Published on the web December 28, 2010
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Synthesis and Photoisomerization of Highly Fluorescent Stilbene Ionic Liquids
Hiroyasu Tamura, Yoshihiro Shinohara, and Tatsuo Arai*
Graduate School of Pure and Applied Sciences, University of Tsukuba, Ibaraki 305-8571
(Received November 10, 2010; CL-100945; E-mail: arai@chem.tsukuba.ac.jp)
Photoresponsive stilbenes with imidazolium cation directly
attached to the para-position of the phenyl ring were prepared
and their properties were examined. Although the cis isomer was
liquid at room temperature and practically gave no fluorescence,
the trans isomer was solid and gave fluorescence emission with
quantum yield as high as ca. 0.2. Furthermore photoisomeriza-
tion from pure cis isomer in the liquid state to trans isomer in the
solid state was observed.
sion between ionic solid and ionic liquid; in this case both cis
and trans isomers are nonfluorescent and the cis isomer
thermally reverts to the trans isomer. We are interested in
preparing photoreversible ionic salts, which can undergo photo-
chemical change between ionic liquid and ionic solid with
thermal stability and considerably high fluorescence efficiency.
In this respect we have prepared stilbenes with imidazolium
cation directly attached to the phenyl ring of stilbene 320 and
their photochemical properties are examined. We wish to report
here the preparation and characteristic properties of ionic liquids
3 with photoresponsive stilbene group.11-13
Ionic liquids are salts which are liquid at low temperatures
(<100 °C)1,2 and consist of only cations and anions.3,4 Ionic
liquids have been recognized as useful reaction media5,6 with
characteristic features.7,8 They are expected to be practical
solvents because they have almost no vapor pressure and have
interesting properties9 such as high dielectric constant with
considerably high viscosity compared with common organic
solvents. In the study of photochemical reaction in ionic liquid
as a solvent ultrafast photochemical reactions such as trans-cis
isomerization in the excited singlet state have been studied. In
this case, photoisomerization of trans-stilbene took place
efficiently with faster rate constant than that expected from
the relatively high viscosity of an ionic liquid.10 As to the
photochemical reaction of ionic liquid compounds, only a few
have been reported. One of them is the stilbene type ionic liquid
1, where photochemical cis-trans isomerization could change
the properties of ionic salts from room temperature ionic liquid
of cis isomers to the ionic solid of the trans isomers
(Scheme 1).11 In this case one can change the fluorescence
properties of these compounds between fluorescent trans isomers
with quantum yield ca. 0.02 and the nonfluorescent cis isomers.
Another example is the azobenzene type salts 2,12,13 where
photochemical cis-trans isomerization also induces interconver-
Stilbene ionic salts 3 showed different properties depending
on the conformation around the central C=C double bond. For
example, the cis isomer is a room temperature ionic liquid with a
melting point of ¹35.6 °C, but the trans isomer is ionic solid at
room temperature with a melting point of 92.6 °C. These melting
points are similar to those of 1 with 53.5 °C for trans-1 and
¹39.4 °C for cis-1. The extinction coefficient of the absorption
spectra of 3 is larger than that of 1 and the absorption maximum
and fluorescence maximum of 3 shifted to a longer wavelength
than that of 1. In addition the quantum yield of fluorescence
emission of 3 is more than 10 times larger than that of 1 and the
quantum yield of trans-to-cis isomerization of 3 is lower than
that of 1. These values are summarized in Table 1, together with
those of parent stilbene and phenylazo ionic liquid.
If we discuss details of the physicochemical and photo-
chemical properties between 1 and 3, the melting points of
trans- and cis-3 were higher than that of trans- and cis-1 with
methylene chain, respectively (Table 1). Direct attachment of
imidazolium cation to the phenyl ring caused the melting point
to rise due to a more rigid structure. The absorption, fluores-
cence, and fluorescence excitation spectra of trans-3 in aceto-
nitrile are shown in Figure 1. cis-3 did not give detectable
fluorescence, and, therefore, only the absorption spectrum is
included in the figure. Similarity in absorption and fluorescence
excitation spectra for trans-3 indicated that the fluorescence is
really observed from the ionic compounds. Compared with the
absorption spectrum of trans-1 in acetonitrile, the absorption
maximum shifted slightly to longer wavelength, and the molar
extinction coefficient increased. The fluorescence maximum of
trans-3 shifted considerably to longer wavelength than that of
trans-1, and the fluorescence quantum yield increased by more
than 10 times from 0.016 to 0.19 under Ar (Table 1). trans-3
showed quite a high fluorescence quantum yield in comparison
with trans-4-cyanostilbene (Φf = 0.005),17 and even donor-
substituted stilbenes such as trans-4-aminostilbene (Φf = 0.03)18
and trans-4-dimethylaminostilbene (Φf = 0.037).19
Figure 2 shows absorption spectra of cis-3 and trans-3 in
acetonitrile and the spectral change on photoirradiation. On
irradiation with 275-nm light from a xenon lamp, the absorption
spectrum of trans-3 changed to approach a spectral profile of a
mixture of cis and trans isomers, and the spectrum of cis-3
Scheme 1. Structure of compounds 1, 2, and 3 and photo-
chemical cis-trans isomerization.
Chem. Lett. 2011, 40, 129-131
© 2011 The Chemical Society of Japan
Tamura, Hiroyasu
