Synthesis and the effect of substituent position upon unsymmetrical isomeric diarylethenes bearing a pyrrole unit

Article abstract of DOI:10.1016/j.jphotochem.2010.07.001

Four unsymmetrical diarylethenes bearing a pyrrole moiety were synthesized, and the structures of two isomeric compounds were determined by single-crystal X-ray diffraction analysis. The properties of these compounds, including photochromism, fluorescence

Full text of DOI:10.1016/j.jphotochem.2010.07.001

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and the effect of substituent position upon unsymmetrical isomeric
diarylethenes bearing a pyrrole unit
Gang Liua,b, Shouzhi Pub,∗, Xiaomei Wanga,∗
a College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
b 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:
Received 7 April 2010
Received in revised form 27 June 2010
Accepted 2 July 2010
Available online 8 July 2010
Four unsymmetrical diarylethenes bearing a pyrrole moiety were synthesized, and the structures of
two isomeric compounds were determined by single-crystal X-ray diffraction analysis. The properties
of these compounds, including photochromism, fluorescence and electrochemical properties of these
diarylethenes were also investigated systematically. Each of the four diarylethene derivatives exhibited
good photochromism and functioned as an effective fluorescent photoswitch both in solution and in
PMMA films. The absorption maxima, the quantum yields of cyclization and cycloreversion, and the flu-
orescence quantum yields of the open-ring isomers increased whereas the emission peaks decreased
evidently when the methoxy group was attached at any of the three positions on the terminal ben-
zene ring. Compared with the analogous diarylethenes bearing a pyrazole moiety, the introduction of
the pyrrole moiety could increase the absorption maxima and decrease the cycloreversion quantum
yields significantly. Cyclic voltammograms testified that methoxy group and its substituted position
could effectively modulate the electrochemical behaviours of these diarylethene derivatives.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Photochromism
Diarylethene
Pyrrole group
Substituent position effect
Photochemical and electrochemical
property
1. Introduction
On the one hand, the distinguishable features of diarylethenes
mainly depended on the categories of heteroaryl moieties and their
Photochromic compounds have been extensively investigated
for their potential applications in erasable optical memories and
photoswitches [1–3]. Among the photochromic compounds hith-
erto reported, diarylethenes are regarded as the best candidates
for such devices, because of their advantages such as a high effi-
ciency of photoisomerization, sufficient thermal stability of both
the open-ring and the close-ring isomers, remarkable resistance to
photo-fatigue, and ease monitoring by using UV–vis spectroscopy
[4–7].
The photochromic process of diarylethenes is based on a
reversible transformation between the open-ring isomer with a
hexatriene structure and the closed-ring isomer with a cyclohexa-
diene structure, according to the Woodward-Hoffmann rule [8].
The open and closed isomers of diarylethenes differ from each other
not only in their absorption spectra but also in various physical
and chemical properties such as refractive index [9,10], oxida-
tion/reduction potentials [11–13], and chiral properties [14–16],
among others. These molecular property changes are of great inter-
est because of their potential applications in photonic apparatus
such as optical memories [1,17–20], multi-stable photoswitches
[3,21–24], and full color displays [25].
substitution positions [26]. For instance, diarylethenes bearing
thiophene/benzothiophene moieties show good thermal stability
and remarkable fatigue resistance [1], but those bearing pyrrole
groups are thermally unstable and return to the open-ring iso-
mers even in the dark [27]. The electron-donating substituents
of the bis(3-thienyl)ethene diarylethenes could be effective to
increase the absorption coefficient of the closed-ring isomers
and decrease the cycloreversion quantum yield [28,29], however,
those attached bis(2-thienyl)ethene diarylethenes could increase
the maxima absorption of the open-ring isomers and reduce the
cyclization quantum yield [30]. Previously, most of the heteroaryl
moieties have been thiophene or benzothiophene rings, with just
a few reports concerning other heteroaryl moieties such as furan
[31], thiazole [32,33], indole [34,35], and so on. Pyrrole is an attrac-
tive aryl unit due to its biological characteristics [36,37], and its
structure is similar to those of thiophene and furan. However,
diarylethene derivatives with a pyrrole unit hitherto reported is
very rare. As far as we are aware, there are only a few examples
of photochromic diarylethenes bearing a pyrrole unit. Uchida et
al. [27] reported a symmetrical diarylethene bearing two pyrrole
groups that showed thermally unstable and reverted to the open-
ring form even at room temperature in the dark. In a previous
paper, we synthesized two isomeric hybrid diarylethenes bearing
both pyrrole and thiophene moieties and investigated their pho-
tochromic features, indicating that the hybrid diarylethenes had
∗
Corresponding authors. Tel.: +86 791 3831996; fax: +86 791 3831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.07.001

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
231
using a SHG-200 UV lamp, CX-21 ultraviolet fluorescence analy-
sis cabinet and a BMH-250 visible lamp. The required wavelength
was isolated by the use of the appropriate filters. Electrochemical
examinations were performed in a one-compartment cell by using
a Model 263 potentiostat–galvanostat (EG&G Princeton Applied
Research) under computer control at room temperature. Platinum-
electrodes (diameter 0.5 mm) served as the working electrode
and counter electrode. Platinum wire served as a quasireference
electrode. It was calibrated using an internal ferrocene (Fc/Fc+)
standard with a formal potential of E1/2 = +0.35 V versus platinum
wire. The typical electrolyte was acetonitrile (5 mL) containing
0.1 mol/L tetrabutylammonium tetrafluoroborate ((TBA)BF4) and
1.0 × 10−3 mol/L diarylethene sample. All solutions were deaer-
ated by bubbling with a dry argon stream and maintained at
a slight argon overpressure during electrochemical experiments.
Solvents used were spectroscopic grade and were purified by dis-
tillation.
Suitable crystals of 3o and 4o were obtained by slow evapora-
tion of a hexane solution. All the measurements were made on a
Bruker SMART APEX II CCD diffractometer using a MULTI scan tech-
nique at room temperature using Mo K␣ radiation. The structures
were solved by direct methods and refined by full-matrix least-
squares procedures on F2 by full-matrixc least-squares techniques
using SHELXTL-97 program. All non-hydrogen atoms were refined
anisotropically. Crystallographic data for of the structures in this
paper have been deposited with in the Cambridge Crystallographic
Data Centre as supplemental publication CCDC 762279 for 3o and
CCDC 639786 for 4o. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44 0 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).]
Scheme 1. Photochromism of diarylethenes 1–4.
good thermal stability and fatigue resistance in the solid medium
[26].
On the other hand, electron donor/acceptor substituents and
their substitution positions have also a significant effect on the
photochromic properties of diarylethene derivatives [38,39]. So
far, many publications concerning the substituent effect have been
reported, but reports on the effect of substituent position are rare
[40]. Previously, we reported the electron-donating methoxy sub-
stituent position effect not only on the properties of unsymmetrical
photochromic dithienylethene derivatives, but also on those of
unsymmetrical photochromic hybrid diarylethenes bearing both
pyrazole and thiophene moieties [40–43]. The results indicated that
the substituent position of methoxy group had a significant effect
on the properties of these diarylethene compounds. However,
there existed a great difference between the methoxy substituent
effect of the isomeric dithienylethene derivatives and those of
the isomeric diarylethenes bearing both pyrazole and thiophene
moieties. That is to say, the substituent position effect of the same
functional group is remarkable different from each other for differ-
ent photochromic diarylethene systems. The result give us valuable
insight and encourage us to explore the methoxy substituent posi-
tion effect on the properties of hybrid photochromic diarylethenes
bearing both pyrrole and thiophene moieties. Therefore, we have
synthesized four hybrid diarylethenes bearing both pyrrole and
thiophene moieties in order to clarify these effects in this work.
The synthesized diarylethenes are 1-(2-cyano-1,5-dimethyl-4-
pyrryl)-2-[2-methyl-5-phenyl-3-thienyl]perfluorocyclopentene
2.2. Synthesis
2.2.1. 3-Bromo-5-phenyl-2-methylthiophene (6a)
Compound 6a was prepared by reacting 3-bromo-2-methyl-
5-thienylboronic acid [43] (3.1 g, 14.0 mmol) with bromobenzene
(2.20 g, 14.0 mmol) in the presence of Pd(PPh3)4 (0.27 g, 0.23 mmol)
and Na2CO3 (6.36 g, 60.0 mmol) in tetrahydrofuran (THF) (80 mL
containing 10% water). After refluxing for 15 h, the product was
extracted with ether. The organic layer was dried over MgSO4, fil-
trated and evaporated. The crude product was purified by column
chromatography on SiO2 using hexane as the eluent and 3.0 g of 6a
obtained as a yellowish solid 83% yield. 1H NMR (400 MHz, CDCl3):
ı 2.34 (s, 3H, –CH3), 7.02 (s, 1H, thiophone-H), 7.20 (d, J = 8.0 Hz, 1H,
benzene-H), 7.29 (t, J = 8.4 Hz, 2H, benzene-H), 7.42 (d, J = 8.0 Hz, 2H,
benzene-H).
(1o),
5-(2-methoxyphenyl)-3-thienyl]perfluorocyclopentene
(2o), 1-(2-cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-
(3-methoxyphenyl)-3-thienyl]perfluorocyclopentene (3o),
and 1-(2-cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-(4-
1-(2-cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-
methoxyphenyl)-3-thienyl]perfluorocyclopentene (4o) of which,
1o, 2o and 3o are new compounds. The photochromic process of
the diarylethenes is shown in Scheme 1. Although diarylethene
4o has been reported previously [26], it is presented here for
comparison with those of other three diarylethene compounds.
2.2.2. 3-Bromo-5-(2-methoxyphenyl)-2-methylthiophene (6b)
Compound 6b was prepared by an analogous method similar
to that used for to 6a and obtained as a buff solid in 68% yield. 1
H
NMR (400 MHz, CDCl3): ı 2.41 (s, 3H, –CH3), 3.91 (s, 3H, –OCH3),
6.95–6.99 (m, 2H, benzene-H), 7.27 (s, 1H, thiophene-H), 7.30 (s,
1H, benzene-H), 7.54 (d, 1H, J = 8.0 Hz, benzene-H).
2. Experimental
2.2.3. 3-Bromo-5-(3-methoxyphenyl)-2-methylthiophene (6c)
Compound 6c was prepared by an analogous method similar to
that used for to 6a and obtained as a buff solid in 72% yield. 1H
NMR (400 MHz, CDCl3): ı 2.41 (s, 3H, –CH3), 3.83 (s, 3H, –OCH3),
6.83 (d, 1H, J = 8.0 Hz, benzene-H), 7.03 (s, 1H, thiophene-H), 7.09
(t, 2H, J = 4.0 Hz, benzene-H), 7.27 (t, 1H, J = 8.0 Hz, benzene-H).
2.1. General
NMR spectra were recorded on a Bruker AV400 (400 MHz) spec-
trometer with CDCl3 as the solvent and tetramethylsilane as an
internal standard. Infrared spectra (IR) were performed using a
Bruker Vertex-70 spectrometer. Elemental analysis was measured
with an elemental analyzer labeled the PE 2400 CHN analyzer.
Melting point was taken on a WRS-1B melting point apparatus.
Absorption spectra were measured using an Agilent 8453 UV/VIS
spectrophotometer. Fluorescence spectra were measured using
a Hitachi F-4500 fluorimeter. Photoirradiation was carried out
2.2.4. 3-Bromo-5-(4-methylphenyl)-2-methylthiophene (6d)
Compound 6d was prepared by an analogous method similar to
that used for to 6a and obtained as a buff solid in 72% yield. 1H NMR
(400 MHz, CDCl3): ı 2.40 (s, 3H, –CH3), 3.83 (s, 3H, –OCH3), 6.85 (d,

232
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
2H, J = 8.8 Hz, benzene-H), 6.99 (s, 1H, thiophene-H), 7.43 (d, 2H,
J = 8.8 Hz, benzene-H).
(m, 1H, benzene-H), 7.41 (s, 1H, thiophene-H), 7.58–7.60 (m, 1H,
benzene-H); 13C NMR (400 MHz, CDCl3): ı 11.3, 14.2, 33.0, 55.6,
76.7, 77.0, 77.2, 77.4, 105.4, 110.2, 111.3, 111.8, 113.0, 113.7, 118.9,
121.1, 122.3, 124.8, 124.9, 127.8, 138.9, 134.8, 136.1, 137.6, 140.7,
155.6; IR (ꢀ, KBr, cm−1): 739, 845, 898, 987, 1030, 1054, 1115, 1128,
1184, 1278, 1335, 1384, 1436, 1482, 1599, 1656, 2218, 2372, 2923,
3130, 3450.
2.2.5. 3-Bromo-1,5-dimethyl-2-pyrrolecarbonitrile (8)
To a stirred solution of 1,5-dimethyl-2-pyrrolecarbonitrile (7)
(3.0 g, 25.0 mmol) in acetic acid (100 mL) at 0 ◦C was slowly added
Br2/acetic acid (v/v = 29/40) solution. The reaction mixture was
stirred overnight at this temperature. After pouring the reaction
mixture into a beaker containing some water, a large quantity of
yellowish precipitate was produced. The precipitate was washed
by water, filtered and dried, and then purified by silica gel col-
umn chromatography using hexane as the eluent. Compound 8 was
obtained as yellowish solid of 4.2 g in 85% yield. 1H NMR (400 MHz,
CDCl3): ı 2.30 (s, 3H, –CH3), 3.68 (s, 3H, –CH3), 6.74 (s, 1H, pyrrole-
H).
2.2.9. 1-(2-Cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-(3-
methoxyphenyl)-3-thienyl]perfluorocyclopentene
(3o)
Diarylethene 3o was prepared by an analogous method
similar to that used for diarylethene 1o using 3-Bromo-5-(3-
methoxyphenyl)-2-methylthiophene (6c) instead of 6a, and was
obtained as a solid 0.48 g in 40% yield. M.p. 156.7–158.2 ◦C; Anal.
Calcd for C24H18F6N2OS (%): Calcd C, 58.06; H, 3.65; N, 5.64. Found
C, 58.11; H, 3.61; N, 5.66; 1H NMR (400 MHz, CDCl3): ı 1.76 (s, 3H,
–CH3), 1.95 (s, 3H, –CH3), 3.61 (s, 3H, –CH3), 3.86 (s, 3H, –OCH3),
6.87 (s, 1H, pyrrole-H), 6.86 (d, 1H, J = 6.2 Hz, benzene-H), 7.06 (s,
1H, thiophene-H), 7.14 (d, 1H, J = 7.7 Hz, benzene-H), 7.56 (s, 1H,
benzene-H), 7.30 (t, 1H, J = 7.8 Hz, benzene-H); 13C NMR (400 MHz,
CDCl3): ı 11.3, 14.5, 33.1, 55.4, 76.7, 77.0, 77.3, 77.3, 105.5, 109.9,
111.4, 112.9, 113.3, 116.1, 118.1, 118.8, 122.8, 126.0, 130.1, 134.53,
136.0, 140.4, 142.3, 160.1; IR (ꢀ, KBr, cm−1): 688, 780, 975, 1049,
1103, 1189, 1272, 1332, 1434, 1581, 1606, 1643, 2217, 2957.
2.2.6. 2-Cyano-1,5-dimethyl-4-pyrryl-perfluorocyclopentene (9)
To a stirred THF solution (80 mL) of compound 8 (4.0 g,
20.1 mmol) was added dropwise a 2.5 M n-BuLi/hexane solution
(8.8 mL, 22 mmol) at −78 ◦C under nitrogen atmosphere. Stirring
was continued for 30 min at −78 ◦C, octafluorocyclopentene (C5F8)
(2.8 mL, 20.5 mmol) was slowly added and the reaction mixture was
stirred for 2.0 h at this low temperature. The reaction was quenched
by water. The product was extracted with diethyl ether. The organic
layer was washed with 1 M aqueous HCl and water, and then dried
over anhydrous MgSO4, filtrated and evaporated in vacuo. The crude
product was purified by column chromatography on silica gel using
petroleum ether as the eluent to give 3.6 g compound 9 obtained
as yellowish solid in 57% yield. 1H NMR (400 MHz, CDCl3): ı 2.32
(s, 3H, –CH3), 3.72 (s, 3H, –CH3), 6.98 (s, 1H, pyrrole-H).
2.2.10. 1-(2-Cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-(4-
methoxyphenyl)-3-thienyl]perfluorocyclopentene
(4o)
Diarylethene 4o was prepared by an analogous method
similar to that used for diarylethene 1o using 3-Bromo-5-(4-
methoxyphenyl)-2-methylthiophene (6d) instead of 6a, and was
obtained as a solid 0.41 g in 38% yield. M.p. 143.9–144.5 ◦C; Anal.
Calcd for C24H18F6N2OS (%): Calcd C, 58.06; H, 3.65; N, 5.64. Found
C, 58.10; H, 3.59: N, 5.62; 1H NMR (400 MHz, CDCl3): ı 1.69 (s,
3H, –CH3), 1.87 (s, 3H, –CH3), 3.53 (s, 3H, –CH3), 3.77 (s, 3H,
–OCH3), 6.84 (s, 1H, pyrrole-H), 6.86 (s, 2H, benzene-H), 7.04 (s,
1H, thiophene-H), 7.38 (d, 2H, J = 8.4 Hz, benzene-H); 13C NMR
(400 MHz, CDCl3): ı 11.2, 14.4, 30.8, 32.9, 55.4, 105.5, 110.1, 112.8,
114.5, 116.2, 118.8, 121.5, 125.9, 126.2, 126.9, 135.9, 139.3, 142.5,
159.7; IR (ꢀ, KBr, cm−1): 630, 743, 822, 845, 895, 985, 1031, 1056,
1108, 1183, 1256, 1335, 1399, 1442, 1466, 1514, 1553, 1611, 2219,
2840, 3128.
2.2.7. 1-(2-Cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-
phenyl-3-thienyl]perfluorocyclopentene
(1o)
To a stirred anhydrous THF (80 mL) of compound 6a (0.7 g,
2.5 mmol) was added dropwise a 2.5 M n-BuLi/hexane solution
(1 mL, 2.5 mmol) at −78 ◦C under argon atmosphere. After 30 min,
15 mL THF containing compound 9 (0.8 g, 2.5 mmol) was added and
the reaction mixture was stirred for 2 h at this low temperature.
The reaction was allowed to slowly warm to the room temperature
and quenched by water. The product was extracted with diethyl
ether, and then dried with MgSO4, filtered, and evaporated in vacuo.
The crude product was purified by column chromatography using
petroleum ether as the eluent to give 0.39 g compound 1o as white
solid in 36% yield. M.p. 140.2–141.2 ◦C; Anal. Calcd for C23H16F6N2S
(%): C, 59.22; H, 3.46; N, 6.01. Found C, 59.27; H, 3.51; N, 6.03; 1H
NMR (400 MHz, CDCl3): ı 1.77 (s, 3H, –CH3), 1.96 (s, 3H, –CH3), 3.61
(s, 3H, –CH3), 6.93 (s, 1H, pyrrole-H), 7.23 (s, 1H, thiophene-H), 7.39
(t, 1H, J = 8.0 Hz, benzene-H), 7.53–7.56 (m, 2H, benzene-H), 7.54 (d,
2H, J = 7.6 Hz, benzene-H); 13C NMR (400 MHz, CDCl3): ı 10.7, 13.9,
32.4, 76.1, 76.4, 76.6, 76.7, 104.9, 109.4, 112.03, 125.0, 125.4, 127.4,
128.5, 132.7, 135.4, 139.7, 142.0; IR (ꢀ, KBr, cm−1): 756, 845, 987,
1097, 1183, 1277, 1334, 1385, 1548, 1624, 2221, 2372, 2920, 3125,
3450.
3. Results and discussion
3.1. Discussion of the synthetic strategy
The synthesis method of diarylethenes 1o–4o is shown
in Scheme 2. Suzuki coupling of four bromobenzene deriva-
tives with a thiophene boronic acid [43] gave the compounds
6a–6d. 2-Cyano-1,5-dimethyl-4-pyrryl-perfluorocyclopentene (9)
was derived originally from 1,5-dimethyl-2-pyrrolecarbonitrile (7)
according to the same method reported previously [26]. Finally,
compounds 6a–6d were lithiated separately and then coupled with
compound 9 to give diarylethenes 1o–4o, respectively. The struc-
tures of 1o–4o were confirmed by NMR, IR, and elemental analysis.
2.2.8. 1-(2-Cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-(2-
methoxyphenyl)-3-thienyl]perfluorocyclopentene
(2o)
Diarylethene 2o was prepared by an analogous method
similar to that used for diarylethene 1o using 3-Bromo-5-(2-
methoxyphenyl)-2-methylthiophene (6b) instead of 6a, and was
obtained as a solid 0.45 g in 43% yield. M.p. 98.7–99.8 ◦C; Anal. Calcd
for C24H18F6N2OS (%): Calcd C, 58.06; H, 3.65; N, 5.64. Found C,
58.01; H, 3.69; N, 5.59; 1H NMR (400 MHz, CDCl3): ı 1.74 (s, 3H,
–CH3), 1.97 (s, 3H, –CH3), 3.59 (s, 3H, –CH3), 3.92 (s, 3H, –OCH3),
6.95 (s, 1H, pyrrole-H), 6.97–7.02 (m, 2H, benzene-H), 7.26–7.28
3.2. Photochromic behaviours of diarylethenes
The photochromic behaviours of diarylethenes 1–4 induced by
photoirradiation at room temperature were measured both in hex-
ane (2.0 × 10−5 mol/L) and in PMMA amorphous film (10%, w/w). In
hexane, the absorption spectral changes of diarylethene 1 induced
by alternating irradiation with UV light and visible light with appro-
priate wavelength are shown in Fig. 1. Diarylethene 1o exhibited

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
233
Scheme 2. Synthetic route for diarylethenes 1–4.
closed-ring isomer 1c. This could be seen with the naked eye, as
the colorless solution of 1o turned blue. Alternatively, the blue col-
ored solution could be bleached to colorless upon irradiation with
visible light (ꢂ > 450 nm), indicating 1c was returning to the ini-
tial state 1o. The coloration–decoloration cycle could be repeated
more than 20 times and a clear isosbestic point was observed
at 308 nm. Diarylethenes 2–4 showed analogous photochromism
similar to diarylethene 1 in hexane. Upon irradiation with 297 nm
light, absorption bands in visible region appeared and the solutions
containing 2o–4o turned blue as a result of the cyclization reactions
to produce the closed-ring isomers 2c–4c, which the absorption
maxima were observed at 594, 589 and 592 nm, respectively. The
colorchangesofdiarylethenes 1–4 upon photoirradiation inhexane
are also shown in Fig. 1. The blue colored solutions of 2c–4c can be
decolorized upon irradiation with visible light (ꢂ > 450 nm) again,
due to the compounds returning to the open-ring isomers 2o–4o,
respectively. In the photostationary state, the isosbestic points for
diarylethenes 2, 3 and 4 were observed at 330, 318 and 320 nm,
respectively. In addition, the photoconversion ratios of the four
derivatives were analyzed by HPLC in the photostationary state,
which the value was 90% for 1, 91% for 2, 89% for 3, and 94% for 4,
respectively (Table 1).
For practical applications in optical devices, it is very impor-
tant that photochromic materials can keep good photochromism
in a polymer film, such as the PMMA film [18]. The PMMA
films were prepared by dissolving 10 mg diarylethene sample and
100 mg polymethylmethacrylate (PMMA) into chloroform (1 mL),
with the aid of ultrasound, and then the homogeneous solution
was spin-coated on a quartz substrate (10 mm × 10 mm × 1 mm).
In PMMA amorphous film, diarylethenes 1–4 also showed simi-
lar photochromism as in solution. The absorption spectral change
of diarylethene 1 and the color changes of diarylethenes 1–4 are
shown in Fig. 2. Compared to those in hexane, the maximum
absorption peaks of both the ring-opened and the ring-closed iso-
Fig. 1. Absorption spectral changes of diarylethene 1 and the color changes of
diarylethenes 1–4 upon alternating irradiation with UV and visible light in hex-
ane (2.0 × 10−5 mol/L) at room temperature: (A) spectral changes for compound 1
and (B) color changes for compounds 1–4.
a sharp absorption peak at 290 nm in hexane, which was arisen
from ꢁ → ꢁ* transition [44]. Upon irradiation with 297 nm light, a
new visible absorption band centered at 584 nm emerged while the
original peak at 290 nm decreased, indicating the formation of the
Table 1
Absorption spectral characteristics and photochromic reactivity of diarylethenes 1–4 in hexane at 2.0 × 10−5 mol/L, and in PMMA films (10%, w/w) at room temperature.
c
Compound
ꢂo,max/nma (ε/L mol−1 cm−1
)
ꢂc,max/nmb (ε/L mol−1 cm−1
)
˚
Conversion at PSS in hexane
Hexane
PMMA film
Hexane
PMMA film
˚
o–c
˚
c–o
1
2
3
4
290 (2.49 × 104)
308 (2.23 × 104)
293 (2.04 × 104)
296 (2.83 × 104)
302
314
307
307
584 (1.00 × 104)
594 (1.11 × 104)
589 (9.36 × 103)
592 (1.15 × 104)
607
614
607
614
0.20
0.25
0.22
0.44
0.054
0.088
0.072
0.10
90%
91%
89%
94%
a
Absorption maxima of open-ring forms.
Absorption maxima of closed-ring forms.
Quantum yields of cyclization reaction (˚o–c) and cycloreversion reaction (˚c–o), respectively.
b
c

234
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
open-ring and the closed-ring isomers varied with the same trend
in hexane, i.e., they decreased in the order of para- > ortho- > meta-
substituent by the methoxy group. Therefore, the molar absorption
coefficients of both the open-ring and the closed-ring isomers of the
para-substituted derivative 4 are the biggest; while those of the
meta-substituted derivative 3 are the smallest. The values of the
ortho-substituted derivative 2 are in between those of the para and
meta-substituted derivatives. As shown in Table 1, the cyclization
quantum yields of diarylethenes 1–4 are much higher than their
respective cycloreversion quantum yields. This is completely dif-
ferent from unsymmetrical diarylethenes bearing a pyrazole unit
whose cyclization quantum yields are lower than their respective
cycloreversion quantum yields [42]. The cyclization and cyclore-
version quantum yields of these three isomeric diarylethene
derivatives showed an analogous changing trend similar to that
of their molar absorption coefficients, i.e., they also decreased in
the order of para- (˚o–c,4 = 0.44, ˚c–o,4 = 0.10) > ortho- (˚o–c,2 = 0.25,
˚c–o,2 = 0.088) > meta-substituent (˚o–c,3 = 0.22, ˚c–o,3 = 0.072) by
the methoxy group. This varying pattern is different from those
of isomeric diarylethenes reported previously [40–43]. In addition,
the pyrrole moiety has also a significant effect on the photochromic
properties of diarylethenes 1–4. When replacing the pyrrole moi-
ety with the thiophene moiety in the same molecular skeleton of
photochromic diarylethenes, the absorption maxima and the molar
absorption coefficients of both of the open-ring and closed-ring iso-
mers decreased remarkably, and the absorption maxima of their
closed-ring isomers showed so big hypsochromic shift that they
exhibited the purple colors in hexane [41]. Similarly, when replac-
ing the pyrrole moiety with the pyrazole unit, the molar absorption
coefficients and the quantum yields of cyclization and cyclorever-
sion increased significantly, but the absorption maxima decreased
clearly [42], compared with pyrrole moiety. The result indicates
that the substituent position effect of methoxy group is remark-
able different from each other for diarylethenes bearing different
aryl moiety.
Fig. 2. Absorption spectral changes of diarylethene 1 and the color changes of
diarylethenes 1–4 upon alternating irradiation with UV and visible light in PMMA
film (10% w/w) at room temperature: (A) spectral changes for compound 1 and (B)
color changes for compounds 1–4.
mers of diarylethenes 1–4 in PMMA film are at longer wavelengths
than those in hexane. The red shift values of the absorption maxima
of the open-ring isomers are 12 nm for 1o, 6 nm for 2o, 14 nm for 3o,
and 11 nm for 4o, and those of the closed-ring isomers are 23 nm for
1c, 20 nm for 2c, 18 nm for 3c, and 22 nm for 4c, respectively. The
red shift phenomena are consistent with those of the reported stud-
ies of most of the diarylethenes [40,41,45], which may be attributed
to the polar effect of the polymer matrix and the stabilization of
molecular arrangement in solid state [46,47].
The thermal stabilities of the open-ring and closed-ring iso-
mers of unsymmetrical diarylethenes 1–4 were tested in hexane
both at room temperature and at 80 ◦C. Storing these solutions
in the dark and then exposing them to air for more than 10 days
at room temperature and at 80 ◦C, respectively, we found that no
changes in the UV/vis spectra of diarylethenes 1–4 were observed.
The result indicated that the thermal stability of these unsymmetri-
cal diarylethenes 1–4 was much better than that of the symmetrical
diarylethene bearing two pyrrole moieties [27].
The photochromic parameters of compounds 1–4 are summa-
rized in Table 1. It could be seen from these data that the methoxy
substituent position had a significant effect on the photochromic
properties of these diarylethene derivatives, including the absorp-
tion maxima, molar absorption coefficients, and quantum yields of
cyclization and cycloreversion. As shown in Table 1, the unsubsti-
tuted parent diarylethene 1 has the smallest absorption maxima
and the smallest cyclization and cycloreversion quantum yields
among these diarylethene derivatives. When introduction of the
electron-donating methoxy group into the terminal benzene ring,
the absorption maxima and the quantum yields of diarylethenes
2–4 increased at some extent. For isomeric diarylethenes 2–4, the
absorption maxima of both the open-ring isomer and the closed-
ring isomer of the ortho-substituted derivative 2 are at the longest
wavelength in hexane; while those of the meta-substituted deriva-
tive 3 are the shortest. The result is in good agreement with that
reported for the methoxy substituent in a previous work [40], but
it is quite different from those reported for chlorine substituents
where the absorption maximum of diarylethene with the ortho-
substituted chlorine atom is the shortest, and that of para- or
meta-substituted derivatives is the longest [48–50]. For isomeric
diarylethenes 2–4, the molar absorption coefficients of both of the
Colorless crystals of 3o and 4o, suitable for X-ray analysis, were
obtained by the slow evaporation of hexane solutions. The X-ray
crystallographic analysis data are listed in Table 2. Their ORTEP
drawings of the single crystals 3o and 4o are shown in Fig. 3.
The molecules of both diarylethenes 3o and 4o crystallize with
an approximate C2 symmetry with the photoactive anti-parallel
conformation in the crystalline phase, which can undergo pho-
tocyclization [51,52]. Both the two molecules include four planar
rings, which can form three dihedral angles between every two
adjacent planar rings. The dihedral angles of diarylethenes 3o and
4o are listed in Table 3. From these data, it could be clearly seen
that all dihedral angles of the meta-substituted derivative 3 are
bigger than those of the para-substituted derivative 4. The reason
may be attributed to the molecular stereo-structure of methoxy
group at different position. The distance between the two reactive
C atoms is 3.681(2) Å for 3o and 3.539(6) Å for 4o (Table 3), which
is close enough for the photocyclization to take place [52–54].
In fact, the crystals 3o and 4o showed good photochromism in
accordance with the expected analysis (Fig. 4). Upon irradiation
with 297 nm light, the colorless crystal of 3o and 4o turned blue
quickly, as a result of the formation of the closed-ring isomers 3c
and 4c. Alternatively, the blue colored crystals 3c and 4c returned

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
235
Fig. 3. ORTEP drawings of crystals 3o and 4o, showing 30% probability displacement ellipsoids: (A) 3o and (B) 4o.
Table 3
Distances between the reacting carbon atoms (d, Å) and dihedral angles (Â,◦) of
diarylethenes 3o and 4o.
Compound
d (Å)
 (◦)a
Table 2
Â1
Â2
Â3
Crystal data and structure refinements for diarylethenes 3o and 4o.
3o
4o
C5. . .C9
C5. . .C14
3.681(2)
3.539(6)
46.5(3) 47.0(3)
38.5(5) 45.3(5)
33.0(3)
23.7(5)
Compound 3o
24H18F6N2OS
496.46
294(2)
Triclinic
P-1
Compound 4o
a
Formula
C
C24H18F6N2OS
496.46
291(2)
Monoclinic
P2(1)/c
Â1, dihedral angle between the cyclopentene ring the pyrrole ring; Â2, dihedral
angle between the cyclopentene ring and the thiophene ring; Â3, dihedral angle
between the thiophene ring and the adjacent benzene ring.
Formula weight
Temperature
Crystal system
Space group
Unit cell dimensions
a (Å)
8.8364(18)
11.236(2)
11.943(2)
91.277(3)
108.206(3)
92.082(4)
1124.9(4)
2
23.120(3)
8.9474(11)
11.4416(13)
90.00
101.835(1)
90.00
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
ꢃ (◦)
Volume (Å3)
Z
2316.6(5)
4
Reflections collected
Reflections observed
Number of parameters
5769
3957
330
0.213
1.466
1.016
0.71073
0.0473
0.1116
0.0795
0.1313
36076
12597
967
0.202
1.509
1.038
0.71073
0.0537
0.1364
0.0715
0.1524
ꢄ (mm−1
)
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Radiation (Å)
Final R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 (all data)
Fig. 4. Photographs of photochromic processes for diarylethenes 3 and 4 in the
crystalline phase.
wR2 (all data)

236
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
completely contrary to those of the unsymmetrical dithienylethene
derivatives whose the emission intensity of the ortho-substituted
derivative is the smallest and that of the para-substituted derivative
is the strongest both in hexane and in PMMA film [41]. Com-
pared to isomeric diarylethenes 2o–4o, the unsubstituted parent
diarylethene 1o showed the strongest emission intensity and the
lowest energy emission peak both in hexane and in PMMA film.
The results showed that the electron-donating group could be
effective to reduce the emission peak and weaken the fluorescent
intensity of diarylethenes bearing a pyrrole unit. The fluorescence
quantum yields of the open isomers of 1o, 2o, 3o, and 4o were
determined to be 0.010 0.024, 0.044, and 0.025, respectively, using
anthracene (0.27 in acetonitrile) as the reference. From the data,
we can see that the unsubstituted parent diarylethene 1 has the
smallest fluorescence quantum yield. That is to say, the introduc-
tion of electron-donating methoxy group in the terminal benzene
ring can significantly enhance the fluorescence quantum yield of
diarylethenes bearing a pyrrole unit.
As has been observed for most of the reported diarylethenes
[40–43,58–62], diarylethenes 1–4 exhibited relatively strong fluo-
rescence switch on changing from open-ring isomers to closed-ring
isomers. When irradiated by UV light, the photocyclization was
occurred and the fluorescence of diarylethenes 1–4 effectively was
quenched, due to producing the non-fluorescence closed-ring iso-
mers. The back irradiation by appropriate wavelength visible light
regenerated their respective open-ring isomers and recovered the
original emission spectra. During the process of photoisomeriza-
tion, the fluorescent spectral changes of diarylethene 1–4 in hexane
solutions (5.0 × 10−5 mol/L) and in PMMA films (10%, w/w) are
shown in Figs. 6 and 7, respectively. Upon irradiation with 297 nm
light, the samples arrived at the photostationary sate, resulting in
the emission intensities of diarylethenes 1o–4o being quenched
to ca. 7% for 1, 7% for 2, 12% for 3, and 4% for 4. That is to say,
the fluorescent modulation efficiencies of diarylethenes 1–4 at
photostationary state were 93% for 1, 93% for 2, 88% for 3, and
96% for 4. The result is well consistent with the photoconver-
sion ratios of diarylethenes 1–4 in the photostationary state. To
some extent, the fluorescent modulation efficiency depends on the
photoconversion ratio of diarylethenes. The greater the photocon-
version ratio is, the bigger the fluorescent modulation efficiency
is achieved in the photostationary state. Therefore, the fluores-
cent modulation efficiency of the para-substituted derivative 4 is
the biggest and the meta-substituted derivative 3o is the small-
est among isomeric diarylethenes 2–4 in hexane. Similarly, their
emission intensities decreased to the photostationary state upon
irradiation with 313 nm light in PMMA films, where the emission
intensities of diarylethenes 1–4 were quenched to ca. 17% for 1, 28%
for 2, 19% for 3, and 24% for 4, respectively. Therefore, the fluores-
cent modulation efficiencies of diarylethenes 1–4 in hexane were
much higher than those in PMMA film. This is completely contrary
to those of the unsymmetrical dithienylethene derivatives whose
fluorescent modulation efficiencies in hexane are much lower
than those in PMMA film [40,41]. Compared with those reported
diarylethenes [40,41,48,49], the fluorescent modulation efficien-
cies of diarylethenes 1–4 were significantly enhanced both in the
liquid and solid media. This indicated that diarylethenes bearing
a pyrrole unit have the potential use as fluorescent modulation
switches [6,63]. In addition, the “on” and ‘off” state of the fluores-
cence was measured by changing the power of the UV and visible
light. The average “on” and “off” times shortened with increasing
light power, indicating that the switching effect is indeed photo-
chemical [44]. Therefore, these diarylethene compounds could be
potentially applied to optical memory with fluorescence readout
method and fluorescence modulation switches.
Fig. 5. Fluorescence emission spectra of diarylethenes 1–4 both in hexane solution
(5.0 × 10−5 mol/L) and in PMMA films (10%, w/w) at room temperature: (A) emission
spectra in hexane, excited at 340 nm and (B) emission spectra in PMMA films, excited
at 350 nm.
to colorless and reproduce 3o and 4o upon irradiation with vis-
ible light (ꢂ > 500 nm). After 500 repeat cycles, the two isomeric
diarylethenes still showed good photochromism by alternating
irradiation with UV and visible light in the single crystalline phase.
So, they can be potentially used for the construction of certain
optoelectronic devices [55,56].
3.3. Fluorescence of diarylethenes
Fluorescent properties can be useful not only in molecular scale
optoelectronics [57], but in digital fluorescence photoswitches
[58]. The fluorescence emission spectra of 1o–4o in hexane
(5.0 × 10−5 mol/L) and in PMMA films (10%, w/w) at room tem-
perature are illustrated in Fig. 5. In hexane, the emission peaks of
diarylethenes 1o–4o were observed at 439, 425, 425 and 426 nm
when excited at 340 nm, and were observed at 424, 422, 422 and
422 nm when monitored at 350 nm in PMMA films. When compar-
ing samples in hexane to those in PMMA film, the emission peaks of
diarylethenes 1o–4o consistently exhibit a hypsochromic shift with
values of 15 nm for 1o, 3 nm for 2o, 3 nm for 3o, and 4 nm for 4o.
The result is contrary to those reported previously [40,41,48,49].
Among isomeric diarylethenes 2o–4o, the emission intensity of
the ortho-substituted derivative 2o is the strongest and that of the
para-substituted derivative 4o is the smallest both in hexane and
in PMMA film, which may be attributed to the steric hindrance of
ortho-methoxy group in addition to its strong electron-donating
nature [40]. This is in good agreement with those of the unsym-
metrical diarylethenes bearing a pyrazole unit [42], however, its
The concentration dependence on the florescence spectrum
of diarylethenes 1o was measured in hexane at room tempera-

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
237
Fig. 6. Emission intensity changes of diarylethene 1–4 by photoirradiation at room temperature in hexane (5.0 × 10−5 mol/L), excited at 340 nm.
Fig. 7. Emission intensity changes of diarylethene 1–4 by photoirradiation at room temperature in PMMA film (10%, w/w), excited at 350 nm.

238
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
Fig. 8. Emission spectral changes of diarylethene 1o in various concentrations in
hexane at room temperature, excited at 340 nm.
ture and the result is shown in Fig. 8. When the concentration
of diarylethene 1o in hexane was increased from 1.0 × 10−6 to
5.0 × 10−5 mol/L, the emission peak showed a clear bathochromic
shift from 427 to 439 nm when excited at 340 nm, and the emission
intensity increased dramatically with the increase of the concen-
tration. However, when the concentration increased further from
5.0 × 10−5 to 1.0 × 10−3 mol/L, the emission peak shifted slightly
along the longer wavelength direction, and the emission intensity
decreased remarkably. The hexane solution showed very weak flu-
orescence when the concentration arrived at 1.0 × 10−3 mol/L. Just
like diarylethene 1o, compounds 2o–4o showed similar fluorescent
properties depending on the concentration employed. The results
are summarized in Table 4. The result elucidated that diarylethenes
1o–4o exhibited a significant concentration effect on their flu-
orescent properties. The emission peaks of diarylethenes 1o–4o
showed an evident bathochromic shift along with the increase of
concentration. However, the emission intensities of them showed
a remarkable initial increase with subsequent dramatic decrease
with the increase of concentration. The strongest and the smallest
values were obtained at 5.0 × 10−5 and 1.0 × 10−3 mol/L, respec-
tively. The fluorescent quenching of diarylethenes 1o–4o at higher
concentration may be ascribed to the molecular aggregation [64].
Fig. 9. Cyclic voltammograms (second scan) of diarylethene 1–4 in acetonitrile with
the scanning rate of 50 mV s−1: (A) 1, (B) 2, (C) 3, and (D) 4.
1o–4o were observed at +1.63, 1.43, 1.51, and 1.31 V, and those of
1c–4c were observed at +0.72, 1.15, 0.98, and 0.79 V, respectively.
The result indicated that the oxidation process of the open-ring
isomers 1o–4o occurs at higher potentials than that of the corre-
sponding closed-ring isomers 1c–4c. This is in accordance with the
theory that longer conjugation length generally leads to less pos-
itive potentials, with the addition of each heterocyclic ring [66].
After cyclization reaction, the ␲-conjugation of the ring-closed iso-
mer extends across the perfluorocyclopentene ring causing a lower
oxidation onset. Moreover, there are great differences amongst the
electronic current and polarization curve shapes between the open-
ring and closed-ring isomers of diarylethenes 1–4 at the scanned
voltage region. The difference of oxidation onset between the open-
ring and closed-ring isomers of diarylethenes 1–4 ( Vo–c) was
0.91 V for 1, 0.28 V for 2, 0.53 V for 3, and 0.52 V for 4. Compared to
isomeric diarylethenes 2–4, the unsubstituted parent diarylethene
1 showed the biggest Vo–c, indicating that the electron-donating
group could be effective to decrease the Vo–c of these diarylethene
derivatives.
3.4. Electrochemistry of diarylethenes
The electrochemical properties of diarylethenes can be poten-
tially applied to molecular scale electronic switches [65]. The
oxidative cyclization and cycloreversion and the reductive elec-
trochemical cyclization of some diarylethene derivatives have
been reported [12,13,66,67]. Cyclic voltammograms (CV) were
performed on the diarylethenes 1–4 under identical experimen-
tal conditions at a scanning rate of 50 mV s−1. The CV curves of
diarylethenes 1–4 are shown in Fig. 9. The oxidation onsets of
According to the same method reported previously [68–70],
the Highest Occupied Molecular Orbitals (HOMO) and the Lowest
Unoccupied Molecular Orbitals (LUMO) energy levels can be esti-
mated by using the energy level of ferrocene as reference. Based
on the HOMO and LUMO energy level, the band-gap (Eg) of each
compound can be calculated approximately and the results are
summarized in Table 5. These data showed that the band-gaps of
the ring-closed isomers 1c–4c are lower than those of their cor-
responding ring-opened isomers 1o–4o. Among these compounds,
the Eg of 1o is the highest (Eg = 2.60 eV) and that of 1c is the lowest
(Eg = 1.35 eV), implying that the charge transfer must be faster in
1c compared to that in others [71]. When introducing the methoxy
group into the terminal benzene ring of the same diarylethene
Table 4
The effect of concentration on the fluorescence spectra of diarylethenes 1–4 at room
temperature in hexane, monitored at 340 nm.
Concentrations (mol/L) ꢂem,max (emission intensity)
1o
2o
3o
4o
1.0 × 10−6
5.0 × 10−6
1.0 × 10−5
5.0 × 10−5
5.0 × 10−4
1.0 × 10−3
427 (2338) 403 (3299) 401 (2426) 402 (2011)
430 (3231) 408 (3928) 407 (2796) 409 (2300)
435 (4276) 417 (5520) 421 (3383) 414 (2935)
439 (5780) 425 (3810) 425 (3621) 426 (3242)
441 (4138) 430 (2554) 427 (2492) 426 (2351)
441 (637)
436 (274)
430 (333)
430 (499)

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
239
Table 5
[5] K. Uchida, A. Takata, M. Saito, A. Murakami, S. Nakamura, M. Irie, A novel pho-
tochromic film by oxidation polymerization of a bisbenzothienylethene with
phenol groups, Adv. Mater. 15 (2003) 785–788.
[6] H. Tian, B.Z. Chen, H.Y. Tu, K. Müllen, Novel bisthienylethene-based pho-
tochromic tetraazaporphyrin with photoregulating luminescence, Adv. Mater.
14 (2002) 918–923.
Electrochemical properties of diarylethenes 1–4 in acetonitrile.
Compounds
Oxidation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg (eV)
[7] F.M. Raymo, M. Tomasulo, Electron and energy transfer modulation with pho-
tochromic switches, Chem. Soc. Rev. 34 (2005) 327–336.
[8] K. Matsuda, M. Irie, Diarylethene as a photoswitching unit, J. Photochem. Pho-
tobiol. C: Photochem. Rev. 5 (2004) 169–182.
[9] C. Bertarelli, A. Bianco, F. D’Amore, M.C. Gallazzi, G. Zerbi, Effect of substitution
on the change of refractive index in dithienylethene: An ellipsometric study,
Adv. Funct. Mater. 14 (2004) 357–363.
[10] E. Kim, Y.-K. Choi, M.-H. Lee, Photoinduced refractive index change of a pho-
tochromic diarylethene, Macromolecules 32 (1999) 4855–4860.
[11] G. Guirado, C. Coudret, M. Hliwa, J.-P. Launay, Understanding electrochromic
processes initiated by dithienylcyclopentene cation-radicals, J. Phys. Chem. B
109 (2005) 17445–17459.
1o
1c
2o
2c
3o
3c
4o
4c
1.63
0.72
1.43
1.15
1.51
0.98
1.31
0.79
−6.43
−5.52
−6.23
−5.95
−6.31
−5.78
−6.11
−5.59
−0.97
−0.63
−0.90
−0.77
−0.97
−0.79
−0.89
−0.76
−3.83
−4.17
−3.9
2.60
1.35
2.33
1.92
2.48
1.77
2.20
1.55
−4.03
−3.81
−4.01
−3.91
−4.04
skeleton, the band-gaps of the open-ring isomers 2o–4o decreased,
and those of the closed-ring isomers 2o–4o increased significantly,
compared with the unsubstituted parent diarylethene 1. The results
suggest that the methoxy group and its substituted position have a
great effect on the electrochemical properties of these diarylethene
compounds but further work is required to quantify these effects.
It should be noted here that calculation absolute HOMO and LUMO
levels from electrochemical data in combination with the energy
gap is still in debate [70].
[12] W.R. Browne, J.J.D. de Jong, T. Kudernac, M. Walko, L.N. Lucas, K. Uchida,
J.H. van Esch, B.L. Feringa, Oxidative electrochemical switching in dithienyl-
cyclopentenes, part 1: effect of electronic perturbation on the efficiency and
direction of molecular switching, Chem. Eur. J. 11 (2005) 6414–6429.
[13] W.R. Browne, J.J.D. de Jong, T. Kudernac, M. Walko, L.N. Lucas, K. Uchida, J.H.
van Esch, B.L. Feringa, Oxidative electrochemical switching in dithienylcy-
clopentenes, part 2: effect of substitution and asymmetry on the efficiency and
direction of molecular switching and redox stability, Chem. Eur. J. 11 (2005)
6430–6441.
[14] T. Okuyama, Y. Tani, K. Miyake, Y. Yokoyama, Chiral helicenoid diarylethene
with large change in specific optical rotation by photochromism, J. Org. Chem.
72 (2007) 1634–1638.
[15] Y. Tani, T. Ubukata, Y. Yokoyama, Y. Yokoyama, Chiral helicenoid diarylethene
with highly diastereoselective photocyclization, J. Org. Chem. 72 (2007)
1639–1644.
4. Conclusions
[16] K. Matsuda, S. Yamamoto, M. Irie, Diastereoselective cyclization of
a
In summary, four unsymmetrical diarylethene derivatives bear-
ing a pyrrole moiety were synthesized, and their photochemical
and electrochemical properties were discussed systematically.
All compounds undergo good photochromism both in solution
and in PMMA films. In comparison of the unsubstituted par-
ent diarylethene 1, the absorption maxima, the quantum yields
of cyclization and cycloreversion, and the fluorescence quantum
yields of the open-ring isomers of diarylethenes 2–4 bearing a
methoxy group at the ortho-, meta-, and para-position of the termi-
nal benzene ring increased remarkably. However, the fluorescence
emission peaks and the Egs of the open-ring isomers of 2–4 were
much lower than those of 1. The methoxy group and its substi-
tuted position had a significant effect on the photochemical and
electrochemical properties of these isomeric diarylethene deriva-
tives. The pyrrole moiety induced new features which differed
from those of diarylethenes bearing thiophene or pyrazole rings
which had been reported previously. The results will be help-
ful for in understanding the substituent position effect on the
properties of diarylethenes bearing a pyrrole moiety and for the
synthesis of efficient photoactive diarylethenes with tunable prop-
erties.
diarylethene having a chiral N-phenylethylamide substituent in crystals, Tetra-
hedron Lett. 42 (2001) 7291–7293.
[17] C.C. Corredor, Z.-L. Huang, K.D. Belfield, Two-photon 3D data storage via fluores-
cence modulation of an efficient fluorene dye by a photochromic diarylethene,
Adv. Mater. 18 (2006) 2910–2914.
[18] S.Z. Pu, H.H. Tang, B. Chen, J.K. Xu, W.H. Huang, Photochromic diarylethene for
two-photon 3D optical storage, Mater. Lett. 60 (2006) 3553–3557.
[19] S.Z. Pu, F.S. Zhang, J.K. Xu, L. Shen, Q. Xiao, B. Chen, Photochromic diarylethenes
for three-wavelength optical memory, Mater. Lett. 60 (2006) 485–489.
[20] S.Z. Pu, T.S. Yang, B. Yao, Y.L. Wang, M. Lei, J.K. Xu, Photochromic diarylethene
for polarization holographic optical recording, Mater. Lett. 61 (2007) 855–859.
[21] H. Tian, Y.L. Feng, Next step of photochromic switches, J. Mater. Chem. 18 (2008)
1617–1622.
[22] J.J. Zhang, W.J. Tan, X.L. Meng, H. Tian, Soft mimic gear-shift with a multi-
stimulus modified diarylethene, J. Mater. Chem. 19 (2009) 5726–5729.
[23] H. Tian, S. Wang, Photochromic bisthienylethene as multi-function switches,
Chem. Commun. (2007) 781–792.
[24] Y.L. Feng, Y.L. Yan, S. Wang, W.H. Zhu, S.X. Qian, H. Tian, Photochromic thio-
phene oligomers based on bisthienylethene: syntheses, photochromic and
two-photon properties, J. Mater. Chem. 16 (2006) 3685–3692.
[25] K. Higashiguchi, K. Matsuda, N. Tanifuji, M. Irie, Full-color photochromism of a
fused dithienylethene trimer, J. Am. Chem. Soc. 127 (2005) 8922–8923.
[26] S.Z. Pu, G. Liu, L. Shen, J.K. Xu, Efficient synthesis and properties of isomeric pho-
tochromic diarylethenes having a pyrrole unit, Org. Lett. 9 (2007) 2139–2142.
[27] K. Uchida, T. Matsuoka, K. Sayo, M. Iwamoto, S. Hayashi, M. Irie, Thermally
reversible photochromic systems. Photochromism of a dipyrrolylperfluorocy-
clopentene, Chem. Lett. (1999) 835–836.
[28] M. Irie, K. Sakemura, M. Okinaka, K. Uchida, Photochromism of dithienylethenes
with electron-donating substituents, J. Org. Chem. 60 (1995) 8305–8309.
[29] S.Z. Pu, C.H. Zheng, Z.G. Le, G. Liu, C.B. Fang, Substituent effects on the properties
of photochromic diarylethene, Tetrahedron 64 (2008) 2576–2585.
[30] K. Uchida, T. Matsuoka, S. Kobatake, T. Yamaguchi, M. Irie, Substituent effect on
the photochromic reactivity of bis(2-thienyl)perfluorocyclopentenes, Tetrahe-
dron 57 (2001) 4559–4565.
Acknowledgements
This work was supported by Program for the NSFC of China
(20962008, 50973077), New Century Excellent Talents in Univer-
sity (NCET-08-0702), the Key Scientific Project from Education
Ministry of China (208069), National Natural Science Foundation
of Jiangxi Province (2008GZH0020, 2009GZH0034) and the Youth
Science Funds of the Education Office of Jiangxi, China (GJJ09302).
[31] X.D. Deng, L.S. Liebeskind,
A contribution to the design of molecular
switches: novel acid-mediated ring-closing-photochemical ring-opening of
2,3-bis(heteroaryl)quinones, J. Am. Chem. Soc. 123 (2001) 7703–7704.
[32] K. Uchida, T. Ishikawa, M. Takeshita, M. Irie, Thermally irre-
versible
photochromic
systems.
Reversible
photocyclization
of
1,2-bis(thiazolyl)perfluorocyclopentenes, Tetrahedron 54 (1998) 6627–6638.
[33] S. Takami, M. Irie, Synthesis and photochromic properties of novel yellow
developing photochromic compounds, Tetrahedron 60 (2004) 6155–6161.
[34] K. Yagi, M. Irie, Fluorescence property of photochromic diarylethenes with
indole groups, Bull. Chem. Soc. Jpn. 76 (2003) 1625–1628.
[35] K. Yagi, C.F. Soong, M. Irie, Synthesis of fluorescent diarylethenes having a 2,4,5-
triphenylimidazole chromophore, J. Org. Chem. 66 (2001) 5419–5423.
[36] K. Abu-Rabeah, B. Polyak, R.E. Ionescu, S. Cosnier, R.S. Marks, Synthesis and
characterization of a pyrrole-alginate conjugate and its application in a biosen-
sor construction, Biomacromolecules 6 (2005) 3313–3318.
References
[1] M. Irie, Diarylethenes for memories and switches, Chem. Rev. 100 (2000)
1685–1716.
[2] S. Kawata, Three-dimensional optical data storage using photochromic mate-
rials, Chem. Rev. 100 (2000) 1777–1788.
[3] H. Tian, S.J. Yang, Recent progresses on diarylethene based photochromic
switches, Chem. Soc. Rev. 33 (2004) 85–97.
[4] S.H. Chen, H.M.P. Chen, Y. Geng, D.J. Stephen, K.L. Marshall, T.N. Blanton, Novel
glassy nematic liquid crystals for non-destructive rewritable optical memory
and photonic switching, Adv. Mater. 15 (2003) 1061–1065.
[37] T. Bando, H. Sugiyama, Synthesis and biological properties of sequence-specific
DNA-alkylating pyrrole–imidazole polyamides, Acc. Chem. Res. 39 (2006)
935–944.

240
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 230–240
[38] K. Yumoto, M. Irie, K. Matsuda, Control of the photoreactivity of diarylethene
derivatives by quaternarization of the pyridylethynyl group, Org. Lett. 10 (2008)
2051–2054.
[39] S.Z. Pu, T.S. Yang, G.Z. Li, J.K. Xu, B. Chen, Substituent position effect on the
optoelectronic properties of photochromic diarylethenes, Tetrahedron Lett. 47
(2006) 3167–3171.
[56] M. Berberich, A.-M. Krause, M. Orlandi, F. Scandola, F. Würthner, Toward
fluorescent memories with nondestructive readout: photoswitching of fluores-
cence by intramolecular electron transfer in a diaryl ethene-perylene bisimide
photochromic system, Angew. Chem. 120 (2008) 6718–6721.
[57] B. Gorodetsky, H.D. Samachetty, R.L. Donkers, M.S. Workentin, N.R.
Branda, Reductive electrochemical cyclization of
a photochromic 1,2-
[40] S.Z. Pu, C.B. Fan, W.J. Miao, G. Liu, The effect of substituent position upon unsym-
metrical isomeric diarylethenes bearing a methoxy group, Dyes Pigments 84
(2010) 25–35.
[41] S.Z. Pu, W.J. Liu, W.J. Miao, Photochromism of new unsymmetrical isomeric
diarylethenes bearing a methoxyl group, J. Phys. Org. Chem. 22 (2009) 954–963.
[42] S.Z. Pu, T.S. Yang, J.K. Xu, B. Chen, Syntheses and properties of new pho-
tochromic diarylethene derivatives having a pyrazole unit, Tetrahedron Lett.
47 (2006) 6473–6477.
[43] T.S. Yang, S.Z. Pu, B. Chen, J.K. Xu, Electron-donating methoxy group position
effect on properties of a new unsymmetrical diarylethene derivatives having a
pyrazole unit, Can. J. Chem. 85 (2007) 12–20.
[44] Z.X. Li, L.Y. Liao, W. Sun, C.H. Xu, C. Zhang, C.J. Fang, C.H. Yan, Reconfigurable
cascade circuit in a photo- and chemical-switchable fluorescent diarylethene
derivative, J. Phys. Chem. C 112 (2008) 5190–5196.
[45] S.Z. Pu, C.B. Fan, W.J. Miao, G. Liu, New photochromic diarylethenes with a
six-membered aryl unit, Tetrahedron 64 (2008) 9464–9470.
[46] K. Kasatani, S. Kambe, M. Irie, Photochromic reaction and fluorescence of
dithienylethenes in the solid state, J. Photochem. Photobiol. A 122 (1999)
11–15.
[47] M. Hoshino, F. Ebisawa, T. Yoshida, K. Sukegawa, Refractive index change in
photochromic diarylethene derivatives and its application to optical switching
devices, J. Photochem. Photobiol. A 105 (1997) 75–81.
[48] C.B. Fan, S.Z. Pu, G. Liu, T.S. Yang, Substituent position effect on the properties
of isomeric photochromic diarylethenes bearing chlorine atoms, J. Photochem.
Photobiol. A: Chem. 194 (2008) 333–343.
[49] C.B. Fan, S.Z. Pu, G. Liu, T.S. Yang, Substituent position effect on the proper-
ties of new unsymmetrical isomeric diarylethenes having a chlorine atom, J.
Photochem. Photobiol. A: Chem. 197 (2008) 415–425.
dithienylcyclopentene dication, Angew. Chem. Int. Ed. 43 (2004) 2812–2815.
[58] T. Fukaminato, T. Sasaki, T. Kawai, N. Tamai, M. Irie, Digital photoswitching of
fluorescence based on photochromism of diarylethene derivatives at a single-
molecule level, J. Am. Chem. Soc. 126 (2004) 14843–14849.
[59] H. Tian, B. Qin, R.X. Yao, X.L. Zhao, S.J. Yang, A single photochromic molecular
switch with four optical outputs probing four inputs, Adv. Mater. 15 (2003)
2104–2107.
[60] S.Z. Xiao, T. Yi, Y.F. Zhou, Q. Zhao, F.Y. Li, C.H. Huang, Multi-state molecular
switches based on dithienylperfluorocyclopentene and imidazo [4,5-f] [1,10]
phenanthroline, Tetrahedron 62 (2006) 10072–10078.
[61] Y. Zou, T. Yi, S.Z. Xiao, F.Y. Li, C.Y. Li, X. Gao, J.C. Wu, M.X. Yu, C.H. Huang,
Amphiphilic diarylethene as a photoswitchable probe for imaging living cells,
J. Am. Chem. Soc. 130 (2008) 10750–15751.
[62] C. Zhang, H.-P. Zhou, L.-Y. Liao, W. Feng, W. Sun, Z.-X. Li, C.-H. Xu, C.-J. Fang, L.-D.
Sun, Y.-W. Zhang, C.-H. Yan, Luminescence modulation of ordered upconver-
sion nanopatterns by photochromic diarylethene: rewritable optical storage
with nondestructive readout, Adv. Mater. 22 (2010) 633–637.
[63] B.Z. Chen, M.Z. Wang, Y.Q. Wu, H. Tian, Reversible near-infrared fluorescence
switch by novel photochromic unsymmetrical-phthalocyanine hybrids based
on bisthienylethene, Chem. Commun. (2002) 1060–1061.
[64] T. Fukaminato, T. Kawai, S. Kobatake, M. Irie, Fluorescence of photochromic
1,2-bis(3-methyl-2-thienyl)ethene, J. Phys. Chem. B 107 (2003) 8372–8377.
[65] D. Bonifazi, M. Scholl, F. Song, L. Echegoyen, G. Accorsi, N. Armaroli, F. Diederich,
Exceptional redox and photophysical properties of a triply fused diporphyrin-
C60 conjugate: novel scaffolds for multicharge storage in molecular scale
electronics, Angew. Chem. Int. Ed. 42 (2003) 4966–4970.
[66] S.Z. Pu, T.S. Yang, J.K. Xu, L. Shen, G.Z. Li, Q. Xiao, B. Chen, Syntheses and opto-
electronic properties of four photochromic dithienylethenes, Tetrahedron 61
(2005) 6623–6629.
[50] S.Z. Pu, L.S. Yan, Z.D. Wen, G. Liu, L. Shen, Synthesis and chlorine atom posi-
tion effect on the properties of unsymmetrical photochromic diarylethenes, J.
Photochem. Photobiol. A: Chem. 196 (2008) 84–93.
[51] S. Kobatake, K. Uchida, E. Tsuchida, M. Irie, Single-crystalline photochromism
of diarylethenes: reactivity–structure relationship, Chem. Commun. (2002)
2804–2805.
[67] Y. Moriyama, K. Matsuda, N. Tanifuji, S. Irie, M. Irie, Electrochemical cycliza-
tion/cycloreversion reactions of diarylethenes, Org. Lett. 7 (2005) 3315–3318.
[68] C.H. Zheng, S.Z. Pu, J.K. Xu, M.B. Luo, D.C. Huang, L. Shen, Synthesis and the effect
of alkyl chain length on optoelectronic properties of diarylethene derivatives,
Tetrahedron 63 (2007) 5437–5449.
[52] M. Morimoto, M. Irie, Photochromism of diarylethene single crystals: crys-
tal structures and photochromic performance, Chem. Commun. (2005)
3895–3905.
[69] F.C. Tsai, C.C. Chang, C.L. Liu, W.C. Chen, S.A. Jenekhe, New thiophene-linked
conjugated poly(azomethine)s: theoretical electronic structure, synthesis, and
properties, Macromolecules 38 (2005) 1958–1966.
[53] M. Morimoto, M. Irie, Photochromic reactions of diarylethenes in single crystals
with intermolecular O–H· · ·N hydrogen-bonding networks, Chem. Eur. J. 12
(2006) 4275–4282.
[70] X.W. Zhan, Y.Q. Liu, X. Wu, S. Wang, D.B. Zhu, New series of blue-emitting and
electron-transporting fluorene-based copolymers, Macromolecules 35 (2002)
2529–2537.
[54] V. Ramamurthy, K. Venkatesan, Photochemical reactions of organic crystals,
Chem. Rev. 87 (1987) 433–481.
[55] M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, A digital fluorescent molec-
ular photoswitch, Nature 420 (2002) 759–760.
[71] E. Kim, M. Kim, K. Kim, Diarylethenes with intramolecular donor–acceptor
structures for photo-induced electrochemical change, Tetrahedron 62 (2006)
6814–6821.