Solvent effect on photochromism of a dithienylperfluorocyclopentene having diethylamino group

Article abstract of DOI:10.1016/j.tet.2009.05.053

Some photochromic diarylethenes having polar substituents were synthesized, and their photochromic behavior was examined. They exhibited photochromism with the similar photoreactivity in a non-polar solvent. However, the photocyclization quantum yield of

Full text of DOI:10.1016/j.tet.2009.05.053

Tetrahedron 65 (2009) 6104–6108
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Solvent effect on photochromism of a dithienylperfluorocyclopentene
having diethylamino group
,
b
,
a
a
Seiya Kobatake a , Yuko Terakawa , Hiroyuki Imagawa
*
a Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
b Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Kawaguchi Center Building,
4-1-8 Honcho, Kawaguchi 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 April 2009
Received in revised form 20 May 2009
Accepted 20 May 2009
Available online 27 May 2009
Some photochromic diarylethenes having polar substituents were synthesized, and their photochromic
behavior was examined. They exhibited photochromism with the similar photoreactivity in a non-polar
solvent. However, the photocyclization quantum yield of dithienylperfluorocyclopentene having diethyl-
amino group was found to decrease in polar solvents. The yield in acetonitrile was estimated to be 60
times smaller than that in hexane. The yields determined in various solvents were found to be correlated
with the solvent polarity parameter ET(30). Such an effect was not observed in dithienyl-
fluorocyclopentenes bearing methoxy or acetoxy group and in non-fluorinated dithienylcyclopentene
bearing diethylamino group.
Keywords:
Photochromism
Diarylethene
Ó 2009 Elsevier Ltd. All rights reserved.
Solvent effect
Quantum yield
Solvent polarity parameter
1. Introduction
solvents and low reactivity in polar solvents.27 It has been reported
to be due to the formation of a twisted intramolecular charge
Photochromic compounds have attracted much attention to
cause reversible property changes of not only coloration but also
geometrical structure, dielectric constant, refractive index, and
fluorescence by photoirradiation.1,2 Photochromic compounds are
classified into two types, thermally unstable type (T-type) and
thermally stable type (P-type) of the photogenerated isomers.1,3
Azobenzene and spiropyran belong to T-type photochromic com-
pounds. Among various photochromic compounds, diarylethenes
with heterocyclic aryl rings have been developed as a P-type photo-
chromic compound because of some excellent characteristics such as
thermal stability of both isomers and fatigue-resistant property.4–8
The diarylethenes can be potentially used for application to optical
memory media,9–11 switching devices,12–14 display materials,15,16
and photo-mechanical actuators.17–20
The solvent effects on the reactivity and the absorption band of
photochromic compounds have been reported for spiropyrans,21–24
spirooxazines,25 furylfulgides,26 and diarylethenes.27–31 In most
cases the dipole moment of the molecules plays an important role
of the solvent effects. Diarylethenes having maleic anhydride and
maleimide at the ethene part have high reactivity in non-polar
transfer (TICT) state in the open-ring isomer in polar solvents at the
excited state, which has a photoinactive conformation.32
We have recently designed and synthesized some diarylethenes
for novel material applications. Diarylethene molecules that show
photochromism and acidichromismwere designed.33 P-Type photo-
chromic diarylethenes with diethylamino group can switch to T-
type photochromic system by the addition of an acid as the external
stimuli.33 Introduction of substituents to photochromic diary-
lethenes may give us new possibility of novel properties.34 In this
work, we have investigated the photochromic reactivities of diaryl-
ethene 1a in various solvents and found that the photocyclization
quantum yield of 1a is strongly dependent on the solvent polarity.
Theeffect was unique in comparisonwith otherdiarylethenes 2a–5a
(Scheme 1). The solvent effect was correlated with the solvent po-
larity parameter ET(30), which represents the molar electronic
transition energies of a betaine as the overall polarity of solvents.35
2. Results and discussion
2.1. Photochromic behavior
Photochromic reactivity of 1a was examined in various sol-
vents. Figure 1 shows absorption spectral changes of 1a in hexane
and acetonitrile. Upon irradiation with 313-nm light, the color of
* Corresponding author. Tel./fax: þ81 6 6605 2797.
E-mail address: kobatake@a-chem.eng.osaka-cu.ac.jp (S. Kobatake).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.05.053

S. Kobatake et al. / Tetrahedron 65 (2009) 6104–6108
6105
X
X
X
X
the solution turned to blue and green in hexane and acetonitrile,
respectively. The colored solution was bleached by irradiation
with visible light (l>520 nm) in both solutions. Table 1 shows
X
X
X
X
X
X
X
X
UV
Vis.
absorption maxima and absorption coefficients of the open- and
closed-ring isomers. The absorption bands of 1a and 1b were
shifted to bathochromic by the solvent polarity. The solvent effect
of the absorption band was observed in the closed-ring isomer
more effectively than in the open-ring isomer. The absorption
maximum of the colored isomer 1b was observed at 607 and
630 nm in hexane and acetonitrile, respectively, depending on the
solvent polarity. The absorption coefficient of 1b increased
according to the shift of the absorption band to the longer
wavelength. These findings indicate that the molecule 1b has
Me
Me
Me
Me
S
S
S
S
R
R
1a: X=F, R=NEt2
2a: X=F, R=OMe
3a: X=F, R=OCOMe
4a: X=F, R=H
1b: X=F, R=NEt2
2b: X=F, R=OMe
3b: X=F, R=OCOMe
4b: X=F, R=H
5a: X=H, R=NEt2
5b: X=H, R=NEt2
Scheme 1. Photochromic reactions of diarylethenes 1a–5a.
relatively large dipole moment and the molecule at the
excited state is more stable in polar solvents. This is due to
p
p
–
–
p
*
p
*
excitation solvent effect called solvatochromism.36 Sol-
vatochromism is arised from the interaction between the dipole
moments of the solvent and the molecule. As a result, sol-
vatochromism appears to a large effect if the difference in the
dipole moments at the ground state and the excited state is
large.35
Figure 2 shows absorption spectral changes of 2a in hexane
and acetonitrile upon irradiation with 313-nm light. The ab-
sorption maximum of 2a was not shifted by the difference in the
solvents. Diarylethene 3a also exhibited the similar photochro-
mic behavior in hexane and in acetonitrile. The absorption
maxima of the closed-ring isomers in acetonitrile were shifted to
only 10 nm longer wavelength than that in hexane, whereas that
of 1b in acetonitrile was shifted to 23 nm longer than that in
hexane. The results are similar to that of diarylethene without
any substituent (R¼H) (4a). Even in non-fluorinated diaryl-
ethene with diethylamino group (5a) and its closed-ring isomer
(5b), the absorption maxima were not changed by the solvent in
spite of having diethylamino group. These results indicate that
both the strong electron-donating character of diethylamino
group and the electron-accepting character of the per-
fluorocyclopentene play an important role of the solvent effect
for solvatochromism.
The solvent effect was also observed for the photoreactivity.
Upon irradiation with 313-nm light, the photocyclization of 1a
proceeded to 100% conversion in hexane, whereas in acetonitrile
the conversion was only 66% in the photostationary state (PSS)
at the slower rate than that in hexane. The difference of the
photocyclization conversion between in hexane and in acetoni-
trile was not observed for other diarylethenes 2a–5a. This in-
dicates that the solvent effect of the photocyclization of 1a is
unique. The electron-donating and electron-accepting characters
for diethylamino group and perfluorocyclopentene ring were
also strongly affected by the solvent effect of the photochromic
reaction.
Figure 1. Absorption spectral changes of diarylethene 1a in hexane (1.4ꢀ10ꢁ5 M) (a)
and acetonitrile (1.5ꢀ10ꢁ5 M) (b) upon irradiation with 313-nm light.
Table 1
Photochemical properties of 1–5 in hexane and acetonitrile
Diarylethene
In hexane
In acetonitrile
Open-ring isomer
Closed-ring isomer
Fa/b
Conversiona/%
Open-ring isomer
Closed-ring isomer
Fa/b
Conversiona/%
lmax/nm
3
/Mꢁ1 cmꢁ1
lmax/nm
3
/Mꢁ1 cmꢁ1
lmax/nm
3
/Mꢁ1 cmꢁ1
lmax/nm
3
/Mꢁ1 cmꢁ1
1b
2
323
290
284
280
321
35,100
41,200
40,500
35,600
32,500
607
580
575
575
538
25,100
19,500
19,100
15,600
23,400
0.66
0.57
0.53
0.59
0.51
100
97
92
97
100
335
291
286
285
329
32,500
42,000
39,100
37,100
26,600
630
590
585
587
540
28,300
21,000
17,700
16,800
22,400
0.011
0.55
0.50
0.56
0.70
66
99
94
97
100
3
4c
5b
a
Upon irradiation with 313-nm light.
Ref. 33.
The results in hexane were cited from Ref. 37.
b
c

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S. Kobatake et al. / Tetrahedron 65 (2009) 6104–6108
acid was added to 1a in acetonitrile, the photocyclization quan-
tum yield increased to be 0.63. On the other hand, the photo-
cycloreversion quantum yields were similar in both solutions. The
slower photocoloration rate and the lower conversion in PSS in
acetonitrile are due to the smaller photocyclization quantum
yield.
The quantum yields were also measured in mixed solvents of
toluene and acetonitrile. Figure 3 shows the photocyclization
quantum yields measured in the mixed solvents. The quantum
yields decreased with the increasing volume fraction of acetoni-
trile. This indicates that the solvent polarity strongly affects the
photocyclization quantum yield. A decrease in the photocyclization
quantum yields with increasing solvent polarity was ascribed to the
presence of the TICT state in the excited state.27 In non-polar sol-
vents, diarylethenes have a photoreactive planar antiparallel con-
formation and a photoinactive parallel conformation both in the
excited state and in the ground state. However, in polar solvents,
the photoinactive twisted antiparallel conformation in the TICT
state has an advantage rather than the photoreactive planar anti-
parallel conformation in the excited state because the TICT state is
more stable than the planar conformation in the excited state. As
a result, the photocyclization quantum yields decreased in polar
solvents. The sensitivity of the solvent effect to the quantum yields
depends on the strength of electron-donating and electron-
accepting characters in the molecules.
In order to correlate the relationship between the photo-
cyclization quantum yields and the solvent polarity, we employed
ET(30) as a polar parameter of solvents.35 Figure 4 shows the re-
lationship between the photocyclization quantumyields and ET(30).
The ET(30) values for the mixed solvent of toluene and acetonitrile
were cited from Ref. 23. When we used solvents, of which ET(30) is
smaller than 146 kJ molꢁ1, diarylethene 1a exhibited the photo-
cyclization reaction with the high cyclization quantum yield. This
indicates that such solvents have no interaction with 1a in the ex-
cited state. However, when ET(30) is larger than 146 kJ molꢁ1, the
cyclization quantum yield decreased. In such cases, the population
Figure 2. Absorption spectral changes of diarylethene 2a in hexane (1.7ꢀ10ꢁ5 M) (a)
and acetonitrile (1.7ꢀ10ꢁ5 M) (b) upon irradiation with 313-nm light.
2.2. Quantum yield
The photochromic conversion is related to the photocyclization
and photocycloreversion quantumyields. Therefore, their yields of 1–
5 were examined. As shown in Table 1, the photocyclization quantum
yields of 1a were largely different in hexane and acetonitrile. How-
ever, other diarylethenes 2a–5a showed similar quantum yields in
hexane and acetonitrile. These results indicate that only diarylethene
1a exhibits the solvent effect to the reactivity.
To investigate the difference in the photoreactivity of 1a by the
solvent polarity in more detail, the photocyclization and cyclo-
reversion quantum yields were examined in various solvents.
Table 2 shows photocyclization and photocycloreversion quantum
yields in hexane, toluene, tetrahydrofuran (THF), and acetonitrile.
The photocyclization quantum yields were estimated to be 0.66
and 0.011 in hexane and acetonitrile, respectively. The value in
acetonitrile was found to be 60 times smaller than that in hexane.
The yields remarkably decreased in the polar solvents. When the
Figure 3. Photocyclization and photocycloreversion quantum yields in mixed solvents
of toluene and acetonitrile. Photocyclization and photocycloreversion reactions were
carried out upon irradiation with 313- and 500-nm light, respectively.
Table 2
Photochemical properties of 1a in various solvents
Solvent
1a
F1a/1b
1b
F1b/1a
Conversiona/%
lmax/nm
3
/Mꢁ1 cmꢁ1
lmax/nm
3
/Mꢁ1 cmꢁ1
Hexane
Toluene
THF
Acetonitrile
Acetonitrileb
323
335
334
335
293
35,100
30,200
33,600
32,500
37,400
0.66
0.66
0.32
0.011
0.63
607
626
631
630
590
25,100
24,500
28,100
28,300
15,800
0.0053
0.0084
0.0074
0.0099
0.015
100
99
98
66
99
a
Upon irradiation with 313-nm light.
b
In the presence of 10 equiv of CF3SO3H relative to 1a.33

S. Kobatake et al. / Tetrahedron 65 (2009) 6104–6108
6107
3. Conclusion
We examined the photochromic property and reactivity of some
diarylethenes bearing polar substituents in polar and non-polar
solvents. Diarylethene 1a and its closed-ring isomer showed sol-
vatochromism because the excitation of diarylethene 1a is con-
sidered to have a large dipole moment in comparison with the
ground state. The solvent dependence of 2a–5a was hardly ob-
served. The photocyclization quantum yield of 1a also showed the
solvent effect and was correlated with ET(30). The solvent effect of
diarylethene 1a is due to the formation of the TICT state in the
excited state by the strong electron-donating character of the
diethylamino group and the strong electron-accepting character of
perfluorocyclopentene ring.
4. Experimental section
4.1. General
Figure 4. Relationship between photocyclization quantum yield and ET(30) of solvents
(-: hexane, C: toluene, :: THF, 0: acetonitrile, B: mixture of toluene and
acetonitrile).
Solvents used were spectroscopic grade and purified by distil-
lation. 1H NMR and 13C NMR spectra were recorded on a Bruker
AV300N NMR spectrometer. IR spectra were recorded on a JASCO
FT/IR-410 spectrometer. Mass spectra were obtained on a Jeol JMS-
700T (FAB) spectrometer. Elemental analysis was performed with
an Elementar vario EL III. Absorption spectra were performed with
a JASCO V-560 spectrophotometer. Photoirradiation was carried out
using a 200 W mercury–xenon lamp (Moritex MUV-202) as the
light source. Monochromic light was obtained by passing the light
through a monochromator and a UV filter or a colored filter. The
relative quantum yields were determined by comparing the re-
action yield with a reference sample.
in the photoinactive twisted antiparallel conformation increases
relative to the photoreactive antiparallel conformation in the ex-
cited state. The good relationship between the cyclization quantum
yield and ET(30) was observed. The linear relationship was observed
by the plot of ln(1/F) against ET(30), as shown in Figure 5.
4.2. Synthesis
Diarylethenes 1a,33 2a,38 4a,37 and 5a33 were prepared accord-
ing to the methods in the literature. Diarylethene 3a was prepared
according to the synthetic routes as shown in Scheme 2.
4.2.1. 1-(2-Methyl-5-(4-(methoxymethoxy)phenyl)thien-3-yl)-2-
(2-methyl-5-phenylthien-3-yl)perfluorocyclopentene (9)
Under an argon atmosphere, 1.6 M n-butyllithium in hexane
(4.0 mL, 6.4 mmol) was added dropwise to a solution of 739 (1.7 g,
5.4 mmol) in dry THF (100 mL) at ꢁ78 ꢂC. The solution was stirred
for 2 h at the same temperature. To the solution was added drop-
wise the mixture of 1-(2-methyl-5-phenylthien-3-yl)hepta-
fluorocyclopentene (8)40 (1.2 g, 3.3 mmol) and dry THF (10 mL) at
ꢁ78 ꢂC, and then the mixture was stirred for 4 h at the same
Figure 5. Relationship between ln(1/
ene, :: THF, 0: acetonitrile, B: mixture of toluene and acetonitrile).
F) and ET(30) of solvents (-: hexane, C: tolu-
F
F
F
F
F
F
Br
Me
Br
Me
n-BuLi / THF
1) n-BuLi / THF
2) B(OBu)3
Me
F
F
F
F
F
Br
S
S
F
S
Me
S
3) CH3OCH2O
/ Pd(PPh3)4
Br
F
OCH2OCH3
OCH2OCH3
6
7
9
Me
S
8
F
F
F F
F
F
F
F
F
F
F
F
HCl
THF
Ac2O
Pyridine
Me
Me
S
Me
S
Me
S
S
OH
OCOCH3
10
3a
Scheme 2. Synthetic routes of diarylethene 3a.

6108
S. Kobatake et al. / Tetrahedron 65 (2009) 6104–6108
temperature. The reaction was stopped by the addition of water.
The solution was extracted with ether. The organic layer was
washed with water, dried over MgSO4, filtered, and concentrated.
The residue was purified by silica gel column chromatography with
hexane/ethyl acetate (9:1) as the eluent. Compound 9 was obtained
as a colorless solid by further purification by HPLC (0.81 g, 42%).
578.0809. Anal. Calcd for C29H20F6O2S2: C, 60.20; H, 3.48%. Found:
C, 60.27; H, 3.56%.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific
Research (C) (No. 19550142) from the Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan, and PRESTO, Japan
Science and Technology Agency.
1H NMR (300 MHz, CDCl3, TMS)
d¼1.95 (s, 3H, CH3), 1.96 (s, 3H,
CH3), 3.49 (s, 3H, OCH3), 5.20 (s, 2H, OCH2O), 7.05 (d, J¼8.8 Hz, 2H,
Ar), 7.17 (s, 1H, thienyl H), 7.28 (s, 1H, thienyl H), 7.30 (t, J¼7.4 Hz,
1H, Ar), 7.39 (t, J¼7.4 Hz, 2H, Ar), 7.46 (d, J¼8.8 Hz, 2H, Ar), 7.54 (d,
References and notes
J¼7.4 Hz, 2H, Ar). 13C NMR (75 MHz, CDCl3):
¼14.5, 14.6, 56.1, 94.4,
d
111.1 (t of quintet, JC–F¼271, 271, 25, 25, 25, 25 Hz), 116.2 (tt, JC–
1. Brown, G. H. Photochromism; Wiley-Interscience: New York, NY, 1971.
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3. Kobatake, S.; Irie, M. Annu. Rep. Prog. Chem., Sect. C 2003, 99, 272–313.
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¼257, 257, 24, 24 Hz), 116.7, 121.5, 122.4, 125.6, 125.7, 125.9, 126.9,
F
127.3, 127.9, 129.0, 133.3, 136.0, 140.5, 141.3, 142.0, 142.2, 157.1.
HRMS (FAB) calcd for C29H22F6O2S2 580.0965, found 580.0985.
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4.2.2. 1-(2-Methyl-5-(4-hydroxyphenyl)thien-3-yl)-2-(2-methyl-5-
phenylthien-3-yl)perfluorocyclopentene (10)
Into a flask containing diarylethene 9 (0.70 g, 1.2 mmol) in THF
(15 mL) was added concd hydrochloric acid (2 mL), and the solution
was stirred overnight at room temperature. The solution was
neutralized and extracted with ether. The organic layer was washed
with water, dried over MgSO4, filtered, and concentrated. The res-
idue was purified by silica gel column chromatography with hex-
ane/ethyl acetate (8:2) as the eluent. Compound 10 was obtained as
a solid by further purification by HPLC (0.59 g, 92%).
1H NMR (300 MHz, CDCl3, TMS)
d
¼1.94 (s, 3H, CH3), 1.96 (s, 3H,
CH3), 4.90 (br, 1H, OH), 6.85 (d, J¼8.1 Hz, 2H, Ar), 7.16 (s, 1H, thienyl
H), 7.28 (s, 1H, thienyl H), 7.30 (t, J¼7.6 Hz, 1H, Ar), 7.38 (t, J¼7.6 Hz,
2H, Ar), 7.42 (d, J¼8.1 Hz, 2H, Ar), 7.54 (d, J¼7.6 Hz, 2H, Ar). 13C NMR
(75 MHz, CDCl3)
d
¼14.5, 14.6, 111.1 (t of quintet, JC–F¼271, 271, 25,
25, 25, 25 Hz), 115.9, 116.2 (tt, JC–F¼257, 257, 24, 24 Hz), 121.3, 122.4,
125.6, 125.7, 125.9, 126.4, 127.2, 127.9, 129.0, 133.3, 136.1, 140.3,
141.3, 142.0, 142.2, 155.5. IR (KBr):
(FAB) calcd for C27H18F6OS2 536.0703, found 536.0701.
n
¼3380 cmꢁ1 (br, OH); HRMS
4.2.3. 1-(2-Methyl-5-(4-acetoxyphenyl)thien-3-yl)-2-(2-methyl-5-
phenylthien-3-yl)perfluorocyclopentene (3a)
Into a flask were added diarylethene 10 (0.27 g, 0.50 mmol),
acetic anhydride (0.24 g, 2.4 mmol), and pyridine (0.080 g,
1.0 mmol). The solution was stirred overnight at room temperature.
The solution was extracted with ether. The organic layer was
washed with water, dried over MgSO4, filtered, and concentrated.
The residue was purified by silica gel column chromatography with
hexane/ethyl acetate (8:2) as the eluent. Compound 3a was
obtained as a solid by further purification by HPLC (0.24 g, 83%).
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1H NMR (300 MHz, CDCl3, TMS)
d¼1.952 (s, 3H, CH3), 1.959 (s,
3H, CH3), 2.32 (s, 3H, COCH3), 7.11 (d, J¼8.8 Hz, 2H, Ar), 7.24 (s, 1H,
thienyl H), 7.28 (s, 1H, thienyl H), 7.30 (t, J¼7.4 Hz, 1H, Ar), 7.39 (t,
J¼7.4 Hz, 2H, Ar), 7.54 (d, J¼8.8 Hz, 2H, Ar), 7.54 (d, J¼7.4 Hz, 2H,
Ar). 13C NMR (75 MHz, CDCl3)
d
¼14.5, 21.1, 111.0 (t of quintet, JC–
¼271, 271, 25, 25, 25, 25 Hz), 116.2 (tt, JC–F¼257, 257, 24, 24 Hz),
F
122.2, 122.3, 122.6, 125.6, 125.8, 125.9, 126.7, 127.9, 129.0, 131.2,
133.3, 136.1, 141.3, 141.5, 142.3, 150.3, 169.4. IR (KBr):
(s, C]O); HRMS (FAB) calcd for C29H20F6O2S2 578.0809, found
n
¼1770 cmꢁ1