Extraordinarily low cycloreversion quantum yields of photochromic diarylethenes with methoxy substituents

Article abstract of DOI:10.1246/cl.2001.618

Diarylperfluorocyclopentenes having 2-methoxy-5-phenylthiophene as the aryl groups were synthesized. Introduction of two methoxy substituents instead of methyl groups at the reactive carbons of 1,2-bis(5-phenyl-3-thienyl)perfluorocyclopentene decreased the photocyclorever-sion quantum yield by a factor of 10³, while the cyclization quantum yield was similar.

Full text of DOI:10.1246/cl.2001.618

618
Chemistry Letters 2001
Extraordinarily Low Cycloreversion Quantum Yields
of Photochromic Diarylethenes with Methoxy Substituents
Katsunori Shibata, Seiya Kobatake, and Masahiro Irie*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, and CREST,
Japan Science and Technology Corporation, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581
(Received March 21, 2001; CL-010246)
Diarylperfluorocyclopentenes having 2-methoxy-5-
0.37 mmol) were added to the mixture, and the mixture was
refluxed for 5 h at 70 °C to form 15 g of 3-bromo-2-methoxy-
5-phenylthiophene (4) (Yield: 63%). To the mixture of 4 (14 g,
52 mmol) and 140 mL of dry THF, 36 mL of a n-BuLi hexane
solution (59 mmol) was added at –78 °C. Then, to the solution
was added 3.5 mL (26 mmol) of octafluorocyclopentene to
yield 2.5 g of 1-(2-methoxy-5-phenyl-3-thienyl)perfluorocyclo-
pentene (5) (Yield: 25% based on octafluorocyclopentene) and
7.2 g (1.3 mmol) of 1a (Yield: 50%).6 A n-BuLi hexane solu-
tion (0.84 mL, 1.4 mmol) was added to a dry THF solution of
3-bromo-2-methyl-5-phenylthiophene7 (0.33 g, 1.3 mmol) at
–78 °C, and to the mixture was added 5 (0.50 g, 1.3 mmol) to
yield 0.47 g (0.87 mmol) of 2a (Yield: 67%).8 1,2-Bis(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene (3a) were
synthesized by a similar method according to the literature.7
phenylthiophene as the aryl groups were synthesized.
Introduction of two methoxy substituents instead of methyl
groups at the reactive carbons of 1,2-bis(5-phenyl-3-
thienyl)perfluorocyclopentene decreased the photocyclorever-
sion quantum yield by a factor of 103, while the cyclization
quantum yield was similar.
Photochromism is referred as a reversible photoisomeriza-
tion between two isomers having different absorption spectra.1
Among a number of photochromic compounds, diarylethenes
with heterocyclic aryl groups, such as thiophene or benzothio-
phene aryl groups, are the most promising compounds for
applications to optical memories and switches because of their
thermally irreversible and fatigue resistant photochromic per-
formance.2 Diarylethenes having various substituents have
been so far synthesized. Introduction of electron-donating sub-
stituents, such as methoxy or diethylamino groups, at para-posi-
tions of the phenyl groups of 1,2-bis(2,4-dimethyl-5-phenyl-3-
thienyl)perfluorocyclopentene decreased photocycloreversion
quantum yields by a factor of 2 to 6.3 Bulky substituents at the
reactive carbons of diarylethenes increased the photocyclization
quantum yields and decreased the thermal stability of the
closed-ring isomers.4,5 Here, we examined the effect of
methoxy substituents at the reactive carbons on the quantum
yields of photocyclization and photocycloreversion. The quan-
tum yields of methoxy-substituted derivatives (1 and 2) were
measured and compared with those of 3.
1,2-Bis(2-methoxy-5-phenyl-3-thienyl)perfluorocyclo-
pentene (1a) and 1-(2-methoxy-5-phenyl-3-thienyl)-2-(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a) were
synthesized as follows. To dry ether containing 3,5-dibromo-2-
methoxythiophene (24 g, 88 mmol) was added a 15% n-BuLi
hexane solution (56 mL, 92 mmol) at –78 °C. Tributylborate
(32 mL, 123 mmol) was slowly added to the reaction mixture at
–78 °C, and the mixture was stirred for 2 h. Iodobenzene (18 g,
88 mmol), 20 wt% Na2CO3 (aq) (90 mL), and Pd(PPh3)4 (4.3 g,
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
619
Figure 1a shows the absorption spectral change of 1 by
irradiation with 313 nm light. 1a has the absorption maxima at
267 and 309 nm in hexane. Upon irradiation with 313 nm light,
the colorless solution of 1a turned blue, in which the visible
absorption maximum was observed at 625 nm. The photoirra-
diated sample was analyzed by HPLC (silica gel; hexane/ethyl
acetate = 4/1 as the eluent). The sample gave only one peak
(monitoring wavelength: 326 nm, elution time: 14 min), which
was different from that of the open-ring isomer 1a (elution
time: 24 min). This indicates 1a completely converted to the
colored product. The structure of the isolated colored product
The polar factor rather than the resonance factor of the methoxy
group is considered to affect the photocycloreversion reaction,
because the methoxy groups in the closed-ring isomers are not
conjugated with the π-conjugation of the closed-ring isomers.
The colored isomers of almost all photochromic
diarylethenes are unstable under room visible light and the col-
ors are bleached. Therefore, they can not be used for display
applications, such as memory cards. The above photochemical-
ly stable photochromic compound can be potentially used to
memory cards as well as write-once memory media and color
dosimeters.11
1
was analyzed by mass spectrum, H NMR spectrum, and ele-
mental analysis.9 All data agreed well with the closed-ring iso-
mer 1b. The absorption spectrum of 1b is also shown in Figure
1a. The spectrum is identical with that in the photostationary
state. The conversion from 1a to 1b in the photostationary state
under irradiation with 313 nm light was 100%. The λmax of 1b
in the visible light region showed a bathochromic shift as much
as 50 nm in comparison with that of 3b. The blue color was
stable and never returned to the colorless form under room
light. Upon irradiation with the visible light (λ > 500 nm) for 4
h, it slowly reformed the open-ring isomer, while the blue color
of 3b was bleached immediately by irradiation with the light for
1 min. The photocycloreversion reaction of 1b was much slow-
er than that of 3b.
Figure 1b shows the absorption spectral change of 2 by
irradiation with 313 nm light. 2a has the absorption maxima at
271 and 298 nm in hexane. Upon irradiation with 313 nm light,
the colorless solution of 2a turned blue, in which the visible
absorption maximum was observed at 600 nm. The colored
product was isolated by HPLC (silica gel; hexane as the eluent),
and the structure was analyzed by mass spectrum and 1H NMR
spectrum, and elemental analysis.10 All data agreed well with
the closed-ring isomer 2b. The absorption spectrum of 2b is
also shown in Figure 1b. The conversion from 2a to 2b in the
photostationary state under irradiation with 313 nm light was
98%. The blue color was bleached by irradiation with the visi-
ble light (λ > 500 nm) for 5 min, and the absorption spectrum
returned to that of 2a.
This work was supported by CREST (Core Research
Evolutional Science and Technology) of Japan Science and
Technology Corporation.
References and Note
1
H. Dürr and H. Bouas-Laurent, “Photochromism
Molecules and Systems,” Elsevier, Amsterdam (1990).
M. Irie, Chem. Rev., 100, 1685 (2000).
M. Irie, K. Sakemura, M. Okinaka, and K. Uchida, J. Org.
Chem., 60, 8305 (1995).
2
3
4
5
6
K. Uchida, E. Tsuchida, Y. Aoi, S. Nakamura, and M. Irie,
Chem. Lett., 1999, 63.
S. Kobatake, K. Uchida, E. Tsuchida, and M. Irie, Chem.
Lett., 2000, 1340.
1
1a: pale yellow colored crystals: mp 171.9–172.3 °C; H
NMR (200 MHz, CDCl3) δ 3.71 (s, 6H), 7.16 (s, 2H),
7.2–7.5 (m, 10H); MS m/z (M+) 552. Anal. Calcd for
C27H18F6O2S2: C, 58.69; H, 3.28%. Found: C, 58.87; H,
3.29%.
7
8
M. Irie, T. Lifka, S. Kobatake, and N. Kato, J. Am. Chem.
Soc., 122, 4871 (2000).
2a: pale yellow colored crystals: mp 138.1–138.6 °C; H
1
NMR (200 MHz, CDCl3) δ 2.04 (s, 3H), 3.64 (s, 3H), 7.07
(s, 1H), 7.2–7.5 (m, 11H); MS m/z (M+) 536. Anal. Calcd
for C27H18F6OS2: C, 60.44; H, 3.38%. Found: C, 60.48; H,
3.37%.
1
9
1b: dark blue colored crystals: mp 194–195 °C; H NMR
To compare the photocyclization and photocycloreversion
reactivities of 1 and 2 with that of 3, the quantum yields were
measured. Table 1 summarizes the quantum yields, the absorp-
tion maxima of the open- and closed-ring isomers, and their
absorption coefficients. The cyclization quantum yields of 1a
(Φ = 0.44) and 2a (Φ = 0.44) were slightly smaller than that of
3a (Φ = 0.59). On the other hand, the cycloreversion quantum
yields of 1b and 2b were < 2.0 × 10–5 and 0.0016, respectively,
which were 1000 and 10 times smaller than that of 3b. The
methoxy substituents at the reactive carbons of the thienyl
groups remarkably decreased the cycloreversion quantum yield.
(200 MHz, CDCl3) δ 3.77 (s, 6H), 6.57 (s, 2H), 7.3–7.6 (m,
10H); MS m/z (M+) 552. Anal. Calcd for C27H18F6O2S2:
C, 58.69; H, 3.28%. Found: C, 58.77; H, 3.28%.
1
10 2b: dark blue colored crystals: mp 192–193 °C; H NMR
(200 MHz, CDCl3) δ 2.30 (s, 3H), 3.56 (s, 3H), 6.45 (s,
1H), 6.71 (s, 1H), 7.3–7.6 (m, 10H); MS m/z (M+) 536.
Anal. Calcd for C27H18F6OS2: C, 60.44; H, 3.38%. Found:
C, 60.68; H, 3.37%.
11 a) S. Irie, T. Yamaguchi, H. Nakazumi, S. Kobatake, and
M. Irie, Bull. Chem. Soc. Jpn., 72, 1139 (1999). b) S. Irie
and M. Irie, Bull. Chem. Soc. Jpn., 73, 2385 (2000).