Photochromism of new unsymmetrical isomeric diarylethenes bearing a pyridine group

Article abstract of DOI:10.1016/j.tet.2012.01.058

A new class of unsymmetrical isomeric diarylethenes bearing a terminal pyridine group was synthesized and their structures were determined by single-crystal X-ray diffraction analysis. The effects of the nitrogen atom position in the pyridine group on their properties including photochromism, acidchromism, and fluorescence were investigated systematically. Among the three isomeric derivatives, the cyclization quantum yield and the absorption maxima of the ortho-substituted diarylethene were the biggest, while the fluorescence quantum yield of the para-substituted diarylethene was the biggest. Furthermore, the three isomeric diarylethenes exhibited multi-addressable switching behaviors by the stimulation of acid/base and light. Addition of trifluoroacetic acid to solutions of these diarylethenes produced the protonated derivatives, which also showed excellent photochromic behaviors, but only the para-substituted diarylethene exhibited a notable color change of solution from crimson to violet. The results revealed that the effects of the nitrogen atom position in the terminal pyridine group played a very important role during the process of photoisomerization for these isomeric diarylethene derivatives.

Full text of DOI:10.1016/j.tet.2012.01.058

Tetrahedron 68 (2012) 2267e2275
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Photochromism of new unsymmetrical isomeric diarylethenes bearing a pyridine
group
*
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:
A new class of unsymmetrical isomeric diarylethenes bearing a terminal pyridine group was synthesized
and their structures were determined by single-crystal X-ray diffraction analysis. The effects of the ni-
trogen atom position in the pyridine group on their properties including photochromism, acidchromism,
and fluorescence were investigated systematically. Among the three isomeric derivatives, the cyclization
quantum yield and the absorption maxima of the ortho-substituted diarylethene were the biggest, while
the fluorescence quantum yield of the para-substituted diarylethene was the biggest. Furthermore, the
three isomeric diarylethenes exhibited multi-addressable switching behaviors by the stimulation of acid/
base and light. Addition of trifluoroacetic acid to solutions of these diarylethenes produced the pro-
tonated derivatives, which also showed excellent photochromic behaviors, but only the para-substituted
diarylethene exhibited a notable color change of solution from crimson to violet. The results revealed
that the effects of the nitrogen atom position in the terminal pyridine group played a very important role
during the process of photoisomerization for these isomeric diarylethene derivatives.
Received 22 November 2011
Received in revised form 10 January 2012
Accepted 18 January 2012
Available online 24 January 2012
Keywords:
Diarylethene
Pyridine
Photochromism
Nitrogen atom position
Acidchromism
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
photomodulation of the chemical reactivity, and biochemistry.
Recently, Piard et al. developed a ratiometric fluorescent photo-
Organic photochromic systems have recently received in-
creasing attention in materials science because of the widespread
potential application in photonic devices. Various types of photo-
chromic compounds, such as spirobenzopyrans,1 azobenzenes,2
fulgides,3 and diarylethenes,4 have hitherto been developed.
Among such compounds, diarylethene derivatives are one of the
most promising candidates for optoelectronic devices, such as op-
tical memories and photoswitches, mainly due to the excellent
thermal stability of the respective isomers, notable fatigue re-
sistance, and high reactivity in the solid state.5
The six-membered pyridine ring has good aromaticity with the
similar structure to the benzene ring.6 The nitrogen atom of pyri-
dine ring can easily occur to nucleophilicity substituted reaction
resulted from its vigoroso electronegativity.7 Additionally, pyridine
bases have high biological activities, which are widely used as core
components or modifiers of pharmaceutical products and agricul-
tural chemicals.8 Recently, diarylethenes bearing a pyridine group
have attracted more and more attention, and the pyridine ring is
regarded as side group connected with other aromatic rings, such
as thiazolyl,9 thiophene,10 and benzothiophene.11 These diary-
lethene derivatives exhibited some unusual characteristics, which
switch based on a diarylethene with two terminal pyridine rings
linked with thiazolyl moieties.9 Branda et al. reported a photo-
controlled molecular switch based on a symmetrical dithienyle-
thene bearing two terminal pyridine rings and found it could
control biological organism and retain its photoswitching behavior
in vivo.10b Furthermore, the pyridyl group can directly connect to
perfluorocyclopentene ring. Our group explored a new class of
unsymmetrical diarylethenes bearing a pyridine unit directly
linked with the ethene moiety, which exhibited excellent photo-
chromism even in the crystalline phase.12 Diarylethenes bearing
pyridine units could be used as fluorescent probes of recognizing
ions due to the excellent coordinating ability of pyridyl with some
specific ions.13 For example, Yi et al. developed a multiresponsive
fluorescence switch based on diarylethene and terpyridine units,
which could serve as a detector for the biological process of metal
ion transmembrane transport.13a Tian et al. prepared a new Iridium
complex coordinating to the photochromic diarylethene, which
showed excellent near-Infrared photochromic behaviors accom-
panied by efficient quenching of phosphorescence emission.13b All
of the results reveal that diarylethenes bearing pyridine units have
attracted much attention as highly promising candidates for opto-
electronic devices due to their specific characteristics.
could
be
potentially
applied
in
photoswitches,
the
Generally, the photochromic behaviors of diarylethene de-
rivatives can be effectively modified by suitable functionalization of
the aromatic rings with different substituents.14 Apart from the fact
* Corresponding author. Tel./fax: þ86 791 83831996; e-mail address: pushouzhi@
tsinghua.org.cn (S. Pu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.01.058

2268
G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
that the substituent effects have an important role to play in
modifying the properties of diarylethenes, the position of the
substituent can also affect their properties significantly, which can
be used for the fine tuning of the optoelectronic behaviors of dia-
rylethene derivatives. Although many reports concerning the sub-
stituent position effects on the photochromic properties have been
hitherto reported, the results mainly emphasis on effects of a cer-
tain same substituent linked with the different position of the
terminal benzene ring on their photochromic properties.15 To the
best of our knowledge, the nitrogen atom position effect of pyridine
ring on the properties of diarylethenes has not been hitherto
reported. In this study, we have explored a new class of un-
symmetrical isomeric diarylethenes bearing both isoxazole and
thiophene moieties with a terminal pyridine unit and mainly dis-
cussed the nitrogen atom position effect of pyridine ring on their
physicochemical properties. The synthesized diarylethenes are
1-(3,5-dimethyl-4-isoxazolyl)-2-[2-methyl-5-(2-pyridyl)-3-thienyl]
perfluorocyclopentene (1o), 1-(3,5-dimethyl-4-isoxazolyl)-2-[2-
methyl-5-(3-pyridyl)-3-thienyl]perfluorocyclopentene (2o), and
1-(3,5-dimethyl-4-isoxazolyl)-2-[2-methyl-5-(4-pyridyl)-3-thienyl]
perfluorocyclopentene (3o). All of these derivatives exhibited
excellent photochromism in solution, in PMMA amorphous films,
and even in their respective crystalline phase. The photochromic
scheme of diarylethenes 1e3 is shown in Scheme 1.
Suitable crystals 1oe3o were obtained by slow evaporation of
the mixture solutions containing chloroform and hexane. All the
measurements were made on a Bruker SMART APEX II CCD dif-
fractometer 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.
All nonhydrogen atoms were refined anisotropically. Crystallo-
graphic data for the structures of diarylethenes 1oe3o have been
deposited with in the Cambridge Crystallographic Data Centre as
supplemental publication CCDC 846204 for 1o, CCDC 846205 for 2o,
and CCDC 846206 for 3o. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (e-mail: deposit@ccdc.cam.ac.uk; fax: þ44 0 1223 336033).
2.2. Synthesis
The synthetic route used to obtain diarylethenes 1oe3o is
shown in Scheme 2. Suzuki coupling of three bromopyridine de-
rivatives with a thiophene boronic acid gave the pyridylthiophene
derivatives 5aec. The mono-substituted (3,5-dimethyl-4-
isoxazolyl)perfluorocyclopentene (7) was synthesized by the sim-
ilar reported method using 3,5-dimethyl-4-iodoisoxazole (6) as raw
material.16 Finally, compounds 5aec were lithiated and then sep-
arately coupled with compound 7 to give diarylethenes 1oe3o,
respectively. The structures of 1oe3o were confirmed by elemental
analysis, NMR, and IR.
F
F
F
F
F
F
F
F
UV
Vis
F
F
F
F
2.2.1. 3-Bromo-2-methyl-5-(2-pyridyl)thiophene (5a). Compound
5a was prepared by reacting 3-bromo-2-methyl-5-thienylboronic
acid17 (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 slowly cool to the
room temperature. After being extracted with ether, the organic
layer was dried over MgSO4, filtrated, and evaporated. The crude
product was purified by column chromatography on SiO2 using
hexane as the eluent and 3.00 g of 5a obtained as a pale yellow solid
N
R
N
R
O
S
N
O
S
N
1o:R=
2o: R=
3o: R=
1c:R=
2c: R=
3c: R=
N
N
in 83% yield. Mp 71e72 ꢀC; 1H NMR (400 MHz, CDCl3):
d 2.44 (s, 3H, e
CH3), 7.13e7.16 (m, 1H, pyridineeH), 7.36 (s, 1H, thiopheneeH),
7.55 (d, J¼8.0 Hz, 1H, pyridineeH), 7.65e7.69(m, 1H, pyridineeH),
N
N
8.53 (d, J¼8.0 Hz, 1H, pyridineeH); IR (
n
, KBr, cmꢁ1): 732, 771, 882,
1012, 1094, 1158, 1288, 1327, 1481, 1628, 3458.
Scheme 1. Photochromism of diarylethenes 1e3.
2.2.2. 3-Bromo-2-methyl-5-(3-pyridyl)thiophene (5b). Compound
5b was prepared by a method similar to that used for 5a. The crude
product was purified by column chromatography on SiO2 using pe-
troleum ether/ethyl acetate (v/v¼8/1) as an eluent to give 2.60 g 5b
as a pale yellow solid in 72% yield. Mp 39e40 ꢀC; 1H NMR (400 MHz,
2. Experiment
2.1. General methods
Melting point was taken on a WRS-1B melting point apparatus.
NMR spectra were recorded on a Bruker AV400 (400 MHz) spec-
trometer with CDCl3 as the solvent and tetramethylsilane as an in-
ternal standard. Infrared spectra (IR) were recorded on a Bruker
Vertex-70 spectrometer. Elemental analysis was measured with an
elemental analyzer labeled the PE CHN 2400 analyzer. Fluorescent
spectra were measured using a Hitachi F-4500 fluorimeter. Ab-
sorption spectra were measured using an Agilent 8453 UV/vis
spectrophotometer. Photoirradiation was carried out using an SHG-
200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet, and
a BMH-250 Visible lamp. The required wavelength was isolated by
the use of the appropriate filters. The quantum yields of cyclization/
cycloreversion were determined by comparing the reaction yield
with the known yield of 1,2-bis(2-methyl-5-phenyl-3-thienyl)per-
fluorocyclopentene in hexane.5c Dissolved ultrasonically 10 mg of
diarylethene sample and 100 mg PMMA into 1.0 mL chloroform, the
filmwas prepared by spin-coating on the surface of quartz substrate.
CDCl3):
1H, pyridineeH), 7.77(d, J¼8.0 Hz,1H, pyridineeH), 8.51(d, J¼8.0 Hz,
1H, pyridineeH), 8.78 (s,1H, pyridineeH); IR (
, KBr, cmꢁ1): 703, 797,
d 2.44 (s, 3H, eCH3), 7.16 (s,1H, thiopheneeH), 7.29e7.32 (m,
n
946, 1017, 1124, 1164, 1320, 1454, 1489, 1614, 3437.
2.2.3. 3-Bromo-2-methyl-5-(4-pyridyl)thiophene (5c). Compound
5c was prepared by a method similar to that used for 5a. The crude
product was purified by column chromatography on SiO2 using
petroleum ether/ethyl acetate (v/v¼6/1) as an eluent to give 2.82 g
5c as a pale yellow solid in 78% yield. Mp 88e89 ꢀC; 1H NMR
(400 MHz, CDCl3):
7.37 (d, 2H, J¼8.0 Hz, pyridineeH), 8.58(d, J¼8.0 Hz, 2H,
pyridineeH); IR (
, KBr, cmꢁ1): 714, 810, 864, 1011, 1163, 1220, 1323,
d 2.44 (s, 3H, eCH3), 7.30 (s, 1H, thiopheneeH),
n
1458, 1495, 1597, 3431.
2.2.4. (3,5-Dimethyl-4-isoxazolyl)perfluorocyclopentene
(7). To
a stirred THF solution (80 mL) of compound 6 (3.62 g, 20.10 mmol)

G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
2269
Br
Br
Br
R
R
S
Pd(PPh3)4
Na2CO3
B(OH)2
S
4
N
F
F
5a: R =
5b: R =
5c: R =
F
F
F
F
N
n-BuLi/THF
195 K
N
R
O
S
N
N
1o: R =
2o: R =
3o: R =
F
F
N
F
F
I
F
n-BuLi/THF
C5F8,195 K
F
N
F
O
N
N
O
6
7
Scheme 2. Synthetic route for diarylethenes 1oe3o.
was added dropwise a 2.5 M n-BuLi/hexane solution (8.80 mL,
22 mmol) at 195 K under nitrogen atmosphere. Stirring was con-
tinued for 30 min at 195 K, octafluorocyclopentene (C5F8) (2.80 mL,
20.50 mmol) was slowly added and the reaction mixture was stir-
red for 2 h at this low temperature. The reaction was quenched by
water. After being extracted with ether, the organic layer was
washed with 1 M aqueous HCl and water. The organic layer dried
over anhydrous MgSO4, filtrated, and evaporated. The crude prod-
uct was purified by column chromatography on silica gel using
petroleum ether as the eluent to give 2.80 g compound 7 as pale
using petroleum ether/ethyl acetate (v/v¼3/1) as the eluent to give
0.47 g 2o as a pale yellow solid in 43% yield. Mp 160e161 ꢀC; calcd
for C20H14F6N2OS: Calcd C, 54.05; H, 3.18; N, 6.30. Found C, 53.89; H,
3.26; N, 6.41; 1H NMR (400 MHz, CDCl3):
d 1.79 (s, 3H, eCH3),1.99(s,
3H, eCH3), 3.61 (s, 3H, eCH3), 6.91 (s, 1H, pyrroleeH), 7.29 (s, 1H,
thiopheneeH), 7.34 (t, 1H, J¼8.0 Hz, pyridineeH), 7.82 (d, 1H,
J¼8.0 Hz, pyridineeH), 8.54 (d, 1H, J¼8.0 Hz, pyridineeH) 8.80 (t,
1H, J¼8.0 Hz pyridineeH); 13C NMR (100 MHz, CDCl3):
d 11.3, 14.5,
33.0, 105.7, 112.7, 118.7, 123.7, 123.9, 126.3, 128.8, 129.3, 130.8, 132.7,
135.8, 138.6, 146.6, 148.8; IR (n
, KBr, cmꢁ1): 698, 747, 804, 847, 893,
yellow liquid in 48% yield. 1H NMR (400 MHz, CDCl3):
d
2.26 (s, 3H, e
10.7, 12.2,
945, 989, 1065, 1128, 1192, 1276, 1341, 1424, 1493, 1594, 1657, 2921,
3677.
CH3), 2.43 (s, 3H, eCH3); 13C NMR (100 MHz, CDCl3):
d
99.9, 158.5, 170.6.
2.2.7. 1-(3,5-Dimethyl-4-isoxazolyl)-2-[2-methyl-5-(4-pyridyl)-3-
thienyl]perfluorocyclopentene (3o). Diarylethene 3o was prepared
by a method similar to that used for diarylethene 1o using 3-
bromo-5-(4-pyridyl)-2-methylthiophene (5c) instead of 5a. The
crude product was purified by column chromatography on silica gel
using petroleum ether/ethyl acetate (v/v¼3/1) as the eluent to give
0.50 g 3o as a pale yellow solid in 48% yield. Mp 153e154 ꢀC; calcd
for C20H14F6N2OS: calcd C, 54.05; H, 3.18; N, 6.30. Found C, 54.36; H,
2.2.5. 1-(3,5-Dimethyl-4-isoxazolyl)-2-[2-methyl-5-(2-pyridyl)-3-
thienyl]perfluorocyclopentene (1o). To a stirred anhydrous THF
(80 mL) of compound 5a (0.63 g, 2.50 mmol) was added dropwise
a 2.5 M n-BuLi/hexane solution (1.00 mL, 2.50 mmol) at 195 K under
argon atmosphere. After 30 min, 15 mL THF containing compound 7
(0.70 g, 2.50 mmol) was added and the reaction mixture was stirred
for 2 h at this low temperature. The reaction was allowed to slowly
warm to the room temperature and quenched by water. The
product was extracted with ether, dried with MgSO4, filtrated, and
evaporated in vacuo. The crude product was purified by column
chromatography on silica gel using petroleum ether/ethyl acetate
(v/v¼4/1) as the eluent to give 0.34 g diarylethene 1o as a pale
yellow solid in 31% yield. Mp 88e89 ꢀC; calcd for C20H14F6N2OS:
calcd C, 54.05; H, 3.18; N, 6.30. Found C, 54.42; H, 3.04; N, 6.18; 1H
3.09; N, 6.14; 1H NMR (400 MHz, CDCl3):
d 1.78 (s, 3H, eCH3), 2.04(s,
3H, eCH3), 3.61 (s, 3H, eCH3), 6.90 (s, 1H, pyrroleeH), 7.41 (d, 2H,
J¼8.0 Hz, pyridineeH), 7.44 (s, 1H, thiopheneeH), 8.61 (d, 2H,
J¼8.0 Hz, pyridineeH); 13C NMR (100 MHz, CDCl3):
d 11.3,14.6, 33.0,
105.7, 109.7,112.6,118.7, 119.4,125.0, 126.5, 135.8, 139.3,140.2, 142.7,
150.5; IR (n
, KBr, cmꢁ1): 749, 812, 844, 895, 989, 1067, 1135, 1198,
1276, 1318, 1346, 1430, 1500, 1594, 1653, 2924, 3030, 3416.
NMR (400 MHz, CDCl3): d 2.04 (s, 3H, eCH3), 2.29(s, 3H, eCH3), 3.59
(s, 3H, eCH3), 6.93 (s, 1H, pyrroleeH), 7.18 (t, 1H, J¼8.0 Hz,
pyridineeH), 7.49 (s, 1H, thiopheneeH), 7.62 (d, 1H, pyridineeH),
7.71 (t, 1H, J¼8.0 Hz, pyridineeH), 8.55 (d, 1H, pyridineeH); 13C
3. Results and discussion
3.1. Photochromism of diarylethenes in solution and solid
medium
NMR (100 MHz, CDCl3):
d 11.3, 14.7, 105.6, 109.9, 112.7, 115.9, 118.5,
118.7, 122.3, 124.0, 126.0, 135.9, 136.7, 143.0, 143.2, 149.6, 151.5; IR (n,
KBr, cmꢁ1): 743, 774, 810, 894, 989, 1068, 1128, 1196, 1272, 1344,
1432, 1488, 1588, 1642, 3405.
The photochromic behaviors of diarylethenes 1oe3o induced by
photoirradiation at room temperature were measured both in
hexane (2.0ꢂ10ꢁ5 mol Lꢁ1) and in PMMA films (10%, w/w). The
changes in the absorption spectra of diarylethene 1 and the color of
diarylethenes 1oe3o in hexane induced by alternating irradiation
with UV and visible light are shown in Fig. 1. Diarylethene 1o
exhibited a sharp absorption peak at 303 nm in hexane, which
2.2.6. 1-(3,5-Dimethyl-4-isoxazolyl)-2-[2-methyl-5-(3-pyridyl)-3-
thienyl]perfluorocyclopentene (2o). Diarylethene 2o was prepared
by a method similar to that used for diarylethene 1o using 3-
bromo-5-(3-pyridyl)-2-methylthiophene (5b) instead of 5a. The
crude product was purified by column chromatography on silica gel
aroused from
pep
* transition.18 Upon irradiation with 297 nm

2270
G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
A
B
0.44
0.33
0.22
0.11
0.00
Vis
UV
UV
Vis
300 s
UV
200
100
40
20
5
Vis
0
315
420
525
630
735
Wavelength (nm)
Fig. 1. Absorption spectral changes of diarylethene 1 (A) and color changes of diarylethenes 1e3 (B) upon alternating irradiation with UV and visible light in hexane
(2.0ꢂ10ꢁ5 mol Lꢁ1) at room temperature.
light, a new visible absorption band centered at 534 nm emerged
while the original peak at 303 nm decreased due to the formation
of the closed-ring isomer 1c. This could be seen with the naked eye,
as the colorless solution of 1o turned purple red. Alternatively, the
purple red solution could be bleached completely to colorless upon
at the photostationary state in solution. But they were much lower
than that of diarylethenes bearing a pyrrole moiety.19 In PMMA
films, diarylethenes 1e3 also showed similar photochromism to
that in hexane. The changes in the absorption spectra of diary-
lethene 1 and the color changes of diarylethenes 1e3 are shown in
Fig. 3. Upon irradiation with 297 nm UV light, the colorless diary-
lethene 1o/PMMA film turned violet, of which the absorption
maximum was observed at 544 nm; while the colors of other two
diarylethene/PMMA films changed from colorless to purple red
with the appearance of a new broad absorption band centered at
531 nm for 2 and 532 nm for 3 due to the formation of the closed-
ring isomers 2c and 3c. All colored diarylethene/PMMA films could
be bleached completely by irradiating of visible light with appro-
priate wavelength. Compared with that in solution, the absorption
maxima of the closed-ring isomers 1ce3c exhibited an evidently
bathochromic shift in their respective PMMA film. The red shifts of
the absorption maxima of 1ce3c were 10 nm for 1c, 12 nm for 2c,
and 11 nm for 3c. This red shift phenomenon may be attributed to
the polar effect of the polymer matrix in amorphous polymer state
as observed for the majority of the reported diarylethenes.15d,20
The photochromic features of diarylethenes 1e3 in hexane and
in PMMA films are summarized in Table 1. These data revealed that
the position of the nitrogen atom in the terminal pyridine unit had
a significant effect on the photochromic properties of these diary-
lethene derivatives, such as the absorption maxima, molar ab-
sorption coefficients, and quantum yields of cyclization/
cycloreversion. Among the three isomeric compounds, the ab-
sorption maxima of the open- and closed-ring isomers of diary-
lethene 1 were at the longest wavelength both in hexane and in
a PMMA film. When the nitrogen atom was located at the meta- or
para-position of the terminal pyridine unit, the absorption maxima
of other two isomeric diarylethenes had notably decreased in both
hexane and PMMA films. The molar absorption coefficients in-
creased in the order of para-<ortho-<meta-substitution by the ni-
trogen atom in pyridine ring for the closed-ring isomers. Among
the closed-ring isomers 1ce3c, the molar absorption coefficient of
the para-substituted derivative 3c is the smallest; while that of the
meta-substituted derivative 2c is the biggest. For the open-ring
isomers 1oe3o, the molar absorption coefficient of 1o is the
smallest and that of 2o is the biggest. The cyclization and cyclo-
reversion quantum yields of diarylethene 1 were 0.47 and 0.10,
respectively. When the nitrogen atom was located at the meta- or
para-position of the terminal pyridine unit, the cyclization quan-
tums of other two isomeric diarylethenes decreased significantly
and their values were almost equal to each other (Fo-c,2¼0.37 and
Fo-c,3¼0.36). The cycloreversion quantum yields increased in the
order of meta-<ortho-<para-substitution by the nitrogen atom in
irradiation with visible light (l>450 nm), indicating that 1c
returned to the initial state 1o. As with compound 1, diarylethenes
2 and 3 also exhibited notable photochromism in hexane. Upon
irradiation with 297 nm light, absorption bands in the visible region
appeared and the colorless solutions 2o and 3o turned crimson
(Fig. 1B) due to cyclization reactions resulting in the formation of
the closed-ring isomers 2c and 3c, which their absorption maxima
were appeared at 519 and 521 nm, respectively. Both of the
crimson-colored solutions 2c and 3c could be decolorized by irra-
diation with visible light of wavelength greater than 450 nm, sug-
gesting return to the open-ring isomers 2o and 3o. In the
photostationary state, the isosbestic points for diarylethenes 1e3 in
hexane were observed at 318, 299, and 313 nm, respectively. The
photoconversion ratios from open- to closed-ring isomers of
the three derivatives in the photostationary state were analyzed
by HPLC in hexane. As shown in Fig. 2, the photoconversion
ratios of diarylethenes 1e3 were 80% for 1, 75% for 2, and 79%
for 3. Compared to the reported analogous derivative, namely 1-
(3,5-dimethyl-4-isoxazolyl)-2-(2-methyl-5-phenyl-3-thienyl)per-
fluorocyclopentene (DIMPTP),16 the photoconversion ratios of the
three isomeric diarylethenes were notably increased when arrived
1o
1c
0
0
0
2
4
6
8
10
2o
2c
5
10
15
20
3c
3o
5
10
15
20
T/min
Fig. 2. The photoconversion ratios of diarylethenes 1e3 at photostationary state in
hexane analyzed by HPLC method.

G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
2271
B
A
0.68
0.51
0.34
0.17
0.00
Vis
UV
300 s
UV
200
Vis
UV
100
60
30
10
0
Vis
240
360
480
600
720
Wavelength (nm)
Fig. 3. Absorption spectral changes of diarylethene 1 (A) and color changes of diarylethenes 1e3 (B) upon alternating irradiation with UV and visible light in PMMA films (10%, w/w)
at room temperature.
Table 1
Absorption parameters and photochromic reactivity of diarylethenes 1e3 in hexane (2.0ꢂ10ꢁ5 mol Lꢁ1) and in PMMA films (10%, w/w)
Compound
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
303 (2.13ꢂ104)
269 (2.53ꢂ104)
272 (2.17ꢂ104)
304
276
278
534 (1.06ꢂ104)
519 (1.33ꢂ104)
521 (9.99ꢂ103)
544
531
532
0.47
0.37
0.36
0.10
0.080
0.11
80%
75%
79%
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization reaction (Fo-c) and cycloreversion reaction (Fc-o), respectively.
b
c
pyridine ring. Compared to the analogous diarylethene DIMPTP,16
diarylethenes 1e3 had bigger molar absorption coefficients and
cyclization quantum yields as well as smaller cycloreversion
quantum yields. The result indicated that the terminal pyridine ring
could be effective to enhance the cyclization quantum yield and
suppress the cycloreversion quantum yield.
So far, the thermal stability and fatigue resistance of diary-
lethenes have been extensively reported.21 Especially, the fatigue-
resistant property, i.e., how many times photocyclization and
photocycloreversion reaction cycles could be repeated without loss
of performance, was a very important factor for practical applica-
tions in optical devices.21,22 On the one hand, the thermal stabilities
of the open- and closed-ring isomers of diarylethenes 1e3 were
tested in hexane at room temperature and at 341 K, respectively.
Storing these solutions in hexane at room temperature in the dark
and then exposing them to air for more than 1000 h, no changes in
their colors and spectra were observed. Moreover, no de-
composition was detected when the three isomeric diarylethenes
were exposed to air for more than one month. At 341 K, diary-
lethenes 1e3 also exhibited good thermal stabilities for more than
8 h. After refluxing more than 8 h in hexane in the dark, no changes
in their absorption spectra were observed and no any de-
composition was detected by NMR method. On the other hand, the
fatigue resistances of diarylethenes 1e3 were examined both in
hexane and in PMMA films by alternating irradiation with UV and
visible light in air at room temperature, as shown in Fig. 4. In
hexane, the coloration and decoloration cycles of diarylethenes 1e3
could be repeated more than 50 times with only ca. 4% degradation
of 1c, 12% degradation of 2c, and 19% degradation of 3c. In PMMA
films, they also exhibit good photochromism with only ca. 14%
degradation of 1c, 19% degradation of 2c, and 25% degradation of 3c
after 200 repeated cycles. Therefore, the fatigue resistance of
A
B
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
1
2
3
0.6
1
2
3
0.4
0.2
0.0
0
10
20
30
40
50
0
50
100
150
200
Repeat Cycles
Repeat Cycles
Fig. 4. Fatigue resistances of diarylethenes 1e3 in hexane (A) and in PMMA films (B) in air atmosphere at room temperature. Initial absorptance of the sample was fixed to 1.0.

2272
G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
diarylethene 1 was the strongest and that of diarylethene 3 was the
weakest in both solution and solid medium among the three iso-
meric compounds. The result revealed that the fatigue resistance of
diarylethenes 1e3 in PMMA films was much better than that in
hexane as observed for the reported diarylethenes bearing a thia-
zole ring.23
diarylethene 2o, the molecule crystallizes with an appropriate C2
symmetry with the photoactive anti-parallel conformation in the
crystalline phase and the distance between the two reactive C
ꢀ
atoms (C9/C18) is 3.647 A. There are two independent molecules
in the asymmetric unit of diarylethene 3o and each of them adopts
an anti-parallel conformation in the crystalline phase. The dis-
tances between the potentially photoactive C atoms (C9/C18 and
ꢀ
ꢀ
C29/C37) in each molecule were 3.635 A and 3.643 A, respectively.
From the data, it could be concluded that all molecules of
crystals 1oe3o were fixed in an anti-parallel mode in the crystal-
line phase and the distances of the two reactive C atoms were less
3.2. Photochromism of diarylethenes in the crystalline phase
To gain a deeper understanding of the relationship between the
conformation and the photochromism of these isomeric diary-
lethene derivatives in the crystalline phase, the structural confir-
mations of diarylethenes 1oe3o were provided by X-ray
crystallographic analysis. The X-ray crystallographic analysis data
are listed in Table S1. Their ORTEP drawings and photochromism
induced by alternating irradiation with UV and visible light in the
crystalline phase are shown in Fig. 5. For diarylethene 1o, the
molecule crystallizes with an appropriate C2 symmetry with the
photoactive anti-parallel conformation in the crystalline phase,
which can be expected to undergo photocyclization.24 The dihedral
angles between the hexafluorocyclopentene ring and the two ad-
jacent heteroaryl rings was 48.3ꢀ for O1/N1/C16eC18 and 51.3ꢀ for
S1/C6eC9, and that between the thiophene ring and the linked
pyridine ring was 19.5ꢀ. The intermolecular distance between the
ꢀ
than 4.2 A. So they could be expected to undergo photochromism in
the single crystalline phase. As a matter of fact, the crystals of
1oe3o underwent excellent photochromism in accordance with
the expected ring closure upon irradiation with UV light, as shown
in Fig. 5E. Upon irradiation with 297 nm light, these colorless
crystals of 1oe3o turned to purple red quickly due to the formation
of the closed-ring isomers 1ce3c. When the colored crystals were
dissolved separately in hexane, an intense absorption band was
appeared at the same wavelength as that of their respective closed-
ring isomer in solution. Alternatively, the colored crystals returned
to colorless upon irradiation with the appropriate visible light.
Furthermore, these crystals exhibited remarkable fatigue-
resistance greater than 200 cyclization/cycloreversion repeated
cycles and their closed-ring isomers remained stable for more than
a half year in the dark at room temperature. As such, they could
be potentially used for the promising candidates for certain
ꢀ
two photoactive C atoms (C9/C18) was 3.500 A, which was close
enough to perform the cyclization reaction.24,25 The corresponding
data of other two diarylethenes are summarized in Table S2. For
Fig. 5. ORTEP drawings of crystals 1oe3o and their color changes by photoirradiation in the single crystalline phase: (A) ORTEP drawing of 1o; (B) ORTEP drawing of 2o; (C) ORTEP
drawing of 3oeI; (D) ORTEP drawing of 3oeII; (E) color changes for crystals 1e3.

G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
2273
optoelectronic devices such as high density three-dimensional
0.6
0.4
0.2
0.0
1c
optical recording media, optical switches, or color displays.21,24,26
A
4c
3.3. Acidchromism of diarylethenes
300
400
500
600
5c
700
800
Here, the multiple switching behaviors of diarylethenes 1e3
were discussed by the stimulation of acid/base and light. Fig. 6 il-
lustrates the structural and color changes between 1e3 and 4e6 in
acetonitrile solution (2.0ꢂ10ꢁ5 mol Lꢁ1). Addition of trifluoroacetic
0.6
0.4
0.2
0.0
2c
B
acid in acetonitrile solution (1.0
m
L, 6.8ꢂ10ꢁ3 mol Lꢁ1) to the so-
300
400
500
600
6c
700
800
lutions of 1oe3o produced the protonated diarylethenes 4oe6o,
0.6
0.4
0.2
0.0
whose absorption maxima bathochromic shifted from 303 nm to
338 nm for 4o (
3
¼1.48ꢂ104 L molꢁ1 cmꢁ1), from 270 nm to 295 nm
3c
C
for 5o (
6o (
3
¼1.75ꢂ104 L molꢁ1 cmꢁ1), and from 284 nm to 341 nm for
3
¼2.11ꢂ104 L molꢁ1 cmꢁ1). The protonated 4oe6o could return
to the initial state 1oe3o by neutralization with triethylamine base
L, 3.6ꢂ10ꢁ3 mol Lꢁ1). Diarylethenes 4oe6o also exhibited
300
400
500
600
700
800
(2.0
m
Wavelength (nm)
notable photochromism by photoirradiation. Upon irradiation
with UV light, the colorless solutions containing 4oe6o separately
turned the closed-ring isomers 4ce6c with different colors,
whose absorption maxima in the visible region were observed
Fig. 7. The absorption changes of diarylethenes 1e3 with addition of acid in the
photostationary state: (A) 1; (B) 2; (C) 3.
at 538 nm for 4c (
3
¼1.08ꢂ104 L molꢁ1 cmꢁ1), 529 nm for 5c
(
(
3
¼1.04ꢂ104
L
molꢁ1 cmꢁ1), and 546 nm for 6c
emission peaks of diarylethenes 1oe3o showed an evident hyp-
sochromic shift in PMMA films with values of 13 nm for 1o, 15 nm
for 2o, and 9 nm for 3o when compared with that in hexane.
Among diarylethenes 1oe3o, the emission intensity of diary-
lethene 2o was the weakest and that of 3o was the strongest both in
hexane and in a PMMA film. By using anthracene as the reference,
the fluorescence quantum yields of diarylethenes 1oe3o were
determined to be 0.0129, 0.0139, and 0.0368, respectively. The re-
sult suggested that the fluorescence quantum yield could be sig-
nificantly enhanced when the nitrogen atom was located at the
para-position of the terminal pyridine unit. Compared to the
3
¼8.26ꢂ103 L molꢁ1 cmꢁ1). Similarly, a reversible transformation
between the colored diarylethenes 1ce3c and 4ce6c could be
performed by the stimulation of acid/base. As depicted in Fig. 7,
addition of trifluoroacetic acid to the solution of 3c produced the
protonated 6c whose absorption maximum shifted from 528 to
546 nm, and this process was accompanied by a notable color
change of solution from crimson to violet. This phenomenon is
consistent with that of reported diarylethenes bearing a pyridine
ring.27 However, for diarylethenes 1c and 2c, no changes in their
absorption maxima and colors were observed under the same ex-
perimental condition. The result revealed that the nitrogen atom
position in the terminal pyridine ring had a significant effect on the
acidchromism of these isomeric diarylethene derivatives.
analogous diarylethene DIMPTP
lethenes 1e3 had much bigger fluorescence quantum yields, in-
dicating that the terminal pyridine ring could be effective to
(
Ff,DIMPTP¼0.0094),16 diary-
Fig. 6. The structural and color changes between diarylethenes 1e3 and diarylethenes 4e6 in acetonitrile (2.0ꢂ10ꢁ5 mol Lꢁ1).
3.4. Fluorescence of diarylethenes
enhance the fluorescence quantum yield as compared a terminal
benzene ring.
The fluorescence properties of diarylethenes 1oe3o both in
solution (5.0ꢂ10ꢁ5 mol Lꢁ1) and in PMMA films (10%, w/w) were
measured at room temperature. As shown in Fig. 8, the emission
peaks of the diarylethenes 1oe3o were observed at 399, 403, and
394 nm when excited at 290 nm in hexane, and were observed at
412, 418, and 403 nm when excited at 310 nm in PMMA films. As
have been observed for most of the reported diarylethenes,28 the
Diarylethenes 1e3 exhibited an excellent fluorescent switch on
changing from the open-ring isomers to closed-ring isomers by
photoirradiation both in hexane and in PMMA films. When irradi-
ated by UV light, the photocyclization reaction was carried out and
the emission intensity of 1oe3o quenched evidently due to the
formation of the non-fluorescent closed-ring isomers 1ce3c.
The back irradiation by appropriate wavelength visible light

2274
G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
A
B
3o
3o
1o
4200
3500
2800
2100
1400
700
2000
1o
2o
2o
1500
1000
500
0
0
350
400
450
500
550
400
450
500
550
Wavelength(nm)
Wavelength(nm)
Fig. 8. Emission spectra of diarylethenes 1e3 in hexane (5.0ꢂ10ꢁ5 mol Lꢁ1) when excited at 290 nm (A) and in PMMA films (10%, w/w) when excited at 310 nm (B) at room
temperature.
A
B
2000
1500
1000
500
0
4400
3300
2200
1100
0
Vis
Vis
UV
UV
300 s
250
300 s
200
250
150
80
160
120
80
40
20
5
20
0
0
400
450
500
550
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
Fig. 9. Emission intensity changes of diarylethene 1 upon irradiation with UV and visible light at room temperature: (A) in hexane (5.0ꢂ10ꢁ5 mol Lꢁ1), excited at 290 nm; (B) in
a PMMA film (10%, w/w), excited at 310 nm.
regenerated their open-ring isomers and recovered their original
emission intensity. Fig. 9 shows the fluorescent changes of diary-
lethene 1 by photoirradiation in hexane and in a PMMA film. When
arrived at the photostationary state, the emission intensity of 1o
was quenched to ca. 30% in hexane and 33% in a PMMA film. Just
like diarylethene 1o, the emission intensities of diarylethenes 2 and
3 were quenched to ca. 28% for 2o and 39% for 3o in hexane, and
those in PMMA films were quenched to ca. 29% for 2o and 44% for
3o when arrived at the photostationary state. The residues of
fluorescence for diarylethenes 1e3 in the photostationary state
may be attributed to the incomplete cyclization reaction and the
existence of parallel conformations.15d,20c For practical applications
in optoelectronic devices, it is very important that photochromic
diarylethenes can function as an effective fluorescent switch in
a solid state, such as PMMA film. Moreover, the fluorescent
switching efficiencies of the three isomeric diarylethenes were
significantly enhanced both in hexane and in PMMA films as
compared to some reported dithienylethenes derivatives.15d,20c
Therefore, these diarylethenes can be potentially suitable for us-
ing as optical memory with fluorescence readout method or
a fluorescent photoswitch.29
position on their multiple switching behaviors by the stimulation of
acid/base and light. Each of the compounds showed notable pho-
tochromism in solution, in PMMA films, as well as in the crystalline
phase. The isomeric diarylethenes also performed a reversible
isomerization by acid/base stimuli, and all of them functioned as
a fluorescence switch in both solution and solid medium. The be-
haviors of the three isomeric derivatives were notably different
from each other, which might be attributed to the effects of nitro-
gen atom position in the terminal pyridine group. The results will
be helpful in understanding the new type of the substituent posi-
tion effect on the tunable behaviors of isomeric diarylethenes
bearing a terminal pyridine unit.
Acknowledgements
This work was supported by Program for the NSFC of China
(20962008, 21162011), the Project of Jiangxi Academic and Tech-
nological leader (2009DD00100), National Natural Science Foun-
dation of Jiangxi Province (2009GZH0034, 2010GZH0040), and the
Science Funds of the Education Office of Jiangxi, China (GJJ09646,
GJJ11026).
4. Conclusion
Supplementary data
Three new isomeric diarylethenes bearing a terminal pyridine
group were synthesized to investigate the effects of nitrogen atom
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2012.01.058.

G. Liu et al. / Tetrahedron 68 (2012) 2267e2275
2275
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