Photoinduced formation of an azobenzene-based CD-active supramolecular cyclic dimer

Article abstract of DOI:10.1002/chem.201406054

A series of new photo-responsive amino acid-derived azobenzenedicarboxylic acid derivatives (S)-a-e were synthesized. Compound (S)-a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)-a] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)-a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis-azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)-4, an analogue of (S)-b, confirmed the formation of a dimer. A theoretical CD study revealed that (S)-a in the cis form should be present as a cyclic chiral dimer.

Full text of DOI:10.1002/chem.201406054

DOI: 10.1002/chem.201406054
Full Paper
&
Supramolecular Chemistry
Photoinduced Formation of an Azobenzene-Based CD-Active
Supramolecular Cyclic Dimer**
[
a, b]
[c]
[d]
[e]
[f]
Hiromitsu Sogawa,
Kayo Terada, Yu Miyagi, Masashi Shiotsuki, Yoshihito Inai,
[g]
[d]
Toshio Masuda, and Fumio Sanda*
Abstract: A series of new photo-responsive amino acid-de-
rived azobenzenedicarboxylic acid derivatives (S)-1a–e were
synthesized. Compound (S)-1a in the trans form exhibited
no circular dichroism (CD) signal in DMF under ambient con-
ditions, whereas intense Cotton effects were observed upon
UV irradiation, indicating the formation of a chiral supra-
molecular structure in the cis form. The CD signals disap-
peared when trifluoroacetic acid (TFA) was added to the so-
lution. The ester counterpart [(S)-1a’] showed no CD signal.
Hydrogen bonding between the carboxy groups seemed
necessary for constructing the supramolecular structure. The
kinetic studies of cis to trans isomerization of (S)-1a demon-
strated that the formation of a chiral supramolecule enhan-
ces the stability of the cis-azobenzene structure. The ESI
mass spectrum of stilbenedicarboxylic acid (S)-4, an ana-
logue of (S)-1b, confirmed the formation of a dimer. A theo-
retical CD study revealed that (S)-1a in the cis form should
be present as a cyclic chiral dimer.
Introduction
Stimuli-responsive molecules, which adopt two and more iso-
meric forms having different structures and properties, gather
much attention due to their potential applicability in optical
[
a] Dr. H. Sogawa
Department of Polymer Chemistry
Graduate School of Engineering, Kyoto University
Katsura Campus, Nishikyo-ku
Kyoto 615-8510 (Japan)
[
1,2]
[3–6]
memory devices
and molecular machines.
Among the
various external stimuli, light has several advantages, including
easy control of the irradiation wavelength, intensity, and time.
Over the past few decades, azobenzene has been widely re-
searched as one of the most promising photochromic mole-
cules undergoing trans-to-cis isomerization with large confor-
[b] Dr. H. Sogawa
Present Address: Department of Organic and
Polymeric Materials, Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8552 (Japan)
[
c] Dr. K. Terada
[7]
Graduate School of Materials Science
Nara Institute of Science and Technology
Takayama-cho, Ikoma 630-0192 (Japan)
mational and polarity changes. A large variety of azoben-
zene-containing photo-responsive systems have been reported
[8]
[9]
so far, including molecular recognition, catalysis, sol–gel for-
[
d] Y. Miyagi, Prof. Dr. F. Sanda
[10,11]
[1,2,4,12–18]
mation,
etc.
Department of Chemistry and Materials Engineering
Faculty of Chemistry, Materials and Bioengineering
Kansai University, 3-3-35 Yamate-cho
Suita, Osaka 564-8680 (Japan)
Meanwhile, molecules consisting of simple building blocks
that can form highly sophisticated structures fascinate re-
searchers with interests in supramolecular chemistry and nano-
materials. Noncovalent interactions including hydrogen bond-
E-mail: sanda@kansai-u.ac.jp
[e] Prof. Dr. M. Shiotsuki
[19–24]
[25–28]
[29]
ing,
p-stacking,
and coordination bonding are com-
Faculty of Engineering, Tokyo City University
Tamadutsumi 1-28-1, Setagaya-ku
Tokyo 158-8557 (Japan)
monly used to construct self-assembled structures consisting
of two or more monomeric units. In particular, hydrogen-
bonded supramolecules form a wide range of structures such
[
f] Prof. Dr. Y. Inai
Department of Frontier Materials
Shikumi College, Graduate School of Engineering
Nagoya Institute of Technology, Gokiso-cho
Showa-ku, Nagoya 466-8555 (Japan)
[19]
[20,21]
[22–24]
as helices, linear tapes,
and so on,
some of which
[19]
exhibit interesting features such as chiral amplification,
[20]
[22]
a sol–gel transition,
and enhanced electroluminescence.
[
g] Prof. Dr. T. Masuda
Although many researchers have synthesized azobenzene-de-
rived photo-responsive supramolecules that form two different
structures, utilization of a cis-azobenzene moiety as a building
Department of Polymer Materials
School of Materials Science and Engineering
Shanghai University, Materials Building
Nanchen 333, Shanghai 200444 (P.R. China)
[30–32]
block is limited.
This is presumably because cis-azoben-
zene gradually converts to the trans form under visible light at
room temperature, thereby making the construction of stable
supramolecules difficult. Therefore, most reports focus their at-
[
**] CD=Circular dichroism.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406054.
Chem. Eur. J. 2015, 21, 6747 – 6755
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tention on the degradation behavior of well-defined structures
by trans-to-cis photo-isomerization. Sleiman and co-workers
have synthesized cyclic tetramers of p-azobenzenedicarboxylic
were synthesized by condensation of 3,3’-azobenzenedicarbox-
ylic acid with various amino acid methyl esters. Compound (S)-
2 was synthesized from 3,3’-azobenzenedicarboxylic acid and
l-lactic acid methyl ester in a manner similar to (S)-1a; trans-
(S)-4 and cis-(S)-4 were synthesized by the reaction of trans-
and cis-3,3’-stilbenedicarboxylic acid, respectively, with l-leu-
cine methyl ester hydrochloride.
[
33]
acid derivatives in their cis forms. This is a quite rare exam-
ple of a molecular assembly formed of cis-azobenzene-based
units as well-defined building blocks.
Amino acids are naturally derived optically active reagents,
which are useful as chiral auxiliaries in asymmetric organic syn-
thesis. Various attempts have been made to utilize amino
acids, not only for peptide synthesis, but also as key compo-
nents in self-assembled molecules to enhance biodegradabil-
The CD and UV/Vis absorption spectra of (S)-1a were mea-
sured in DMF at room temperature with UV (300 nm<l<
400 nm) irradiation. As shown in Figure 2, compound (S)-1a
[
34]
[30,31]
ity,
gel formation,
and regulated chiral higher-order
structures based on hydrogen-bonding ability and optical ac-
[
35]
tivity. Sanders and co-workers have assembled helical nano-
tubes consisting of N,N’-dimethylnaphthalenediimide having
pendant carboxy groups derived from amino acids, in which
the tubular structure is stabilized by hydrogen bonding be-
[36]
tween the carboxy groups.
In the present article, we report the synthesis of new amino
acid-derived azobenzenedicarboxylic acid derivatives 1a–
e (Figure 1), and demonstrate the formation of chiral supra-
molecular structures in a cis form, a rare example among azo-
benzene-derived supramolecules reported thus far. The photo-
isomerization upon UV and visible-light irradiation and thermal
isomerization of azobenzene moieties accompanying the for-
mation of CD-active structures were monitored by CD and UV/
Vis spectroscopies. We also estimated the probable self-assem-
bled structures by using conformational analysis and CD simu-
lation, together with the comparison with trans- and cis-stilbe-
nedicarboxylic acid derivatives.
Figure 2. CD and UV/Vis absorption spectra of (S)-1a measured in DMF
(c=0.05 mm) at 208C after irradiation at 300<l<400 nm. Arrows indicate
increasing irradiation time.
exhibited a strong UV/Vis absorption at 320 nm, which is at-
tributable to the p–p* transition band of trans-azobenzene
units. The intensity decreased upon UV irradiation, whereas
a new peak appeared at 420 nm corresponding to the n–p*
transition band of cis-azobenzene units. A photo-stationary
state was reached at 16 min, at which the trans/cis ratio of azo-
Results and Discussion
Compounds (R)-/(S)-1a–e were synthesized by alkaline hydroly-
sis of the corresponding methyl esters (R)-/(S)-1a’–e’, which
[7e,37]
benzene units was 28/72 from UV/Vis spectroscopy.
taneously, compound (S)-1a showed an intense CD signal at
75 and 420 nm corresponding to the p–p* and n–p* transi-
tion bands of cis-azobenzene units, respectively, upon photo-ir-
Simul-
2
[38]
radiation. These results indicate that the azobenzene moiety
of (S)-1a isomerized from trans to cis, with concomitant forma-
tion of a chiral supramolecular structure, which will be con-
cluded later as a supramolecular cyclic dimer.
The CD signal of (S)-1a disappeared after the UV irradiation
was stopped and 1 vol% of trifluoroacetic acid (TFA) was
added. On the other hand, (S)-1a’ showed no CD signals upon
UV irradiation, indicating the absence of a chiral higher-order
structure. These results suggest that cis-(S)-1a forms a chiral
supramolecular structure by hydrogen bonding between the
carboxy groups in a manner similar to p-azobenzenedicarbox-
[33]
ylic acid. However, compound (S)-3, which was synthesized
from p-azobezenedicarboxylic acid, showed no CD signal
either before or after UV irradiation (the Supporting Informa-
[39]
tion, Figure S1).
Thus, in the present case, the 3,3’-linked
azobenzene structure should be indispensable to the forma-
tion of a chiral supramolecule in contrast to the behavior of
Figure 1. Structures of azobenzenedicarboxylic acid and stilbenedicarboxylic
acid derivatives.
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the structure formed in the case of p-azobenzenedicarboxylic
sembled structure, as mentioned above. It is presumed that
hydrogen bonds irregularly form between amide groups as
well as carboxy groups in the high concentration region, and
thereby disordered self-assemblies form, leading to small CD
signals.
[
33]
acid.
Compound (S)-2, having ester groups in place of amide
groups, was synthesized to confirm the effect of amide groups
on the formation of a chiral supramolecular structure. Al-
though (S)-2 showed no CD signal before UV irradiation, a CD
signal was observed after irradiation, but the intensity of the
photoinduced CD signal was weak compared with that of (S)-
The CD and UV/Vis absorption spectra of (S)-1a returned to
the patterns before UV irradiation after the sample had been
subjected to light in the visible region, confirming that the
azobenzene moieties of (S)-1a underwent reversible cis-to-
trans photo-isomerization with accompanying assembly–disas-
sembly of a chiral higher-order structure (the Supporting Infor-
mation, Figure S2).
1
a (Figure 3). Meanwhile, it is estimated that the rotational
To investigate the substituent effect on molecular assembly,
we next measured the CD and UV/Vis absorption spectra of
(
S)-1b–e and (R)-1a,b after UV irradiation for 16 min. Before UV
irradiation, all the compounds showed negligibly small CD sig-
nals. They reached photo-stationary states after UV irradiation
for 16 min, wherein the trans/cis ratios ranged from 34/68 to
[42]
2
5/75, similar to that of (S)-1a (trans/cis=28/72). As depict-
ed in Figure 5, (S)- and (R)-1a, and (S)- and (R)-1b exhibited re-
spective mirror-image CD signals, as expected from the oppo-
sitely signed optical rotations.
Figure 3. CD and UV/Vis absorption spectra of (S)-1a and (S)-2 measured in
DMF (c=0.05 mm) at 208C after irradiation at 300<l<400 nm for 16 min.
À1
barrier for C(=O)ÀNH is 3.6 kJmol larger than that of C(=O)À
[40]
O, which possibly leads to the difference of conformational
stability. It is likely that the difference between (S)-1a and (S)-2
arises from the difference of rotatability of the amide and ester
[41]
linkages, and hydrogen bonding involving amide groups.
Figure 4 plots the concentration dependence of the CD
signal intensity of (S)-1a induced by UV irradiation. The [q]420nm
gradually increased as the sample concentration was increased
up to 0.05 mm, but it decreased when the concentration was
increased above 0.05 mm. It is suggested that the chiral supra-
molecular structure of (S)-1a is formed most efficiently at
Figure 5. CD and UV/Vis absorption spectra of (S)-/(R)-1a,b (A) and (S)-1c–
e (B) measured in DMF (c=0.05 mm) at 208C after irradiation at
300<l<400 nm for 16 min.
0.05 mm. Well-regulated hydrogen bonding between carboxy
groups seems to be a key factor for constructing a chirally as-
The chirality of the supramolecular structure was controlla-
ble by the chirality of the amino acid. Aspartic acid-derived (S)-
c, having ester pendants, showed CD signals with almost the
1
same intensity as those of (S)-1a and (S)-1b with aliphatic in-
stead of ester groups. Thus, it seems that the ester groups do
not participate in hydrogen bonding. On the other hand, phe-
nylalanine-derived (S)-1d showed CD signals almost twice as
strong as those of (S)-1a. Serine-derived (S)-1e having hydroxy
groups showed weak CD signals compared with (S)-1a. It is
presumed that the hydroxy groups disturb the formation of
regulated hydrogen bonding between the carboxy groups to
some extent, resulting in less efficient formation of molecular
assembly.
Figure 6 shows the CD and UV/Vis absorption spectra of (S)-
Figure 4. Plot of [q] at 420 nm of (S)-1a measured in DMF at various concen-
trations at 208C after irradiation at 300<l<400 nm for 16 min.
1b measured in DMF/H O mixtures with varying compositions
2
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bility of the cis form, resulting in the slow cis-to-trans thermal
isomerization in DMF compared with that in CH OH. This is
3
one of the rare examples of stabilization of cis-azobenzene
[46]
units by molecular assembly.
Although (S)-1a maintains the relatively stable cis form to
some extent, it is not sufficiently long-lived to examine the
presence of a chiral supramolecular structure by methods
other than CD spectroscopy. Since cis-stilbene stably exists and
hardly isomerizes into a trans form under ambient condi-
[47–49]
tions,
the stilbenedicarboxylic acid derivative cis-(S)-4 was
synthesized as a model for (S)-1a to check the self-assembly
property, and trans-(S)-4 was also synthesized for comparison.
As shown in Figure 7, trans-(S)-4 showed a strong absorption
Figure 6. CD and UV/Vis absorption spectra of (S)-1b after irradiation at
00<l<400 nm for 16 min measured in DMF/H O mixtures (c=0.05 mm)
at 208C.
3
2
after UV irradiation for 16 min. The positive first and negative
second Cotton effects in DMF gradually inverted to negative
[43]
first and positive second effects with increasing H O content.
2
Compound (S)-1b showed intense CD signals also in CH CN,
3
DMF/CHCl , DMF/toluene, but not in CH OH and THF (the Sup-
3
3
porting Information, Figure S3).
Kinetic studies were carried out to evaluate the rate con-
stants of thermal cis-to-trans isomerization of the azobenzene
moiety. The time–isomerization courses of (S)-1a, (S)-1a’, (S)-3,
and 3,3’-azobenzene dicarboxylic acid were monitored by UV/
Vis spectroscopy in DMF and CH OH at 308C, and the rate con-
3
[
44]
stants were determined (Table 1). With the exception of (S)-
Figure 7. CD and UV/Vis absorption spectra of trans-(S)-4 and cis-(S)-4 to-
gether with (S)-1b after UV irradiation for 16 min measured in DMF
(c=0.05 mm) at 208C.
Table 1. First-order conversion rate constants for the cis-to-trans thermal
isomerization.
6
À1
Compound
Rate constant of thermal transition k”10 [s
In DMF at T [K] In CH OH at T [K]
03.15 313.15 323.15 303.15 313.15 323.15
]
3
peak assignable to the p–p* transition at 305 nm, but a negligi-
bly small CD signal at that wavelength. It is considered that
trans-(S)-4 does not form a chirally regulated aggregate in
a manner similar to trans-(S)-1a. On the other hand, cis-(S)-4
showed an intense CD signal around 280 nm, although the
corresponding molar absorptivity (e) was less than half that of
trans-(S)-4. The shorter lmax and smaller e of the p–p* absorp-
tion of the cis isomer compared to the trans counterpart are in
good agreement with the behavior of other stilbene deriva-
3
(
(
(
S)-1a
S)-1a’
S)-3
1.10
3.34
4.92
1.99
3.55
10.4
13.3
26.5
1.56
1.19
3.74
1.38
5.00
4.95
17.3
12.5
[
a]
[a]
[a]
[a]
–
–
–
–
–
[
a]
[a]
[a]
[a]
3
,3’-azobenzene
–
–
–
dicarboxylic acid
[a] Not determined.
[50,51]
tives.
The CD spectroscopic pattern of cis-(S)-4 around
1
a, the first-order rate constant was larger in DMF than in
280 nm agreed well with that of cis-(S)-1b, which was formed
by UV irradiation in DMF. These results indicate that cis-(S)-4
also forms a chiral supramolecular structure similar to cis-(S)-
CH OH. As mentioned above, compound (S)-1a forms a chiral
supramolecular structure by UV irradiation in DMF but not in
3
[52]
CH OH. The activation parameters of (S)-1a were determined
according to the Eyring Equation. The DH , DS , and DG in
1b.
3
[
45]
°
°
°
The CD and UV/Vis absorption spectra of cis-(S)-4 were mea-
sured at 10–508C to check the temperature dependence
(Figure 8). The intensity of the Cotton effect gradually de-
creased as the temperature increased, but not to a great
extent. An intense Cotton effect remained at 508C (68% of the
value at 108C) without an accompanying UV/Vis spectroscopic
change. This result suggests that the chiral supramolecular
structure was partly collapsed by heating without thermal iso-
merization of the stilbene moiety.
À1
DMF at 303 K were determined to be 98.7 kJmol ,
À1 À1
À1
À41.8 Jmol
K
and 111.3 kJmol , and those in CH OH were
3
À1
À1 À1
À1
9
5.0 kJmol , À33.5 Jmol
K
and 105.4 kJmol , respectively.
These values are comparable to those of the azobenzene de-
rivatives that adopt unusually stable cis forms due to steric hin-
[
9]
°
drance of cyclic structures. The DG of (S)-1a in DMF was
larger than that in CH OH. It is considered that the formation
3
of a chiral supramolecular assembly enhances the relative sta-
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Figure 8. CD and UV/Vis absorption spectra of cis-(S)-4 measured in DMF
(c=0.05 mm) at 10–508C.
Figure 9 depicts the ESI mass spectra of trans-(S)-4 and cis-
(S)-4. The trans-(S)-4 showed comparatively recognizable peaks
according to monomer, dimer, trimer, and tetramer. On the
other hand, cis-(S)-4 clearly showed peaks attributable to mo-
nomer and dimer, whereas the peaks of trimer and tetramer
were negligibly small. Dicarboxylic acids possibly assemble
into both linear and multimeric cyclic structures, wherein the
dominant structure depends on the shape of the molecular
[
24,37,53]
backbone.
It is likely that trans-(S)-4 molecules favorably
form oligomeric linear chains, whereas cis-(S)-4 molecules
prefer to form cyclic dimers through hydrogen bonding as
shown in Figure 10. The different aggregation fashion of cis-
Figure 10. Possible aggregated structures of trans-(S)-4 and cis-(S)-4.
(
S)-4 compared with trans-(S)-4 seems to be the key factor for
supramolecule assisted by hydrogen bonding, which shows in-
[
54–56]
[57]
forming a chiral supramolecular structure.
This result is
tense CD signals. In the present study, it is assumed that the
similar to that of a 1208 spacer-linked ditopic hydrazide-based
molecular chirality of the cyclic dimer is brought about by the
asymmetrically twisted conformation, leading to the intense
CD signals based on the chirally positioned chromophores. De-
tailed conformational studies are discussed below.
Energy minimization of the cyclic dimer cis-(S)-1a or cis-(R)-
[58]
1
a led to a rectangular form, as depicted in Figure 11. Their
energy values, dipole moments, and alanine dihedral angles
[59]
[59–63]
(
f) are listed in Table 2.
Based on the four f values, the
dimers of structures #1 and #2 both show a roughly enantio-
meric relation, whereas they should be completely in mirror
symmetry. A small difference between the two structures may
have resulted from the energy minimization procedure used
here. Each monomer molecule in the dimer has two alanine
Figure 11. Structures of dimers cis-(S)-1a or cis-(R)-1a (#’s 1–3), and of mon-
omers cis-(S)-1a (#’s 4–7): These structures correspond to #’s 1–7 in Table 2,
respectively. Each arrow also represents the dipole moment in Table 2. For
the molecular graphics, see ref. [58].
Figure 9. ESI mass spectra of trans-(S)-4 and cis-(S)-4. Samples were prepared
3 3
as solutions in CHCl /CH OH=1/1 (v/v, 0.20 mm) and injected into the ESI
compartment.
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Table 2. Structural properties of dimers and monomers of cis-1a.
[
b]
[c]
Structure
Species of
cis-1a
State
Energy difference from #1 (for dimers)
Dipole moment (D)
f
f
1
1
(M
(M
1
2
)/f
)/f
2
(M
(M
1
2
)
)
[
a]
[c]
À1
[d]
or #4 (for monomers) [kJmol
]
2
#
#
#
#
#
1
2
3
4
5
(S)-dimer
optimized from Type I
optimized from Type I
Type II
0.0
0.89
2.22
8.57
6.04
5.75
À166/À94
[
À2889.154825]
À164/+79
+166/+98
(R)-dimer
1.8
+
161/À74
+179/À175
166/+84
(S)-dimer
227.5
+
[
e]
e]
(S)-monomer
(S)-monomer
optimized from monomer #5
1st one extracted from dimer #1
0.0
À159/À89
[
À1444.558097]
[
4.1
À166/À94
À160/+73
À164/+79
[
[
e]
e]
#6
7
(S)-monomer
(S)-monomer
optimized from monomer #7
2nd one extracted from dimer #1
27.0
31.7
5.11
5.36
#
[
a] (S)-Monomer, cis-(S)-1a; (S)- or (R)-dimer, dimer of cis-(S)-1a or cis-(R)-1a. [b] Geometry optimization was performed with MM computation using
[60]
MMFF94. In #3, a glycine analogue of cis-(S)-1a was geometry-optimized from the Type II backbone, and then the two glycine moieties were replaced
by (S)-alanine through side chain modification. [c] Energies and dipole moments were evaluated using the single-point DFT calculation in Gaussian 03
from the respective structure. The values in brackets are energies in Hartree obtained from the DFT calculation. [d] Two dihedral angles f and f [8] of
1 2
alanine in one monomer unit are defined as C-N-C -C. [e] Monomers #’s 5 and 7 were extracted from the two monomer units (1st and 2nd, respectively)
[61,62]
[63]
a
[59]
of dimer #1. Monomers #’s 4 and 6 were geometry-optimized from #’s 5 and 7, respectively.
moieties that adopt a fully-extended conformation (fꢀ1808)
and a semi-extended conformation (fꢀ Æ908). Here, the two
carbonyl groups in each monomer molecule are arranged with
an up-down orientation (designated as the “Type I” conforma-
tion in Table 2). The Type II conformation (#3), in which the
two carbonyl groups of one molecule are arranged with a simi-
lar direction, was also modeled. On the whole, a non-flat cyclic
structure that bends at the two azobenzene units is required.
However, this shape is considerably unstable compared with
tion of structure #1. The corresponding CD spectra of mono-
mers #4 to #7 indicate marked positive or negative peaks at
around 440 nm. The most stable monomer structure is #4,
whose CD spectrum possesses a negative peak at around
440 nm followed by more intense negative signals below
290 nm. This profile is completely different from the experi-
mental data for cis-(S)-1a. Thus, cis-(S)-1a in the current experi-
mental condition should essentially favor the dimer rather
than the monomeric state. As for the two extracts of dimer #1,
a large positive peak at around 435 nm is found in #7, whereas
#5 shows a negative peak at around 435 nm with about half
the intensity of #7. The summed CD spectrum of #5 and #7
#1, thus being substantially prohibited.
Their theoretical CD and UV/Vis absorption spectra are given
[
61,64]
in Figure 12
(for the details of the simulation, see the Ex-
perimental Section). The UV/Vis absorption spectra of dimers
1 and #2 show a small band at 435 nm and a more intense
#
band at 285 nm. These absorption positions and profiles agree
well with experimental data for cis-(S)-1a in DMF. The CD spec-
trum of dimer #1 shows a positive signal at 435 nm, followed
by a negative weak signal at shorter wavelengths. This pattern
is essentially consistent with the experimental CD spectrum of
cis-(S)-1a. Although a similar pattern is found in dimer #2, its
CD signal intensity is much smaller than the experimental data.
Dimer #3 gives a marked negative CD signal at 465 nm, which,
however, does not reflect the experimental pattern. Therefore,
the dimer of cis-(S)-1a in solution should prefer a structure
similar to that of #1.
To simulate the CD pattern of monomer molecules of cis-(S)-
1
a, two monomer components were separated from dimer #1.
Each monomer molecule was also geometry-optimized by mo-
[
60]
lecular mechanics (MM) computation using MMFF94.
The
structure, energy, and dipole moment of each result are sum-
marized in Figure 11 and Table 2. Two extracts (#5 and #7), as
well as their geometry-optimized monomers (#4 and #6) adopt
a V-shape that bends at the central azo group. These monomer
molecules commonly possess a larger dipole moment than
dimer #1. Obviously, the two dipole moments of the monomer
Figure 12. Theoretical CD and UV/Vis absorption spectra of dimers cis-(S)-1a
or cis-(R)-1a (#’s 1–3), and of monomers cis-(S)-1a (#’s 4–7): these structures
correspond to #’s 1–7 in Table 2, respectively. [q] and e are calculated with
respect to azobenzene concentration. The experimental spectra of cis-(S)-1a
in DMF are also superimposed. #’s 1–3: TD=58 (Gaussian 09), the others:
[61,64]
molecules cancel out according to the C -symmetric dimeriza-
TD=29 (Gaussian 03).
2
Chem. Eur. J. 2015, 21, 6747 – 6755
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agrees well with the CD spectrum of dimer #1, indicating that
the CD profile of dimer #1 originates simply from the respec-
tive monomer components. In conclusion, the theoretical CD
study demonstrates that cis-(S)-1a in DMF should be present
as a dimer with the rectangular-type cyclic structure of #1
boxylic acid derivatives (S)-1a–e form CD-active chiral cyclic di-
meric structures through trans-to-cis isomerization of the azo-
benzene moieties by UV irradiation, and return to CD-inactive
states through cis-to-trans isomerization of the azobenzene
moieties by visible-light irradiation. The theoretical CD study
also suggested that (S)-1a in the cis form assumes a rectangu-
lar-type cyclic dimeric structure, allowing it to exhibit intense
CD signals. Kinetic studies of thermal isomerization of (S)-1a
revealed that the formation of a chiral supramolecular struc-
ture featuring molecular chirality enhanced the relative stabili-
ty of the normally unstable cis form. The present system is
a quite rare example in which a bent cis-azobenzene unit
serves as a building block of a chiral supramolecular struc-
(
Figure 11).
We also attempted a similar CD calculation for the backbone
of stilbene cis-(S)-4(alanine). The structures and energies of the
two dimers adopting a Type I (#8) or Type II (#9) form are
shown in Figure S5 and Table S1 (the Supporting Information),
including the monomer components of #8 (#10 and #11). Re-
garding the stability of the dimer, the Type I form (#8) is found
to be more stable than the Type II (#9), similar to the case with
azobenzene. Their theoretical CD and UV/Vis absorption spec-
tra are shown in Figure S6 (the Supporting Information). The
two dimers give an absorption band at 230–240 nm, which is
at a shorter wavelength than the experimental position of cis-
[65]
ture.
Experimental Section
(
S)-4 in DMF (Figure 7). In both Types I and II, theoretical CD
Measurements
spectra for the transition states yield a negative band, as ex-
perimentally observed. On the other hand, the peak values of
13
Proton (400 MHz) and C (100 MHz) and 2D NMR (NOESY, DOSY)
spectra were recorded on a JEOL EX-400 spectrometer or a Bruker
Biospin AVANCE HD-500 spectrometer. IR spectra were measured
on a JASCO FT/IR-4100 spectrophotometer. Melting points (M.p.)
were measured on a Yanaco micro melting point apparatus. Specif-
[q] and e in Type I are closer to the experimental data, com-
pared with those in Type II, thus suggesting that such a Type I
form is preferential for dimerization. The two monomers ex-
tracted from the Type I dimer, #10 and #11, yield extremely
large CD signals below 230 nm. Here a positive CD band is ob-
tained for more stable monomer #10. The summed CD spec-
trum of monomers #10 and #11 affords a positive band, where-
as a negative CD band is obtained for dimer #8 and the experi-
mental data of cis-(S)-4 in DMF. Therefore, the structural and
spectral simulations support the view that cis-(S)-4 in solution
should favor a dimer species of the Type I form.
ic rotations ([a] ) were measured on a JASCO DIP-100 digital polar-
D
imeter with a sodium lamp as the light source. Elemental analyses
were performed at the Microanalytical Center of Kyoto University.
Mass spectra were measured on a JEOL JMS-HX110A or a JEOL
JMS-T100CS mass spectrometer. CD spectra were recorded on
a JASCO J-820 spectropolarimeter. UV/Vis absorption spectra were
recorded on a JASCO J-820 spectropolarimeter or JASCO V-550
spectropolarimeter.
Photo-irradiation
Conclusion
Photo-irradiation was carried out with a 400 W high-pressure mer-
cury lamp equipped with a power source (HB-400, Fuji Glass Work)
at room temperature. The appropriate wavelengths were selected
either with a Pyrex glass and a UV-D33S filter (Toshiba) for irradia-
tion at 300 nm<l<400 nm or with an L-42 filter (Toshiba) for irra-
diation at 420 nm<l. Sample solutions were introduced into
a quartz cell, which was placed 20 cm away from the lamp. The
photo-isomerization was monitored by UV/Vis absorption spectros-
copy. The trans/cis ratio of azobenzene moiety was determined
from the intensity of the p–p* transition absorption of trans-azo-
benzene. The intensity of pre-irradiated state was set to 100%
In the present study, we have demonstrated the photo-respon-
siveness of amino acid-derived new optically active azobenze-
nedicarboxylic acid derivatives (S)-1a–e. They exhibited no CD
signals in DMF under ambient conditions, whereas intense CD
signals were observed upon UV irradiation, indicating the for-
mation of highly ordered chiral structures upon isomerization
of the azobenzene moieties from the trans to cis form. The
compounds reversibly returned to the initial states showing no
CD signals by further visible-light irradiation. Hydrogen bond-
ing between the carboxy groups plays an important role in for-
mation of the chiral structures, because the ester counterpart
[7e,37]
trans.
Materials
did not show this phenomenon. Addition of H O to a DMF so-
2
lution of (S)-1b resulted in the inversion of the signs of the CD
signals. Compounds trans-(S)-4 and cis-(S)-4, stilbenedicarboxyl-
ic acid analogues of trans-(S)-1b and cis-(S)-1b, showed chirop-
tical properties similar to those of the trans- and cis-azobenze-
nedicarboxylic acid derivatives. Only cis-(S)-4 showed intense
CD signals. ESI mass spectral data indicated that trans-(S)-4
and cis-(S)-4 exist as differently aggregated structures. Namely,
trans-(S)-4 molecules favorably form oligomeric linear chains,
whereas cis-(S)-4 molecules prefer to form cyclic dimers
through hydrogen bonding, thereby resulting in the intense
CD peaks. Consequently, it is concluded that azobenzenedicar-
All the reagents and solvents were used as purchased without pu-
rification. Compounds (S)-3, trans-3,3’-stilbenedicarboxylic acid
methyl ester, and cis-3,3’-stilbenedicarboxylic acid methyl ester
[47,48,66]
were synthesized according to the literature.
The details of
the synthesis of photo-responsive compounds, as well as analytical
and spectroscopic data, are described in the Supporting Informa-
tion.
CD Simulation
We theoretically estimated supramolecular structures of cis-(S)-1a
or cis-(R)-1a, and their CD spectra. As described in the preceding
Chem. Eur. J. 2015, 21, 6747 – 6755
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results, cis-(S)-1a or cis-(S)-1b should be present predominantly as
dimer species. Thus, we focused on a dimeric structure of cis-1a.
Two cis monomers were combined by two bridges between car-
boxy groups to create a cyclic dimer. The initial conformation was
energy-minimized by MM computation with the MMFF94 force
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[
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Acknowledgements
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This research was supported by JSPS Research Fellowships for
Young Scientists and Kyoto University Global COE Program “In-
ternational Center for Integrated Research and Advanced Edu-
cation in Materials Science”, a Grant-in-Aid for Scientific Re-
search on Innovative Areas “New Polymeric Materials Based on
Element-Blocks” (No. 2401) from the Ministry of Education, Cul-
ture, Sports, Science, and Technology, Japan, and the Kansai
University Grant-in-Aid for progress of research in graduate
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Chem. Eur. J. 2015, 21, 6747 – 6755
www.chemeurj.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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À10
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À1
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6
[
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In cis-1a, the (D/2) value
should be about 43 or 50 nm on the basis of the experimental e–l ab-
sorption band around 380 nm (shown in Figure 12). The experimental
values might be overestimated, because the absorption profile is
weakly broadening and tailing. Thus a somewhat smaller (D/2), 40 nm,
was chosen. For cis-(S)-4(alanine), a (D/2) value of 34 nm was used on
the basis of the absorption profile around 290 nm (shown in Figure 7).
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[
Received: November 11, 2014
Published online on March 12, 2015
Chem. Eur. J. 2015, 21, 6747 – 6755
www.chemeurj.org
6755
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim