The photochromism of unsymmetrical diarylethene isomers with an electron-withdrawing cyano substituent

Article abstract of DOI:10.1016/j.dyepig.2010.01.015

Three unsymmetrical isomeric diarylethenes bearing an electron-withdrawing cyano group were synthesized and their structures determined using single-crystal X-ray diffraction analysis. Each of the compounds displayed excellent photochromism in solution, in PMMA film, as well as in the crystalline phase. The isomeric compounds also functioned as a fluorescence switch in PMMA films. The cyclization quantum yield and the absorption maxima of both the ring-opened and ring-closed isomers increased in the order: ortho- < meta- < para-substitution of the cyano group. Compared with the unsubstituted parent compound, the cycloreversion quantum yield decreased whereas the molar absorption coefficient of the ring-opened and the ring-closed isomers increased significantly when the cyano group was attached at any of the three positions on the terminal benzene ring. Cyclic voltammetry studies indicated that the position of cyano substitution had a marked effect on the electrochemical behaviours of these isomeric diarylethenes.

Full text of DOI:10.1016/j.dyepig.2010.01.015

Dyes and Pigments 87 (2010) 1e9
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Dyes and Pigments
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The photochromism of unsymmetrical diarylethene isomers
with an electron-withdrawing cyano substituent
*
Shouzhi Pu , Weijun Liu, Gang Liu
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:
Three unsymmetrical isomeric diarylethenes bearing an electron-withdrawing cyano group were
synthesized and their structures determined using single-crystal X-ray diffraction analysis. Each of the
compounds displayed excellent photochromism in solution, in PMMA film, as well as in the crystalline
phase. The isomeric compounds also functioned as a fluorescence switch in PMMA films. The cyclization
quantum yield and the absorption maxima of both the ring-opened and ring-closed isomers increased in
the order: ortho- < meta- < para-substitution of the cyano group. Compared with the unsubstituted
parent compound, the cycloreversion quantum yield decreased whereas the molar absorption coefficient
of the ring-opened and the ring-closed isomers increased significantly when the cyano group was
attached at any of the three positions on the terminal benzene ring. Cyclic voltammetry studies indicated
that the position of cyano substitution had a marked effect on the electrochemical behaviours of these
isomeric diarylethenes.
Received 2 November 2009
Received in revised form
21 January 2010
Accepted 23 January 2010
Available online 4 February 2010
Keywords:
Photochromism
Diarylethene
Crystal
Cyano substituent position effect
Photochemistry
Electrochemistry
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
introduction of electron-donating groups to bis(3-thienyl)ethene
diarylethenes increases the absorption coefficient of the ring-closed
The exploration of photochromic diarylethenes with good
optical and electromagnetic properties for molecule-scale infor-
mation processing has become an active research area of organic
chemistry [1], mainly due to their potential applications in photonic
devices such as erasable memory media [2], optical switching
[3e5], and full color displays [6]. Among the diarylethene deriva-
tives hitherto reported, diarylethenes with heterocyclic aryl groups,
especially those bearing two thiophene or benzothiophene rings,
are the most promising candidates for these applications because of
their excellent thermally irreversible photochromic property,
remarkable fatigue-resistance, and high sensitivity [2,3,7e10].
Diarylethenes bearing phenyl groups have been of particular
interest in the last few years because the phenyl group can increase
the absorption coefficient of the photochromic molecule and the
conversion at the photostationary state [11]. Moreover, the opto-
electronic properties of the phenyl-substituted diarylethene deriv-
atives can be further modified by suitable functionalization of the
aromatic rings with electron-donating or electron-withdrawing
groups [12e15]. To date, many reports concerning the substituent
effect on the properties of diarylethene derivatives have been
reported. For instance, some reports demonstrated that the
isomers and decreases the cycloreversion quantum yield. Therefore,
substitution with various functional groups could be an effective
way to increase the maxima absorption of the ring-opened isomers
and to reduce the cyclization quantum yield [16e18]. Moreover,
Irie et al. demonstrated that the cycloreversion quantum yield
was strongly suppressed by the introduction of electron-donating
methoxy groups at the reactive 2- and 20-positions of the thiophene
rings [19,20]. Yamaguchi et al. investigated the substituent effect on
the photochromic properties of diarylethenes bearing two benzo-
furan aryl groups [21], and Tanifuji et al. reported the effect of the
radical substituents on the photochromic reactivity of bis(3-ben-
zothiophene)perfluorocyclopentene [22]. In previous work, we also
illustrated that different substituents had a significant effect on the
properties of diarylethene derivatives, including their photochro-
mism, fluorescence and electrochemical properties [23e26].
Apart from the fact that various functional groups have signifi-
cant effects on the photochromic properties of diarylethenes, the
position of the substituent can affect photochromic properties
significantly, which can be used for the fine tuning of the opto-
electronic properties of diarylethene compounds. Previously, the
current authors focused on the effects of electron-donating
methoxy groups or halide substituents [27e34]. This paper
concerns the effect of the position of the cyano substitution on
diarylethenes. The cyano group is a strongly electron-withdrawing
substituent and has a distinguishable effect on the photochromic
* 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.2010.01.015

2
S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
features of diarylethenes comparing with the effect of electron-
donating group [16]. We synthesized three unsymmetrical isomeric
diarylethenes bearing a cyano group in order to elucidate these
effects. The three isomeric compounds (1, 2 and 3) showed favor-
able photochromic properties in solution, in PMMA film as well as
in the single crystalline phase.
5a was purified by column chromatography on SiO2 using hexane
as the eluent and 3.04 g obtained as baby yellow solid in 60% yield.
1H NMR (400 MHz, CDCl3)
d 2.42 (s, 3H, eCH3), 7.14 (d, 2H,
J ¼ 8.8 Hz, phenyl-H), 7.24 (s, 1H, thienyl-H), 7.27 (s, 1H, phenyl-H),
7.53 (d, 1H, J ¼ 8.0 Hz, phenyl-H).
2.2.2. Synthesis of 3-bromo-2-methyl-5-(3-cyanophenyl)
thiophene 5b
2. Materials and methods
Compound 5b was prepared by an analogous method similar to
that used for to 5a and was obtained as a buff colored solid (3.21 g)
2.1. Experimental
in 59% yield. 1H NMR (400 MHz, CDCl3)
d 2.43 (s, 3H, eCH3), 6.96 (t,
General: Melting point was taken on a WRS-1B melting point
1H, J ¼ 8.0 Hz, phenyl-H), 7.13 (s,1H), 7.21 (d, H, J ¼ 5.6 Hz, phenyl-
apparatus. NMR spectra were recorded on
a Bruker AV400
H), 7.29 (s, 1H, phenyl-H), 7.53 (t, 1H, J ¼ 6.8 Hz, phenyl-H).
(400 MHz) spectrometer with CDCl3 as the solvent and tetrame-
thylsilane as aninternalstandard. Elementalanalysis was performed
with the PE 2400 CHN analyzer. Mass spectra were measured
with an Agilent MS Trap VL spectrometer. Infrared spectra (IR)
were recorded on a Bruker Vertex-70 spectrometer. Fluorescent
spectra were measured using a Hitachi F-4500 spectrophotometer.
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 wavelength was isolated
by the use of the appropriate filters. The relative quantum yields
were determined by comparing the reaction yield with the known
yield of the compound 1,2-bis(2-methyl-5-phenyl-3-thienyl)per-
fluorocyclopentene in hexane (Foec ¼ 0.59, Fceo ¼ 0.013) [35].
Electrochemical examinations were performed in a one-compart-
ment 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 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 tetrafuoroborate ((TBA)
BF4) and 4.0 ꢀ 10ꢁ3 mol/L dithienylethene. All solutions were dea-
erated 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
distillation.
Suitable crystals of 1oe3o 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 at
room temperature using MoK\a 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 nonhydrogen atoms were refined aniso-
tropically. Crystallographic data of the structures have been
deposited in the Cambridge Crystallographic Data Centre as
supplemental publication CCDC 751281 for 1o, CCDC 615594 for 2o
and CCDC 722282 for 3o. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44 1223 336033 or e-mail:deposit@ccdc.cam.ac.uk).
2.2.3. Synthesis of 3-bromo-2-methyl-5-
(4-cyanophenyl)thiophene 5c
Compound 5c was prepared by an analogous method similar to
that used for to 5a and was obtained as a buff colouredsolid (3.57 g)
in 61% yield.1H NMR (400 MHz, CDCl3)
d 2.41 (s, 3H, eCH3), 7.03
(s, 1H, thienyl-H), 7.06 (t, 2H, J ¼ 8.0 Hz, phenyl-H), 7.48 (t, 2H,
J ¼ 6.2 Hz, phenyl-H).
2.2.4. Synthesis of 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-
(2-cyanophenyl)-3-thienyl] perfluorocyclopentene 1o
Compound 5a (2.0 g, 7.2 mmol) in anhydrous THF was added
dropwise to a 2.5 mol/L n-BuLi solution (3.1 mL) at ꢁ78 ꢂC under
an argon atmosphere. Stirring was continued for 30 min and
(2,5-dimethyl-3-thienyl)perfluorocyclopentene [31,32,36] (2.19 g,
7.2 mmol) was slowly added to the reaction mixture at ꢁ78 ꢂC and
stirred for 1 h at this temperature. Then, the reaction was allowed
to slowly warn to the room temperature and stirred there for 1 h.
The reaction was quenched with 10 mL distilled water. The mixture
was extracted with ether. The organic layer was collected and dried
over MgSO4, filtered and evaporated. The crude product was puri-
fied by column chromatography on silica gel using petroleum ether
as the eluent resulting in 1.56 g of 1o being obtained as white solid
in 45% yield. M.p. 104.6e105 ꢂC; MS m/z (M þ 1) 484.58; 1H NMR
(400 MHz, CDCl3): d 1.91 (s, 3H, -CH3),1.97 (s, 3H, eCH3), 2.45 (s, 3H,
eCH3), 6.8 (s, 1H, thienyl-H), 7.15 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.10
(t, 1H, J ¼ 8.0 Hz, phenyl-H), 7.20 (d, 1H, J ¼ 5.6 Hz, phenyl-H), 7.44
(s, 1H, thienyl-H), 7.58 (t, 1H, J ¼ 7.6 Hz, phenyl-H); 13C NMR
(400 MHz, CDCl3): d 14.58, 15.00, 55.59, 112.1, 121.0, 121.9, 125,
126.9, 128.0, 130.7, 131.8, 136.0, 137.5, 139.5, 142.2, 156.0; IR (n, KBr,
cmꢁ1): 735, 772, 822, 860, 898, 986, 1051, 1103, 1188, 1274, 1336,
1441, 1555, 1594, 1637, 2225, 2927. Anal. Calcd. for C23H15F6NS2 (%):
C, 57.14; H, 3.13. Found: C, 57.67; H, 3.54.
2.2.5. Synthesis of 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-
(3-cyanophenyl)-3-thienyl] perfluorocyclopentene 2o
Compound 2o was prepared by an analogous method similar to
that used for to 1o and was obtained as a white solid (1.66 g) in 48%
yield. M.p. 138.8e138.9 ꢂC; MS m/z (M þ 1) 484.77; 1H NMR
(400 MHz, CDCl3):
d 1.90 (s, 3H, eCH3), 1.97 (s, 3H, eCH3), 2.45 (s,
3H, eCH3), 6.75 (s, 1H, thienyl-H), 7.02 (t, 2H, J ¼ 8.0 Hz, phenyl-H),
7.24 (d, 1H, J ¼ 10.0 Hz, phenyl-H), 7.28 (s, 1H, thienyl-H), 7.35 (m,
2.2. Synthesis
2H, J ¼ 8.0 Hz, phenyl-H); 13C NMR (400 MHz, CDCl3):
d 14.25, 14.41,
15.07,112.4, 114.5,121.2, 123.4, 124.5,126.1, 130.5, 135.5, 137.9, 139.8,
2.2.1. Synthesis of 3-bromo-2-methyl-5-
140.5, 141.8, 161.9, 164.4; IR (n
, KBr, cmꢁ1): 732, 798, 856, 892, 981,
(2-cyanophenyl)thiophene 5a
1051, 1110, 1189, 1270, 1336, 1446, 1602, 2228, 2924. Anal. Calcd. for
C23H15F6NS2 (%): C, 57.14; H, 3.13. Found: C, 57.38; H, 3.29.
Compound 5a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid (4) [30] (4.0 g; 18.2 mmol) with 1-bromo-2-
cyanobenzene (3.31 g; 18.2 mmol) in the presence of Pd(PPh3)4
(caution: light sensitive, air sensitive; avoid strong oxidizing
agents; 0.4 g) and Na2CO3 (6.40 g; 60 mmol) in tetrahydrofuran
(THF) (80 mL, containing 10% water) for 15 h at 70 ꢂC. The product
2.2.6. Synthesis of 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-
(4-cyanophenyl)-3-thienyl] perfluorocyclopentene 3o
Compound 3o was prepared by an analogous method similar to
that used for to 1o and was obtained as a white solid (1.73 g) in 50%

S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
3
yield. M.p. 149.0e149.5 ꢂC; MS m/z (M þ 1) 484.39; 1H NMR
F
F
F
F
F
F
F
F
(400 MHz, CDCl3):
d 1.85 (s, 3H, eCH3), 1.90 (s, 3H, eCH3), 2.36 (s,
UV
Vis
3H, eCH3), 6.68 (s, 1H, thienyl-H), 7.02 (d, 2H, J ¼ 8.0 Hz, phenyl-H),
F
F
F
F
7.08 (s, 1H, thienyl-H), 7.42e7.45 (m, 2H, phenyl-H); 13C NMR
(400 MHz, CDCl3):
d 14.25, 14.35, 15.07, 115.8, 116.1, 122.5, 124.5,
126.1, 127.3, 129.7, 137.8, 139.7, 140.9, 141.1, 161.3, 163.7; IR (n, KBr,
S
S
S
S
R
R
cmꢁ1): 742, 831, 887, 986, 1049, 1105, 1185, 1273, 1338, 1440, 1489,
1560, 1604, 2223, 2924. Anal. Calcd. for C23H15F6NS2 (%): C, 57.14; H,
3.13. Found: C, 57.35; H, 3.41.
1o: R = ortho-CN
2o: R = meta-CN
3o: R = para-CN
DT4o: R = H
1c: R = ortho-CN
2c: R = meta-CN
3c: R = para-CN
DT4c: R = H
3. Results and discussion
Fig. 2. Photochromism of diarylethenes 1e3.
3.1. Discussion of the synthetic stragegy
isomer 1o. Just like diarylethene 1, compounds 2 and 3 also show
good photochromism in hexane (Fig. 3). Upon irradiation with
297 nm light, absorption bands in the visible region appeared and
the solutions of 2o and 3o turned purple due to the cyclization
reactions, leading to the production of the ring-closed isomers 2c
and 3c. Their absorption maxima in the visible region were
observed at 544 and 552 nm, respectively. The purple-colored
solutions of 2c and 3c can be decolorized upon irradiation with
The synthesis route for the diarylethenes 1o, 2o and 3o is shown
in Fig. 1. Suzuki coupling of the three bromobenzene derivatives
with a thiophene boronic acid [30] gave the cyanophenylthiophene
derivatives (5a, 5b and 5c). They were lithiated and coupled to (2,5-
dimethyl-3-thienyl)perfluorocyclopentene [31,32,36] to give the
unsymmetrical diarylethene derivatives 1o, 2o and 3o, respec-
tively. The structures of 1o, 2o and 3o were confirmed by NMR, IR,
and mass spectrometry, elemental analysis, and X-ray crystallo-
graphic analysis.
visible light (l > 450 nm) again, due to the conversion of the
compounds back to the ring-opened isomers 2o and 3o. This
reversion also causes the reappearance of the absorption maxima
at 295 and 312 nm, respectively. At the photostationary state, the
isosbestic points for diarylethenes 1, 2 and 3 were observed at 333,
331 and 341 nm, respectively.
3.2. Photochromic behaviours
The photochromic scheme of diarylethenes 1, 2 and 3 is shown
in Fig. 2.
In amorphous PMMA films, diarylethenes 1, 2 and 3 also showed
similar photochromism as in hexane, and their spectral and color
changes are shown in Fig. 4. The PMMA films were prepared by
dissolving 10 mg of diarylethene sample and 100 mg of poly-
methylmethacrylate (PMMA) in chloroform (1 mL) with the aid of
ultrasound, and the homogeneous solution was spin-coated on
a quartz substrate (20 ꢀ 10 ꢀ 1 mm3) at 1500 rpm. Compared to
those in hexane, the maximum absorption peaks of both the ring-
opened and the ring-closed isomers of diarylethenes 1 and 2 in
PMMA film are at longer wavelengths than those in hexane. The red
shift phenomena are consistent with those of the reported studies
of most of the diarylethenes [23,28e31,38], which may be attrib-
uted to the polar effect of the polymer matrix and the stabilization
of molecular arrangement in solid state [39,40]. However, for dia-
rylethene 3, the absorption maxima of both its ring-opened and
ring-closed isomers in PMMA film are at shorter wavelengths than
those in hexane. The blue shift of its ring-opened isomer is 25 nm,
and that of its ring-closed isomer is only one nanometer. To out best
knowledge, this is a unique diarylethene example that its absorp-
tion maximum shows a blue shift in the solid state comparing to
that in solution. The reason for this blue shift is not clear and
further investigation is in progress.
The photochromic behaviours of diarylethenes 1, 2 and 3
induced by photoirradiation at room temperature were measured
both in hexane and in PMMA films. In hexane, the absorption
spectrum and color changes of diarylethenes 1, 2 and 3 induced by
alternating irradiation with UV light and visible light at appropriate
wavelength are shown in Fig. 3. The absorption maximum of
compound 1 was observed at 272 nm (
in hexane, as the result of a pe
3
¼ 3.08 ꢀ 104 L molꢁ1 cmꢁ1
)
p* transition [37]. This colorless
solution turned purple and a new absorption band centered at
539 nm (
3
¼ 1.77 ꢀ 104 L molꢁ1 cmꢁ1) appeared when it was irra-
diated with 297 nm violet light, resulting from a ring-closure
reaction and the production of the ring-closed isomer 1c. Upon
irradiation with visible light (l > 450 nm), the colored 1c under-
went a cycloreversion reaction to the initial colorless ring-opened
Br
Br
R
Br
S
B(OH)2
S
4
R
Pd(Pph3)4
Na2CO3
5a: R=ortho-CN
5b: R=meta-CN
5c: R=para-CN
The photochromic parameters of diarylethenes 1, 2 and 3 are
summarized in Table 1. These data showed that the cyano substit-
uent position had a remarkable effect on the photochromic prop-
erties of these isomeric diarylethenes, including the absorption
maxima, molar absorption coefficients, and quantum yields of
cyclization and cycloreversion reactions. For these three isomeric
diarylethene derivatives, the absorption maxima of both of the
ring-opened and the ring-closed isomers varied with the same
trend in hexane, i.e., they increased in the order of ortho- < meta-
< para-substitution. Therefore, the absorption maxima of the ring-
opened isomer and the ring-closed isomer of diarylethene 3 are the
longest; while those of 1 are the shortest. The result is quite
different from those reported for electron-donating methoxy
groups, where the absorption maximum of diarylethene bearing
the ortho-substituted methoxy group is the longest, and that of
F
F
F
F
n-BuLi,-78 °C
F
F
F
F
F
F
F
F
S
S
R
F
S
1o: R=ortho-CN
2o: R=meta-CN
3o: R=para-CN
Fig. 1. Synthetic route for diarylethenes 1oe3o.

4
S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
0.8
A
B
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Vis
Vis
UV
0.6
UV
0.4
UV
UV
Vis
Vis
0.2
0.0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
1o 2o 3o
C
D
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Vis
UV
Vis
UV
UV
Vis
300
400
500
600
700
800
Wavelength (nm)
1c 2c 3c
Fig. 3. Absorption spectral and color changes of diarylethenes 1e3 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) color changes for 1e3.
A
B
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Vis
Vis
UV
UV
UV
Vis
UV
Vis
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1o
2o
3o
C
D
Vis
UV
UV
Vis
Vis
UV
300
400
500
600
700
Wavelength (nm)
2c
3c
1c
Fig. 4. Absorption spectral and color changes of diarylethenes 1e3 by photoirradiation in PMMA film (10%, w/w) at room temperature: (A) spectral changes for 1, (B) spectral
changes for 2, (C) spectral changes for 3, (D) color changes for 1e3.

S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
5
Table 1
Table 2
Absorption characteristics and photochromic reactivity of diarylethenes 1e3 in
Crystal data and structure refinements for diarylethenes 1oe3o.
hexane (2.0 ꢀ 10ꢁ5 mol/L) and in PMMA film (10%, w/w).
Compound 1o
Compound 2o
Compound 3o
Compound lo,max (nm)a
/L molꢁ1 cmꢁ1
lc,max (nm)b
/L molꢁ1 cmꢁ1
)
Fc
Formula
C23H15F6NS2
483.48
296(2)
Triclinic
P-1
8.588(3)
11.536(4)
13.043(5)
99.488(4)
107.867(4)
109.715(4)
1104.6(7)
2
C23H15F6NS2
483.48
294(2)
Monoclinic
P21/c
16.598(4)
8.7178(18)
17.250(4)
90
116.899(4)
90
2226.1(8)
4
C23H15F6NS2
483.48
291(2)
Monoclinic
P21/c
21.4209(19)
9.1529(8)
11.2875(10)
90
102.1450(10)
90
2163.5(3)
4
(
3
)
(
3
Formula weight
Temperature
Crystal system
Space group
Unit cell dimensions a (Å)
b (Å)
Hexane
PMMA Hexane
film
PMMA Foec Fceo
film
1
2
3
272 (3.08 ꢀ 104) 278
295 (3.62 ꢀ 104) 297
312 (5.50 ꢀ 104) 287
539 (1.77 ꢀ 104) 554
544 (1.76 ꢀ 104) 557
552 (2.29 ꢀ 104) 551
0.45 0.028
0.46 0.041
0.54 0.037
c (Å)
a
b
c
a
b
g
(o)
(o)
(o)
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring
(Foec) and closed-ring isomers (Fceo),
Volume (Å3)
respectively.
Z
Reflections collected
Reflections observed
Number of parameters
8462
4080
292
0.302
1.454
1.031
0.71073
0.0438
0.1139
0.0571
0.1246
10978
3935
346
0.299
1.443
15950
4010
322
0.308
1.484
1.038
0.71073
0.035
0.0976
0.0470
0.1077
para-substituted methoxy group is the shortest [30,31]. The molar
absorption coefficients of the ring-opened isomers 1oe3o also
clearly increased in the order of ortho- < meta- < para-substitution
by the cyano group. Among the ring-closed isomers 1ce3c,
the molar absorption coefficient of the para-substituted derivative
3c is the highest in hexane; while that of the meta-substituted
derivative 2c is the lowest. As shown in Table 1, the cyclization
quantum yields of diarylethenes 1e3 are much higher than
their respective cycloreversion quantum yields. This is completely
different from unsymmetrical diarylethenes bearing a pyrazole
unit whose cyclization quantum yields are lower than their
respective cycloreversion quantum yields [33]. For diarylethenes
1e3, the cyclization quantum yield increases in the order of
m
(mmꢁ1
)
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Radiation (Å)
1.02
0.71073
0.0443
0.1098
0.0988
0.1396
Final R1[I > 2s(I)]
wR2[I > 2s(I)]
R1 (all data)
wR2 (all data)
ORTEP drawings and photochromism induced by alternating irra-
diation with UV light and visible light in the crystalline phase
are shown in Fig. 5. For diarylethene 1o, the molecule crystallizes
with an approximate C2 symmetry with the photoactive anti-
parallel conformation in the crystalline phase, which can undergo
photocyclization [7,42e44]. The molecule includes four planar
rings, which can form three dihedral angles between every two
adjacent planar rings. The dihedral angles between the hexa-
fluorocyclopentene ring and the two thiophene rings are 45.7(6)ꢂ
for S1/C6eC9 and 52.3(6)ꢂ for S2/C18eC21, and that between the
thiophene ring and the linked benzene ring is 37.5(6)ꢂ. In hexa-
fluorocyclopentene ring, the distances clearly show that the C1eC5
bond (1.341(3) Å) is a double bond, being significantly shorter than
other carbonecarbon single bonds (1.508(4) Å to 1.526(4) Å) of the
ring. Two symmetrical thiophene moieties are linked by the C1]C5
double bond, with both of them attached to the ethylene group via
the 2-position of the thiophene ring. The two methyl groups are
located on opposite sides of the double bond, reflected in the
torsion angles C5eC1eC7eC6 [ꢁ44.3(4)ꢂ] and C1eC5eC19eC18
[ꢁ53.5(4)ꢂ], and are thus trans with respect to the double bond.
Such a conformation is crucial for the compound to exhibit
photochromic and photo-induced properties [45e47]. The inter-
molecular distance between the two reactive C atoms (C6.C18) is
3.664(3) Å.
The corresponding data of compounds 2o and 3o are summa-
rized in Table 3. For diarylethenes 2o and 3o, both of them also
adopt the anti-parallel conformation in the crystalline phase, and
the distance between the two reactive C atoms is 3.552(6) Å for 2o
[48] and 3.594 (6) Å for 3o. As a result, all molecules of crystals 1o,
2o and 3o are fixed in an anti-parallel mode in the crystalline phase
and the distances of the two reactive C atoms are less than 4.2 Å,
which is close enough for the photocyclization reaction to take
place [44,49,50]. In fact, the crystals of 1o, 2o and 3o underwent
photochromic reaction, in accordance with the expected ring
closure, to form 1c, 2c and 3c upon irradiation with UV light. Fig. 5
shows the color changes of crystals 1o, 2o and 3o upon photo-
irradiation in the crystalline phase. When these colored crystals
were dissolved in hexane, an intense absorption maximum was
appeared at the same wavelength as that of their respective ring-
closed isomer in solution. Alternatively, the purple-colored crystals
ortho-
(
Foec,1 ¼0.45) < meta-
(
Foec,2 ¼ 0.46) < para-substitution
(Foec,3 ¼ 0.54) by the cyano group. However, the trend in the
cycloreversion quantum yield is different. The cycloreversion
quantum yield of the meta-substituted derivative 2 is the highest
(Fceo,2 ¼ 0.041), that of the ortho-substituted derivative 1 is the
lowest (Fceo,1 ¼0.028), and the para-substituted derivative 3 is
intermediate (Fceo,3 ¼ 0.037). This pattern of variation is different
from those of isomeric diarylethenes reported in previous work
[31,32]. When substituting with the methoxy group in the same
diarylethene system, the cyclization and cycloreversion quantum
yields decreased evidently, and other photochromic parameters
were also very different [31], compared with cyano substitution.
However, the cyclization quantum yields increased and the
cycloreversion quantum yields decreased significantly when
substituting with chlorine atom in the same position of diary-
lethene system [32], compared with cyano substitution. The
unsubstituted parent compound, namely 1-(2,5-dimethyl-
3-thienyl)-2-(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene
(DT4, as shown in Fig. 2), show quite different photochromic
properties. The molar absorption coefficients of DT4
(
3o
¼
2.47 ꢀ 104 L molꢁ1 cmꢁ1
,
3c ¼ 3.68 ꢀ 103 L molꢁ1 cmꢁ1) are much
lower than that of 1, 2 and 3; while the cycloreversion quantum
yield of DT4 (Fceo ¼ 0.119) is higher [32]. The result demonstrates
that the photochromic features of diarylethenes can be effectively
controlled by the introduction of the electron-withdrawing
cyano group on the terminal benzene ring. Compared to the
symmetric
analogue,
1,2-bis(2-methyl-5-(4-cyanophenyl)-3-
thienyl)perfluorocyclopentene [41], the cycloreversion quantum
yields of the isomeric derivatives are higher, but their molar
absorption coefficients and the absorption maxima are much lower.
Single crystals 1o, 2o and 3o, suitable for X-ray analysis, were
obtained by slow evaporation of hexane solutions. To gain a deeper
understanding of the relationship between the conformation and
the photochromism of these isomeric diarylethene derivatives in
the crystalline phase, the structural conformations of diarylethenes
1o, 2o and 3o were confirmed by X-ray crystallographic analysis.
The X-ray crystallographic analysis data are listed in Table 2. Their

6
S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
Fig. 5. ORTEP drawings of crystals 1oe3o and their color changes by photoirradiation in the single crystalline phase: (A) ORTEP drawing of 1o, (B) ORTEP drawing of 2o, (C) ORTEP
drawing of 3o, (D) color changes for crystals 1e3.
returned to colorless upon irradiation with the appropriate visible
light ( > 450 nm). Moreover, these photochromic crystals exhibi-
diarylethene 1o was slightly decreased upon irradiation with
297 nm UV light. Irradiation by visible light at appropriate wave-
lengths regenerated the ring-opened isomer 1o and restored the
original emission spectrum. When reaching the photostationary
state, the emission intensity of diarylethene 1 was quenched to ca.
88%. The lower fluorescent conversion may be mainly caused by the
incomplete cyclization reaction and the existence of parallel
conformations with relatively strong fluorescence [27e29].
l
ted remarkable fatigue-resistance greater than 100 cyclization/
cycloreversion repeated cycles and their ring-closed isomers
remained stable for more than 300 days in the dark at room
temperature. Therefore, they can be potentially used for the
construction of certain optoelectronic devices [51].
3.3. Fluorescence of diarylethenes
Using a Hitachi F-4500 spectrophotometer, the fluorescence
properties of diarylethenes 1o, 2o and 3o both in solution
(2.0 ꢀ 10ꢁ5 mol/L) and in amorphous PMMA films (10% w/w) were
measured at room temperature. Because diarylethenes 2o and 3o
show very faint fluorescence in solution, we herein only discuss the
fluorescent property of diarylethene 1o in hexane. In hexane, the
emission peak of diarylethene 1o was observed at 384 nm when
excited at 300 nm. As shown in Fig. 6, the emission intensity of
400
UV Vis
300
200
100
0
Table 3
Distances between the reacting carbon atoms (d, Å) dihedral angles (
q,
ꢂ) of 1oe3o.
Compound
d (Å)
q
(ꢂ)a
q1
q2
q3
1o
2o [48]
3o
C8.C16
C5.C13
C4.C13
3.664(3)
3.552(6)
3.594(6)
52.3(6)
45.8(5)
42.3(8)
45.7(6)
44.0(5)
41.1(8)
37.5(6)
24.4(3)
23.8(8)
350
400
450
500
550
a
q1, Dihedral angle between the hexafluorocyclopentene ring and 2,5-dime-
thylthiophene ring; q2, dihedral angle between the hexafluorocyclopentene ring and
the thiophene ring attached t a benzene ring; q3, dihedral angel between the thio-
phene ring and the adjacent benzene ring.
Wavelength (nm)
Fig. 6. Emission intensity changes of diarylethene 1 in hexane (2.0 ꢀ 10ꢁ5 mol/L) upon
irradiation with 297 nm UV light at room temperature, excited at 300 nm.

S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
7
In PMMA films, the emission peaks of diarylethenes 1o, 2o and
3o were observed at 411, 407, and 413 nm when excited at 300 nm.
Their emission intensity changes in PMMA films during the process
of photoisomerization are shown in Fig. 7. As observed for most of
the reported diarylethenes [34,52e56], diarylethenes 1, 2 and 3
displayed a relatively strong fluorescence switch when they are
converted from the ring-opened isomers to ring-closed isomers by
photoirradiation in PMMA film. Upon irradiation with 313 nm light,
their emission intensities decreased to the photostationary state,
where the emission intensities of diarylethenes 1, 2 and 3 were
quenched to ca. 50%, 60%, and 54%, respectively. The result showed
that the position of cyano substitution had a significant effect not
only on the emission spectra but also on the fluorescence switching
properties of diarylethenes 1, 2 and 3. This characteristic may be
useful for increasing selectivity in their potential application as the
fluorescence photo-switches. In addition, Compared to diary-
lethenes 1e3, the analogues bearing an electron-donating methoxy
group exhibited stronger emission intensity [31], and other
analogues bearing a fluorine atom displayed stronger fluorescence
switching property [38].
A
600
UV Vis
450
300
150
0
3.4. Electrochemistry of diarylethenes
Because of the differences in the
p-conjugation of the two
isomers of diarylethenes, the photoresponsive system described
here also allows for changes in their electrochemical properties. To
date, the oxidative cyclization and cycloreversion of diarylethene
derivatives have been attracted much attention due to their
potentially important applications in photoelectronic devices
[57e61].
Cyclic voltammograms (CV) were performed on the diary-
lethenes 1, 2 and 3 under identical experimental conditions at
50 mV/s and the results are shown in Fig. 8. The oxidation onsets of
their ring-opened isomers 1o, 2o and 3o were initiated at 1.83, 1.82
and 1.70 V, and those of their ring-closed isomers 1c, 2c and 3c
350
350
350
400
450
500
500
500
550
550
550
Wavelength (nm)
B
600
450
300
150
0
UV Vis
A
1o
1c
400
450
Wavelength (nm)
-2
-2
-2
-1
-1
-1
0
0
0
1
1
1
2
2
2
2o
2c
B
C
900
750
600
450
300
150
0
UV Vis
B
C
C
400
450
Wavelength (nm)
Potential/V vs. SCE
Fig. 7. Emission intensity changes of diarylethenes 1e3 in PMMA film (10%, w/w) upon
irradiation with 313 nm UV light at room temperature, excited at 300 nm: (A) 1, (B) 2,
and (C) 3.
Fig. 8. Cyclic voltammetry (second scan) of diarylethenes 1e3 in acetonitrile at a scan
rate of 50 mV/s: (A) 1, (B) 2, and (C) 3.

8
S. Pu et al. / Dyes and Pigments 87 (2010) 1e9
Table 4
Acknowledgements
Electrochemical parameters of diarylethenes 1e3.a
This work was supported by Program for the NSFC of China
(20962008), New Century Excellent Talents in University (NCET-
08-0702), the Project of Jiangxi Youth Scientist, the Key Scientific
Project from Education Ministry of China (208069) and the
Youth Science Funds of the Education Office of Jiangxi, China
(GJJ09572).
Compound
Oxidation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg
1o
1c
2o
2c
3o
3c
þ1.83
þ1.75
þ1.82
þ1.77
þ1.70
þ1.61
ꢁ6.63
ꢁ6.55
ꢁ6.62
ꢁ6.57
ꢁ6.50
ꢁ6.41
ꢁ0.84
ꢁ0.69
ꢁ0.64
ꢁ0.57
ꢁ0.61
ꢁ0.66
ꢁ3.96
ꢁ4.11
ꢁ4.16
ꢁ4.23
ꢁ4.19
ꢁ4.14
2.67
2.44
2.46
2.34
2.31
2.27
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