Photochromism of new unsymmetrical diarylethenes based on the hybrid of azaindole and thiophene moieties

Article abstract of DOI:10.1016/j.dyepig.2014.03.003

A new class of photochromic diarylethenes with both azaindole and thiophene moieties were synthesized to investigate the effects of the substituents on their photochromic behaviors, and their structures were determined by single crystal X-ray diffraction

Full text of DOI:10.1016/j.dyepig.2014.03.003

Dyes and Pigments 106 (2014) 94e104
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photochromism of new unsymmetrical diarylethenes based on the
hybrid of azaindole and thiophene moieties
*
Zhiyuan Sun, Hui Li, Gang Liu, Congbin Fan, Shouzhi Pu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and 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 photochromic diarylethenes with both azaindole and thiophene moieties were synthe-
sized to investigate the effects of the substituents on their photochromic behaviors, and their structures
were determined by single crystal X-ray diffraction analysis. The azaindole moiety was connected
directly to the central cyclopentene ring as a heteroaryl moiety to participate the photoisomerization
reaction in solution, solid amorphous films, and the single crystalline phase. Each of the diarylethenes
exhibited remarkable fluorescent photo-switches in both solution and solid media. The electron-
donating substituents significantly enhanced their cyclization quantum yields, while the electron-
withdrawing groups greatly increased the molar absorption coefficient of their closed-ring isomers.
Cyclic voltammetry studies indicated that the band-gaps of diarylethenes with an azaindole notably
increased when going from electron-donating substituents to electron-withdrawing substituents. The
results revealed that the azaindole moiety and substituents played a vital role in the process of photo-
isomerization reactions for these diarylethenes.
Received 19 January 2014
Received in revised form
28 February 2014
Accepted 1 March 2014
Available online 11 March 2014
Keywords:
Diarylethene
Photochromism
Azaindole moiety
Substituent effect
Fluorescence switch
Electrochemistry
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
[17,18]. However, the indole moiety is rarely used in photochromic
diarylethene systems because its bromide can be easily oxidized
Photochromic compounds can experience a reversible photo-
isomerization reaction between their two isomers with distin-
guishable absorption spectra upon irradiation with light of
appropriate wavelength [1e3]. Among all the reported photo-
chromic compounds, diarylethenes with different heterocyclic aryl
rings are the most promising candidates for practical applications
to rewritable optical memories and photoswitches due to their
excellent photochromic features, such as remarkable fatigue
resistance, good thermal stability of both two isomers, high sensi-
tivity, as well as rapid response [3e6].
Among the diarylethenes hitherto reported, most of the heter-
oaryl moieties have been thiophene or benzothiophene rings
[3,7,8], with just a few reports concerning other heteroaryl moi-
eties, including benzofuran [9], isoxazole [10,11], pyrrole [12],
thiazole [13], pyrimidine [14], pyridine [15], naphthalene [16].
These different heteroaryl moieties have a vital role to play in
modulating the photochromic properties of diarylethenes during
the processes of photoisomerization. The reported diarylethenes
with indole moieties are well known to exhibit strong fluorescence
when exposed to the air and its high aromatic stabilization energy
results in low thermal stability of the closed-ring forms [19e22].
Recently, azaindoles have been caused growing interest of the
scientific community and pharmaceuticals firms [23,24]. Specially,
7-azaindole is considered as a bioisostere of indole or purine
moiety, and it is found to be integral part of many bioactive mol-
ecules [25]. Therefore, the azaindole derivatives have been widely
used as anthelmintic agents, optical probes for protein structure
and dynamics, as well as coordination reagents [26e30]. Intro-
duction of an azaindole ring into the photochromic diarylethene
system can be expected to undergo favorable photochromism with
special characteristics. However, to the best of our knowledge, the
azaindole-containing diarylethene derivatives have not hitherto
been reported.
In this work, a new class of diarylethenes with an azaindole
moiety were synthesized and their structures and properties were
systematically investigated. The synthesized compounds are 1-
(1,2-dimethyl-7-azaindole-3-yl)-2-[2-methyl-5-(4-trifluorophenyl)-
3-thienyl]perfluorocyclopentene (1o), 1-(1,2-dimethyl-7-azaindole-3-
yl)-2-[2-methyl-5-(4-fluorophenyl)-3-thienyl]perfluorocyclopentene
(2o),
1-(1,2-dimethyl-7-azaindole-3-yl)-2-(2-methyl-5-phenyl-3-
thienyl)perfluorocyclopentene (3o), 1-(1,2-dimethyl-7-azaindole-3-
yl)-2-[2-methyl-5-(4-methylphenyl)-3-thienyl]perfluorocyclopentene
* Corresponding author. Tel./fax: þ86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
http://dx.doi.org/10.1016/j.dyepig.2014.03.003
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
95
(4o),
and
1-(1,2-dimethyl-7-azaindole-3-yl)-2-[2-methyl-5-(4-
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44 0 1223
methoxyphenyl)-3-thienyl]perfluorocyclopentene (5o). All of these
diarylethene derivatives exhibit good thermal irreversible photochro-
mism in solution, in PMMA amorphous films, and even in the single
crystalline phase. The photochromicscheme of the diarylethenes 1e5is
shown in Fig. 1.
336033 or e-mail:deposit@ccdc.cam.ac.uk).
2.2. Synthesis
The synthesis route for diarylethenes 1oe5o is shown in Fig. 2.
First, 1,2-dimethyl-3-bromine-7-azaindole (9) was derived from
7-azaindole by the procedures of acylation, methylation, and
bromination [25]. Compounds 10aee were synthesized according
to the reported method [13]. Finally, the compound 9 was lithiated
and then separately mixed with 10aee afforded the target diary-
lethenes 1oe5o, respectively. The structures of 1oe5o were
confirmed by NMR, IR, mass spectrum, elemental analysis, and
single-crystal X-ray diffraction analysis.
2. Experimental
2.1. General
All solvents were spectroscopic grade and were purified 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 measured 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 appropriate filters. Fluorescence spectra were measured using a
Hitachi F-4600 spectrophotometer. Electrochemical examinations
were performed in a one-compartment cell by using a Model 263
potentiostategalvanostat (EG&G Princeton Applied Research) un-
der computer control at room temperature. Platinum-electrodes
(diameter 0.5 mm) served as working electrode and counter elec-
trode. The PMMA films of 1oe5o were prepared by dissolving
5.00 mg of diarylethene sample and 50.00 mg of PMMA in 0.5 mL
chloroform, and the homogeneous solution was spin-coated on a
quartz substrate (20 ꢀ 20 ꢀ 1 mm3) at a rotating speed of 1500 rpm.
Suitable crystals of 1oe5o were obtained by slow evaporation of
ethyl acetate mixed with petroleum ether solutions of the five
diarylethenes. All the measurements were collected by a Bruker
SMART APEX II CCD diffractometer using a MULTI scan technique at
room temperature using Mo Ka radiation. The structures were
solved by direct methods and refined by full-matrix least-squares
procedures on F2 by full-matrix least-squares techniques using
SHELXTL-97 program. Further details on the crystal structure
investigation have been deposited with The Cambridge Crystallo-
graphic Data Center as supplementary publication CCDC 963071 for
1o, 963070 for 2o, 963067 for 3o, 963068 for 4o, and 963072 for 5o.
Copies of the data can be obtained, free of charge, on application to
2.2.1. 1-Phenylsulfonyl-7-azaindole (6)
Sodium hydroxide (1.25 g) was added to a solution of CH2Cl2
containing benzyltriethylammonium chloride (0.07 g, 0.3 mmol)
and 7-azaindole (1.18 g, 10.0 mmol). The mixtures reacted 10 min at
the 273 K, and then benzenesulfonyl chloride (2.21 g, 12.5 mmol)
was slowly added to the solution. After the addition, the solution
was stirred for 20 min and then warmed to the room temperature.
After 12 h, the suspension was extracted with dichloromethane and
the organic layer was collected and washed with NaCl solution and
then water. The organic layer was dried over MgSO4, filtrated, and
evaporated. The crude product was purified by column chroma-
tography on silica gel using dichloromethane as eluent to give 6
(2.32 g) as a colorless solid in 90% yield. Mp: 402e403 K. 1H NMR
(400 MHz, CDCl3):
d
6.51 (d, 1H, J ¼ 4.0 Hz, azaindolyl-H), 7.06e7.10
(m, 1H, azaindolyl-H), 7.38 (t, 2H, J ¼ 8.0 Hz, phenyl-H), 7.47 (t, 1H,
J ¼ 7.2 Hz, phenyl-H), 7.64 (d, 1H, J ¼ 4.0 Hz, azaindolyl-H), 7.75 (d,
1H, J ¼ 4.0 Hz, azaindolyl-H), 8.10 (s, 1H, phenyl-H), 8.11 (s, 1H,
phenyl-H), 8.34 (d, 1H, J ¼ 4.8 Hz, azaindolyl-H).
2.2.2. 1-Phenylsulfonyl-2-methyl-7-azaindole (7)
1-Phenylsulfonyl-7-azaindole 6 (2.58 g, 10.0 mmol) in anhy-
drous THF was added dropwise to a 2.0 mol Lꢁ1 LDA in hexane/THF
solution (10.0 mL) at 248 K under an argon atmosphere. Stirring
was continued for 0.5 h. Excess iodomethane (3.0 mL) was added.
After 6 h, the mixture was warmed to room temperature and
reacted 4 h. The mixture was extracted with dichloromethane and
the organic layer was collected washing with NaCl solution and
then water. The organic layer was dried over MgSO4, filtrated, and
evaporated. The crude product was purified by column
Fig. 1. Photochromism of diarylethenes 1e5.

96
Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
Fig. 2. Synthetic route for diarylethenes 1oe5o.
chromatography on silica gel using petroleum ether: ethyl acetate
2.2.5. 1-(1,2-Dimethyl-7-azaindol-3-yl)-2-[2-methyl-5-(4-
(3:1) as eluent to give 7 (2.54 g) as a colorless solid in 93% yield. Mp:
trifluoromethylphenyl)-3-thienyl] perfluorocyclopentene (1o)
369e370 K. 1H NMR (400 MHz, CDCl3):
d
2.73 (s, 3H, eCH3), 6.28 (s,
Raw material 1,2-dimethyl-3-bromo-7-azaindole 9 (0.45 g,
2.0 mmol) in anhydrous THF was added to a 2.4 mol Lꢁ1 n-BuLi in
hexane solution (1.0 mL, 2.4 mmol) at 195 K under an argon at-
mosphere. Stirring was continued for 0.5 h and 10a (1.13 g,
2.6 mmol) was slowly added to the reaction mixture at 195 K and
stirred for 2 h at this temperature. The reaction was quenched with
water. The mixture was warmed to room temperature and
extracted with CH2Cl2. The organic layer was dried over MgSO4,
filtrated, and evaporated. The crude product was purified by col-
umn chromatography on SiO2 using petroleum ether: ethyl acetate
(5:1) as eluent to give 1o (0.23 g, 21%) as a yellowish solid. Mp:
457e458 K; Calcd for C26H17F9N2S (%): Calcd C, 55.72; H, 3.06; N,
5.00. Found C, 55.73; H, 3.01; N, 5.04; 1H NMR (400 MHz, CDCl3):
1H, azaindolyl-H), 7.10e7.13 (m, 1H, azaindolyl-H), 7.46 (t, 2H,
J ¼ 8.0 Hz, phenyl-H), 7.54 (t, 1H, J ¼ 7.6 Hz, phenyl-H), 7.68 (d, 1H,
J ¼ 7.6 Hz, azaindolyl-H), 8.14 (d, 2H, J ¼ 7.2 Hz, phenyl-H), 8.35 (d,
1H, J ¼ 4.0 Hz, azaindolyl-H).
2.2.3. 1-Phenylsulfonyl-2-methyl-3-bromo-7-azaindole (8)
At the room temperature, NBS (2.14 g, 12.0 mmol) was slowly
added to a stirring solution of CCl4 containing 1-phenylsulfonyl-2-
methyl-7-azaindole 7 (2.72 g, 10.0 mmol). After 12 h, quenched
with saturated Na2S2O8 and extracted with dichloromethane. The
organic layer was collected washing with NaCl solution and then
water. The organic layer was dried over MgSO4, filtrated, and
evaporated. The crude product was purified by column chroma-
tography on silica gel using petroleum ether: ethyl acetate (5:1) as
eluent to give 8 (2.90 g) as a colorless solid in 83% yield. Mp: 389e
d
1.79 (s, 3H, eCH3), 2.06 (s, 3H, eCH3), 3.77 (s, 3H, eNCH3), 7.08e
7.11 (m, 1H, azaindolyl-H), 7.42 (s, 1H, thienyl-H), 7.63 (s, 4H,
phenyl-H), 7.87 (d, 1H, J ¼ 8.0 Hz, azaindolyl-H), 8.31 (d, 1H,
J ¼ 4.4 Hz, azaindolyl-H). 13C NMR (100 MHz, CDCl3, ppm):
d 11.60,
390 K. 1H NMR (400 MHz, CDCl3):
d 2.77 (s, 3H, eCH3), 7.21e7.24
14.64, 28.50, 99.14, 117.21, 118.34, 124.11, 125.36, 125.62, 125.98,
126.02, 126.84, 127.43, 129.82, 136.73, 138.92, 140.19, 142.12, 143.05,
(m, 1H, azaindolyl-H), 7.48 (t, 2H, J ¼ 8.0 Hz, phenyl-H), 7.58 (t, 1H,
J ¼ 7.6 Hz, phenyl-H), 7.70 (d, 1H, J ¼ 8.0 Hz, azaindolyl-H), 8.16 (t,
2H, J ¼ 7.6 Hz, phenyl-H), 8.42 (d, 1H, J ¼ 4.4 Hz, azaindolyl-H).
148.10. MS m/z (Mþ): 561.1; IR (KBr, , cmꢁ1): 732, 774, 794, 816,
n
832, 850, 891, 976, 993, 1014, 1045, 1067, 1111, 1189, 1263, 1325,
1427, 1466, 1532, 1616, 2930.
2.2.4. 1,2-Dimethyl-3-bromo-7-azaindole (9)
At 273 K, NaH (8.70 g) was added to a stirring solution of DMF
2.2.6. 1-(1,2-Dimethyl-7-azaindol-3-yl)-2-[2-methyl-5-(4-
containing 1-phenylsulfonyl-2-methyl-3-bromo-7-azaindole
8
fluorophenyl)-3-thienyl] perfluorocyclopentene (2o)
(2.14 g, 12.0 mmol). After reaction 30 min, iodomethane (1.5 mL)
was added dropwisely. And then the product was quenched with
water after reacted 24 h. The organic layer was extracted with
dichloromethane and washed with plenty of water. The organic
layer was dried over MgSO4, filtrated, and evaporated. The crude
product was purified by column chromatography on silica gel using
petroleum ether: ethyl acetate (5:1) as eluent to give 9 (0.38 g) as a
Diarylethene 2o was prepared by an analogous method to that
used for compound 1o using 10b instead of 10a. The crude product
was purified by column chromatography on SiO2 using petroleum
ether: ethyl acetate (5:1) as eluent to give 2o (0.23 g, 23%) as a
yellowish solid. Mp: 441e442 K; Calcd for C25H17F7N2S (%): Calcd C,
58.82; H, 3.36; N, 5.49. Found C, 58.84; H, 3.35; N, 5.51; 1H NMR
(400 MHz, CDCl3):
d 1.72 (s, 3H, eCH3), 2.09 (s, 3H, eCH3), 3.71 (s,
faint yellow liquid in 34% yield. 1H NMR (400 MHz, CDCl3):
d
2.46 (s,
3H, eNCH3), 7.10e7.14 (m, 1H, azaindolyl-H), 7.21e7.26 (m, 2H,
phenyl-H), 7.54 (s, 1H, thienyl-H), 7.63e7.67 (m, 2H, phenyl-H), 7.79
(d, 1H, J ¼ 7.6 Hz, azaindolyl-H), 8.26 (d, 1H, J ¼ 3.6 Hz, azaindolyl-
3H, eCH3), 3.81 (s, 3H, eNCH3), 7.09e7.10 (m, 1H, azaindolyl-H),
7.75 (d, 1H, J ¼ 7.6 Hz, azaindolyl-H), 8.27 (d, 1H, J ¼ 7.6 Hz,
azaindolyl-H).
H). 13C NMR (100 MHz, CDCl3, ppm):
d 11.59, 14.54, 28.48, 99.21,

Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
97
115.83, 116.05, 117.15, 118.38, 122.59, 126.45, 127.30, 129.68, 138.94,
140.64, 140.89, 142.93, 148.04. MS m/z (Mþ): 511.1; IR (KBr, , cmꢁ1):
2.2.9. 1-(1,2-Dimethyl-7-azaindol-3-yl)-2-[2-methyl-5-(4-
methoxyphenyl)-3-thienyl] perfluorocyclopentene (5o)
n
775, 794, 810, 830, 852, 890, 975, 995, 1047, 1077, 1124, 1158, 1192,
1234, 1271, 1303, 1335, 1429, 1469, 1511, 2926.
Diarylethene 5o was prepared by an analogous method to that
used for compound 1o using 10e instead of 10a. The crude product
was purified by column chromatography on silica gel using pe-
troleum ether: ethyl acetate (5:1) as the eluent resulting in 0.27 g of
5o being obtained in 26% yield as a pink solid. Mp: 462e463 K;
Calcd for C26H20F6N2OS (%): Calcd C, 59.77; H, 3.86; N, 5.36. Found
2.2.7. 1-(1,2-Dimethyl-7-azaindol-3-yl)-2-[2-methyl-5-phenyl-3-
thienyl]perfluorocyclopentene (3o)
Diarylethene 3o was prepared by an analogous method to that
used for compound 1o using 10c instead of 10a. The crude product
was purified by column chromatography on SiO2 using petroleum
ether: ethyl acetate (5:1) as eluent to give 3o (0.26 g, 26%) as a
yellowish solid. Mp: 413e414 K; Calcd for C25H18F6N2S (%): Calcd C,
50.97; H, 3.68; N, 5.69. Found C, 50.99; H, 3.65; N, 5.71; 1H NMR
C, 59.82; H, 3.84; N, 5.38; 1H NMR (400 MHz, CDCl3):
d 1.73 (s, 3H, e
CH3), 2.05 (s, 3H, eCH3), 3.76 (s, 3H, eNCH3), 3.82 (s, 3H, eOCH3),
6.90 (d, 2H, J ¼ 8.8 Hz, phenyl-H), 7.07e7.10 (m, 1H, azaindolyl-H),
7.21 (s, 1H, thienyl-H), 7.45 (d, 2H, J ¼ 8.4 Hz, phenyl-H), 7.89 (d, 1H,
J ¼ 8.0 Hz, azaindolyl-H), 8.30 (d, 1H, J ¼ 4.8 Hz, azaindolyl-H). 13
C
NMR (100 MHz, CDCl3, ppm):
d 11.59, 14.52, 28.48, 55.38, 99.29,
(400 MHz, CDCl3):
d 1.76 (s, 3H, eCH3), 2.05 (s, 3H, eCH3), 3.76 (s,
114.40, 116.41, 116.61, 117.13, 118.44, 121.50, 126.29, 126.86, 127.49,
3H, eNCH3), 7.07e7.10 (m, 1H, azaindolyl-H), 7.30 (d, 1H, J ¼ 7.6 Hz,
phenyl-H), 7.33 (s, 1H, thienyl-H), 7.38 (t, 2H, J ¼ 8.0 Hz, phenyl-H),
7.53 (d, 2H, J ¼ 8.0 Hz, phenyl-H), 7.89 (d, 1H, J ¼ 8.0 Hz, azaindolyl-
H), 8.30 (d, 1H, J ¼ 4.4 Hz, azaindolyl-H). 13C NMR (100 MHz, CDCl3,
135.30, 139.00, 139.68, 141.93, 142.90, 148.08, 159.49. MS m/z (Mþ):
523.1; IR (KBr, n
, cmꢁ1): 775, 794, 814, 852, 896, 974, 996, 1041,
1077, 1108, 1176, 1194, 1250, 1270, 1334, 1430, 1470, 1513, 1536, 1610,
2915.
ppm):
d 11.60, 14.59, 28.49, 99.25, 117.15, 118.41, 122.63, 125.56,
126.43, 127.49, 127.83, 128.99, 133.43, 138.98, 140.67, 142.01, 142.93,
3. Results and discussion
148.08. MS m/z (Mþ): 493.1; IR (KBr, , cmꢁ1): 691, 734, 752, 776,
n
796, 817, 852, 891, 975, 994, 1047, 1078, 1124, 1192, 1272, 1301, 1336,
1429, 1467, 1597, 2919.
3.1. Photochromism in solution and PMMA films
Diarylethenes 1oe5o could be toggled between the colorless
open-ring isomers (1oe5o) and the colored closed-ring isomers
(1ce5c) in both solution and solid media. The photochromic be-
haviors of diarylethenes 1e5 by photoirradiation were measured in
both hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and PMMA films (10%, w/w) at
room temperature. The absorption spectral changes of 1 and color
changes of 1e5 by alternating irradiation with UV and visible light
in hexane are shown in Fig. 3. In hexane, the maximum absorption
2.2.8. 1-(1,2-Dimethyl-7-azaindol-3-yl)-2-[2-methyl-5-(4-
methylphenyl)-3-thienyl] perfluorocyclopentene (4o)
Diarylethene 4o was prepared by an analogous method to that
used for compound 1o using 10d instead of 10a. The crude product
was purified by column chromatography on SiO2 using petroleum
ether: ethyl acetate (5:1) as eluent to give 4o (0.22 g, 22%) as a
yellowish solid. Mp: 475e476 K; Calcd for C26H20F6N2S (%): Calcd C,
61.65; H, 3.98; N, 5.53. Found C, 61.68; H, 3.94; N, 5.57; 1H NMR
of diarylethene 1o was observed at 296 nm
( ,
3
(400 MHz, CDCl3):
d
1.75 (s, 3H, eCH3), 2.05 (s, 3H, eCH3), 2.36 (s,
2.64 ꢀ 104 L molꢁ1 cmꢁ1) due to
p / p* transition [31]. Upon
3H, eCH3), 3.76 (s, 3H, eNCH3), 7.07e7.10 (m, 1H, azaindolyl-H),
7.18 (d, 2H, J ¼ 8.0 Hz, phenyl-H), 7.28 (s, 1H, thienyl-H), 7.42 (d,
2H, J ¼ 8.0 Hz, phenyl-H), 7.89 (d, 1H, J ¼ 8.0 Hz, azaindolyl-H), 8.30
(d, 1H, J ¼ 4.4 Hz, azaindolyl-H). 13C NMR (100 MHz, CDCl3, ppm):
irradiation with 297 nm light, a new absorption band observed at
584 nm (
3
, 1.74 ꢀ 104 L molꢁ1 cmꢁ1) due to the formation of the
closed-ring isomer 1c, accompanied with an evident color change
from colorless to blue. Reversely, the blue colored solution was
entirely bleached upon irradiation with visible light (l > 450 nm),
d
11.61, 14.56, 21.15, 28.49, 117.13, 118.42, 122.08, 125.47, 126.32,
127.49, 129.64, 130.67, 137.81, 138.97, 140.12, 142.13, 142.91, 148.07.
indicating that 1c returned to the initial state 1o. Diarylethenes 2e5
also exhibited analogous photochromism as observed for 1 in
hexane. Upon irradiation with 297 nm UV light, the solution
changed from colorless to blue and new visible absorption bands
MS m/z (Mþ): 507.1; IR (KBr, , cmꢁ1): 734, 776, 796, 811, 852, 892,
n
975, 995, 1046, 1123, 1193, 1271, 1302, 1334, 1429, 1467, 1540, 1573,
1648, 2920.
Fig. 3. Absorption spectra changes of diarylethene 1 and color changes of 1e5 by photoirradiation in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) at room temperature: (A) absorption spectral
changes of 1 and (B) color changes of 1e5. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

98
Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
arrangement in solid state may be resulted in this redshift phe-
nomenon [32].
The absorption spectral features of these compounds in both
hexane and PMMA films are summarized in Table 1. The results
showed that the azaindole moiety and the different substituents
linked at the para-position of the terminal benzene ring had a sig-
nificant effect on the photochromic features of diarylethenes 1e5.
For both the open-ring and the closed-ring isomers of diarylethenes
1e5, the absorption maxima only changed slightly in both hexane
and PMMA films when different substituents were linked at the
para-position of the terminal benzene ring. However, their molar
absorption coefficients, quantum yields of cyclization and cyclo-
reversion, and photoconversion ratios changed notably when
different substituents were introduced into the para-position of the
terminal benzene ring. The absorption maxima of the closed-ring
isomers 1ce5c were observed at 579e600 nm, which were much
longer than those of analogous diarylethenes with a pyridine [15],
benzothiophene [10], or benzofuran [33]. The results revealed that
the azaindole moiety could be effective to shift the absorption
maxima of these diarylethenes to a longer wavelength. Compared to
diarylethenes with an electron-withdrawing group (trifluoromethyl
or fluoro group, such as in compound 1 or 2), the molar absorption
coefficients of the closed-ring isomers of diarylethenes with an
electron-donating group (methyl or methoxy group, such as in
compound 4 or 5) decreased evidently, but their photoconversion
ratios in the photostationary state increased remarkably. As shown
in Table 1, the cyclization quantum yields of diarylethenes with an
electron-donating group are much bigger than those of diary-
lethenes with an electron-withdrawing group. Compared with the
unsubstituted parent diarylethene 3, the cyclization quantum yields
of diarylethenes with a functional group (such as in compounds 1, 2,
4, and 5) enhanced, and their cycloreversion quantum yields
decreased evidently. As a result, diarylethene 3 has the smallest
cyclization quantum yields (Fo-c, 3 ¼ 0.25) and the biggest cyclo-
reversion quantum yields (Fc-o, 3 ¼ 0.28) among the five derivatives.
Compared to the analogous diarylethenes with an isoxazole [10],
pyrrole [34], or benzofuran moiety [33], the cycloreversion quantum
yields of diarylethenes with an azaindole moiety increased signifi-
cantly. However, the cyclization quantum yields of diarylethenes
with an azaindole moiety are much higher than those of diary-
lethenes with a naphthalene moiety [35]. The results suggested that
the azaindole moiety could significantly influence the photochromic
reactivity of these diarylethene derivatives.
Fig. 4. The photoconversion ratios of 1e5 in the photostationary state by HPLC
analysis.
centered at 581, 579, 579, and 581 nm emerged, respectively. In the
photostationary state, the isosbestic points of 1e5 were observed at
360, 356, 355, 358, and 321 nm, respectively. This indicates that
there exists only a two-component photochromic reaction between
the open-ring and the closed-ring isomers of these diarylethene
derivatives. The photoconversion ratios of 1e5 from open-ring to
closed-ring isomers in hexane were measured by HPLC analysis,
with the value of 62% for 1, 51% for 2, 61% for 3, 67% for 4, and 74%
for 5 (Fig. 4).
In PMMA films, 1e5 also showed similar photochromism as
observed in hexane, and the absorption spectral changes of 1 and
color changes of 1e5 are shown in Fig. 5. Upon irradiation with UV
light, the colors of diarylethene-containing films 1e5 changed from
colorless to blue, and the new absorption bands were observed at
600 nm for 1c, 593 nm for 2c, 592 nm for 3c, 595 nm for 4c, and
598 nm for 5c. Compared to those in hexane, the absorption
maxima of the closed-ring isomers 1ce5c in PMMA films are at
longer wavelengths, with the redshift value of 16 nm for 1c, 12 nm
for 2c, 13 nm for 3c, 16 nm for 4c, and 17 nm for 5c. The polar effect
of the polymer matrix and the stabilization of molecular
The thermal stability of the open-ring and the closed-ring iso-
mers of 1e5 was tested in ethanol at room temperature and 351 K.
Fig. 5. Absorption spectra changes of diarylethene 1 and color changes of 1e5 by photoirradiation in PMMA films (10%, w/w) at room temperature: (A) absorption spectral changes
of 1 and (B) color changes of 1e5. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
99
Table 1
Absorption characteristics and photochromic reactivity of diarylethenes 1e5 in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and in PMMA films (10%, w/w).
Compd
lo,max/nma
(
3
/L molꢁ1 cmꢁ1
)
lc,max/nmb
(
3
/L molꢁ1 cmꢁ1
)
Fc
Conversion at PSS in hexane (%)
Hexane
PMMA film
Hexane
PMMA film
Fo-c
Fc-o
1
2
3
4
5
296 (2.64 ꢀ 104)
290 (2.58 ꢀ 104)
291 (2.42 ꢀ 104)
292 (2.61 ꢀ 104)
296 (2.63 ꢀ 104)
298
292
293
295
296
584 (1.74 ꢀ 104)
581 (1.84 ꢀ 104)
579 (1.63 ꢀ 104)
579 (1.60 ꢀ 104)
581 (1.66 ꢀ 104)
600
593
592
595
598
0.32
0.26
0.25
0.43
0.45
0.12
0.21
0.28
0.26
0.13
62
51
61
67
74
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring (Fo-c) and closed-ring isomers (Fc-o), respectively.
b
c
Storing these solutions in the dark and then exposing them to air
for more than 30 days at room temperature, we found that no
changes in the UV/Vis spectra of 1e5 were observed. At 351 K, the
ethanol solutions of diarylethenes 1e5 still exhibited excellent
thermal stability for more than 15 days. Fig. 6 shows the thermal
fading of 1c in hexane at various temperatures. The result revealed
that the absorbance decay of 1c became fast with the increase of
temperature, but the thermal fading change was very slight in the
range of 298 e 343 K. As like 1c, the thermal fading change of 2ce5c
is very small in the range of 298 e 343 K. The result indicated that
diarylethenes with an azaindole moiety possessed very excellent
thermally irreversible photochromic behaviors. In addition, the
fatigue resistance of diarylethenes 1e5 was tested in both hexane
and PMMA films at room temperature, and the result is presented
in Fig. 7. In hexane, the coloration and decoloration cycles of 1e5
could repeat more than 100 times with the degradation of 5% for 1c,
6% for 2c, 10% for 3c, 4% for 4c, and 7% for 5c. After repeating 200
times in PMMA films, these derivatives still exhibited favorable
photochromism with the degradation of 10% for 1c, 7% for 2c, 5% for
3c, 8% for 4c, and 11% for 5c, respectively. Therefore, diarylethenes
with an azaindole moiety exhibited very strong fatigue resistance
in both hexane and PMMA films.
conformations of 1oe5o were provided by X-ray crystallographic
analysis. Their ORTEP drawings and photochromism in the crys-
talline state are shown in Fig. 8, and X-ray crystallographic analysis
data are listed in Table 2. The five azaindole-containing diary-
lethenes crystallized with photoactive anti-parallel conformations
in the crystalline phase, and the distances between the two reactive
ꢀ
ꢀ
carbon atoms are 3.629 A for 1o (C11/C19), 4.021 A for 2o
3.2. Crystal structures and photochromism in the crystalline phase
In general, diarylethene has two conformations with two het-
erocyclic rings in C2 symmetry (anti-parallel conformation) and in
mirror symmetry (parallel conformation) [3,36,37]. In order to
know the relation between conformation and photochromic be-
haviors of diarylethenes in the crystalline phase, the final structural
Fig. 7. Fatigue resistance of diarylethenes 1e5 in air atmosphere at room temperature:
(A) in haxane and (B) in PMMA films. Initial absorptance of the sample was fixed to 1.0.
Fig. 6. Thermal fading of 1c in hexane at various temperature.

100
Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
Fig. 8. ORTEP drawings of crystals of 1oe5o and their photochromism in the single crystalline phase: (A) 1o, (B) 2o, (C) 3o, (D) 4o, (E) 5o, and (F) photochromism of 1e5 in the
crystalline phase.
Table 2
Crystal data for diarylethenes 1oe5o.
1o
2o
3o
4o
5o
Formula
C26H17F9N2S
560.48
296(2)
Triclinic
P-1
C25H17F7N2S
510.47
296(2)
Triclinic
P-1
C25H18F6N2S
492.45
296(2)
Triclinic
P-1
C26H20F6N2S
506.50
296(2)
Triclinic
P-1
C26H20F6N2OS
522.50
296(2)
Triclinic
P-1
Formula weight
Temperature
Crystal system
Space group
Unit cell dimensions
ꢀ
a (A)
9.4766(3)
11.933(3)
11.979(3)
68.5330(10)
83.125(2)
76.820(2)
1226.48(6)
2
9.3690(14)
11.431(15)
11.794(16)
85.061(8)
70.229(8)
73.707(8)
1141.0(3)
2
9.3685(9)
11.390(12)
11.909(12)
85.357(5)
69.878(4)
72.830(5)
1139.8(2)
2
9.4712(7)
11.378(8)
11.775(9)
85.840(4)
71.517(4)
76.946(4)
1172.4(15)
2
9.3224(3)
11.583(3)
11.610(3)
85.3150(10)
74.2740(10)
81.5730(10)
1192.60(6)
2
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
3
ꢀ
Volume (A )
Z
Reflections collected
Reflections observed
Number of parameters
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Final R1 [I > 2s(I)]
wR2[I > 2s(I)]
5614
3933
392
1.518
4005
3016
319
1.486
5188
4259
311
1.426
5333
4477
339
1.435
4179
3674
339
1.455
1.112
0.0710
0.1859
0.0647
1.062
1.031
1.049
1.055
0.0864
0.2140
0.0641
0.1911
0.0930
0.2230
0.0733
0.1925
0.0789
0.2052
0.0660
0.1899
0.0664
0.1828
0.0562
0.1684
R1 (all data)
wR2 (all data)
0.1803

Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
101
ꢀ
ꢀ
(C10/C18), 4.059 A for 3o (C8/C16), 3.960 A for 4o (C11/C19),
As observed for most of the reported diarylethenes [48e50],
diarylethenes 1e5 exhibited notable fluorescent switches by pho-
toirradiation in both hexane and PMMA films. The fluorescence
changes of 1 by photoirradiation in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and
in a PMMA film (10%, w/w) are shown in Fig. 10. Upon irradiation
with 297 nm UV light, the emission intensity of diarylethene 1o
decreased significantly due to the producing of its closed-ring
isomer 1c. The back irradiation by appropriate visible light regen-
erated its open-ring isomer 1o and recovered the original emission
intensity. When arrived at the photostationary state, the emission
intensity of 1o was quenched to ca. 22% in hexane and 6% in a
PMMA film. Therefore, its fluorescent modulation efficiency in the
photostationary state was 78% in hexane and 94% in a PMMA film.
Just like 1, 2e5 also showed remarkable fluorescent switches in
both hexane and PMMA films upon alternating irradiation with UV
and visible light. In the photostationary state, the fluorescent
modulation efficiency of 2e5 in hexane was 68% for 2, 72% for 3,
69% for 4, and 76% for 5, and that in PMMA films was 91% for 2, 94%
for 3, 92% for 4, and 88% for 5. The result indicated that the fluo-
rescent modulation efficiency of diarylethenes 1e5 in PMMA films
was much higher than that in their respective solutions. This is in
accordance with that of diarylethenes with an isoxazole,
ꢀ
and 3.755 A for 5o (C7/C13), 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 A [38,39], all of these crystals should be expected to display
favorable photochromism by photoirradiation in the single crys-
talline phase. As expected, 1oe5o exhibit favorable photochromism
in the crystalline phase (Fig. 8F). Upon irradiation with UV light, the
crystals of 1oe5o turned blue. When the colored crystals were
dissolved in hexane, intense absorption maxima were observed at
the same wavelengths as those of their respective closed-ring iso-
mers in solution. Reversely, the colored crystals were bleached
completely and their absorption spectra were returned to their
initial states upon irradiation with visible light of the appropriate
wavelength. This characteristic could be potentially used for the
construction of certain optoelectronic devices [40].
3.3. Fluorescence of diarylethenes
Fluorescent property of diarylethene can be useful in molecular-
scale optoelectronics and fluorescence photoswitching materials
[41e43]. Recently, the fluorescent properties of many diarylethene
derivatives have been extensively attracted [44e47]. The fluores-
cence emission spectra of diarylethenes 1oe5o in both hexane
(2.0 ꢀ 10ꢁ5 mol Lꢁ1) and PMMA films (10%, w/w) were investigated
at room temperature, and the results are shown in Fig. 9. In hexane,
the emission peaks of diarylethenes 1oe5o were observed at 426
(lex, 294 nm), 426 (lex, 292 nm), 428 (lex, 290 nm), 431 (lex
,
287 nm), and 437 nm (lex, 289 nm), respectively. In PMMA films,
the emission peaks were observed at 445 (lex, 350 nm), 447 (lex
345 nm), 445 (lex, 340 nm), 448 (lex, 337 nm), and 452 nm (lex
,
,
342 nm). Compared to those in hexane, the emission peaks in
PMMA films exhibited a remarkable redshift with the value of
19 nm for 1o, 21 nm for 2o, 17 nm for 3o, 17 nm for 4o, and 15 nm
for 5o. The result is in accordance with that of the analogous dia-
rylethenes with an indole moiety [21]. In addition, the emission
intensity of diarylethenes with an electron-withdrawing group
(trifluoromethyl or fluorine) was much stronger than that of dia-
rylethenes with an electron-donating group (methyl or methoxy)
in hexane. Among the five derivatives, the emission intensity of 1o
is the strongest in both hexane and a PMMA film. The results
indicated that different substituents attached at the terminal of
benzene ring had a significant effect not only on the emission peak
but also on the emission intensity. The fluorescence quantum yields
of 1oe5o in acetonitrile were determined to be 0.073 for 1o, 0.062
for 2o, 0.079 for 3o, 0.066 for 4o, and 0.044 for 5o with anthracene
as a reference. The result indicated that either the electron-
withdrawing or electron-donating substituents could effectively
suppress the fluorescence quantum yield of diarylethene with an
azaindole unit, as compared to the unsubstituted parent 3o.
Table 3
q
(ꢂ) of
ꢀ
Distances between the reacting carbon atoms d (A) and dihedral angles
diarylethenes 1oe5o.
q
(ꢂ)a
ꢀ
Compd
d (A)
q1
q2
q3
1o
2o
3o
4o
5o
C11/C19
C10/C18
C8/C16
C11/C19
C7/C13
3.629
4.021
4.059
3.960
3.755
48.1
66.3
68.6
60.8
52.1
46.0
58.1
59.5
59.5
50.8
14.1
26.3
26.3
17.1
2.9
a
q1, Dihedral angle between the cyclopentene ring and the azaindole ring; q2
,
dihedral angle between the cyclopentene ring and the thiophene ring; q3, dihedral
angle between the thiophene ring and the adjacent benzene ring.
Fig. 9. Emission spectra of diarylethenes 1e5 at room temperature: (A) in hexane
(2.0 ꢀ 10ꢁ5 mol Lꢁ1) and (B) in PMMA films (10%, w/w).

102
Z. Sun et al. / Dyes and Pigments 106 (2014) 94e104
naphthalene, or pyridine moiety [10,15,16]. Compared to the pyri-
dine- or naphthalene-containing diarylethenes with the similar
molecular skeleton [15,16], the fluorescent modulation efficiency of
diarylethenes with an azaindole moiety enhanced notably during
the process of photoisomerization. This characteristic is ideal for
practical applications in certain optical devices [51,52].
3.4. Electrochemistry of diarylethenes
So far, the electrochemical properties of photochromic diary-
lethenes have been widely reported [53e56]. In order to investigate
the electrochemical properties of 1e5, the cyclic voltammetry tests
were performed under the same experimental conditions at a
scanning rate of 50 mV sꢁ1. The electrolyte was acetonitrile (5.0 mL)
containing 0.5 mmol tetrabutylammonium tetrafuoroborate ((TBA)
BF4) and 0.2 mmol diarylethene sample. Fig. 11 shows the cyclic
voltammetry curves of diarylethenes 1e5. The oxidation onsets
(Eonset) of 1e5 were initiated at 1.32 and 0.85 V for 1o and 1c, 1.29
and 0.81 V for 2o and 2c, 1.28 and 0.77 V for 3o and 3c, 1.25 and
0.73 V for 4o and 4c, and 1.11 and 0.72 V for 5o and 5c, respectively.
The difference of oxidation onset between the open-ring and
Fig. 11. Cyclic voltammetry curve of diarylethenes 1e5 in 0.1 mol Lꢁ1 ((TBA)BF4) with a
scan rate of 50 mV sꢁ1
.
the oxidation process for the open-ring isomers 1oe5o occurred at
higher potentials than in the corresponding closed-ring isomers
1ce5c, indicating that shorter conjugation length leads to larger
positive potentials [41]. Furthermore, the oxidation potentials of
diarylethenes with an electron-donating group (such as in 4 or 5)
were much lower than those of diarylethenes with an electron-
withdrawing group (such as in 1 or 2).
closed-ring isomers of 1e5 (DVo-c) was 0.47 V for 1, 0.48 V for 2,
0.51 V for 3, 0.52 V for 4, and 0.39 V for 5. The result indicated that
According to the reported method [57,58], the Highest Occupied
Molecular Orbitals (HOMO) and the 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. Their electrochemical characteristics are
summarized in Table 4. When replacing the hydrogen atom at the
para-position of the terminal benzene ring with an electron-
withdrawing substituent (trifluoromethyl or fluorine group, such
as in 1 or 2), the band-gaps enhanced significantly. Reversely, the
band-gaps decreased notably when replacing the hydrogen atom
with an electron-donating group (methyl or methoxy group, such
as in 4 or 5). As a result, the band-gaps of 1e5 decreased in order of
1 > 2 > 3 > 4 > 5. The result revealed that different substituents
linked at the terminal of benzene ring had a significant effect on the
electrochemical properties of these diarylethenes with an azain-
dole, but further work is required to quantify the effects.
4. Conclusion
Five unsymmetrical photochromic diarylethenes based on the
hybrid of azaindole and thiophene moieties have been synthesized
Table 4
Electrochemical properties of diarylethenes 1e5 in acetonitrile.
Compd
Oxidation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg
1o
1c
2o
2c
3o
3c
4o
4c
5o
5c
þ1.32
þ0.85
þ1.29
þ0.81
þ1.28
þ0.77
þ1.25
þ0.73
þ1.11
þ0.72
ꢁ6.12
ꢁ5.65
ꢁ6.09
ꢁ5.61
ꢁ6.08
ꢁ5.57
ꢁ6.05
ꢁ5.53
ꢁ5.91
ꢁ5.52
ꢁ0.75
ꢁ0.71
ꢁ0.76
ꢁ0.73
ꢁ0.76
ꢁ0.67
ꢁ0.77
ꢁ0.68
ꢁ0.79
ꢁ0.69
ꢁ4.05
ꢁ4.09
ꢁ4.04
ꢁ4.07
ꢁ4.04
ꢁ4.13
ꢁ4.03
ꢁ4.12
ꢁ4.01
ꢁ4.11
2.07
1.56
2.05
1.54
2.04
1.44
2.02
1.41
1.90
1.41
Fig. 10. Emission intensity changes of diarylethene 1 by photoirradiation at room
temperature: (A) in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and (B) in PMMA films (10%, w/w).

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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (51373072, 21262015, 21363009), the Science
Funds of Natural Science Foundation of Jiangxi Province
(20122BAB213004, 20132BAB203005), and the Project of the Sci-
ence Funds of Jiangxi Education Office (KJLD12035, GJJ12587).
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