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Figure 1. Chemical structure of 1(n,m)-X.
(Tf2N) were introduced as counterions. The oligo(ethylene
oxide) units were introduced in the ammonium group with the
expectation that such a flexible and bulky head group would
lower the melting temperature of ICs. As expected, com-
pounds with short alkyl chains, 1(1,2)-Cl and 1(1,4)-Cl, gave
an IL phase at ambient temperature, thus indicating that the
bulky oligoether-based ammonium group lowered the crys-
tallinity by weakening van der Waals interactions, p–p
stacking, and the Madelung energy. In contrast, the longer-
alkyl chain compounds, 1(4,6)-Br, 1(6,4)-Cl, 1(6,4)-Br, and
1(8,2)-Br, were obtained in crystalline form (see Figure S1
and Table S1 in the Supporting Information). The substitution
of the halide counterion for Tf2N significantly lowered the
melting point and 1(6,4)-Tf2N gave an IL at ambient temper-
ature. This feature is ascribed to weakened electrostatic
interactions as a result of the enhanced delocalization of
anionic charge on the Tf2N ion.[1]
Photochromic properties of the cationic azobenzene
compounds were then investigated for neat ILs, ICs, and
methanol solutions thereof. All these methanol solutions
naturally showed reversible photoisomerization character-
istics (see Figure S3). The reversible photoisomerization was
also observed for the neat ILs 1(1,2)-Cl, 1 (1,4)-Cl, and 1(6,4)-
Tf2N, as revealed by color changes, UV/Vis spectra (for neat
IL specimens), and also by 1H NMR spectra obtained for
CDCl3 solutions. Very interestingly, the ICs 1(6,4)-Cl, 1(6,4)-
Br, 1(4,6)-Br, and 1(8,2)-Br all displayed photoisomerization
characteristics at ambient temperatures. Figure 2 shows the
effect of photoillumination on X-ray diffraction (XRD)
patterns of 1(6,4)-Br and their polarizing optical microscopy
(POM) images under crossed polarizers (inset) as a typical
example. Before UV irradiation, the yellow 1(6,4)-Br sample
showed birefringence in POM images under the crossed
polarizer (Figure 2a), and is consistent with the crystalline
lamellar structure as observed in XRD measurements (see
Figures S4–S6). After illumination of UV light by a high-
pressure mercury lamp [band pass filter, l = (365 Æ 10) nm],
the yellow crystal sample melted to give a red liquid phase
within a few minutes (Figure 2b; see Movie S1). The irradi-
ated part of the sample became dark in the POM images
under the crossed polarizer, and was accompanied by an
increase in n–p* absorption intensity at l = 450 nm (see
Figures S8a,b and S9a). These changes were also accompa-
nied by disappearance of XRD patterns (Figure 2b). These
observations clearly indicate that photoisomerization of
Figure 2. X-ray diffraction patterns obtained for 1(6,4)-Br prepared on
silicon wafer: a) as prepared film, b) after UV irradiation, and c) after
Vis irradiation. A picture of each sample is shown as an inset. Optical
microscopy images under crossed polarizers are shown in the right
(substrate, glass slide). A high-pressure mercury lamp [band pass
filter, l=(365Æ10) nm) was used as UV light illumination for bright-
field and polarized optical microscopy. Xenon lamp (band pass filter,
l=(365Æ10) nm] was used for XRD samples.
trans-1(6,4)-Br induced its liquefaction to the isotropic IL
phase. The molar ratio of the cis isomer, determined by
1H NMR spectroscopy (in CDCl3), was about 80%. Mean-
while, upon illumination of the visible light [l = (470 Æ
20) nm] to the photoliquefied ILs, crystallization occurred
reversibly within 1 minute (Figure 2c; see Movie S1), and was
accompanied by a decrease in the n–p* absorption intensity of
cis isomer at l = 450 nm (see Figure S8c,d) and reappearance
of the XRD patterns characteristic to the trans-azobenzene
compound (Figure 2c). The reversible photoliquefaction and
photocrystallization were similarly observed for the ICs of
1(6,4)-Cl, 1(4,6)-Br, and 1(8,2)-Br at room temperature (see
Figures S10–12). Thus, these ICs show totally reversible
photoliquefaction and photocrystallization phenomena
which are superior features surpassing the previously
reported solid–liquid phototransition systems.[13]
The photon energy storage capacity of the present photo-
induced IC–IL phase-transition system was then investigated
for 1(6,4)-Br. The ionic liquid 1(6,4)-Tf2N was taken as
a reference to determine the photon energy converted and
stored in the IL phase. These samples were photoisomerized
in advance and stored in the cis form. Figures 3 a and b shows
DSC thermograms obtained for ILs of cis-1(6,4)-Tf2N and cis-
1(6,4)-Br, respectively. Upon heating cis-1(6,4)-Tf2N, a broad
exothermic peak was observed around at 498C with
a DH value of 46.1 kJmolÀ1, which corresponds to the
DH value of 51.8 kJmolÀ1 for the pure cis isomer (Figure 3a).
This peak is associated with the thermally induced cis–trans
isomerization of the IL, and the observed change in enthalpy
(DH) is consistent with the reported energy (DHcis–trans) stored
in a cis-azobenzene chromophore.[10a] In contrast, when the
photoliquefied cis-1(6,4)-Br was heated, two exothermic
2
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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