Synthesis and photochromic properties of novel pyridine-containing diarylethenes

Article abstract of DOI:10.1016/j.dyepig.2012.05.016

Three isomeric pyridine-containing diarylethenes were synthesized to study the effects of nitrogen atom position (ortho, para, meta) on their photochromic properties. Among these diarylethenes, the example with the nitrogen atom at the ortho-position of p

Full text of DOI:10.1016/j.dyepig.2012.05.016

Dyes and Pigments 95 (2012) 553e562
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photochromic properties of novel pyridine-containing
diarylethenes
*
Gang Liu, Ming Liu, Shouzhi Pu , Congbin Fan, Shiqiang Cui
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 isomeric pyridine-containing diarylethenes were synthesized to study the effects of nitrogen atom
position (ortho, para, meta) on their photochromic properties. Among these diarylethenes, the example
with the nitrogen atom at the ortho-position of pyridine displayed the largest absorption maximum and
molar absorption coefficients. The cyclization quantum yields increased in order of para < meta < ortho,
whereas their cycloreversion quantum yields decreased in order of para > meta > ortho. Compared to the
diarylethene with terminal phenyl ring, those with a terminal pyridine showed enhanced cyclization
quantum yields and emission intensities. Moreover, these pyridine-containing diarylethenes exhibited
multi-addressable switching behavior under the stimulation of both proton and light. Addition of tri-
fluoroacetic acid to the solutions of the diarylethenes resulted in notable color change, and their N-
protonated forms also possessed excellent photochromism. These results indicated that the nitrogen
atom position played a pivotal role in the process of photoisomerization of the diarylethenes.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 20 February 2012
Received in revised form
16 May 2012
Accepted 17 May 2012
Available online 24 May 2012
Keywords:
Pyridine-containing diarylethene
Photochromism
Position of nitrogen atom
Acidichromism
Fluorescence
Multi-addressable switching
1. Introduction
The pyridine ring has good aromaticity with a similar structure to
benzene [34]. But the nitrogen atom of pyridine can be protonated or
Photochromic molecules which exhibit reversible color change
upon light irradiation have attracted significant attention in recent
years [1e5]. Currently, diarylethene derivatives are one of the most
extensively investigated photochromic molecules [6]. The original
studies on the photochromic properties of diarylethenes were re-
ported by Irie and his coworkers [7e9]. Exploration of new diary-
lethene structures with improved properties has been a hot area of
research in organic material science ever since [10e12].
It is well known that suitable functionalization of the aromatic
rings with different substituents can effectively modify the
photochromic behavior of diarylethenes [13e17]. The position of
substituents is also important for fine tuning of their optoelectronic
properties [18e22]. So far, related research has been mainly
focused on the effects of substituted benzene ring in diarylethenes
on their photochromic properties.
alkylated to respond to external stimuli [35,36]. In the past few
years, novel pyridine-based diarylethenes have been prepared
[37e42]. The unusual characteristics of these new compounds have
many potential applications in photoswitching, photomodulation of
chemical reactivity, and control of biological systems. Piard et al.
designed a ratiometric fluorescent photoswitch on the basis of
a diarylethene with two pyridylthiazoles, which could reach a trade-
off between fluorescence and photochromism [37]. Using
a symmetrical dithienylethene with two terminal pyridines, Branda
et al. developed a photo-controlled molecular switch, which could
be applied in living cells [39]. Yi et al. prepared an amphiphilic
pyridine-containing diarylethene with high membrane perme-
ability and low cytotoxicity. This molecule could be potentially
utilized as an in vivo cellular marker and fluorescence switch with
high-ratio signal change and excellent fatigue resistance [41].
Moreover, pyridine-containing diarylethenes are capable of recog-
nizing ions due to the excellent coordination of pyridine with
specific ions. Recently, we prepared a series of new asymmetric
pyridine-containing diarylethenes, which selectively bound Cu(II)
ions [43]. A multi-responsive fluorescence switch based on a ter-
pyridine-containing diarylethene was synthesized by Yi and
coworkers and applied as a detector for the biological process of
metal ion transmembrane transport [44]. All the above research
Among diarylethenes reported up to date, those based on ben-
zothiophene [23e30] exhibited remarkable fatigue resistance and
excellent thermal stability, which are essential to many applica-
tions, such as optical storage [29], fluorescence switching [31], dual
image formation [32], and liquid crystals [33].
* Corresponding author. Tel./fax: þ86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.05.016

554
G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
revealed that pyridine-containing diarylethenes have versatile
applications due to their unique physicochemical properties.
To the best of our knowledge, the effects of both pyridine moiety
and position of nitrogen atom on the properties of diarylethenes
have rarely been reported. Therefore, we designed a series of three
new asymmetric diarylethenes with a benzothiophene and a pyr-
idylthiophene. In the present paper, we report the synthesis and
photochromic properties of the three novel diarylethenes. The
effects of pyridine ring and position of nitrogen atom on their
photochromic properties are also discussed. The photo-
isomerization of the diarylethenes with nitrogen atom at ortho-,
para-, meta-position of the pyridine moiety (1e3) along with
a diarylethene with a phenylthiophene as a reference compound
(4) is described in Fig. 1.
diarylethenes 1e4. The structures of 1e4 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).
2.2.1. 3-Bromo-2-methyl-5-(2-pyridyl)thiophene (5a)
Compound 5a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid [46] (3.10 g, 14.00 mmol) with 2-
bromopyridine (2.20 g, 14.00 mmol) in the presence of Pd(PPh3)4
(0.27 g, 0.23 mmol) and Na2CO3 (6.36 g, 60.00 mmol) in THF (80 mL
containing 10% water) for 15 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 3.00 g of 5a as a pale
yellow solid in 83% yield. M.P. 344e345 K; 1H NMR (400 MHz,
2. Experimental
2.1. General
CDCl3):
d 2.44 (s, 3H, eCH3), 7.13e7.16 (m, 1H, pyridine-H), 7.36 (s,
1H, thiophene-H), 7.55 (d, J ¼ 8.0 Hz, 1H, pyridine-H), 7.65e7.69 (m,
Melting points were measured with a WRS-1B melting point
apparatus. NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer, with CDCl3 as solvent and tetramethylsilane as internal
standard. Infrared spectra (IR) were recorded on a Bruker Vertex-70
spectrometer. Elemental analysis was measured with an elemental
analyzer labeled PE CHN 2400 analyzer. Fluorescent spectra were ob-
tained with a Hitachi F-4500 fluorimeter. Absorption spectra were
measured using an Agilent 8453 UV/VIS spectrophotometer. Photo-
irradiation was carried out with an SHG-200 UV lamp, a CXe21
ultraviolet fluorescence analysis cabinet, and a BMH-250 Visible
lamp. The required wavelength was isolated by appropriate filters. The
quantum yields of cyclization/cycloreversion were determined by
comparing the reaction yields with that of 1,2-bis(2-methyl-5-phenyl-
3-thienyl)perfluorocyclopentene in hexane [45].
1H, pyridine-H), 8.53 (d, J ¼ 8.0 Hz, 1H, pyridine-H); IR (
n, KBr,
cmꢁ1): 732, 771, 882, 1012, 1094, 1158, 1288, 1327, 1481, 1628, 3458.
2.2.2. 3-Bromo-2-methyl-5-(3-pyridyl)thiophene (5b)
Compound 5b was prepared according to the method used for
5a. The crude product was purified by column chromatography on
silica gel using petroleum ether/ethyl acetate (v/v ¼ 8/1) as eluent
to afford 2.60 g of 5b as a pale yellow solid in 72% yield. M.P.
312e313 K; 1H NMR (400 MHz, CDCl3):
d 2.44 (s, 3H, eCH3), 7.16 (s,
1H, thiophene-H), 7.29e7.32 (m, 1H, pyridine-H), 7.77(d, J ¼ 8.0 Hz,
1H, pyridine-H), 8.51 (d, J ¼ 8.0 Hz, 1H, pyridine-H), 8.78 (s, 1H,
pyridine-H); IR (n
, KBr, cmꢁ1): 703, 797, 946, 1017, 1124, 1164, 1320,
1454, 1489, 1614, 3437.
2.2.3. 3-Bromo-2-methyl-5-(4-pyridyl)thiophene (5c)
2.2. Synthesis
Compound 5c was prepared according to the method used for
5a. The crude product was purified by column chromatography on
silica gel using petroleum ether/ethyl acetate (v/v ¼ 6/1) as eluent
to afford 2.82 g of 5c as a pale yellow solid in 78% yield. M.P.
The synthetic route for diarylethenes 1e4 is shown in Fig. 2.
Suzuki coupling of bromobenzene and three bromopyridines with
thiophene boronic acid gave the phenylthiophene and pyridylth-
iophenes 5ae5d. Mono-substituted (2-methyl-3-benzothiophene)
perfluoro- cyclopentene 7 was synthesized by lithiation of 3-
bromo-2-methylbenzothiophene and reaction with per-
fluorocyclopentene. Lithiation of 5aed and coupling with 7 gave
361e362 K; 1H NMR (400 MHz, CDCl3):
d
2.44 (s, 3H, eCH3), 7.30 (s,
1H, thiophene-H), 7.37 (d, 2H, J ¼ 8.0 Hz, pyridine-H), 8.58 (d,
, KBr, cmꢁ1): 714, 810, 864, 1011,
J ¼ 8.0 Hz, 2H, pyridine-H); IR (
n
1163, 1220, 1323, 1458, 1495, 1597, 3431.
F
F
2.2.4. 3-Bromo-2-methyl-5-phenyl-thiophene (5d)
F
F
F
F
F
F
Compound 5d was prepared according to the method used for
5a. The crude product was purified by column chromatography on
silica gel using petroleum ether as eluent to afford 2.91 g of 5d as
a pale yellow solid in 82% yield. M.P. 339e340 K; 1H NMR (400 MHz,
UV
Vis
F
F
F
F
R
CDCl3):
J ¼ 8.0 Hz, phenyl-H), 7.29 (t, 2H, J ¼ 8.4 Hz, phenyl-H), 7.42 (d, 2H,
, KBr, cmꢁ1): 754, 792, 827, 904, 946,
d 2.34 (s, 3H, eCH3), 7.02 (s, 1H, thienyl-H), 7.20 (d, 1H,
R
S
S
S
S
J ¼ 8.0 Hz, phenyl-H); IR (
n
N
N
1007, 1032, 1072, 1155, 1326, 1443.
1c:R=
2c:R=
3c: R=
4c: R=
1o:R=
2.2.5. 2-Methyl-3-benzothiophene-perfluorocyclopentene (7)
N
N
To a stirred THF solution (80 mL) of compound 6 (4.56 g,
20.10 mmol) was added dropwise a 2.5 M n-BuLi/hexane solution
(8.80 mL, 22.00 mmol) at 195 K under argon atmosphere. Stirring
was continued for 30 min at 195 K, octafluorocyclopentene (C5F8)
(2.80 mL, 20.50 mmol) was slowly added and the reaction mixture
was stirred for 2 h at this temperature. The reaction was quenched
by addition of water. After being extracted with ether, the organic
layer was washed with 1 M aqueous HCl and water. The organic
layer was dried over anhydrous MgSO4, filtered, and concentrated.
The crude product was purified by column chromatography on
2o:R=
3o: R=
4o: R=
N
N
Fig. 1. Photochromism of diarylethenes 1e4.

G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
555
Br
Br
Br
R
R
N
S
Pd(PPh3)4
Na2CO3
B(OH)2
S
4
F
F
5a:R=
5b:R=
F
F
F
F
N
R
S
S
N
5c: R=
5d: R=
N
1o:R=
2o:R=
3o: R=
4o: R=
n-BuLi
195 K
N
N
F
F
F
F
F
F
Br
n-BuLi/THF
C5F8,195 K
F
S
6
S
7
Fig. 2. Synthetic route for diarylethenes 1e4.
0.75
0.75
0.60
0.45
0.30
0.15
0.00
A
C
E
B
Vis
Vis
UV
0.60
UV
0.45
UV
Vis
UV
Vis
0.30
0.15
0.00
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
0.75
0.60
0.45
0.30
0.15
0.00
0.75
D
Vis
Vis
UV
UV
0.60
0.45
0.30
0.15
0.00
UV
Vis
UV
Vis
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. Absorption spectra and color changes of diarylethenes 1e4 by photoirradiation in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) at room temperature: (A) Spectral changes for 1; (B) Spectral
changes for 2; (C) Spectral changes for 3; (D) Spectral changes for 4; (E) Color changes for 1e4.

556
G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
silica gel using petroleum ether as eluent to afford 4.30 g compound
by addition of water. The product was extracted with diethyl ether.
The combined organic layers was dried over MgSO4, filtered, and
concentrated. The crude product was purified by column chroma-
tographyon silica gel using petroleum ether/ethyl acetate(v/v ¼ 6/1)
as eluent to afford 0.39 g compound 1 as a reddish brown liquid in
32% yield. Calcd for C24H15F6NS2: Calcd C, 58.17; H, 3.05; N, 2.83.
7 as a white crystalline solid in 63% yield. M.P. 322e323 K; 1H NMR
(400 MHz, CDCl3):
benzothiophene-H), 7.48 (d, 1H, J ¼ 8.0 Hz, benzothiophene-H),
, KBr, cmꢁ1): 723,
d 2.52 (s, 3H, eCH3), 7.40 (m, 2H,
7.80 (t, 1H, J ¼ 8.0 Hz, benzothiophene-H); IR (
n
754, 827, 858, 968, 1028, 1137, 1204, 1282, 1330, 1354, 1387, 1435,
1462, 1536, 1706, 2930, 3469.
Found C, 58.44; H, 2.87; N, 2.67; 1H NMR(400 MHz, CDCl3):
d 1.92 (s,
3H, eCH3), 2.29(s, 3H, eCH3), 7.13 (t,1H, J ¼ 8.0 Hz, pyridine-H), 7.36
(t, 2H, J ¼ 8.0 Hz, benzothiophene-H), 7.45 (s, 1H, thiophene-H),7.49
(d, 1H, J ¼ 8.0 Hz, benzothiophene-H), 7.58 (d, 1H, J ¼ 8.0 Hz,
pyridine-H), 7.64 (t,1H, J ¼ 8.0 Hz, pyridine-H), 7.73 (d,1H, J ¼ 8.0 Hz,
benzothiophene-H), 8.51 (d, 1H, J ¼ 8.0 Hz, pyridine-H); 13C NMR
2.2.6. 1-(2-Methyl-3-benzothiophene)-2-[2-methyl-5-(2-pyridyl)-
3-thienyl]perfluorocyclopentene (1)
To a stirred anhydrous THF (80 mL) of compound 5a (0.70 g,
2.50 mmol) was added dropwise a 2.5 M n-BuLi/hexane solution
(1 mL, 2.50 mmol) at 195 K under argon atmosphere. After 30 min,
THF (15 mL) containing compound 7 (0.85 g, 2.50 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
(100 MHz, CDCl3): d 14.9, 15.0, 118.4, 120.3, 122.1, 122.2, 123.8, 124.5,
124.9,125.2,126.4,136.7,138.2,139.5,142.4,142.6,144.6,149.6,151.5;
IR (n
, KBr, cmꢁ1): 773, 893, 965, 993,1051,1112,1142,1192,1275,1341,
1400, 1619, 1685, 3130, 3414.
1.80
1.80
A
B
Vis
Vis
UV
UV
1.35
1.35
0.90
0.45
0.00
0.90
UV
UV
Vis
Vis
0.45
0.00
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
C
1.80
1.35
0.90
0.45
0.00
D
1.80
1.35
0.90
0.45
0.00
Vis
Vis
UV
UV
UV
Vis
UV
Vis
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
E
Fig. 4. Absorption spectra and color changes of diarylethenes 1e4 by photoirradiation in PMMA films (10%, w/w) at room temperature: (A) Spectral changes for 1; (B) Spectral
changes for 2; (C) Spectral changes for 3; (D) Spectral changes for 4; (E) Color changes for 1e4.

G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
557
2.2.7. 1-(2-Methyl-3-benzothiophene)-2-[2-methyl-5-(3-pyridyl)-
2.2.9. 1-(2-Methyl-3-benzothiophene)-2-[2-methyl-5-phenyl-3-
3-thienyl]perfluorocyclopentene (2)
thienyl]perfluorocyclopentene (4)
Diarylethene 2 was prepared according to the method used for
Diarylethene 4 was prepared according to the method used for
diarylethene 1 from 3-bromo-5-phenyl-2-methylthiophene (5d).
The crude product was purified by column chromatography on
silica gel using petroleum ether/ethyl acetate (v/v ¼ 10/1) as eluent
to afford 0.53 g compound 1 as a reddish brown liquid in 38% yield.
Calcd for C24H16F6S2: Calcd C, 60.72; H, 3.26; S, 12.97. Found C,
diarylethene
1 from 3-bromo-5-(3-pyridyl)-2-methylthiophene
(5b). The crude product was purified by column chromatography
on silica gel using petroleum ether/ethyl acetate (v/v ¼ 2/1) as
eluent to afford 0.49 g compound 2 as a reddish brown liquid in 40%
yield. Calcd for C24H15F6NS2: Calcd C, 58.17; H, 3.05; N, 2.83. Found
C, 57.97; H, 3.21; N, 2.94; 1H NMR(400 MHz, CDCl3):
d
1.98 (s, 3H,
60.49; H, 3.41; S, 13.13; 1H NMR(400 MHz, CDCl3):
d 1.93 (s, 3H,
eCH3), 2.31(s, 3H, eCH3), 7.21 (s, 1H, thiophene-H), 7.34 (m, 3H,
benzothiophene-H), 7.56 (d, 1H, J ¼ 8.0 Hz, benzothiophene-
H),7.68e7.78 (m, 2H, pyridine-H), 8.50 (d, 1H, J ¼ 8.0 Hz, pyridine-
H), 8.67 (d, 1H, J ¼ 8.0 Hz, pyridine-H); 13C NMR (100 MHz, CDCl3):
eCH3), 2.30(s, 3H, eCH3), 7.19 (s, 1H, thiophene-H), 7.27e7.37 (m,
5H, phenyl-H), 7.44 (t, 2H, J ¼ 8.0 Hz, benzothiophene-H),7.58 (d,
1H, J ¼ 8.0 Hz, benzothiophene-H), 7.76 (d, 1H, J ¼ 8.0 Hz,
benzothiophene-H); 13C NMR (100 MHz, CDCl3):
d 14.9, 15.3, 107.2,
d
14.8,15.3,120.2, 122.0,122.1,122.1,123.6,123.8,124.6,125.0,125.5,
109.3, 120.3, 122.0, 122.2, 122.4, 124.5, 124.9, 125.1, 125.5, 127.8,
129.3, 132.6, 138.1, 138.2, 142.5, 142.8, 146.7, 148.8; IR (
n
, KBr, cmꢁ1):
128.9, 133.2, 136.4, 138.2, 141.7, 141.8, 142.5; IR (
n
, KBr, cmꢁ1): 730,
706, 756, 801, 892, 966, 993,1049, 1113, 1141, 1192, 1275, 1339, 1400,
1619, 1684, 3128, 3414.
754, 837, 892, 967, 993, 1020, 1051, 1135, 1192, 1276, 1340, 1400,
1436, 1630, 3130, 3452.
2.2.8. 1-(2-Methyl-3-benzothiophene)-2-[2-methyl-5-(4-pyridyl)-
3-thienyl]perfluorocyclopentene (3)
3. Results and discussion
Diarylethene 3 was prepared according to the method used for
diarylethene 1o from 3-bromo-5-(4-pyridyl)-2-methylthiophene (5c).
The crude product was purified by column chromatography on silica
gel using petroleum ether/ethyl acetate (v/v ¼ 2/1) as eluent to give
0.44 g compound 3 as a reddish brown liquid in 36% yield. Calcd for
C24H15F6NS2: Calcd C, 58.17; H, 3.05; N, 2.83. Found C, 58.37; H, 2.96; N,
3.1. Photochromism of diarylethenes 1e4
The photochromic behavior of diarylethenes 1e4 induced by
photoirradiation was measured in both hexane (2.0 ꢀ 10ꢁ5 mol
L
ꢁ1) and PMMA films (10%, w/w) at room temperature. Their
absorption spectra and color changes induced by alternating
irradiation with UV and visible light were determined (Fig. 3.). The
absorption maximum of diarylethene 1o in hexane was observed
2.96; 1H NMR(400 MHz, CDCl3):
d 1.97 (s, 3H, eCH3), 2.31(s, 3H, eCH3),
7.29 (t, 2H, J ¼ 8.0 Hz, pyridine-H), 7.31 (s,1H, thiophene-H), 7.32e7.37
(m, 2H, benzothiophene-H), 7.54 (t,1H, J ¼ 8.0 Hz, benzothiophene-H),
7.74 (t, 1H, J ¼ 8.0 Hz, benzothiophene-H), 8.55 (d, 2H, J ¼ 8.0 Hz,
at 263 nm due to a
light (
emerged. The formation of the ring-closed isomer 1c resulted in
a change in the color of solution from colorless to purple red. For
the reverse reaction, the purple red solution of 1c faded to
pep* transition [47]. Upon irradiation with UV
¼ 297 nm), a new absorption band centered at 546 nm
l
pyridine-H); 13C NMR (100 MHz, CDCl3):
d 14.8,15.3,119.5,122.0,122.1,
124.6, 125.0, 125.7, 128.9, 130.9, 138.1, 138.3, 138.6, 140.3, 142.5, 144.0,
150.4; IR (
, KBr, cmꢁ1): 756, 814, 893, 965, 992, 1050, 1113, 1141, 1193,
1275, 1340, 1402, 1596, 1721, 3129, 3414.
n
colorless upon irradiation with visible light
(l > 450 nm).
1o
2o
2c
1c
0
2
4
6
8
10
0
2
4
6
8
10
3o
3c
4o
4c
0
2
4
6
8
10
0
2
4
6
8
10
T/min
Fig. 5. The photoconversion ratios of diarylethenes 1e4 analysized by HPLC method in the photostationary state in hexane.

558
G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
Table 1
Absorption parameters and photochromic reactivity of diarylethenes 1e4 in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and in PMMA films (10%, w/w).
Compound
lo,max/nma (ε/L molꢁ1 cmꢁ1
)
lc,max/nmb (ε/L molꢁ1 cmꢁ1
)
Fc
Conversion at PSS in hexane
Hexane
PMMA film
Hexane
PMMA film
Foec
Fceo
1
2
3
4
263 (3.32 ꢀ 104)
261 (2.18 ꢀ 104)
261 (3.26 ꢀ 104)
262 (3.63 ꢀ 104)
263
262
262
264
546 (1.94 ꢀ 104)
534 (1.40 ꢀ 104)
541 (1.51 ꢀ 104)
536 (1.50 ꢀ 104)
558
547
552
548
0.53
0.40
0.34
0.16
0.012
0.020
0.023
0.020
79%
78%
75%
94%
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization reaction (Foec) and cycloreversion reaction (Fceo), respectively.
b
c
Diarylethenes 2e4 exhibited similar photochromic properties with
absorption maxima at 534, 541, and 536 nm, respectively. As
shown in Fig. 4, the absorption maxima of diarylethenes 1ce4c in
PMMA films were redshifted compared to those in hexane, which
could be possibly ascribed to polar effect of polymer matrix as well
as to stabilization of molecular arrangement in solid state
[19e22,48]. In addition, the photoconversion ratios of 1e4 from
ring-opened to ring-closed isomers in photostationary state in
hexane were analyzed by HPLC (Fig. 5.). Compared to that of
diarylethene 4 (94%), the photoconversion ratios of pyridine-
containing diarylethenes 1e3 (75 w 79%) in hexane were signifi-
cantly lower, suggesting that the presence of nitrogen atom in the
pyridine moiety may increase the portion of molecules with
a parallel conformation.
3.2. Acidichromism of diarylethenes 1e3
The multiple switching behavior of diarylethenes 1e3 were
studied with stimulation of acid/base and light. The structural and
color changes among 1e3 and their N-protonated forms 10e30 in
acetonitrile (2.0 ꢀ 10ꢁ5 mol Lꢁ1) are illustrated in Fig. 7. Addition of
trifluoroacetic acid in acetonitrile (5.0
m
L, 6.8 ꢀ 10ꢁ3 mol Lꢁ1) to the
solution of 1oe3o produced the N-protonated diarylethenes
1o0e3o0. The absorption maxima bathochromically shifted from
306 nm to 342 nm for 1o0 (ε ¼ 8.03 ꢀ 104 L molꢁ1ꢁc1mꢁ1), from
290 nm to 299 nm for 2o0 (ε ¼ 6.60 ꢀ 104 L molꢁ1 cm ), and from
299 nm to 345 nm for 3o0 (ε ¼ 3.58 ꢀ 104 L molꢁ1 cmꢁ1). 1o0e3o0
could be converted back into 1oe3o by neutralization with
From the data of the photochromic properties of diarylethenes
1e4 summarized in Table 1, it was noticed that compound 1, in
which the nitrogen atom is located at an ortho-position of the
pyridine moiety, has the largest molar absorption coefficients both
for ring-opened and ring-closed isomers. The molar absorption
coefficients of 1e3 for both ring-opened and ring-closed isomers
decreased in order of 1 > 3 > 2 (ortho > para > meta). For the ring-
closed isomers 1ce3c, the absorption maxima decreased in order of
1 > 3 > 2 (ortho >para > meta) in both hexane and PMMA films.
However, no significant changes in the absorption maxima of the
ring-opened isomers 1oe3o were observed.
A 1.0
0.8
1
0.6
0.4
0.2
0.0
2
3
4
The cyclization quantum yields increased in order of 3 < 2 < 1
(para (Foec,3 ¼ 0.34) < meta (Foec,2 ¼ 0.40) < ortho (Foec,1 ¼ 0.53)),
whereas the cycloreversion quantum yields decreased in order of
3 > 2 > 1 (para (Fceo,3 ¼ 0.023) > meta (Fceo,2 ¼ 0.020) > ortho
(Fceo,1 ¼ 0.012)). While the effect of position of nitrogen atom in
0
10
20
30
40
50
pyridine is consistent with that of chlorine atom in
chlorobenzene-containing diarylethenes [22], it is contrary to
that of cyano group in cyanobenzene-containing diarylethenes
[19]. Furthermore, the cyclization quantum yields of 1e3 were
significantly larger than that of 4, indicating that the presence of
nitrogen atom in the terminal aromatic ring enhanced the cycli-
zation quantum yields of the corresponding diarylethenes.
The thermal stability of the ring-opened and ring-closed isomers
of diarylethenes 1e4 was tested. At ambient temperature, exposure
of the solution of 1e4 in hexane to air for 100 h in the dark caused no
changes in color and absorption spectra. Compound 1e4 were stable
in the dark for at least one year. When the solution of 1e4 in hexane
was heated under reflux (342 K) for 24 h in darkness, no decom-
position was detected by NMR spectroscopy.
The fatigue resistance of photochromic compounds is a crucial
factor for their practical applications in optical devices [6,49]. Upon
alternating irradiation with UV and visible light, the fatigue resis-
tance of 1e4 was examined in both hexane and PMMA films. As
depicted in Fig. 6, the coloration and decoloration cycle of 1e4 could
repeat 50 times in hexane with only ca. 5% w 15% degradation, and
repeat 200 times in PMMA films with only ca. 19% w 24%
degradation.
Repeat Cycles
1.0
0.8
0.6
0.4
0.2
0.0
B
1
2
3
4
0
50
100
150
200
Repeat Cycles
Fig. 6. Fatigue resistances of diarylethenes 1e4 in air atmosphere at room tempera-
ture: (A) In hexane; (B) In PMMA films. Initial absorptance of the sample was fixed to
1.0.

G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
559
Fig. 7. The structural and color changes between diarylethenes 1e3 and diarylethenes 10e30 in acetonitrile (2.0 ꢀ 10ꢁ5 mol Lꢁ1).
triethylamine (10.0
m
L, 3.6 ꢀ 10ꢁ3 mol Lꢁ1). Upon irradiation with
3.3. Fluorescence of diarylethenes 1e4
UV light, the colorless solution of 1o0e3o0 turned into different
colors, indicating the formation of N-protonated ring-closed
isomers 1c0e3c0. Their absorption maxima were at 573 nm for 1c0
The fluorescence properties of diarylethenes 1e4 in both hexane
(5.0 ꢀ 10ꢁ5 mol Lꢁ1) and PMMA films (10%, w/w) were evaluated at
room temperature. Their emission spectra are shown in Fig. 9.
Compared to those in hexane, the emission peaks of 1e4 in PMMA
films exhibited an evident hypsochromic shift (3 nm for 1, 4 nm for
(ε
(ε
¼
1.13
1.32
ꢀ
104
104
L
molꢁ1 cmꢁ1), 549 nm for 2c0
¼
ꢀ
L
molꢁ1 cmꢁ1), and 584 nm for 3c0
(ε ¼ 1.14 ꢀ 104 L molꢁ1 cmꢁ1). Alternatively, an interconversion
between diarylethenes 1ce3c and 1c0e3c0 could be conducted by
stimulation with acid/base. Similar to reported pyridine-containing
diarylethenes [36,43], the N-protonation process was accompanied
with a notable change in the color of solution (Fig. 7) from purple
red (1c) to blue (1c0), red (2c) to purple red (2c0), and purple red (3c)
to blue (3c0). The bathochromic shift of absorption spectra of
1c0e3c0 (Fig. 8) was possibly due to the lowering of excited state
energy levels of protonated species [50]. Among another series of
isomeric asymmetric diarylethenes with a pyridylisoxazole and
pyridylthiophene, only the one with nitrogen atom at para-position
of the pyridine moiety showed obvious acidichromism [51], sug-
gesting that structure of the other aryl moiety of diarylethene and
position of the nitrogen atom in pyridine moiety have a synergic
effect on the acidichromism of pyridine-containing diarylethenes.
A
1o
4000
3000
2o
3o
2000
4o
1000
0
400
450
500
550
600
0.6
1c
A
Wavelength(nm)
1c'
0.4
0.2
0.0
B
1o
4000
3000
2000
1000
0
300
400
500
600
700
0.6
0.4
0.2
0.0
2o
3o
2c 2c
'
B
300
400
500
600
700
0.6
0.4
0.2
0.0
C
4o
3c
3c'
350
400
450
500
550
300
400
500
600
700
Wavelength(nm)
Wavelength (nm)
Fig. 9. Emission spectra of diarylethenes 1e4 at room temperature: (A) In hexane
(5.0 ꢀ 10ꢁ5 mol Lꢁ1), excited at 300 nm: (B) In PMMA films (10%, w/w), excited at
310 nm.
Fig. 8. The absorption changes of diarylethenes 1e3 with addition of acid in the
photostationary state: (A) 1; (B) 2; (C) 3.

560
G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
A
B
3000
2400
1800
1200
600
Vis
4000
3000
2000
1000
0
Vis
UV
UV
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
C
D
2500
1280
Vis
UV
Vis
UV
2000
1500
1000
500
0
960
640
320
0
350
400
450
500
550
360
420
480
540
600
Wavelength(nm)
Wavelength(nm)
Fig. 10. Emission intensity changes of diarylethenes 1e4 by photoirradiation in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) at room temperature, excited at 300 nm: (A) 1, (B) 2, (C) 3, (D) 4.
2, 5 nm for 3, and 10 nm for 4), which was in agreement with the
results in our previous reports [19e21]. The emission intensities of
1e4 decreased in order of 1 > 2 > 3 > 4 in both hexane and PMMA
films. The fluorescence quantum yields of 1e4 were determined as
0.034, 0.059, 0.049, and 0.048, respectively (referenced to anthra-
cene). The result showed that the fluorescence quantum yield could
A
B
3500
2800
2100
1400
700
4250
Vis
Vis
UV
UV
3400
2550
1700
850
0
0
350
400
450
500
550
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
C
D
900
750
600
450
300
150
0
2800
2100
1400
700
0
Vis
UV
Vis
UV
376
423
470
517
564
400
450
500
550
600
Wavelength(nm)
Wavelength(nm)
Fig. 11. Emission intensity changes of diarylethenes 1e4 by photoirradiation in PMMA films at room temperature, excited at 310 nm: (A) 1, (B) 2, (C) 3, (D) 4.

G. Liu et al. / Dyes and Pigments 95 (2012) 553e562
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position in pyridine.
Similar to most reported diarylethenes [52e56], 1e4 exhibited
an evident fluorescent switch upon photoisomerization from ring-
opened isomers to ring-closed isomers in both hexane and PMMA
films. As shown in Figs. 10 and 11, irradiation of 1oe4o with UV
light caused the formation of the non-fluorescent ring-closed
isomers 1ce4c. Reversely, irradiation 1ce4c with visible light
regenerated the ring-opened isomers 1oe4o and recovered the
original fluorescence emission. The emission intensities of 1oe4o
were quenched to ca. 55% for 1, 35% for 2, 25% for 3, and 34% for
4, when photostationary state was reached. Compound 3, which
has the nitrogen atom at para-position of pyridine, displayed the
highest fluorescent modulation efficiency of 75%. Similarly, fluo-
rescent modulation efficiencies of 1e4 in PMMA films were
determined as 73% for 1, 76% for 2, 76% for 3, and 67% for 4. The
residual fluorescence for 1e4 in photostationary state may be
attributed to the incomplete cyclization reaction as well as to the
existence of parallel conformation [21,57]. In comparison with the
diarylethenes reported in our previous research [19,21], 1e4 had
significantly enhanced fluorescent modulation efficiencies in both
liquid and solid media, which is ideal for applications, such as
optical recording media and fluorescent photoswitches [23,58e61].
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4. Conclusions
In this paper, three new asymmetric diarylethenes containing
a benzothiophene and a pyridylthiophene were synthesized. These
compounds showed notable photochromic characteristics and
functioned as fluorescence switches in hexane and PMMA films.
These compounds also underwent reversible isomerizations under
acid/base stimuli, and the absorption maxima of the ring-closed
isomers exhibited remarkable bathochromic shifts accompanied
with significant color change in acetonitrile. These results revealed
that both the pyridine ring and the position of nitrogen atom in
pyridine had significant effects on the photochromic behavior of
the corresponding diarylethenes.
Acknowledgments
[30] Matsuda K, Irie M.
A diarylethene with two nitronyl nitroxides: photo-
This work was supported by the National Natural Science
Foundation of China (20962008, 21162011), the Project of Jiangxi
Academic and Technological leader (2009DD00100), the Project of
Jiangxi Advantage Sci-Techn Innovative Team (20113BCB24023),
the Project of Jiangxi Youth Scientist, and the Project of the Science
Funds of Jiangxi Education Office (GJJ10241, GJJ1067).
switching of intramolecular magnetic interaction. J Am Chem Soc 2000;122:
7195e201.
[31] Kawai T, Kim M-S, Sasaki T, Irie M. Fluorescence switching of photochromic
diarylethenes. Opt Mater 2002;21:275e8.
[32] Utsumi H, Nagahama D, Nakano H, Shirota Y. Synthesis of a novel family of
photochromic amorphous molecular materials based on dithienylethene,
their photochromic properties and application for dual image formation.
J Mater Chem 2002;12:2612e9.
[33] Yamaguchi T, Inagawa T, Nakazumi H, Irie S, Irie M. Photoinduced pitch
changes in chiral nematic liquid crystals formed by doping with chiral dia-
rylethene. J Mater Chem 2001;11:2453e8.
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