Efficient Photocycloreversion Reaction of Diarylethenes by Introduction of Cyano Subsutituents to the Reactive Carbons

Article abstract of DOI:10.1246/cl.2003.858

Highly efficient photocycloreversion reaction was observed for diarylethene derivatives with cyano groups at the reactive carbons of the thiophene rings. The cycloreversion quantum yield of 1,2-bis(2-cyano-5-phenyl-3- thienyl)perfluorocyclopentene was fou

Full text of DOI:10.1246/cl.2003.858

858
Chemistry Letters Vol.32, No.9 (2003)
Efficient Photocycloreversion Reaction of Diarylethenes
by Introduction of Cyano Subsutituents to the Reactive Carbons
Kentaro Morimitsu, Seiya Kobatake, Shinichiro Nakamura,y and Masahiro IrieÃ
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581
yYokohama Research Center, Mitsubishi Chemical Corporation, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502
(Received May 16, 2003; CL-030426)
Highly efficient photocycloreversion reaction was observed
Figure 1 shows the absorption spectra of 1a and 1b in hex-
ane. 1a has the absorption maxima at 273, 315, and 327 nm in
hexane. Upon irradiation with 313-nm light, the colorless solu-
tion of 1a turned to orange which is due to the formation of the
closed-ring isomer 1b.9 The visible absorption maximum of 1b
was observed at 496 nm, which is 21 nm shorter than that of
methyl derivative 2b. The orange color of 1b in hexane imme-
diately disappeared upon irradiation with visible light
(ꢀ > 500 nm) and the absorption spectrum returned to that of
1a. The conversion from 1a to 1b in the photostationary state
upon irradiation with 313-nm light was 58%. The bisphenylth-
ienylethene 4 having cyano groups at the reactive carbons also
showed similar photochromic behavior to 1. The colorless 4a
has the absorption maximum at 284 nm, and changed to purple
colored isomer 4b10 upon irradiation with 313-nm light in hex-
ane. 4b returned to 4a immediately upon irradiation with visible
light. Thermal decoloration of 1b and 4b was not observed even
after a day at room temperature. The absorption maxima of 1b–
6b are summarized in Table 1. The methoxy derivatives 3b and
6b shifted the visible absorption maxima 30 and 50 nm longer
than those of methyl derivatives 2b and 5b, respectively. On
the other hand, the cyano derivatives 1b and 4b shifted the max-
ima 21 and 30 nm shorter than those of methyl derivatives 2b
and 5b, respectively.
for diarylethene derivatives with cyano groups at the reactive
carbons of the thiophene rings. The cycloreversion quantum
yield of 1,2-bis(2-cyano-5-phenyl-3-thienyl)perfluorocyclopen-
tene was found to be ca. 30 times larger than that of 1,2-
bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene.
Photochromism is defined as a reversible isomerization be-
tween two isomers having different absorption spectra by pho-
toirradiation.1 Various types of photochromic compounds have
been so far developed in an attempt to apply the compounds to
optoelectronic devices, such as memories and switches. Among
these compounds, diarylethenes are the most promising candi-
dates for the application because of their thermal irreversibility
and fatigue-resistance.2,3 Recently, we have found that dithieny-
lethenes having alkoxy groups at the reactive carbons of the
thiophene rings remarkably decrease the cycloreversion quan-
tum yield.4,5 The cycloreversion quantum yields are lower than
10À3. A strategy to increase the cycloreversion quantum yield
of diarylethenes, however, has not been found out yet. Here,
we report that cyano groups at the reactive carbons of diaryle-
thenes increase the cycloreversion quantum yield considerably
and the theoretical calculation supports the effect.
To examine the substituent effect on photochromic reactiv-
ity we measured cyclization and cycloreversion quantum yields
of the diarylethenes in hexane. Table 1 summarizes the quantum
yields of 1–6. The cyano groups at the reactive carbons did not
affect the cyclization quantum yields of 1 and 4, and the values
are similar to those of the methyl substituted derivatives 2 and
1,2-Bis(2-cyano-1-benzothien-3-yl)perfluorocyclopentene
(1a)6 was synthesized by reacting 1,2-bis(2-formyl-5-phenyl-3-
thienyl)perfluorocyclopentene with hydroxylammonium chlo-
ride in the presence of sodium hydrogencarbonate in ethanol
at room temperature and by dehydration. The formyl diaryle-
thene derivative was obtained by coupling 3-bromo-2-(2,5-di-
oxolanyl)benzothiophene and octafluorocyclopentene followed
by deprotection. 1,2-Bis(2-cyano-5-phenyl-3-thienyl)perfluoro-
cyclopentene (4a)7 was also prepared according to a procedure
similar to that used for 1a. 1,2-Bis(2-methoxy-1-benzothien-3-
yl)perfluorocyclopentene (3a)8 was prepared from 2-methoxy-
benzothiophene and octafluorocyclopentene.
0.8
0.4
0
F2
F2
F2
F2
F2
F2
UV
R
R
R
R
Vis.
S
S
S
S
1a: R=CN, 2a: R=Me, 3a: R=OMe
1b: R=CN, 2b: R=Me, 3b: R=OMe
F2
F2
300
400
500
600
F2
S
F2
S
F2
S
F2
S
Wavelength / nm
UV
R
R
R
R
Figure 1. Absorption spectral change of 1 in hexane
(3:0 Â 10À5 mol dmÀ3) by photoirradiation: 1a (- - - -), 1
in the photostationary state under irradiation with 313-
nm light (– - –), and 1b (—).
Vis.
4a: R=CN, 5a: R=Me, 6a: R=OMe
4b: R=CN, 5b: R=Me, 6b: R=OMe
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.9 (2003)
859
5, respectively. On the contrary, the cycloreversion quantum
yields are strongly affected by the substituents at the reactive
carbons. As shown in Table 1, the cycloreversion quantum
yields of 1 and 4 are 0.59 and 0.41, which are 1.7 and 30 times
larger than those of 2 and 5, respectively. The cyano groups in-
crease the cycloreversion quantum yields. The cycloreversion
quantum yield of 1 is larger than the cyclization quantum yield
of 1. Consequently, we can control the cycloreversion reactivity
of the diarylethenes as much as 24000 times for 4 and 6.
The theoretical calculations in excited states of a model
system of the diarylethenes (scheme in Figure 2) were carried
out with Gaussian 9813 and GAMESS14 program packages in
order to confirm the substituent effect.15 As shown in Figure
2, upon excitation to the allowed 1B state which is so close
in energy to the 2A state, very fast internal conversion to the
2A state is assumed. The excited state potential energy surface
possesses two minima (2Ac and 2Ao) in the proximity of the
closed-ring isomer at the transition state. Consequently, an en-
ergy barrier exists on the pathway from closed- to open-ring iso-
mers in excited state. When the 2Ac point is populated, the cy-
cloreversion quantum yield depends on the ability of the system
to overcome the energy barrier between 2Ac and 2Ao. The bar-
rier is related to the energy difference, ÁE (¼ E(2Ao) À
E(2Ac)). When the energy difference is small, the barrier is ex-
pected to become small. The calculated energy differences were
1.1,16 5.9, and 13.9 kcal molÀ1 for the cyano, methyl, and me-
thoxy derivatives, respectively.15 The order of ÁE, which de-
pended on the substituents, is well correlated with that of the cy-
cloreversion quantum yields obtained experimentally.
The present work was partly supported by the Grant-in-
Aids for Scientific Research on Priority Areas (No. 15033252
and 12131211) and a Grant-in-Aid for the 21st Century COE
Program, ‘‘Functional Innovation of Molecular Informatics’’
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. We thank NIPPON ZEON CO., Ltd. for
their supply of octafluorocyclopentene.
References and Notes
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2
3
4
5
6
7
8
9
1a: colorless crystals: mp 232.5–233.5 ꢀC; 1H NMR
(200 MHz, CDCl3) ꢁ 7.5–8.2 (m, 8H); MS m=z (Mþ) 490.
Anal. Calcd. for C23H8F6N2S2: C, 56.33; H, 1.64; N,
5.71%. Found: C, 56.30; H, 1.60; N, 5.69%.
4a: pale yellow colored crystals: mp 165.5–166.5 ꢀC; 1H
NMR (200 MHz, CDCl3) ꢁ 7.4–7.7 (m, 12H); MS m=z (Mþ)
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5.16%. Found: C, 59.91; H, 2.31; N, 5.22%.
3a: colorless crystals: mp 171.9–172.5 ꢀC; 1H NMR
(200 MHz, CDCl3) ꢁ 3.6 (s, 6H), 7.1–7.6 (m, 8H); MS m=z
(Mþ) 500. Anal. Calcd. for C23H14F6O2S2: C, 55.20; H,
2.82%. Found: C, 55.17; H, 2.81%.
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91/9 as the eluent), 1a and 1b were eluted at 52 and 68 min,
respectively.
Table 1. Absorption maxima of closed-ring isomers and quan-
tum yields in hexane
b
c
ꢀ
max/nma
Èo!c
Èc!o
0.59
0.35
1
496
517
547
545
575
625
0.28
0.35
0.34
0.42
0.59
0.44
211
3
10 4b was isolated by HPLC(silica gel; hexane/ethyl acetate =
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respectively.
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0.030
0.41
0.013
4
512
64
1:7 Â 10À5
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1B
2A
2Ao
2Ac
h
ν
1A
Vis.
R
R
R
R
H3C
H3C
CH3
CH3
S
S
S
S
closed-ring isomer
open-ring isomer
Distance between the reactive carbons
Figure 2. Schematic representation of the potential ener-
gy profile of the model system (R=CN, Me, OMe).
16 The value was determined for 1,2-bis(2-cyano-5-phenyl-3-
thienyl)ethene in this work.
Published on the web (Advance View) August 25, 2003; DOI 10.1246/cl.2003.858