Photochromism of diarylethene derivatives bearing a benzo[b]silole unit

Article abstract of DOI:10.1016/j.tetlet.2011.08.063

Photochromic benzo[b]silole derivatives, 1-(1,1-dimethyl-2-phenylbenzo[b] silol-3-yl)-2-(2-phenylbenzo[b]thien-3-yl)perfluorocyclopentene and 1-(1,1-dimethyl-2-phenylbenzo[b]silol-3-yl)-2-(2-phenyl-1-benzofuran-3-yl) perfluorocyclopentene, were synthesize

Full text of DOI:10.1016/j.tetlet.2011.08.063

Tetrahedron Letters 52 (2011) 5601–5604
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Photochromism of diarylethene derivatives bearing a benzo[b]silole unit
Tadatsugu Yamaguchi a, , Manabu Hosaka a, Toru Ozeki a, Masakazu Morimoto b, Masahiro Irie b
⇑
a Hyogo University of Teacher Education, 942-1 Shimokume, Kato, Hyogo 673 1494, Japan
b Department of Chemistry, Research Center for Smart Molecules, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171 8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2011
Revised 10 August 2011
Accepted 10 August 2011
Available online 22 August 2011
Photochromic benzo[b]silole derivatives, 1-(1,1-dimethyl-2-phenylbenzo[b]silol-3-yl)-2-(2-phen-
ylbenzo[b]thien-3-yl)perfluorocyclopentene and 1-(1,1-dimethyl-2-phenylbenzo[b]silol-3-yl)-2-(2-phe-
nyl-1-benzofuran-3-yl)perfluorocyclopentene, were synthesized and their photochromic performance
was examined in solution.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Photochromism
Diarylethene
Benzo[b]silole
Photochromism has attracted considerable attention because of
its potential application to molecular devices, such as optical mem-
ories and switches.1 Among various thermally irreversible photo-
chromic compounds, diarylethene derivatives are the most
promising because of their fatigue resistance and thermally irre-
versible properties.2,3 The diarylethene has two isomers which
are called open- and closed-ring isomers. The diarylethene deriva-
tives thus far synthesized have hetero-aryl moieties such as thio-
reaction of 3-bromo-1,1-dimethyl-2-phenylbenzo[b]silole with
1-(2-phenyl-1-benzofuran-3-yl)heptafluorocyclopentene gave
diarylethene 2a in 13% yield. 1,2-Bis(2-methylbenzo[b]thiophen-
3-yl)perfluorocyclopentene (3a) and 1,2-bis(2-methyl-1-benzofu-
ran-3-yl)perfluorocyclopentene (4a) were also synthesized as
reference compounds. These structures were confirmed by 1H
NMR, mass spectroscopy, and elemental analysis. The compounds
1a and 2a were also confirmed by X-ray crystallography.19,20
Figure 1a shows the absorption spectral change of 1a upon
photoirradiation in hexane. At the photostationary state under
irradiation with 313 nm light, 16% of the open-ring isomer 1a,
phene,4
benzothiophene,5
thiazole,6
oxazole,7
furan,8
benzofuran,9 indene,10 pyrrole,11 indole,12 imidazole,13 pyrazole,14
and isooxazole15 rings. Benzene derivatives16 also show photo-
chromism in solution. Their photochromic properties are strongly
dependent on their aryl moieties. The aryl units contain organic
aromatic rings, which are carbon, hydrogen, oxygen, nitrogen,
and sulfur atoms. The thermal stability of both isomers of diary-
lethene-type photochromic compounds can be attained by intro-
ducing heterocyclic aryl groups, which have low aromatic
stabilization energy.3
Recently, interesting studies on silole and benzo[b]silole rings
have been reported.17,18 In this study, the novel diarylethene deriv-
atives 1a and 2a having a benzo[b]silole group were synthesized
and their photochromic performance was examined in solution
(Scheme 1).
which has the maxima at 277 nm (
e
: 1.82 Â 104 MÀ1 cmÀ1), con-
verted to the closed-ring isomer 1b, which has the absorption max-
ima at 482 nm (
e
: 1.31 Â 104 MÀ1 cmÀ1). The colorless solution
became orange. Upon irradiation with light of wavelength longer
than 440 nm light, the closed-ring isomer returned to the original
1a. However, the thermal stability of 1b was absent at room tem-
perature (25 °C). When we plot the logarithmic scale of absorbance
changes of 1b at 482 nm versus the time, we get a straight line
according to first order kinetics (Fig. 2). The rate constant (k) of
the thermal cycloreversion reaction is 0.00283 hÀ1, and the life-
time of half value (s) from the closed isomer to the open isomer
is 354 h. No degradation process occurs during the thermal stabil-
The diarylethenes 1a and 2a were synthesized according to
Scheme 2. The coupling reaction of 3-bromo-1,1-dimethyl-2-phen-
ylbenzo[b]silole with 1-(2-phenylbenzo[b]thiophene-3-yl)hepta-
fluorocyclopentene gave diarylethene 1a in 12% yield (Scheme 1).
The synthetic route of 2a was the same as that of 1a. The coupling
ity experiment.
Compound 2a also underwent photochromism upon irradiation
with UV and visible light in hexane (Fig. 1b). 2a showed the
absorption maximum at 280 nm (
irradiation with 313 nm light, the diarylethene transformed into
the closed-ring isomer 2b (emax 1.68 Â 104 MÀ1 cmÀ1) at 435 nm.
At the photostationary state, the conversion ratio from the open-
to the closed-ring isomer was 67%. Upon irradiation with light of
wavelength longer than 440 nm, the closed-ring isomer returned
e
max 2.35 Â 104 MÀ1 cmÀ1). Upon
⇑
Corresponding author. Tel./fax: +81 725 44 2200.
E-mail address: tyamagu@hyogo-u.ac.jp (T. Yamaguchi).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.063

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T. Yamaguchi et al. / Tetrahedron Letters 52 (2011) 5601–5604
F2
F2
F2
F2
F2
X
F2
Si
(a)
hν
0.6
0.4
Ph
Ph
1b: X=S
2b: X=O
Ph
hν'
Si
X
Ph
1a: X=S
2a: X=O
F2
F2
F2
F2
Y
F2
F2
0.2
0.0
hν
Ph
Ph
3b: Y=S
4b: Y=O
Ph
Ph
3a: Y=S
hν'
Y
Y
Y
300
400
500
600
700
4a: Y=O
Wavelength / nm
Scheme 1. Photochromic reactions of diarylethenes.
0.8
0.6
0.4
(b)
F2
F2
Si
F2
X
i), ii)
I
Ph
Ph
Ph
Si
1a: X = S
2a: X = O
0.2
0.0
Scheme 2. Synthesis of benzo[b]silole derivatives 1 and 2. Reagents and conditions:
(i) n-BuLi(1.1 equiv), THF at À78 °C; (ii) 1-(2-phenylbenzo[b]thiophen-3-yl)hepta-
fluorocyclopentene(12%); (iii) 1-(2-phenyl-1-benzofuran-3-yl)heptafluorocyclo-
pentene(13%).
300
400
500
600
700
Wavelength / nm
to the original 2a. The closed-ring isomer 2b was stable at 25 °C in
hexane, and it was also stable at 70 °C in toluene for more than 3 h.
The cyclization and cycloreversion quantum yields were mea-
sured for compounds 1a–2a (Table 1). The data for the diaryleth-
enes 3a–4a are also shown in the table. The diarylethene 3a,
which has two benzo[b]thiophene rings, did not dissolve in hexane,
and the absorption data of 3a were determined in chloroform. The
molar absorption coefficient of 1b was determined from the com-
bined data of the HPLC measurement and the absorption spectra.
The closed-ring isomer diarylethene (4b), which has a benzofuran
ring, shows an absorption maximum at 482 nm in hexane. By
Figure 1. (a) Absorption spectra of 1a (dotted line) and at the photostationary state
(solid line) under irradiation with 313 nm light in hexane (3.02 Â 10À5 mol/L). (b)
Absorption spectra of 2a (dotted line), 2b (dashed line), and at the photostationary
state (solid line) under irradiation with 313 nm light in hexane (2.94 Â 10À5 mol/L).
changing an aryl unit from a benzofuran ring to a benzo[b]silole
ring, the absorption maximum of the closed-ring isomer (2b)
shifted to shorter wavelength by as much as 47 nm. Although the
solvents of 1b and 3b were different, the same tendency was ob-
served. For compound 3b, by changing an aryl unit from a benzo-
thiophene ring to a benzo[b]silole ring, the absorption maximum
of the closed-ring isomer (1b) shifted to the shorter wavelength.
The differences of the absorption band of the closed-ring isomers
1b–4b originate from the differences of the aromaticity.
-0.78
-0.80
-0.82
It was reported that the closed-ring isomer for 1,2-bis(2-
methyl-1-benzofuran-3-yl)perfluorocyclopentene (5a) is 469 nm
Table 1
Absorption characteristics and quantum yields of 1–4
-0.84
-0.86
-0.88
Compound (solvent)
e
/104 dm3 molÀ1 cmÀ1
Quantum yield
a
b
Cyclization Cycloreversion
1
1.82
(277 nm)
2.35
(280 nm)
2.81
(273 nm)
3.42
1.31
(482 nm)
1.68
(435 nm)
0.705
(524 nm)
1.80
—
—
(hexane)
2
0.46
0.38
(hexane)
(313 nm)
0.29
(436 nm)
0.29
3
0
10 20 30 40 50
Time / h
60 70 80
(chloroform)
(313 nm)
0.39
(524 nm)
0.27
4
(hexane)
(277 nm)
(482 nm)
(313 nm)
(482 nm)
Figure 2. Plot of logA versus time for first order process.

T. Yamaguchi et al. / Tetrahedron Letters 52 (2011) 5601–5604
5603
S2
C52
Si1
C1
C17
C36
S1
Si2
Figure 3. The ORTEP drawing of 1a. The ellipsoids represent 50% displacement of atoms.
O2
C36
C52
Si2
Si1
C1
C17
O1
Figure 4. The ORTEP drawing of 2a. The ellipsoids represent 50% displacement of atoms.
in hexane.9 The closed-ring isomer of 1-(2-methyl-1-benzofuran-
3-yl)-2-(2-methyl-1-inden-3-yl)perfluorocyclopentene (6a)10 is
435 nm in hexane. By changing an aryl unit from a benzothiophene
ring to an indene ring, the absorption maximum of the closed-ring
isomer (2b) shifted to a shorter wavelength by as much as 34 nm.
The hypsochromic effect is slightly larger for the benzo[b]silole
ring than for the indene ring.
ran derivative 5a has the methyl unit at the reactive carbon atoms.
By changing the methyl unit to the phenyl unit, the cyclization
quantum yield of 4 become slightly larger than that of 5. For com-
pound 3, the cyclization quantum yield is slightly smaller than that
of compound 4.
The previous works on 1,1-dimethyl-2-phenylbenzo[b]silole17
showed that the compound had a high fluorescence quantum yield
Table 1 also shows the cyclization and cycloreversion quantum
yields. Those of compound 1 could not be measured because the
closed-ring isomer 1b is unstable at room temperature in solution.
Those of compound 2 could be measured. The values for diaryleth-
ene 2 are similar to those for the reference compound 4 in hexane.
The cyclization quantum yield of 5 is 0.38 in hexane. The benzofu-
(Ufl=0.55(300 nm)) in dichloromethane. But the fluorescence
quantum yields of 1a and 2a are 0.054 and 0.010 (313 nm) in hex-
ane, respectively.19 These diarylethenes have a perfluorocyclopen-
tene moiety, and they are not good groups of fluorescent properties
due to their strong deficient-electronic function. The fluorescent
quantum yield of 2b is 0.003 (313 nm) in hexane. The fluorescence

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T. Yamaguchi et al. / Tetrahedron Letters 52 (2011) 5601–5604
intensity reversibly changed upon irradiation with UV and visible
light.
471)’ from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan.
A single crystal was obtained by recrystallization from hexane,
and X-ray crystallographic analysis of the crystal was carried out.
Figure 3 shows the ORTEP drawing of 1a.20 The drawing shows that
this diarylethene has benzo[b]thiophene and benzo[b]silole rings.
The aryl moieties are in an anti-parallel conformation, and the dis-
tances between the reactive carbon atoms are 0.386 nm (C1–C17)
and 0.374 nm (C36–C52). The distances are suitable for the mole-
cule to undergo the photochromic reaction in the crystal.9 The col-
orless crystals of 1a were irradiated with 365 nm light, and the
crystal color became pale violet. After the colored crystals were al-
lowed to stand for 3 days, the crystal color became colorless.
Figure 4 shows the ORTEP drawing of 2a.21 The drawing shows
that this diarylethene has benzofuran and benzo[b]silole rings. The
aryl moieties are in an antiparallel conformation, and the distances
between the reactive carbon atoms are 0.386 nm (C1–C17) and
0.374 nm (C36–C52). The colorless crystals of 2a were irradiated
with 365 nm light, and the crystal became brown. After the colored
crystals were irradiated with >440 nm light, the crystal became
colorless.
Supplementary data
Supplementary data associated with this Letter can be found, in
the online version, at doi:10.1016/j.tetlet.2011.08.063.
References and notes
1. Dürr, H.; Bouas-Laurent, H. Photochromism Molecules and Systems; Elsevier:
Amsterdam, 1990.
2. Irie, M. Photo-Reactive Materials for Ultrahigh-Density Optical Memory; Elsevier:
Amsterdam, 1994.
3. Irie, M. Chem. Rev. 2000, 100, 1685.
4. Nakayama, Y.; Hayashi, K.; Irie, M. Bull. Chem. Soc. Jpn. 1991, 64, 202.
5. Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem. Soc., Chem. Commun.
1992, 206.
6. Uchida, K.; Ishikawa, T.; Takeshita, M.; Irie, M. Tetrahedron 1998, 54, 6627.
7. Kuroki, L.; Takami, S.; Shibata, K.; Irie, M. Chem. Commun. 2005, 6005.
8. Yamaguchi, T.; Irie, M. J. Mater. Chem. 2006, 16, 4690.
9. Yamaguchi, T.; Irie, M. J. Org. Chem. 2005, 25, 10323.
10. Yamaguchi, T.; Fujita, Y.; Irie, M. Tetrahedron Lett. 2006, 47, 1267.
11. Uchida, K.; Matsuoka, T.; Sayo, K.; Iwamoto, M.; Hayashi, S.; Irie, M. Chem. Lett.
1999, 835.
12. Yagi, K.; Irie, M. Bull. Chem. Soc. Jpn. 2003, 76, 1623.
13. Nakashima, T.; Goto, M.; Kawai, S.; Kawai, T. J. Am. Chem. Soc. 2008, 130, 14570.
14. Pu, S.; Yang, T.; Xu, J.; Chen, B. Tetrahedron Lett. 2006, 47, 6473.
15. Pu, S.; Li, H.; Liu, G.; Liu, W.; Cui, S.; Fan, C. Tetrahedron 2011, 67, 1438.
16. Pu, S.; Fan, C.; Miao, W.; Liu, G. Tetrahedron 2008, 64, 9464.
17. Ilies, L.; Tsuji, H.; Sato, Y.; Nakamura, E. J. Am. Chem. Soc. 2008, 130, 4240.
18. Ilies, L.; Tsuji, H.; Nakamura, E. Org. Lett. 2009, 11, 3966.
19. Guilbault, G. G. Practical Fluorescence; Marcel Dekker: New York, 1990.
In conclusion, new photochromic compounds having
a
benzo[b]silole unit were synthesized and their photochromic reac-
tivity was examined in solution. Although the benzofuran deriva-
tive (2b) showed thermal stability at 70 °C in toluene, the
benzo[b]thiophene derivative (1b) showed thermal instability at
25 °C in hexane. They showed photochromism in single-crystalline
phase. Further modification of the benzo[b]silole derivatives is
now in progress in our laboratory, and the reason why the diary-
lethene 1b is thermally unstable will be clarified.
ꢀ
20. Crystal data for 1a: C35H24F6SSi, MW = 618.69, triclinic, space group P1,
a = 10.5709(8) Å, b = 10.8900(8) Å, c = 28.972(2) Å,
a = 95.2270(10)°,
b = 91.9470(10)°,
c
= 117.5680(10)°, V = 2933.3(4)Å3, Z = 4, Dc = 1.401 g cmÀ3
,
R1 = 0.0523 for 11921 observed reflections with I > 2r(I) from 32078 unique
reflections. CCDC deposition number: 825052.
Acknowledgment
ꢀ
21. Crystal data for 2a:
a = 10.3316(7) Å, b = 10.9624(8) Å, c = 28.891(2) Å,
b = 91.8890(10)°,
= 116.7640(10)°, V = 2901.5(4)Å3, Z = 4, Dc = 1.380 g cmÀ3
C
35H24F6OSi, MW = 602.63, triclinic, space group P1,
a
= 94.9020(10)°,
c
,
This work was supported by a Grant-in-Aid for Scientific Re-
search in a Priority Area ‘New Frontiers in Photochromism (No.
R1 = 0.0528 for 11747 observed reflections with I > 2r(I) from 31626 unique
reflections. CCDC deposition number: 825051.