The synthesis of novel photochromic diarylethenes bearing a biphenyl moiety and the effects of substitution on their properties

Article abstract of DOI:10.1016/j.dyepig.2010.04.005

A new class of diarylethene bearing a biphenyl moiety was synthesized and the effects of substitution on photochromism, fluorescence and electrochemical character, were investigated. Under alternating irradiation with UV and visible light, the compounds exhibited good photochromism and functioned as?fluorescent photoswitches both in solution and in PMMA film. Electron-donating substituents shifted the λ_max of the diarylethenes to longer wavelengths and decreased their cyclization quantum yield, while electron-withdrawing substituents greatly increased both cyclization and cycloreversion quantum yield. In addition, cyclic voltammetry revealed that the substituents had a significant effect on the electrochemical behaviour of the diarylethene derivatives.

Full text of DOI:10.1016/j.dyepig.2010.04.005

Dyes and Pigments 87 (2010) 257e267
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis of novel photochromic diarylethenes bearing a biphenyl
moiety and the effects of substitution on their properties
*
Shouzhi Pu , Wenjuan Miao, Shiqiang Cui, Gang Liu, Weijun 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:
A new class of diarylethene bearing a biphenyl moiety was synthesized and the effects of substitution on
photochromism, fluorescence and electrochemical character, were investigated. Under alternating irra-
diation with UV and visible light, the compounds exhibited good photochromism and functioned
as fluorescent photoswitches both in solution and in PMMA film. Electron-donating substituents shifted
the lmax of the diarylethenes to longer wavelengths and decreased their cyclization quantum yield, while
electron-withdrawing substituents greatly increased both cyclization and cycloreversion quantum yield.
In addition, cyclic voltammetry revealed that the substituents had a significant effect on the electro-
chemical behaviour of the diarylethene derivatives.
Received 10 December 2009
Received in revised form
10 April 2010
Accepted 12 April 2010
Available online 21 April 2010
Keywords:
Photochromism
Diarylethenes
Ó 2010 Elsevier Ltd. All rights reserved.
Biphenyl moiety
Substitution effect
Photochemistry
Electrochemistry
1. Introduction
group can result in significant modification of the photochromic
properties of each type of diarylethene molecule. For example,
Photochromic compounds can undergo a light-induced revers-
ible transformation between two forms that differ not only in
their absorption characteristics but also in terms of many physical
and chemical characteristics, such as geometry (structure), refrac-
tive index and oxidation/reduction potential [1e6]. The photo-
responsive character associated with this fascinating class of
organic molecules offers a wealth of potential application in widely
divergent areas, ranging from biomedical research to information
technology [2,7,8]. Diarylethene derivatives, especially those with
heterocyclic aryl groups, have recently received much attention as
they have notable fatigue resistance, high thermal stability, rapid
response and high reactivity in the solid state [2,9e11].
Currently, the design and synthesis of new photochromic
compounds are an active area of research, with many reports on the
synthesis and investigation of the properties of diarylethenes with
heterocyclic aryl rings. Among diarylethenes hitherto reported, the
majority of the heteroaryl compounds bear thiophene or benzo-
thiophene rings [12e19], with only a few reports concerning other
heteroaryl moieties, such as furan [20], thiazole [21,22], indole [23],
benzofuran [24], pyrrole [25e27] and pyrazole [28]. A change in aryl
diarylethenes comprising thiophene or benzothiophene moieties
exhibit excellent thermal stability and outstanding fatigue resis-
tance [2,9], whereas symmetrical diarylethenes with two pyrrole
rings are thermally unstable and return to the open-ring isomer,
even in the dark [26]. Moreover, Kawai et al. [29] reported triangular
terthiophene derivatives that displayed reversible photochromic
reactions of high cyclization quantum yield. Yamaguchi et al. [30]
developed a novel type of 6
p conjugated photochromic system
that contained a bis(2,30-benzothienyl) unit, which showed efficient
photochromism and high thermal stability in the case of the colored
isomers. Yokoyama et al. [31] and Branda et al. [32] synthesized
independently a new class of photochromic compound based on
a modified hexatriene skeleton. This novel molecular scaffold offers
the chance to decorate photoresponsive systems with a wide range
of functional groups without sacrificing photochromic behaviour.
Studies on the hexatriene backbone of diarylethenes are mainly
confined to the five-membered heteroaryl moieties. In the case of
six-membered, aryl ring moieties, only a few symmetrical diary-
lethene derivatives bearing two phenyl/naphthyl groups have been
reported [33e35], the majority of which are thermally reversible
with poor photochromism. Previous reports by the present authors
have concerned a new class of photochromic diarylethene deriva-
tives bearing both five-membered and six-membered moieties
that show some novel characteristics which differ to 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.2010.04.005

258
S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
containing five-membered aryl rings [36e38]. The design of a dia-
rylethene molecule bearing both five-membered and six-membered
groups prompted the synthesis of unsymmetrical diarylethene
derivatives by the introduction of a biphenyl unit as one aryl moiety
of the diarylethene skeleton. Compared with reported diarylethene
derivatives, diarylethenes bearing a biphenyl unit have two distinct
advantages insofar as, the greater the number of benzene rings the
molecule contains, the easier it is to modify the molecular structure
as the reactive sites of the benzene ring can be easily modified using
different electron-donating/withdrawing substituents, thereby
influencing the optoelectronic properties of the diarylethene
derivatives. In addition, diarylethenes bearing a biphenyl unit also
have a unique molecular skeleton compared to those reported
bearing five-membered heteroaryl groups. As an aryl moiety, the
biphenyl unit connected directly to the central ethene unit can
yields were determined by comparing the reaction yields of these
diarylethene compounds in hexane against 1,2-bis(2-methyl-5-
phenyl-3-thienyl)perfluorocyclopentene in hexane (with their
respective absorption maxima visible light irradiation) [39]. Fluo-
rescence spectra were measured using a Hitachi F-4500 fluorimeter.
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
working electrode and counter electrode. Platinum electrodes served
as a quasi reference electrode and was calibrated using the ferrocene
(Fc/Fþc ) redox couple which has a formal potential E1/2 ¼ þ0.35 V
versus platinum wire. The typical electrolyte was acetonitrile (5 mL)
containing 0.1 mol Lꢀ1 tetrabutylammonium tetrafluoroborate
((TBA)BF4) and 4.0 ꢁ 10ꢀ3 mol Lꢀ1 dithienylethene. All solutions were
deaerated by a dry argon stream and maintained at a slight argon
overpressure during electrochemical experiments.
efficiently modulate the extent of p-conjugation in the diarylethene
between the open-ring and the closed-ring isomers. It was inter-
esting to discover that in the biphenyl segment, the second phenyl
participated in the
p-conjugation of the open-ring isomer, but not in
2.2. Synthesis of diarylethenes
the closed-ring. As a result, the
p-conjugation is delocalized from the
biphenyl moiety to the ethene unit in the open-ring isomer while
in the closed-ring isomer, the second phenyl is separated from the
first one. This inevitably influences the optical and electrochemical
properties of the corresponding diarylethene derivatives.
The synthetic route used to obtain 1oe5o is shown in Fig. 2.
Five bromobenzene derivatives were firstly coupled with phenyl
boronic acid (6) [40,41] by means of the Suzuki reaction to yield the
biphenyl derivatives (7ae7e). Separately, 2-methyl-5-phenyl-3-
thienyl-perfluorocyclpentene (9) was prepared from 2-methyl-
thiophene according to the procedure described previously [32,42]
and, finally, compounds 7ae7e were separately lithiated and then
coupled with compound 9 to give 1oe5o, respectively.
The purposes of this work were to develop a new class of
diarylethenes bearing a biphenyl unit and to investigate substituent
effects on optoelectronic properties. Accordingly, five new unsym-
metrical diarylethenes with different substituents at the meta-
position of the second phenyl ring of the biphenyl were prepared
(1oe5o as shown in Fig. 1). Each of the diarylethenes showed good
photochromism both in solution and in amorphous PMMA film; to
the best of our knowledge, they are the first examples of this class of
photochromic diarylethene derivatives bearing both biphenyl and
thiophene moieties.
2.2.1. 1-Bromo-2-methyl-4-(3-methoxylphenyl)benzene (7a)
7a was prepared by reacting 1-bromo-2-methyl-4-phenyl boronic
acid (6) [40,41] (2.50 g; 11.6 mmol) with 3-bromoanisole (2.18 g,
11.6 mmol) in the presence of Pd(PPh3)4 (0.8 g, 0.53 mmol) and
Na2CO3 (6.40 g, 60 mmol) in tetrahydrofuran (THF) (80 mL containing
10% water) for 16 h at 90 ꢂC. The product 7a was purified by column
chromatography on SiO2 using petroleum ether as an eluent and
2.22 g obtained as a colorless oil in 69.1% yield. 1H NMR (400 MHz,
2. Experimental
CDCl3):
d
2.46 (s, 3H, eCH3), 3.86 (s, 3H, eCH3), 6.90 (d,1H, J ¼ 8.0 Hz,
2.1. General methods
phenyl-H), 7.08 (s, 1H, phenyl-H), 7.13 (d, 1H, J ¼ 7.8 Hz, phenyl-H),
7.25 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.35 (t, 1H, J ¼ 8.0 Hz, phenyl-H),
7.43 (s, 1H, phenyl-H), 7.57 (d, 1H, J ¼ 8.0 Hz, phenyl-H).
Solvents were purified by distillation before use. Mass spectra
were measured with an Agilent MS Trap VL spectrometer. Elemental
analyses were determined with a PE CHN 2400 analyzer. NMR
spectra were recorded on a Bruker AV400 (400 MHz) spectrometer
with CDCl3 as the solvent and tetramethylsilane as an internal stan-
dard. IR spectra were recorded on a Bruker Vertex-70 spectrometer.
The absorption spectra were measured using an Agilent 8453 UV/VIS
spectrophotometer. Photoirradiation was carried out using SHG-200
UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and BMH-
250 visible lamp. The required wavelength was isolated by the use of
the appropriate HB ML UV Narrow Bandpass Filters. The quantum
2.2.2. 1-Bromo-2-methyl-4-(3-methylphenyl)benzene (7b)
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 an eluent to give 7b (2.13 g, 70.3%) as
a colorless oil. 1H NMR (400 MHz, CDCl3):
d 2.44 (s, 3H, eCH3), 2.48
(s, 3H, eCH3), 7.13 (t,1H, J ¼ 7.0 Hz, phenyl-H), 7.20 (d,1H, J ¼ 8.0 Hz,
phenyl-H), 7.34e7.38 (m, 3H, phenyl-H), 7.47 (s, 1H, phenyl-H), 7.59
(d, 1H, J ¼ 8.0 Hz, phenyl-H).
2.2.3. 1-Bromo-2-methyl-4-phenylbenzene (7c)
F
F
F
F
7c 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 an eluent to give 7c (1.96 g, 68.3%) as
F
F
F
F
F
F
F
F
a colorless oil. 1H NMR (400 MHz, CDCl3):
d 2.45 (s, 3H, eCH3), 7.24
UV
Vis
(d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.34 (t, 1H, J ¼ 7.2 Hz, phenyl-H),
S
S
7.40e7.44 (m, 3H, phenyl-H), 7.53e7.58 (m, 3H, phenyl-H).
R
R
2.2.4. 1-Bromo-2-methyl-4-(3-cyanophenyl)benzene (7d)
1c: R=OCH3
2c: R=CH3
3c: R=H
1o: R=OCH3
2o: R=CH3
3o: R=H
7d was prepared by a method similar to that used for 7a. The
crude product was purified by column chromatography on SiO2
using the mixture of petroleum ether and acetic ether (v/v ¼ 6/1) as
an eluent to give 7d (2.17 g, 68.7%) as a white solid. M.p. 57e59 ꢂC;
4c: R=CN
5c: R=CF3
4o: R=CN
5o: R=CF3
Fig. 1. Photochromism of diarylethenes 1e5.
1H NMR (400 MHz, CDCl3):
d 2.47 (s, 3H, eCH3), 7.22 (d, 1H,

S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
259
Br
Br
Br
R
Pd(Pph3)4
B(OH)2
Na2CO3
R
7a: R=OCH3
7b: R=CH3
F
F
6
F
F
F
F
7c: R=H
n-BuLi,-78 oC
7d: R=CN
7e: R=CF3
S
F
F
F
F
Br
1 ) n-BuLi,-78 oC
2 ) C5F8
R
F
F
1o: R=OCH3
2o: R=CH3
3o: R=H
F
S
4o: R=CN
5o: R=CF3
S
8
9
Fig. 2. Synthetic route for diarylethenes 1oe5o.
J ¼ 8.0 Hz, phenyl-H), 7.41 (s, 1H, phenyl-H), 7.54 (t, 1H, J ¼ 7.6 Hz,
phenyl-H), 7.59e7.64 (m, 2H, phenyl-H), 7.77 (d, 1H, J ¼ 8.0 Hz,
phenyl-H), 7.82 (s, 1H, phenyl-H).
THF (5 mL) was added. The reaction mixture was further stirred at
ꢀ78 ꢂC for 1 h and allowed to slowly warm to the room tempera-
ture. The reaction was stopped with distilled water. The product
was extracted with ether, dried with MgSO4, filtrated and evapo-
rated in vacuo. The crude product was purified by column chro-
matography using petroleum ether as the eluent toþgive 1o (0.34 g,
32%) as a yellow solid. M.p. 116e118 ꢂC; MS m/z (M ) 545.09; Calcd
for C30H22F6OS (%): Calcd C, 66.17; H, 4.07. Found C, 66.51; H, 4.12;
2.2.5. 1-Bromo-2-methyl-4-(3-trifluoromethylphenyl)benzene (7e)
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 an eluent to give 7e (2.39 g, 65.5%) as
a colorless oil. 1H NMR (400 MHz, CDCl3):
d
2.51 (s, 3H, eCH3), 7.29
1H NMR (400 MHz, CDCl3):
d 1.99 (s, 3H, eCH3), 2.05 (s, 3H, eCH3),
(d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.47 (s, 1H, phenyl-H), 7.57 (t, 1H,
J ¼ 7.8 Hz, phenyl-H), 7.64 (d, 2H, J ¼ 8.0 Hz, phenyl-H), 7.75 (d, 1H,
J ¼ 7.6 Hz, phenyl-H), 7.82 (s, 1H, phenyl-H).
3.78 (s, 3H, eOCH3), 6.87 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.09 (s, 1H,
thienyl-H), 7.12 (d, 1H, J ¼ 7.6 Hz, phenyl-H), 7.21e7.24 (m, 2H,
phenyl-H), 7.28e7.32 (m, 3H, phenyl-H), 7.37 (s, 1H, phenyl-H),
7.44e7.49 (m, 4H, phenyl-H); 13C NMR (400 MHz, CDCl3):
d 13.0,
2.2.6. 2-Methyl-5-phenyl-3-thienyl-perfluorocyclopentene (9)
19.0, 54.1, 111.8, 112.2, 118.4, 121.6, 123.8, 124.4, 124.5, 125.7, 126.8,
127.9, 128.5, 128.8, 132.3, 136.4, 140.1, 140.3, 141.0, 141.6, 159.0; IR
This compound was synthesized by the same method as that
reported in Ref. [32]. To a stirred solution of 3-bromo-2-methyl-5-
phenylthiophene (5.06 g, 20 mmol) in THF (80 mL) was added
dropwise a 2.5 mol Lꢀ1 n-BuLi/hexane solution (8.8 mL, 22 mmol) at
ꢀ78 ꢂC under an argon atmosphere. Stirring was continued for
30 min at this low temperature. Perfluorocyclopentene (3.3 mL,
24 mmol) was then added to the reaction mixture at ꢀ78 ꢂC, which
was stirred for 2 h at this temperature; the reaction mixture was
then warmed to room temperature spontaneously. Reaction
was stopped by the addition of methanol (5 mL) and the product was
extracted three times with ether (150 ml). The organic layer was
washed with 1 M aqueous HCl and water respectively. The organic
layer was dried over MgSO4, filtrated, and evaporated. The crude
product was purified by column chromatography on SiO2 using
petroleum ether as an eluent to give 9 (5.3 g, 72.6%) as a white solid.
(KBr, n
, cmꢀ1): 691, 756, 784, 834, 858, 893, 923, 990, 1065, 1164,
1222, 1271, 1401, 1453, 1615, 1667, 2839, 3036.
2.2.8. [1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-methyl-4-(3-
methylphenyl)-1-phenyl)]perfluorocyclopentene (2o)
Compound 2o was prepared by a method similar to that used for
1o. The crude product was purified by column chromatography
on SiO2 using petroleum ether as an eluent to give 2o (0.39 g, 37%) as
a yellow solid. M.p. 123e124 ꢂC; MS m/z (Mþ) 529.19; Calcd for
C30H22F6S (%): Calcd C, 68.17; H, 4.20. Found C, 68.14; H, 4.22; 1H
NMR (400 MHz, CDCl3): d 1.92 (s, 3H, eCH3), 1.96 (s, 3H, eCH3), 2.34
(s, 3H, eCH3), 7.12 (s, 2H, thienyl-H, phenyl-H), 7.23 (t,1H, J ¼ 7.4 Hz,
phenyl-H), 7.26e7.29 (m, 4H, phenyl-H), 7.31 (d, 2H, J ¼ 7.0 Hz,
phenyl-H), 7.37 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.41e7.45 (m, 3H,
M.p. 35e37 ꢂC [32]; 1H NMR (400 MHz, CDCl3):
d
2.51 (s, 3H, eCH3),
phenyl-H); 13C NMR (400 MHz, CDCl3):
d 14.8, 19.9, 21.5, 122.7,
7.25 (s, 1H, thienyl-H), 7.28 (t, 1H, J ¼ 11.4 Hz, phenyl-H), 7.40 (t, 2H,
124.2, 124.9, 125.5, 125.6, 126.5, 127.8, 128.6, 128.8, 129.0, 129.5,
J ¼ 7.4 Hz, phenyl-H), 7.55 (d, 2H, J ¼ 7.2 Hz, phenyl-H); 13C NMR
129.6, 133.4, 137.4, 138.5, 140.0, 141.1, 142.0, 142.9; IR (KBr,
n
, cmꢀ1):
(100 MHz, CDCl3):
d
14.69, 120.45, 122.28, 125.58, 125.63, 125.78,
757, 786, 838, 861, 893, 992,1071,1166,1276,1400,1455,1618,1669,
2839, 3035.
128.10, 128.80, 129.05, 133.16, 142.59, 143.05. IR (KBr, n
, cmꢀ1): 689,
758, 835, 855, 894, 972,1028,1074,1120,1151,1200,1278,1330,1360,
1386, 1446, 1471, 1502, 1602, 1696, 2929, 3066.
2.2.9. [1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-methyl-4-phenyl-1-
phenyl)]perfluorocyclopentene (3o)
2.2.7. [1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-methyl-4-(3-
methoxylphenyl)-1-phenyl)]perfluorocyclopentene (1o)
To stirred anhydrous THF (50 mL) containing 7a (0.55 g, 2 mmol)
was added, dropwise, a 2.5 mol Lꢀ1 n-BuLi solution (0.96 mL) at
ꢀ78 ꢂC under argon atmosphere. After the mixture has been stirred
for 30 min at ꢀ78 ꢂC, compound 9 (0.73 g, 2 mmol) in anhydrous
3o was prepared by a method similar to that used for 1o. The
crude product was purified by column chromatography on SiO2
using petroleum ether as an eluent to give 3o (0.47 g, 46%) as
a yellow solid. M.p. 121e122 ꢂC; MS m/z (Mþ) 515.15; Calcd for
C29H20F6S (%): Calcd C, 67.70; H, 3.92. Found C, 67.78; H, 3.98; 1H
NMR (400 MHz, CDCl3): d 2.02 (s, 3H, eCH3), 2.05 (s, 3H, eCH3), 7.17

260
S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
(s, 1H, thienyl-H), 7.28 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.32e7.38
(m, 3H, phenyl-H), 7.41 (d, 2H, J ¼ 7.0 Hz, phenyl-H), 7.44 (s, 1H,
phenyl-H), 7.46e7.49 (m, 3H, phenyl-H), 7.53 (t, 1H, J ¼ 7.8 Hz,
phenyl-H), 7.58 (d, 2H, J ¼ 7.4 Hz, phenyl-H); 13C NMR (400 MHz,
using the mixture of petroleum ether and acetic ether (v/v ¼ 8/1) as
an eluent to give 4o (0.44 g, 41%) as a yellow solid. M.p. 151e153 ꢂC;
MS m/z (Mþ) 540.17; Calcd for C30H19F6NS (%): Calcd C, 66.78; H,
3.55; N, 2.60. Found C, 66.71; H, 3.59; N, 2.61; 1H NMR (400 MHz,
CDCl3):
d
14.9, 19.9, 122.7, 124.9, 125.5, 125.6, 126.6, 127.1, 127.8,
CDCl3): d 1.95 (s, 3H, eCH3), 2.00 (s, 3H, eCH3), 7.11 (s,1H, thienyl-H),
127.9, 128.9, 129.0, 129.6, 133.4, 137.5, 139.9, 141.2, 142.0, 142.7; IR
7.23 (d, 1H, J ¼ 7.4 Hz, phenyl-H), 7.30 (t, 3H, J ¼ 7.4 Hz, phenyl-
H), 7.42 (d, 4H, J ¼ 7.4 Hz, phenyl-H), 7.49 (t,1H, J ¼ 7.8 Hz, phenyl-H),
7.59 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.74 (d, 1H, J ¼ 8.0 Hz, phenyl-H),
(KBr,
n
, cmꢀ1): 757, 785, 857, 893, 991, 1067, 1164, 1275, 1401, 1453,
1616, 1666, 2622, 2856, 3043.
7.80 (s, 1H, phenyl-H); 13C NMR (400 MHz, CDCl3):
d 13.7, 18.8, 112.2,
2.2.10. [1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-methyl-4-(3-
cyanophenyl)-1-phenyl)]perfluorocyclopentene (4o)
117.5, 121.5,123.7, 124.2, 124.5,126.9,128.0, 128.5,128.7, 128.9,129.6,
130.2, 130.3, 132.2, 137.0, 139.3, 140.0, 140.2, 141.1; IR (KBr,
n
, cmꢀ1):
4o was prepared by a method similar to that used for 1o. The
crude product was purified by column chromatography on SiO2
757, 784, 831, 862, 891, 991, 1068, 1164, 1273, 1401, 1453, 1616, 1667,
2230, 2843, 3039.
0.5
A
B
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
Vis
0.4
UV
Vis
UV
0.3
UV
390 s
330 s
260
110
50
UV
270
0.2
80
UV
UV
40
15
0
Vis
Vis
20
0.1
0.0
0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
0.6
0.4
0.2
0.0
C
D
Vis
0.6
0.4
0.2
0.0
UV
Vis
UV
310 s
250
110
50
UV
UV
240 s
180
80
20
40
UV
Vis
UV
Vis
0
15
0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
E
F
0.6
0.4
0.2
0.0
Vis
UV
UV
270 s
180
80
35
15
0
UV
Vis
300
400
500
600
700
Wavelength (nm)
Fig. 3. Absorption spectra and color changes of diarylethenes 1e5 by photoirradiation in hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1) at room temperature: (A) spectral changes for 1; (B) spectral
changes for 2; (C) spectral changes for 3; (D) spectral changes for 4; (E) spectral changes for 5; (F) color changes for 1e5.

S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
261
Table 1
2.2.11. [1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-methyl-4-(3-
trifluoromethylphenyl)-1-phenyl)]perfluorocyclopentene (5o)
Absorption characteristics and photochromic reactivity of diarylethenes 1e5 in
hexane (2.0 ꢁ 10ꢀ5 mol/L) and in PMMA film (10%, w/w).
5o was prepared by a method similar to that used for 1o. The
crude product was purified by column chromatography on SiO2
using petroleum ether as an eluent to give 5o (0.49 g, 42%) as an
yellow oil. MS m/z (Mþ) 583.18; Calcd for C30H19F9S (%): Calcd C,
61.86; H, 3.29. Found C, 61.93; H, 3.35; 1H NMR (400 MHz, CDCl3):
Compound l0,max (nm)a
/L molꢀ1 cmꢀ1
lc,max (nm)b
/L molꢀ1 cmꢀ1
)
Fc
(
3
)
(3
Hexane
Hexane
PMMA film Foec Fceo
1
2
3
4
5
288 (2.47 ꢁ 104) 563 (5.16 ꢁ 103) 581
286 (2.88 ꢁ 104) 559 (3.44 ꢁ 103) 575
285 (3.42 ꢁ 104) 556 (6.11 ꢁ 103) 573
280 (2.89 ꢁ 104) 560 (5.32 ꢁ 103) 573
281 (3.33 ꢁ 104) 560 (4.65 ꢁ 103) 577
0.29 0.14
0.32 0.13
0.37 0.11
0.40 0.26
0.39 0.30
d
1.96 (s, 3H, eCH3), 2.00 (s, 3H, eCH3), 7.10 (s, 1H, thienyl-H), 7.21
(d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.28 (t, 2H, J ¼ 7.6 Hz, phenyl-H), 7.33
(s, 1H, phenyl-H), 7.39e7.41 (m, 3H, phenyl-H), 7.48 (t, 2H,
J ¼ 7.8 Hz, phenyl-H), 7.55 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.68 (d, 1H,
J ¼ 7.8 Hz, phenyl-H), 7.74 (s, 1H, phenyl-H); 13C NMR (400 MHz,
a
b
c
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring Foec) and closed-ring isomers (Fceo),
CDCl3):
d 14.8, 19.9, 122.6, 123.9, 124.5, 124.9, 125.3, 125.6, 127.5,
(
127.9, 129.0, 129.3, 129.6, 129.8, 130.4, 131.2, 131.5, 133.3, 137.9,
140.8, 141.1, 141.2, 142.1; IR (KBr,
, cmꢀ1): 752, 785, 829, 860, 889,
992, 1071, 1165, 1401, 1451, 1617, 1668, 2850, 3035.
respectively.
n
Fig. 4. Absorption spectra and color changes of diarylethenes 1e5 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) spectral changes for 4; (E) spectral changes for 5; (F) color changes for 1e5.

262
S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
3. Results and discussion
quantum yield of 4c is smaller than that of 5c. The absorption
maxima and cyclization quantum yields of diarylethenes 4 and 5 are
almost equal. Moreover, both the cyclization and cycloreversion
quantum yields of diarylethenes 4 and 5 bearing electron-with-
drawing groups are much higher than those of diarylethenes 1 and 2
bearing electron-donating groups. These results are quite different
from those reported for diarylethenes bearing a phenyl moiety,
where absorption maxima markedly decreased with the increase of
electron-withdrawing ability, and cycloreversion quantum yields
showed the reverse trend [36].
For practical applications in optical devices, it is very important
that photochromic materials maintain good photochromism in
a polymer film, such as PMMA [36,45]. Films were prepared by
dissolving 10 mg diarylethene sample and 100 mg PMMA in 1.0 mL
chloroform by ultrasonication, followed by spin-coating on the
surface of quartz substrate. In PMMA amorphous film, diarylethenes
1e5 also showed good photochromism (Fig. 4) similar to samples in
solution. The photochromic properties of diarylethenes 1e5 in PMMA
film are also summarized in Table 1. Upon irradiation with 313 nm
light, the colors of the diarylethene/PMMA films 1e5 changed from
colorless to blue, with the appearance of a new broad absorption band
centered at 581, 575, 573, 573 and 577 nm, respectively. This band was
assigned to the formation of the closed-ring isomers 1ce5c. In PMMA
solid media, the photostationary equilibriums of diarylethenes 1e5
are achieved by irradiation with light of wavelength 313 nm for 570,
555, 475, 405, and 445 s, respectively. All colored diarylethene/PMMA
films can revert to colorless upon irradiation with visible light
3.1. Photochromism of diarylethenes
The photochromic properties of diarylethenes 1e5 were
measured at room temperature both in hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1
)
and in PMMA medium (10%, w/w). Fig. 3 shows the changes in
the absorption spectra and color of diarylethenes 1e5 in hexane,
induced by alternating irradiation with UV and visible light of
appropriate wavelength. As shown in Fig. 3A, compound 1o
exhibited a sharp absorption peak at 288 nm in hexane, as a result of
a
p
ꢀ
p
* transition [43]. Upon irradiation with 297 nm light, a new
visible absorption band centered at 563 nm emerged while the
original peak at 288 nm decreased, indicating the formation of
the closed-ring isomer 1c; this change in absorption was manifest in
the colorless solution becoming violet. The photostationary state is
achieved upon irradiation with light of wavelength 297 nm for 390 s.
Alternatively, the violet solution could be bleached completely to
colorless upon irradiation with visible light (l > 450 nm) for 110 s,
indicating that 1c was returning to the initial state 1o. As with
diarylethene 1, compounds 2e5 also show good photochromism in
hexane (Fig. 3BeE). Upon irradiation with 297 nm light, absorption
bands in the visible region appeared and the solutions 2oe5o turned
violet due to cyclization reactions leading to the formation of the
closed-ring isomers 2ce5c. The photostationary state is achieved by
irradiation 330 s for 2, 310 s for 3, 240 s for 4, and 270 s for 5,
respectively. These compounds had the maxima absorption wave-
lengths in the visible region at 559, 556, 560, and 560 nm, respec-
tively. All of the violet-colored solutions 2ce5c can be decolorized by
irradiating with visible light of wavelengths greater than 450 nm,
indicating return to the open-ring isomers 2oe5o. The bleaching
irradiation time is 125 s for 2, 150 s for 3, 80 s for 4, and 65 s for 5,
respectively. In the photostationary state, the isosbestic points for
diarylethenes 1e5 were observed at 308, 304, 302, 303, and 305 nm,
respectively. The absorption spectral features of these compounds
are summarized in Table 1. The quantum yields were determined by
comparing the reaction yields of diarylethenes 1e5 in hexane
against 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene
in hexane at room temperature [39], and the results are also in
Table 1. The results showed that altering the substituent at 3-posi-
tion of the benzene ring had a significant effect on the photochromic
features of diarylethenes 1e5, including the absorption maxima,
molar absorption coefficients and quantum yields. In hexane, the
absorption maxima of diarylethenes 1oe5o decreased slightly with
the increasing electron-withdrawing ability. Among diarylethenes
1e5, the unsubstituted parent compound 3 has the greatest molar
absorption coefficient, both for its open-ring and closed-ring
isomers, but the lowest cycloreversion quantum yield. When
replacing the hydrogen atom at the meta-position of the terminal
phenyl ring in the biphenyl moiety with an electron-donating
substituent (methoxy or methyl group, such as in compound 1 or 2),
the absorption maxima and the cycloreversion quantum yields
increase to some extent, but the molar absorption coefficients and
the cyclization quantum yields show a clear reduction. When the
same position instead contains an electron-withdrawing substituent
(cyano or trifluoromethyl group, such as in compound 4 or 5), the
molar absorption coefficients are still significantly below that of
compound 3, but their absorption maxima, cyclization and cyclo-
reversion quantum yields have notably increased. The absorption
maximum and molar absorption coefficient of diarylethene 1c are
much larger than those of diarylethene 2c, due to the greater elec-
tron-donating ability of methoxy group as compared to a methyl
group [44], but the cyclization quantum yield of 1c is smaller than
that of 2c. Although the molar absorption coefficient of diarylethene
4c is much larger than that of diarylethene 5c, the cycloreversion
(l > 450 nm). The bleaching irradiation time is 125 s for 1, 130 s for 2,
160 s for 3, 108 s for 4, and 90 s for 5, respectively. As has
been observed for most of the reported diarylethenes [46e49], the
maximum absorption peaks of the closed-ring isomers 1ce5c are
found at longer wavelengths in PMMA film than those in hexane
solution. The red shift of the absorption maxima for the closed-ring
isomers are 18 nm for 1c,16 nm for 2c,17 nm for 3c,13 nm for 4c, and
17 nm for 5c. This red shift phenomena may be attributed to the polar
effect of the polymer matrix and the stabilization of molecular
arrangement in solid state [50,51].
3.2. Fluorescence of diarylethenes
Fluorescent properties can be useful in molecular scale opto-
electronics [52], digital fluorescence photoswitches [53,54] and ion-
sensors [55e58]. In our present work, the fluorescence properties
of diarylethenes 1e5, both in solution and in PMMA film, were
5000
In hexane
In PMMA film
4000
λ
em, 326
λ
em, 323
3000
2000
1000
0
350
400
450
Wavelength (nm)
500
550
Fig. 5. Emission spectra of diarylethene 1o both in hexane (1.0 ꢁ 10ꢀ4 mol Lꢀ1, excited
at 323 nm) and in PMMA film (10%, w/w, excited at 326 nm) at room temperature.

S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
263
Table 2
respectively. The Stokes shift of the fluorescence was relatively large
and red shifted, in comparison with the absorption edge. This type of
large Stokes shift has been discussed in detail by Sekiya and co-
workers [59]. When comparing samples in hexane to those in
a PMMA film, the emission peaks of diarylethenes 1oe5o consis-
tently exhibit a bathochromic shift across their maxima absorption
wavelengths with values of 14 nm for 1o,14 nm for 2o,14 nm for 3o,
9 nm for 4o, and 8 nm for 5o. When going from electron-donating to
electron-withdrawing substituents, the emission peaks of 1oe5o
gradually increased both in hexane (from 415 to 432 nm) and in
PMMA film (from 429 to 441 nm); but their emission intensity
decreased significantly when the electron-donating groups were
replaced with electron-withdrawing groups. The emission intensity
of 1o is the strongest and that of 4o is the weakest, however, the
emission peak of 1o is at highest energy while 4o is the lowest both
in hexane solution and in PMMA film. By using anthracene (0.27 in
acetonitrile) as the reference, the fluorescence quantum yields of
The fluorescence emission features of diarylethenes 1oe5o both in hexane
(1.0 ꢁ 10ꢀ4 mol Lꢀ1) and in PMMA film (10%, w/w) at room temperature.
lem, max (Relative intensity)
1o
2o
3o
4o
5o
Hexane
415 (3558) 420 (3329) 422 (3204) 432 (2228) 431 (2380)
PMMA film 429 (4386) 434 (3738) 436 (3467) 441 (2597) 439 (2753)
measured at room temperature using a Hitachi F-4500 fluorimeter.
The fluorescence emission spectra of diarylethene 1o both in hexane
(1.0 ꢁ 10ꢀ4 mol Lꢀ1) and in PMMA film (10%, w/w) are illustrated in
Fig. 5. The emission features of diarylethenes 2oe5o are summa-
rized in Table 2. When dissolved in hexane, diarylethenes 1oe5o,
excited at 323 nm, exhibited emission peaks at 415, 420, 422, 432,
and 431 nm, respectively. When excited at 326 nm, emission peaks
of 1oe5o in PMMA were observed at 429, 434, 436, 441, and 439 nm,
A
B
4000
3000
2000
1000
0
4000
UV
UV
0
0
Vis
20
30
Vis
UV
30
60
UV
3000
80
105
220
580 s
170
650 s
2000
1000
0
350
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
C
D
2500
2000
1500
1000
500
UV
0
Vis
UV
0
3000
2000
1000
0
20
Vis
UV
20
50
UV
30
160
320
550 s
80
180
400 s
0
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
E
2500
UV
Vis
0
UV
30
2000
1500
1000
500
0
90
130
190
460 s
400
450
500
550
Wavlength (nm)
Fig. 6. Emission intensity changes of diarylethenes 1e5 in hexane (1.0 ꢁ 10ꢀ4 mol Lꢀ1) upon irradiation with 313 nm UV light at room temperature, excited at 323 nm: (A) 1; (B) 2;
(C) 3; (D) 4; (E) 5.

264
S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
1oe5o were determined to be 0.0221, 0.0214, 0.0205, 0.0201 and
0.0203, respectively. The results showed that the electron-donating
substituent could be effective to increase the efficiency of the fluo-
rescence and decrease the emission peak of diarylethenes bearing
a biphenyl unit.
As has been observed for most of the reported diarylethenes
[25,60e65], diarylethenes 1e5 exhibited a very good fluorescent
switch on changing from the open-ring isomers to closed-ring
isomers by photoirradiation both in hexane and in PMMA film. Upon
irradiation with UV light, the photocyclization reaction was carried
out and the non-fluorescent closed-ring isomers 1ce5c were
produced. Back irradiation of the appropriate wavelength of visible
light regenerated the open-ring isomers 1oe5o and duplicated the
original emission spectra. During the process of photoisomerization,
the five compounds exhibited changes in their fluorescence in
hexane as shown in Fig. 6. Upon irradiation with 297 nm UV light for
several hundred seconds varying from 400 to 650 s, the samples
arrived at the photostationary state, resulting in emission intensities
of diarylethenes 1e5 quenched to ca. 22%, 18%, 20%, 16%, and 16%,
respectively. The back irradiation with visible light (l > 450 nm)
regenerated the open-ring isomers and reproduced the original
emission spectra. When their emission intensities reverted to those
of original values, the irradiation times were 200 s for 1, 225 s for 2,
240 s for 3, 160 s for 4, and 150 s for 5, respectively. The similar
emission intensity changes of diarylethenes 1e5 in PMMA film
during the process of photoisomerization are shown in Fig. 7. Upon
irradiation with 313 nm UV light, emission intensities decreased
remarkably along with the photoisomerization from the open-ring
isomers to the closed-ring isomers when excited at 326 nm. When
the samples arrived at the photostationary state by irradiation with
UV light for several hundreds seconds varying from 408 to 568 s, the
emission intensities of diarylethenes 1e5 were quenched to ca. 28%,
A
B
4000
5000
UV
0
Vis
0
UV
3
UV
Vis
3
8
UV
4000
3000
2000
1000
0
8
3000
2000
1000
0
18
78
548 s
38
158
568 s
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
C
D
4000
3000
2000
1000
0
3000
2000
1000
0
0
0
UV
UV
5
3
Vis
Vis
18
8
UV
UV
38
18
78
476 s
158
408 s
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
E
3000
Vis
UV
0
UV
3
8
18
78
2000
1000
0
448 s
400
450
500
550
Wavelength (nm)
Fig. 7. Emission intensity changes of diarylethenes 1e5 in PMMA film (10%, w/w) upon irradiation with 313 nm UV light at room temperature, excited at 326 nm: (A) 1; (B) 2; (C) 3;
(D) 4; (E) 5.

S. Pu et al. / Dyes and Pigments 87 (2010) 257e267
265
16%, 20%, 25%, and 30%, respectively. In PMMA film, the recovering
emission intensities of diarylethenes 1e5 are obtained by irradiation
with visible light for 130, 145, 150, 110, and 100 s, respectively. After
20 repeat cycles, no obvious changes in the fluorescent modulation
efficiency of these diarylethenes were observed, both in hexane
and in PMMA films. Compared with those reported diarylethenes
[36,47,48], the fluorescent modulation efficiencies of diarylethenes
1e5 were significantly enhanced both in the liquid and solid states.
This indicated that diarylethenes bearing a biphenyl unit have the
potential use as fluorescent modulation switches [66,67]. The resi-
dues of fluorescent modulation efficiencies of diarylethenes 1oe5o
may be attributed to the incomplete cyclization reaction and the
existence of parallel conformations [28,47,48,68,69]. In addition,
the average time to switch from an ‘‘on’’ and ‘‘off’’ state shortened in
proportion to the reciprocal power of radiated light as the power of
the UV and visible light was changed, indicating that the switching
effect is indeed photochemical [61].
The oxidation onsets of 1oe5o were at þ1.65, 1.68, 1.65, 1.64, and
1.67 V, respectively. The CV curves for 1ce5c were at þ1.70, 1.55,
1.63, 1.62, and 1.69 V, respectively. The differences of oxidation
onset between the open-ring and closed-ring isomers of diary-
lethenes 1e5 were ꢀ0.05 V for 1, 0.13 V for 2, 0.02 V for 3, 0.02 V for
4, and ꢀ0.02 V for 5. This shows that the oxidation process for the
open-ring isomers 2oe4o occurs at higher potentials than in
the corresponding closed-ring isomers 2ce4c whereas the reverse
occurs for diarylethenes 1 and 5. The reason for the anomalous high
oxidation potentials of 1c and 5c is currently not clear and further
work is in progress. To the best of our knowledge, this is the first
diarylethene example for which oxidation onset of the open-ring
isomers is lower than that of their closed-ring isomers. Moreover,
there are great differences amongst the electronic current and
polarization curve shapes between the open-ring and closed-ring
isomers of diarylethenes 1e3, but not for diarylethenes 4 and 5 at
the scanned voltage region. These results suggest that the different
electron-donating/withdrawing substituents have a significant
effect on the electrochemical properties of these diarylethene
derivatives bearing a biphenyl moiety but further work is required
to quantify these effects.
3.3. Electrochemistry of diarylethenes
It is well known that the ring opening and closing trans-
formation of some diarylethenes can be initiated not only by UV or
visible light irradiation, but also by an electrochemical redox
process [70]. The electrochromic behaviour of diarylethenes have
attracted much attention [1,5,18,71].
Cyclic voltammetry (CV) was performed on the diarylethenes
1e5 under identical experimental conditions at a scanning rate of
100 mV/s. The CV curves of diarylethenes 1e5 are shown in Fig. 8.
4. Conclusion
In summary, five new photochromic diarylethenes possessing
a biphenyl moiety were synthesized to investigate substituent
effects upon their properties. These new photochromic systems
showed good photochromism and acted as a remarkable fluorescent
switch both in solution and in PMMA film. Their distinguishable
optical and electrochemical characteristics may be attributed to the
different substituent effects. The biphenyl aryl moiety induced
some new dramatic properties differing from other diarylethenes
with five-membered aryl moieties reported so far. The results will be
helpful for the synthesis of efficient photoactive diarylethene
derivatives with new molecular skeletons and to understand the
substitution effects in diarylethenes bearing a biphenyl unit to aid
the rational design of novel diarylethenes for potential application in
optoelectronics and other fields.
1o
A
1c
B
2o
2c
Acknowledgements
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 Science
Funds of the Education Office of Jiangxi, China (GJJ08365).
C
3o
3c
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