Synthesis and photochromism of isomeric unsymmetrical diarylethenes bearing both naphthalene and thiophene moieties

Article abstract of DOI:10.1016/j.jphotochem.2012.06.003

Three new isomeric unsymmetrical diarylethenes bearing both naphthalene and thiophene moieties were synthesized. Their photochromism, fluorescence and electrochemistry properties were investigated in detail. Each of the diarylethene derivatives showed not

Full text of DOI:10.1016/j.jphotochem.2012.06.003

Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and photochromism of isomeric unsymmetrical diarylethenes bearing
both naphthalene and thiophene moieties
Renjie Wang, Shouzhi Pu∗, Gang Liu, Shiqiang Cui, 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:
Three new isomeric unsymmetrical diarylethenes bearing both naphthalene and thiophene moieties
were synthesized. Their photochromism, fluorescence and electrochemistry properties were investigated
in detail. Each of the diarylethene derivatives showed notable photochromism and functioned as an
effective fluorescent switch both in solution and in PMMA films. The cyclization quantum yield and the
molar absorption coefficients of both the open- and closed-ring isomers were found to decrease based
on the position of the chloro substituent attached to the terminal benzene ring as follows: para- > meta-
> the ortho-position. Consequently, the cycloreversion quantum yield would increase para- to meta- to
ortho-chloro substitution. When compared with the unsubstituted parent compound, the presence of
the chloro substituent on the terminal benzene ring of these diarylethenes, regardless of the substitution
position, would considerably lower the band gaps of the open- and closed-ring isomers. Our results
indicated that the chlorine atom and its substitution position have significant influence on the optical
and electrochemical properties of the diarylethenes bearing a naphthalene moiety.
Received 12 January 2012
Received in revised form 25 May 2012
Accepted 7 June 2012
Available online 16 June 2012
Keywords:
Photochromism
Diarylethene
Chlorine atom position effect
Fluorescent switch
Electrochemistry
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
capability. Therefore, it is necessary to develop new photochromic
materials with bistable photoswitching of fluorescence emission
Photochromic materials have received intense interest because
of their potential promising applications in optical memory and
switching devices [1–5]. In the past few years, various types of
photochromic compounds have been synthesized in an attempt to
apply them to optoelectronic devices. In particular, a lot of attention
was focused on molecular devices due to the possibility of incor-
porating them as molecular switches and into integrated circuits
[6]. Typical molecular switches are compounds that interconvert
between two different states reversibly using an external stimulus
such as light, electricity, chemical reagents, or any non-destructive
trigger that can be employed to induce a reversible change in
the system [7]. These properties enable the storage of informa-
tion at the molecular level, which has the potential to significantly
influence the development of optoelectronic materials science and
information technologies [2,8]. Photochromic molecular switches
could meet this requirement, because they possess reversible
photo-induced transformations that regulate electrical conductiv-
ity from a conjugated “on” state to a non-conjugated “off” state
upon photoirradiation. In order to realize the application of molec-
ular switches as optical memory media, one essential requirement
is that photochromic materials must have non-destructive readout
for the sake of achieving detection with minimal damage to data.
Diarylethenes with heterocyclic aryl groups are the most
promising candidates for practical applications owing to their
notable fatigue resistance, high reactivity and excellent thermal
stability of the respective isomers [9–15]. In recent years, some
studies aiming at the potential application of diarylethenes have
focused considerably on the design of diarylethene derivatives with
tunable optoelectronic properties [16–19], and the subsequent
investigation into their fundamental properties have contributed
to a broad understanding of the photochromism of diarylethenes
[20–24]. Generally, the photochromic reactivity of diarylethenes
mainly depends on the types of heteroaryl groups and elec-
tron donor/acceptor substituents. The nature of heteroaryl moiety
greatly influences the photo-reactivity and the unique features of
diarylethenes. For example, the diarylethenes bearing two indole
units are excellent fluorescent photoswitches [25,26], and those
with thiophene/benzothiophene moieties show excellent thermal
stability and outstanding fatigue resistance [9,11]. However, the
diarylethenes bearing two pyrrole groups are thermally unstable
and return to the open-ring isomers even in the dark [27]. Similarly,
the types of substituents and their substitution positions also have
notable effects on diarylethene photochromic properties [28–30].
In general, the introduction of electron-donating substituents to
bis(3-thienyl)ethene diarylethenes would decrease the cyclorever-
sion quantum yields and increase the absorption coefficients of the
∗
Corresponding author. Tel.: +86 791 83831996; fax: +86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.06.003

48
R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
Br
F
F
F
F
F
Br
F
F
R
F
F
F
Br
6a:R = para
6b:R = meta
6c:R = ortho
Cl
Cl
Cl
F
F
UV
S
Pd(PPh3)4
NaCO3 aq.
R
F
B(OH)2
S
5
6d:R =
H
Vis
6
S
S
F
F
R
R
F
Br
para Cl
meta Cl
ortho Cl
H
para Cl
meta Cl
ortho Cl
H
1o: R =
2o: R =
3o: R =
4o: R =
1o: R =
2o: R =
3o: R =
4o: R =
F
F
1) n-BuLi/THF
F
2) C5F8,195 K
7
8
Scheme 1. Photochromism of diarylethenes 1o–4o.
F
F
F
F
F
1o:R = para
2o:R = meta
3o:R = ortho
Cl
Cl
Cl
F
closed-ring isomers, while those attached to bis(2-thienyl)ethene
diarylethenes would reduce the cyclization quantum yields and
shift the absorption maxima of the open-ring isomers to a longer
wavelength [31–33].
n-BuLi/THF
4o:R =
H
195 K
S
R
Presently, the design of new photochromic diarylethene with
different aryl moieties has become an active area of research.
So far, the reported diarylethene systems are mainly confined
to the five-membered heteroaryl rings, but diarylethenes based
on the hybrid backbone of five- and six-membered aryl moieties
are very rare. In a previous report, we have developed a new
class of diarylethenes bearing both naphthalene and thiophene
and found that they exhibit excellent photochromism with
good fatigue resistance and thermal stability [34]. Our results
also indicated that the substituents have a significant effect on
the properties of these diarylethenes. In this study, in order to
further elucidate the effect of substituent position on the pho-
tochromic features of diarylethenes bearing both naphthalene
and thiophene moieties, we have synthesized three new iso-
meric diarylethenes with a chlorine atom at either the para-,
meta- or ortho-position on the terminal benzene ring: 1-(2-
methyl-1-naphthyl)-2-[2-methyl-5-(4-chlorophenyl)-3-thienyl]
Scheme 2. Synthetic route for diarylethenes 1o–4o.
with a thiophene boronic acid (5) [35]. Then compounds 6a–d
were separately lithiated, and then coupled with (2-methyl-1-
naphthyl)perfluorocyclopentene (8) [34] to give the unsymmetrical
diarylethene derivatives 1o–4o. The structures of 1o–4o were con-
firmed by elemental analysis, NMR, and IR.
2.2.1. 3-Bromo-2-methyl-5-(4-chlorophenyl)thiophene (6a)
Compound 6a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid (5) [36] (2.58 g, 11.3 mmol) with 1-bromo-4-
chlorobenzene (2.17 g, 11.3 mmol) in the presence of Pd(PPh3)4
(0.15 g, 0.01 mmol) and Na2CO3 (2 mol L−1, 50 mL) in tetrahydro-
furan (THF) (90 mL). After refluxing for 16 h, the organic layer was
dried over MgSO4, filtrated and evaporated. The crude product was
purified by column chromatography on SiO2 using petroleum ether
as the eluent to give 2.6 g 6a as a pale yellow solid in 81% yield. M.p.
342–343 K. 1H NMR (400 MHz, CDCl3, TMS): ı 2.44 (s, 3H, CH3),
7.11 (s, 1H, thiophene–H), 7.36 (d, 2H, benzene–H, J = 8.0), 7.45 (d,
1H, benzene–H, J = 8.0).
perfluorocyclopentene
(1o),
1-(2-methyl-1-naphthyl)-2-[2-
methyl-5-(3-chlorophenyl)-3-thienyl] perfluorocyclopentene (2o)
and 1-(2-methyl-1-naphthyl)-2-[2-methyl-5-(2-chlorophenyl)-3-
thienyl] perfluorocyclopentene (3o). Moreover, the unsubstituted
parent diarylethene 1-(2-methyl-1-naphthyl)-2-(2-methyl-5-
phenyl-3-thienyl)perfluorocyclopentene (4o) previously reported
[34] is also included for comparison. The photochromic scheme of
the four diarylethenes is shown in Scheme 1.
2.2.2. 3-Bromo-2-methyl-5-(3-chlorophenyl)thiophene (6b)
Compound 6b was prepared by an analogous method similar
to that used for to 6a and obtained as a pale yellow liquid in 66%
yield. 1H NMR (400 MHz, CDCl3, TMS): ı 2.44 (s, 3H, CH3), 7.12
(s, 1H, thiophene–H), 7.24–7.32 (m, 2H, benzene–H), 7.49 (m, 1H,
benzene–H, J = 8.0), 7.62 (d, 1H, benzene–H, J = 8.0).
2. Experimental
2.1. General
All solvents used were spectroscopic grade and were puri-
fied by distillation before use. NMR spectra were recorded on a
Bruker AV400 (400 MHz) spectrometer with CDCl3 as the solvent
and tetramethylsilane as an internal standard. Infrared spectra
(IR) were recorded on a Bruker Vertex-70 spectrometer. Melting
point was taken on a WRS-1B melting point apparatus. Absorption
spectra were measured using an Agilent 8453 UV/VIS spectropho-
tometer. Photoirradiation was carried out using an SHG-200 UV
lamp, a CX-21 ultraviolet fluorescence analysis cabinet and a BMH-
250 visible lamp. The required wavelength was isolated by the
use of the appropriate filters. Fluorescence spectra were measured
using a Hitachi F-4500 spectrophotometer.
2.2.3. 3-Bromo-2-methyl-5-(2-chlorophenyl)thiophene (6c)
Compound 6c was prepared by an analogous method similar
to that used for to 6a and obtained as a pale yellow liquid in 75%
yield. 1H NMR (400 MHz, CDCl3, TMS): ı 2.42 (s, 3H, CH3), 7.15
(s, 1H, thiophene–H), 7.19–7.25 (m, 2H, benzene–H), 7.27–7.31
(t, 1H, benzene–H, J = 8.0), 7.38 (d, 1H, benzene–H, J = 8.0).
2.2.4. 3-Bromo-2-methyl-5-phenylthiophene (6d)
Compound 6d was prepared by an analogous method similar to
that used for to 6a and obtained as a light yellow solid in 72% yield.
M.p. 339–341 K; 1H NMR (400 MHz, CDCl3): ı 2.34 (s, 3H, −CH3),
7.02 (s, 1H, thiophene–H), 7.20 (d, 1H, benzene–H, J = 8.0), 7.29
(t, 2H, benzene–H, J = 8.0) and 7.42 (d, 2H, benzene–H, J = 8.0).
2.2. Synthesis
The synthesis route for diarylethenes 1o–4o is shown in
Scheme 2. First, the phenylthiophene derivatives (6a–d) were
prepared by Suzuki coupling of five bromobenzene derivatives

R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
49
2.2.5. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(4-
chlorophenyl)-3-thienyl]perfluorocyclopentene
(1o)
2.2.7. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(2-
chlorophenyl)-3-thienyl]perfluorocyclopentene
(3o)
To a stirred anhydrous THF containing 6a (0.5 g, 1.74 mmol)
was added dropwise
Diarylethene 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 the eluent to give 0.32 g 3o as
a pale yellow solid in 36% yield. Calcd. for C27H17ClF6S (%): Calcd.
C, 62.01; H, 3.28. Found C, 62.00; H, 3.26; M.p. 341–342 K; 1H NMR
(400 MHz, CDCl3, ppm): ı 2.32 (s, 3H, −CH3), 2.35 (s, 3H, −CH3), 6.90
(s, 1H, thiophene–H), 7.10–7.18 (m, 3H, benzene–H), 7.27–7.29 (m,
1H, benzene–H), 7.34 (d, 1H, naphthalene–H, J = 8.0 Hz), 7.42–7.57
(m, 2H, naphthalene–H), 7.68 (d, 1H, naphthalene–H, J = 8.0 Hz),
7.84 (t, 2H, naphthalene–H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3,
TMS): ı = 14.61, 20.32, 114.34, 116.09, 116.34, 116.64, 123.38,
124.24, 124.83, 125.60, 126.50, 126.81, 127.03, 127.81, 128.62,
128.53, 129.91, 130.52, 131.77, 131.87, 131.94, 135.51, 136.89,
140.91, 141.87; IR (ꢀ, KBr, cm−1): 754, 775, 817, 835, 850, 865, 892,
986, 1047, 1102, 1134, 1186, 1225, 1261, 1277, 1344, 1433, 1491,
1509, 1597, 2362, 3349, 3652.
a
2.4 mol L−1 n-BuLi/hexane solution
(0.77 mL, 1.91 mmol) at 195 K under argon atmosphere.
After the mixture has been stirred for 30 min, (2-methyl-1-
naphthyl)perfluorocyclopentene (8) (0.53 g, 1.74 mmol) in solvent
of anhydrous THF was added. The reaction was further stirred at
195 K for 2 h, and the reaction was allowed to slowly warm to
the room temperature. The reaction was quenched with distilled
water. The product was extracted with diethyl ether, dried with
MgSO4, and concentrated under reduced pressure. The crude
product was purified by column chromatography using petroleum
ether as the eluent to afford to 0.36 g compound 1o as a colorless
solid in 40% yield. Calcd. for C27H17ClF6S (%): Calcd. C, 62.01; H,
3.28. Found C, 62.00; H, 3.27; M.p. 379–380 K 1H NMR (400 MHz,
CDCl3, ppm): ı 2.18 (s, 3H, −CH3), 2.34 (s, 3H, −CH3), 6.81 (s,
1H, thiophene–H), 7.09 (d, 2H, benzene–H, J = 8.0), 7.21 (d, 1H,
benzene–H, J = 8.0 Hz), 7.29 (d, 1H, benzene–H), 7.32 (d, 1H,
naphthalene–H, J = 8.0 Hz), 7.41–7.57 (m, 2H, naphthalene–H),
7.69 (d, 1H, naphthalene–H, J = 8.0 Hz), 7.85 (t, 2H, naphthalene–H,
J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS): ı = 15.25, 20.43,
123.10, 123.32, 123.63, 123.76, 124.22, 124.91, 125.65, 126.49,
126.63, 126.99, 127.61, 128.31, 128.53, 128.98, 130.05, 130.51,
131.77, 131.85, 133.43, 135.48, 139.87, 141.25; IR (ꢀ, KBr, cm−1):
731, 744, 771, 810, 866, 893, 983, 1011, 1048, 1107, 1131,
1194, 1274, 1339, 1439, 1466, 1500, 1548, 1640, 2860, 3409,
3695.
2.2.8. 1-(2-Methyl-1-naphthyl)-2-(2-methyl-5-phenyl-3-
thienyl)perfluorocyclopentene
(4o)
Diarylethene 4o was prepared by a method similar to that used
for 1o. The crude product was purified by column chromatogra-
phy on SiO2 using petroleum ether as the eluent to give 0.87 g
4o as a pale green oil liquid in 60% yield. Calcd. for C27H18F6S
(%): Calcd. C, 66.39; H, 3.71. Found C, 66.46; H, 3.79; 1H NMR
(400 MHz, CDCl3, ppm): ı 2.13 (s, 3H, CH3), 2.30 (s, 3H, CH3),
6.81 (s, 1H, thiophene–H), 7.20 (d, 2H, benzene–H, J = 8.0 Hz),
7.33 (t, 2H, benzene–H, J = 8.0 Hz), 7.42 (d, 1H, naphthalene–H),
7.42 (m, 3H, naphthalene–H), 7.64 (d, 1H, benzene–H), 7.76 (t,
2H, naphthalene–H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS):
ı = 14.85, 20.44, 123.38, 123.73, 124.97, 125.37, 125.52, 125.64,
126.96, 127.63, 128.31, 128.55, 128.85, 130.04, 130.47, 131.91,
133.31, 135.54, 140.88, 141.23; IR (KBr, ꢀ, cm−1): 756, 774, 815,
871, 891, 984, 1031, 1134, 1191, 1272, 1339, 1445, 1470, 1505,
1554, 1599, 1739, 1944, 2862, 2926, 3062.
2.2.6. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(3-
chlorophenyl)-3-thienyl]perfluorocyclopentene
(2o)
Diarylethene 2o was prepared by a method similar to that used
for 1o. The crude product was purified by column chromatogra-
phy on SiO2 using petroleum ether as the eluent to give 0.34 g
2o as a colorless solid in 38% yield. Calcd. for C27H17ClF6S (%):
Calcd. C, 62.01; H, 3.28. Found C, 62.01; H, 3.28; M.p. 349–350 K;
1H NMR (400 MHz, CDCl3, ppm): ı 2.18 (s, 3H, −CH3), 2.34 (s, 3H,
−CH3), 6.85 (s, 1H, thiophene–H), 7.06–7.07 (m, 2H, benzene–H),
7.13 (s, 2H, benzene–H), 7.33 (d, 1H, naphthalene–H, J = 8.0 Hz),
7.46–7.56 (m, 2H, naphthalene–H), 7.69 (d, 1H, naphthalene–H,
J = 8.0 Hz), 7.84 (t, 2H, naphthalene–H, J = 8.0 Hz); 13C NMR
(100 MHz, CDCl3, TMS): ı = 15.30, 20.48, 123.29, 123.43, 124.21,
124.86, 125.00, 125.36, 125.70, 127.05, 127.54, 128.37, 128.56,
130.11, 131.86, 134.78, 134.96, 135.49, 139.51, 141.69; IR (ꢀ,
KBr, cm−1): 743, 773, 811, 832, 870, 893, 976, 987, 1048, 1083,
1105, 1133, 1192, 1274, 1341, 1425, 1497, 1572, 1596, 3061,
3632.
3. Results and discussion
3.1. Photoisomerization of diarylethenes
The photochromic behaviors of diarylethenes 1–4 induced by
photoirradiation at room temperature have been measured both
in hexane (2.0 × 10−5 mol L−1) and in ploy(methyl methacrylate)
(PMMA) films (10%, w/w). The changes in the absorption spectra
of diarylethene 1 by alternating irradiation with UV and visible
light are shown in Fig. 1. In hexane, the colorless solution contain-
ing the open-ring isomer 1o exhibited a sharp absorption peak at
1.2
0.72
A
B
Vis
Vis
UV
UV
0.9
0.6
0.3
0.0
0.54
UV
0.36
0.18
0.00
Vis
UV
Vis
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption spectral changes of diarylethene 1 by photoirradiation at room temperature: (A) in hexane (2.0 × 10−5 mol L−1), (B) in a PMMA film.

50
R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
Fig. 2. The color changes of diarylethenes 1–4 by photoirradiation at room temperature: (A) in hexane, (B) in PMMA films. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of the article.)
263 nm (ε, 3.24 × 104 L mol−1 cm−1), which was due to the ␲ → ␲*
derivatives, such as the absorption maxima, molar absorption
coefficients, and the quantum yields of cyclization and cyclorever-
sion reactions. For the isomeric diarylethenes 1–3, the absorption
maxima of the closed-ring isomers 1c–3c shifted to shorter wave-
lengths in the order of para- > meta- > the ortho-substitution. As a
result, the absorption maximum of the closed-ring isomer 1c is
the longest (513 nm), while that of 3c is the shortest (499 nm).
The results are in well agreement with those of the reported
diarylethenes bearing an electron-withdrawing cyano group [42].
In contrast, the absorption maximum of diarylethene bearing the
ortho-substituted methoxy group was found to be the longest, and
that of para-substituted methoxy group was the shortest [43,44].
In hexane, the molar absorption coefficients of 1o–3o and 1c–3c
decreased in the order of para- > meta- > the ortho-substitution by
the chlorine group. The cyclization and cycloreversion quantum
yields have been measured by the reported method [31], and the
values are also summarized in Table 1. The results revealed that
the cyclization quantum yield increased based on the position of
chlorine atom in the order of ortho- < meta- < the para-substitution.
Among the three isomeric derivatives, the cyclization yield of 1 is
the highest (˚o–c = 0.29) and that of 3 is the lowest (˚o–c = 0.20).
However, the cycloreversion quantum yield trend of diarylethenes
1–3 was opposite to that of the cyclization quantum yield, such
that the yield increases as the chlorine atom attached to the ben-
zene ring was substituted at the para- < meta- < the ortho-position.
In comparison with diarylethenes 1–3, the unsubstituted parent
diarylethene 4 has the greatest cyclization and cycloreversion
quantum yields. The results may be explained by considering the
different dihedral angle between the phenyl ring and the thiophene
ring due to the different position of chloro-substituent. The para-
substituted chloro atom does not ever introduce any torsion on the
dihedral angle between the phenyl ring and the thiophene ring,
while the angle is larger when the chloro-substituent is attached
on the ortho-position. As a result, diarylethene 3 has the lowest
conjugation among the three derivatives, which leads to the small-
est absorption coefficients and blue shift of the absorption spectra
of the closed ring isomer. Meanwhile, the lowest conjugation also
makes the cyclization quantum yield lowest and the cycloreversion
transition [37]. Upon irradiation with 297 nm light, the colorless
solution of 1o turned red and a new visible absorption band was
observed at 513 nm (ε, 2.23 × 104 L mol−1 cm−1) due to the forma-
tion of the closed-ring isomer 1c. Alternatively, the red colored
solution could be bleached completely upon irradiation with visible
light (ꢁ > 500 nm), resulting from the regeneration of the open-ring
isomer 1o. When arrived at the photostationary state, the isosbestic
point of diarylethene 1 was observed at 294 nm, which supported
the reversible two component photochromic reaction schemes
[38]. Similarly, diarylethenes 2–4 also showed photochromism in
hexane. As shown in Fig. 2, the solutions containing 2o–4o turned
from colorless to red upon irradiation with 297 nm light as a result
of the cyclization reactions to produce the closed-ring isomers
2c–4c. The corresponding absorption maxima were observed at 510
for 2c, 499 for 3c, and 508 nm for 4c. All the solutions of 2c, 3c and
4c can be decolorized by irradiation with visible light (ꢁ > 500 nm)
to regenerate the open-ring isomers 2o, 3o and 4o, respectively.
The isosbestic points for diarylethenes 2, 3 and 4 were observed
at 286, 283 and 289 nm, respectively, in the photostationary state.
Additionally, the photoconversion ratios from the open-ring to the
closed-ring isomers of diarylethenes 1–4 at the photostationary
state were analyzed by HPLC, and the values were 65% for 1, 62%
for 2, 55% for 3, and 63% for 4 (Table 1). For practical applications in
optical devices, it is very important for photochromic materials to
maintain good photochromism in a polymer film (e.g. PMMA film)
[39]. PMMA amorphous films with diarylethenes 1–4 also showed
similar photochromism as in hexane, and their color changes upon
photoirradiation are shown in Fig. 2. When compared to those in
hexane, the absorption maxima of the closed-ring isomers in PMMA
films are shifted to longer wavelengths; the observed increase were
6 nm for 1c, 9 nm for 2c, 2 nm for 3c, and 16 nm for 4c. This may
be attributed to the polar effect of the polymer matrix and the
stabilization of molecular arrangement in a solid medium [40,41].
The photochromic parameters of compounds 1–4 in hexane and
in PMMA films are summarized in Table 1. It is apparent that the
position of the substituent at the terminal benzene ring has a signif-
icant effect on the photochromic properties of these diarylethenes
Table 1
Absorption spectral properties of diarylethenes 1–4 in hexane (2.0 × 10−5 mol L−1) and in PMMA films (10%, w/w) at room temperature.
c
Compound
ꢁo,max/nma (ε/L mol−1 cm−1
)
ꢁc,max/nmb (ε/L mol−1 cm−1
)
˚
Conversion at PSS in hexane
Hexane
PMMA film
Hexane
PMMA film
˚
o–c
˚
c–o
1
2
3
4
263 (3.24 × 104)
263 (3.00 × 104)
263 (2.62 × 104)
261 (2.76 × 104)
293
265
267
264
513 (2.23 × 104)
510 (2.00 × 104)
499 (1.49 × 104)
508 (1.50 × 104)
519
519
501
524
0.29
0.23
0.20
0.31
0.10
0.12
0.15
0.21
65
62
55
63
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring (˚o–c) and closed-ring isomers (˚c–o), respectively.
b
c

R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
51
quantum yield highest. The results were well consistent with that
of the reported diarylethenes bearing two thiophene rings [31,45].
Moreover, the naphthalene moiety also has a significant effect on
the photochromic properties of diarylethenes 1–4. When the naph-
thalene unit was replaced with a thiophene moiety in the same
molecular skeleton of photochromic diarylethenes, the absorption
maxima of both the open- and closed-ring isomers showed a big
bathochromic shift [30]. Therefore, the introduction of naphtha-
lene moiety into the hexatriene skeleton of diarylethene could
effectively shift the absorption maximum to a shorter wavelength.
The thermal stability of diarylethene mainly depends on the
aromatic stabilization energy of the aryl moiety [46], and it is
an indispensable property for optical memory media application
[47]. Uchida et al. reported some photochromic dinaphthylethene
derivatives with excellent thermal stability, which the absorption
intensity of the closed-ring isomer remains almost constant for
more than 500 h even at 343 K [48]. Based on the semi-empirical
MOPAC AM 1 method, the energy of dinaphthalene derivative was
calculated to be 30.27 kcal mol−1. This value is much lower than
that of diphenylethene derivative (43.87 kcal mol−1), but it is much
a significant effect on the fatigue resistances of the three isomeric
diarylethenes.
3.2. Fluorescence of diarylethenes
The fluorescent features of diarylethenes 1o–4o both in hexane
(2.0 × 10−5 mol L−1) and in PMMA films (10%, w/w) were measured
at room temperature. As shown in Fig. 5, their emission peaks in
hexane and in PMMA films were observed at 432 nm (ꢁex, 320 nm)
for 1o, 402 nm (ꢁex, 309 nm) for 2o, 410 nm (ꢁex, 307 nm) for 3o,
and 420 nm (ꢁex, 298 nm) for 4o, and at 435 nm (ꢁex, 321 nm) for
1o, 429 nm (ꢁex, 300 nm) for 2o, 422 nm (ꢁex, 300 nm) for 3o, and
428 nm (ꢁex, 300 nm) for 4o, respectively. In comparison with the
hexane samples, the fluorescence emission peaks of diarylethenes
1o–4o in PMMA films consistently exhibit a bathochromic shift,
where the shifts observed were 3 nm for 1o, 17 nm for 2o, 12 nm
for 3o, and 8 nm for 4o. Among the isomeric derivatives 1o–3o,
the fluorescence emission intensity of the ortho-substituted deriva-
tive 3o is the strongest and that of the meta-substituted derivative
2o is the weakest in hexane and in a PMMA film, which may be
attributed to the steric hindrance of ortho-substituted chlorine and
the resonance and polar effect of chlorine atom. On the one hand,
the thiophene–phenyl bond rotation is strained by ortho-steric hin-
drance. Although the chlorine atom can exhibit an electro-donating
character in terms of resonance effect, it can also exhibit an electro-
withdrawing character in terms of polar effect [30]. Compared with
diarylethenes 1–3, the unsubstituted parent diarylethene 4 showed
the weakest fluorescence emission intensity both in hexane and in
a PMMA film.
The solvent effects on the fluorescence features of diarylethenes
1o–4o were measured in different solvents at room temperature,
and the results are summarized in Table 2. From these data, it
can be easily seen that the emission peaks of diarylethenes 1o–4o
exhibit a remarkable bathochromic shift with the increase of the
solvent polarity. However, their emission intensities decrease sig-
nificantly with the increase of the solvent polarity. As a result, the
four derivatives have the shortest emission peak and the strongest
emission intensity in hexane; however, they have the longest emis-
sion peak and the weakest emission intensity in acetonitrile. The
result indicated that the solvent polarity had a significant effect on
the fluorescencefeatures of these diarylethenederivatives. By using
anthracence as the reference, the fluorescence quantum yields
of diarylethenes 1o–4o were determined in hexane, ethanol and
acetonitrile, respectively (Table 2). The result showed that the flu-
orescence quantum yields of diarylethenes 1o–4o were increased
with the decrease of solvent polarity. In various solvents, the fluo-
rescence quantum yields of 1o–4o are the biggest in hexane, with
the values were 0.115 for 1, 0.143 for 2, 0.175 for 3, and 0.098 for 4.
Among the four derivatives, the ortho-substituted derivative 3o has
the biggest fluorescence quantum yield either in hexane, ethanol,
or acetonitrile. Compared with diarylethenes 1o–3o bearing a chlo-
rine atom, the unsubstituted parent diarylethene 4 has the smallest
fluorescence quantum yield, suggesting that the introduction of
chlorine atom into any position of the terminal benzene ring can
notably improve the fluorescence quantum yield of diarylethenes
bearing a naphthalene moiety.
higher than that of dithienylethene derivative (12.1 kcal mol−1
)
[9,48,49]. So it can be expected to predict that the diarylethenes
bearing both naphthalene and thiophene moieties had a relatively
low energy, which leads to good thermal stability [9], as compared
with dinaphthylethene derivatives. The thermal stabilities of the
open- and closed-ring isomers of diarylethenes 1–4 were tested in
ethanol both at room temperature and at 351 K. Storing these solu-
tions in ethanol in the dark at room temperature and then exposed
them to air for more than one month, we observed no changes in
the UV/Vis spectra of diarylethenes 1–4. Furthermore, no decom-
position was detected even after they were exposed to air for more
than six months. At 351 K, diarylethenes 1–4 still showed excellent
thermal stabilities for more than 8 h.
In order to estimate the activation energy of the thermal fading,
the decay processes of the closed isomers 1c–4c were measured
in hexane at 298 K, 318 K, and 338 K, and the results are shown
in Fig. 3. The decay of the red color at the maximum absorption
peak obeyed the first-order kinetics. The half-life time of 1c–4c
at 298 K was 39 h for 1c, 37 h for 2c, 26 h for 3c, and 40 h for 4c.
For the closed-ring isomers 1c–4c, the activation energies of the
thermal reversion could be calculated according to the Arrhenius
equation, and the values were 90.4 kJ mol−1 for 1c, 85.2 kJ mol−1 for
2c, 59.6 kJ mol−1 for 3c, and 75.6 kJ mol−1 for 4c. The results sug-
gest that the activation energy increases based on the position of
chlorine atom in the order of ortho- < meta- < the para-substitution.
Among these diarylethenes, the example with the chlorine atom
at the ortho-position of terminal phenyl ring displayed the lowest
activation energy. Compared to the reported diarylethene bear-
ing two pyrrole rings (32.5 kJ mol−1), diarylethenes 1c–4c showed
enhanced activation energies, but they are lower as compared with
that of the 1,2-bis(2-naphtyl)cyclopentene (121 kJ mol−1 at 298 K)
[27].
The fatigue resistance is another critical factor for practical
applications in optical devices [9,50]. The fatigue resistances of
diarylethenes 1–4 were examined both in hexane and in PMMA
films by alternating irradiation with UV and visible light in air at
room temperature, as shown in Fig. 4. In hexane, the coloration
and decoloration cycles of diarylethenes 1–4 can be repeated at
least 100 times with 10% degradation of 1c, 12% of 2c, 26% of 3c,
and 14% of 4c, which may result from the formation of an epox-
ide [51]. In PMMA films, diarylethenes 1–4 also exhibited excellent
photochromism after 200 cycles with only 20% degradation of
1c, 25% degradation of 2c, 40% of 3c, and 32% of 4c. The results
showed that diarylethene 1 had a stronger fatigue resistance than
the other three diarylethenes both in hexane and in a PMMA film,
suggesting that the substitution position of the chlorine atom had
Diarylethenes 1–4 exhibited excellent fluorescent switching
capabilities both in hexane and in PMMA amorphous films that
are comparable to most of the reported diarylethenes [52–55].
When irradiated at 313 nm, the photocyclization reactions would
yield the non-fluorescent closed-ring isomers 1c–4c, which was
evident by the decrease in emission intensities as compared to
the open-ring diarylethenes 1o–4o. Subsequent irradiation under
appropriate wavelength visible light regenerated their open-ring
isomers 1o–4o and recovered their original emission intensities.
The fluorescence changes of diarylethene 1 by photoirradiation in

52
R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
0.00
0.00
-0.02
-0.04
-0.06
-0.08
-0.10
A
B
-0.02
-0.04
298 K
318 K
338 K
-0.06
-0.08
-0.10
298 K
318 K
338 K
0
2
4
6
8
10
0
2
4
6
8
10
Time (min)
Time (min)
0.00
-0.02
-0.04
-0.06
-0.08
-0.10
0.00
-0.05
-0.10
-0.15
C
D
298 K
318 K
338 K
298 K
318 K
338 K
0
2
4
6
8
10
0
2
4
6
8
10
Timie (min)
Time (min)
Fig. 3. The thermal fading of 1c (A), 2c (B), 3c (C), and 4c (D) at 298 K, 318 K, and 338 K.
Fig. 4. Fatigue resistances of diarylethenes 1–4 in air atmosphere at room temperature: (A) in hexane, (B) in PMMA films. Initial absorbance of the sample was fixed to 1.0.
hexane (2.0 × 10−5 mol L−1) and in a PMMA film (10%, w/w) are
in hexane and 68% in a PMMA film. Similarly, at photostationary
state, the fluorescent modulation efficiencies of diarylethenes 2,
3 and 4 in hexane were 43%, 56%, and 43%, and those in PMMA
films were 60%, 56%, and 47%, respectively. The results showed
that the position of chlorine atom had a significant effect on the
shown in Fig. 6. At the photostationary state, the emission inten-
sity of diarylethenes 1 was quenched to ca. 16% in hexane and 32%
in a PMMA film upon irradiation with UV light. Thus, its fluores-
cent modulation efficiency in the photostationary state was 84%
A 3000
B
10000
7500
5000
2500
0
2400
3o
2o
3o
2o
1800
1o
1o
4o
4o
1200
600
0
350
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 5. Fluorescence emission spectra of diarylethenes 1–4 at room temperature: (A) in hexane (2.0 × 10−5 mol L−1), (B) in PMMA films (10%, w/w).

R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
53
Table 2
The fluorescence features of diarylethenes 1o–4o in different solvents (c = 2.0 × 10−5 mol L−1).
ꢁem (nm) (emission intensity), ˚fa
a
˚
s
1o
2o
3o
4o
Hexane
Ethanol
Acetonitrile
434 (2539), 0.12
446 (558), 0.030
473 (425), 0.016
402 (1824), 0.14
435 (720), 0.031
454 (654), 0.019
410 (2932), 0.18
436 (906), 0.040
455 (787), 0.023
420 (1588), 0.10
424 (435), 0.025
463 (414), 0.011
0.36
0.31
0.27
a
˚f = fluorescence quantum yield; ˚s = the fluorescence quantum yield of anthracence.
A
B 7200
2400
5400
3600
1800
0
1800
1200
600
0
Vis
UV
350
420
490
560
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 6. Emission intensity changes of diarylethene 1 upon irradiation with UV light at room temperature: (A) in hexane (excited at 320 nm), (B) in a PMMA film (excited at
321 nm).
fluorescent switching properties of the isomeric diarylethenes 1–4.
Compared to diarylethene 1, the fluorescent modulation efficien-
cies of diarylethenes 2–4 were evidently lowered both in the liquid
Initial states
PSS states
and solid media. This may be attributed to the incomplete cycliza-
tion reaction and the existence of parallel conformations [56].
2o
2c
1o
1c
3.3. Electrochemistry of diarylethenes
It has been demonstrated that diarylethenes readily undergo
reversible photochromic reactions between the colorless and col-
ored isomers when stimulated by alternating irradiation with UV
and visible light. Owing to the remarkable differences in the nature
of ꢂ-conjugation between the two isomers, these photo-responsive
systems can be tuned not only in optical properties but also in elec-
trochemical properties, both of which are useful in optoelectronic
device applications [57]. Therefore, the electrochemical property of
diarylethenes has attracted much attention because of their molec-
ular switching capability, and thus their potential application in
molecular-scale electronic switches [58–60]. The electrochemical
properties of 1–4 were evaluated by cyclic voltammetry (CV) under
the same experimental conditions previously reported [32]. The CV
curves of diarylethenes 1–4 are shown in Fig. 7. The onset potentials
(Eonset) of oxidation and reduction for 1o were initiated at +0.21
and −0.73 V, and those of 1c were at +0.07 and −0.81 V, respec-
tively. According to the reported method [61,62], the ionization
potential and electron affinity of 1o were calculated to be −5.01
and −4.07 eV, and those of 1c were −4.87 and −3.99 eV, respec-
tively. Based on the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) energy level, the
band gap Eg (Eg = LUMO–HOMO) of 1o and 1c can be determined
as +0.94 and +0.88 eV, respectively. Similarly, the oxidation of 2o,
3o and 4o is initiated at +0.24, +0.21 and +0.36 V, and that of 2c,
3c and 4c is initiated at +0.22, +0.07, and +0.29 V, respectively. The
results indicate that the oxidation process for the open-ring iso-
mers 1o–4o occurs at higher potentials than the corresponding
closed-ring isomers 1c–4c. This is in accordance with the theory
that closed-ring isomers with longer conjugation length generally
lead to a lower positive potential [63,64]. As shown in Table 3, all
Egs of the open-ring isomers 1o–4o were higher than those of the
-0.8
-0.4
0.0
0.4
-0.8
-0.8
-0.4
-0.4
0.0
0.4
0.4
4o
4c
3o
3c
0.0
-0.8
-0.4
0.0
0.4
Potential/V vs. SCE
Fig. 7. Cyclic voltammetry of diarylethenes 1–4 in acetonitrile with a scanning rate
of 50 mV/s.
closed-ring isomers 1c–4c. Among these compounds, the Eg of 3c
was the smallest, which implies that the charge transfer of 3c must
be faster compared to that in others [65]. Compared with the three
isomeric diarylethenes bearing a chlorine atom, the unsubstituted
Table 3
Electrochemical properties of diarylethenes 1–4.
Compound
Oxidation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg
1o
1c
2o
2c
3o
3c
4o
4c
+0.21
+0.07
+0.24
+0.22
+0.21
+0.07
+0.36
+0.29
−5.01
−4.87
−5.04
−5.02
−5.01
−4.87
−5.16
−5.09
−0.73
−0.81
−0.68
−0.69
−0.76
−0.75
−0.78
−0.73
−4.07
−3.99
−4.28
−4.29
−4.16
−4.15
−4.18
−4.13
0.94
0.88
0.76
0.73
0.85
0.72
1.02
0.96

54
R. Wang et al. / Journal of Photochemistry and Photobiology A: Chemistry 243 (2012) 47–55
parent diarylethene 4 possessed the largest Eg both for the open-
and closed-ring isomers. Furthermore, there exist clear differences
among diarylethenes 1–4 not only in the shape of the polarization
curve but also in all electrochemical parameters at the scanned
voltage region. The results indicate that the presence of a chlo-
rine atom and its substitution position can significantly affect the
electrochemical properties of these diarylethene derivatives, but
further work is required to quantify these effects.
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4. Conclusion
Three new unsymmetrical diarylethenes bearing a chlorine
atom at either the para-, meta-, or ortho-position of the terminal
benzene ring were synthesized, and their optical and electrochem-
ical properties were also investigated. The results revealed that
the presence and position of the chloro substituent significantly
influenced the properties of these isomeric diarylethenes. Com-
pared with the unsubstituted parent diarylethene, diarylethenes
bearing a chlorine atom at the para-, meta-, or ortho-position of
the terminal benzene ring had lower quantum yields of cycliza-
tion/cycloreversion and band gaps, but they had higher fluorescent
quantum yields. This work provided a better understanding of
the effect of chloro substituent position on the properties of
diarylethenes bearing a naphthalene moiety, which may help guide
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This work was supported by the NSFC (20962008, 21162011),
the Project of Jiangxi Academic and Technological Leader
(2009DD00100), the Project of Jiangxi Youth Scientist, and the
Project of the Science Funds of Jiangxi Education Office (GJJ10241,
GJJ09646).
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