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is well known that azobenzene and its derivatives are photo-
isomerizable and undergo a reversible transformation between
croscopy (POM). As shown in Figure S10 in the Supporting In-
formation, four different LC cells with 10 mm cell gap were pre-
[
7]
trans and cis isomers in the presence of light. Therefore, the
cooperative conformational changes of azobenzene depending
on the wavelength of light is a most effective method for ap-
plication in photo-modulated LC devices. Furthermore, since
the cross-linked structure of polydiacetylene is simply formed
by irradiating 254 nm ultraviolet (UV) light to the diacetylene
moiety without chemical initiator and catalysts, the stability of
LC alignment can be significantly increased by introducing a di-
acetylene moiety. Therefore, the diacetylene photo-polymeriz-
able function is selectively chosen for programming the target-
pared by varying the ADMA /NLC weight ratio from 0.4:99.6,
1
0.3:99.7, 0.2:99.8 to 0.1:99.9. Here, the ADMA /NLC mixtures
1
were filled into the LC cells on a 1008C hot plate utilizing the
one drop filling (ODF) process and slowly cooled to room tem-
[9]
perature; this method can prevent the flow effect. When the
content of ADMA in the LC mixture was reduced from 0.4 to
1
0.1 wt%, the dark-state area in the magnified POM image was
significantly increased and the Maltese cross in the conoscopic
image became obvious (Figure S10). Note that the orientation
of homeotropic LC should exhibit a dark state under the cross-
polarized POM because the molecular orientations of LC are
aligned normal to the surface of test LC cells and do not exhib-
[
8]
ed smart amphiphile molecule.
The dual photochemical behavior of ADMA was first investi-
1
[10]
gated by ultraviolet–visible (UV/Vis) spectroscopy. As shown in
Figure 2a, by irradiating UV light with a maximum intensity at
it any birefringence (zero birefringence effect).
Since the alignment of LC is closely related, not only opti-
365 nm, the thermodynamically stable trans conformer of azo-
mizing the concentration of ADMA in the LC mixture but also
1
benzene transforms to its metastable cis conformer and reach-
es a new photostationary state in 60 s. The absorption band
between 300 and 410 nm resulting in the p-to-p* transition
originates from the trans conformer of azobenzene, whereas
the ground state (n) of the cis conformer of azobenzene is ex-
cited to the p* state by absorbing UV light in the range 410–
controlling the rates of phase separation from the NLC
medium and diffusion of ADMA onto the substrates, the com-
1
patibility and interaction of ADMA with NLC should be finely
1
tuned by attaching the hydrophilic carboxylic acid and hydro-
phobic alkyl chains (Figure 3a). To confirm this proposal, the
alignment behaviors of 0.1 wt% ADMA LC cell were investi-
1
530 nm. The transition between the two photostationary equi-
gated by a specifically designed experiment. When the LC cell
librium states is a fully reversible process by alternating UV-
and Vis-light irradiation (Figure S9 in the Supporting Informa-
tion). It is worth mentioning that not only the cycle is reversi-
ble, but also the amplitude of the change remained without
deterioration. When 254 nm UV is irradiated, the absorption
band at 630 nm gradually increases with irradiation time, as
shown in Figure 2b. This result clearly reflects the formation of
polydiacetylenes as the consequence of photo-polymerization
of the diyne groups in the self-assembled diacetylenes. The
with 0.1 wt% ADMA was fabricated by a rapid cooling rate
1
À1
(40.08Cmin ) from 100 to 258C, a homogeneously orientated
LC was obtained (Figure S11 in the Supporting Information).
This means that, due to the insufficient times of the phase-sep-
aration from the NLC medium and diffusion to the substrate,
the ADMA amphiphiles dimerize in the NLC medium by inter-
1
molecular hydrogen boding between carboxylic acid groups
and do not form the monolayer on the substrate. On the other
À1
hand, when the LC cell is slowly cooled at 1.08Cmin , a perfect
30 min irradiation was sufficient for the saturated polymeri-
dark state without any light leakage is obtained. This result
zation state. This peculiar dual photochemical property of
shows that the initially dissolved ADMA in the NLC gradually
1
ADMA can be very useful for stabilizing the photo-alignment
phase-separated from the NLC medium and diffused onto the
1
layer and allowed us to fabricate a reversible photo-modulat-
ing device.
substrate for the construction of self-assembled ADMA mono-
1
layer which is an ideal surface structure for the homeotropic
LC alignment layer (Figure 3b). Since the interaction of the
polar heads with the substrate is much higher than that of hy-
The homeotropic alignment of nematic LC (NLC) was exam-
ined by both orthoscopic and conoscopic polarized optical mi-
drophobic tails, ADMA amphiphile can diffuse and be deposit-
1
ed on the surface with molecular orientation perpendicular to
the surface.
For enhancing the stability of LC cell, photo-polymerization
of the self-assembled ADMA monolayer was conducted by ir-
1
radiating with 254 nm UV light (Figure 3c). Polymer-stabilized
ADMA LC cell securely maintains the homeotropic state. This
1
result indicates that the photo-polymerization of the self-as-
sembled ADMA monolayer does not disturb the LC orienta-
1
tion. By switching the wavelength of light from 365 nm (trans
isomer) to 450 nm (cis isomer), the NLC alignment can be re-
versibly controlled from homeotropic to homogenous states,
as demonstrated in Figure 3c and d, respectively.
To investigate the surface morphological changes of the
Figure 2. UV/Vis absorption spectra of ADMA
1
: a) after irradiation with
ADMA layer according to light irradiation, the self-assembled
1
365 nm UV light for trans–cis photo-isomerization of the azobenzene moiety,
ADMA monolayer was analyzed by atomic force microscopy
and b) after irradiation with 254 nm UV light for photo-polymerization of
diacetylene groups.
1
(AFM). The surface of bare ITO glass substrate is relatively
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Chem. Eur. J. 2014, 20, 1 – 5
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!