Photoinduced phase transitions in chiral binaphthyl-diol-doped smectic liquid crystals by a photochromic azobenzene

Article abstract of DOI:10.1246/cl.2010.1144

The liquid crystalline states of mixtures of smectic 4-octyl4′- cyanobiphenyl, chiral binaphthyl-diol, and a photochromic azobenzene derivative, 4-n-hexyl-4′-(l-bromopropyloxy)azobenzene, are dependant on the chiral dopant and azobenzene concentrations. T

Full text of DOI:10.1246/cl.2010.1144

doi:10.1246/cl.2010.1144
1144
Published on the web September 28, 2010
Photoinduced Phase Transitions in Chiral Binaphthyl-diol-doped Smectic Liquid Crystals
by a Photochromic Azobenzene
Guojie Wang,* Mingzhi Zhang, Qiang Yang, Fang Liu, Zihui Cheng, Renwei Guo, and Huai Yang*
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China
(Received August 11, 2010; CL-100694; E-mail: guojie.wang@mater.ustb.edu.cn)
The liquid crystalline states of mixtures of smectic 4-octyl-
H13C6
N=N
(AB)
OCH2CH2CH2Br
4¤-cyanobiphenyl, chiral binaphthyl-diol, and a photochromic
azobenzene derivative, 4-n-hexyl-4¤-(1-bromopropyloxy)azo-
benzene, are dependant on the chiral dopant and azobenzene
concentrations. The trans-to-cis isomerization of azobenzene
induces the phase transition from smectic to cholesteric and then
to isotropic states.
OH
OH
H17C8
CN
(BD)
Scheme 1.
(8CB)
Photoinduced control of phase structures of liquid crystals
(LCs) has received great attention for its extensive applications
in optical memory, optical display, and optical switching.1
Azobenzene chromophores are well-known photoresponsive
molecules, which possess a unique photochromism between
trans form with a rod-like shape and cis form with a bent shape.
A small amount of azobenzene chromophores could be used as
guest molecules to affect the structures and properties of host
LCs, due to their trans-to-cis photoisomerization. The trans form
stabilizes the phase structure of LCs, while the cis form tends to
destabilize the phase structure of the mixture because of its bent
shape.2 The change in molecular shapes of azobenzene has been
first studied to induce the nematic-isotropic transition in nematic
LC systems by photoisomerization, where the change in the
optical properties such as birefringence could be detected by the
use of polarizers. Recently, much effort has been devoted to
photoresponsive cholesteric liquid crystals (CLCs), which
possess an inherent selective reflectivity, primarily for tunable
reflectors and lasing elements. Three approaches have been
examined for the azobenzene photochemical modulation of
CLCs: 1) mixing azobenzene as a photoresponsive dopant with
a CLC;3 2) mixing photoresponsive azobenzene nematic LCs
with CLCs;4 and 3) mixing photoresponsive chiral azobenzene
derivatives with nematic LCs.5 Here we report the effect of
photoisomerization of azobenzene on the structures and proper-
ties of smectic LCs doped with a chiral molecule and an
azobenzene derivative. The phase structures and transmittance
(reflectivity) of the smectic LC mixtures could be controlled by
photoirradiation.
a)
b)
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The photoresponsive LC mixtures examined herein were
formed by mixing a homemade azobenzene derivative and a
chiral molecule with a smectic LC host. The corresponding
chemical structures of the materials used are shown in
Scheme 1. The photoresponsive 4-n-hexyl-4¤-(1-bromopropyl-
oxy)azobenzene (AB) was synthesized by Williamson ether
synthesis of 4-(4-n-hexylphenylazo)phenol, which was obtained
by azo coupling.6 (S)-(¹)-1,1¤-Binaphthyl-2,2¤-diol (BD) and
4-octyl-4¤-cyanobiphenyl (8CB) are commercial compounds.
8CB is a smectic LC around room temperature (28 °C Sm 37 °C
N 44 °C I). All the experiments were carried out at room
temperature using a sandwich cell (thickness: ca. 5 ¯m) coated
with rubbed poly(vinyl alcohol).
Figure 1. a) Polarized optical micrographs of the 8CB mixture
containing 1% BD and 3% AB before (left image) and after
UV irradiation for 60 s (right image). Magnification: 400. b)
Transmission spectra of the 8CB mixture during UV irradiation
of the cell.
The 8CB mixtures doped with photoresponsive AB and
chiral BD exhibit a smectic A phase when the chiral BD
concentration is as low as 1 wt %. Figure 1a shows the polarized
optical micrographs (POMs) of the 8CB mixture containing 1%
BD and 3% AB before and after UV irradiation (365 nm,
10 mW cm¹2). First, the smectic A phase could be observed
Chem. Lett. 2010, 39, 1144-1145
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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before UV irradiation where the azobenzene was in trans form;
after UV irradiation the azobenzene became cis form and the
mixture was induced to exhibit a cholesteric phase. Although the
azobenzene could be isomerized completely to its cis form,
measured by its UV-vis absorbent spectra,7 the isomerization
could not proceed from cholesteric to isotropic, as could be
ascribed to the low concentration of the azobenzene. To confirm
this, the amount of AB was increased to 5% or more, and then
the cholesteric to isotropic transition was observed during
photoirradiation. The smectic A phase of the mixture could be
recovered after the sample was kept in the dark for 12 h. The
photoinduced phase transition from smectic to cholesteric and
then to isotropic in the LC mixtures is brought out by the
photoisomerization. It has been proven that the photoinduced
phase instabilities in smectic A LCs were due to the photo-
isomerization, and a subsequent increase in the smectic layer
spacing was observed.8 The trans form of the azobenzene, which
possesses a rod-like shape, stabilizes the smectic LC phase.
After UV irradiation, the trans form is converted to cis form,
which possesses a bent shape and decreases the order of the LC.
The cis form induces the phase transition from smectic to
nematic and then to isotropic when the amount of azobenzene is
sufficient in the 8CB mixtures. The doped chiral BD converts the
photoinduced nematic structure to cholesteric, as it is well-
known that nematic LC mixing with chiral molecules can exhibit
cholesteric structure. So the smectic 8CB LC doped with chiral
BD and photoresponsive AB exhibited phase transition from
smectic to cholesteric and then to isotropic after UV irradiation.
The spectral properties during UV irradiation of the mixture of
1% BD and 3% AB in 8CB are shown in Figure 1b. The
transmission increased with the increase of the irradiation time
until it was saturated when all the trans isomers were converted
to the cis. Combining the phase transition of the mixture
revealed by POM, it can be concluded that the cholesteric phase
exhibited higher transmittance than that of the smectic. Yet no
selective reflectivity was found in this mixture doped with 1%
chiral BD, as could be caused by the low helical twisting power
at the low concentration of the chiral BD.
To investigate the effect of photoisomerization of AB on the
smectic 8CB mixture doped with high concentration of the chiral
BD, we prepared a sample of 8CB containing 3% BD and 5%
AB. This mixture with high concentration of chiral molecules
exhibited cholesteric phase at room temperature instead of the
smectic shown at low concentration of chiral BD. After UV
irradiation, the trans isomer of AB in this mixture was converted
to the cis, and the cholesteric phase was induced to isotropic.
Figure 2 shows the transmission spectra of the mixture during
UV irradiation. The reflectivity of the mixture decreased, and the
reflection notch was red-shifted a little from 1904 to 1906 and
then to 1908 nm when UV irradiating for 0, 10, and 30 s. The
reflection disappeared when irradiating for 60 s. After irradiating
for 180 s where the cholesteric phase was completely induced
to the isotropic revealed by POM, no reflection was found and
the transmission was increased. White, Bunning, et al. reported
that an azobenzene-based chiral molecule could phototune the
reflection notch around 2000 nm.5d Thus the structure of
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Figure 2. Transmission spectra of the 8CB mixture containing
3% BD and 5% AB during UV irradiation of the cell.
photoresponsive azobenzene doped plays a great role in photo-
tuning the properties of the cholesteric LCs.
In summary, we have demonstrated that the structures and
properties of smectic LCs could be phototuned by doping with
photoresponsive azobenzene and chiral molecules. The photo-
induced phase transition from smectic to cholesteric and then
to isotropic is dependent on the concentration of the dopants.
By adjusting the amount of the dopants in 8CB, a selective
reflection could be controlled by photoirradiation.
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Chem. Lett. 2010, 39, 1144-1145
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/