A photoresponsive liquid crystal based on (1-cyclohexenyl)phenyldiazene as a close analogue of azobenzene

Article abstract of DOI:10.1039/b906716g

A new photoresponsive calamitic liquid crystal molecule possessing (1-cyclohexenyl)phenyldiazene as a close analogue of azobenzene was synthesized and a comparative study with the corresponding azobenzene analogue carried out.

Full text of DOI:10.1039/b906716g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
A photoresponsive liquid crystal based on (1-cyclohexenyl)phenyldiazene
as a close analogue of azobenzenew
Masanao Sato, Shusaku Nagano and Takahiro Seki*
Received (in Cambridge, UK) 2nd April 2009, Accepted 28th April 2009
First published as an Advance Article on the web 19th May 2009
DOI: 10.1039/b906716g
A new photoresponsive calamitic liquid crystal molecule possessing
(1-cyclohexenyl)phenyldiazene as a close analogue of azo-
benzene was synthesized and a comparative study with the
corresponding azobenzene analogue carried out.
[4-pentyl-(1-cyclohexenyl)]-(4-cyanophenyl)diazene (5CPDCN,
Scheme 1) was found to have the most favorable stability and
photoswitching properties. Thus, the present exploration is
focused on this compound, and its spectroscopic and thermal
properties are compared with those of the corresponding
Az derivative possessing identical substituents (4-cyano-
40-pentylazobenzene (5AzCN, Scheme 1)).
Azobenzene (Az) is a typical and widely used photochromic
molecule that exhibits E(trans)/Z(cis) reversible isomeriza-
tion.1 The structure of this molecule in its E-isomer is
rod-like, and it can be utilized as the mesogen of photo-
responsive calamitic liquid crystals (LCs). A great number of
photoresponsive molecular systems have been explored to
exert various functions, such as surface photoalignment,2
photoinduced phase transition,3 photoinduced mass transport,4
photomechanical operations,5 etc.
The synthesis of 5CPDCB was performed according to the
modified procedures of Caglioty et al.8 2-Bromo-4-pentyl-
cyclohexanone and 4-cyanophenylhydrazine were coupled
via the 1,4-elimination of hydrogen bromide in a toluene/
tetrahydrofuran (THF) mixed solvent at room temperature
for 30 min. 5AzCN was synthesized by the coupling reaction
of 4-pentylaniline and 4-nitrosobenzonitrile in acetic acid at
room temperature for 9 h. Detailed synthesis procedures,
characterization of the compounds, and experimental details
of the measurements and instrumentation are described in the
ESI.w
UV-visible absorption spectra of 5CPDCN and 5AzCN in
THF are shown in Fig. 1. The absorption peaks (lmaxs) of the
p–p* bands of 5CPDCN and 5AzCN were observed at 315
and 340 nm, respectively. Thus, the band of 5CPDCN showed
a significant hypsochromic shift of 25 nm from that of 5AzCN.
Also, the whole band existed at shorter wavelengths, below the
UV edge of the visible region. In comparison with the non-
substituted CPD,7 The lmax of the p–p* transition of
5CPDCN showed a slight bathochromic shift by 11 nm. The
n–p* absorption band feature of 5CPDCN was almost the
same as that of 5AzCN. Therefore, the solution of 5CPDCN
appeared pale yellow in color. The band edge of the p–p* band
of 5CPDCN of longer wavelength was positioned around
360 nm, and the band separation between the p–p* (315 nm)
and n–p* (430 nm) bands was large. This means that an optical
window, without light absorption, exists in the g-line (365 nm)
region of the Hg lamp. Since the g-line is generally used to
excite almost all Az derivatives, this feature should be useful
for constructing dual switching systems composed of both
CPD and Az compounds.
(1-Cyclohexenyl)phenyldiazene (CPD) possesses an analogous
structure to Az, i.e. one of the phenyls of Az is substituted
by a cyclohexenyl ring. The photochemistry of this unit
has previously been reported by Schantl and Margaretha,6
and was recently revisited by Conti et al.7 According to these
reports, CPD reversibly undergoes the E - Z photo-
isomerization about the NQN bond, as for Az. The quantum
efficiencies of CPD for both the E - Z and Z - E processes
are comparable or slightly higher than those of Az, and the
thermal back reaction behaves substantially as for Az.7
Although the fundamental photochemistry of this unit was
intensively investigated by Conti et al.,7 there was no attempt
to introduce functional substituent(s) into this unit. One of
the most fascinating functionalizations is to impart liquid
crystallinity by introducing flexible alkyl chain(s) onto the
4,40-positions of the two rings. Through this strategy, one may
expect similar functions of Az-based LCs with CPD. However,
in terms of practical applications and molecular assembly
character, at least two matters will be of interest: (i) due to
the reduced p-conjugation aromaticity, the p–p* absorption
band will be shifted to shorter wavelengths, far beyond the
visible region;7 (ii) the non-planar cyclohexenyl ring should
affect the molecular assembly properties. The loss of planarity
in the cyclohexene ring should reduce the p–p stacking inter-
actions and modify the thermophysical properties.
This Communication reports the synthesis, photoreactive
and thermophysical properties of a newly designed LC
molecule containing the CPD mesogen. We have syn-
thesized several rod-like CPD derivatives, and among them
Department of Molecular Design and Engineering, Graduate School of
Engineering, Nagoya Universiy, Furo-cho, Chikusa, Nagoya 464-8603,
Japan. E-mail: tseki@apchem.nagoya-u.ac.jp
w Electronic supplementary information (ESI) available: Synthesis of
compounds and experimental procedures. See DOI: 10.1039/b906716g
Scheme 1
ꢀc
This journal is The Royal Society of Chemistry 2009
3792 | Chem. Commun., 2009, 3792–3794

View Article Online
Fig. 1 UV-visible absorption spectra of 5CPDCN (solid line) and
5AzCN (dotted line) in THF.
Irradiation of a THF solution of 5CPDCN (concentration:
2.5 ꢁ 10ꢂ5 mol dmꢂ3) with 313 nm light from a Hg lamp
brought about its photoisomerisation to the Z-form. For the
non-substituted CPD, the photostationary state provided 80%
content of the Z-isomer. However, the introduction of a cyano
group onto the phenyl ring reduced the Z-content of the
photostationary state down to ca. 50% (Fig. 2a). Irradiation
with 436 nm light invoked the Z - E back reaction to nearly
90% content of the E-isomer (Fig. 2b). Thus, the reversible
photoisomerization reaction was also observed for the cyano-
substituted CPD. The E/Z isomerization could be repeated
many times by alternate irradiation (Fig. 2c).
Fig. 3 The thermal phase change behaviour of 5CPDCN and 5AzCN
(above), POM images of 5CPDCN during the cooling process (middle)
and X-ray diffraction patterns of 5CPDCN in a capillary holder
(bottom), corresponding to the conditions of the POM observations.
Thermal properties with structural evaluations are shown in
Fig. 3. 5CPDCN was found to exhibit a nematic LC phase, as
confirmed by the appearance of a Schlieren texture in the
polarized optical microscopic (POM) image and the absence of
layer spacing order in the X-ray diffraction (XRD) profile of
the LC state (60 1C in the cooling process). Differential
scanning calorimetry (DSC) and POM observations revealed
two different aspects to the thermal property differences
between 5CPDCN and 5AzCN, as follows: (i) 5CPDCN
exhibited enantiotropic LC behaviour, whereas 5AzCN
showed a monotropic character, which agrees well with the
literature data for 5AzCN;9 (ii) the melting and crystallization
temperatures of 5CPDCN were lowered. During the heating
process, the crystal to nematic transition was observed at
64.5 1C, which can be compared with the melting temperature
of 85.0 1C for 5AzCN. Also, during the cooling process, the
crystallization temperature became significantly lower; 35.0
and 53.6 1C for 5CPDCN and 5AzCN, respectively. These
results can be understood by assuming weakened interactions
between the molecules for 5CPDCN. Since the molecular mass
is almost the same for the two compounds, the lowered
transition temperature for 5CPDCN can be ascribed to the
effect of non-planarity and the dissymmetrical structure of
the cyclohexenyl ring in the 5CPDCN molecule, providing
reduced p–p stacking interactions.
As in one of its possible applications, 5CPDCN was doped
(4 mol%) in a nematic LC matrix of 40-pentyl-4-cyanobiphenyl
(5CB, Merck), and spot exposure (diameter: 350 mm) to
313 nm light was achieved at 34 1C (Fig. 4). The mixture
initially showed the nematic LC phase, as indicated by the
Fig. 2 The photoisomerisation behaviour of 5CPDCN in THF.
Exposure to a: 313 nm and b: 436 nm light, which induces Z- and
E-isomers, respectively. c: Absorbance changes at 315 nm after
alternate irradiation are indicated.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 3792–3794 | 3793

View Article Online
In summary, this work proposes for the first time an Az
analogue of a photoresponsive calamitic LC molecule based
on CPD, whose structure differs only in one ring of the
mesogen, namely, cyclohexenyl for CPD or phenyl for Az.
The similarities and differences between the two rod-like
photoresponsive molecules presented in this work are expected
to expand the design and choice of photoresponsive LC
systems. Extended studies in polymer systems will be attractive
in terms of device applications.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Photochromism (no. 471),’’ and
‘‘Scientific Research (B) (19350053)’’ from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
Notes and references
1 G. S. Hartley, Nature, 1937, 140, 281; H. Rau, in Photochemistry
and Photophysics, ed. J. Rebek, Jr, CRC Press, Boca Raton, 1990,
ch. 4, pp. 119; H. Rau, Photoisomerization of Azobenzenes, in
Photoreactive Organic Thin Films, ed. Z. Sekkat and W. Knoll,
Academic Press, San Diego, 2002, ch. 1, pp. 3; N. Tamai and
H. Miyasaka, Chem. Rev., 2000, 100, 1875.
Fig. 4 The photoinduced nematic to isotropic phase transition
observed by POM. A sandwiched cell (4 mm spacer) of 5CB containing
4 mol% 5CPDCN was placed on a temperature controlled stage at
34 1C. Spot irradiation (diameter: 350 mm) was achieved in the center.
2 K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki and K. Aoki, Langmuir,
1988, 4, 1214; T. Seki, M. Sakuragi, Y. Kawanishi, T. Tamaki,
R. Fukuda and K. Ichimura, Langmuir, 1993, 9, 211; K. Ichimura,
Chem. Rev., 2000, 100, 1847.
Schlieren texture over the entire area (POM image at 0 s).
Upon irradiation with a circular beam spot, the texture was
rapidly modulated within the irradiated area and eventually
changed to an isotropic state after 7 s in this case. As expected,
the isotropic region reverted to the original nematic phase by
irradiation with 436 nm light. Such reversible photoinduced
phase transitions have been extensively investigated for
Az-containing LC materials.3 The present results with
5CPDCN essentially coincide with the behavior of LC Az
derivatives. This is reasonably explained by the structural
similarity of the two photoresponsive mesogenic units, and it
can be assumed that the CPD unit is a close analogue in terms
of both photoisomerization behavior and molecular assembly
functions. Exposure to 365 nm light led to no trace of any
phase transition in the 5CPDCN LC system. This fact is a
distinct difference from that seen for Az systems. In this way,
the phase switching behavior is similar but the working
wavelength is clearly distinguished for Az systems.
3 S. Tazuke, S. Kurihara and T. Ikeda, Chem. Lett., 1987, 911;
J.-H. Sung, S. Hirano, O. Tsutsumi, A. Kanazawa, T. Shiono and
T. Ikeda, Chem. Mater., 2002, 14, 385; T. Ikeda and O. Tsutsumi,
Science, 1995, 268, 1873; T. Ikeda, J. Mater. Chem., 2003, 13, 2037.
4 A. Natansohn and P. Rochon, Chem. Rev., 2002, 102, 4139;
N. K. Viswanathan, J. Mater. Chem., 1999, 9, 1941; K. J. Yager
and C. J. Barrett, Curr. Opin. Solid State Mater. Sci., 2001, 5, 487;
T. Seki, Curr. Opin. Solid State Mater. Sci., 2007, 10, 241.
5 T. Ikeda, M. Nakano, Y. Yu, O. Tsutsumi and A. Kanazawa, Adv.
Mater., 2003, 15, 201; Y. Yu, M. Nakano and T. Ikeda, Nature,
2003, 425, 145; T. Ikeda, J. Mamiya and Y. Yu, Angew. Chem., Int.
Ed., 2007, 46, 506; T. Seki, H. Sekizawa, S. Morino and
K. Ichimura, J. Phys. Chem. B, 1998, 102, 5313.
6 J. G. Schantl and P. Margaretha, Helv. Chim. Acta, 1981, 64, 2492.
7 I. Conti, F. Marchioni, A. Credi, G. Orlandi, G. Rosini and
M. Garavelli, J. Am. Chem. Soc., 2007, 129, 3198.
8 L. Caglioty, G. Roshini and F. Rossi, J. Am. Chem. Soc., 1966, 88,
3865.
9 U. Hrozhyk, S. Serak, N. Tabiryan and T. J. Bunning, Mol. Cryst.
Liq. Cryst., 2006, 454, 235.
ꢀc
This journal is The Royal Society of Chemistry 2009
3794 | Chem. Commun., 2009, 3792–3794