Trans–cis isomerization energies of azopyridines: a calorimetric and computational study

Article abstract of DOI:10.1007/s10973-017-6913-0

Azobenzenes undergo reversible trans–cis photo-isomerization and have been studied extensively as photo-responsive materials. Despite their similar photochemistry, azopyridines have received relatively little attention; for example, their isomerization energies are presently unknown. In comparison with azobenzenes, azopyridines offer additional opportunities for materials design through hydrogen bonding and coordination chemistry. Here we report the isomerization energies for all three symmetrical azopyridines (i.e., the 2,2′-, 3,3′-, and 4,4′-isomers) through a combined experimental and computational study. Heat of isomerization was measured in the liquid state, with o-terphenyl introduced to suppress crystallization. We obtain ?E_iso?=?25.2?±?0.6, 42.6?±?0.6, and 35.0?±?1.8?kJ?mol^?1 for 2,2′, 3,3′, and 4,4′-azopyridine, respectively. For azobenzene, we obtain ?E_iso?=?47.0?±?1.3?kJ?mol^?1, in agreement with the literature value and validating our method. Theoretical calculations yielded gas-phase ?E_iso in reasonable agreement with experiment and explain the low isomerization energy of 2,2′-azopyridine on the basis of a low-energy cis conformer. Because of the smaller van der Waals volume of the pyridine N relative to the phenyl CH, the two aromatic rings in the cis isomer can approach closer to coplanarity, leading to greater π-conjugation and lower conformational energy.

Full text of DOI:10.1007/s10973-017-6913-0

Journal of Thermal Analysis and Calorimetry
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Trans–cis isomerization energies of azopyridines: a calorimetric
and computational study
Men Zhu1 Lian Yu1
•
Received: 8 August 2017 / Accepted: 7 December 2017
´
´
Ó Akademiai Kiado, Budapest, Hungary 2018
Abstract
Azobenzenes undergo reversible trans–cis photo-isomerization and have been studied extensively as photo-responsive
materials. Despite their similar photochemistry, azopyridines have received relatively little attention; for example, their
isomerization energies are presently unknown. In comparison with azobenzenes, azopyridines offer additional opportu-
nities for materials design through hydrogen bonding and coordination chemistry. Here we report the isomerization
energies for all three symmetrical azopyridines (i.e., the 2,20-, 3,30-, and 4,40-isomers) through a combined experimental
and computational study. Heat of isomerization was measured in the liquid state, with o-terphenyl introduced to suppress
crystallization. We obtain DEiso = 25.2 0.6, 42.6 0.6, and 35.0 1.8 kJ mol-1 for 2,20, 3,30, and 4,40-azopyridine,
respectively. For azobenzene, we obtain DEiso = 47.0 1.3 kJ mol-1, in agreement with the literature value and vali-
dating our method. Theoretical calculations yielded gas-phase DEiso in reasonable agreement with experiment and explain
the low isomerization energy of 2,20-azopyridine on the basis of a low-energy cis conformer. Because of the smaller van
der Waals volume of the pyridine N relative to the phenyl CH, the two aromatic rings in the cis isomer can approach closer
to coplanarity, leading to greater p-conjugation and lower conformational energy.
Keywords Photochemistry Á Isomerization energy Á DSC Á Ab initio
Introduction
molecular machines [6], data storage media [7], and
protein switches [8].
For nearly a century, azobenzene and its derivatives have
been studied for their reversible photo-induced trans–cis
isomerization and applications as photo-responsive
materials [1–3]. Upon irradiation, the trans isomer of
azobenzene converts to the cis (Scheme 1). This reaction
is reversible on storage in the dark or upon irradiation at
a different wavelength. This isomerization reaction cau-
ses large changes in physical properties, for example,
dipole moment (from 0 to 3.08 D) [4] and energy (in-
crease by 47 kJ mol-1) [5]. It is possible to cycle the
system many times with little fatigue and chemical
decomposition, suggesting applications as durable mate-
rials. Azobenzene-based materials have been explored as
Azopyridines have similar structures as azobenzenes
and similar photochemistry [9], but have to date received
less attention. From the standpoint of materials design, the
pyridyl group offers additional control of molecular pack-
ing through hydrogen bonding and coordination chemistry.
Zhao and coworkers [10] have demonstrated that azopy-
ridine-containing polymers assemble with aliphatic and
aromatic carboxylic acids to create photo-responsive liquid
crystals, whose smectic/isotropic transition temperature is
tunable by light. Photo-controlled swelling–shrinking are
reported for the micro vesicles of an azopyridine amphi-
philic polymer [11]. Similar vesicles formed by a poly(-
ethylene oxide)-polymethacrylate-azopyridine copolymer
undergo rapid disintegration and fusion under UV irradia-
tion, with potential use for controlled release [12]. Certain
azopyridines self-assemble into nano-fibers whose length
can be reduced by an order of magnitude under UV light
[13]. A common ligand in coordination chemistry [14],
azopyridine can be organized by metal coordination to
& Lian Yu
lian.yu@wisc.edu
1
Department of Chemistry and School of Pharmacy,
University of Wisconsin-Madison, Madison, WI 53705, USA
123

M. Zhu, L. Yu
size of 5 mg. Nuclear magnetic resonance (NMR) was
performed in CDCl3 with a Varian Unity-Inova 400 MHz
NMR spectrometer for assessment of chemical purity. UV–
visible spectra were collected with a Hitachi U-3000
spectrophotometer.
N N
UV
N
N
Vis, kT
2,20- and 3,30-azopyridine were synthesized according to
a literature method [16]. 1 g of 2-aminopyridine or
3-aminopyridine was dissolved in 20 mL of deionized
water, and the solution was added dropwise into 60 mL
NaClO solution (10–15% w/w) in an ice-water batch under
constant stirring in the dark. After one hour, the product
was extracted with ethyl ether (3 times, each time 50 mL).
The solvent was removed with a rotary evaporator, and the
product was further separated by column chromatography
(mobile phase: acetone/hexane (1:10); column: basic
Al2O3). After solvent removal by rotary evaporation, the
product was recrystallized from hexane to obtain 2,20-
azopyridine or 3,30-azopyridine. Purities by NMR: 92% for
2,20-azopyridine, [ 99% for 3,30-azopyridine.
trans – azobenzene
(trans – AB)
cis – azobenzene
(cis – AB)
N
N
N
N
2,2′ – azopyridine
(2,2′ – APy)
N
N
N
N
N
N
N
N
4,4′ – azopyridine
(4,4′ – APy)
3,3′ – azopyridine
(3,3′ – APy)
The mixture of azobenzene or an azopyridine with OTP
was prepared by weighing the two components into a
capped glass vial and heating the vial to obtain a homo-
geneous solution. For photo-isomerization, an azobenzene/
azopyridine-OTP solution was placed between two UV-
transmitting quartz slides (2.5 cm 9 2.5 cm) to form a
liquid film approximately 10 lm thick. This liquid film was
irradiated at room temperature with the 365 nm light from
a Spectroline ENF-240C UV Lamp for a few minutes to
several hours to obtain different cis isomer contents, and
the longest irradiation time used was enough to reach the
photo-stationary state. During irradiation, the sample was
placed on a large aluminum block, which provided efficient
heat dissipation and maintained the sample at the room
temperature. The sample remained fully liquid throughout
irradiation. The liquid was then transferred into a pre-
weighed Tzero Hermetic DSC pan with a capillary tube
under dim light. The filled DSC pan was weighed again (to
obtain the sample mass), sealed, and scanned to determine
the heat of isomerization. Before each heating scan, the
sample was cooled to approximately - 70 °C. A portion of
the sample was dissolved in ethanol or hexane, and the
solution’s UV–visible spectrum was recorded to calculate
the fraction of cis isomer based on the known extinction
coefficients of the isomers [12].
Scheme 1 Molecular structures of azobenzene (AB) and azopyridines
(APy). The cis–trans isomerization is indicated for azobenzene as an
example
produce MRI agents whose contrast is tunable by light
[15].
Despite the potential of azopyridine-based materials, the
current understanding of their photochemistry remains
limited. To our knowledge, there has been no report on the
isomerization energies of azopyridines, a fundamental
parameter for materials design. This missing information is
in contrast to the detailed characterization of azobenzene
systems. In this study, we report the isomerization energies
of all three symmetrical azopyridines (the 2,20-, 3,30-, and
4,40-isomers, Scheme 1). Isomerization energy was
obtained from the heat of isomerization measured in the
liquid state using a method validated against the literature
value on azobenzene. Theoretical calculations yielded
results in reasonable agreement with experiment and
explained the surprisingly low isomerization energy of
2,20-azopyridine on the basis of its closer approach to
planarity in the cis state.
Experimental methods
Ab initio calculations were performed with Gaussian 09
[17]. Isomerization energies were computed using G3MP2,
which provided accurate results for azobenzene [18].
Molecular geometries were visualized with VMD 1.9.2
[19].
Azobenzene ([ 99% pure), 4,40-azopyridine (NMR purity
96%), 2-aminopyridine ([ 99% pure), 3-aminopyridine
(99% pure), NaClO (10–15% w/w solution), and o-ter-
phenyl (OTP, 99% pure) were purchased from Sigma-
Aldrich and used as received.
Differential scanning calorimetry (DSC) was conducted
with a TA DSC Q2000 instrument with a typical sample
123

Trans–cis isomerization energies of azopyridines: a calorimetric and computational study
0.0
–0.1
–0.2
–0.3
–0.4
1200
800
400
0
(a)
Termal
isomerization
(AB)
(a)
Tg
35% trans
65% cis
35% trans
65% cis
100% trans
100% trans
350
350
350
400
400
400
450
500
550
600
–60 –40 –20
0
20 40 60 80 100 120 140 160
1000
–0.1
(b) (2,2′ – APy)
(b)
(c)
(d)
40% trans
60% cis
Tg
–0.2
–0.3
–0.4
40% trans
60% cis
500
100% trans
0
450
500
550
600
100% trans
–60
–30
0
30
60
90
120
150
180
1000
ε
(c) (3,3′ – APy)
0.0
–0.1
–0.2
–0.3
–0.4
30% trans
70% cis
500
Tg
30% trans
70% cis
100% trans
0
450
500
550
600
100% trans
90 120
400
–60
–30
0
30
60
150
180
(d)(4,4′ – APy)
–0.10
–0.15
–0.20
–0.25
–0.30
–0.35
73% trans
27% cis
200
Tg
73% trans
27% cis
100% trans
450 500
0
350
400
550
600
Wavelength/nm
100% trans
Fig. 1 UV–vis spectra of azobenzene/OTP (1:2 w/w), 2,20-azopy-
ridine/OTP (1:2), 3,30-azopyridine/OTP (7:93), and 4,40-azopyridine/
OTP (1:4). The first sample was dissolved in ethanol and the rest in
hexane. The y-axis is extinction coefficient based on azobenzene or
azopyridine concentration. OTP has no absorbance at k [ 300 nm.
(Color figure online)
–60 –40 –20
0
20 40 60 80 100 120 140 160
Temperature/°C
Fig. 2 DSC curves (exo up) of
a azobenzene/OTP (1:2); b
2,20-azopyridine/OTP (1:2); c 3,30-azopyridine/OTP (7:93); d 4,40-
azopyridine/OTP (1:4). Heating rate = 10 °C min-1. During the 1st
heating (red) the cis isomers transform to the trans near 110 °C
releasing heat. The exo- and endothermic events at 20–70 °C for
c and d result from crystallization and melting. (Color figure online)
Results and discussion
Experimental isomerization energies
the efficiency of the photochemical process). This was
accomplished by mixing azopyridine with o-terphenyl
(OTP), a non-crystallizing component. The amount of OTP
was chosen to be the smallest amount possible to suppress
crystallization. In this method, the irradiated sample was
measured by DSC directly, without first separating the cis
isomer by chromatography [5].
In this work, an azopyridine liquid was irradiated to con-
vert a fraction of the trans isomers to the cis and the pro-
duct was heated to measure the heat of thermal conversion
from the cis isomer to the trans. In order to maintain
azopyridine in the liquid state during irradiation, its crys-
tallization must be inhibited (crystals scatter light, reducing
123

M. Zhu, L. Yu
the known extinction coefficients of the isomers [16], the
fractions of the cis- and trans isomers in a solution can be
calculated. These values are indicated in Fig. 1 for the
various samples.
150
100
50
AB
3,3′ – APy
Figure 2 shows the DSC heating traces of the azoben-
zene/OTP and azopyridine/OTP mixtures (the same sam-
ples in Fig. 1). For each system, two curves are shown,
corresponding to the sample in the 100% trans state and the
sample in which a fraction of the trans isomers has been
converted to the cis. For both samples, the first thermal
event is the glass transition Tg.
4,4′ – APy
2,2′ – APy
In the case of azobenzene (Fig. 2a), the sample con-
taining the cis isomers shows a broad exothermic peak at
110 °C, whereas the 100% trans sample does not. This
event corresponds to the thermal conversion from the cis to
the trans isomer [5]. Integrating this peak gives the thermal
isomerization enthalpy, which under ambient pressure is
0
0
0.2
0.4
0.6
0.8
cis fraction
Fig. 3 Isomerization energy per gram vs cis isomer fraction for
azobenzene and 2,20-, 3,30-, and 4,40-azopyridines. (Color
figure online)
approximately the same as the isomerization energy, DEiso
.
In Fig. 3 (solid circles), we plot the DEiso value against the
cis fraction in the sample. The samples with different cis
fractions were obtained by varying the irradiation time. A
linear relation is seen between the two quantities, the slope
of which yields the isomerization energy: DEiso = 258
7 J g-1 = 47.0 1.3 kJ mol-1 for azobenzene. This
value agrees with the literature value of 48.2
0.3 kJ mol-1, also obtained by DSC for samples of pure
cis-azobenzene [5]. There are additional reports on the
azobenzene isomerization energy from other methods;
these values appear to be nearly independent of the
Our method requires that prior to heating, the degree of
photo-isomerization be known. Figure 1 shows the UV–
visible spectra collected for this purpose. For each system,
the lower curve corresponds to the non-irradiated sample
(trans isomers only) and the upper curve to the irradiated
sample (containing both trans and cis isomers). In this
spectral region, azopyridine molecules account for all the
absorbance, since OTP does not absorb in this region. The
peak near 450 nm is the n - p* transition [20], which is
nearly forbidden in the trans isomer and significantly
stronger in the cis isomer. From the spectra in Fig. 1 and
Table 1 Calculated gas-phase properties of azobenzene (AB) and azopyridines (APy)
DE/kJ mol-1
Torsion angle
C–N–N–C/degree
l/D
Calculated aver. l/D
Exp. l/D
Ref [4]
AB-trans
0
180
0
0
0.52
3.08
4.04
AB-cis
45.8
26.8
38.2
36.3
10.6
6.06
0
8.60
10.36
9.39
7.92
180
3.45
4.10
6.19
6.13
0
3.45
4.18
2,20-APy-cis-cc
2,20-APy-cis-fc
2,20-APy-cis-ff
2,20-APy-trans-cc
2,20-APy-trans-fc
2,20-APy-trans-ff
3,30-APy-cis-cc
3,30-APy-cis-fc
3,30-APy-cis-ff
3,30-APy-trans-cc
3,30-APy-trans-fc
3,30-APy-trans-ff
4,40-APy-cis
0.49
2.75
1.25
1.78
2.85
2.42
180
3.34
0
180
49.9
50.9
50.6
6.47
3.52
0
9.35
8.88
8.59
180
0.63
4.20
2.98
0
180
4.01
0
180
41.8
0
7.39
180
0.33
0
0.33
0
N/A
0.42
4,40-APy-trans
Bold font indicates the lowest energy of the three conformers (cc, fc, ff) for 2,20- or 3,30-azopyridine
123

Trans–cis isomerization energies of azopyridines: a calorimetric and computational study
60
cc
50
40
30
20
AB
3,3′ –APy
4,4′ –APy
2,2′ –APy
fc
10
0
0
10
20
30
40
50
60
ΔEiso,comp/KJ mol–1
Fig. 5 Comparison of calculated and experimental isomerization
energies. Circles are results from this work. Standard deviations of
experimental DE are shown or are smaller than the symbol size.
Triangle indicates the literature experimental value of DEiso [18].
(Color figure online)
ff
azopyridine crystallizes more rapidly than the other two
compounds, necessitating a higher concentration of OTP
(93 vs. 67% w/w). Even at a higher OTP concentration,
crystallization still took place during the DSC scan. In the
100% trans sample, crystallization occurs near 30 °C and
the resulting crystals melt near 60 °C. In the sample con-
taining the cis isomer, crystallization occurs near 20 °C
and crystal melting at 40 °C. All these events were con-
firmed by optical microscopy on a temperature-controlled
stage. Despite the complications from crystallization and
melting, the thermal isomerization is still clearly observed
in the liquid state. This event also occurs near 110 °C, prior
to which the system has entered the liquid state (after
crystallization and melting). Applying the same procedure
as above, we obtain DEiso = 231 3 J g-1 = 42.6
0.6 kJ mol-1 for 3,30-azopyridine.
Fig. 4 Illustraion of the cc, fc, and ff conformers with cis 2,20-
azopyridine as an example. (Color figure online)
physical state of the sample (gas, solution, or crystal),
falling in the range 47 3 kJ mol-1 [5, 18, 21, 22]. The
consistency between our result and the literature results
validates our method.
A noteworthy feature in Fig. 2a is the small but real
difference of Tg between the two traces. This difference
indicates that the liquid enriched in the cis isomer has a
slightly higher Tg. The effect is also observed with the
azopyridines (see below).
For 2,20-azopyridine (Fig. 2b), we observe essentially
the same features as azobenzene (Fig. 2a). Figure 3 (red
squares) shows how the heat of isomerization changes with
the fraction of the cis isomer. By the same procedure, we
obtain DEiso = 137 3 J g-1 = 25.2 0.6 kJ mol-1 for
2,20-azopyridine.
The situation for 4,40-azopyridine (Fig. 2d) is similar to
that of 3,30-azopyridine just described (Fig. 2c). Here the
100% trans sample showed weak thermal events corre-
sponding to crystallization (20 °C) and crystal melting
(45 °C), whereas the cis-containing sample was free of
these events. For this system, we obtain DEiso = 190
10 J g-1 = 35.0 1.8 kJ mol-1
.
Viewing the above
results together, we note that the thermal conversions of the
cis isomers to the trans all occur at approximately the same
temperatures, indicating a similar kinetic barrier for iso-
merization. This similarity is not surprising given the
similar structures of these molecules. There is a significant
variation in the DEiso values within this group of mole-
cules; the value for 2,20-azopyridine is almost half of the
For 3,30-azopyridine (Fig. 2c), the essential features of
the DSC results are still similar to those of azobenzene
(Fig. 2a) and 2,20-azopyridine (Fig. 2b), with the exception
that crystallization occurred during the DSC analysis. This
123

M. Zhu, L. Yu
Table 2 Torsion angles for
azobenzene (AB) and
azopyridines (APy)
h (C–N–N–C)/degree
u1/degree
u2/degree
cosh ? cosu1 ? cosu2
AB-cis
8.60
10.36
9.39
7.92
9.35
8.88
8.59
7.39
51.81
48.67
42.01
63.23
53.12
53.74
52.29
56.21
51.81
48.67
59.87
63.23
53.12
49.75
52.29
56.21
2.23
2.30
2.23
1.89
2.19
2.23
2.21
2.10
2,20-APy-cis-cc
2,20-APy-cis-fc
2,20-APy-cis-ff
3,30-APy-cis-cc
3,30-APy-cis-fc
3,30-APy-cis-ff
4,40-APy-cis
u1 and u2 are the torsion angles of the aromatic rings relative to the azo bond
values for the other systems. This result was studied further
through theoretical calculations (see below).
isomerization temperature 398 K, in good agreement with
25.2 kJ mol-1 from experiment. For 3,30-azopyridine, the
same procedure yields DEiso = 48.7 kJ mol-1 at 393 K,
compared to 42.6 kJ mol-1 from experiment, a fair
agreement. In Fig. 5, the foregoing comparison between
theory and experiment is shown graphically. Overall, there
is a broad agreement between theory and experiment. The
residual difference is likely a result of the combined errors
in both methods.
Quantum chemical calculations
Table 1 shows the results of G3MP2 calculations for the
molecules of this study. For each azo compound, calcula-
tions were performed for both the cis isomer and the trans
isomer in the gas phase. For 2,20- and 3,30-azopyridines,
each cis or trans configuration must be further specified
depending on whether the pyridine N is close (c) or far
(f) relative to the central N = N bond, resulting in the cc,
fc, and ff combinations, as illustrated in Fig. 4 [4]. This
complexity does not arise for azobenzene or 4,40-azopy-
ridine because of their symmetry. In Table 1, E is the
energy at 298.15 K, and DE is the energy of a given con-
former relative to the lowest energy conformer.
It is noteworthy that G3MP2 can reproduce the surpris-
ingly low DEiso for 2,20-azopyridine. According to the cal-
culations, the cc isomer of cis 2,20-azopyridine has especially
low energy among its three conformers (cc, fc, and ff). In this
conformer, the two aromatic rings approach coplanarity the
closest. This can be seen by summing the cosines of the three
torsional angles that describe the departure from planarity at
the three sites of torsion (Table 2). For a trans isomer (pla-
nar), this sum is three. For a cis isomer, this value is between
zero and three. In the cc conformer of cis-2,20-azopyridine,
this sum is the largest (2.3), suggesting the closest approach
to coplanarity and the greatest extent of p conjugation among
all the cis isomers. This presumably leads to a lower con-
formational energy [25, 26]. The ability for the cc conformer
of cis 2,20-azopyridine to closely approach coplanarity is
likely a result of the smaller van der Waals volume of the
pyridine N relative to the phenyl CH.
For azobenzene, our results reproduce the literature
values [18]. The calculated isomerization energy
DEiso = 45.8 kJ mol-1 agrees with the experimental value
in the gas phase (47.4 3.2 kJ mol-1) [18]. The torsion
angles C–N–N–C (180° for the trans and 8.60° for the cis)
agree with those in the crystals (180° and 7.68°) [23, 24].
For each azopyridine, the trans isomer has planar
geometry, as expected for a system attempting to optimize
the delocalization of p electrons. For the trans isomers, the
dipole moments are expected to vanish by symmetry
except for the fc conformer of 2,20- or 3,30- azopyridine;
this is indeed the case. For the cis isomers, the C–N–N–C
torsion angles are about 9°, similar to that of azobenzene.
Concerning the isomerization energies, the calculated
value for 4,40-azopyridine (41.8 kJ mol-1) is in fair
agreement with the experimental value (35.0 kJ mol-1).
For 2,20- or 3,30-azopyridine, each trans or cis isomer has
multiple conformers (cc, fc, ff). Assuming Boltzmann
distribution between the three conformers and noting their
degeneracies (1 for cc and ff; 2 for fc), we obtain
DEiso = 26.1 kJ mol-1 for 2,20-azopyridine at its
Table 1 shows the calculated dipole moment at
298.15 K for each isomer of the 3 azopyridines. For an
isomer that has multiple conformers, an average dipole
moment is computed assuming Boltzmann distribution
between the conformers. The results agree reasonably well
with the experimental data [4]. The residual difference is
likely a result of the combined errors in both techniques.
Bullock et al. noted that the dipole moment should vanish
for trans azobenzene and trans 4,40-azopyridine, but their
experimental values are nonzero. This could be a result of
contamination from the cis isomer, perhaps formed in
solutions not in complete darkness [4].
123

Trans–cis isomerization energies of azopyridines: a calorimetric and computational study
11. Han K, Su W, Zhong M, Yan Q, Luo Y, Zhang Q, Li Y. Rev-
Conclusions
ersible photocontrolled swelling-shrinking behavior of micron
vesicles self-assembled from azopyridine-containing diblock
copolymer. Macromol Rapid Commun. 2008;29:1866–70.
12. Lin L, Yan Z, Gu J, Zhang Y, Feng Z, Yu Y. UV-responsive
behavior of azopyridine-containing diblock copolymeric vesicles:
photoinduced fusion, disintegration and rearrangement. Macro-
mol Rapid Commun. 2009;30:1089–93.
13. Aoki K, Nakagawa M, Ichimura K. Self-assembly of amphoteric
azopyridine carboxylic acids: organized structures and macro-
scopic organized morphology influenced by heat, pH change, and
light. J Am Chem Soc. 2000;122:10997–1004.
In this work we obtained the isomerization energies of all
three symmetrical azopyridines for the first time by
experiment and quantum chemical calculations. Our
method was validated against the literature value on
azobenzene. The computational results are in reasonable
agreement with experiment and explain the low isomer-
ization energy of 2,20-azopyridine on the basis of a low-
energy cis conformer. Because of the smaller van der
Waals volume of the pyridine N relative to the phenyl CH,
the two aromatic rings in the cc cis isomer can approach
closer to coplanarity, leading to greater p-conjugation and
lower conformational energy. Our results are relevant for
developing azopyridine-based materials. Given their ability
to form hydrogen bonds and metal coordination bonds,
azopyridine molecules may expand the design space for
photo-responsive materials.
14. Baldwin DA, Lever ABP, Parish RV. Complexes of 2,20-azopy-
ridine with iron(II), cobalt(II), nickel(II), copper(I), and cop-
per(II). Infrared study. Inorg Chem. 1969;8:107–15.
¨
15. Dommaschk M, Peters M, Gutzeit F, Schu¨tt C, Nather C,
Sonnichsen FD, Tiwari S, Riedel C, Boretius S, Herges R. Pho-
¨
toswitchable magnetic resonance imaging contrast by improved
light-driven coordination-induced spin state switch. J Am Chem
Soc. 2015;137:7552–5.
16. Brown EV, Granneman GR. Cis–trans isomerism in the pyridyl
analogs of azobenzene. Kinetic and molecular orbital analysis.
J Am Chem Soc. 1975;97:621–7.
17. Frisch MJ, et al. Gaussian 09, revision E.01. Wallingford:
Gaussian, Inc.; 2009.
18. Cammenga HK, Emel’yanenko VN, Verevkin SP. Re-investiga-
tion and data assessment of the isomerization and 2,20-cyclization
of stilbenes and azobenzenes. Ind Eng Chem Res.
2009;48:10120–8.
Acknowledgements We thank the National Science Foundation for
supporting this work through Grant No. DMR-1234320 and through
the University of Wisconsin—Madison MRSEC (DMR-1720415).
19. Humphrey W, Dalke A, Schulten K. VMD—visual molecular
dynamics. J. Molec. Graphics. 1996;14:33–8.
20. Cusati T, Granucci G, Persico M, Spighi G. Oscillator strength
References
1. Hartley GS. The cis-form of azobenzene. Nature. 1937;140:281.
2. Bandara HMD, Burdette SC. Photoisomerization in different
classes of azobenzene. Chem Soc Rev. 2012;41:1809–25.
3. Yager KG, Barrett CJ. Novel photo-switching using azobenzene
functional materials.
2006;182:250–61.
4. Bullock DJW, Cumper CWN, Vogel AI. Physical properties and
chemical constitution. Part 63. The electric dipole moments of
azobenzene, azopyridines, and azoquinolines. J Chem Soc.
1965;5316–23.
5. Wolf E, Cammenga HK. Thermodynamic and kinetic investiga-
tion of the thermal isomerization of cis-azobenzene. Z. Phys.
Chem. (Neue Folge). 1977;107:21–38.
6. Yamada M, Kondo M, Mamiya JI, Yu Y, Kinoshita M, Barrett
CJ, Ikeda T. Photomobile polymer materials: towards light-driven
plastic motors. Angew Chem Int Ed. 2008;47:4986–8.
7. Natansohn A, Rochon P. Photoinduced motions in azo-containing
polymers. Chem Rev. 2002;102:4139–76.
8. Banghart MR, Mourot A, Fortin DL, Yao JZ, Kramer RH,
Trauner D. Photochromic blockers of voltage-gated potassium
channels. Angew Chem Int Ed. 2009;48:9097–101.
and polarization of the forbidden n- [ p* band of trans-
azobenzene:
2008;128:194312.
a
computational study.
J
Chem Phys.
21. Dias AR, da Piedade MEM, Simoes JAM, Simoni JA, Teixeira C,
Diogo HP, Yang M, Pilcher G. Enthalpies of formation of cis-
J Photochem Photobiol A: Chem.
azobenzene and trans-azobenzene.
1992;24:439–47.
J
Chem Thermodyn.
22. Adamson AW, Vogler A, Kunkely H, Wachter R. Pho-
tocalorimetry. Enthalpies of photolysis of trans-azobenzene, fer-
rioxalate and cobaltioxalate ions, chromium hexacarbonyl,
dirhenium dicarbonyl. J Am Chem Soc. 1978;100:1298–300.
23. Harada J, Ogawa K. X-ray diffraction analysis of nonequilibrium
states in crystals: observation of an unstable conformer in flash-
cooled crystals. J Am Chem Soc. 2004;126:3539–44.
24. Mostad A, Rømming C. A refinement of the crystal structure of
cis-azobenzene. Acta Chem Scand. 1971;25:3561–8.
25. Ishikawa T, Noro T, Shoda T. Theoretical study on the photoi-
somerization of azobenzene. J. Chem. Phys. 2001;115:7503–12.
26. Cembran A, Bernardi F, Garavelli 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;126:3234–43.
9. Campbell N, Henderson AW, Taylor D. Geometrical Isomerism
of Azo-compounds. J. Chem. Soc. 1953;1281–5.
10. Cui L, Zhao Y. Azopyridine side chain polymers: an efficient
way to prepare photoactive liquid crystalline materials through
self-assembly. Chem Mater. 2004;16:2076–82.
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