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36
Chemistry Letters 2001
Does Photoisomerization Proceed in an Ionic Liquid?
Ryosuke Ozawa and Hiro-o Hamaguchi*
Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
(Received May 1, 2001; CL-010400)
Photoisomerization of trans-stilbene has been investigated
In order to evaluate the polarity of [bmim]PF , we first
6
in an ionic liquid, 1-butyl-3-methylimidazolium hexafluoro-
examined the E (30) value, which is equal to the transition
T
phosphate([bmim]PF ). It has been found that the photo-iso-
energy of the charge transfer absorption band of N-phenoxide
6
merization indeed proceeds in this ionic liquid with a rate much
larger than that expected from its polarity and shear viscosity.
betaine dye. E (30) is commonly used as an empirical indicater
T
5
of solvent polarity. We obtained 54 kcal/mol for [bmim]PF .
6
This value is comparable to those of highly polar solvents such
as alcohols; E (30) is 55.5 kcal/mol for methanol and 47.6
T
Liquids that are composed solely of ions are called ionic
liquids. Ionic liquids are expected to afford a unique environ-
ment for chemical reactions. They are considered as one of the
most promising new solvents in the “green chemistry”.
Although a number of synthetic and electrochemical studies
kcal/mol for decanol .
We then observed steady-state absorption change of trans-
stilbene in [bmim]PF with irradiation by a Xe flash lamp with
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a filter cutting below 300 nm. The result is shown in Figure 1.
The absorption band of trans-stilbene at 300 nm decreases and
the band of cis-stilbene at 250 nm increases as the irradiation
time increases. A clear isosbestic point appears at 266 nm, indi-
cating that there is no other side-reactions in the trans–cis pho-
toisomerization process. Therefore, we exclude the possibility
1
have already been reported, there are only few that are con-
cerned with the physicochemical properties of ionic liquids.
In the present paper, we investigate photoisomerization of
trans-stilbene in 1-butyl-3-methylimidazolium hexafluoro-
phosphate([bmim]PF ), which is a well studied ionic liquid that
that the stilbene reacts with [bmim]PF . However, from a sepa-
6
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is air and moisture stable. Photoisomerization of trans-stilbene
is known to be strongly influenced by the solvent polarity and
viscosity.2 Therefore this system is suitable for studying the
rate experiment, we find that the [bmim]PF itself generates
photoproducts when irradiated with a wavelength shorter than
300 nm.
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properties of ionic liquids as solvents. Since [bmim]PF is an
extremely viscous solvent (312 cP at 303 K) that may well hin-
der the internal rotation, primary interest here is to elucidate
whether or not the photoisomerization of trans-stilbene proceed
The time profile of the observed fluorescence at 320–420
nm is shown in Figure 2. The decay curve is fitted well by a
double exponential function. The fast component has a lifetime
138 ps and the slow component has a lifetime as long as a few
nanoseconds. The intensity ratio is about 10:1 with the fast
component stronger than the slow. From the fluorescence spec-
tral shape, the fast component of the decay is ascribed to the
fluorescence of trans-stilbene. The slow component is ascribed
to the fluorescence from unknown photoproduct of [bmim]PF6,
which is genarated by the laser excitation at around 266 nm. As
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3
efficiently in [bmim]PF6.
4
[
bmim]PF was synthesized as described in the literature.
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trans-Stilbene was recrystallized from ethanol before use.
Picosecond time-resolved fluorescence spectra were obtained
with the use of the third harmonic of a titanium-sapphire laser
(
c.a. 266 nm, 5–10 mW, 1kHz) as an excitation source and a
streak camera (Hamamatsu C2909) for detection. A polarizer
was placed at magic angle to eliminate the effect of rotational
diffusion. Typical time resolution of the system was 15 ps.
a result, the S state lifetime of trans-stilbene is obtained as τ =
1
138 ps in [bmim]PF . In the case of S trans-stilbene, the iso-
6
1
merization is considered to be the only non-radiative decay
–
3
–3
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Sample concentration was 3 × 10 mol dm .
process both in non-polar and polar solvents. Though we can
not rule out the possibility that a new non-radiative decay chan-
nel opens in an ionic liquid, we assume here that the isomeriza-
Copyright © 2001 The Chemical Society of Japan