Photochromism of new unsymmetrical diarylethene derivatives bearing both benzofuran and thiophene moieties

Article abstract of DOI:10.1016/j.dyepig.2012.01.003

A new class of unsymmetrical photochromic diarylethenes bearing both benzofuran and thiophene moieties was synthesized and the effects of substitution on their properties were discussed systematically. Two compounds among them show distinctly different conformation in the single crystalline phase: one crystallizes with an anti-parallel conformation which exhibit good photochromism, whereas the other crystallizes with a parallel conformation which exhibits no photochromism in the crystalline phase. Each of the compounds exhibited remarkable photochromism and functioned as a fluorescent switch in both solution and PMMA films. The electron-donating groups significantly increased the cyclization quantum yield, depressed the cycloreversion quantum yield, shifted the emission peak to a longer wavelength, and decreased the emission intensity; while the electron-withdrawing groups functioned as an inverse action for these diarylethene derivatives. The cyclic voltammograms results revealed that these diarylethenes exhibited evident electrochromism during electrolysis and the oxidative cyclization is thermodynamically.

Full text of DOI:10.1016/j.dyepig.2012.01.003

Dyes and Pigments 94 (2012) 195e206
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photochromism of new unsymmetrical diarylethene derivatives bearing both
benzofuran and thiophene moieties
*
Shouzhi Pu , Renjie Wang, Gang Liu, Weijun Liu, Shiqiang Cui, Peijian Yan
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of unsymmetrical photochromic diarylethenes bearing both benzofuran and thiophene
moieties was synthesized and the effects of substitution on their properties were discussed systemati-
cally. Two compounds among them show distinctly different conformation in the single crystalline
phase: one crystallizes with an anti-parallel conformation which exhibit good photochromism, whereas
the other crystallizes with a parallel conformation which exhibits no photochromism in the crystalline
phase. Each of the compounds exhibited remarkable photochromism and functioned as a fluorescent
switch in both solution and PMMA films. The electron-donating groups significantly increased the
cyclization quantum yield, depressed the cycloreversion quantum yield, shifted the emission peak to
a longer wavelength, and decreased the emission intensity; while the electron-withdrawing groups
functioned as an inverse action for these diarylethene derivatives. The cyclic voltammograms results
revealed that these diarylethenes exhibited evident electrochromism during electrolysis and the
oxidative cyclization is thermodynamically.
Received 30 July 2011
Received in revised form
24 November 2011
Accepted 8 January 2012
Available online 12 January 2012
Keywords:
Photochromism
Diarylethene
Benzofuran unit
Substituent effect
Optical property
Electrochromism
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the nature of the heteroaryl moieties greatly influences the photo-
reactivity and the distinguishable features of diarylethenes. For
Currently, organic photochromic materials have received
considerable attention because of their potential for photonic
applications such as optical storage materials, photoswitches, and
organic semiconductor devices [1e5]. To date, various types of
photochromic compounds such as diarylethenes [6], fulgides [7],
and phenoxynaphthacenequinines [8,9], have been developed in an
attempt to satisfy the requirements of these optoelectric devices.
Among the reported photochromic systems, photochromic diary-
lethenes are the most promising candidates for such applications
because of their good thermal stability, excellent fatigue resistance,
rapid response, and high reactivity in the solid state [2,10e15].
Especially, dithienylethenes bearing terminal benzene rings are of
considerable interest because the hydrogen atoms of the terminal
benzene ring can be replaced by different electron-donating/
withdrawing substituents, which efficiently modulate their
photochromic characteristics [16].
example, diarylethenes with thiophene/benzothiophene moieties
have good thermal stability and remarkable fatigue resistance
[6,17e19], whereas diarylethenes bearing two pyrrole groups are
thermally unstable and return to the open-ring isomers even in the
dark [20]. On the other hand, electron donor/acceptor substituents
and their substitution positions also have a significant effect on the
photochromic properties of diarylethene derivatives [21e24]. In
a typical example, the electron-donating substituents of the bis(3-
thienyl)ethene diarylethenes can be effective to increase the
absorption coefficients of the closed-ring isomers and decrease the
cycloreversion quantum yields [25,26], however, those attached
bis(2-thienyl)ethene diarylethenes can increase the absorption
maxima of the open-ring isomers and reduce the cyclization
quantum yields [27].
Maybe for the above reasons, the design and synthesis of new
photochromic diarylethene skeletons with different substituents
has become an active area of research at the present stage. Among
the reported diarylethenes, most of the heteroaryl diarylethenes
bear thiophene or benzothiophene rings which exhibit good
thermal stability and outstanding fatigue resistance [2,6], with only
a few reports concerning other heteroaryl moieties such as furan
[28], thiazole [29e31], pyrrole [32], indene [33], indole [34,35],
among others. Benzofuran is an attractive aryl unit with low
As we all know, the photochromic reactivity of diarylethene
mainly depends on the categories of heteroaryl groups and
different electron donor/acceptor substituents. On the one hand,
* Corresponding author. Tel./fax: þ86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.01.003

196
S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
aromatic stabilization energy and its molecular structure is similar
to that of benzothiophene, suggesting that introduction of the
benzofuran ring into the heteroaryl moieties of diarylethenes can
be expected to undergo thermally irreversible photochromic reac-
tions [36]. Up to date, there are a few examples of photochromic
diarylethenes bearing two benzofuran rings [36e39]. The diary-
lethenes bearing benzofuran moieties exhibit good photochromism
and outstanding fatigue resistance in solution as well as in the
single crystalline phase [36]. The results give us valuable insight
and encourage us to develop a new class of hybrid photochromic
diarylethene derivatives bearing a benzofuran moiety. To date,
most of the reported diarylethenes bearing benzofuran rings are
symmetrical compounds, unsymmetrical diarylethenes bearing
a benzofuran moiety are very rare [33,39]. As far as our knowledge,
no affiliation concerning photochromic hybrid diarylethene
bearing both benzofuran and thiophene moieties has been reported
so far.
potentiostatgalvanostat (EG&G Princeton Applied Research) under
computer control at room temperature. Platinum-electrodes
(diameter 0.5 mm) served as working electrode and counter elec-
trode. Platinum wire served as a quasi reference electrode. It was
calibrated using the ferrocene (Fc/Fcþ) redox couple which has
a formal potential E1/2 ¼ þ0.35 V versus platinum wire. The typical
electrolyte was acetonitrile (5.0 mL) containing 0.1 mol Lꢀ1 tetra-
butylammonium
tetrafuoroborate
((TBA)BF4)
and
4.0 ꢁ 10ꢀ3 mol Lꢀ1 diarylethene sample. All solutions were dea-
erated by bubbling with a dry argon stream and maintained at
a slight argon overpressure during electrochemical experiments.
The PMMA films of diarylethenes 1e5 were prepared by dissolving
10 mg of diarylethene sample and 100 mg of poly-
methylmethacrylate (PMMA) in chloroform (1 mL) with the aid of
ultrasound, and the homogeneous solution was spin-coated on
a quartz substrate (20 ꢁ 20 ꢁ 1 mm3) with a rotating speed at
1500 rpm. The films dried in air and kept in darkness at room
In this work, one of the main purposes was to explore a new
class of hybrid photochromic diarylethene derivatives bearing both
benzofuran and thiophene moieties and to discuss the effects of
substitution on their physicochemical properties. Accordingly, five
new hybrid diarylethene derivatives with different substituents at
the para-position of the terminal benzene ring were synthesized.
The synthesized diarylethenes are 1-(2-ethyl-3-benzofuranyl)-2-
[2-methyl-5-(4-methoxyphenyl)-3-thienyl]perfluorocyclopentene
(1o), 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-methylphenyl)-
3-thienyl]perfluorocyclopentene (2o), 1-(2-ethyl-3-benzofuranyl)-
2-(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (3o), 1-(2-
ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-fluorophenyl)-3-thienyl]
perfluorocyclopentene (4o), and 1-(2-ethyl-3-benzofuranyl)-2-[2-
methyl-5-(4-cyanophenyl)-3-thienyl]perfluorocyclopentene (5o).
The photochromic scheme of the diarylethenes is shown in Fig. 1.
temperature. The thickness of each film was about 3 mm.
Suitable crystals of 3o and 5o were obtained by slow evapora-
tion of a hexane solution. All the measurements were collected by
a Bruker SMART APEX II CCD diffractometer using a MULTI scan
technique at room temperature using Mo Ka radiation. The struc-
tures were solved by direct methods and refined by full-matrix
least-squares procedures on F2 by full-matrix least-squares tech-
niques using SHELXTL-97 program. Further details on the crystal
structure investigation have been deposited with The Cambridge
Crystallographic Data Centre as supplementary publication CCDC
823604 for 3o and 823605 for 5o. Copies of the data can be ob-
tained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: þ44 0 1223 336033 or e-mail:depo-
sit@ccdc.cam.ac.uk).
2.2. Synthesis of diarylethenes
2. Experiment
The synthetic route for diarylethenes 1oe5o is shown in Fig. 2.
Suzuki coupling of five bromobenzene derivatives with thiophene
boronic acid (10) [40,41] gave alkylphenylthiophene derivatives
(11aee). 1-(2-Ethyl-1-benzofuran-3-yl)perfluorocyclopentene (9)
was prepared by alkylation, bromination, and lithiation reactions
from benzofuran. Finally, compounds 11aee was separately lithi-
ated and then coupled with the mono-substituted compound 9 to
give the unsymmetrical diarylethene derivatives 1oe5o, respec-
tively. The structures of 1oe5o were confirmed by elemental
analysis, NMR, and IR.
2.1. General
The solvents were purified by distillation before use. 1H NMR
and 13C NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer with CDCl3 as the solvent and tetramethylsilane as an
internal standard. IR spectra were performed on a Bruker Vertex-70
spectrometer. Elemental analyses were determined with a PE CHN
2400 analyzer. Melting point was determined by a WRS-1B melting
point determination apparatus. The absorption spectra were
measured using an Agilent 8453 UV/VIS spectrophotometer. Photo-
irradiation was carried out using an SHG-200 UV lamp, CX-21
ultraviolet fluorescence analysis cabinet and a BMH-250 visible
lamp. Light of appropriate wavelength was isolated by different
light filters. Fluorescence spectra were measured using a Hitachi F-
4500 spectrophotometer. Electrochemical examinations were per-
2.2.1. 2-Ethylbenzofuran (7)
To a stirred anhydrous THF solution (100 mL) containing
benzofuran (6) (6.1 g, 51.64 mmol) was slowly added 20 mL n-BuLi/
hexane (2.84 mol Lꢀ1) at 195 K under nitrogen atmosphere, and the
solution was stirred for 45 min. Then the ethyl-bromide (4.63 mL,
62 mmol) was added slowly to the reaction mixture, and the
reaction mixture was warmed to room temperature, kept stirring
for another 16 h. Then the reaction mixture was poured into
concentrated sodium chloride solution and extracted with diethyl
ether. The organic layer was dried, filtrated, and concentrated. The
residue was purified by column chromatography using petroleum
ether as the eluent to give 6.3 g of 7 as a colorless liquid in 84% yield.
formed in
a
one-compartment cell by using a Model 263
F
F
F
F
F
F
F
F
F
F
F
F
UV
1H NMR (400 MHz, CDCl3, TMS):
d
1.33 (t, 3H, J ¼ 7.4 Hz, eCH3),
Vis
O
S
2.76e2.82 (m, 2H, eCH2), 6.36 (s, 1H, thienyleH), 7.15e7.23 (m,
2H, phenyleH), 7.40 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.47 (d, 1H,
J ¼ 6.8 Hz, phenyleH).
S
O
OCH3
OCH3
2o : R = CH3
1c : R =
2c : R =
3c : R =
4c : R =
5c : R =
R
1o : R =
R
CH3
H
H
3o : R =
4o : R =
5o : R =
F
F
2.2.2. 3-Bromo-2-ethyl-benzofuran (8)
CN
CN
To a stirred THF solution (100 mL) containing compound 7
(3.0 g, 20.53 mmol) was slowly added 4.02 g N-bromosullirimide
Fig. 1. Photochromism of diarylethenes 1e5.

S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
197
F
F
F
F
F
Br
NBS
C5F8
n-BuLi,195 K
F
O
O
n-BuLi,195 K
CH3CH2Br
THF
O
O
9
6
8
7
F
F
F
F
F
Br
Br
F
Br
R
n-BuLi,195 K
B(OH)2
S
S
Pd(Pph3)4
Na2CO3
9
S
O
R
10
OCH3
1o : R =
11 a : R =
11 b : R =
11 c : R =
OCH3
CH3
H
R
2o : R = CH3
H
3o : R =
4o : R =
5o : R =
F
11 d : R =
11 e : R =
F
CN
CN
Fig. 2. Synthetic route for diarylethenes 1oe5o.
(22.58 mmol) at 273 K. The reaction mixture was stirred for 24 h,
then was poured into sodium thiosulfate solution and extracted
with diethyl ether. The organic layer was dried, filtrated, and
concentrated. The residue was purified by column chromatography
using petroleum ether as the eluent to give 3.62 g of 8 as a pale
2.2.6. 3-bromo-2-methyl-5-phenyl-thiophene (11c)
Compound 11c was prepared by a method similar to that used
for 11a and obtained as a colorless solid in 55% yield. M.p.
66e68 ꢂC; 1H NMR (400 MHz, CDCl3, TMS):
d 2.44 (s, 3H, eCH3),
7.13 (s, 1H, thienyleH), 7.30 (d, 2H, J ¼ 8.0 Hz, phenyleH), 7.37 (t,
yellow liquid in 75% yield. 1H NMR (400 MHz, CDCl3, TMS):
d 1.32 (t,
2H, J ¼ 7.6 Hz, phenyleH), 7.53 (d, 2H, J ¼ 7.6 Hz, phenyleH).
3H, J ¼ 7.6 Hz, eCH3), 2.82e2.87 (m, 2H, eCH2), 7.24e7.29 (m, 2H,
phenyleH), 7.39e7.45 (m, 2H, phenyleH).
2.2.7. 3-bromo-2-methyl-5-(4-fluorophenyl)thiophene (11d)
Compound 11d was prepared by a method similar to that used
for 11a and obtained as a colorless solid in 33% yield. M.p. 61e63 ꢂC;
2.2.3. (2-Ethyl-1-benzofuran-3-yl) perfluorocyclopentene (9)
To a stirred THF solution (100 mL) containing compound 8
(3.0 g, 13.33 mmol) was slowly added 5.2 mL n-BuLi/hexane
(2.84 mol Lꢀ1) at 195 K under nitrogen atmosphere. After 40 min,
octaperfluoropentene (2.2 mL, 0.136 mL/mol) was added quickly to
the reaction mixture, and kept stirring for 2 h at 195 K under
nitrogen atmosphere. Then the reaction mixture was extracted
with diethyl ether and evaporated in vacuum. The residue was
purified by column chromatography using petroleum ether as the
eluent to give 2.6 g of 9 as a yellow oil liquid in 61% yield. 1H NMR
1H NMR (400 MHz, CDCl3, TMS):
d 2.41 (s, 3H, eCH3), 7.03 (s, 1H,
thienyleH), 7.06 (d, 2H, J ¼ 8.0 Hz, phenyleH), 7.46 (d, 2H,
J ¼ 8.0 Hz, phenyleH).
2.2.8. 3-Bromo-2-methyl-5-(4-cyanophenyl)thiophene (11e)
Compound 11e was prepared by a method similar to that used
for 11a and obtained as a pale yellow solid in 51% yield. M.p.
79e80 ꢂC; 1H NMR (400 MHz, CDCl3, TMS):
d 2.45 (s, 3H, eCH3),
7.23 (s, 1H, thienyleH), 7.59 (d, 2H, J ¼ 8.4 Hz, phenyleH), 7.65 (d,
(400 MHz, CDCl3, TMS):
(m, 2H, eCH2), 7.27e7.36 (m, 2H, phenyleH), 7.49e7.54 (m, 2H,
phenyleH).
d
1.36 (t, 3H, J ¼ 7.6 Hz, eCH3), 2.76e2.82
2H, J ¼ 8.4 Hz, phenyleH).
2.2.9. 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-
methoxyphenyl)-3-thienyl]perfluorocyclopentene (1o)
2.2.4. 3-Bromo-2-methyl-5-(4-methoxylphenyl)thiophene (11a)
Compound 11a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid (10) [40,41] (3.32 g, 15.0 mmol) with 1-bromo-
4-methoxylbenzene (2.8 g, 15.0 mmol) in the presence of Pd(PPh3)4
(0.40 g, 0.35 mmol) and Na2CO3 (6.36 g, 60 mmol) in tetrahydro-
furan (THF) (80 mL). After refluxing for 16 h at 95 ꢂC, the product
was extracted with ether. The organic layer was dried over MgSO4,
filtrated, and evaporated. The crude product was purified by
column chromatography on SiO2 using petroleum ether as the
eluent and 2.09 g of 11a obtained as a pale yellow solid in 49% yield.
To a stirred anhydrous THF (50 ml) containing 11a (0.57 g,
2.0 mmol) was added dropwise 0.75 mL n-BuLi/hexane solution
(2.93 mol Lꢀ1) at 195 K under argon atmosphere. After the mixture
has been stirred for 35 min, compound 9 (0.71 g, 2.1 mmol) in
solvent of anhydrous THF was added. The reaction was further
stirred at 195 K for 2 h, and the reaction was allowed to slowly
warm to the room temperature. The reaction was quenched with
water. The product was extracted with ether, dried with MgSO4,
filtered, and evaporated. The crude product was purified by column
chromatography (ether/acetyl acetate mixture, v/v ¼ 5:1) to give
0.54 g of 1o as a colorless solid in 52% yield. M.p. 64e65 ꢂC; Anal.
Calcd for C27H20F6O2S (%): C, 62.06; H, 3.86. Found C, 62.01; H, 3.73;
M.p. 106e107 ꢂC; 1H NMR (400 MHz, CDCl3, TMS):
d 2.40 (s, 3H,
eCH3), 3.83 (s, 3H, eOCH3), 6.90 (d, 2H, J ¼ 8.7 Hz, phenyleH), 6.98
(s, 1H, thienyleH), 7.43 (d, 2H, J ¼ 8.7 Hz, phenyleH).
1H NMR(400 MHz, CDCl3, TMS):
d
0.98 (t, 3H, J ¼ 7.5 Hz, eCH3), 1.87
(s, 3H, eCH3), 2.43e2.48 (m, 2H, eCH2), 3.82 (s, 3H, eOCH3), 6.90
(d, 2H, J ¼ 8.4 Hz, phenyleH) 7.17 (s, 1H, thienyleH), 7.21 (d, 1H,
J ¼ 8.0 Hz, phenyleH), 7.28 (t, 1H, J ¼ 8.0 Hz, phenyleH), 7.44 (t, 3H,
J ¼ 8.4 Hz, phenyleH), 7.51 (d, 1H, J ¼ 7.6 Hz, phenyleH); 13C NMR
2.2.5. 3-Bromo-2-methyl-5-(4-methylphenyl)thiophene (11b)
Compound 11b was prepared by a method similar to that used
for 11a and obtained as a colorless solid in 57% yield. M.p. 68e69 ꢂC;
1H NMR (400 MHz, CDCl3, TMS):
d
2.36 (s, 3H, eCH3), 2.41 (s, 3H,
(100 MHz, CDCl3, TMS): d 11.34, 14.64, 20.84, 55.38, 104.35, 111.09,
eCH3), 7.06 (s, 1H, thienyleH), 7.17 (d, 2H, J ¼ 8.0 Hz, phenyleH),
114.24, 114.42, 120.13, 121.42, 121.84, 123.56, 124.49, 125.33, 126.04,
126.22, 126.48, 126.79, 126.99, 140.36, 142.30, 154.29, 159.58,
7.40 (d, 2H, J ¼ 8.0 Hz, phenyleH).

198
S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
160.40; IR (
n
, KBr, cmꢀ1): 747, 803, 826, 893, 986, 1107, 1249, 1339,
cmꢀ1): 747, 805, 892, 977, 1123, 1194, 1276, 1401, 1567, 1509, 1591,
1629, 1684, 2224.
1452, 1476, 1516, 1555, 1573, 1638, 1657, 2839, 2958.
3. Results and discussion
2.2.10. 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-
methylphenyl)-3-thienyl]perfluorocyclopentene (2o)
3.1. Photochromism of diarylethenes 1e5
Diarylethene 2o was prepared by a method similar to that used
for 1o and 0.38 g of 2o obtained as a colorless solid in 38% yield.
M.p. 95e96 ꢂC; Anal. Calcd for C27H20F6OS (%): C, 64.03; H, 3.98.
Diarylethenes 1e5 exhibited typical absorption changes upon
alternating irradiation with UV and visible light in both hexane and
polymer matrix at room temperature due to the photochromic
reaction. The exposure energy of UV light was determined to ca.
Found C, 63.99; H, 3.85; 1H NMR(400 MHz, CDCl3, TMS):
d 0.97 (t,
3H, J ¼ 7.5 Hz, eCH3), 1.87 (s, 3H, eCH3), 2.37 (s, 3H, eCH3),
2.41e2.46 (m, 2H, eCH2), 7.19 (d, 2H, J ¼ 7.8 Hz, phenyleH), 7.23
(s, 1H, thienyleH), 7.25 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.29 (d, 1H,
J ¼ 8.0 Hz, phenyleH), 7.42 (t, 3H, J ¼ 8.2 Hz, phenyleH), 7.51 (d, 1H,
29 m
w cmꢀ2. Fig. 3 shows the absorption spectral and color changes
of diarylethenes 1L5 in hexane. For diarylethene 1, the absorption
maximum of the open-ring isomer 1o was observed at 294 nm.
Upon irradiation with 297 nm light, the colorless solution of 1o
turned to be magenta, in which the characteristic absorption
maximum was observed at 542 nm arising from the closed-ring
isomer 1c. The photostationary state is achieved upon irradiation
J ¼ 7.6 Hz, phenyleH); 13C NMR (100 MHz, CDCl3, TMS):
d 11.34,
14.71, 20.82, 21.18, 104.37, 111.09, 120.12, 121.91, 123.56, 124.48,
125.32, 125.56, 126.39, 129.65, 130.57, 137.92, 140.80, 142.46, 154.24,
160.39; IR (n
, KBr, cmꢀ1): 747, 798, 815, 836, 892, 990, 1043, 1073,
1121, 1143, 1191, 1270, 1345, 1400, 1595, 1639, 3122.
with UV light for 600 s. Upon visible light irradiation (l > 450 nm)
for 100 s, the magenta color was bleached indicating that 1c
returned to the initial state 1o. Diarylethenes 2e5 also exhibited
a diarylethenes-specific reversible photoisomerization, generating
the closed-ring isomers 2ce5c upon irradiation with UV light,
which their absorption maxima were observed at 537, 535, 533, and
541 nm, respectively. In the photostationary state, the isosbestic
points for diarylethenes 1e5 were observed at 313, 303, 297, 295,
and 337 nm in hexane. For diarylethenes 2-5, the irradiation time of
achieving the photostationary state is 820 s for 2, 730 s for 3, 700 s
for 4, and 610 s for 5, and the bleaching time is 90 s for 2, 110 s for 3,
95 s for 4, and 85 s for 5. When arrived at the photostationary state,
the photoconversion ratios from the open-ring to the closed-ring
isomers of diarylethenes 1e5 were analyzed by HPLC method,
with the values were 92% for 1, 88% for 2, 80% for 3, 83% for 4, and
88% for 5. The photochromic parameters of diarylethenes 1e5 are
summarized in Table 1. Compared to the unsubstituted parent
diarylethene 3, the absorption maxima of diarylethenes bearing an
electron-donating group (such as diarylethenes 1 and 2) showed
a red shift in both hexane and PMMA films. Although the electron-
withdrawing fluorine atom (diarylethene 4) exhibited a shorter
wavelength, the strong electron-withdrawing cyano group shifted
the absorption maxima of diarylethene 5 to a longer wavelength,
compared with diarylethene 3. As shown in Table 1, the absorption
maxima of the closed-ring isomers 1ce5c were appeared at the
region 533e542 nm in hexane, which were shorter than that of
diarylethenes with the similar molecular skeleton including pyr-
azole [42], pyrrole [43e45], pyridine [46], or biphenyl moiety [47],
but much longer than that of similar skeleton diarylethenes bearing
a thiazole or isoxazole moiety [48,49]. This indicated that the
benzofuran moiety could be effective to shift the absorption
maximum of diarylethene to a shorter wavelength direction, but its
action was less than thiazole or isoxazole ring. For the open-ring
isomers 1oe5o, the molar absorption coefficients increased
notably when going from electron-donating to electron-
withdrawing groups, but an inverse trend appeared for the
closed-ring isomers 1ce5c. The cyclization and cycloreversion
quantum yields of the unsubstituted parent diarylethene 3 are 0.38
and 0.079, respectively. Replacing the hydrogen atom at the para-
position of the terminal benzene ring with an electron-donating
substituent (methoxy or methyl group), the cyclization quantum
yields increased and the cycloreversion quantum yields decreased
notably with the increase in electron-donating ability; an inverse
changing trend appeared when replacing with an electron-
withdrawing group (fluorine or cyano group) at the same posi-
tion. As a result, diarylethene 1 has the biggest cyclization quantum
yield and the smallest cycloreversion quantum yield, and
2.2.11. 1-(2-ethyl-3-benzofuranyl)-2-(2-methyl-5-phenyl-3-
thienyl)perfluorocyclopentene (3o)
Diarylethene 3o was prepared by a method similar to that used
for 1o and 0.35 g of 3o obtained as a colorless solid in 36% yield.
M.p. 90e91 ꢂC. Anal. Calcd for C26H18F6OS (%): C, 63.41; H, 3.68.
Found C, 63.37; H, 3.62; 1H NMR(400 MHz, CDCl3, TMS):
d 0.98 (t,
3H, J ¼ 7.4 Hz, eCH3), 1.89 (s, 3H, eCH3), 2.39e2.47 (m, 2H, eCH2),
7.24 (s,1H, thienyleH), 7.30 (t, 3H, J ¼ 8.0 Hz, phenyleH), 7.38 (t, 2H,
J ¼ 7.4 Hz, phenyleH), 7.44 (d,1H, J ¼ 8.0 Hz, phenyleH), 7.52 (d, 3H,
J ¼ 7.2 Hz, phenyleH); 13C NMR (100 MHz, CDCl3, TMS):
d 11.35,
14.72, 20.83, 104.35, 111.09, 120.10, 122.49, 123.57, 124.50, 125.66,
126.40, 127.92, 128.99, 133.35, 141.32, 142.33, 154.26, 160.38; IR (n,
KBr, cmꢀ1): 751, 803, 839, 891, 983, 1074, 1126, 1195, 1272, 1337,
1436, 1474, 1552, 1630, 1647, 2932.
2.2.12. 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-
fluorophenyl)-3-thienyl]perfluorocyclopentene (4o)
Diarylethene 4o was prepared by a method similar to that used
for 1o and 0.65 g of 4o obtained as a colorless solid in 64% yield.
M.p. 102e103 ꢂC; Anal. Calcd for C26H17F7OS (%): C, 61.18; H, 3.36.
Found C, 61.11; H, 3.30; 1H NMR(400 MHz, CDCl3, TMS):
d 0.98 (t,
3H, J ¼ 7.4 Hz, eCH3), 1.88 (s, 3H, eCH3), 2.41e2.47 (m, 2H, eCH2),
7.08 (t, 2H, J ¼ 8.0 Hz, phenyleH), 7.25 (d, 2H, J ¼ 6.7 Hz, phenyleH),
7.29 (s, 1H, thienyleH), 7.44 (d, 1H, J ¼ 8.0 Hz, phenyleH), 7.45e7.50
(m, 3H, phenyleH); 13C NMR (100 MHz, CDCl3, TMS):
d 11.37, 14.69,
20.85, 104.35, 111.13, 115.89, 116.11, 116.28, 120.10, 122.52, 123.60,
124.56, 125.50, 126.41, 127.38, 127.46, 129.66, 141.27, 141.33, 154.29,
160.38, 161.33, 163.79; IR (n
, KBr, cmꢀ1): 747, 807, 895, 980, 1066,
1124, 1157, 1190, 1230, 1254, 1275, 1338, 1402, 1468, 1512, 1588,
1629, 1681, 2980.
2.2.13. 1-(2-ethyl-3-benzofuranyl)-2-[2-methyl-5-(4-
cyanophenyl)-3-thienyl]perfluorocyclopentene (5o)
Diarylethene 5o was prepared by a method similar to that used
for 1o and 0.23 g of 5o obtained as a colorless solid in 22% yield.
M.p.120e121 ꢂC; Anal. Calcd for C27H17F6NOS (%): Calcd C, 62.67; H,
3.31; N, 2.71. Found C, 62.62; H, 3.27; N, 2.68; 1H NMR(400 MHz,
CDCl3, TMS):
d
1.00 (t, 3H, J ¼ 7.6 Hz, eCH3), 1.94 (s, 3H, eCH3),
2.42e2.48 (m, 2H, eCH2), 7.23 (t, 1H, J ¼ 7.6 Hz, phenyleH), 7.30
(t, 1H, J ¼ 7.4 Hz, phenyleH), 7.40 (s, 1H, thienyleH), 7.44 (d, 1H,
J ¼ 8.4 Hz, phenyleH), 7.48 (d,1H, J ¼ 7.6 Hz, phenyleH), 7.59 (d, 2H,
J ¼ 8.4 Hz, phenyleH), 7.65 (d, 2H, J ¼ 8.4 Hz, phenyleH); 13C NMR
(100 MHz, CDCl3, TMS):
d 11.38, 14.82, 20.84, 104.18, 111.18, 118.54,
119.95,119.99,120.02,123.66,124.63,124.66,125.86,125.99,126.08,
126.29, 132.83, 137.49, 139.96, 143.55, 154.29, 160.35; IR ( , KBr,
n

S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
199
0.6
A
B
0.6
0.4
0.2
0.0
Vis
Vis
UV
UV
UV
0.4
UV
Vis
600 s
510 s
450 s
270 s
150 s
90 s
820 s
580 s
450 s
270 s
150 s
40 s
Vis
0.2
40 s
10 s
10 s
0 s
0 s
0.0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
C
D
0.6
0.6
Vis
Vis
UV
UV
0.4
0.2
0.0
0.4
0.2
0.0
700 s
450 s
270 s
150 s
90 s
UV
Vis
730 s
570 s
450 s
270 s
90 s
UV
Vis
40 s
10 s
10 s
0 s
0 s
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
E
F
0.6
0.4
0.2
0.0
Vis
UV
610 s
270 s
150 s
90 s
40 s
0 s
UV
Vis
300
400
500
600
700
Wavelength (nm)
Fig. 3. Absorption spectra and color changes of diarylethenes 1e5 by photoirradiation in hexane (C ¼ 2.0 ꢁ 10ꢀ5 mol Lꢀ1) at room temperature: (A) spectral changes for 1, (B)
spectral changes for 2, (C) spectral changes for 3, (D) spectral changes for 4, (E) spectral changes for 5, (F) color changes for 1e5.
Table 1
Absorption spectral properties of diarylethenes 1L5 in hexane (C ¼ 2.0 ꢁ 10ꢀ5 mol Lꢀ1) and in PMMA films (10%, w/w) at room temperature.
Compound
lo,max/nma (ε/L molꢀ1 cmꢀ1
)
lc,max/nmb (ε/L molꢀ1 cmꢀ1
)
Fc
Conversion at PSS in hexane
Hexane
PMMA film
Hexane
PMMA film
Foꢀc
Fcꢀo
1
2
3
4
5
294 (2.61 ꢁ 104)
257 (2.70 ꢁ 104)
256 (2.81 ꢁ 104)
255 (2.89 ꢁ 104)
311 (2.91 ꢁ 104)
298
257
257
257
313
542 (1.91 ꢁ 104)
537 (1.78 ꢁ 104)
535 (1.71 ꢁ 104)
533 (1.69 ꢁ 104)
541 (1.00 ꢁ 104)
553
547
544
541
552
0.46
0.42
0.38
0.30
0.28
0.053
0.062
0.079
0.089
0.11
92%
88%
80%
83%
88%
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring (Foꢀc) and closed-ring isomers (Fcꢀo), respectively.
b
c

200
S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
diarylethene 5 has the smallest cyclization quantum yield and the
biggest cycloreversion quantum yield. The result indicated that the
substituent at the para-position of the terminal benzene ring plays
an important role during the process of the photochromic reaction
for these diarylethenes.
Just as in hexane, diarylethenes 1e5 also showed photochro-
mism in amorphous PMMA films. Their spectral and color changes
are shown in Fig. 4. Upon irradiation 313 nm light, the colors of
diarylethenes 1e5 in PMMA films changed from colorless to
magenta, with the appearance of a new broad absorption band
centered at 553, 547, 544, 541 and 552 nm, respectively. This band
was assigned to the formation of the closed-ring isomers 1ce5c. In
PMMA films, the photostationary equilibriums of diarylethenes
1e5 are achieved by irradiation with UV light for 960, 1050, 1000,
940, and 780 s, respectively. All colored diarylethene/PMMA films
can revert to colorless upon irradiation with visible light
(l > 450 nm). The bleaching irradiation time is 120 s for 1, 135 s for
2, 125 s for 3, 115 s for 4, and 100 s for 5. As has been observed for
most of the reported diarylethenes [16,50,51], the absorption
maxima of the closed-ring isomers 1ce5c are found at longer
wavelengths in PMMA films than those in hexane solution. The red
shift values of the absorption maxima for the closed-ring isomers
are 11 nm for 1c, 10 nm for 2c, 9 nm for 3c, 8 nm for 4c and 11 nm
for 5c, respectively. The red shift phenomena may be ascribed to
the polar effect of the polymer matrix in the solid state [52,53].
The thermal stability of diarylethene is an indispensable prop-
erty for the application to optical memory media [36], and it is
mainly dependent on the aromatic stabilization energy of the aryl
A
B
1.4
1.6
1.2
0.8
0.4
0.0
Vis
Vis
1.2
UV
UV
1.0
960 s
UV
1050 s
780 s
450 s
270 s
150 s
80 s
UV
750 s
Vis
Vis
0.8
450 s
270 s
0.6
0.4
0.2
0.0
150 s
90 s
40 s
0 s
20 s
0 s
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
C
D
1.5
1.2
0.9
0.6
0.3
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Vis
Vis
UV
UV
1000 s
750 s
450 s
270 s
90 s
940 s
680 s
550 s
270 s
150 s
90 s
UV
UV
Vis
Vis
10 s
0 s
40 s
0 s
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
E
F
1.5
1.2
0.9
0.6
0.3
0.0
Vis
UV
780 s
550 s
270 s
150 s
90 s
UV
Vis
0 s
300
400
500
600
700
Wavelength (nm)
Fig. 4. Absorption spectra and color changes of diarylethenes 1e5 by photoirradiation in PMMA films (10%, w/w) at room temperature: (A) spectral changes for 1, (B) spectral
changes for 2, (C) spectral changes for 3, (D) spectral changes for 4, (E) spectral changes for 5, (F) color changes for 1e5.

S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
201
moiety [19]. The thermal stabilities of the open- and closed-ring
3.2. Photochromic reactions in the crystalline phase
isomers of diarylethenes 1e5 were examined in hexane both at
room temperature and at 341 K. Storing these solutions in the dark
and then exposing them to air for more than 30 days at room
temperature, no changes in the UV/Vis spectra of diarylethenes 1e5
were observed. Moreover, no decomposition was detected when
the five compounds were exposed to air for more than a half year.
At 341 K, diarylethenes 1e5 still showed good thermal stabilities
for more than 50 h. The result suggests that these diarylethene
derivatives have notable thermally irreversible photochromic
behaviors, which is similar the bis(3-benzothienyl)per-
fluorocyclopentene derivatives that undergo thermally irreversible
photochromic reactions [18]. The fatigue resistances of diary-
lethenes 1e5 were examined both in hexane and in PMMA films in
air at room temperature, as shown in Fig. 5. In hexane, the color-
ation and decoloration cycles of diarylethenes 1e5 can be repeated
at least 20 times with almost no photo-degradation. After 50 repeat
cycles, only 2% of 1c, 2% of 2c, 5% of 3c, 8% of 4c, and 2% of 5c were
destroyed at the same condition. Similarly, diarylethenes 1e5
showed remarkable fatigue resistance in PMMA films. After 200
repeated cycles, they still showed notable photochromism with
only ca. 19% degradation of 1c, 14% of 2c, 16% of 3c, 14% of 4c, and
14% of 5c.
It is well known that a diarylethene has two conformations with
the two heterocyclic rings in C2 symmetry (anti-parallel confor-
mation) and in mirror symmetry (parallel conformation). Only anti-
parallel conformation can undergo effective photocyclization
reaction by a conrotatory mechanism, while the parallel confor-
mation is photochemically inactive [6]. Although many diary-
lethene crystals with anti-parallel conformation have been so far
reported, diarylethene crystals with parallel conformation are very
rare [11,54,55]. To know better the relation between the confor-
mation and the photochromic behaviors of photochromic diary-
lethenes in the crystalline phase, gaining diarylethene crystals with
the two structural conformations is very necessary. Fortunately,
two single crystals of diarylethenes 3o and 5o with different
conformation were obtained by slow evaporation of a hexane
solution and their final structural confirmations were provided by
X-ray crystallographic analysis in this work. The ORTEP drawings of
the single crystals are shown in Fig. 6 and the X-ray crystallographic
analysis data are listed in Table 2. For diarylethene 3o, the molecule
crystallizes with an anti-parallel conformation in the crystalline
phase, which can be expected to undergo photocyclization reaction.
The dihedral angles between the hexafluorocyclopentene ring and
the two heteroaryl rings are 62.9(5)ꢂ for O1/C17-C24 and 44.8(5)ꢂ
for S1/C6-C9, and that between the thiophene ring and the linked
benzene ring is 12.1(5)ꢂ. The distance between the two reactive C
atoms (C6 and C17) is 3.775 Å. For diarylethene 5o, the molecule
crystallizes with a parallel conformation in the crystalline phase,
which cannot possibly undergo photocyclization reaction. The
dihedral angles between the hexafluorocyclopentene ring and the
two heteroaryl rings are 58.3(4)ꢂ for O1/C18eC25 and 50.5(4)ꢂ for
S1/C6eC9, and that between the thiophene ring and the linked
benzene ring is 27.0(4)ꢂ. The distance between the two reactive C
atoms (C9 and C18) is 4.200 Å. Based on the empirical rule that the
molecule undergoes the photocyclization reaction if the molecule is
fixed in an anti-parallel mode and the distance between reacting
carbon atoms on the aryl rings is less than 4.2 Å [56e58], the two
crystals should exhibit different photochemical activity by photo-
irradiation in the single crystalline phase. In fact, crystal 3o showed
good photochromism, in accordance with the expected ring
closure, to form 3c upon irradiation with UV light, but crystal 5o
didn’t show any photochromism upon irradiation with UV light in
the crystalline phase. The color changes of diarylethenes 3o and 5o
upon photoirradiation are shown in Fig. 6C. Upon irradiation with
254 nm light, the colorless crystal of 3o turned magenta quickly.
When the colored crystal was dissolved in hexane, an intense
absorption maximum was observed at the same wavelength as that
of its ring-closed isomer 3c in solution. Alternatively, the magenta
crystal returned to colorless upon irradiation with the appropriate
A
B
visible light (l > 450 nm). However, crystal 5o didn’t undergo
photochromic reaction in the single crystalline phase, which was
verified by the fact that irradiating crystal 5o with UV light for 2 h
resulted in no observable color change. When the crystal was dis-
solved in hexane, the solution remained colorless and the absorp-
tion spectrum was the same as that of diarylethene 5o in solution.
Furthermore, diarylethene crystal 3o exhibited excellent fatigue
resistance greater than 100 cyclization/cycloreversion repeated
cycles and its closed-ring isomer remained stable for more than 90
days in the dark at room temperature. So, it will be good candidates
for the construction of certain optoelectronic devices [59].
3.3. Fluorescence of diarylethenes 1e5
Fig. 5. Fatigue resistance of diarylethenes 1e5 in hexane and in PMMA films in air
atmosphere at room temperature: (A) in hexane, (B) in PMMA films. Initial absorbance
of the sample was fixed to 1.0.
The fluorescence spectra of diarylethenes 1e5 in both hexane
and PMMA amorphous films at room temperature are illustrated in

202
S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
Table 2
Crystal data for diarylethenes 3o and 5o.
Compound
3o
5o
Formula
C26 H18 F6 O S
492.46
296(2)
Orthorhombic
Pbca
10.3270(14)
19.115(3)
23.068(3)
90.00
90.00
90.00
4553.4(11)
8
C27 H17 F6 N O S
517.48
296(2)
Monoclinic
P2(1)/c
10.806(5)
21.700(10)
10.203(5)
90.00
96.485(6)
90.00
2377.2(18)
4
Formula weight
Temperature
Crystal system
Space group
Unit cell dimensionsa (Å)
b (Å)
c (Å)
a
b
g
(o)
(o)
(o)
Volume (Å3)
Z
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Final R1[I > 2s(I)]
wR2[I > 2s(I)]
R1 (all data)
1.437
1.016
0.0514
0.1130
1.446
1.017
0.0634
0.1549
0.0924
0.1345
wR2 (all data)
0.1354
0.1988
and 20 nm for 5, respectively. Among compounds 1oe5o, the
emission peak of 1o is the longest and that of 5o is shortest; while
the emission intensity of 1o is the smallest and that of 5o is the
biggest in both hexane and a PMMA film. This indicated that the
A
350
280
210
5o
4o
140
3o
2o
1o
70
0
350
400
450
500
550
Wavelength (nm)
B
10000
8000
6000
4000
2000
0
Fig. 6. ORTEP drawings of crystals 3o and 5o and their color changes by photo-
irradiation in the single crystalline phase: (A) ORTEP drawing of 3o, (B) ORTEP drawing
of 5o, (C) color changes for crystals 3 and 5.
5o
4o
3o
2o
1o
Fig. 7. When monitored at 297 nm, the fluorescent emission peaks
of 1o, 2o, 3o, 4o, and 5o were observed at 438, 423, 413, 417 and
401 nm in hexane. Their emission peaks in PMMA films were
observed at 423, 425, 419, 420 and 382 nm when monitored at
332 nm. Compared with those in hexane, the emission peaks of
diarylethenes 2o, 3o, and 4o showed a minor bathochromic shift in
PMMA film, which is well consistent with those of their maxima
absorption wavelengths. The red shift values of their emission
peaks are 2 nm for 2, 6 nm for 3, and 3 nm for 4, respectively. But for
diarylethenes 1o and 5o, their emission peaks showed a remark-
able hypochromic shift in PMMA films with values of 15 nm for 1
400
450
500
550
Wavelength (nm)
Fig. 7. Fluorescence emission spectra of diarylethenes 1e5 both in hexane solution
(C ¼ 2.0 ꢁ 10ꢀ5 mol Lꢀ1) and in PMMA films (10%, w/w) at room temperature; (A)
emission spectra in hexane, excited at 297 nm; (B) emission spectra in PMMA films,
excited at 332 nm.

S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
203
electron-donating group could be effective to shift the emission
peak to a longer wavelength and to decrease the emission intensity
of diarylethenes bearing a benzofuran moiety, but the electron-
withdrawing group functioned as an inverse action. The result is
quite different from that of diarylethenes bearing a pyrrole or
thiazole moiety [43,48].
can be repeated greater than 50 times with almost no attenuation
of fluorescent intensity. Similarly, diarylethenes 2ꢀ5 also func-
tioned as a notable fluorescent switch upon photoirradiation both
in hexane and in PMMA films. In the photostationary state, their
emission intensities quenched to ca. 24% for 2, 36% for 3, 23% for 4,
and 24% for 5 in solution and ca. 20% for 2, 15% for 3, 13% for 4, and
57% for 5, respectively. Except for diarylethene 5, the fluorescent
modulation efficiencies of diarylethenes 1e4 in PMMA films were
much bigger than those in hexane. For example, the fluorescent
modulation efficiency of diarylethene 1 in a PMMA film was
increased about 19% more than that in hexane. Among the five
diarylethene derivatives, diarylethene 4 exhibited the biggest
fluorescent modulation efficiency both in solution and in a PMMA
film, suggesting that it is the most promising candidate for appli-
cation to the fluorescent switchable devices [64e66].
As has been observed for most of the reported diarylethenes
[60e63], diarylethenes 1e5 exhibited an evident fluorescent
switch along with the photochromism from open-ring isomers to
closed-ring isomers by photoirradiation in both solution and
PMMA films. When irradiated by UV light with the irradiating time
of several hundred seconds, the photocyclization reaction was
carried out and the non-fluorescent closed-ring isomers of these
compounds were produced leading to the decrease of their emis-
sion intensity. The back irradiation by appropriate wavelength
visible light regenerated their open-ring isomers 1oe5o and
recovered the original emission intensity. During the process of
photoisomerization, diarylethene 1 exhibited changes in its fluo-
rescence in hexane and in a PMMA film as shown in Fig. 8. Upon
irradiation with UV light, the emission intensity of diarylethene 1
decreased because of the formation of its non-fluorescent closed-
ring isomer 1c by photocyclization. When arrived at the photo-
stationary state, its emission intensity quenched to ca. 39% in
hexane and 20% in a PMMA film, respectively. Back irradiation of
the appropriate visible light regenerated the open-ring isomer 1o
and duplicated its original emission spectrum. The periodic cycles
3.4. Electrochemical properties of diarylethenes 1e5
The electrochemical properties of diarylethenes can be used for
molecular switching and also can be potentially applied to
molecular-scale electronic switches [67]. The electrochemical
property of diarylethenes has attracted much attention [68e71].
Herein, the electrochemical properties of 1e5 were tested by cyclic
voltammetry (CV) methods under the same experimental condi-
tions. The CV curves of diarylethenes 1e5 are shown in Fig. 9. The
onset potentials (Eonset) of oxidation and reduction for 1o were
initiated at þ1.09 and ꢀ0.83 V, and those of 1c were at þ0.97
and ꢀ0.73 V, respectively. According to the reported method
[72,73], the ionization potential and electron affinity of 1o were
calculated to be ꢀ5.89 and ꢀ3.97 eV, and those of 1c were ꢀ5.77
and ꢀ4.07 eV, respectively. Based on the highest occupied molec-
ular orbitals (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy level, the band gap Eg (Eg ¼ LUMO-HOMO) of 1o
and 1c can be determined as 1.92 eV and 1.70 eV, respectively. Just
as for diarylethene 1, the corresponding data for diarylethenes 2e5
are summarized in Table 3. The oxidation of 2oe5o is initiated at
1.46,1.54,1.34, and 1.77 V, and that of 2ce5c is initiated at 1.05,1.09,
1.11, and 1.18 V, respectively. The result indicates that the oxidation
process for the open-ring isomers 1oe5o occurs at higher poten-
tials than in the corresponding closed-ring isomers 1ce5c. This is
because the longer conjugation length of the closed-ring isomers
generally leads to a lower positive potential [16,74]. As shown in
Table 3, all Egs of the open-ring isomers 1oe5o were higher than
those of the closed-ring isomers 1ce5c. Among these compounds,
the Eg of 1c was the smallest, which implies that the charge transfer
of 1c must be faster compared to that in others [75]. There are great
differences amongst the electronic current and polarization curve
shapes between the open-ring and closed-ring isomers of diary-
lethenes 1e5 at the scanned voltage region. The results suggest that
different substituents have a significant effect on the electro-
chemical behaviors of these diarylethenes. It should be noted here
that the absolute HOMO and LUMO levels in combination with the
energy gap calculated by electrochemical data is still require
further discussion [73].
A
50
Vis
40
UV
30
0 s
90 s
450 s
550 s
650 s
20
10
0
350
400
450
500
550
Wavelength (nm)
B
2000
1500
1000
500
0
Vis
UV
0 s
90 s
270 s
420 s
650 s
800 s
1000 s
Furthermore, electrochromism was observed with the closed-
ring isomers 1ce5c when the CV measurement was carried out.
During the cyclic voltammetry experiments, the colorless solutions
containing diarylethenes 1oe5o around the platinum electrode
changed to magenta. This indicates that the open-ring isomers
1oe5o underwent oxidative cyclization to produce the closed-ring
isomers 1ce5c, respectively, although no marked changes in the
absorption spectra were detected because of the too small amount
of the closed-ring isomers 1ce5c in solution. The colored solutions
of these diarylethenes in photostationary state were obtained from
the conversion of their respective open-ring isomers upon
400
450
500
550
Wavelength (nm)
Fig. 8. Emission intensity changes of diarylethene 1 in hexane (C ¼ 2.0 ꢁ 10ꢀ5 mol Lꢀ1
)
and in a PMMA film (10%, w/w) upon irradiation with UV light at room temperature:
(A) in hexane (excited at 297 nm), (B) in a PMMA film (excited at 332 nm).

204
S. Pu et al. / Dyes and Pigments 94 (2012) 195e206
Fig. 9. Cyclic voltammetry of diarylethenes 1e5 in 0.1 mol/L ((TBA)BF4) with a scan rate of 50 mV/s: (A) 1, (B) 2, (C) 3, (D) 4, (E) 5.
Table 3
diarylethenes bearing a benzofuran moiety, which may shed some
lights on the further application of photochromic diarylethene
derivatives.
Electrochemical properties of diarylethenes 1e5.
Compound
Oxidation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg
1o
1c
2o
2c
3o
3c
4o
4c
5o
5c
þ1.09
þ0.97
þ1.46
þ1.05
þ1.54
þ1.09
þ1.34
þ1.11
þ1.77
þ1.18
ꢀ5.89
ꢀ5.77
ꢀ6.26
ꢀ5.85
ꢀ6.34
ꢀ5.89
ꢀ6.14
ꢀ5.91
ꢀ6.57
ꢀ5.98
ꢀ0.83
ꢀ0.73
ꢀ0.73
ꢀ0.84
ꢀ0.87
ꢀ0.89
ꢀ0.72
ꢀ0.92
ꢀ0.93
ꢀ0.86
ꢀ3.97
ꢀ4.07
ꢀ4.07
ꢀ3.96
ꢀ3.93
ꢀ3.91
ꢀ4.08
ꢀ3.88
ꢀ3.87
ꢀ3.94
1.92
1.70
2.19
1.89
2.41
1.98
2.06
2.03
2.70
2.04
Acknowledgements
This work was supported by Program for NSFC (20962008,
21162011), the Project of New Century Excellent Talents in
University (NCET-08-0702), the Project of Jiangxi Academic and
Technological Leader (2009DD00100), and the Project of the
Science Funds of Jiangxi Education Office (GJJ10241, GJJ09646).
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