on the utility of the azobenzenes for optical information storage

applications.

ACKNOWLEDGMENTS

I would like to thank the Blanchard group for all of the help presented in

completion of this study and the NSF for funding.

TABLE OF CONTENTS

Introduction ........................................................................................... 1
Chapter 1: Synthesis of Azobenzene Derivatives .......................................... 9
Chapter 2: Calculations ......................................................................... 29
Chapter 3: Time Resolved Spectroscopy ................................................... 41
Conclusion .......................................................................................... 62
Appendix A: Mass Spectra of Compounds ................................................. 66

LIST OF TABLES

Table 1. Reaction conditions and yield for synthesized compounds ............... 20

Table 2. Absorption maxima and molar absorptivities of azobenzene, p-
diaminoazobenzene and the p-diamidoazobenzenes 1-5 .............................. 23

Table 3. Transient optical decay functionalities and time constants of
azobenzene, p-diaminoazobenzene and the p-diamidoazobenzenes 1-5 ......... 47

LIST OF FIGURES

Figure 1. Schematic of one possible layered polymer system for separations
based on change in local free volume ......................................................... 4

Figure 2. lsomerization mechanisms for azobenzene .................................... 5
Figure 3. Calculation of change in position and spacing of azobenzene terminal
groups, R, upon isomerization for both meta and para disubstituted

azobenzene ........................................................................................ 10

Scheme 1. Synthetic route to disubstituted azobenzene ............................... 14

Figure 4. Proton NMR spectrum of p-diamidoazobenzene before (middle) and

after (bottom) recrystallization ................................................................. 16
Figure 5. Proton NMR spectrum of azobenzene ......................................... 17
Figure 6. Proton NMR spectrum of p-diaminoazobenzene ........................... 18
Figure 7. Proton NMR spectrum of p-diamidoazobenzene ........................... 18
Figure 8. Proton NMR spectra of a series of p-diamidoazobenzenes .............. 19
Figure 9. Absorbance spectrum of azobenzene ......................................... 26
Figure 10. Absorbance spectrum of p-diaminoazobenzene .......................... 26

Figure 11. Absorbance spectra of azobenzene, p-diaminoazobenzene, and p-
diamidoazobenzene ............................................................................. 27

Figure 12. Calculated energy levels and oscillator strengths for azobenzene,
diaminoazobenzene, and diamidoazobenzene ........................................... 32

Figure 13. Calculated isomerization surface for inversion mechanism of
azobenzene ........................................................................................ 33

Figure 14. Calculated isomerization surface for rotation mechanism of
azobenzene ........................................................................................ 34

Figure 15. Calculated isomerization surface for inversion mechanism of p-
diaminoazobenzene .............................................................................. 35

Figure 16. Calculated isomerization surface for rotation mechanism of p-
diaminoazobenzene .............................................................................. 36

vi

Figure 17. Calculated isomerization surface for inversion mechanism of p-
diamidoazobenzene .............................................................................. 37

Figure 18. Calculated isomerization surface for rotation mechanism of p-
diamidoazobenzene .............................................................................. 38

Figure 19. Possible experiment outcomes available in a transient absorbance
experiment .......................................................................................... 42

Figure 20. Transient absorbance response of a) azobenzene b) p-
diaminoazobenzene c) p-diamidoazobenzene ............................................ 45

Figure 21. Transient absorbance response of two p-diamidoazobenzenes ...... 49

Figure 22. Absorbance spectrum of 5 upon varying durations of ultraviolet
irradiation ........................................................................................... 52

Figure 23. Block diagram of UVNisible instrument modiﬁcation ..................... 53

Figure 24. Example of an absorbance versus concentration graph used to
determine molar absorptivity ................................................................... 53

Figure 25. Recovery of the absorption due to the trans band in azobenzene at
318nm ............................................................................................... 56

Figure 26. Recovery of the absorption due to the trans band in p-
diaminoazobenzene at 370nm ................................................................. 57

Figure 27. Recovery of the absorption due to the trans band in p-
diamidoazobenzene at 370nm ................................................................. 58

vii

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.

viii

INTRODUCTION

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
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 n* 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.(Figure1) 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.18 The two proposed mechanisms for
azobenzene isomerization are illustrated in Figure 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 32 to the S1

manifold.

   

I1>I2

Figure 1: Schematic of one possible layered polymer
system for separations based on change in free volume.

N=N—©

Inversion lnversional
Transition State
5 N:
N=N
Rotation Q

Rotational Transition State

Figure 2: lsomerization mechanisms for azobenzene.

Recent elegant work by both the Tahara18 and Kobayashi19 groups has
cast doubt on the “standard” picture. Using femtosecond optical techniques,
Tahara’s group has shown that excitation to the S; state of azobenzene
undergoes rapid relaxation to the 81 state, where isomerization proceeds.18
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.19 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

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.

Literature Cited

10.

11.

12.

13.

14.

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

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

Howe, L. A.; Jaycox, G. D. J. Poly. Sci. A. Poly. Chem, 1998, 36, 2827.

Yoshii, K.; Machida, S.; Horie, K. J. Poly. Sci. 8. Poly. Phys, 2000, 38, 3098.

. Aoshima, Y.; Egami, C.; Kawata, Y.; Sugihara, 0.; Tsuchimori, M.; Watanabe,

0.; Fugimara, H.; Okamoto, N. Polym. Adv. Tech., 2000, 11, 575.

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

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

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

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

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

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

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.

Chapter 1

SYNTHESIS OF AZOBENZENE DERIVATIVES

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.“<3 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) 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)

 

 

 

 

 

 

 

 

 

A
N N’ :R
119A TN p || 6.4A
hNRO I
l
—— R
A R T
i ’ 'i
N N 9.6A
10.7A hv©
__i_ R _Y_

Figure 3: Calculation of change in position and spacing of
azobenzene terminal groups, R, upon isomerization for both
meta and para disubstituted azobenzene.

10

The synthesis of substituted azobenzenes, of necessity, requires reactions
targeted to specific ring positions. Any of the ring substituents used in the
reactions acts 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
speed 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 must be
considered before the molecule can be planned, and swing to the meta-
deactivation behavior of the azo moiety, subsequent loss of meta substituents is
a common result. Most of the connections to polymer sheets considered for the
azobenzene were of the covalent type that the Blanchard group has extensive
experience. 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

11

makes optical storage possible and simultaneously provides a probe into local
chemical environment, it limits the overall usefulness of the devices. 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 odisubstituted 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.

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 1. 4,4’-Azodianiline was

12

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 CHzclz. Following completion of the reaction, the
solution was washed with 150 mL of 2M HCl. 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 NaCl) 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 clue 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 CHzclz. 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

13

int—QR
N NH2
0
o O
l

 

R Cl
Room
Temperature
~30 minutes
CHZCIZ
H v
TN
0 NéN i
N R
1 R = CH3 H
2 R = CHch3

3 R = CH2CH20H3
4 R = CH2CH20H20H3
5 R = CH2CH2CH2CH20H20H2CH2CH3

Scheme 1: Synthetic route to disubstituted azobenzene.

14

compounds synthesized was determined by proton NMR and UVNisible
spectroscopy. Figure 4 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 5. 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 6) 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 7 and Figure 8) The
associated proton shifts, synthetic yields, and general descriptions for the
compounds are presented below. The reaction conditions and yields are

presented in Table 1.

15

 

 

 

 

 

 

 

 

 

I
. I I
I I
i

I I l' i‘jﬁ—T—r‘VT r *ivltiﬁ'PT T—T‘T"7"T_T’ iT't“‘1'"i—r—r 'T‘I—‘T—l" I T—l_F—T!1 T'T‘rnj‘T‘FT-T—

9 8 7 6 4 3 2 l

5 (iapm)

Figure 4: Proton NMR spectrum of p-diaminoazobenzene (top), p-
diamidoazobenzene before (middle) and after (bottom) recrystallization.

16

Solvent

Phenyl Protons

 

 

 

 

 

 

(D—i
on
\I
on?
01
r:
N

10;

Figure 5: NMR spectrum of azobenzene in
deuterated methanol.

17

—4
.4

Solvent

Phenyl Protons

 

 

 

 

 

Solvent
J; \_ JI¥ JL
i-T—YhTT‘T—T-Y T T {Tr Y T T T T T T . l T T T T IT T T T I T T T T I T T T T I T T T T 1 TT T T
10 9 8 7 6 5 4 3 2 1

Sppm
Figure 6: Proton NMR spectrum of p-diaminoazobenzene.

Solvent

Methylene Protons

Phenyl Protons

 

 

 

 

 

 

[—T—I__T—'T__ ‘T T T T T _T T T T T I T T Ti T ‘T I f Tﬁﬁ T

10 9 8 7 6 s
8ppm
Figure 7: Proton NMR spectrum of p-diamidoazobenzene.

4

I
3 2 1

18

I T T T T I

J Diaminoazobenzene
t JL c - .

 

 

 

R: iII - .Jx .. LI 0 .
= 3 IL. I JIMLJ

R ‘LI... -- 4L

FTTTT‘II III IITTIIIITIIITTI

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 2.15 201.510 0.5

 

 

 

 

 

 

 

 

Figure 8: NMR spectra of a series of p-diamidoazobenzenes.

19

 

Reaction Conditions I Results

 

 

 

Compound Time Solvent Crude Recrystallized
Stirred Yield Yield
(min)

1 28 CH2C|2 80% 53%

2 28 CHzClz 73% 54%

3 30 CHZCIZ 46% 44%

4 28 CHzciz 34% 30%

5 40 THF 69% No

Recrystallization

 

Table 1 : Reaction conditions and yield for synthesized compounds.

20

 

Compound 1. (N-[4-(4-Acetylamino-phenylazo)-phenyl]-acetamide) This
compound was produced with a crude yield of 80% and recrystallized yield of
53% using CHZClz as reaction solvent. The resulting brown ﬂakes showed
proton NMR peaks (300 MHz, d-methanol) at 8: 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 mlz 43, 65, 92, 134,
162, and 296.

Compound 2. (N-[4-(4-Propionylamino-phenylazo)—phenyl]-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 or 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 mlz 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 CH2C|2 as reaction solvent. The resulting orange needles showed
proton NMR peaks (300 MHz, d-methanol) at 8: 0.92 ppm (m, 6H 7 to carbonyl),
1.63 ppm (m, 4H [3 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 mlz 43, 71, 92, 120, 162,

190, 211, 282, 352, and 353.

21

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 CHzClz 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 7 to carbonyl), 1.61 ppm (m, 4H [3 to
carbonyl), 2.33 ppm (m, 4H or 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
I to carbonyl), 1.25 ppm (m; 20H 11, 4), e, 8, and y to carbonyl), 1.55 ppm (m, 4H 8
to carbonyl), 2.33 ppm (m, 4H a 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 2.

22

 

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

 

 

 

 

 

 

 

 

Azobenzene 313 22400 500

p-

394 27800 1630
d iaminoazobenzene

1 364 17900 1300
2 364 23750 2400
3 364 24200 2530
4 364 24900 2500
5 364 25300 2450

 

 

 

 

Table 2. Absorption maxima and molar absorptivities of azobenzene, p-

diaminoazobenzene and the p-diamidoazobenzenes 1-5.

23

 

The steady state absorption spectroscopy of azobenzene is well
understood. The S1 <— 80 transitions for both the trans and cis forms are
characterized by weak absorption bands ( e~ 500 L/mol-cm for cis, 8 ~ 2,000
Umol-cm for trans), and these bands are centered at ~ 434 nm. The 82 (— So
absorption bands for both isomers are stronger, with the trans band at 313 nm (2
~ 22,400 L/mol-cm) and the cis band at 254 nm (8 ~ 12,000 eroI-cm).""12 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 L/mol-cm), with weaker bands seen at 310 nm and
250 nm. The spectra can be seen in Figures 9,10, and 11. 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 12), where the dominant absorption transition (8; +— 80) 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 12), and for all of the azobenzenes, it

24

appears that multiple Sn <— 80 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
1 using a classic polymerization route 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.

25

 

 

trans

 

 

 

 

(D
O
C
(U
9
O
(I)
.0
<
“O
a:
.u
a .
g crs + trans
0
2 S1 (— So
' T T T ‘ T ' T
200 300 400 500 600

Wavelength (nm)

 

Figure 9: Absorbance spectrum of azobenzene.

 

 

 

 

1 .0 -
'l ”f i\\
8 If X
g 0.8 — I I‘ ‘7.
e l" \
8 0 .6 - I \i\
2 i \
lI \

8 0 4 - I \~
.5 ' \‘2
Tu .
g 0 .2 4 x a g
z .1 / / h"\\

0.0 - ' ‘_

' I ' I t l ' l
200 300 400 500 600

Wavelength (nm)

 

Figure 10: Absorbance spectrum of p-diaminoazobenzene.

26

 

 

1.0

.0
00

.9
ox

absorbance (a.u.)

.o
4:.

0.2

0.0

 

 

azobenzene diaminoazobenzene
1 - 5

 

 

l 1 l n I

' n I 1 :5.
200 250 300 350 400 450 500 550

wavelength (nm)

Figure 11 : Absorbance spectra of azobenzene, p-
diaminoazobenzene, and p-diamidoazobenzene.

27

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, 8.; 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, I. 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.

28

Chapter 2

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 n-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 n-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 rr—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

29

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 12). Based on these stepped geometry calculations, the isomerization
surfaces for azobenzene have been calculated and are presented in Figures 13
thru 18.

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 on 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 from the 82 state to the ground state?“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 13 and 14,
show the calculated energy surfaces for the So (a) and S1 (b) states, for
isomerization by rehybridization of one N (Figure 13) or rotation about the N=N
bond (Figure 14). 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

30

calculated results for isomerization on the So and S1 surfaces, a relatively small
81 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 So 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 15,
16, 17, and 18. In Figure 16 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.

31

-1
Energy (cm ) Energy (cm")

WTHHVIH

Energy (cm’I)

A . A
I Y’ I l“ ’l

40000 =———-———
30000 L——_—__.

20000 i

 

 

10000 e

508 E

40000

30000

 

20000

508 E

 

 

\_
‘Tf

40000
30000

20000

508T

 

 

trans cis

 

 

 

 

0 04 ..___ 34f =0. 39
i5 = '
f3r=1147 , ii: 823
T2 azobenzene “T” E f= 0.11
31 r: 0.001 ,.
T1 ,
sO . L— 80
trans crs
1F5 :::::::::: 5
§3 f: 0.007 4
4 d. . b 83 f: 1.05
raminoazo enzene :_:__ = .
11:8.38
82 f: 1.61
i. r: 0.01 ‘_~——~— T3
3 ——— T,
T1 —— T1
80 I __ SO
trans cis
g3 f: 0.21 81 f: 0.35
s} r: 1.29 ‘E— T1
T3 diamidoazobenzene
2 4
S1 f= 0.03
'T. I,
So 80

Figure 12: Calculated energy levels and oscillator strengths for
azobenzene, diaminoazobenzene, and diamidoazobenzene.

32

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37

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38

Semi-empirical calculations were conducted on the model series of p-
disubstituted azobenzenes using Hyperchem® v. 6.0 and Microsoft Exce|®. From
these calculations energy levels and oscillator strengths can be calculated as
presented in Figure 12. 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.

39

Literature Cited

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

2. Dewar, M. J. 8.; Theil, W. J. Am. Chem. 800., 1977, 99, 4907.

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

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

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

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

8. Zimmerman, G.; Chow, L. Y.; Paik, U. J. J. Am. Chem. Soc., 1958, 80, 3528.
9. Shultz, T.; Quenneville, J.; Levine, 8.; Toniolo, A.; Martinez, T. J.;

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

40

Chapter 3

TIME RESOLVED SPECTROSCOPY

While steady state spectroscopy is useful for molecular analysis and
identiﬁcation, it does not provide direct information on molecular motion or
isomerization for labile systems. We have used pump-probe transient
absorbance spectroscopy and time correlated UVNisible spectroscopy to
investigate the isomerization process and the effects of substitution. The
transient absorbance gives an insight into the interactions of photons with
azobenzenes while time correlated UVNisible absorption spectroscopy can yield
time constants for the ground state back isomerization process.

Pump-probe spectroscopy utilizes two tuned laser beams to excite and
monitor a molecular system. Chromophore concentration was kept low enough
that the probability of dimer formation was not a major concern (10‘3 M in 1-
Octanol). This particular experiment can give insight into the overall excitation
and relaxation processes of the system of interest. The technique selected for
this investigation was transient absorbance. In such an experiment the
magnitude, time scale, and sign of the signal all yield information. Figure 19
demonstrates that a positive-going signal is indicative of a bleaching experiment
where the second photon measures absorption depletion induced by the ﬁrst
photon. A negative going signal is indicative of an induced absorption, where an
electron is promoted from the ﬁrst excited singlet state to a higher excited

electron singlet state.

41

 

 

 

 

 

 

 

Sn
op~ oR I OR
81 81
0P TOR T (I
So I e 80 P
. Induced
Bleachlng Absorption
Up = pump OR = probe

 

Figure 19 : Possible experiments available in a transient
absorbance experiment.

42

 

The picosecond pump-probe laser spectrometer used in this experiment
has been described in detail elsewhere,1 and we provide only a brief synopsis of
the system here. A mode-locked CW Nd:YAG laser (Coherent Antares 76-8)
produces 30 W of average power (1064 nm, 100 ps pulses, 76 MHz repetition
rate). The output of this laser is frequency-tripled to produce ~1.1 W average
power at 355 nm with nominally the same pulse characteristics. The third
harmonic light is used to excite two cavity-dumped dye lasers (Coherent 701-3)
synchronously, with the output of both lasers being ~ 60 mW average power at 8
MHz repetition rate, producing 7 ps fwhm autocorrelation traces using a three
plate birefringent ﬁlter. The pump and probe dye lasers are operated with
Stilbene 420 dye (Exciton), at 435 nm and 445 nm, respectively. These
wavelengths were selected to access the 81e80 transitions of the azobenzenes.
Encoding of the transient signals was accomplished using radio-.and audio—
frequency triple modulation, with synchronous demodulation detection.2M
Samples for the transient absorbance experiments were prepared by dissolving
the azobenzene derivative of interest in 1-octanol to achieve a 1x104 M
concentration.

We have measured the transient optical response of the several
disubstituted azobenzenes. For all of the measurements we report here, we
have excited the compound at 445 nm and interrogated the result of that
excitation at 435 nm. These wavelengths access the lowest allowed singlet

transition in each compound. Azobenzene produces a transient response

different from that of the substituted species. Speciﬁcally, we observe an

43

induced absorption centered at zero delay time, with no resolvable longer time
components. This feature is independent of solvent identity and it possesses a
unique characteristic; the transient response exhibits a time proﬁle that is
reproducibly narrower than the instrument response function (Figure 20a). This
anomalous behavior results from the transient signal being a multiphoton
process. The instrument response function is linear in the intensity of each
incident beam, and for the experimental signal to be shorter in time than the
instrument response, it must depend on the intensity of the incident beam(s) in a
manner different than that of the response function; i.e. the signal must scale with
more than two photons overall. The negative sign of the experimental signal
indicates that it results from an induced absorption, and given the low intensity of
the probe beam, it is most likely that the pump beam undergoes resonance-
enhanced two photon absorption, with the probe sensing induced absorption
from the ﬁnal state of the two photon transition to a higher excited singlet state.
Given the low intensity of the probe relative to the pump, the probability of the
nonlinear response being dominated by the probe is negligible, even though it is
at essentially the same wavelength as the pump. This is the simplest
explanation consistent with both the anomalous temporal width and the sign of
the transient response, and we note that this sequence of events involves at

least 8.43 eV of photon energy, slightly less than the ionization threshold of

44

0.2 7 a
0.0 ”AN“ -A‘ V ‘. . 1". A“- A V"
.02 :- ' "w
-0.4 :- 5.
-0.6 :-
-O.8 :- -, .:
"1.0"I I l .11"111 1.1]..1I1IIIIIIJ
-40 -20 0 20 40 60 80 100

  

 

b

=5 0.6"-
(U .

 

 

 

 

 

 

 

delay time (ps)

Figure 20 : Transient absorbance response of a) azobenzene b) p-
diaminoazobenzene c) p-diamidoazobenzene.

45

azobenzene.5 Understanding the details of this multiphoton process will clearly
require further investigation. Because of the small signal intensities, we were not
able to acquire the intensity-dependence of this transient response. Given the
uncertainty in the number of photons involved in this process, accurate
assignment of the states involved is not feasible.

The transient response of p-diaminoazobenzene differs substantially from
both azobenzene and p-diamidoazobenzene. We observe for the diamino
compound a positive (bleaching) transient signal with a 116 ps single exponential
decay time constant (Figure 20b, Table 3). The time constant is not sensitive to
the identity of the solvent in which p-diaminoazobenzene is dissolved, and its
functionality implies that excitation at ~440 nm is to the lowest lying excited
singlet state. Calculations (Figure 12) suggest that the dominant transition for
the trans conformer of this compound is the 82 <— 80 transition, with the 81 <- 80
transition being characterized by a vanishingly small oscillator strength. The
decay time constant seen for the transient data suggest that we are accessing
the 81 +— 80 transition, and the steady state data show the oscillator strength for
this transition to be signiﬁcant. In contrast to the results for azobenzene, the
positive transient signal implies that excited state absorption (Sn <— 81) is
inefﬁcient for p-diaminoazobenzene at ~ 440 nm, and the observed transient is

simply a ground state recovery response.

46

 

 

 

 

 

 

 

 

 

Time constant
Compound Decay functionality
(PS)

Azobenzene induced absorption < IRFa

p-

bleaching 116 :t 12
diaminoazobenzene

1 induced absorption 13 :l: 3

2 induced absorption 14 i 1

3 induced absorption 16 :l: 1

4 induced absorption 16 :l: 1

5 induced absorption 15 :t 1

 

 

 

 

a IRF = instrument response function, 10 ps fwhm.

Table 3. Transient optical decay functionalities and time constants of

azobenzene, p-diaminoazobenzene and the p-diamidoazobenzenes 1-5.

47

The diamidoazobenzenes exhibit a transient response that is independent
of length of the amido aliphatic chain. For all measurements we recover an
induced absorption signal, with a characteristic time constant of ~ 15 ps,
measurably longer than the instrument response function, in contrast to that seen
for azobenzene (Figure 21, Table 3). The induced absorption is likely the result
of Sn <— 81 absorption of the probe pulse following population of the 81 state by
the pump pulse. In this model, the time constant of ~15 ps is determined by the
lifetime of the 81 state. For azobenzene, 3 ~15 ps transient has been seen
previously.6'7 We do not observe such a decay for azobenzene in this work
because we are not pumping the 82 +- 80 transition for the trans form. The origin
of the 15 ps transient in azobenzene was ascribed to a “bottleneck” state on the
82 isomerization surface. It is unlikely that the diamidoazobenzene ~15 ps
recovery time is analogous to that seen for azobenzene (we are not exciting the
82 +— 80 transition of diamidoazobenzene), although it is possible that a
“bottleneck” could be present on the S1 isomerization surface of
diamidoazobenzenes. Clearly, more investigation is required to assign the origin
of this transient. As with the calculations for p-diaminoazobenzene, we ﬁnd that
the p-diamidoazobenzenes are characterized by a low lying ﬁrst excited singlet
state, and the 81 <— 80 transition is calculated to have a small oscillator strength.
This theoretical result appears to be at odds with the experimental steady state

and transient optical responses.

48

0.2 -

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

‘ r
“\\ A A, \. .-\ I"
\VAl‘k - fl 1' “'1 1" ‘1 l 1 (x; I “1' IL,
0'0 1M 77w. i 4 T jj QT \
l , ‘ ~ ’
q ‘l I \‘\‘ flv/VJ
-02- ‘3
I; ,f
a E N
l 3
-o.4- l ,/
I, l
cl 1. l' I,
-0.6- l I
{I E l
-O.8 — g
4 .‘lf/r
-------- Instrument Response (Inverted)
-1.0- ‘V .___ 2
r 1' 1’ V I I r f I l l I I U r I I I
-60 -40 -20 0 20 40 60 80 100
Time (ps)
0.2-
00 -_ HM. AMA‘
-o.2~
-o.4-
-0.6-
1
-0.8-
-------- Instrument Response (inverted)
-1.0- 4
r ' r r l ' I ' r ' 1 ' F ' ‘
50 -40 -20 o 20 4o 60 80 100
Time (ps)

Figure 21 : Transient absorbance response of
two p-diamidoazobenzenes.

49

The azobenzene derivatives we study here exhibit remarkable substituent-
dependent variation in their steady state and time-resolved optical responses,
and these ﬁndings are mirrored by 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
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.

50

 

This can be seen in Figure 22 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 on the spectrometer previously mentioned. Upon varying reaction
conditions and evaluating 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 23.

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
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 24 where the slope is the extinction coefﬁcient and the line

51

1.0-

 

 

 

 

 

 

 

—Room
———UV30
0.8-4 ‘ UVGO
—UVQO
————UV1140
0.6-
w'
.0
<
0.4-
0.2-
O'Ol'l 'l'l l'l'l
200 250 350 400 450 500 550
Wavelength (nm)

Figure 22 : Absorbance spectra of 5 upon varying durations of

ultraviolet irradiation.

52

Neutral Density Filter

 

 

 

 

Source

""" 'l

 

 

 

 

 

 

 

 

 

 

Reference Cell

Figure 23 : Block diagram of UVNis instrument

 

Absorbance

 

1.63
1.4—
1.2-
1.0:
0.8-
0.6:
0.4:
02$

0.0 -

 

Concentration (Molar X105)

 

 

Figure 24 : Example of an absorbance versus concentration
graph used to determine molar absorptivity.

53

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 2.

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.

54

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 25), for the p-
diamidoazobenzenes, the recovery time is 317 minutes (Figure 26), and for p-
diaminoazobenzene, the recovery time is 4.7 minutes (Figure 27). 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 a monomolecular we have used

the Arrhenius equation to calculate isomerization barrier heights. Where A is a
1 ﬂ
k=—=Ae”
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

55

Absorbance (a.u.)

1.4-

     

 

 

1.2-
J
1.0-
0.8-
0.6- Q'lk .
0.4- ,
trans recovery, I = 10904 i 1 min
,I/"/
0.2- '
l ' l ' l ' l ' l ' l
0.0 sodo’ 1on0“ 1.5m“ zoao‘ 2500‘

Turre(rrin)

Figure 25 : Recovery of the absorption due to the trans
band in azobenzene at 318 nm.

56

Absorbance (a.u.)

220-

2151

N

_x

o
l

N
E51

N
5.3

 

I trans recovery, 1: = 2.4 i 0.1 min

 

 

 

TIrre(m'n)

Figure 26 : Recovery of the absorption due to the trans
band in p-diaminoazobenzene at 370 nm.

57

.0 .O
O) \l
l J

Absorbance (a.u.)
,0
‘l‘

0.4 -

0.3 -

trans recovery, 1: = 317 i 2 min

 

 

-2CD

1
2(1)

I I

l'l'l'l'lTl'l‘
60080010001200140016001800

Tme(rrin)

l
400

Figure 27 : Recovery of the absorption due to the trans band
in p-diamidoazobenzene at 370 nm.

58

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 1r* (antibonding)
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 11), so it is not the energy of the 81
+— 80 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 S1 4— 80 transition coordinate is thought to lie along the long

59

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 81 manifold is possible, and the strength of this
coupling is reﬂected in the transition cross sections of the 81 <— 80 transitions for
both the cis and trans forms. Thus, the larger the transition cross section for the
81 «— So transition (Table 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 both 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 Table 3 and Figures 29 to

31.

60

Literature Cited

1. Jiang, Y.; Hambir, S. A.; Blanchard, G. J. Opt. Commun., 1993, 99, 216.
2. Bado, P.; Wilson, 8. B.; Wilson, K. R. Rev. Sci. Instrum, 1982, 53, 706.

3. Andor, L.; Lorincz, A.; Siemion, J.; Smith, D. D.; Rice, 8. A. Rev. Sci. Instrum,
1984, 55, 64.

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

5. Shultz, T.; Quenneville, J.; Levine, B.; Toniolo, A.; Martinez, T. J.;
Lochbrunner, 8.; Schmitt, M.; Shaffer, J. P.; Zgierski, M. Z.; Stolow, A. J. Am.
Chem. Soc., 2003, 125, 8098.

6. 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.

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

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

61

Conclusion

The goal of this research was to evaluate azobenzene derivatives as
possible probes and interlayer linkages in stacked polymer systems. The use of
an isomerizable interlayer linkage 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 azobenzene must fully be understood before
any development can be achieved. Selection of the proper azobenzene to work
with was determined by engineering necessities and ability to be synthesized.
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. These
calculations suggested that the isomerization mechanism was independent of
substitution in the para positions. To validate the calculations a series of time
correlated spectroscopy techniques were performed. Pump-probe transient
absorbance spectroscopy was used to determine the affects of photon
absorption on the molecules. Time correlated UVNisible spectroscopy was
conducted to determine the time constant for isomerization. Upon analyzing the
data it was determined that further calculations were needed to fully understand
the results.

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

62

compounds. The yields obtained for each compound are presented in Table 1.
Careful puriﬁcation techniques greatly improved purity and only mildly affected
yield. The UVNis spectrum shifted with substitution as suspected. All of the p-
diamidoazobenzenes absorb at the same wavelength, which is shifted from pure
azobenzene and p-diaminoazobenzene. The NMR spectroscopy showed the
shifts expected and was used as a tool to analyze the purity of the synthesized
compounds.

Semi-empirical calculations were performed on the series of azobenzenes
synthesized, azobenzene, and the 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 12. 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.

From the transient absorbance experiment it was found that azobenzene
experiences a multi-photon process upon photon absorption.
Diaminoazobenzene demonstrates a bleaching experiment, and
diamidoazobenzene exhibits an induced absorption upon irradiation. This
change in process upon irradiation is indicative of an electronic state alteration.

This is understandable in that the conjugated pi system of the azobenzene

63

moiety is also in conjugation with either the amine or amide nitrogen depending
on which species is being evaluated. 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 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.

The future work on this project leads to the adoption of a different
molecule for the probing and storage applications. This decision is 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 still exhibit the wanted ability to drastically 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.

64

APPENDICES

65

APPENDIX A

MASS SPECTRA OF SYNTHESIZED COMPOUNDS

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

diamidoazobenzenes.

66

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