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Chemistry Letters Vol.37, No.8 (2008)
Fluorescence Switching of a Diarylethene Derivative Having Oxazole Rings
Katsunori Shibata,1 Lumi Kuroki,1 Tuyoshi Fukaminato,1 and Masahiro Irieꢀ2
1Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
2Department of Chemistry, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501
(Received April 21, 2008; CL-080417; E-mail: iriem@rikkyo.ac.jp)
A diarylethene derivative, in which oxazole rings are
(a)
(b)
attached to the ethene moiety at the 5-position, was synthesized,
and its photochromic and fluorescent properties were examined.
The derivative undergoes photochromism in solution and the
open-ring isomer exhibits strong fluorescence. The fluorescence
intensity was found to reversibly change along with the photo-
chromism.
O
O
N
N
Various types of diarylethene derivatives have been synthe-
sized and their optical absorption as well as emission properties
has been so far examined. Diarylethenes undergo reversible
photoisomerization between two isomers (open- and closed-ring
isomers) having different optical properties. The optical absorp-
tion of the closed-ring isomer strongly depends on the kind and
the connected position of the heteroaryl moieties.1–9 Among
many kinds of heteroaryl moieties, oxazole derivatives have
characteristic properties such as fluorescent, laser emissive prop-
erties, and pH sensitivity.10 In a previous study, we reported di-
arylethene derivatives having oxazole rings, 1,2-bis(5-methyl-2-
phenyloxazol-4-yl)perfluorocyclopentene,11 in which oxazole
rings are attached to the ethene moiety at 4-position. Although
the absorption maximum of the closed-ring isomer is shorter
than 500 nm, the maximum is still in the visible region and
the UV-irradiated solution was orange. To further shift the
maximum to UV region, we have prepared 1,2-bis(4-methyl-
2-phenyloxazol-5-yl)perfluorocyclopentene, in which oxazole
rings are attached to the ethene moiety at 5-position, and exam-
ined its photochromic performance and fluorescent property.
The synthesis of 1a (Scheme 1) was carried out according
to Scheme 2. 5-Bromo-4-methyl-2-phenyloxazole (3) was
prepared by the bromination of 4-methyl-2-phenyloxazole (2)
with N-bromosuccinimide in CHCl3. The coupling reaction
Figure 1. ORTEP drawings of 1a showing 50% probability dis-
placement ellipsoids: a) top view and b) side view. The CF2
groups in the cyclopentene ring were disordered. Hydrogen atom
and the disordered structure are omitted for clarify.
between 3 and octafluorocyclopentene gave 1a in 66% yield.
1a was characterized by 1H NMR, MS, and elemental analysis.12
Detail synthetic procedures were described in Supporting
Information.13
The molecular structure in crystal was determined by the
X-ray crystallographic analysis.14 A crystal of 1a was obtained
by recrystallization from hexane solution. The ORTEP drawings
of 1a are shown in Figure 1. 1a is packed in a parallel conforma-
tion and the oxazole rings are attached to ethene moiety at the
5-position. As expected from the parallel conformation, the
crystal did not show any photochromism. But, the crystal
exhibited strong fluorescence.
Figure 2a shows the absorption spectral change of 1 by
photoirradiation in hexane. 1a has absorption maxima at
287 and 373 nm. Upon irradiation with ꢀ > 380-nm light, the
absorption bands decreased and a new band appeared at
252 nm. In contrast to typical photochromism of diarylethene
derivatives, the absorption spectrum showed a hypsochromic
shift. The photogenerated isomer was isolated by HPLC (Wako,
Wakosil 5SIL; hexane/ethyl acetate = 80/20 as the eluent) and
found to be closed-ring isomer 1b by NMR and MS.15 The
closed-ring isomer 1b gave the absorption maxima at 252 and
353 nm. From the spectrum of 1b, the conversion from 1a to
1b in the photostationary state was estimated to be 63% under
irradiation with longer than 380-nm light. The absorption
spectrum of 1b almost returned to that of 1a upon irradiation
with 254-nm light. The cyclization and cycloreversion quantum
yields of 1 in hexane were determined by using the standard
procedure.16 The absorption maxima, coefficients, and quantum
yields are summarized in Table 1.
F2
F2
F2
F2
F2
F2
h
υ
O
O
Me
Me
O
O
Me
Me
hυ'
N
N
N
N
1a
1b
Scheme 1. Photochromism of dioxazolylethene 1.
Me
Me
Br
N
N
a)
b)
Upon irradiation with 365-nm light, the hexane solution of
1a exhibited relatively intense blue-green fluorescence.
Figure 2b shows the fluorescence spectral change of 1 along
with the photochromic reaction in hexane. The fluorescence
maximum of 1a was observed at 470 nm. The fluorescence
quantum yield was 19% by excitation with 365-nm light. This
1a
O
O
2
3
Scheme 2. a) NBS, CHCl3, 92%; b) n-BuLi, octafluorocyclo-
pentene, THF, 66%.
Copyright Ó 2008 The Chemical Society of Japan