Effects of substitution on the optoelectronic properties of photochromic diarylethenes bearing a pyrrole moiety

Article abstract of DOI:10.1016/j.dyepig.2010.11.001

Five diarylethenes bearing a pyrrole moiety were synthesized in order to investigate the effects of the substituents on the properties of these compounds. The structures of three of the diarylethenes were determined by single-crystal X-ray diffraction ana

Full text of DOI:10.1016/j.dyepig.2010.11.001

Dyes and Pigments 90 (2011) 71e81
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Effects of substitution on the optoelectronic properties of photochromic
diarylethenes bearing a pyrrole moiety
,
,
a
,
b
b
Gang Liu a b, Shouzhi Pu b , Xiaomei Wang
, Weijun Liu , Tianshe Yang
*
**
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:
Five diarylethenes bearing a pyrrole moiety were synthesized in order to investigate the effects of the
substituents on the properties of these compounds. The structures of three of the diarylethenes were
determined by single-crystal X-ray diffraction analysis. All of the diarylethene derivatives exhibited
notable photochromism and functioned as effective fluorescent photo-switches in solution and in poly
(methyl methacrylate) films. The absorption maxima and the quantum yields of cyclization and cyclo-
reversion increased whereas the emission peaks and the emission intensities decreased evidently when
the electron-withdrawing/donating substituent was attached at the para-position on the terminal
benzene ring. Cyclic voltammograms indicated that the electron-withdrawing/donating substituents
decreased the bang gaps of the open-ring isomers but enhanced those of the closed-ring isomers,
compared with the unsubstituted parent diarylethene. The properties of these diarylethene derivatives
were significantly different from each other, which might be attributed to the different substituent
effects.
Received 30 July 2010
Received in revised form
3 November 2010
Accepted 7 November 2010
Available online 14 November 2010
Keywords:
Diarylethene
Pyrrole moiety
Photochromism
Substituent effect
Photochemical and electrochemical
property
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
decreased the cycloreversion quantum yield [10], but the same
substituents on bis(2-thienyl)ethene diarylethenes increased the
Photochromic materials have attracted much attention as highly
promising candidates for optical memories and photo-switches
[1,2]. During the past several decades, there have been some
achievements in the synthesis of new families of organic photo-
chromic molecules [3e5]. Among various photochromic molecules,
diarylethenes with heterocyclic aryl rings are regarded as the best
candidates for photoelectronic applications mainly due to the
excellent thermal stability of the respective isomers, notable
fatigue resistance, rapid response and high reactivity in the solid
state [6,7].
Diarylethenes bearing terminal phenyl groups have raised
considerable interest in the last few years because the properties of
the phenyl-substituted diarylethene derivatives can be further
modified by suitable functionalization of the aromatic rings with
electron-donating groups or electron-withdrawing groups [8e11].
For example, some reports indicated that the introduction of elec-
tron-donating substituents to bis(3-thienyl)ethene diarylethenes
increased the absorption coefficient of the closed-ring isomers and
absorption maxima of the open-ring isomers and decreased the
cyclization quantum yield [11]. Moreover, Bertarelli et al. demon-
strated that the strong electron-donating substituents directly
linked to the 5,50-position of the diarylethene moieties gave rise to
an increase in the change of the refractive index [12]. Tanifuji et al.
reported the effect of radical substituents on the photochromic
reactivity of bis(3-benzothiophene)perfluorocyclopentene [13], and
Yamaguchi et al. investigated the substituent effect on the photo-
chromic properties of diarylethenes bearing two benzofuran aryl
groups [14]. Previously, we have reported that different substituents
and the substituted-position had a significant effect on the prop-
erties of diarylethenes such as their photochromism, fluorescence,
and electrochemical properties [15e21].
Apart from the fact that various functional substituents have
notable effects on the photochromic features of diarylethenes,
assorted heteroaryl moieties can also notably affect photochromic
properties. A change in heteroaryl group can result in remarkable
modification of the optical and electrochemical properties of each
type of diarylethene molecule [22e25]. For instance, diarylethenes
with thiophene or benzothiophene moieties exhibit excellent
thermal stability and outstanding fatigue resistance [1,2], whereas
symmetrical diarylethenes with two pyrrole rings are thermally
unstable and return to the open-ring isomer, even in the dark [26].
* Corresponding author. Tel./fax: þ86 791 3831996.
** Corresponding author.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.11.001

72
G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
Diarylethenes bearing a pyrazole unit have a relatively high
cycloreversion quantum yield [27], whereas dithienylethenes
bearing bulky alkoxy groups at 2- and 20-positions of the thiophene
rings have a very small cycloreversion quantum yield [28]. Kawai
et al. reported the triangular terthiophene derivatives which
showed reversible photochromic reactions with high cyclization
quantum yields [29]. Yamaguchi et al. developed a new type of
platinum wire. The typical electrolyte was acetonitrile (5 mL) con-
taining 0.1 mol/L tetrabutylammonium tetrafluoroborate ((TBA)BF4)
and 1.0 ꢀ 10ꢁ3 mol/L diarylethene sample. All solutions were dea-
eratedbybubblingwith adryargonstreamandmaintained ata slight
argon overpressure during electrochemical experiments. Solvents
used were spectroscopic grade and were purified by distillation.
Suitable crystals of 2o, 4o and 5o were obtained by slow evap-
oration of a hexane solution. All the measurements were collected
by a Bruker SMART APEX II CCD diffractometer using a MULTI scan
6p
conjugated photochromic system with a bis(2,30-benzothienyl)
unit which demonstrated efficient photochromism and thermal
stability for the colored isomers [30]. Yokoyama et al. and Branda
et al. independently synthesized a new class of photochromic
compounds based on a modified hexatriene skeleton [24,31]. This
novel molecular scaffold offers the chance to decorate photo-
responsive systems with a wide range of functional groups without
sacrificing the photochromic behavior.
As described above, it can be concluded that most of the het-
eroaryl diarylethenes bear thiophene or benzothiophene rings,
with just a few reports concerning other heteroaryl moieties such
as furan, thiazole, indole, and so on. Pyrrole is an attractive aryl unit
due to its biological characteristics [32], and its structure is similar
to those of thiophene and furan. However, reports of diarylethene
derivatives with a pyrrole unit are rare. As far as we know, there are
only a few examples of photochromic diarylethenes bearing
a pyrrole unit [26,33]. This present work a series of five unsym-
metrical diarylethenes bearing a pyrrole moiety and different
substituents at the para-position of the terminal phenyl ring was
synthesized. Each of the diarylethene derivatives showed notable
photochromism both in solution and in the solid state. Their
photochromic scheme is shown in Fig. 1.
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
762280 for 2o, 762281 for 4o, and 639785 for 5o. Copies of the data
can be obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: þ44 0 1223 336033 or e-
mail:deposit@ccdc.cam.ac.uk).]
2.2. Synthesis
The synthesis method of diarylethenes 1oe5o was shown in
Fig. 2. Suzuki coupling of the five bromobenzene derivatives with
a thiophene boronic acid [34,35] gave the alkylphenylthiophene
derivatives
(7ae7e).
1-(2-Cyano-1,5-dimethyl-4-pyrryl)per-
fluorocyclopentene (10) [33] was synthesized by bromination and
lithiation reactions from 5-cyano-1,2-dimethylpyrrole. Finally,
compounds 7ae7e were separately lithiated and then coupled with
compound 10 to give diarylethenes 1oe5o, respectively.
2. Experimental
2.2.1. 3-Bromo-2-methyl-5-(4-cyanophenyl) thiophene (7a)
Compound 7a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid (6) [34,35] (5.0 g; 22.6 mmol) with 4-bromo-
benzonitrile (4.11 g, 22.6 mmol) in the presence of Pd(PPh3)4 (0.6 g)
and Na2CO3 (6.4 g, 60 mmol) in tetrahydrofuran (THF) (80 mL,
containing 10% water). After refluxing for 15 h at 70 ꢂC, the reaction
mixture was allowed to slowly warm to the room temperature and
then extracted with ether. The organic layer was collected and
dried over MgSO4, filtrated and evaporated. The crude product was
purified by column chromatography on SiO2 using hexane as the
eluent resulting in 4.46 g of 7a being obtained as a pale yellow solid
in 71% yield. M.p. 79e80 ꢂC; Anal. Calcd for C12H8BrNS (%): C, 51.81;
H, 2.90; N, 5.04. Found C, 51.99; H, 3.01; N, 4.96; 1H NMR (400 MHz,
2.1. General
All solvents were purified by distillation before use. 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.
Meltingpointswere measuredonaWRS-1B meltingpointapparatus.
Absorption spectra were measured using an Agilent 8453 UV/VIS
spectrophotometer. Photoirradiation was carried out using a SHG-
200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet and
a BMH-250 visible lamp. The required light wavelength was isolated
by the use of the appropriate filters. Fluorescence spectra were
measured using a Hitachi F-4500 spectrophotometer. Electro-
chemical examinations were performed in a one-compartment cell
by using a Model 263 potentiostategalvanostat (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 quasir-
eference electrode. It was calibrated using an internal ferrocene (Fc/
Fcþ) standard with a formal potential of E1/2 ¼ þ0.35 V versus
CDCl3, TMS): d 2.41 (s, 3H, eCH3), 7.03 (s, 1H, thienyl-H), 7.06 (t, 2H,
J ¼ 8.0 Hz, phenyl-H), 7.44e7.48 (m, 2H, phenyl-H); IR (
n, KBr,
cmꢁ1): 808, 829, 1013, 1067, 1164, 1329, 1465, 1504, 1604, 2224,
2919.
2.2.2. 3-Bromo-5-2-methyl-(4-fluorophenyl)thiophene (7b)
Compound 7b was prepared by a method similar to that used for
7a. The crude product was purified by column chromatography on
SiO2 using petroleum ether as eluent to give 7b (2.54 g, 75%) as
a yellow solid. M.p. 61e63 ꢂC; Anal. Calcd for C11H8BrFS (%): C,
48.73; H, 2.97. Found C, 48.91; H, 2.84; 1H NMR (400 MHz, CDCl3,
F
F
F
F
F
F
F
TMS):
1H, phenyl-H), 7.56 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.73 (d, 1H,
, KBr, cmꢁ1): 789,
d
2.45 (s, 3H, eCH3), 7.18 (s, 1H, thienyl-H), 7.50 (t, J ¼ 7.6 Hz,
F
F
UV
Vis
F
F
F
J ¼ 8.0 Hz, phenyl-H), 7.79 (s, 1H, phenyl-H); IR (
n
R
818, 837, 945, 1011, 1098, 1158, 1243, 1277, 1303, 1326, 1375, 1469,
1516, 1541, 1602, 1885, 2917.
R
S
NC
N
S
NC
N
1c: R =
2c: R =
3c: R =
4c: R =
CN
F
1o: R =
2o: R =
3o: R =
4o: R =
5o: R =
CN
F
H
2.2.3. 3-Bromo-2-methyl-5-phenyl-thiophene (7c)
H
CH3
CH3
OCH3
Compound 7c was prepared by a method similar to that used for
7a. The crude product was purified by column chromatography on
SiO2 using hexane as eluent to give 7c (2.54 g, 83%) as a pale yellow
5c: R = OCH3
Fig. 1. Photochromism of diarylethenes 1e5.

G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
73
Br
Br
Br
R
R
S
Pd(PPh3)4
7a: R = CN
B(OH)2
S
Na2CO3
7b: R =
7c: R =
7d: R =
F
F
F
H
F
F
6
CH3
7e: R = OCH3
F
F
n-BuLi
F
F
F
R
F
Br
S
NC
N
F
n-BuLi/THF
Br2
F
1o: R =
2o: R =
3o: R =
4o: R =
5o: R =
CN
F
F
C5F8,
CN
CN
N
8
H
N
9
CH3
OCH3
NC
N
10
Fig. 2. Synthetic route for diarylethenes 1oe5o.
solid. M.p. 66e68 ꢂC; Anal. Calcd for C11H9BrS (%): C, 52.19; H, 3.58.
Found C, 52.34; H, 3.43; 1H NMR (400 MHz, CDCl3):
2.34 (s, 3H,
eCH3), 7.02 (s, 1H, thienyl-H), 7.20 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.29
(8.80 mL, 22.00 mmol) at ꢁ78 ꢂC under nitrogen atmosphere. Stir-
ring was continued for 30 min at ꢁ78 ꢂC, octafluorocyclopentene
(C5F8) (2.80 mL, 20.51 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 evapo-
rated in vacuo. The crude product was purified by column chroma-
tography on silica gel using petroleum ether as the eluent to give
3.62 g compound 10 obtained as a pale yellow solid in 57% yield. M.p.
101e103 ꢂC; Anal. Calcd for C12H7F7N2 (%): C, 46.17; H, 2.26; N, 8.97.
d
(t, 2H, J ¼ 8.4 Hz, phenyl-H), 7.42 (d, 2H, J ¼ 8.0 Hz, phenyl-H); IR (
n,
KBr, cmꢁ1): 754, 792, 827, 904, 946, 1007, 1032, 1072, 1155, 1326,
1443, 1467, 1498, 1537, 2918.
2.2.4. 3-Bromo-2-methyl-5-(4-methylphenyl)thiophene (7d)
Compound 7d was prepared by a method similar to that used for
7a. The crude product was purified by column chromatography on
SiO2 using petroleum ether as eluent to give 7d (2.94 g, 83%) as
a pale yellow solid. M.p. 67e68 ꢂC; Anal. Calcd for C12H11BrS (%): C,
53.94; H, 4.15. Found C, 54.09; H, 4.03; 1H NMR (400 MHz, CDCl3,
Found C, 46.39; H, 2.11; N, 9.12; 1H NMR (400 MHz, CDCl3):
d
2.32 (s,
, KBr,
3H, eCH3), 3.72 (s, 3H, eCH3), 6.98 (s, 1H, pyrrole-H); IR (
n
TMS):
7.19 (d, 2H, J ¼ 8.0 Hz, phenyl-H), 7.38 (d, 2H, J ¼ 8.0 Hz, phenyl-H);
IR (
, KBr, cmꢁ1): 784, 807, 944, 1007, 1019, 1120, 1162, 1274, 1309,
d
2.37 (s, 3H, eCH3), 2.41 (s, 3H, eCH3), 7.01 (s, 1H, thienyl-H),
cmꢁ1): 845, 890, 974, 1031, 1155, 1203, 1281, 1330, 1372, 1488, 1547,
1696, 2219, 3450.
n
1327, 1374, 1442, 1511, 2852, 2915.
2.2.8. 1-[2-Methyl-5-(4-cyanophenyl)-3-thienyl]-2-(2-cyano-1,5-
dimethyl-4-pyrryl)perfluorocyclopentene (1o)
2.2.5. 3-Bromo-2-methyl-5-(4-methoxyphenyl)thiophene (7e)
Compound 7e was prepared by a method similar to that used for
7a. The crude product was purified by column chromatography on
SiO2 using petroleum ether as eluent to give 7e (2.51 g, 76%) as
a pale yellow solid. M.p.107e108 ꢂC; Anal. Calcd for C12H11BrOS (%):
C, 50.90; H, 3.92. Found C, 50.72; H, 4.04; 1H NMR (400 MHz,
To a stirred anhydrous THF (70 mL) of compound 7a (0.59 g,
2.12 mmol) was added dropwise a 2.5 M n-BuLi/hexane solution
(0.95 mL) at ꢁ78 ꢂC under argon atmosphere. After 30 min, 15 mL
THF containing compound 10 (0.66 g, 2.30 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
a colorless solid in 38% yield. M.p. 180e181 ꢂC; Anal. Calcd for
C24H15F6N3S (%): C, 58.65; H, 3.08; N, 8.55. Found C, 58.69; H, 3.05;
CDCl3):
phenyl-H), 7.27 (s, 1H, thienyl-H), 7.30 (s, 1H, phenyl-H), 7.54 (d, 1H,
, KBr, cmꢁ1): 797, 818, 946, 1033, 1111,
d 2.41 (s, 3H, eCH3), 3.91 (s, 3H, eOCH3), 6.95e6.99 (m, 2H,
J ¼ 8.0 Hz, phenyl-H); IR (
n
1161, 1179, 1255, 1288, 1309, 1455, 1541, 1603, 2917.
2.2.6. 3-Bromo-1,5-dimethyl-2-pyrrolecarbonitrile (9)
To a stirred solution of 1,5-dimethyl-2-pyrrolecarbonitrile (8)
(3.01 g, 25.00 mmol) in acetic acid (100 mL) at 0 ꢂC was slowly
added Br2/acetic acid (v/v ¼ 29/40) solution. The reaction mixture
was stirred overnight at this temperature. After pouring the reac-
tion mixture into a beaker containing some water, a large quantity
of yellow precipitate was produced. The precipitate was washed by
water, filtered and dried, and then purified by silica gel column
chromatography using hexane as the eluent. Compound 8 was
obtained as a pale yellow solid (4.21 g) with a yield of 85%. M.p.
124e125 ꢂC; Anal. Calcd for C7H7BrN2 (%): C, 42.24; H, 3.54; N,
14.07. Found C, 42.05; H, 3.62; N, 14.23; 1H NMR (400 MHz, CDCl3):
N, 8.63; 1H NMR (400 MHz, CDCl3):
d 1.81 (s, 3H, eCH3), 2.00 (s, 3H,
eCH3), 3.62 (s, 3H, eCH3), 6.90 (s, 1H, pyrrole-H), 7.35 (s, 1H,
thiophene-H), 7.42e7.45 (m, 4H, phenyl-H); 13C NMR (400 MHz,
CDCl3):
d
11.4, 14.6, 33.0, 105.8, 109.8, 111.3, 112.7, 118.5, 118.8, 124.7,
, KBr, cmꢁ1): 744,
125.8, 126.7, 132.9, 135.8, 137.4, 140.2, 142.6; IR (
n
824, 897, 980, 1057, 1119, 1194, 1269, 1333, 1436, 1552, 1606, 2219.
2.2.9. 1-[2-Methyl-5-(4-fluorophenyl)-3-thienyl]-2-(2-cyano-1,5-
dimethyl-4-pyrryl)perfluorocyclopentene (2o)
Diarylethene 2o was prepared by an analogous method similar
to that used for diarylethene 1o using 7b instead of 7a, and was
obtained as a colorless solid 0.39 g in 34% yield. M.p. 161e163 ꢂC;
Anal. Calcd for C23H15F7N2S (%): C, 57.02; H, 3.12; N, 5.78. Found C,
d
2.30 (s, 3H, eCH3), 3.68 (s, 3H, eCH3), 6.74 (s, 1H, pyrrole-H); IR (n,
KBr, cmꢁ1): 717, 811, 1045, 1133, 1175, 1334, 1375, 1410, 1470, 1529,
2214, 2953, 3130.
57.07; H, 3.08; N, 5.86; 1H NMR (400 MHz, CDCl3):
d 1.71 (s, 3H,
eCH3), 1.88 (s, 3H, eCH3), 3.55 (s, 3H, eCH3), 6.86 (s, 1H, pyrrole-H),
7.02 (t, 2H, J ¼ 8.4 Hz, phenyl-H), 7.10 (s, 1H, thiophene-H),
2.2.7. 2-Cyano-1,5-dimethyl-4-pyrryl-perfluorocyclopentene (10)
To a stirred THF solution (80 mL) of compound 9 (4.02 g,
20.10 mmol) was added dropwise a 2.5 M n-BuLi/hexane solution
7.42e7.45 (m, 2H, phenyl-H). 13C NMR (400 MHz, CDCl3):
d 11.3,
14.5, 77.0, 33.1, 105.5, 109.9, 112.9, 115.9, 116.1, 118.8, 122.5, 126.0,

74
G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
127.3, 127.4, 129.5, 135.9, 140.3, 141.4, 161.3, 163.8; IR (n
, KBr, cmꢁ1):
741, 807, 828, 894, 987, 1056, 1095, 1183, 1231, 1275, 1335, 1397,
1469, 1513, 1548, 1623, 2221, 3123.
2.2.10. 1-[2-Methyl-5-phenyl-3-thienyl]-2-(2-cyano-1,5-dimethyl-
4-pyrryl)perfluorocyclopentene (3o)
Diarylethene 3o was prepared by an analogous method similar
to that used for diarylethene 1o using 7c instead of 7a, and was
obtained as a yellow solid 0.47 g in 36% yield. M.p. 140e142 ꢂC;
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):
d 1.77 (s, 3H,
eCH3), 1.96 (s, 3H, eCH3), 3.61 (s, 3H, eCH3), 6.93 (s, 1H, pyrrole-H),
7.23 (s, 1H, thiophene-H), 7.39 (t, 1H, J ¼ 8.0 Hz, benzene-H),
7.53e7.56 (m, 2H, benzene-H), 7.54 (d, 2H, J ¼ 7.6 Hz, benzene-H);
13C NMR (400 MHz, CDCl3):
d 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 (
n
, KBr, cmꢁ1): 756, 845, 987, 1097, 1183, 1277, 1334, 1385,
1548, 1624, 2221, 2372, 2920, 3125, 3450.
2.2.11. 1-[2-Methyl-5-(4-methylphenyl)-3-thienyl]-2-(2-cyano-1,5-
dimethyl-4-pyrryl)perfluorocyclopentene (4o)
Diarylethene 4o was prepared by an analogous method similar
to that used for diarylethene 1o using 7d instead of 7a, and was
obtained as a colorless solid 0.37 g in 40% yield. M.p. 135e137 ꢂC;
Anal. Calcd for C24H18F6N2S (%): C, 59.99; H, 3.78, N, 5.83. Found C,
59.94; H, 3.81; N, 5.72; 1H NMR (400 MHz, CDCl3):
d 1.68 (s, 3H,
eCH3), 1.87 (s, 3H, eCH3), 2.30 (s, 3H, eCH3), 3.54 (s, 3H, eCH3),
6.88 (s, 1H, pyrrole-H), 7.13 (d, 3H, J ¼ 7.6 Hz, phenyl-H), 7.36 (d, 2H,
J ¼ 8.0 Hz, phenyl-H); 19F NMR (400 MHz, CDCl3):
d
131.71 (2F),
110.36 (2F), 109.84 (2F). 13C NMR (400 MHz, CDCl3):
d 11.1,11.3, 14.5,
21.2, 33.1, 105.5, 110.0, 112.9, 118.8, 120.2, 122.0, 125.5, 125.9, 129.7,
130.5,136.0,138.0,139.8,142.6; IR (
n
, KBr, cmꢁ1): 740, 808, 844, 894,
982, 1055, 1111, 1192, 1262, 1332, 1436, 1551, 1642, 225, 2919, 3129.
2.2.12. 1-[2-Methyl-5-(4-methoxyphenyl)-3-thienyl]-2-(2-cyano-
1,5-dimethyl-4-pyrryl)perfluorocyclopentene (5o)
Fig. 3. Absorption spectral change of diarylethene 1 and color changes of diary-
lethenes 1e5 upon alternating irradiation with UV and visible light in hexane
(2.0 ꢀ 10ꢁ5 mol/L) at room temperature: (A) spectral changes for compound 1 and (B)
color changes for compounds 1e5.
Diarylethene 5o was prepared by an analogous method similar
to that used for diarylethene 1o using 7e instead of 7a, and was
obtained as a colorless solid 0.41 g in 38% yield. M.p. 144e145 ꢂC;
Anal. Calcd for C24H18F6N2OS (%): C, 58.06; H, 3.65; N, 5.64. Found C,
58.10; H, 3.59; N, 5.62; 1H NMR (400 MHz, CDCl3):
d
1.69 (s, 3H,
(
3
, 3.32 ꢀ 103 L/mol cm), indicating the formation of the closed-ring
eCH3), 1.87 (s, 3H, eCH3), 3.53 (s, 3H, eCH3), 3.77 (s, 3H, eOCH3),
6.84 (s, 1H, pyrrole-H), 6.86 (s, 2H, benzene-H), 7.04 (s, 1H, thio-
phene-H), 7.38 (d, 2H, J ¼ 8.4 Hz, benzene-H); 13C NMR (400 MHz,
isomer 1c. Alternatively, the blue-colored solution bleached
completely to colorless upon irradiation with visible light
(l > 500 nm), indicating that 1c returned to the initial state 1o. The
CDCl3):
d
11.2, 14.4, 30.8, 32.9, 55.4, 105.5, 110.1, 112.8, 114.5, 116.2,
colorationedecoloration cycle could be repeated more than 100
times and a clear isosbestic point was observed at 335 nm. Just like
diarylethene 1, upon irradiation with 297 nm light, the absorption
bands of compounds 2e5 in the visible region appeared and the
solutions turned blue as a result of the cyclization reaction leading
to the production of the closed-ring isomers 2ce5c (Fig. 3). Their
absorption maxima in the visible region were observed at 586 nm
for 2, 584 nm for 3, 592 nm for 4, and 592 nm for 5, respectively. All
the blue-colored solutions of 2ce5c could be decolorized upon
irradiation with visible light of wavelength greater than 500 nm
again, due to the conversion of the compounds to the open-ring
isomers 2oe5o. After repeating 100 colorationedecoloration
cycles, the isosbestic points of compounds 2e5 were observed at
308, 308, 313, and 320 nm, respectively. In addition, the photo-
conversion ratios of the five derivatives were analyzed by HPLC in
the photostationary state, with the value of 96% for 1, 89% for 2, 90%
for 3, 90% for 4, and 94% for 5, respectively (Table 1). In PMMA films,
diarylethenes 1e5 also showed similar photochromism to that in
hexane, and the changes in the absorption spectra of diarylethene 1
and the color changes of diarylethenes 1e5 are shown in Fig. 4. A
10 mg sample of each diarylethene 1e5 and 100 mg of poly(methyl
118.8, 121.5, 125.9, 126.2, 126.9, 135.9, 139.3, 142.5, 159.7; IR (n, KBr,
cmꢁ1): 743, 822, 845, 895, 985, 1031, 1056, 1108, 1183, 1256, 1335,
1399, 1442, 1466, 1514, 1553, 1611, 2219, 2840, 3128.
3. Results and discussion
3.1. Photochromism in solution and solid medium
The photochromic behavior of diarylethenes 1e5 induced by
photoirradiation at room temperature was measured in hexane
(2.0 ꢀ 10ꢁ5 mol/L) and in PMMA amorphous films (10%, w/w) as
well as the color of diarylethenes 1e5, induced by alternating
irradiation with UV and visible light of appropriate wavelength. The
changes in the absorption spectra of diarylethene 1 and the color
changes of diarylethenes 1e5 in hexane induced by alternating
irradiation with UV light and visible light at appropriate wave-
length are shown in Fig. 3. The absorption maximum of the color-
less compound 1o was observed at 312 nm (
Upon irradiation with 297 nm UV light, the colorless solution of 1o
3
, 8.82 ꢀ 103 L/mol cm).
turned blue with a new visible absorption band centered at 603 nm

G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
75
Table 1
Absorption spectral properties and photochromic reactivity of diarylethenes 1e5 in hexane (2.0 ꢀ 10ꢁ5 mol/L) and in PMMA films (10%, w/w).
Compound
lo,max/nma
(3
, L/mol cm)
lc,max/nmb
(3
, L/mol cm)
Fc
Conversion at PSS in hexane
Hexane
PMMA film
Hexane
PMMA film
Foec
Fceo
1
2
3
4
5
312 (8.82 ꢀ 103)
290 (2.11 ꢀ 104)
290 (2.49 ꢀ 104)
291 (2.34 ꢀ 104)
296 (2.83 ꢀ 104)
319
312
302
302
307
603 (3.32 ꢀ 103)
586 (7.19 ꢀ 103)
584 (1.00 ꢀ 104)
592 (6.02 ꢀ 103)
592 (1.15 ꢀ 104)
622
614
607
608
614
0.41
0.26
0.20
0.22
0.44
0.20
96%
89%
90%
90%
94%
0.098
0.054
0.090
0.10
a
Absorption maxima of open-ring forms.
Absorption maxima of closed-ring forms.
Quantum yields of cyclization reaction (Foec) and cycloreversion reaction (Fceo), respectively.
b
c
methacrylate) (PMMA) were dissolved in chloroform (1 mL) with
the aid of ultrasound, and the homogeneous solution was spin-
coated on a quartz substrate (10 mm ꢀ 10 mm ꢀ 1 mm). Upon
irradiation with 297 nm light, the colorless diarylethene 1o/PMMA
film turned blue, of which the absorption maxima was observed at
622 nm; while the colors of other four diarylethene/PMMA films
changed from colorless to blue with the appearance of a new broad
absorption band centered at 614, 607, 608 and 614 nm, respectively,
which was assigned to the formation of the closed-ring isomers
2ce5c. All colored diarylethene/PMMA films reverted to colorless
ring isomers for diarylethenes 1e5 in PMMA film were at longer
wavelengths than those in hexane. The red shifts of the absorption
maxima of the open-ring isomers were 7 nm for 1o, 22 nm for 2o,
12 nm for 3o, 11 nm for 4o, and 11 nm for 5o, respectively, and
those of the closed-ring isomers were 19 nm for 1c, 28 nm for 2c,
23 nm for 3c, 16 nm for 4c and 22 nm for 5c, respectively. The red
shift phenomena is ascribed to the polar effect of the polymer
matrix in the amorphous solid state [38,39].
The photochromic features of diarylethenes 1e5 in hexane and
in PMMA films are summarized in Table 1. The results showed that
different substituents at the para-positions of the benzene ring had
a significant effect on the photochromic features of diarylethenes
1e5 such as the absorption maxima, molar absorption coefficients,
and quantum yields. Among these five compounds, the absorption
maximum of the unsubstituted parent compound 3 was at the
shortest wavelength in hexane and in a PMMA film. Replacing the
hydrogen atom at the para-position of the terminal benzene ring
with either an electron-withdrawing substituent (cyano or fluorine
group, such as in compound 1 or 2) or with an electron-donating
group (methyl or methoxy group, such as in compound 4 or 5)
resulted in a red shift in the absorption maxima, the magnitude of
which increased with electron-withdrawing and electron-donating
ability. Thus, the stronger the electron-withdrawing/donating
ability of the substituent is, the longer the absorption maximum of
diarylethene is achieved in both open-ring and closed-ring isomers.
For instance, the absorption maximum of 1 was longer than that of
diarylethene 2 due to the stronger electron-withdrawing ability of
the cyano group as compared to a fluorine atom. The result is
consistent with that of diarylethene derivatives bearing a six-
membered aryl unit [18]. When substituting with the electron-
withdrawing group (cyano or fluorine group, such as in compound
1 or 2) in the same diarylethene system, the molar absorption
coefficients decreased notably, compared with the unsubstituted
parent compound 3. However, the strong electron-donating
methoxy group could effectively enhance the molar absorption
coefficient of diarylethene 5, both for its open-ring and closed-ring
isomers. The quantum yields of the unsubstituted parent
compound 3 were 0.20 for the cyclization reaction and 0.054 for
the cycloreversion reaction, were the smallest among the five
diarylethene derivatives. When replacing the hydrogen atom at
the para-position of the terminal benzene ring with an electron-
withdrawing substituent (cyano or fluorine group, such as
compound 1 or 2), the quantum yields increased to some extent
with the increase in the electron-withdrawing ability. Similarly,
when replacing the hydrogen atom with an electron-donating
substituent (methyl or methoxy group, such as compound 4 or 5),
the values also increased with the increase in the electron-donating
ability. Compared to that of the unsubstituted parent compound 3,
the cyclization quantum yields of diarylethenes 1 and 5, which had
a strong electron-withdrawing/donating substituent, were twofold
higher. Furthermore, for diarylethenes 1e5, all of the cyclization
quantum yields were higher than their respective cycloreversion
upon irradiation with visible light (l > 500 nm). As has been
observed for most of the reported diarylethenes [36,37], the
maximum absorption peaks of both the open-ring and the closed-
Fig. 4. Absorption spectral change of diarylethene 1 and color changes of diary-
lethenes 1e5 upon alternating irradiation with UV and visible light in PMMA films
(10%, w/w) at room temperature: (A) spectral changes for compound 1 and (B) color
changes for compounds 1e5.

76
G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
ꢀ
quantum yields (Table 1). The result is quite different from that of
diarylethenes bearing pyrazole moiety whose cyclization
the distances of the two reactive carbon atoms were less than 4.2 A,
a
which were close enough for the photocyclization reaction, indi-
cating that they could be expected to undergo photochromism in
the single crystalline phase. In fact, the crystals of 2o, 4o and 5o
underwent photochromic reaction, in accordance with the expec-
ted ring closure, to form 2c, 4c and 5c upon irradiation with UV
light. The color changes of crystals 2o, 4o and 5o upon photo-
irradiation in the crystalline phase are indicated in Fig. 6. Upon
irradiation with 297 nm light, the colorless crystals of 2o, 4o and 5o
turned blue quickly. When these colored crystals were dissolved in
hexane, the solution turned blue and the absorption maximum was
observed at the same wavelength as that of their respective closed-
ring isomer in solution. Alternatively, the blue-colored crystals
returned to colorless upon irradiation with the appropriate wave-
quantum yields were much smaller than their respective cyclo-
reversion quantum yields [27]. In addition, diarylethenes bearing
a pyrrole unit have a larger absorption maximum of the closed-ring
isomers compared with diarylethenes bearing thiophene, thiazole
or pyrazole unit [17,27,40].
3.2. Photochromism in the crystalline phase
Among diarylethenes 1oe5o, single crystals of diarylethenes 2o,
4o and 5o were obtained by slow evaporation of hexane solutions.
To gain a deeper understanding of the relationship between the
conformation and the photochromism of these diarylethene
derivatives in the crystalline phase, their structural confirmations
were provided by X-ray crystallographic analysis. The ORTEP
drawings of the single crystals of 2o, 4o and 5o are shown in Fig. 5,
and the X-ray crystallographic analysis data are listed in Table 2. For
compound 2o, there were two independent molecules in the
asymmetric unit and both of them adopted an approximate C2
symmetry with the photoactive anti-parallel conformation in the
crystalline phase, which could undergo photocyclization [7,41,42].
The two planar aryl moieties exhibited similar geometries in each
molecule, with dihedral angles between the cyclopentene ring and
the adjacent aryl rings of 33.8ꢂ for N1/C17eC20 and 52.8ꢂ for S1/
C1eC4 in one molecule and 34.5ꢂ for N3/C40eC43 and 50.5ꢂ for S4/
C24eC27 in the other. The distances between the potentially pho-
toactive C atoms (C4.C18 and C27.C41) in each molecule were
length visible light
(l > 500 nm). Furthermore, these crystals
exhibited remarkable fatigue resistance greater than 100 cycliza-
tion/cycloreversion repeated cycles and their closed-ring isomers
remained stable for more than 100 days in the dark at room
temperature. As such, they could be potentially used for the
construction of certain optoelectronic devices [6,45].
3.3. Fluorescence of diarylethenes
The fluorescent properties of diarylethenes 1oe5o in solution
(5.0 ꢀ 10ꢁ5 mol/L) and in PMMA films (10%, w/w) were measured
at room temperature using a Hitachi F-4500 spectrophotometer.
The fluorescence emission spectra of 1oe5o are illustrated in
Fig. 7. In hexane, the emission peaks of diarylethenes 1oe5o were
observed at 416, 415, 439, 430 and 426 nm when excited at
340 nm, and were observed at 416, 412, 423, 415, and 422 nm
when excited at 350 nm in PMMA films. When comparing samples
in hexane to those in PMMA films, the emission peaks of diary-
lethenes 2oe5o consistently exhibited a hypsochromic shift with
values of 3 nm for 2o, 16 nm for 3o, 15 nm for 4o, and 4 nm for
5o, respectively. The result is completely contrary to those
reported previously [16,34,36,46]. Among diarylethenes 1oe5o,
ꢀ
3.570 and 3.582 A, respectively, which were close enough for the
cyclization reaction [43,44]. The distances between the reacting
ꢀ
carbon atoms d (A) and dihedral angles
q
(ꢂ) of diarylethenes 4o and
5o were also measured as shown in Table 3. The distance between
the potentially photoactive carbon atoms (C5.C9) in crystal 4o
ꢀ
ꢀ
was 3.566 A, and that (C1.C13) in crystal 5o was 3.539 A. From the
data, it could be concluded that all molecules of crystals 2o, 4o and
5o were fixed in an anti-parallel mode in the crystalline phase and
Fig. 5. ORTEP drawings of crystals 2o, 4o and 5o, showing about 30% probability displacement ellipsoids: (A) 2o(I), (B) 2o(P), (C) 4o, and (D) 5o.

G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
77
Table 2
Crystal data for diarylethenes 2o, 4o and 5o.
Compound
2o
4o
5o
Formula
C23H15F7N2S
484.43
113(2)
Orthorhombic
Pca2(1)
16.626(3)
6.3555(13)
38.695(8)
90.00
90.00
90.00
4088.8(14)
8
C24H18F6N2S
480.46
294(2)
Triclinic
P-1
8.058(3)
8.805(3)
15.818(6)
90.194(7)
98.116(7)
96.102(7)
1104.6(7)
2
C24H18F6N2OS
496.46
291(2)
Monoclinic
P2(1)/c
23.120(3)
8.9474(11)
11.4416(13)
90.00
101.835(1)
90.00
2316.6(5)
4
Formula weight
Temperature
Crystal system
Space group
ꢀ
Unit cell dimensions a (A)
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
3
ꢀ
Volume (A )
Z
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Final R1[I > 2s(I)]
wR2[I > 2s(I)]
R1 (all data)
1.574
1.089
0.0674
0.1502
0.0828
0.1610
1.445
1.043
0.074
0.1996
0.1066
0.2212
1.509
1.038
0.0537
0.1364
0.0715
0.1524
wR2 (all data)
Fig. 6. Photographs of photochromic processes of diarylethenes 2, 4 and 5 in the
crystalline phase.
the emission peak of the unsubstituted parent diarylethene 3o
was at the longest wavelength and its emission intensity was the
strongest in hexane and in PMMA films. When the hydrogen atom
at the para-position of the terminal benzene ring was replaced
with either an electron-withdrawing or with an electron-donating
substituent, both the emission peak and the emission intensity
decreased notably with the increase in the electron-withdrawing
or electron-donating ability. The result was different from diary-
lethene derivatives bearing a thiazole moiety [40]. For diary-
lethenes bearing a thiazole moiety, the emission peak was shifted
to a longer wavelength when replacing the hydrogen atom with an
electron-donating substituent, while it was shifted to a shorter
wavelength when replacing with an electron-withdrawing
substituent. Moreover, when going from electron-withdrawing to
electron-donating groups, the emission peaks of these diary-
lethenes were shifted to a longer wavelength in hexane (from 416
to 430 nm), and their emission intensities were significantly
increased at a lower concentration (5.0 ꢀ 10ꢁ5 mol/L). Therefore,
the electron-donating group could be effective to increase the
emission peak and the emission intensity of diarylethenes bearing
a pyrrole moiety. The result is quite different from that of diary-
lethenes bearing a biphenyl moiety where the electron-donating
group could be effective to increase the emission intensity but
decrease the emission peak [8]. In addition, by using anthracene
(0.27 in acetonitrile) as the reference, the fluorescence quantum
yields of the open-ring isomers of 1oe5o were determined to be
0.047, 0.028, 0.010, 0.013, and 0.025, respectively. The result
indicated that the unsubstituted parent diarylethene 3 had the
smallest fluorescence quantum yield. Replacing the hydrogen
atom at the para-position of the terminal benzene ring with an
electron-withdrawing substituent (such as in compound 1 or 2) or
with an electron-donating substituent (such as in compound 4 or
A
6000
4800
3o
4o
5o
3600
2400
1200
0
1o
2o
350
400
450
500
550
600
Wavelength(nm)
B
6000
4800
3600
2400
1200
0
3o
4o
5o
1o
2o
Table 3
ꢀ
Distances between the reacting carbon atoms d (A) and dihedral angles
diarylethenes 2o, 4o and 5o.
q
(ꢂ) of
ꢀ
Compound
d (A)
q
(ꢂ)a
q1
q2
q3
2o(I)
C4.C18
C27.C41
C5.C9
3.570
3.582
3.566
3.539
33.8
34.5
37.2
38.5
52.8
50.5
53.2
45.3
29.1
28.6
26.4
23.7
400
450
500
550
600
2o(
4o
P)
Wavelength(nm)
5o
C1.C13
Fig. 7. Fluorescence emission spectra of diarylethenes 1e5 both in hexane solution
(5.0 ꢀ 10ꢁ5 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.
a
q1, Dihedral angle between the cyclopentene ring and the pyrrole ring; q2
,
dihedral angle between the cyclopentene ring and the thiophene ring; q3, dihedral
angle between the thiophene ring and the adjacent benzene ring.

78
G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
A
B
3500
2800
2100
1400
700
3500
Vis
Vis
2800
2100
1400
700
0
UV
UV
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
C
D
4500
6000
4500
3000
1500
0
Vis
UV
3600
2700
1800
900
0
Vis
UV
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
E
3500
Vis
UV
2800
2100
1400
700
0
350
400
450
500
550
600
Wavelength(nm)
Fig. 8. Emission intensity changes of diarylethene 1e5 upon irradiation with 297 nm UV light at room temperature in hexane (5.0 ꢀ 10ꢁ5 mol/L).
5) resulted in an increase in the fluorescence quantum yield with
the increase in either electron-withdrawing or electron-donating
ability. Furthermore, the fluorescence quantum yields of diary-
lethenes 1 and 2 bearing an electron-withdrawing group were
much higher than those of diarylethenes 4 and 5 bearing an
electron-donating group.
As has been observed for most of the reported diarylethenes
[33,47e51], diarylethenes 1e5 exhibited a very good fluorescent
switch upon changing from open-ring isomers to closed-ring
isomers by photoirradiation both in hexane and in PMMA films.
When irradiated by UV light, the photocyclization reaction
occurred and the emission intensity of diarylethenes 1oe5o
decreased significantly due to the production of the non-fluores-
cence closed-ring isomers 1ce5c. The back irradiation by visible
light with appropriate wavelength regenerated their open-ring
isomers and recovered the original emission intensity. Figs. 8 and 9
showed the fluorescence changes of diarylethenes 1e5 in hexane
(5.0 ꢀ 10ꢁ5 mol/L) and in PMMA films (10%, w/w), respectively.
Upon irradiation with 297 nm light, the emission intensity of dia-
rylethenes 1o was quenched to ca. 7% in hexane and 28% in a PMMA
film when arrived at the photostationary state. Therefore, the
fluorescent modulation efficiency of diarylethene 1 was 93% in
hexane and 72% in a PMMA film. Like diarylethene 1, the fluores-
cent modulation efficiencies of diarylethenes 2e5 in hexane were
90% for 2, 93% for 3, 91% for 4, and 96% for 5, respectively, and those
in PMMA films were 74% for 2, 83% for 3, 78% for 4, and 76% for 5,
respectively, at the photostationary state. The results showed that
the fluorescent modulation efficiencies of diarylethenes 1e5 in
hexane were much greater than those in PMMA films. The fluo-
rescent modulation efficiency of diarylethene 1 in hexane was
about 21% higher than that in PMMA films. Similarly, the fluores-
cent modulation efficiency was increased by 16% for 2, 10% for 3,
13% for 4, and 20% for 5 in hexane, respectively, compared with that
in PMMA films. The result was completely contrary to those of the
unsymmetrical diarylethene derivatives whose fluorescent modu-
lation efficiencies in hexane are much lower than those in PMMA
films [18,34,46]. However, our findings were in agreement with
that of diarylethene derivatives bearing a thiazole moiety [40,52].
Compared with diarylethenes bearing two thiophene moieties
[16,36,37], the fluorescent modulation efficiencies of diarylethenes

G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
79
A
B
1500
2500
2000
1500
1000
500
Vis
Vis
1200
900
600
300
0
UV
UV
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
C
D
4000
6000
4500
3000
1500
0
Vis
UV
Vis
UV
3200
2400
1600
800
0
350
400
450
500
550
600
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
E
3000
Vis
UV
2400
1800
1200
600
0
400
450
500
550
600
Wavelength(nm)
Fig. 9. Emission intensity changes of diarylethene 1e5 upon irradiation with 297 nm UV light at room temperature in PMMA films (10%, w/w), excited at 350 nm.
1e5 were significantly enhanced both in solution and in solid
states. This indicated that diarylethenes bearing a pyrrole unit
might have the potential use as fluorescent modulation switches
[53,54].
with the theory that longer conjugation length generally leads to
less positive potentials, with the addition of each heterocyclic ring
[9,61]. The difference of oxidation onset between the open-ring and
closed-ring isomers of diarylethenes 1e5 (DVoec) was 0.93 V for 1,
0.85 V for 2, 0.91 V for 3, 0.74 V for 4, and 0.52 V for 5, respectively.
This indicated that the difference of oxidation onsets of these dia-
rylethenes gradually decreased (from 0.93 to 0.52 V) when going
3.4. Electrochemical properties of diarylethenes 1e5
The electrochemical properties of diarylethenes can be poten-
tially applied to molecular scale electronic switches [55]. The
oxidative cyclization and cycloreversion and the reductive elec-
trochemical cyclization of some diarylethene derivatives have been
reported [56e60]. Herein, cyclic voltammograms (CV) were per-
formed on the diarylethenes 1e5 under identical experimental
conditions at a scanning rate of 50 mV/s. Fig. 10 shows the CV
curves of diarylethene 1e5 with the scanning rate of 50 mV/s. The
oxidation onsets of 1oe5o were observed at 1.71, 1.52, 1.63, 1.49,
and 1.31 V, and those of 1ce5c were observed at 0.79, 0.67, 0.72,
0.75, and 0.79 V, respectively. Therefore, the oxidation process of
the open-ring isomers 1oe5o occurred at higher potentials than
the corresponding closed-ring isomers 1ce5c. This is in accordance
from electron-withdrawing to electron-donating groups. The DVoec
of diarylethene 1 was the biggest and that of diarylethene 5 was the
smallest.
According to the same method reported [19,62], the HOMO and
LUMO energy level can also be estimated by using the energy level
of ferrocene as reference. Based on the HOMO and LUMO energy
level, the band gap (Eg, Eg ¼ LUMO ꢁ HOMO) of each compound
could be calculated approximately and the results were summa-
rized in Table 4. From this table, it could be easily seen that the
different substituents at the para-position of the benzene ring had
a significant effect on the electrochemical properties of diary-
lethenes 1e5. Among these diarylethene derivatives, the Eg of the
unsubstituted parent compound 3o was the biggest and that of 3c

80
G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
1o
2o
1c
2c
A
B
-1.5 -1.0 -0.5
0.0
0.0
0.0
0.5
1.0
1.5
2.0
-1.5 -1.0 -0.5 0.0
0.5
1.0
1.5
2.0
3o
4o
3c
4c
C
D
-1.5 -1.0 -0.5
0.5
1.0
1.5
2.0
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
2.0
5o
5c
E
-1.5
-1.0
-0.5
0.5
1.0
1.5
2.0
Potential/V
Fig. 10. Cyclic voltammetry (second scan) of diarylethene 1e5 in acetonitrile with the scanning rate of 50 mV/s: (A) 1, (B) 2, (C) 3, (D) 4, and (E) 5.
was the smallest, implying that the charge transfer must be faster
in 3c compared to that in others [60]. Introducing either an elec-
tron-withdrawing or an electron-donating group into the terminal
benzene ring of the same diarylethene skeleton resulted in
a decrease in the Egs of the open-ring isomers 1o, 2o, 4o, and 5o
whilst those of the closed-ring isomers 1c, 2c, 4c, and 5c increased
significantly, compared with the unsubstituted parent diary-
lethene 1. Furthermore, the Egs of diarylethenes 1 and 2 bearing an
electron-withdrawing group were much higher than those of
diarylethenes 4 and 5 bearing an electron-donating group. It
should be noted here that calculation of the absolute HOMO and
LUMO levels from the electrochemical data in combination with
the energy gap is still in debate [63]. As shown in Fig. 10, there
were great differences in the electronic current and polarization
curve shapes between the open-ring and closed-ring isomers of
diarylethenes 1e5 at the scanned voltage region. The results
indicated that the different substituents might have a notable
effect on the electrochemical characteristics of these diarylethenes
bearing a pyrrole moiety but further work is required to quantify
the effects.
4. Conclusions
In conclusion, five unsymmetrical diarylethenes bearing a pyrrole
moiety were synthesized and the substituent effects on their
photochemical and electrochemical features were discussed. All of
them underwent remarkable photochromism both in solution and in
PMMA films, and three compounds among them also underwent
notable photochromism in the crystalline phase. Compared with the
unsubstituted parent diarylethene 3, the absorption maxima, the
quantum yields of cyclization and cycloreversion of other diary-
lethenes bearing an electron-withdrawing/donating group at the
para-position of the terminal benzene ring were notably increased.
However, their fluorescence emission peaks, the emission intensi-
ties, and the DVoecs were much lower than those of diarylethene 3.
The Egs of the open-ring isomers of diarylethenes bearing an elec-
tron-withdrawing group were much higher than those of diary-
lethenes bearing an electron-donating group whereas that of the
closed-ring isomers showed an inverse trend for these diarylethene
derivatives. The properties induced by the introduction of the
pyrrole moiety, which were different from those of diarylethenes
bearing thiophene or pyrazole rings reported previously, may
shed some lights on the further application of photochromic
diarylethenes.
Table 4
Electrochemical properties of diarylethenes 1e5 in acetonitrile.
Compound
Oxidation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg
Acknowledgements
1o
1c
2o
2c
3o
3c
4o
4c
5o
5c
1.71
0.79
1.52
0.67
1.63
0.72
1.49
0.75
1.31
0.79
ꢁ6.51
ꢁ5.59
ꢁ6.32
ꢁ5.47
ꢁ6.43
ꢁ5.52
ꢁ6.29
ꢁ5.55
ꢁ6.11
ꢁ5.59
ꢁ0.84
ꢁ0.69
ꢁ1.04
ꢁ0.75
ꢁ0.97
ꢁ0.63
ꢁ0.90
ꢁ0.75
ꢁ0.89
ꢁ0.77
ꢁ3.96
ꢁ4.11
ꢁ3.76
ꢁ4.05
ꢁ3.83
ꢁ4.17
ꢁ3.90
ꢁ4.05
ꢁ3.91
ꢁ4.04
2.55
1.48
2.56
1.42
2.60
1.35
2.39
1.50
2.20
1.55
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), the Project of Jiangxi Academic and
Technological leader (2009DD00100), the Project of Jiangxi Youth
Scientist, National Natural Science Foundation of Jiangxi Province
(2008GZH0020, 2009GZH0034) and the Science Funds of the
Education Office of Jiangxi, China (GJJ09646, GJJ09567).

G. Liu et al. / Dyes and Pigments 90 (2011) 71e81
81
[33] Pu SZ, Liu G, Shen L, Xu JK. Efficient synthesis and properties of isomeric
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