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Chemistry Letters Vol.36, No.10 (2007)
Photoswitching of Solvatochromism Using Diarylethenes with 2,5-Disubstituted 3-Thienyl Unit
Naoki Tanifuji,Ã1 Masahiro Irie,2 and Kenji MatsudaÃ1;2
1Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST)
2Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Motooka, Fukuoka 819-0395
(Received June 26, 2007; CL-070685; E-mail: kmatsuda@cstf.kyushu-u.ac.jp)
Novel photochromic bis(3-thienyl)ethene with a 2,5-disub-
stituted thienyl unit, which shows photoswitching of solvato-
chromic property, was synthesized. The open-ring isomer
showed solvatochromic behavior, while the closed-ring isomer
did not. It was suggested that the alteration of the ꢀ-conjugated
bond structure is the main cause of the switching of the solvato-
chromic property.
interaction gets weaker. Therefore, this molecular system can al-
ter the electronic interaction via the ꢀ-conjugated chain between
substituents.
Solvatochromism is mostly attributed to the donor–acceptor
interaction through the ꢀ-conjugated chain.9 Therefore, by intro-
ducing the donor and the acceptor moiety into one of the thienyl
groups of bis(3-thienyl)ethene, solvatochromic property can
be integrated into the photochromic diarylethene. Practically,
a methoxy group was chosen for the donor moiety and a nitro
group was chosen for the acceptor group.
Photochromic compounds have been developed not only for
the photoinduced color changes but also for the photoswitching
of the physical properties originating from the molecular struc-
tural changes.1 Recently, several examples are reported on the
application of diarylethenes, which is one of the most robust
photochromic units, to the photoswitching of physical proper-
ties, including intramolecular magnetic interaction,2 organogel
characteristics,3 morphology of single crystals,4 fluorescence,5
and electric conductivity.6 Beside these applications, photochro-
mic compounds are also suitable for ‘‘molecular electronics’’ be-
cause ‘‘molecular electronics’’ deals with the molecular structure
for the control of the flow of information signals through them.7
In this study, we report the use of the photochromic compounds
for the photoswitching of the solvatochromism, which also
originates from the electronic interaction through a ꢀ-conjugat-
ed chain.
Photochromism of diarylethenes changes the molecular
structure from hexatriene to cyclohexadiene at the photoreactive
moiety.1a We have already reported the photoswitching of the
electronic interaction using the change of hybrid orbital at the
reactive carbon of bis(3-thienyl)ethenes with a 2,5-bis(aryl-
ethynyl)-3-thienyl group.2k,8 The photoswitching mechanism is
explained by the model compound having a 2,5-disubstituted
thienyl unit (Figure 1). In the open-ring isomer, the two function-
al substituents are linked by a ꢀ-conjugated chain. Therefore, the
intramolecular electronic interaction occurs between the two
substituents. In the closed-ring isomer, the hybrid orbital of
the carbon atom at the 2-position of the thiophene ring changes
from sp2 to sp3. Accordingly, the ꢀ-conjugated chain between
the two substituents is disconnected and the intramolecular
According to this strategy, we synthesized new compounds
1 and 2 which have both a bis(3-thienyl)ethene structure as a
photochromic unit and a donor–spacer–acceptor structure as a
solvatochromic unit (Figure 2).10 Trimethylsilyl-protected com-
pounds 3 and 4 were deprotected to give desilylated compounds.
Target molecules 1 and 2 were prepared by mild nitration using
nitric acid in acetic anhydride solution as nitration reagent into
ꢁ-position of thiophene ring of the desilylated compounds.11
Bis(3-thienyl)ethene derivatives 1–4 underwent reversible
isomerization by irradiation with UV and visible light. The
absorption spectra of the open- and the closed-ring isomers
are shown in Figure 3. Upon irradiation with 365 nm light, the
open-ring isomer 1–4a underwent photocyclization reaction
in n-hexane and methanol solution. All the closed-ring isomer
1–4b underwent cycloreversion reaction to the open-ring
isomers 1–4a completely by photoirradiation with 578 nm light.
The absorption spectra of open-ring isomer 1a depended on
the solvents. The color of the methanol solution of 1a was orange
and the hexane solution of 1a was yellow. The absorption
maximum was observed at 403 nm and the absorption edge
was located at 460 nm in hexane. The methanol solution of 1a
was orange and the absorption maximum was observed at
421 nm. The absorption maximum in methanol was red-shifted
about 20 nm in comparison with the hexane solution. In addition,
the absorption edge in methanol is 530 nm, which is also 70 nm
F2
F2
F2
F2
F2
F2
UV
NO2
n
NO2
n
S
S
S
S
Vis.
S
S
MeO
MeO
1b (n = 1), 2b (n = 2)
OFF State
1a (n = 1), 2a (n = 2)
ON State
Solvatochromism
Solvatochromism
UV
F2
S
S
C
C
F2
F2
Vis.
S
S
sp2
open-ring isomer : ON
sp3
MeO
NO2
n
TMS
n
S
S
closed-ring isomer : OFF
S
S
S
MeO
: functional substituent
5 (n = 1), 6 (n = 2)
3a (n = 1), 4a (n = 2)
Figure 1. Photoswitching mechanism of bis(3-thienyl)ethene
using 2-position carbon atom of the thiophene ring.
Figure 2. Photochromic reaction of diarylethenes having
solvatochromic unit 1 and 2 and reference compounds 3–6.
Copyright Ó 2007 The Chemical Society of Japan
Tanifuji, Naoki
