The effect of the cyano group position upon unsymmetrical isomeric diarylethenes bearing a pyrrole unit

Article abstract of DOI:10.1016/j.dyepig.2010.12.007

Four photochromic diarylethenes bearing a pyrrole moiety were synthesized, and the structures of two of the new isomeric compounds were determined by single-crystal X-ray diffraction analysis. Each of the diarylethene derivatives exhibited good photochrom

Full text of DOI:10.1016/j.dyepig.2010.12.007

Dyes and Pigments 90 (2011) 89e99
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The effect of the cyano group position upon unsymmetrical isomeric
diarylethenes bearing a pyrrole unit
,
,
b
a
a
Gang Liu a b, Shouzhi Pu a , Xiaomei Wang , Weijun Liu , Congbin Fan
*
a Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China
b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four photochromic diarylethenes bearing a pyrrole moiety were synthesized, and the structures of two
of the new isomeric compounds were determined by single-crystal X-ray diffraction analysis. Each of the
diarylethene derivatives exhibited good photochromism both in solution and in poly(methyl methac-
rylate) films, which their photoconversion ratios from open-ring to closed-ring isomers were larger than
89% in hexane at photostationary state. They also functioned as effective fluorescent photoswitches both
in hexane and in poly(methyl methacrylate) films. The absorption maxima, cyclization/cycloreversion
quantum yields, and fluorescence quantum yields increased whereas the molar absorption coefficients
and the emission peaks decreased notably when the cyano group was attached at any of the three
positions on the terminal benzene ring. Cyclic voltammograms indicated that the introduction of cyano
group decreased the band-gap of the open-ring isomer and increased that of the closed-ring isomer.
The cyano group and its -substituted position can availably modulate the optical and electrochemical
behaviours of these diarylethene derivatives.
Received 7 November 2010
Received in revised form
15 December 2010
Accepted 16 December 2010
Available online 29 December 2010
Keywords:
Photochromism
Diarylethene
Pyrrole group
Substituent position effect
Optical and electrochemical property
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
strongly depend on the kind of the heteroaryl moieties and the
electron donor/acceptor substituents. The nature of the heteroaryl
Photochromic molecules attract much attention from funda-
mental as well as practical points of view because of their potential
for applications to optical devices, such as optical memories and
switches [1e3]. Among the photochromic compounds, diary-
lethenes are regarded as the best candidates for such devices, owing
to its excellent thermal stability, fatigue resistance, and easy moni-
toring using UVevis spectroscopy [4,5]. As components of molecular
electronics, it is desirable to develop various types of molecular
switch devices with diarylethene compounds. During the past
decades, there have been important achievements in the synthesis
of new families of organic photochromic molecules [3,6,7].
Generally, most of the diarylethene derivatives show very large
spectral shifts upon photoisomerization from the open-ring to
closed-ring isomers [1]. The open-ring isomer is colourless in most
cases, while the closed-ring isomer shows some colours such as
yellow, red, or blue. The open-ring and closed-ring isomers differ
from each other not only in their absorption but also in various
moieties greatly influences the photo-reactivity and the distin-
guishable features of diarylethenes. For instance, diarylethenes with
thiophene or benzothiophene moieties exhibit excellent thermal
stability [1,5,15], whereas symmetrical diarylethene bearing pyrrole
groups is thermally unstable and returns to the open-ring isomers
even in the dark [16]. Moreover, diarylethenes with benzofuran
moieties have outstanding fatigue resistance and small photo-
conversion ratio [17], whereas diarylethenes bearing both thiazole
and benzene moieties have relatively weak fatigue resistance [18].
For the diarylethene derivatives reported, most of the heteroaryl
moieties have been thiophene or benzothiophene rings, with just
a few reports on other heteroaryl moieties such as thiazole [19,20],
oxazole [21], indole [22,23], and so on. Pyrrole is an attractive
aryl unit due to its biological characteristics and has been applied in
biosensors widely [24,25]. However, diarylethene derivatives with a
pyrrole unit hitherto reported is very rare. Uchida et al. [16] 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 photochromic features, indicating
that the hybrid diarylethenes had good thermal stability and fatigue
resistance in the solid medium [26]. On the other hand, electron
physical and chemical properties such as
p-conjugation length [8],
refractive indices [9,10], geometrical structures [11,12], and chiral
properties [13,14]. The photochromic characteristics of diarylethenes
* Corresponding author. Tel./fax: þ86 791 3831996.
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.2010.12.007

90
G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
donor/acceptor substituents and their substitution positions have
also a significant effect on the photochromic properties of diary-
lethene derivatives [27,28], which can be used for the fine tuning
of the optoelectronic properties of diarylethene compounds. For
instance, 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 cyclo-
reversion quantum yield [29,30], however, those attached bis(2-
thienyl)ethene diarylethenes could increase the maxima absorption
of the open-ring isomers and reduce the cyclization quantum
yield [31]. In addition, Morimitsu et al. revealed that bulky alkoxy
substituents at 2- and 20-position of thiophene rings could extraor-
dinarily decrease the cycloreversion quantum yield and the thermal
stability of the colored closed-ring isomer at high temperature [32].
Although there are many reports concerning the substituent
effects on the properties of diarylethenes, studies on the substit-
uent position effect are extremely rare. Previously, we reported
the electron-withdrawing cyano group position effect not only on
the properties of unsymmetrical dithienylethenes with thiophene
moieties [33], but also on those of symmetrical dithienylethenes
[34]. Similarly, we also studied the electron-donating methoxy
substituent position effect on the properties of different diary-
lethene systems [35e39]. The results revealed that the substituent
position had a significant effect on the properties of these diary-
lethene compounds, and there existed a notable difference in
the substituent position effect of the same functional group for
different photochromic diarylethene systems. In this work, we
have synthesized four hybrid photochromic diarylethenes bearing
both pyrrole and thiophene moieties (1oe4o) in order to better
understand the cyano substituent position effect on the properties
of these compounds. The synthesized diarylethenes are 1-(2-
cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-phenyl-3-thienyl]per
fluorocyclopentene (1o), 1-(2-cyano-1,5-dimethyl-4-pyrryl)-2-[2-
methyl-5-(2-cyanophenyl)-3-thienyl]perfluorocyclopentene (2o),
1-(2-cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-(3-cyanophenyl)-
3-thienyl]perfluorocyclopentene (3o), and 1-(2-cyano-1,5-dimet
hyl-4-pyrryl)-2-[2-methyl-5-(4-cyanophenyl)-3-thienyl]perfluoro
cyclopentene (4o) of which, 2o, 3o and 4o are new compounds.
Although diarylethene 1o has been reported previously [37], it
is presented here for comparison with those of other three diary-
lethene compounds. All of these diarylethenes showed good
photochromism both in solution and in poly(methyl methacrylate)
(PMMA) amorphous films. The photochromic scheme of diary-
lethenes 1oe4o is shown in Fig. 1.
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.
Photoconversion ratios from open-ring to closed-ring isomers at
photostationary state were measured using an Agilent 1100 HPLC
chromatographic analyzer. Fluorescence spectra were measured using
a Hitachi F-4500 fluorimeter. Photo-irradiation was carried out using
a SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis 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
potentiostategalvanostat (EG&G Princeton Applied Research) under
computer control at room temperature. Platinum-electrodes (diam-
eter 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/Fcþ) standard with a formal potential
of E1/2 ¼ þ0.35 V versus platinum wire. The typical electrolyte
was acetonitrile (5 mL) containing 0.1 mol/L tetrabutylammonium
tetrafluoroborate ((TBA)BF4) and 1.0 ꢀ 10ꢁ3 mol/L dithienylethene
sample. All solutions were deaerated by bubbling with a dry argon
stream and maintained at a slight argon overpressure during elec-
trochemical experiments. Solvents used were spectroscopic grade and
were purified by distillation. The PMMA films were prepared by dis-
solving 10 mg sample of each diarylethene 1e4 and 100 mg of poly
(methyl methacrylate) into 1.0 mL chloroform with aid of ultrasound,
then spin coating the homogeneous solution on a quartz substrate
(20 mm ꢀ 20 mm ꢀ 1 mm) at 1500 rpm.
Suitable crystals of 2o and 3o were obtained by slow evaporation
of a hexane solution. All the measurements were made on a Bruker
SMART APEX II CCD diffractometer using a MULTI scan technique
using Mo Ka radiation. The structures were solved by direct methods
and refined by full-matrix least-squares procedures on F2 by full-
matrix least-squares techniques using SHELXTL-97 program. All
non-hydrogen atoms were refined anisotropically. Crystallographic
data for the structures of the two diarylethenes in this paper have
been deposited in the Cambridge Crystallographic Data Centre as
supplemental publication CCDC 764125 for 2o and CCDC 764126 for
3o. 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 1oe4o was shown in
Fig. 2. Suzuki coupling of the four bromobenzene derivatives with
a thiophene boronic acid [27] gave the alkylphenylthiophene
derivatives (5aed) [24,28]. 1-(2-cyano-1,5-dimethyl-4-pyrryl)per-
fluorocyclopentene (6) [17] was synthesized by bromination and
lithiation reactions from 5-cyano-1,2-dimethylpyrrole. Finally,
compounds 5aed were separately lithiated and then coupled with
compound 6 to give diarylethenes 1oe4o, respectively.
2. Experimental
2.1. General
NMR spectra were recorded on a Bruker AV400 (400 MHz) spec-
trometer with CDCl3 as the solvent and tetramethylsilane as an
F
F
F
F
F
F
F
F
2.2.1. 1-(2-Cyano-1,5-dimethyl-4-pyrryl)-2-[2-methyl-5-phenyl-3-
thienyl]perfluorocyclopentene (1o)
F
UV
Vis
F
F
F
To a stirred anhydrous THF (80 mL) of compound 5a (0.54 g,
2.12 mmol) [24] was added dropwise a 2.5 M n-BuLi/hexane solution
(0.95 mL, 2.38 mmol) at ꢁ78 ꢂC under argon atmosphere. After
30 min, 15 mL THF containing compound 6 (0.66 g, 2.30 mmol) [17]
was added and the reaction mixture was stirred for 2 h at this low
temperature and quenched by water. The product was extracted
with diethyl ether, and then dried with MgSO4, filtered, and evap-
orated in vacuo. The crude product was purified by column
chromatography using petroleum ether as the eluent to give 0.35 g
S
R
NC
N
S
R
NC
N
1c: R = H
1o: R = H
2c: R = ortho
CN
CN
2o: R = ortho
CN
CN
3c: R = meta
4c: R = para
3o: R = meta
4o: R = para
CN
CN
Fig. 1. Photochromism of diarylethenes 1e4.

G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
91
F
F
F
F
F
F
F
F
F
Br
F
F
F
+
F
n-BuLi
S
R
S
R
N
°C
NC
N
5a: R = H
1o: R = H
6
5b: R = ortho
5c: R = meta
CN
CN
CN
2o: R = ortho
CN
CN
3o: R = meta
4o: R = para
5d: R = para
CN
Fig. 2. Synthetic route for diarylethenes 1e4.
compound 1o as a colourless solid in 36% yield. M.p. 140.2e141.2 ꢂC;
142.0; IR (n
, KBr, cmꢁ1): 756, 845, 987, 1097, 1183, 1277, 1334, 1385,
Anal. Calcd for C23H16F6N2S (%): C, 59.22; H, 3.46; N, 6.01. Found C,
1548, 1624, 2221, 2372, 2920, 3125, 3450.
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);
2.2.2. Synthesis of diarylethenes 2oe4o
Diarylethenes 2oe4o were prepared by an analogous method
similar to that used for diarylethene 1o. Compound 2o: colorless
solid, yield: 52%; M.p. 137.3e138.5 ꢂC; Anal. Calcd for C24H15F6N3S
(%): C, 58.65; H, 3.08; N, 8.55. Found C, 58.71; H, 3.03; N, 8.49; 1H
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,
0.35
A
B
0.60
0.28
0.21
0.14
0.07
0.00
Vis
Vis
0.45
UV
UV
0.30
UV
Vis
UV
Vis
0.15
0.00
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength(nm)
C 0.16
D
0.20
0.15
0.10
0.05
0.00
Vis
Vis
UV
UV
0.12
0.08
UV
UV
Vis
Vis
0.04
0.00
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
E
Fig. 3. Absorption spectral and color changes of diarylethenes 1e4 by photoirradiation in hexane (2.0 ꢀ 10ꢁ5 mol/L) at room temperature: (A) spectral changes for 1, (B) spectral
changes for 2, (C) spectral changes for 3, (D) spectral changes for 4, (E) color changes for 1e4.

92
G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
Initial state
PSS state
1o
2o
1c
2c
0
2
4
6
8
10
0
2
4
6
8
10
12
14
4o
3o
Initial state
PSS state
4c
3c
0
2
4
6
8
10 12 14 16 18
0
2
4
6
8
10 12 14 16
Time (s)
Fig. 4. The photoconversion ratios of diarylethenes 1e4 at photostationary state in hexane by HPLC method.
NMR (400 MHz, CDCl3):
d
1.83 (s, 3H, eCH3), 2.10 (s, 3H, eCH3), 3.63
3. Results and discussion
(s, 3H, eCH3), 6.98 (s, 1H, pyrrole-H), 7.41 (s, 1H, thiophene-H), 7.47
(t,1H, J ¼ 7.6 Hz, phenyl-H), 7.57 (d, 1H, J ¼ 7.8 Hz, phenyl-H), 7.63 (t,
1H, J ¼ 7.2 Hz, phenyl-H), 7.76 (d, 1H, J ¼ 7.8 Hz, phenyl-H). 13C NMR
3.1. Photochromic behaviours of diarylethenes
(400 MHz, CDCl3):
d
12.9, 15.8, 34.5, 107.1, 110.9, 112.1, 114.3, 119.6,
The photochromic behaviour of diarylethenes 1e4 induced by
photoirradiation at room temperature was measured both in hexane
(2.0 ꢀ 10ꢁ5 mol/L) and in PMMA amorphous films (10%, w/w).
In hexane, the absorption spectral and color changes of diarylethene
1e4 induced by alternating irradiation with UV light and visible light
with appropriate wavelength are shown in Fig. 3. Diarylethene 1o
exhibited a sharp absorption peak at 290 nm in hexane, which was
120.1, 127.7, 129.3, 129.6, 131.1, 134.4, 135.5, 137.7, 138.1, 138.7, 144.2;
IR (n
, KBr, cmꢁ1): 545, 578, 632, 655, 685, 763, 823, 835, 851, 895, 983,
1056,1116,1187,1274,1332,1385,1438,1492,1553,1595,1633, 2222.
Compound 3o: colorless solid, yield: 44%; M.p.183.6e184.4 ꢂC; Anal.
Calcd for C24H15F6N3S (%): C, 58.65; H, 3.08; N, 8.55. Found C, 58.71;
H, 3.02; N, 8.62; 1H NMR (400 MHz, CDCl3):
d 1.80 (s, 3H, eCH3), 2.00
(s, 3H, eCH3), 3.64 (s, 3H, eCH3), 6.93 (s, 1H, pyrrole-H), 7.32 (s, 1H,
thiophene-H), 7.54 (t,1H, J ¼ 8.0 Hz, phenyl-H), 7.60 (d,1H, J ¼ 7.8 Hz,
phenyl-H), 7.78 (d, 1H, J ¼ 7.8 Hz, phenyl-H), 7.83 (s, 1H, phenyl-H).
arisen from p / p* transition [40]. 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 forma-
tion of the closed-ring isomer 1c. This could be seen with the naked
eye, as the colorless solution of 1o turned blue. Similarly, the blue
colored solution could be bleached to colorless upon irradiation with
13C NMR (400 MHz, CDCl3):
d 12.8,15.9, 34.4,107.2, 111.3,114.1,114.9,
119.6, 120.2, 125.5, 127.9, 130.4, 131.0, 131.3, 132.5, 135.9, 137.2, 141.2,
143.3; IR (
n
, KBr, cmꢁ1): 454, 545, 619, 633, 649, 679, 705, 740, 754,
793, 840, 850, 893, 981, 1057, 1111, 1137, 1194, 1274, 1336, 1384, 1397,
1431,1459,1490,1581,1599,1639, 2219. Compound 4o: yellow solid,
yield: 38%; M.p. 180.1e181.1 ꢂC; Anal. Calcd for C24H15F6N3S (%): C,
58.65; H, 3.08; N, 8.55. Found C, 58.69; H, 3.05; N, 8.48; 1H NMR
visible light (l >450 nm), indicating 1c was returning to the initial
state 1o. The colorationedecoloration cycle could be repeated more
than 50 times and a clear isosbestic point was observed at 308 nm.
Just like diarylethene 1, diarylethenes 2oe4o also showed excellent
photochromism in hexane. In the photostationary state, the iso-
sbestic points for diarylethenes 2, 3 and 4 were observed at 335, 321
and 334 nm, respectively. When arrived at the photostationary state,
the photoconversion ratios from open-ring to closed-ring isomers of
the four derivatives were analyzed under UV irradiation in hexane
solution by HPLC, and the results are shown in Fig. 4. From this
(400 MHz, CDCl3):
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):
11.4, 14.6, 33.0,
105.8, 109.8, 111.3, 112.7, 118.5, 118.8, 124.7, 125.8, 126.7, 132.9, 135.8,
137.4, 140.2, 142.6; IR (
, KBr, cmꢁ1): 503, 744, 824, 897, 980, 1057,
d 1.81 (s, 3H, eCH3), 2.00 (s, 3H, eCH3), 3.62 (s, 3H,
d
n
1119, 1194, 1269, 1333, 1436, 1552, 1606, 2219.
Table 1
Absorption spectral characteristics and photochromic reactivity of diarylethenes 1e4 in hexane at 2.0 ꢀ 10ꢁ5 mol/L, and in PMMA films (10%, w/w) at room temperature.
Compound
lo,max/nma
(
3
/L molꢁ1 cmꢁ1
)
lc,max/nmb
(3
/L molꢁ1 cmꢁ1
)
Fc
Conversion at
PSS in hexane
hexane
PMMA film
hexane
PMMA film
Foec
Fceo
1
2
3
4
290 (2.49 ꢀ 104)
292 (1.44 ꢀ 104)
295 (7.34 ꢀ 103)
312 (8.82 ꢀ 103)
302
306
312
319
584 (1.00 ꢀ 104)
590 (7.92 ꢀ 103)
592 (2.88 ꢀ 103)
603 (3.32 ꢀ 103)
607
611
611
622
0.20
0.27
0.36
0.41
0.054
0.078
0.15
90%
89%
91%
96%
0.20
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

G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
93
figure, it can be easily calculated the photoconversion ratios of dia-
rylethenes 1e4 in the photostationary state, which their values are
90% for 1, 89% for 2, 91% for 3, and 96% for 4 (Table 1).
the absorption maxima and the quantum yields of both the
open-ring isomers and the closed-ring isomers varied with the same
trend, i.e., they increased in the order of ortho- < meta- < para-
substituent by the cyano group. Therefore, both the absorption
maxima and the quantum yields of cyclization and cycloreversion of
the para-substituted derivative 4 are the biggest; while those of the
ortho-substituted derivative 2 are the smallest. The values of the
meta-substituted derivative 3 are in between those of the para and
ortho-substituted derivatives. However, the molar absorption coef-
ficients of diarylethenes 2e4 showed a different changing trend,
compared with that of their absorption maxima and quantum
yields. In hexane, they decreased in the order of meta- < para-
< ortho-substituent by the cyano group. As a result, the molar
absorption coefficients of both the open-ring and the closed-ring
isomers of the ortho-substituted derivative 2 are the biggest; while
The photochromic features of these compounds are summarized
in Table 1. The results showed that the cyano substituent position
had a significant effect on the photochromic properties of these
diarylethene derivatives, including the absorption maxima, molar
absorption coefficients, and quantum yields of cyclization and
cycloreversion. Among these diarylethene derivatives, the unsub-
stituted parent diarylethene 1 has the smallest absorption maxima
and quantum yields of cyclization and cycloreversion, but it has the
biggest molar absorption coefficients of both the ring-opened and
ring-closed isomers. When introduction of the electron-with-
drawing cyano group into any position of the terminal benzene ring,
the absorption maxima and the quantum yields of diarylethenes
2e4 increased to some extent, whereas their molar absorption
coefficients showed a reverse trend. For isomeric diarylethenes 2e4,
those of the meta-substituted derivative
3 are the smallest.
Compared to that of the diarylethene 2, the molar absorption
A
B
1.6
Vis
0.8
Vis
UV
UV
1.2
0.6
0.8
UV
Vis
UV
0.4
Vis
0.4
0.2
0.0
0.0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength(nm)
1.2
C
D
Vis
0.60
0.45
0.30
0.15
0.00
Vis
UV
UV
0.9
0.6
0.3
0.0
UV
Vis
UV
Vis
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
E
Fig. 5. Absorption spectral and color changes of diarylethenes 1e4 by photoirradiation in PMMA films (10%, w/w) at room temperature: (A) spectral changes for 1, (B) spectral
changes for 2, (C) spectral changes for 3, (D) spectral changes for 4, (E) color changes for 1e4.

94
G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
coefficients of diarylethenes 3 and 4 were almost twofold lower.
These varying patterns are quite different from those of isomeric
diarylethenes bearing different substituents reported [33e39].
Furthermore, the pyrrole moiety has also a significant effect on the
photochromic properties of diarylethenes 1e4. When replacing
the pyrrole moiety with the thiophene moiety in the same molec-
ular skeleton of photochromic diarylethenes, the molar absorption
coefficients and the cyclization quantum yields increased remark-
ably, and the absorption maxima of their closed-ring isomers
showed so big hypsochromic shift that they exhibited the purple
colours in hexane [33].
The thermal stabilities of the open-ring and closed-ring isomers
of unsymmetrical diarylethenes 1e4 were tested in hexane both at
room temperature and at 80 ꢂC. Storing these solutions in hexane in
the dark and then exposing them to air both at room temperature
and at 80 ꢂC for more than 72 h, we found that no changes in
the UV/vis spectra of diarylethenes 1e4 were observed. That is to
say, no decomposition was detected, which indicates that unsym-
metrical diarylethenes bearing a pyrrole unit had better thermal
stability than symmetrical ones bearing pyrrole units [16].
For practical applications in optical devices, it is very important
that photochromic materials can keep good photochromism in
a polymer film such as poly(methyl methacrylate) (PMMA) [41,42].
In PMMA amorphous films, diarylethenes 1e4 also showed similar
photochromism to that in hexane solution, and their spectral and
color changes are shown in Fig. 5. As has been observed for most of
the reported diarylethenes [33,36,43,44], the maximum absorption
peaks of both the open-ring and the closed-ring isomers in PMMA
films are longer than those in hexane. The red shift values of the
absorption maxima of the open-ring isomers are 12 nm for 1o,
14 nm for 2o,17 nm for 3o, and 7 nm for 4o, and those of the closed-
ring isomers are 23 nm for 1c, 21 nm for 2c, 19 nm for 3c, and 19 nm
for 4c, respectively. The red shift phenomena may be attributed to
the polar effect of the polymer matrix in solid state [45,46].
Single crystals of diarylethenes 2o and 3o were obtained by slow
evaporation of hexane solutions. To gain a deeper understanding of
the relation between the conformation and the photochromic
behaviors of the two diarylethene derivatives in the crystalline
phase, their final structural confirmations were provided by X-ray
crystallographic analysis. The X-ray crystallographic analysis data
Table 2
Crystal data and structure refinements for diarylethenes 2o and 3o.
Compound 2o
Compound 3o
Fig. 6. ORTEP drawings of crystals 2o and 3o and their color changes by photo-
irradiation in the single crystalline phase: (A) ORTEP drawing of 2o, (B) ORTEP drawing
of 3o, (C) color changes for crystals 2o and 3o.
Formula
C24 H15 F6N3
491.45
S
C24 H15 F6N3
491.45
113 (2)
Triclinic
P ꢁ 1
S
Formula weight
Temperature
Crystal system
Space group
Unit cell dimensions
a (Å)
113 (2)
Triclinic
P ꢁ 1
are listed in Table 2. Their ORTEP drawings and photochromism
induced by alternating irradiation with UV light and visible light in
the crystalline phase are shown in Fig. 6 The molecules of both
diarylethenes 2o and 3o crystallize with an appropriate C2 symmetry
1090.1 (3)
9.0882 (13)
9.1285 (12)
15.097 (3)
76.237 (7)
85.251 (8)
63.686 (7)
1090.1 (3)
2
1093 (5)
8.999 (17)
11.24 (4)
12.62 (3)
113.08 (6)
109.49 (5)
90.20 (5)
1093 (5)
2
10272
5116
312
0.217
b (Å)
c (Å)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
Table 3
Volume (Å3)
Distances between the reacting carbon atoms d (Å) and dihedral angles
diarylethenes 2o and 3o.
q
(ꢂ) of
z
Reflections collected
Reflections observed
Number of parameters
10129
5087
311
0.217
Compound
d (Å)
q
(ꢂ)a
m
(mmꢁ1
)
q1
q2
q3
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Final R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 (all data)
1.497
1.016
0.0358
0.0960
0.0507
0.1014
1.493
1.060
0.0419
0.1136
0.0612
0.1249
2o
3o
C4.C19
C4.C19
3.520 (5)
3.629 (4)
36.7 (3)
41.9 (6)
56.6 (3)
60.3 (6)
38.2 (3)
22.1 (6)
a
q1, Dihedral angle between the cyclopentene ring and cyclopentene ring linked
up with 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.
wR2

G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
95
with the photoactive anti-parallel conformation in the crystalline
A
phase, which can undergo photocyclization [15,47]. For diarylethene
2o, the distances clearly show that the C13eC17 bond (1.351 Å) in
hexafluorocyclopentene ring is a double bond, being significantly
shorter than other carbon-carbon single bonds (1.499 Å to 1.550 Å) of
the ring. The thiophene and pyrrole moieties are linked by the C13]
C17 double bond. The two methyl groups are located on opposite
sides of the double bond, reflected in the torsion angles
C13eC17eC18eC19 [ꢁ38.8(6)ꢂ] and C17eC13eC3eC4 [ꢁ52.5(6)ꢂ]
and are thus trans with respect to the double bond. Such a confor-
mation is crucial for the compound to exhibit photochromic and
photo-induced properties [48]. The dihedral angles between the
hexafluorocyclopentene ring and the two heteroaryl rings are 36.7
(3)ꢂ for N2/C18eC21 and 56.6(3)ꢂ for S1/C1eC4, and that between
the thiophene ring and the linked benzene ring is 38.2(3)ꢂ.
The corresponding data of compounds 3o are summarized inTable 3.
The distance between the two reactive C atoms is 3.520(5) Å for 2o
and 3.629(4) for 3o, which is close enough for the photocyclization
to take place [15,48,49]. In fact, the crystals 2o and 3o showed good
photochromism in accordance with the expected analysis (Fig. 6).
Upon irradiation with 297 nm light, the colorless crystal of 2o and 3o
turned blue quickly, as a result of the formation of the closed-ring
isomers 2c and 3c. Alternatively, the blue colored crystals 2c and 3c
returned to colorless and reproduce 2o and 3o upon irradiation with
6000
4800
3600
2400
1200
0
1o
2o
4o
3o
350
400
450
500
550
600
Wavelength(nm)
B
6000
4800
3600
2400
1200
0
1o
visible light (l > 500 nm). After 500 repeat cycles, the two isomeric
2o
4o
3o
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 [50,51].
3.2. Fluorescence of diarylethenes
Fluorescent properties can be useful in molecular-scale opto-
electronics and digital photoswitching of fluorescence [52e54]. The
fluorescence modulation is a particularly intriguing approach due to
the stabilization of diarylethene and versatility in materials selection
[55]. Until now, the fluorescent properties of many diarylethene
derivatives have been extensively explored [33e40,56e60]. In
this work, the fluorescence properties of the four diarylethenes in
hexane solution and in PMMA films were measured using a Hitachi
F-4500 fluorimeter at room temperature. The fluorescence emission
spectra of diarylethenes 1oe4o in hexane (5.0 ꢀ 10ꢁ5 mol/L) and in
PMMA films (10%, w/w) at room temperature are illustrated in Fig. 7.
In hexane, the emission peaks of diarylethenes 1oe4o were
observed at 439, 418, 419 and 415 nm when exited at 340 nm, and
were observed at 424, 417, 414 and 414 nm in PMMA films when
excited at 350 nm. Compared to those in hexane, the emission peaks
of diarylethenes 1oe4o in PMMA films consistently exhibit a hyp-
sochromic shift with values of 15 nm for 1o,1 nm for 2o, 5 nm for 3o,
and 1 nm for 4o. The result is contrary to those reported previously
[36,38,43]. Among isomeric diarylethenes 2oe4o, the emission
intensity of the ortho-substituted derivative 2o is the strongest and
that of the meta-substituted derivative 3o is the smallest both in
hexane and in PMMA film. The result is quite different from those
of the similar unsymmetrical diarylethenes with an electron-
donating methoxy group whose the emission intensity of the
ortho-substituted derivative is the strongest and that of the para-
substituted derivative is the smallest both in hexane and in PMMA
film [37]. Compared to isomeric diarylethenes 2oe4o, the unsub-
stituted 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-withdrawing
cyano group could be effective to reduce the emission peak and
weaken the fluorescent intensity of diarylethenes bearing a pyrrole
unit. This is in good agreement with those of the unsymmetrical
400
440
480
520
560
Wavelength(nm)
Fig. 7. Fluorescence emission spectra of diarylethenes 1e4 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; (B) emission spectra in PMMA films, excited at
350 nm.
diarylethenes with the same skeleton and with an electron-donating
methoxy group in the terminal benzene ring [37]. In addition,
by using anthracene (0.27 in acetonitrile) as the reference, the
fluorescence quantum yields of the open isomers of 1o, 2o, 3o, and
4o were determined to be 0.010, 0.051, 0.027, and 0.047, respectively.
From the data, it can be easily concluded that the unsubstituted
parent diarylethene 1 has the smallest fluorescence quantum yield.
Therefore, the introduction of electron-withdrawing cyano group
into any position of the terminal benzene ring can significantly
improve the fluorescence quantum yield of diarylethenes bearing
a pyrrole moiety.
Diarylethenes 1e4 exhibited an excellent fluorescent switch on
changing from the open-ring isomers to closed-ring isomers by
photoirradiation both in hexane and in PMMA film. When irradiated
by UV light, the photocyclization reaction was occurred and the
emission intensity of diarylethenes 1oe4o decreased significantly
due to producing the non-fluorescence closed-ring isomers 1ce4c.
The back irradiation by appropriate wavelength visible light regen-
erated their open-ring isomers 1oe4o and recovered the original
emission intensity. The fluorescence changes of diarylethenes 1e4 by
photoirradiation in hexane (5.0 ꢀ 10ꢁ5 mol/L) and in PMMA films
(10%, w/w) are showed in Figs. 8 and 9. Upon irradiation with UV
light, the emission intensity of 1o was quenched to ca. 7% in hexane
and 17% in a PMMA film when arrived at the photostationary state.
That is to say, its fluorescent modulation efficiencyat photostationary

96
G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
A
B
6000
6000
4500
3000
1500
0
Vis
4800
UV
Vis
UV
3600
2400
1200
0
350
400
450
500
550
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
C
D
3500
1200
900
600
300
0
Vis
UV
Vis
UV
2800
2100
1400
700
0
350
400
450
500
550
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
Fig. 8. Emission intensity changes of diarylethenes 1e4 upon irradiation with 297 nm UV light at room temperature in hexane (5.0 ꢀ 10ꢁ5 mol/L), excited at 340 nm: (A) 1, (B) 2, (C)
3, and (D) 4.
A
B
5000
4000
3000
2000
1000
0
6000
4500
3000
1500
0
Vis
UV
Vis
UV
400
440
480
520
560
360
400
440
480
520
560
Wavelength(nm)
Wavelength(nm)
C
D
2500
2000
1500
1000
500
2000
1500
1000
500
0
Vis
UV
Vis
UV
0
400
450
500
550
400
450
500
550
Wavelength(nm)
Wavelength(nm)
Fig. 9. Emission intensity changes of diarylethenes 1e4 upon irradiation with 297 nm UV light at room temperature in PMMA films (10%, w/w), excited at 350 nm: (A) 1, (B) 2, (C) 3,
and (D) 4.

G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
97
1o
2o
2c
1c
-1.5 -1.0 -0.5 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
4o
3o
3c
4c
-1.5 -1.0 -0.5 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
Potential/V
Fig. 10. Cyclic voltammetry (second scan) of diarylethenes 1e4 in acetonitrile with the scanning rate of 50 mV sꢁ1
.
state was 93% in hexane and 83% in a PMMA film. As with diary-
lethene 1, the fluorescent modulation efficiencies of other three
isomeric diarylethene derivatives at the photostationary state in
hexane were 94% for 2, 74% for 3, and 93% for 4, and those in PMMA
films were 87% for 2, 71% for 3, and 72% for 4. Therefore, the fluo-
rescent modulation efficiencies of diarylethenes 1e4 in hexane were
much higher than those in PMMA films. For example, the fluorescent
modulation efficiency of diarylethene 1 in PMMA film was reduced
about 10% more than that in hexane, and those of other three
compounds decreased partly in PMMA films. The result is in good
agreement with those of the unsymmetrical diarylethenes with the
same skeleton and with an electron-donating methoxy group in
the terminal benzene ring [37], but it’s completely contrary to those
of the unsymmetrical dithienylethene derivatives whose fluorescent
modulation efficiencies in hexane are much lower than those in
PMMA film [36,38]. Compared with diarylethenes bearing two
thiophene moieties [36,38,43,59], the fluorescent modulation
efficiencies of diarylethenes 1e4 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 [4,60].
voltammograms (CV) were performed on the diarylethenes 1e4
under identical experimental conditions at a scanning rate of
50 mV sꢁ1. The CV curves of diarylethenes 1e4 are shown in Fig. 10.
The oxidation onsets of 1oe4o were observed at þ1.63, 1.81, 1.69,
and 1.71 V, and those of 1ce4c were observed at þ0.72, 0.89, 0.79,
and 0.79 V, respectively. Therefore, the difference of oxidation onset
between the open-ring and closed-ring isomers of diarylethenes
1e4 (DVoec) was 0.91 V for 1, 0.92 V for 2, 0.90 V for 3, and 0.92 V for
4. The result indicated that the oxidation process of the open-ring
isomers 1oe4o occurs at higher potentials than that of the
corresponding closed-ring isomers 1ce4c. This is because the longer
conjugation length of the closed-ring isomers generally leads to
a less positive potential [66,67]. After the cyclization reaction, the
p-conjugation of the closed-ring isomers extends across the per-
fluorocyclopentene ring causing a lower oxidation onset. Compared
to isomeric diarylethenes 2oe4o, the unsubstituted parent
diarylethene 1o showed the smallest oxidation onsets both for the
open-ring and closed-ring isomers. The oxidation onsets of both the
open-ring and closed-ring isomers of diarylethenes 2e4 notably
increased when the cyano group was attached at any of the three
positions on the terminal benzene ring.
According to the same method reported [65,68,69], the Highest
Occupied Molecular Orbitals (HOMO) and the Lowest Unoccupied
Molecular Orbitals (LUMO) energy levels can be estimated 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 4. These data showed that the band-gaps of the open-ring
isomers 1oe4o are higher than those of their corresponding
closed-ring isomers 1ce4c. Among these compounds, the Eg of 1o is
the highest (Eg ¼ 2.60 eV) and that of 1c is the lowest (Eg ¼ 1.35 eV),
implying that the charge transfer must be faster in 1c compared to
that in others [70]. When introducing the cyano group into the
terminal benzene ring of the same diarylethene skeleton, the band-
gaps of the open-ring isomers 2oe4o decreased whilst those of the
closed-ring isomers 2ce4c increased significantly, compared with
the unsubstituted parent diarylethene 1. When substituting with
the methoxy group on the same diarylethene system, the Egs of the
open-ring isomers decreased and those of the closed-ring isomers
increased clearly, and other electronic parameters were also very
3.3. Electrochemistry of diarylethenes
Recently, the electrochemical behaviours of diarylethene deriv-
atives have attracted much attention because of their potential
applications in molecular-scale electronic switches [61e66]. Cyclic
Table 4
Electrochemical properties of diarylethenes 1e4 in acetonitrile.
Compound
Oxidation
Reduction
Band gap
Eonset(V)
IP(eV)
Eonset(V)
EA(eV)
Eg
1o
1c
2o
2c
3o
3c
4o
4c
1.63
0.72
1.81
0.89
1.69
0.79
1.71
0.79
ꢁ6.43
ꢁ5.52
ꢁ6.61
ꢁ5.69
ꢁ6.49
ꢁ5.59
ꢁ6.51
ꢁ5.59
ꢁ0.97
ꢁ0.63
ꢁ0.77
ꢁ0.64
ꢁ0.79
ꢁ0.65
ꢁ0.84
ꢁ0.69
ꢁ3.83
ꢁ4.17
ꢁ4.03
ꢁ4.16
ꢁ4.01
ꢁ4.15
ꢁ3.96
ꢁ4.11
2.60
1.35
2.58
1.53
2.48
1.44
2.55
1.48

98
G. Liu et al. / Dyes and Pigments 90 (2011) 89e99
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In conclusion, four unsymmetrical diarylethenes based on the
hybrid skeleton of thiophene and pyrrole moieties were synthe-
sized in order to investigate the effects of the substituents on
the properties of these compounds. It has been demonstrated that
electron-withdrawing cyano substituent and its -substituted
position have a significant effect on the photochemical and elec-
trochemical properties of these diarylethene derivatives. Compared
with the unsubstituted parent diarylethene 1, diarylethenes 2e4
bearing a cyano group at the ortho-, meta- and para-position of the
terminal benzene ring have larger absorption peaks, quantum
yields of cyclization and cycloreversion, fluorescence quantum
yields, and the oxidation onsets, however, they have smaller molar
absorption coefficients and emission peaks. In addition, the intro-
duction of the pyrrole moiety increased the absorption maxima
and the cycloreversion quantum yields whereas decreased the
molar absorption coefficients and the cyclization quantum yields
significantly, compared with the analogous dithienylethene deriv-
atives. The results may shed some lights on the further application
of photochromic diarylethenes bearing a pyrrole moiety.
[24] Abu-Rabeah K, Polyak B, Ionescu RE, Cosnier S, Marks RS. Synthesis and
characterization of
a pyrrole-alginate conjugate and its application in
a biosensor construction. Biomacromolecules 2005;6:3313e8.
[25] Bando T, Sugiyama H. Synthesis and biological properties of sequence-specific
DNA-alkylating pyrrole-imidazole polyamides. Accounts of Chemical Research
2006;39:935e44.
[26] Pu SZ, Liu G, Shen L, Xu JK. Efficient synthesis and properties of isomeric photo-
chromic diarylethenes having a pyrrole unit. Organic Letters 2007;9:2139e42.
[27] Yumoto K, Irie M, Matsuda K. Control of the photoreactivity of diarylethene
derivatives by quaternarization of the pyridylethynyl group. Organic Letters
2008;10:2051e4.
Acknowledgements
[28] Pu SZ, Yang TS, Li GZ, Xu JK, Chen B. Substituent position effect on the
optoelectronic properties of photochromic diarylethenes. Tetrahedron Letters
2006;47:3167e71.
[29] Irie M, Sakemura K, Okinaka M, Uchida K. Photochromism of dithienylethenes
with electron-donating substituents. The Journal of Organic Chemistry
1995;60:8305e9.
[30] Pu SZ, Zheng CH, Le ZG, Liu G, Fan CB. Substituent effects on the properties of
photochromic diarylethenes. Tetrahedron 2008;64:2576e85.
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the photochromic reactivity of bis(2-thienyl)perfluorocyclopentenes. Tetra-
hedron 2001;57:4559e65.
This work was supported by Program for the NSFC of China
(20962008), New Century Excellent Talents in University (NCET-08-
0702), the Key Scientific Project from Education Ministry of China
(208069), the Project of Jiangxi Academic and Technological leader
(2009DD00100), National Natural Science Foundation of Jiangxi
Province (2008GZH0020, 2009GZH0034) and the Science Funds of
the Education Office of Jiangxi, China (GJJ09646, GJJ09567).
[32] Morimitsu K, Shibata K, Kobatake S, Irie M. Dithienylethenes with a novel
photochromic performance. The Journal of Organic Chemistry 2002;67:4574e8.
[33] Pu SZ, Liu WJ, Liu G. The photochromism of unsymmetrical diarylethene
isomers with an electron-withdrawing cyano substituent. Dyes and Pigments
2010;87:1e9.
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