Photoinduced Isothermal Phase Transition of Ionic Liquid Crystals
a
temperature range much wider than that of pure
light through them (Figure S6 in the Supporting Informa-
tion).[19] The result for 5 is shown in Figure 5a. Before UV
irradiation, the probe light could transmit through the
crossed polarizers because of the birefringence of the
sample. After UV irradiation for 9 seconds, the transmit-
[C12MIm][BF4] was obtained for the MMAB/[C12MIm][BF4]
A
R
ACHTUNGTRENNUNG
mixtures. Remarkably, the higher the MMAB content was,
the wider became the mesogenic range (Figure S3 in the
Supporting Information). For example, compared with that
of pure [C12MIm]ACHTUNGTRENNUNG[BF4] (168C for heating and 368C for cool-
ing), 5 exhibited a liquid–crystalline temperature range as
wide as 268C upon heating and 518C upon cooling. The ex-
panded mesophase range (depressed crystallization and en-
hanced mesophase stability) could be very useful regarding
the widespread applications of ILCs. The entropy change of
the phase transition of smectic/isotropic liquid (clearing
points) was calculated (Figure S4 in the Supporting Informa-
tion). For both heating and cooling processes, the obvious
increase in the absolute value of the entropy change with
addition of MMAB up to 4% suggested that the degree of
order of the mesophase increased with the amount of
MMAB. One possible explanation for this result is that
MMAB forms a sandwich-like structure based on a cation/p
interaction with the imidazolium cation of the IL,[15–18] which
stabilizes the mesophase and expands the temperature
range.
For all mixtures, even including 1 with an MMAB content
of 1 wt%, a significant and reversible photoresponsive S–I
phase transition can be successfully achieved. The isother-
mal phase transition of 5 at 508C under alternating irradia-
tion with UV and visible light is shown in Figure 4 as an ex-
ample. Before irradiation, typical fan-shaped focal conic tex-
tures were observed in the POM images, thereby indicating
a homogeneous smectic A phase. Upon UV irradiation, the
fan-shaped texture disappeared from the direction of irradi-
ation. Within 4 seconds of irradiation, a uniform dark area
(isotropic liquid) was obtained. By contrast, irradiation of
the isotropic liquid with visible light first resulted in the ap-
pearance of small Batꢂnnet rods and then rapidly turned
into a wide area of Batꢂnnet rods. Eventually, a fan-like tex-
ture developed, indicating the recovery of the smectic A
phase. Increasing the UV power density was found to expo-
nentially decrease the response time (Figure S5 in the Sup-
porting Information), which was defined as the time re-
quired for the completion of the photochemical S–I phase
transition in the field of view. It should be stressed that cold
LEDs were used as sources for irradiation with both UV
and visible light in order to prevent heating effects during ir-
radiation. The photochemically induced phase transitions
can be interpreted as follows: As depicted in Figure 1b,
under UV irradiation, the trans form of the guest molecule
MMAB is photoisomerized into the cis form. The former,
which has a rod-like shape, serves to stabilize the liquid
crystal phase, whereas the latter has the opposite effect due
to its bent shape, thus acting as an impurity and changing
the orientation of the adjacent ILC molecules, which then
destabilize the whole liquid crystal phase by the cooperative
motion of ILCs (domino effect).[8]
Figure 5. a) Change in normalized transmittance of the probe light of
a
film of 5 ) and visible
upon irradiation with UV (16 mWcmÀ2
(10 mWcmÀ2) light. Photoirradiation was performed at 508C on cooling
from isotropic liquid and the transmittance was normalized to 1 and 0
for liquid crystal and isotropic liquid, respectively. b) Time required for
the photochemical S–I phase transition (response time) of 5 as a function
of temperature under UV irradiation (16 mWcmÀ2) on cooling from iso-
tropic liquid.
tance was reduced to near zero, thereby indicating that the
photochemical S–I phase transition in this mixture had com-
pletely taken place at this time. Upon subsequent irradiation
with visible light, the transmittance of the probe light barely
changed for 10 seconds but then increased steeply, thus re-
vealing I–S phase transition.
The influence of irradiation with UV and visible light on
the S–I phase transition temperature was investigated by
POM for the entire heating and cooling cycles. As depicted
in Figure 6, under irradiation with visible light, the S–I
phase transition temperatures of 5 with the trans form
(TS–I(trans)) were 55 and 528C upon heating and cooling, re-
spectively, which is in good agreement with the aforemen-
tioned DSC result. However, under UV irradiation, the S–I
phase transition temperature with the cis form (TS–I(cis)) de-
creased to 48 and 458C upon heating and cooling, respec-
tively. If the temperature of the sample is set at a tempera-
ture between TS–I(trans) and TS–I(cis), alternating UV/Vis irradi-
ation can result in reversible S–I phase transition. However,
beyond this temperature range, no phase transition is in-
The photoresponsive phase transition was further dynami-
cally monitored by setting ILC samples between a pair of
crossed polarizers and measuring the transmittance of probe
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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