Polarity controlled reaction path and kinetics of thermal cis-to-trans isomerization of 4-aminoazobenzene

Article abstract of DOI:10.1021/jp4125205

Spectral and kinetic behavior of thermal cis-to-trans isomerization of 4-aminoazobenzene (AAB) is examined in various solvents of different polarities. In contrast to azobenzene (AB), it is found the rate of thermal isomerization of AAB is highly dependent on solvent polarity. Accelerated rates are observed in polar solvents as compared to nonpolar solvents. Moreover, a decrease in the barrier height with an increase in medium polarity is observed. Our observations suggest that inversion is the preferred pathway in cis-to-trans thermal isomerization in a nonpolar medium; however, in a polar medium, the isomerization path deviates from the inversion route and rotational behavior is incorporated. Differences in the kinetics and in mechanisms of isomerization in different media are rationalized in terms of modulation in barrier height by polarity of the medium and solute-solvent interaction. It is found that kinetics as well as the mechanism of thermal isomerization in AAB is controlled by the polarity of the medium. ? 2014 American Chemical Society.

Full text of DOI:10.1021/jp4125205

Article
pubs.acs.org/JPCB
Polarity Controlled Reaction Path and Kinetics of Thermal Cis-to-
Trans Isomerization of 4‑Aminoazobenzene
Neeraj Kumar Joshi,† Masanori Fuyuki,‡ and Akihide Wada*,†
†Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
‡Organization of Advanced Science and Technology, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
ABSTRACT: Spectral and kinetic behavior of thermal cis-to-trans isomerization of 4-
aminoazobenzene (AAB) is examined in various solvents of different polarities. In
contrast to azobenzene (AB), it is found the rate of thermal isomerization of AAB is
highly dependent on solvent polarity. Accelerated rates are observed in polar solvents
as compared to nonpolar solvents. Moreover, a decrease in the barrier height with an
increase in medium polarity is observed. Our observations suggest that inversion is the
preferred pathway in cis-to-trans thermal isomerization in a nonpolar medium;
however, in a polar medium, the isomerization path deviates from the inversion route
and rotational behavior is incorporated. Differences in the kinetics and in mechanisms
of isomerization in different media are rationalized in terms of modulation in barrier
height by polarity of the medium and solute−solvent interaction. It is found that
kinetics as well as the mechanism of thermal isomerization in AAB is controlled by the
polarity of the medium.
AB (260 nm) is weaker, but the n → π* transition (440 nm)
1. INTRODUCTION
absorbs more strongly than trans-AB.14
Photoinduced studies provide information regarding various
radiative and nonradiative relaxation processes by which an
excited molecule returns to its ground state. The family of
azobenzene derivatives (ABD) is photolabile, and photo-
induced modifications in their case have long been
known.1−41 ABD can exist either in cis or trans conformations.
The cis form is energetically less stable as compared to the trans
form. Trans → cis photoisomerization occurs following
irradiation with UV−visible light. On the other hand, cis →
trans thermal isomerization occurs spontaneously in the dark
owing to the thermodynamic stability of the trans isomer. In
addition to thermal isomerization, photoinduced cis → trans
isomerization is also possible. Such easy interconversion
between the two isomers makes ABD a widely used organic
chromophore for technical applications, including optical
waveguides and shutters,1 switching display devices,2,3 optical
memories,4,5 electro-optical modulators,6 photoactive artificial
muscles,7−9 light-triggered nanomachines,10,11 etc.
Azobenzene (AB) has been a subject of interest and
investigation on the mechanism of isomerization process
(either photoinduced or thermal).11−21 X-ray and computa-
tional data indicate trans-AB adopts a planar structure with C2h
symmetry,22,23 while cis-AB possesses a nonplanar conforma-
tion with C2 symmetry.24,25 The absorption spectrum of trans-
AB consists of two well separated bands in the UV−visible
region.26,27 The strong UV band (320 nm) arises from the
symmetry allowed π → π* (S2 - second excited singlet)
transition.11,28 The much weaker band in the visible region
(450 nm) corresponds to the symmetry forbidden n → π* (S1 -
first excited singlet) transition.11 The π → π* transition of cis-
Furthermore, trans ↔ cis interconversion can be controlled
by irradiating either transitions11 and/or by changing the
intensity of irradiating monochromatic radiation.28,29 In
general, for photoisomerization of AB, four possible mecha-
nisms have been proposed, i.e., rotation, inversion, concerted
inversion, and inversion assisted rotation.11,23,30,31 The
transition state formed in concerted inversion has no net
dipole moment, whereas the other three pathways possess polar
transition states.21 Relaxation from all four transition states can
afford either the cis or trans isomer; therefore, all four
mechanisms predict photostationary states consisting of both
isomers.21 It has been shown theoretically23,32−36 and
experimentally37−41 that the thermal cis → trans isomerization
of AB proceeds via an inversion mechanism. Theoretical
calculations predict a significantly larger activation energy for
rotation (36.2 kcal/mol) as compared to inversion (24.9 kcal/
mol).23 The rate of thermal isomerization is also found
independent of solvent polarity, which is also indicative of
inversion as the preferred pathway.21
Substitution by different groups at different positions in the
parent moiety (AB) can alter spectroscopic properties and
dynamics/kinetics of isomerization.36−40 On substituted AB,
solvents also greatly influence the photostationary state and rate
of thermal isomerization reaction.12,36,41−44 Substitution of an
amino group (−NH2) at the 4-position in trans-AB, i.e., trans-4-
aminoazobenzene (trans-AAB), shifts the π → π* absorption
Received: December 20, 2013
Revised: January 24, 2014
Published: January 26, 2014
© 2014 American Chemical Society
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band (compared to trans-AB) toward longer wavelength,
causing an overlap with the n → π* absorption band.40 In
contrast to trans-AB, n → π* absorption is buried under the
intense π → π* absorption band of trans-AAB. An energy gap
(difference between π → π* and n → π* states) of 3000−4000
cm−1 was estimated for trans-AAB in ethanol,40 which is smaller
than the energy gap of trans-AB (∼10 000 cm−1). The
photoisomerization dynamics of the π → π* excited trans-
AAB in ethanol and heptanol were investigated by using UV−
vis transient absorption spectroscopy by Hirose et al.40 The
transient absorption spectra of the π → π* excited trans-AAB
showed that the internal conversion process from the n→ π*
state to the ground state corresponded well to that of the trans-
AB directly excited to the n→ π* state, not to that of the π →
π* excited trans-AB. This result indicates that the photo-
isomerization of π → π* excited trans-AAB proceeded with
different pathways from the π → π* excited trans-AB. Study by
Hirose et al.40 suggests that spectral features as well as
dynamics of photoisomerization of trans-AAB are quite
different from its parent trans-AB.
available which do not encompass the solvent polarity and
viscosity effect in detail. In this paper, to decipher the
mechanism of isomerization in the ground state and to obtain
details about one of the deactivation processes of a photo-
reaction cycle, we focus our study on the kinetics of thermal
isomerization of 4-aminoazobenzene (AAB) under the depend-
ence of solvent polarity, viscosity, and temperature. To the best
of our knowledge, thermal cis-to-trans isomerization of AAB
has not been systematically investigated experimentally yet.
Therefore, systematic experiments are performed to obtain
better insight on the thermal cis-to-trans isomerization of AAB.
2. EXPERIMENTAL SECTION
2.1. Materials. 4-Aminoazobeneze (AAB) obtained from
Wako Pure Chemical Industries, Ltd., Japan (99% pure), was
used as such. All the solvents used were of spectroscopic grades.
The polarity parameter and viscosity of the solvents are listed in
Table 1.
Table 1. Estimated Value of the cis Population in the
Photostationary State in Different Media
Photoinduced trans-to-cis (forward), photoinduced cis-to-
trans (reverse), and thermal cis-to-trans isomerization (back)
reactions are the major relaxation processes in the photo-
reaction network (Figure 1) of the ABD system. The forward
a
b
medium
ETN
η
(cP)
cis population (%)
DEE
0.117
0.355
0.552
0.762
0.781
0.224
0.306
3.95
91.6
88.4
66.8
34.3
11.7
ACTN
IBOH
MEOH
EG
0.544
16.1
a
b
Reference 50. Reference 56, temperature at 298 K.
2.2. Instrumentation. Steady state absorption spectra were
recorded by a dual beam Shimadzu UV-2400PC spectropho-
tometer. For temperature dependent measurements, a facility
(Circulating Bath/Immersion Circulator, Polyscience, USA) is
available with the sample compartment. The temperature
measurement has an accuracy of 1 °C. To generate cis-AAB, a
cw UV laser (405 nm, ∼200 mW, purchased from World Co.
Ltd., Japan) was used. The power of the laser source was
measured through a Gentec TPM 300 power meter. Time
resolved measurements for a time constant longer than 1.0 s
were carried out by sequential accumulation of the spectropho-
tometer. To measure the temporal behavior with a time
constant shorter than 1.0 s, a time-resolved setup was built
(shown in Figure 2). In the setup, a pump UV laser beam was
modulated by a mechanical chopper and the transient
absorption changes were monitored by a probe beam. An
electric lamp (MINI MAGLITE 2AA) was used as the source
for the probe beam. Pump and probe beams were focused and
Figure 1. Schematic diagram of the photoreaction network of
azobenzene derivatives. Details are given in the text.
(covered by a dash-dotted line in Figure 1) and reverse reaction
(covered by only a dashed line in Figure 1) can coexist
depending on the choice of excitation wavelength and generally
occurs in an extremely short time scale (fs to sub-ps regime).
However, once a cis form is generated, it thermally relaxes (in
dark) to its stable trans ground state configuration. For
realization of complete tale of such complex photoreaction
network, detailed knowledge about all deactivation processes of
the photonetwork is quite helpful. The knowledge of the back
reaction (highlighted by a red line in Figure 1), one of the
important reactions in a network, can help in understanding
and control of the whole photoreaction network.
As discussed earlier, the photoinduced dynamics (forward
reaction) of AAB is quite different from its parent AB,40 so it is
also interesting to investigate the back reaction in AAB.
Although based on the theoretical calculations,32,43,44 few
reports on thermal cis-to-trans isomerization of AAB are
Figure 2. Schematic diagram of the home-built setup to observe
temporal absorbance change at a time constant shorter than 1.0 s. Ch,
mechanical chopper; Gr, grating; MC, monochromator; Osci,
oscilloscope; PMT, photomultiplier tube.
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overlapped in the quartz cell of 2 mm path length. The
intensity of the transmitted probe beam was detected by a PMT
(R928, Hamamatsu) coupled with a monochromator (JASCO,
CT10). Temporal behavior of the intensity was observed and
accumulated by a digital oscilloscope (Tektronix, 200 MHz,
2GS/s) which was synchronized to on/off modulation of the
pump beam. In all solvents, the concentration of the AAB was
approximately 4 × 10−5 M. To prevent photoisomerization
caused by any other radiation, e.g., room light, all experiments
were carried out in the dark room.
2.3. Calculation Details. Calculations were performed
using the Gaussian 03 package.45 The trans and cis isomers for
the thermal isomerization of 4-AAB in the ground state were
optimized unrestrainedly by means of the DFT methods. The
6-31G* basis set was used in the B3LYP calculations. In order
to locate and optimize the transition state (TS) structure in the
gas phase, the synchronous transit-guided quasi-Newton
(STQN) method46,47 with QST2 option was employed at the
DFT-B3LYP/6-31G* level. All optimized structures (cis
isomer, trans isomer, and TS structure) were confirmed by
vibrational frequency analysis. Only real frequency values were
obtained for the cis and trans isomer, and a single imaginary
frequency was shown by TS structure. Calculations in solvents
were performed employing the self-consistent reaction field
(SCRF) method based on the polarizable continuum model
(PCM).48,49
Figure 4. Absorption spectra of AAB in MEOH: (a) absorption
spectrum before UV irradiation; (b) absorption spectrum under UV
irradiation (photostationary state); (c) difference spectrum between
spectrum a and spectrum b.
absorption band corresponding to n → π* transition of trans-
AAB is so weak that it is buried under the π → π* transition
and appears as a long wavelength tail.
To generate cis-AAB, the UV laser (405 nm) was irradiated
(approximately for 10−15 s) until it reached a photostationary
state (Figures 3b and 4b). The absorption profile of the
photostationary state is recorded under the irradiation of an UV
laser (405 nm). In DEE, under the irradiation of an UV laser
(photostationary state), a large decrease in absorbance at
around 380 nm with two newly developed absorption bands at
around 330 nm and around 455 nm is observed (Figure 3b).
However, in MEOH (Figure 4b), the profile of the photosta-
tionary state is entirely different from that of DEE.
Subsequently, we observe that the profile of the photosta-
tionary state and decrease in the absorbance strongly depends
on the polarity of the medium (discussed in detail later).
In the case of DEE, in the photostationary state (Figure 3b),
the broad absorption profile with two well separated bands
indicates the presence of cis-AAB. In the difference spectrum
(Figure 3c), negative absorbance at ∼380 nm is assigned to the
photobleaching of trans-AAB and the absorption peaks at
around 330 nm and around 465 nm are attributed to cis-AAB.
In analogy with AB (the energy gap between π → π* and n →
π* transition is larger for cis-AB than trans-AB, and the π → π*
band shows blue shift and weakening11,14,28), the short
wavelength absorption band (330 nm) is assigned as the π →
π* transition of cis-AAB, while the long wavelength absorption
band (465 nm) is attributed to the n → π* transition of cis-
AAB. Here, we would like to mention that, in DEE, the energy
gap (in the π → π* and n → π* states) of cis-AAB is ∼8800
cm−1, which is much larger than the energy gap for trans-AAB
(∼3000−4000 cm−1). On the other hand, assignment of the
bands of cis form from the profile of the photostationary state
in MEOH (Figure 4b) is more complicated than for DEE.
In a particular medium, in the photostationary state, the ratio
between the cis to trans form depends on the intensity of the
pump light, the rate of thermal isomerization reaction, and the
absorption cross section of each isomer. Under the assumption
that the peak intensity of the trans form at the absorption
maximum wavelength (380 nm) is only due to the trans form
(i.e., no cis form is present), the cis isomer present in the
photostationary state can be estimated by the decrease in the
absorbance at this wavelength. Using the absorption spectrum,
the population ratio (PR) of the cis form of AAB in the
photostationary state can be estimated from the following
formula as
3. RESULTS AND DISCUSSION
3.1. Effect of Solvent and Temperature on Thermal
Isomerization. Absorption spectra of AAB are recorded in
solvents of different polarities, viz, diethyl ether (DEE), acetone
(ACTN), isobutanol (IBOH), methanol (MEOH), and
ethylene glycol (EG). The polarity of the solvent is defined
by the dimensionless microscopic solvent polarity parameter ETN
proposed by Reichardt.50
Absorption spectra (at 298 K) of AAB in DEE (ENT = 0.117)
and MEOH (ENT = 0.762) are shown in Figures 3a and 4a,
Figure 3. Absorption spectra of AAB in DEE: (a) absorption spectrum
before UV irradiation; (b) absorption spectrum under UV irradiation
(photostationary state); (c) difference spectrum between spectrum a
and spectrum b.
respectively. AAB exhibits an absorption maximum at ∼380 nm
with a long wavelength absorption tail. In all studied solvents,
no significant effect of polarity on the absorption maximum is
observed for AAB. The absorption maximum of AAB is also
found to be independent of the solute concentration, and
hence, the possibility of solute aggregation is ruled out in all
studied solvents. As discussed,40 the absorption band at ∼380
nm is attributed to π → π* transition of trans-AAB, while the
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[Aλ]wi − [Aλ]ui
Article
⎛
⎞
⎟
t
τ
⎜
A(t) = A∞ + A1 exp −
population ratio of cis: PR =
× 100
⎝
⎠
(2)
[Aλ]wi
(1)
where A(t) and A∞ correspond to the absorbance of AAB at
any time t and absorbance as time approaches infinity,
respectively. A1 is the pre-exponential factor, and τ is the
lifetime of the ground state of cis-AAB. The lifetimes were
recovered from the plots by the least-squares fitting of the data
to eq 2. Lifetimes were determined with an associated error
lower than 5% in all cases.
The effect of temperature on the rate of thermal isomer-
ization reaction was also investigated. Lifetimes (τ) and
reaction rate (k = 1/τ) in different media at different
temperatures are summarized in Table 2. It is indicated from
where the subscripts wi and ui refer to without irradiation and
under irradiation of UV laser, respectively. Aλ is the absorbance
at wavelength λ, where λ is the wavelength of absorption
maximum of the trans form. Using eq 1, the effect of polarity
and viscosity of the solvent on the estimated population of cis-
AAB in the photostationary state is shown in Table 1. From
Table 1, it can be noticed that, compared to the solvent
viscosity, the polarity of the solvent plays a major role on the
photostationary state (discussed in detail later). Lesser change
in absorption profile, namely, decrease of cis-AAB population,
in polar medium indicates a lesser thermodynamic stability of
cis-AAB in the ground state, i.e., the faster ground state
isomerization from cis to trans forms as compared to nonpolar
medium.
Table 2. Temperature Dependent Lifetime and Rate of
Thermal Isomerization Reaction
Once the irradiation of the UV laser (405 nm) is stopped, as
a matter of fact, the absorption intensity of cis peaks (330 and
465 nm) decreases and simultaneously absorption correspond-
ing to the trans peak begins to increase, as shown in Figure 5.
solvent (ENT)
temperature (K)
τ (s)
kc→t (s−1
)
DEE (0.117)
290
295
300
305
310
290
295
300
305
310
290
295
300
305
310
278
283
288
293
298
280
285
290
295
2.61 × 104
2.02 × 104
1.13 × 104
6.55 × 103
3.38 × 103
2.46 × 104
1.92 × 104
1.18 × 104
7.93 × 103
5.11 × 103
1.47 × 102
1.22 × 102
93.1
3.82 × 10−5
4.95 × 10−5
8.84 × 10−5
15.25 × 10−5
29.55 × 10−5
4.10 × 10−5
5.20 × 10−5
8.42 × 10−5
12.60 × 10−5
19.56 × 10−5
6.82 × 10−3
8.21 × 10−3
10.74 × 10−3
13.62 × 10−3
19.07 × 10−3
0.94
ACTN (0.355)
IBOH (0.554)
MEOH (0.762)
EG (0.781)
73.4
52.4
Figure 5. Temporal behavior of the absorption profile of AAB in DEE
after UV irradiation, showing the cis-to-trans conversion in the dark at
295 K. Delay times (after stopping the irradiation of the pump beam)
of observation are presented in the figure.
1.05
0.90
1.11
0.70
1.42
0.58
1.72
0.42
2.38
0.26
3.85
0.22
4.55
Figure 5 exhibits the absorption profiles in DEE monitored at
different time intervals after the removal of the UV laser.
Temporal profiles of the trans peak intensity at different
temperatures are depicted in Figure 6.
0.19
5.26
0.13
7.70
Generally, the rate of thermal isomerization reaction follows
a first-order kinetics. The unimolecular thermal cis-to-trans
isomerization reaction in the dark obeys eq 2
Table 2 that the lifetime of the ground state of the cis form
strongly depends on the polarity of the medium. In DEE, the
estimated lifetime at 300 K is approximately 1.13 × 104 s (∼188
min), while it comes out to be 0.42 s in MEOH. The data in
Table 2 clearly shows accelerated rates in polar media, and
illustrates that in a particular medium reaction is temperature
dependent; on lowering the temperature, the reaction becomes
slower.
Further, the rate of reaction is governed by barrier height and
it can be estimated using temperature dependent reaction rates
and employing the Arrhenius equation as follows
⎛
⎞
⎟
⎠
Ea
RT
⎜
kc→t = A exp −
⎝
(3)
where kc→t is the rate constant of thermal isomerization
reaction from cis to trans, T is the absolute temperature, R is
the universal gas constant, and A is the pre-exponential factor
(frequency factor). The term Ea is used for the barrier height in
the Arrhenius equation. Arrhenius plots for different solvents
Figure 6. Temporal behavior of absorbance at 380 nm of AAB in DEE
after stopping UV irradiation at different temperatures. Solid lines
indicate the results of the least-squares fitting of the data to eq 2.
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Figure 7. Arrhenius plot in different solvents: (a) DEE, (b) ACTN, (c) IBOH, and (d) MEOH.
are shown in Figure.7. The solid straight lines in the Arrhenius
plot represent the linear fitting of the data, and the slope of
these lines is used to estimate the barrier height (Ea). We have
also theoretically calculated the barrier height in the gas phase
(using the STQN method), which is 24.2 ( 1.2) kcal/mol. The
plot between solvent polarity and barrier height is shown in
Figure 8 which shows the polarity dependence of the obtained
K) as compared to the MEOH (0.55 cP at 298 K); on the other
hand, the ENT value of EG (ENT = 0.781) is similar to that of
MEOH (ENT = 0.762). It should be mentioned that, upon UV
laser (405 nm) irradiation in EG, the change in absorption
spectrum (Figure 9) was lesser as compared to MEOH. In EG,
Figure 9. Absorption spectrum of AAB in EG: (a) absorption
spectrum before UV irradiation; (b) absorption spectrum under UV
irradiation (photostationary state); (c) difference spectrum between
spectrum a and spectrum b.
Figure 8. Barrier height and solvent polarity parameter. The error in
the barrier height for gas is assumed similar to that observed in the
experimental value for solvents. 1 kcal/mol = 4.18 kJ/mol.
the estimated cis population of AAB in the photostationary
state is ∼11.7% (Table 1). Additionally, due to the high
viscosity of EG, one may expect a longer lifetime of the ground
state of cis-AAB. On the contrary, faster kinetics is observed in
EG as compared to MEOH (Table 2). Apparently, viscosity
does not slow the thermal reaction rate. This result is in good
agreement with the other reports for similar types of
molecules.20,51 From temperature dependent ground state
decay behavior of cis-AAB in EG, the estimated barrier height
comes out to be 7.3 ( 0.4) kcal/mol which is slightly less than
that in MEOH probably due to the slightly higher value of ETN
of EG as compared to MEOH. This result also indicates that, in
a particular solvent, a decrease in reaction rate on lowering the
temperature is not due to increased viscosity of the solvent.
Therefore, the barrier height for thermal cis-to-trans isomer-
barrier height. It is shown from the figure that the barrier height
almost linearly decreases with an increase in polarity of the
solvent. The decrease in the value of Ea is in the following
order: 24.2 ( 1.2) kcal/mol (gas) >18.5 ( 0.9) kcal/mol
(DEE) > 13.8 ( 0.7) kcal/mol (ACTN) > 9.1 ( 0.5) kcal/mol
(IBOH) > 7.5 ( 0.4) kcal/mol (MEOH). Using the quantum
chemical approach, Dokic et al.32 calculated Ea as approximately
25.1 kcal/mol for AAB in the gas phase.
We have also examined the effect of viscosity on the reaction
rate of thermal isomerization. For this purpose, ethylene glycol
(EG) was chosen, as its viscosity is much higher (16.1 cP at 298
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ization in AAB can be correlated to the solvent polarity
parameter (ENT) of the surrounding medium. In the foregoing
discussion, we see the decrease in the barrier height with an
increase in the polarity of the medium (Figure 8); thus, higher
rates are noticed in higher polar medium. Following our results,
it appears that increased polarity of the medium may affect the
reaction in three possible ways: (i) by affecting the energy of
the transition state but the relative stability of cis-AAB remains
unaffected (Figure 10-i), (ii) by affecting the energy of cis-AAB
Table 3. Theoretically Estimated Dipole Moments in the
Ground State by DFT-B3LYP/6-31G*
medium
μcis (D)
μtrans (D)
gas
4.95
6.36
6.99
7.65
3.11
4.02
4.43
4.49
DEE
ACTN
MEOH
Figure 10. Schematic diagram for various considered models. TS,
transition state; H, high polarity; M, medium polarity; L, low polarity.
Figure 12. Solvent dependence of the energy of cis, trans, and
transition state (TS) of AAB compared to trans-AAB in MEOH.
Solvents are indicated in the figure. The energy of the transition state
structure is estimated by using the experimental value of barrier height
except for gas. Solid lines are guide for eyes. 1 kcal/mol = 4.18 kJ/mol.
but the energy of the transition state remains unaltered (Figure
10-ii), and (iii) by affecting simultaneously both the relative
stability of cis-AAB and the transition state (Figure 10-iii). In all
cases, the barrier height will change and reflects in the rate
constant of the reaction. In order to verify these proposed
models, computational calculations were carried out.
the cis and trans form is thereby rationalized as the higher
dipole moment is stabilized by the polar medium.23
In order to analyze the TS character, the energies of different
structures are plotted as a function of solvent polarity, as shown
in Figure 13. The solid lines indicate the results of linear fitting
3.2. Relative Stability, Dipole Moments, and Tran-
sition State. The optimized structures of trans- and cis-AAB in
the gas phase are shown in Figure 11 and well match with the
previous reports.43 In the gas phase, the ground state dipole
moments of the trans- and cis-AAB are 3.11 and 4.95 D,
respectively. Silva et al.53 have reported the dipole moment
value of trans-AAB, i.e., 3.05 D (in the gas phase). This value is
very similar to that which we obtained (3.11 D). Values of the
ground state dipole moments of optimized trans- as well as cis-
AAB in solvents are shown in Table 3. The results of
calculations in DEE, ACTN, and MEOH show that the ground
state dipole moments of both trans- and cis-AAB increase with
an increase in the polarity of the medium. Further, the ground
state energies of the cis form, trans form, and transition state
(TS) in the gas phase were obtained using computational
calculation through optimization. TS energies in solvents are
obtained from the sum of the ground state energy of the cis
form (obtained using computational calculation) and the
estimated barrier height. The results of calculated and estimated
energies of each form in different media are summarized in
Figure 12. From Figure 12, it is clear that each form is stabilized
with an increase in the polarity of the medium, namely, Figure
10-iii. As listed in Table 3, the dipole moment of the cis and
trans form increases with an increase in polarity. Stabilization of
■
●
Figure 13. Polarity dependence on energy of TS ( ), cis-AAB ( ),
▲
and trans-AAB ( ) compared to trans-AAB in MEOH. Solid lines are
results of linear fitting of the data. The steepness of the slope for TS is
emphasized by the red-colored line. Dashed lines are guides for the
eyes. 1 kcal/mol = 4.18 kJ/mol.
of the data. It is evident from Figure 13 that the slope of the
linear fit for the TS structure is much steeper than the slope of
the linear fit for the cis and trans forms, which fairly indicates
that with an increase in solvent polarity stabilization of TS is
Figure 11. Optimized structures of AAB in the gas phase using DFT-B3LY/6-31G*.
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Article
greater than stabilization of the cis and trans forms. From these
results, it can be predicted that the TS should possess a higher
dipole moment and polar character in polar medium as
compared to less polar medium. Although the possibility of a
different TS structure in a polar medium as compared to that in
a relatively less polar medium cannot be ruled out, it is feasible
only if the mechanism of isomerization is different in a polar
medium than a nonpolar medium.
observations suggest that the mechanism for cis-to-trans
thermal isomerization is greatly influenced by the polarity of
medium. Our investigation supports a model similar to that as
proposed in Figure 10-iii to explain the observed features of
thermal isomerization of AAB. In a nonpolar medium, the
inversion mechanism seems to be preferred. However, it is
believed that the rotation mechanism occurs in a polar medium.
Furthermore, on excitation, either via one photon or
multiphoton can have involvement of higher excited states
such as S2 or Sn but the thermal isomerization is the final
deactivation process in a photoreaction network of the ABD
system. Therefore, these findings on the thermal isomerization
are important to the understanding and control of the
photoreaction network even in the case of multiphoton
excitation.
Further, the TS structure in the gas phase was searched
computationally using the STQN (QST2) method. The
optimized TS structure is shown in Figure 11 which
corresponds to the inversion mechanism.43,44,52 With DEE
being nonpolar in character, thermal isomerization through
inversion can be expected. Faster relaxation times in polar
media are evidence that intermolecular interaction between the
solute and the solvent molecules can influence the rate of the
isomerization process for AAB. In 4-hydroxy azobenzene
(HAB), higher rates in polar solvents have also been repoted.36
In HAB, formation of intermolecular hydrogen bonds favors
rotation,36,54,55 which seems to be the key for the faster
isomerization kinetics in polar medium. In analogy to HAB,36
solute−solvent interaction such as intermolecular hydrogen
bonding is also feasible for AAB in polar medium. Therefore, it
is worth considering interaction then a polar medium. This
results in a higher stabilization of TS and hence lowering in the
barrier height, which results in accelerated rates. Additionally, a
polar TS with charge separation can lower the activation energy
and favor rotation.35 The formation of a charged TS should be
favorable in polar solvents. Therefore, a rotation mechanism
which involves a more polar TS than the TS of the inversion
mechanism may be involved for thermal cis-AAB to trans-AAB
in solvents of higher polarity.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: aki.wada@koala.kobe-u.ac.jp. Phone: +81-78-803-
5695.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Prof. Shunji Kasahara, Molecular Photo Science Research
Center, Kobe University, Kobe, Japan, is greatly acknowledged
for providing us use of their facility of Gaussian 03 software and
system. This work was financially supported by a Grant-in-Aid
for Challenging Exploratory Research (Grant No. 24655015)
from the Japan Society for the Promotion of Science (JSPS).
In comparison to AB, where the reaction path as well as
kinetics of thermal cis-to-trans isomerization is independent of
polarity,21 it is found that thermal cis-to-trans isomerization in
AAB is highly sensitive toward the polarity of the medium. As it
has been observed that substitution of an amino group at the 4-
position in AB alters the dynamics of photoisomerization as
compared to AB,40 similarly, results presented here indicate
that thermal isomerization is also influenced by substitution of
an amino group at the 4-position in AB. It is natural to consider
from the results that the polarity and the electron donor
character of the substituent play an important role for the onset
of the solvent dependency. However, the details on the
mechanism of solvent dependency still need to be investigated
comprehensively by theoretical calculation.
REFERENCES
■
(1) Bang, C. U.; Shishido, A.; Ikeada, T. Azobenzene Liquid-
Crystalline Polymer for Optical Switching of Grating Waveguide
Couplers with a Flat Surface. Macromol. Rapid Commun. 2007, 28,
1040−1044.
(2) Yager, K. G.; Barrett, C. J. Novel Photo-switching using
Azobenzene Functional Materials. J. Photochem. Photobiol., A 2006,
182, 250−261.
(3) Beharry, A. A.; Sadovski, O.; Woolley, G. A. Azobenzene
Photoswitching without Ultraviolet Light. J. Am. Chem. Soc. 2011, 133,
19684−19687.
(4) Gibbons, W. M.; Shannon, P. J.; Sun, S. T.; Swetlin, B. J. Surface-
mediated Alignment of Nematic Liquid Crystals with Polarized Laser
Light. Nature 1991, 351, 49−50.
(5) Ikeda, T.; Tsutsumi, O. Optical Switching and Image Storage by
Means of Azobenzene Liquid-Crystal Films. Science 1995, 268, 1873−
1875.
(6) Luk, Y. Y.; Abbott, N. L. Surface-Driven Switching of Liquid
Crystals Using Redox-Active Groups on Electrodes. Science 2003, 301,
623−626.
(7) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.;
Shelley, M. Fast Liquid-Crystal Elastomer Swims into the Dark. Nat.
Mater. 2004, 3, 307−310.
(8) Yamada, M.; Kondo, M.; Mamiya, J. I.; Yu, Y.; Kinoshita, M.;
Barrett, C. J.; Ikeda, T. Photomobile Polymer Materials: Towards
Light-Driven Plastic Motors. Angew. Chem., Int. Ed. 2008, 47, 4986−
4988.
(9) Yin, R.; Xu, W.; Kondo, M.; Yen, C. C.; Mamiya, J. I.; Ikeda, T.;
Yu, Y. Can Sunlight Drive the Photoinduced Bending of Polymer
Films? J. Mater. Chem. 2009, 19, 3141−3143.
(10) Yu, Y. L.; Nakano, M.; Ikeda, T. Photomechanics: Directed
Bending of a Polymer Film by Light. Nature 2003, 425, 145−145.
CONCLUSION
■
We investigated the kinetics of the thermal isomerization
reaction of AAB in solvents of varying polarities. The effect of
temperature and viscosity on the isomerization reaction is also
investigated. Our investigations reveal some salient features of
the isomerization reaction of AAB. The rate of thermal
isomerization of AAB is highly dependent on solvent polarity.
In contrast to nonpolar solvents, accelerated rates are observed
in polar medium. A positive correlation between barrier height
and the polarity of the medium is observed. Accelerated rates
and stability of various forms in solvents of higher polarity
suggest that solute−solvent interaction especially the hydrogen
bonding network of solute and solvent can influence the
isomerization reaction compared to nonpolar solvents. We have
also observed that increased viscosity of the medium does not
hinder the thermal isomerization reaction of AAB. Our
1897
dx.doi.org/10.1021/jp4125205 | J. Phys. Chem. B 2014, 118, 1891−1899

The Journal of Physical Chemistry B
Article
(11) Rau, H.; Lueddecke, E. On the Rotation-Inversion Controversy
on Photoisomerization of Azobenzenes. Experimental Proof of
Inversion. J. Am. Chem. Soc. 1982, 104, 1616−1620.
(12) Asano, T.; Okada, T.; Thermal, Z-E Isomerization of
Azobenzenes. The Pressure, Solvent, and Substituent Effects. J. Org.
Chem. 1984, 49, 4387−4391.
(13) Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.;
Gaub, H. E. Single-Molecule Optomechanical Cycle. Science 2002,
296, 1103−1106.
(14) Cembran, A.; Bernardi, F.; Garvelli, M.; Gagliardi, L.; Orlandi,
G. On the Mechanism of the cis−trans Isomerization in the Lowest
Electronic States of Azobenzene: S0, S1, and T1. J. Am. Chem. Soc.
2004, 126, 3234−3243.
(15) Lednev, I. K.; Ye, T. Q.; Hester, R. E.; Moore, J. N.
Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of
trans-Azobenzene in Solution. J. Phys. Chem. 1996, 100, 13338−13341.
(16) Satzger, H.; Root, C.; Braun, M. Excited-State Dynamics of
trans- and cis-Azobenzene after UV Excitation in the ππ* Band. J. Phys.
Chem. A 2004, 108, 6265−6271.
(32) Dokic, J.; Gothe, M.; Wirth, J.; Peters, M. V.; Schwarz, J.; Hecht,
S.; Saalfrank, P. Quantum Chemical Investigation of Thermal Cis-to-
Trans Isomerization of Azobenzene Derivatives: Substituent Effects,
Solvent Effects, and Comparison to Experimental Data. J. Phys. Chem.
A 2009, 113, 6763.
(33) Anderson, J. A.; Petterson, R.; Tegner, L. Flash Photolysis
Experiments in the Vapour Phase at Elevated Temperatures I: Spectra
of Azobenzene and the Kinetics of its Thermal cis-trans Isomerization.
J. Photochem. 1982, 20, 17−32.
(34) Brown, E. V.; Granneman, G. R. Cis-trans Isomerism in the
Pyridyl Analogs of Azobenzene. Kinetic and Molecular Orbital
Analysis. J. Am. Chem. Soc. 1975, 97, 621−627.
(35) Jaffe, H. H. A Reexamination of the Hammett Equation. Chem.
̈
Rev. 1953, 53, 191−261.
(36) Amoros, J. G.; Ferrer, A. S.; Massad, W. A.; Nonell, S.; Velasco,
D. Kinetic Study of the Fast Thermal cis-to-trans Isomerisation of
Para-, Ortho- and Polyhydroxyazobenzenes. Phys. Chem. Chem. Phys.
2010, 12, 13238−13242.
(37) Matazo, D. R.; Ando, R. A.; Borin, A. C.; Santos, P. S. Azo−
HydrazoneTautomerism in Protonated Aminoazobenzenes: Reso-
nance Raman Spectroscopy and Quantum-Chemical Calculations. J.
Phys. Chem. A 2008, 112, 4437−4443.
(38) Yi, Y.; Fang, G.; Maclennan, J. E.; Clark, N. A.; Dahdah, J.;
Furtak, T. E.; Kim, K.; Farrow, M. J.; Korblova, E.; Walba, D. M.
Dynamics of Cis Isomers in Highly Sensitive Amino-azobenzene
Monolayers: The Effect of Slow Relaxation on Photo-induced
Anisotropy. J. Appl. Phys. 2011, 109, 103521−5.
(39) Coelho, P. J.; Castro, M. C. R.; Raposo, M. M. M. Reversible
trans−cis Photoisomerization of New Pyrrolidene Heterocyclic
Imines. J. Photochem. Photobiol., A 2013, 259, 59−65.
(40) Hirose, Y.; Yui, H.; Sawada, T. Effect of Potential Energy Gap
between the n-π* and the π-π* State on Ultrafast Photoisomerization
Dynamics of an Azobenzene Derivative. J. Phys. Chem. A 2002, 106,
3067−3071.
(41) Gille, K.; Knoll, H. Quitzsch, Rate Constants of the Thermal cis-
trans Isomerization of Azobenzene Dyes in Solvents, Acetone/Water
Mixtures, and in Microheterogeneous Surfactant Solutions. K. Int. J.
Chem. Kinet. 1999, 31, 337−350.
(17) Cusati, T.; Granucci, G.; Persico, M. Photodynamics and Time-
Resolved Fluorescence of Azobenzene in Solution: A Mixed Quantum-
Classical Simulation. J. Am. Chem. Soc. 2011, 133, 5109−5123.
(18) Bandara, H. M. D.; Friss, T. R.; Enriquez, M. M.; Isley, W.;
Incarvito, C.; Frank, H. A.; Gascon, J.; Burdette, S. C. Proof for the
Concerted Inversion Mechanism in the trans→cis Isomerization of
Azobenzene Using Hydrogen Bonding to Induce Isomer Locking. J.
Org. Chem. 2010, 75, 4817−4827.
(19) Fujino, T.; Arzhantsev, S. Yu.; Tahara, T. Femtosecond Time-
Resolved Fluorescence Study of Photoisomerization of trans-
Azobenzene. J. Phys. Chem. A 2001, 105, 8123−8129.
(20) Serra, F.; Terentjev, E. M. Effects of Solvent Viscosity and
Polarity on the Isomerization of Azobenzene. Macromolecules 2008, 41,
981−986.
(21) Bandara, H. M. D.; Burdette, S. C. Photoisomerization in
Different Classes of Azobenzene. Chem. Soc. Rev. 2012, 41, 1809−
1825.
(22) Brown, C. J. A Refinement of the Crystal Structure of
Azobenzene. Acta Crystallogr. 1966, 21, 146−152.
(23) Crecca, C. R.; Roitberg, A. E. Theoretical Study of the
Isomerization Mechanism of Azobenzene and Disubstituted Azoben-
zene Derivatives. J. Phys. Chem. A 2006, 110, 8188−8203.
(24) Mostad, A.; Romming, C. A Refinement of the Crystal Structure
of cis-Azobenzene. Acta Chem. Scand. 1971, 25, 3561−3568.
(25) Gagliardi, L.; Orlandi, G.; Bernardi, F.; Cembran, A.; Garavelli,
M. A Theoretical Study of the Lowest Electronic States of
Azobenzene: The Role of Torsion Coordinate in the cis−trans
Photoisomerization. Theor. Chem. Acc. 2004, 111, 363−372.
(26) Hamm, P.; Ohline, S. M.; Zinth, W. Vibrational Cooling after
Ultrafast Photoisomerization of Azobenzene Measured by Femto-
second Infrared Spectroscopy. J. Chem. Phys. 1997, 106, 519−529.
(27) Lednev, I. K.; Ye, T. Q.; Matousek, P.; Towrie, M.; Foggi, P.;
Neuwahl, F. V. R.; Umapathy, S.; Hester, R. E.; Moore, J. N.
Femtosecond Time-resolved UV-visible Absorption Spectroscopy of
Trans-azobenzene: Dependence on Excitation Wavelength. Chem.
Phys. Lett. 1998, 290, 68−74.
(28) Liang, Z.; Hai, M.; Wang, P.; Zhang, J.; Xie, J.; Zhang, Q.
Nonlinearly Optical−Optical Isomerization Cycle in Azobenzene
Liquid Crystal Polymers. J. Appl. Phys. 2001, 90, 5866−5870.
(29) Loudwig, S.; Bayley, H. Photoisomerization of an Individual
Azobenzene Molecule in Water: An On−Off Switch Triggered by
Light at a Fixed Wavelength. J. Am. Chem. Soc. 2006, 128, 12404−
12405.
(42) Wildes, P. D.; Pacifici, J. G.; Iric, G.; Whitten, D. G. Solvent and
Substituent Effects on the Thermal Isomerization of Substituted
Azobenzenes. Flash Spectroscopic Study. J. Am. Chem. Soc. 1971, 93,
2004−2008.
(43) Wang, L.; Xie, J.; Zhou, H.; Yi, C.; Xu, W. Cis−trans
Isomerization Mechanism of 4-Aminoazobenzene in the S0 and S1
States: A CASSCF and DFT Study. J. Photochem. Photobiol., A 2009,
205, 104−108.
(44) Wazzan, N. A.; Richardson, P. R.; Jones, A. C. Cis-Trans
Isomerization of Azobenzenes Studied by Laser-Coupled NMR
Spectroscopy and DFT Calculations. Photochem. Photobiol. Sci. 2010,
9, 968−974.
(45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.05; Gaussian,
Inc.: Pittsburgh, PA, 2003.
(46) Peng, C.; Schlegel, H. B. Combining Synchronous Transit and
Quasi-Newton Methods to Find Transition States. Isr. J. Chem. 1993,
33, 449−454.
(47) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. Using
Redundant Internal Coordinates to Optimize Equilibrium Geometries
and Transition States. J. Comput. Chem. 1996, 17, 49−56.
(48) Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic Interaction of a
Solute with a Continuum. A Direct Utilization of ab initio Molecular
Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55,
117−129.
(49) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New
Developments in the Polarizable Continuum Model for Quantum
Mechanical and Classical Calculations on Molecules in Solution. J.
Chem. Phys. 2002, 117, 43−54.
(30) Magee, J. L.; Shand, W., Jr.; Eyring, H. Non-adiabatic Reactions.
Rotation about the Double Bond. J. Am. Chem. Soc. 1941, 63, 677−
688.
(31) Curtin, D. Y.; Grubbs, E. J.; McCarty, C. G. Uncatalyzedsyn-anti
Isomerization of Imines, Oxime Ethers, and Haloimines. J. Am. Chem.
Soc. 1966, 88, 2775−2786.
1898
dx.doi.org/10.1021/jp4125205 | J. Phys. Chem. B 2014, 118, 1891−1899

The Journal of Physical Chemistry B
Article
(50) Reichardt, C. Solvatochromic Dyes as Solvent Polarity
Indicators. Chem. Rev. 1994, 94, 2319−2358.
(51) Marcandalli, B.; Beltrame, P. L.; Pagila, E. D. Solvent and
Viscosity Effect on the Kinetics of the Thermal cis-trans Isomerization
of 3′-Nitro-4-Diethylaminoazobenzene. Dyes Pigm. 1989, 11, 179−
189.
(52) Wang, L.; Wang, X. An ab initio Study of Stable Conformation
and Thermal Isomerization of p-Amionazobenzene. J. Mol. Stuct:
THEOCHEM 2007, 806, 179−186.
(53) Silva, D. L.; Krawczyk, P.; Bartkowiak, W.; Mendonca, C. R.
Theoritical Study of One-and Two-photon Absorption Spectra of
Azoaromatic Compounds. J. Chem. Phys. 2009, 131, 244516−244529.
(54) Kojima, M.; Nebashi, S.; Ogawa, K.; Kurita, N. Effect of Solvent
on Cis-to-Trans Isomerization of 4-Hydroxyazobenzene Aggregated
through Intermolecular Hydrogen Bonds. J. Phys. Org. Chem. 2005, 18,
994−1000.
(55) Kurita, N.; Nebashi, S.; Kojima, M. Density Functional
Molecular Orbital Calculations on the Stability of Hydrogen-bonded
4-Hydroxyazobenzene Dimmers. Chem. Phys. Lett. 2005, 408, 197−
204.
(56) CRC Handbook of Chemistry and Physics, 89th ed.; CRC Press:
Taylor and Francis Group, 2008−2009; pp 6−197.
1899
dx.doi.org/10.1021/jp4125205 | J. Phys. Chem. B 2014, 118, 1891−1899