Syntheses and photochromism of new isomeric diarylethenes bearing an indole moiety

Article abstract of DOI:10.1016/j.dyepig.2013.04.003

Four new photochromic diarylethenes, each bearing an indole moiety, were synthesized and three of their structures were determined by single-crystal X-ray diffraction analysis. Under alternating irradiation with UV and visible light, the compounds showed

Full text of DOI:10.1016/j.dyepig.2013.04.003

Accepted Manuscript
Syntheses and photochromism of new isomeric diarylethenes bearing an indole
moiety
Chunhong Zheng, Shouzhi Pu, Zhiyi Pang, Bing Chen, Gang Liu, Yanfeng Dai
PII:
S0143-7208(13)00130-7
10.1016/j.dyepig.2013.04.003
DYPI 3902
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 22 January 2013
Revised Date: 4 April 2013
Accepted Date: 6 April 2013
Please cite this article as: Zheng C, Pu S, Pang Z, Chen B, Liu G, Dai Y, Syntheses and photochromism
of new isomeric diarylethenes bearing an indole moiety, Dyes and Pigments (2013), doi: 10.1016/
j.dyepig.2013.04.003.
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Highlights for manuscript
ꢀ Isomeric diarylethenes bearing an indole moiety were synthesized > They displayed high
fluorescence efficiency and remarkable fatigue resistance > The effects of the indole ring and
the methoxy position played a very important role.

ACCEPTED MANUSCRIPT
Graphical Abstract
Syntheses and photochromism of new isomeric diarylethenes
bearing an indole moiety
Chunhong Zhenga, Shouzhi Pub, a*, Zhiyi Pangb, Bing Chenb, Gang Liub, Yanfeng Daia
aDepartment of Chemistry, Nanchang University, Nanchang 330031, PR China
bJiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University,
Nanchang 330013, PR China
*Corresponding author: E-mail address: pushouzhi@tsinghua.org.cn (S. Pu); Tel./Fax:
+86-791-83831996.
A new class of isomeric diarylethenes bearing an indole moiety was synthesized and the effects of the
methoxy substituent position on their photochemical and electrochemical properties were investigated.
The results indicated that the electron-donating methoxy group and its substituted position had a
significant effect on the photochromic behaviors of these isomeric diarylethene derivatives.
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Syntheses and photochromism of new isomeric diarylethenes
bearing an indole moiety
Chunhong Zhenga, Shouzhi Pub, a*, Zhiyi Pangb, Bing Chenb, Gang Liub, Yanfeng Daia
aDepartment of Chemistry, Nanchang University, Nanchang 330031, PR China
bJiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University,
Nanchang 330013, PR China
*Corresponding author: E-mail address: pushouzhi@tsinghua.org.cn (S. Pu); Tel./Fax:
+86-791-83831996.
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Abstract Four new photochromic diarylethenes, each bearing an indole moiety, were synthesized and
three of their structures were determined by single-crystal X-ray diffraction analysis. Under alternating
irradiation with UV and visible light, the compounds showed favorable photochromism in hexane,
PMMA films and the crystalline phase. The cyclization quantum yield gradually increased and the
molar absorption coefficients of the closed-ring isomers decreased going from the ortho- to meta- to the
para-position of the substitution. The absorption maxima showed little change when the methoxy
group was attached at any of the three positions on the terminal benzene ring. Each of the compounds
displayed high fluorescence efficiency and remarkable fatigue resistance in both solution and PMMA
films by photoirradiation. Of particular note, their fluorescent modulation efficiency could be achieved
ca. 90% in solid media. Cyclic voltammograms testified that the methoxy group and its substituted
position could effectively modulate the electrochemical behaviors of these diarylethenes.
Keywords: Photochromism; Diarylethene; Indole moiety; Methoxy group position effect; Fluorescence;
Electrochemistry.
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1. Introduction
Photochromic compounds have attracted much attention because of their potential application in
photonic devices, such as optical memory media and photo-optical switches [1-3]. Diarylethenes
derivatives are the most promising candidates among various photochromic molecules, because of their
excellent thermal stability and outstanding fatigue resistance [4-7]. In order to acquire ideal
photochromic properties, chemists usually expand the library of diarylethenes by either altering the
central bridge or the two aryl units.
In the past several decades, significant advancement has been made in the synthesis of novel
photochromic diarylethene systems [8-11]. It has been revealed that the nature of the aryl moieties can
heavily influence the physicochemical properties of the photochromic diarylethenes during the process
of cyclization and cycloreversion reactions induced by photoirradiation. For example, diarylethenes
with thiophene or benzothiophene moieties exhibit excellent thermal stability and outstanding fatigue
resistance [3,4], whereas dipyrrolyl-perfluorocyclopentene groups are thermally unstable and return to
the open-ring isomer even in the dark [12]. Diarylethenes with two thiazole moieties have a high
cyclization quantum yield [13,14], whereas diarylethenes with a pyrazole unit have a relatively high
cycloreversion quantum yield [15]. Apart from the fact that the heteroaryl moieties have influenced the
photochromic characteristics of diarylethene derivatives, the substituents and their substituted positions
have also notably influenced their photochromic properties [16-19]. In particular, the properties of
diarylethenes with terminal phenyl groups could be further modified by introduction of
electron-donating or electron-withdrawing groups to the terminal benzene rings [20,21]. For instance,
introduction of electron-donating substituents to bis(2-thienyl)ethene diarylethenes increased the
absorption maxima of the open-ring isomers and decreased their cyclization quantum yield [20], and
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the same substituents on bis(3-thienyl)ethene diarylethenes increased the absorption coefficient of the
closed-ring isomers and decreased the cycloreversion quantum yield [21]. An alkoxy group, as a strong
electron-donating substituent, played a key role in the photochromic systems. Irie et al. revealed that
the introduction of alkoxy groups at the reactive carbon sites in diarylethenes remarkably suppressed
the cycloreversion quantum yields [17,22,23]. We found that the methoxy group and its substituted
positions at the terminal phenyl groups had a significant effect on the properties of the diarylethenes
with different photochromic skeletons [24-26]. However, its substituted position effect on the
photochromic properties of diarylethenes with an indole moiety has never yet been reported.
To date, many reports have focused on the synthesis of photochromic diarylethenes with different
heteroaryl units including thiophene, benzothiophene, thiazole, pyrrole, pyrazole, oxazole, furan,
benzofuran, indene, benzene, naphthalene, pyridine and benzo[b]silole among others [27-39]. Indole
has the structure of benzene fused with pyrrole and its 2-substituted forms are well known to exhibit
strong fluorescence [40-43]. The indole ring system is probably the most ubiquitous heterocycle in
nature [44]. However, the indole moiety is rarely used in diarylethenes [45-48], due to its high aromatic
stabilization energy, which results in low thermal stability of the closed-ring forms [47,48]. Recently,
we found that thermal fading of the closed-ring isomer could be significantly suppressed by the
introduction of a low aromatic stabilization energy (ASE) aryl unit [49]. Based on this idea, in order
to improve the thermal stability of diarylethenes with an indole moiety and investigate the methoxy
substituted position effect on their properties, we have synthesized four new hybrid photochromic
diarylethenes with both indole and thiophene moieties in this work, three of which are isomeric
compounds with a methoxy at ortho-, meta- and para-positions of the terminal benzene ring. The
synthesized
diarylethenes
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1-(1,2-dimethylindol-3-yl)-2-[2-methyl-5-(2-methoxyphenyl)-3-thienyl]perfluorocyclopentene
1-(1,2-dimethylindol-3-yl)-2-[2-methyl-5-(3-methoxyphenyl)-3-thienyl]perfluorocyclopentene
(1o),
(2o),
1-(1,2-dimethylindol-3-yl)-2-[2-methyl-5-(4-methoxyphenyl)-3-thienyl]perfluorocyclopentene (3o) and
1-(1,2-dimethylindol-3-yl)-2-(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene
(4o).
The
photochromic scheme of diarylethenes 1o−4o is described in Fig. 1.
<Fig. 1>
2.1. General methods
Chemical reagents were purchased from Acros Corp. and used without further purification. All solvents
used were spectroscopic grade and were purified by distillation before use. Reactions were monitored
by analytical thin-layer chromatography on plates coated with 0.25 mm silica gel 60 F254. HPLC
analysis was performed by an Agilent 1200 instrument fitted with a ZORBAX SIL column (5 µm
particle size, 4.6 mm, 25 cm). Melting points were measured with a WRS-1B melting point apparatus.
Infrared spectra were recorded on a Bruker Vertex-70 spectrometer. NMR spectra were obtained with a
Bruker AV-400 spectrometer with teramethyl silane (TMS) as interal reference and CDCl3 as solvent.
Elemental analysis was measured with a PE CHN 2400 analyzer. Absorption spectra were obtained
with an Agilent 8453 UV/Vis spectrometer. Photo-irradiation was carried out with 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 light filters. Fluorescence properties were
measured with a Hitachi F-4500 spectrophotometer.
Suitable crystals of 2o, 3o, and 4o were obtained by slow evaporation of corresponding solutions of
these diarylethenes (2o in diethyl ether, 3o in chloroform, and 4o in chloroform-hexane cosolvent
system). All the measurements were collected by a Bruker SMART APEX II CCD diffractometer using
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a MULTI scan technique at room temperature using Mo Kα 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. Further details on the crystal structure investigation have
been deposited with The Cambridge Crystallographic Data Centre as supplementary publication CCDC
918112 for 2o, 918113 for 3o, and 918114 for 4o. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 0 1223 336033 or
e-mail:deposit@ccdc.cam.ac.uk).
2.2 Synthesis
The synthetic route for diarylethenes 1o−4o is shown in Fig. 2. First, Suzuki coupling of bromobenzene
and three bromobenzene derivatives with thiophene boronic acid gave the phenylthiophene and
methoxyphenylthiophene derivatives 6a−d, which were lithiated and reacted with
perfluorocyclopentene to give compounds 7a−d. Then, brominated 1,2-dimethylindole 8 to give
3-bromo-1,2-dimethylindole 9. Finally, compound 9 was lithiated and then separately coupled with
compound 7a−d to give diarylethene derivatives 1o−4o. The structures of 1o−4o were confirmed by
elemental analysis, NMR, and IR spectroscopy. The PMMA films were prepared by dissolving 10 mg
diarylethene sample and 100 mg polymethylmethacrylate (PMMA) in chloroform (1 mL) with the aid
of ultrasound; then the homogeneous solution was spin-coated on a quartz substrate (10 mm × 10 mm ×
1 mm).
<Fig. 2>
2.2.1 3-Bromo-5-(2-methoxyphenyl)-2-methylthiophene (6a)
Compound 6a was prepared by reacting 3-bromo-2-methyl-5-thienylboronic acid [50] (4.2 g, 19.0
mmol) with 1-bromo-2-methoxybenzene (3.91 g, 20.9 mmol) in the presence of Pd(PPh3)4 (0.73 g, 0.63
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mmol) and Na2CO3 (2 mol L-1, 10 mL) in tetrahydrofuran (THF) (80 mL) for 18 h at 343 K. The
reaction was allowed to cool to room temperature. After being extracted with ether, the organic layer
was dried over MgSO4, filtered, and concentrated. The crude product was purified by column
chromatography on silica gel using petroleum ether as eluent to afford 3.87 g of 6a visible as a buff
1
solid with a 72% yield. H NMR (400 MHz, CDCl3): δ 2.41 (s, 3H, -CH3), 3.91 (s, 3H, -CH3),
6.95-6.99 (m, 2H, phenyl-H), 7.27 (s, 1H, thienyl-H), 7.30 (s, 1H, phenyl-H), 7.54 (d, 1H, J = 8.0 Hz,
phenyl-H).
2.2.2 3-Bromo-5-(3-phenyl)-2-methylthiophene (6b)
Compound 6b was prepared according to the method used for 6a. The crude product was purified by
column chromatography on silica gel using petroleum ether as eluent to afford 6b visible as a buff solid
with a 74% yield. 1H NMR (400 MHz, CDCl3): δ 2.41 (s, 3H, -CH3), 3.83 (s, 3H, -CH3), 6.83 (d, 1H, J
= 8.0 Hz, phenyl-H), 7.03 (s, 1H, thienyl-H), 7.09 (t, 2H, J = 4.0 Hz, phenyl-H), 7.27 (t, 1H, J = 8.0
Hz, phenyl-H).
2.2.3 3-Bromo-5-(4-methoxyphenyl)-2-methylthiophene (6c)
Compound 6c was prepared according to the method used for 6a. The crude product was purified by
column chromatography on silica gel using petroleum ether as eluent to afford 6c visible as a buff solid
with a 76% yield. 1H NMR (400 MHz, CDCl3): δ 2.40 (s, 3H, -CH3), 3.83 (s, 3H, -CH3), 6.90 (d, 2H, J
= 8.8 Hz, phenyl-H), 6.99 (s, 1H, thienyl-H), 7.43 (d, 2H, J = 8.8 Hz, phenyl-H).
2.2.4 3-Bromo-5-phenyl-2-methylthiophene (6d)
Compound 6d was prepared according to the method used for 6a. The crude product was purified by
column chromatography on silica gel using petroleum ether as eluent to afford 6d visible as a yellow
buff solid with a 72% yield. 1H NMR (400 MHz, CDCl3): δ 2.28 (s, 3H, -CH3), 6.84 (s, 1H, phenyl-H),
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7.12 (s, 1H, thienyl-H), 7.33-7.35 (d, 2H, J = 8.0 Hz, phenyl-H), 7.56-7.58 (d, 2H, J = 8.0 Hz,
phenyl-H).
2.2.5 1-[2-Methyl-5-(2-methoxyphenyl)-3-thienyl]perfluorocyclopentene (7a)
To a stirred THF solution (50 mL) of compound 6a (2.67 g, 9.44 mmol) was added dropwise a 2.5 M
n-BuLi/hexane solution (3.8 mL, 9.5 mmol) at 195 K under argon atmosphere. Stirring was continued
for 30 min at 195 K, octafluorocyclopentene (C5F8) (1.3 mL, 9.56 mmol) was slowly added and the
reaction mixture was stirred for 2 additional hours at this temperature. The reaction was quenched by
addition of water. After being extracted with ether, the organic layer was washed with water. The crude
product was purified by column chromatography on silica gel using petroleum ether as eluent to afford
1.52 g of compound 7a visible as a yellow oil with a 41% yield. 1H NMR (400 MHz, CDCl3): δ 2.41 (s,
3H, -CH3), 3.92 (s, 3H, -CH3), 6.98-7.01 (m, 2H, phenyl-H), 7.29-7.31 (t, 1H, phenyl -H), 7.43 (s, 1H,
thienyl -H), 7.57 (d, 1H, J = 8.6 Hz, phenyl-H).
2.2.6 1-[2-Methyl-5-(3-methoxyphenyl)-3-thienyl]perfluorocyclopentene (7b)
Compound 7b was prepared according to the method used for 7a. The crude product was purified by
column chromatography on silica gel using petroleum ether as eluent to afford 7b visible as a yellow
oil with a 48% yield. 1H NMR (400 MHz, CDCl3): δ 2.33 (s, 3H, -CH3), 3.77 (s, 3H, -CH3), 6.75 (d, 1H,
J = 8.2 Hz, phenyl-H), 6.95 (s, 1H, thienyl-H), 7.01 (s, 2H, phenyl-H), 7.19 (t, 1H, J = 8.1 Hz,
phenyl-H).
2.2.7 1-[2-Methyl-5-(3-methoxyphenyl)-3-thienyl]perfluorocyclopentene (7c)
Compound 7c was prepared according to the method used for 7a. The crude product was purified by
column chromatography on silica gel using petroleum ether as eluent to afford 7c visible as a buff solid
with a 35% yield. 1H NMR (400 MHz, CDCl3): δ 2.42 (s, 3H, -CH3), 3.85 (s, 3H, -CH3), 6.92 (d, 2H, J
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= 8.6 Hz, phenyl-H), 7.00 (s, 1H, thienyl-H), 7.45 (d, 2H, J = 8.7 Hz, phenyl-H).
2.2.8 1-(2-Methyl-5phenyl-3-thienyl)perfluorocyclopentene (7d)
Compound 7d was prepared according to the method used for 7a. The crude product was purified by
column chromatography on silica gel using petroleum ether as eluent to afford 7d visible as a buff solid
1
with a 55% yield. H NMR (400 MHz, CDCl3): δ 2.51 (s, 3H, -CH3), 7.24 (s, 1H, thienyl-H), 7.31 (t,
1H, J = 6.8 Hz, phenyl-H), 7.39 (t, 2H, J = 7.2 Hz, phenyl-H), 7.54 (d, 2H, J = 7.8 Hz, phenyl-H).
2.2.9 3-Bromo-1,2-dimethylindole (9)
To a stirred THF solution (60 mL) of compound 8 (2.18 g, 15 mmol) at room temperature was slowly
added N-bromosuccinimide (NBS) (3.0 g, 16.86 mmol). After the stirring was continuted for 24 h, the
mixture was poured into water slowly. After being extracted with ether, the organic layer was washed
with water. Then the organic layer was dried over anhydrous MgSO4, filtered, and concentrated. The
crude product was purified by column chromatography on silica gel using petroleum ether as eluent to
1
afford 2.3 g of compound 9 visible as a white solid with a 68% yield. H NMR (400 MHz, CDCl3): δ
2.45 (s, 3H, -CH3), 3.71 (s, 3H, -CH3), 7.15 (t, 1H, J = 7.26 Hz, phenyl-H), 7.21 (t, 1H, J = 7.42 Hz,
phenyl-H), 7.26 (d, 1H, J = 7.59 Hz, phenyl-H), 7.49 (d, 1H, J = 7.76 Hz, phenyl-H).
2.2.10 1-(1,2-Dimethylindol-3-yl)-2-[2-methyl-5-(2-methoxyphenyl)-3-thienyl]perfluorocyclopentene
(1o)
To a stirred anhydrous THF (30 mL) of compound 9 (0.22 g, 1 mmol) was added dropwise a 2.5 M
n-BuLi/hexane solution (0.4 mL, 1 mmol) at 195 K under argon atmosphere. After 30 min, THF (2 mL)
containing compound 7a (0.40 g, 1 mmol) was added and the reaction mixture was stirred for 2 h at
this temperature. The reaction was allowed to warm to room temperature and quenched by addition of
water. The product was extracted with diethyl ether. Then the combined organic layers were dried over
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MgSO4, filtered and concentrated. The crude product was purified by column chromatography on silica
gel using petroleum ether/trichloromethane (v/v = 9/1) as the eluent to afford 0.22 g of compound 1o
1
visible as a red solid with a 42% yield. mp 99-100 °C; H NMR (400 MHz, CDCl3): δ 1.71 (s, 3H,
-CH3), 1.91 (s, 3H, -CH3), 3.56 (s, 3H, -CH3), 3.81 (s, 3H, -CH3), 6.86-6.93 (m, 2H, phenyl-H), 7.08 (t,
1H, J = 7.2 Hz, phenyl-H), 7.16-7.22 (m, 3H, phenyl-H), 7.45 (s, 1H, thienyl-H), 7.52 (d, 1H, J = 7.8
Hz, phenyl-H), 7.58 (d, 1H, J = 8.0 Hz, phenyl-H); IR (KBr, υ, cm-1): 747, 786, 842, 895, 985, 1047,
1118, 1161, 1273, 1401, 1455, 1622, 1667, 2843, 3201.
2.2.11 1-(1,2-Dimethylindol-3-yl)-2-[2-methyl-5-(3-methoxyphenyl)-3-thienyl]perfluorocyclopentene
(2o)
Compound 2o was prepared according to the method used for 1o. The crude product was purified by
column chromatography on silica gel using petroleum ether/trichloromethane (v/v = 9/1) as eluent to
afford 2o visible as a colorless crystal with a 52% yield. mp 102-103 °C; 1H NMR (400 MHz, CDCl3):
δ 1.75 (s, 3H, -CH3), 2.01 (s, 3H, -CH3), 3.63 (s, 3H, -CH3), 3.84 (s, 3H -OCH3), 6.84 (d, 1H, J = 8.2
Hz, phenyl-H), 7.03 (s, 1H, thienyl-H), 7.12 (d, 1H, J = 6.9 Hz, phenyl-H), 7.15 (d, 1H, J = 7.9 Hz,
phenyl-H), 7.22 (t, 1H, J = 7.3 Hz, phenyl-H), 7.26-7.32 (m, 3H, phenyl-H), 7.61 (d, 1H, J = 7.9 Hz,
phenyl-H); IR (KBr, υ, cm-1): 692, 751, 781, 845, 976, 996, 1047, 1119, 1183, 1268, 1338, 1401, 1454,
1534, 1618, 1668, 2840, 3192.
2.2.12 1-(1,2-Dimethylindol-3-yl)-2-[2-methyl-5-(4-methoxyphenyl)-3-thienyl]perfluorocyclopentene
(3o)
Compound 3o was prepared according to the method used for 1o. The crude product was purified by
column chromatography on silica gel using using petroleum ether/trichloromethane (v/v = 9/1) as
eluent to afford 3o visible as a colorless crystal with a 47% yield. mp 205-206 °C; 1H NMR (400 MHz,
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CDCl3): δ 1.74 (s, 3H, -CH3), 2.01 (s, 3H, -CH3), 3.64 (s, 3H, -CH3), 3.84 (s, 3H, -CH3), 6.91 (d, 2H, J
= 8.7 Hz, phenyl-H), 7.14 (s, 1H, thienyl-H), 7.20-7.29 (m, 3H, phenyl-H), 7.45 (d, 2H, J = 8.7 Hz,
phenyl-H), 7.62 (d, 1H, J = 8.0 Hz, phenyl-H); IR (KBr, υ, cm-1): 744, 812, 842, 895, 974, 1041, 1104,
1183, 1272, 1334, 1401, 1455, 1512, 1622, 1667, 2837, 3200.
2.2.13 1-(1,2-Dimethylindol-3-yl)-2-(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (4o)
Compound 3o was prepared according to the method used for 1o. The crude product was purified by
column chromatography on silica gel using using petroleum ether/trichloromethane (v/v = 9/1) as
eluent to afford 4o visible as a colorless crystal with a 47% yield. mp 160-161 °C; 1H NMR (400 MHz,
CDCl3): δ 1.76 (s, 3H, -CH3), 2.00 (s, 3H, -CH3), 3.62 (s, 3H, -CH3), 7.14 (t, 1H, J = 6.8 Hz, phenyl-H),
7.20-7.30 (m, 3H, phenyl-H), 7.35 (s, 1H, thienyl-H), 7.37 (t, 2H, J = 7.2 Hz, phenyl-H), 7.53 (d, 2H, J
= 7.6 Hz, phenyl-H), 7.61 (d, 1H, J = 8.0 Hz, phenyl-H); IR (KBr, υ, cm-1): 689, 749, 811, 842, 893,
974, 998, 1044, 1107, 1184, 1272, 1335, 1401, 1454, 1625, 1667, 3209.
3. Results and discussion
3.1 Photochromism of diarylethenes in hexane and PMMA films
The photochromic behaviors of diarylethenes 1−4 induced by photoirradiation was measured in both
hexane (2.0 × 10-5 mol L-1) and PMMA films (10%, w/w) at room temperature. The absorption spectral
change of diarylethene 1 and color changes of diarylethenes 1−4 induced by alternating irradiation with
UV and visible light in hexane and in PMMA films are shown in Fig. 3. In hexane, the absorption
maximum of diarylethene 1o was observed at 263 nm due to a π-π* transition. Upon irradiation with
297 nm light, a new absorption band centered at 583 nm emerged due to the formation of the
closed-ring isomer 1c. This could easily be seen with the naked eye, as the colorless solution of 1o
turned blue. In the reverse reaction, the blue colored solution was entirely bleached to become colorless
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upon irradiation with visible light (λ > 500 nm), indicating that 1c was returning to the initial state 1o.
Diarylethenes 2−4 exhibited analogous photochromism similar to diarylethene 1 in hexane. Upon
irradiation with 297 nm light, the solutions containing 2o−4o turned blue as a result of the cyclization
reactions to produce the closed-ring isomers 2c−4c, for which the absorption maxima were observed at
583, 583 and 582 nm, respectively. The blue colored solutions of 2c−4c could again be bleached by
irradiation with visible light because the compounds returned to the open-ring isomers 2o−4o,
respectively. In the photostationary state, the isosbestic points for diarylethenes 1−4 were observed at
331, 324, 313 and 336 nm, respectively. The photoconversion ratios of 1−4 from open-ring to
closed-ring isomers in hexane were analyzed by HPLC in the photostationary state, with the values
being 62% for 1, 71% for 2, 68% for 3 and 64% for 4 (Table 1). In PMMA amorphous films,
diarylethenes 1−4 also showed similar photochromism as observed in hexane (Fig. 3). As the results
obtained for most of the reported diarylethenes show, the maximum absorption peaks of diarylethenes
1−4 in PMMA films shifted at a longer wavelength than that of hexane. The red shift values of the
absorption maxima of the closed-ring isomers are 22 nm for 1o, 22 nm for 2o, 20 nm for 3o and 20 nm
for 4o, respectively. The red shift phenomena may be attributed to the polar effect of the polymer
matrix and the stabilization of molecular arrangement in solid state [51,52].
The photochromic parameters of compounds 1−4 are summarized in Table 1. Although the absorption
maxima did not greatly change when the methoxy group was attached at any of the three positions on
the terminal benzene ring, other photochromic features including molar absorption coefficients,
cyclization and cycloreversion quantum yields were heavily dependent on the methoxy substituted
position of the terminal benzene ring. As shown in Table 1, the unsubstituted parent diarylethene 4 has
the biggest molar absorption coefficient and the smallest cyclization and cycloreversion quantum yields
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among these derivatives. Upon introduction of the methoxy group into any of the three positions on the
terminal benzene ring, the molar absorption coefficients decreased and the quantum yields of
diarylethenes 1−3 increased to some extent. For the isomeric compounds 1−3, the cyclization quantum
yield gradually increased as the molar absorption coefficients of the closed-ring isomers decreased
going from the ortho- to meta- to the para-position of substitution. However, the molar absorption
coefficients of the open-ring isomers decreased in order of meta- > ortho- > para-substituent by the
methoxy group. Among these isomeric derivatives, the molar absorption coefficients of the
para-substituted derivative 3 are the smallest both for its open-ring and closed-ring isomers, however,
its cyclization quantum yield is the biggest (Φo-c,3 = 0.48). The cycloreversion quantum yields of
isomeric diarylethenes 1−3 are almost constant (0.41~0.42), but greater than that of the unsubstituted
parent derivative 4 (Φo-c, 4 = 0.33). This result indicated that introduction of electron-donating methoxy
group in the terminal benzene ring could significantly enhance the cyclorerversion quantum yield of
diarylethenes with an indole unit, but its substituted position hardly influenced their cycloreversion
quantum yields. Compared with the analogous diarylethenes with a pyrazole moiety [15], the
absorption maxima of the closed-ring isomers of diarylethenes with an indole moiety showed a
hypsochromic shift, and their cycloreversion quantum yields decreased markedly. When replacing the
thiophene or phenylthiophene moiety with the indole unit in the same molecular skeleton of
photochromic diarylethenes, the cycloreversion quantum yields increased significantly [24,26].
Similarly, when replacing the pyrrole moiety with the indole unit, both the cyclization quantum yields
and the cycloreversion yields increased whereas the photoconversion ratios in the photostationary state
decreased remarkably [25]. These results suggest that the indole moiety has a significant effect on the
photochromic properties of the diarylethene derivatives.
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The thermal stability of both the open-ring and closed-ring isomers of diarylethenes 1−4 were tested in
hexane at room temperature and 341 K. Storing these solutions in the dark and then exposing them to
air for more than 60 hrs at room temperature and at 341 K, respectively, resulted in no changes in the
observed UV/vis spectra for diarylethenes 1−4. This result showed that the thermal stability of these
unsymmetrical diarylethenes was much better than that of the symmetrical diarylethenes with two
indole moieties [47]. In addition, the fatigue resistance of diarylethenes 1−4 was examined in both
hexane and PMMA amorphous films at room temperature, and the result is presented in Fig. 4. As
depicted in Fig. 4, the coloration and de-coloration cycles of 1−4 could repeat 50 times in hexane with
the degradation of 16% for 1c, 12% for 2c, 18% for 3c and 14% for 4c, while repeating 200 times in
PMMA films with the degradation of 14% for 1c, 10% for 2c, 21% for 3c and 21% for 4c, respectively.
Among these four derivatives, therefore, the fatigue resistance of the meta-substituted derivative 2 is
the strongest and that of the para-substituted derivative 3 is the weakest in both solution and solid
medium.
3.2 Crystal structures and photochromism in the crystalline phase
To better understand the photochromic behavior in the crystalline phase, the final structural
confirmations of diarylethenes 2o, 3o and 4o were provided by X-ray crystallographic analysis. The
X-ray crystallographic analysis data are listed in Table 2, and their ORTEP drawings are shown in Fig.
5. The molecules of the three single crystals crystallized with a C2 symmetry in the photoactive
anti-parallel conformation in the crystalline phase. Such a conformation is crucial for the compound to
exhibit photochromism [53]. There are four planar rings in the three crystal molecules, which can form
three dihedral angles between every two adjacent planar rings. The dihedral angles in diarylethenes
2o−4o are summarized in Table 3. Compared to the meta-substituted derivative 2o, the
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para-substituted derivative 3o has a smaller dihedral angle between the thiophene ring and the adjacent
benzene ring. The reason may be attributed to the steric-structure of methoxy group at different
positions in the terminal benzene ring. The distance between the two reactive carbon atoms is 3.689 Ǻ
for 2o (C1…C19), 3.777 Ǻ for 3o (C1…C19) and 3.772 Ǻ for 4o (C1…C18), respectively (Table
3). Based on the empirical rule that the molecule undergoes the photocyclization reaction if the
molecule is fixed in an anti-parallel mode and the distance between reacting carbon atoms on the aryl
rings is less than 4.2 Å [54,55], the three crystals should exhibit photochromism by photoirradiation in
the single crystalline phase. In fact, the crystals 2o−4o showed favorable photochromism, in
accordance with the expected ring closure, to form 2c−4c upon irradiation with UV light. As shown in
Fig. 6, the colorless crystal of 2o−4o turned green upon irradiation with 297 nm light, and the colored
crystals reverted to a colorless state upon irradiation with visible light (λ > 500 nm). After 200 repeat
cycles, the three crystals still exhibited favorable photochromism in the crystalline phase. Therefore,
they can potentially be used for the construction of certain optoelectronic devices [55].
3.3 Fluorescence of diarylethenes
Fluorescence can be useful in molecular scale optoelectronics and digital fluorescence photoswitches,
and the fluorescence features of many diarylethenes have been extensively reported [56-60]. The
fluorescence emission spectra of diarylethenes 1−4 in both hexane (2.0 × 10-5 mol L-1) and PMMA
films (10%, w/w) were evaluated at room temperature, and the results are shown in Fig. 7. In hexane,
the emission peaks of diaryelthenes 1o−4o were observed at 421, 421, 423 and 425 nm when excited at
350 nm, and were observed at 441, 443, 445 and 440 nm when monitored at 350 nm in PMMA films.
When comparing samples in hexane to those in PMMA films, the emission peaks of 1o−4o consistently
displayed an evident bathochromic shift with values of 20 nm for 1o, 22 nm for 2o, 22 nm for 3o and
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15 nm for 4o. The result is in agreement with the same molecular skeleton with thiophene or
phenylthiophen moieties [24,26]. Among the four derivatives, the emission intensity of the
unsubstituted parent diarylethene 4o was the strongest in both hexane and PMMA films. This result
indicated that the electron-donating methoxy group could decrease the fluorescent intensity of
diarylethenes with an indole unit. This was in agreement with the methoxy-substituted diarylethenes
containing a pyrrole moiety [25]. For the isomeric compounds 1−3, the emission intensity gradually
decreased going from the meta- to ortho- to the para-position of substitution. As a result, the emission
intensity of the para-substituted derivative 3o is the weakest in both hexane and PMMA films.
<Fig. 7>
In general, for photochromic diaryelthene derivatives, the initial fluorescence open-ring isomer is
switched off after the photocyclization reaction due to the formation of the non-fluorescence
closed-ring isomers. The fluorescence can be readily restored upon visible light irradiation. As has been
observed for most of the reported diarylethenes [61-63], diarylethenes 1−4 exhibited evident
fluorescent switches in addition to the photochromism from the open-ring isomers to the closed-ring
isomers by photoirradiation in both hexane and PMMA films. During the process of
photoisomerization, the fluorescent spectral changes of diarylethenes 1−4 in hexane (2.0 × 10-5 mol L-1)
and in PMMA films (10%, w/w) are shown in Figs. 8 and 9, respectively. Upon irradiation with 297
nm light, the emission intensity of diarylethenes 1o−4o in hexane was quenched to ca. 30% for 1, 27%
for 2, 27% for 3 and 29% for 4 when arrived at the photostationary state (Figs. 8). Similarly, the
emission intensity of 1o−4o was quenched to ca. 10% for 1, 10% for 2, 14% for 3 and 13% for 4 in
PMMA films when arrived at the photostationary state (Figs. 9). That is to say, the fluorescent
modulation efficiency of the four derivatives in hexane was 70% for 1, 73% for 2, 73% for 3 and 71%
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for 4, and that in PMMA films was 90% for 1, 90% for 2, 86% for 3 and 87% for 4. The data indicated
that diarylethenes with an indole moiety exhibited relatively high fluorescent modulation efficiency in
both solution and solid media by photoirradiation, especially in solid media. This characteristic is ideal
for practical applications in optical devices, such as optical recording media and fluorescent switches
[64,65].
< Fig. 8 >
< Fig. 9 >
3.4 Electrochemistry of diarylethenes
The electrochemical reactions are being used for molecular switching and also can potentially be
applied to molecular-scale electronic switches [66,67]. Electrocyclic reactions induced by
electrochemical reduction or oxidation have been reported [40,68-71]. Herein, we investigated the
electrochemical properties of 1–4 by cyclic voltammetry (CV). Their electrochemical characteristics
are summarized in Table 4, which is converted into Fig. 10. The methoxy group could effectively
modulate the electrochemical behaviors of these diarylethene derivatives. The oxidation onset potential
of the unsubstituted parent diarylethene 4o is the highest, whereas that of 4c is the lowest. The
oxidation onset potentials of 1o−3o were observed at +1.17, 1.10 and 1.09 V, and those of 1c−3c were
observed at +1.22, 1.25 and 1.17 V, respectively. These results indicated that the oxidation process of
the open-ring isomers 1o−3o occurred at a lower potential, as compared with that of the corresponding
closed-ring isomers 1c−3c.
< Fig. 10 >
<Table 4>
According to the reported method [72,73], the Highest Occupied Molecular Orbitals (HOMO) and the
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Lowest Unoccupied Molecular Orbitals (LUMO) energy levels could be estimated by using the energy
level of ferrocene as a reference. Based on the HOMO and LUMO energy level, the band-gap (Eg) of
each compound could be calculated approximately. The data showed that the band-gaps increased in
order of 1 < 2 < 3 (ortho- < meta- < para-). When introducing the methoxy group into the terminal
benzene ring at meta- or para- positions, the band-gaps of 2 and 3 increased significantly, compared
with the unsubstituted parent diarylethene 4. The results suggested that the methoxy group and its
substituted position have a significant effect on the electrochemical properties of these diarylethenes
but further work is required to quantify the effects.
Conclusion
In conclusion, four unsymmetrical diarylethenes with an indole moiety have been synthesized, and their
photochemical and electrochemical properties have been investigated systematically. The results
suggested that the methoxy group and its substituted position had a significant effect on the properties
of these diarylethene derivatives. The effected properties include photochromism, fluorescence and
electrochemistry and could be effectively modulated by the methoxy group and its substituted position
at the terminal benzene ring. Furthermore, the indole moiety induced some new characteristics that
were distinguishable from those of the reported diarylethenes with thiophene, pyrazole or pyrrole rings.
The results will be helpful in understanding the methoxy substituent position effect on the properties of
diarylethenes with an indole moiety and for the synthesis of new diarylethenes with tunable
photochromic behaviors.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21162011, 21262015),
the Project of Jiangxi Advantage Sci-Tech Innovative Team (20113BCB24023), the Project of Jiangxi
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Youth Scientist, and the Project of the Science Funds of Jiangxi Education Office (KJLD12035,
GJJ11026, GJJ12587).
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