Synthesis, crystal structure and photochromism of new diarylethenes with a benzene moiety

Article abstract of DOI:10.1016/j.saa.2014.04.131

Three unsymmetrical diarylethenes with both thiophene and benzene moieties were synthesized. Their crystal structures and photochromic features were investigated systematically to elucidate the substituent effects of the terminal phenyl group on photochromism. Computational studies were performed to provide further insight into their frontier molecular orbitals and spatial distributions of electronic density. Each compound exhibited favorable photochromism in hexane and functioned as notable fluorescent photo-switches with fluorescence quenching efficiency of ～96% induced by the intensity change of UV/vis light. Among the three diarylethenes, the cyano group on the terminal phenyl group significantly decreased the distance of the two reactive carbon atoms and strengthened the intermolecular hydrogen bond stacking interactions, resulting in improved stability in crystal state. Furthermore, it exhibited a red-shifted absorption maximum, an enhanced molar absorption coefficient, cyclization quantum yield, and fluorescence quantum yield in solution.

Full text of DOI:10.1016/j.saa.2014.04.131

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, crystal structure and photochromism of new diarylethenes
with a benzene moiety
⇑
Congbin Fan, Shouzhi Pu , Gang Liu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Three new diarylethenes with
thiophene and benzene moieties were
synthesized.
Their crystal structures and
photochromic features were
investigated systematically.
Computational studies were
performed.
a r t i c l e i n f o
a b s t r a c t
Article history:
Three unsymmetrical diarylethenes with both thiophene and benzene moieties were synthesized. Their
crystal structures and photochromic features were investigated systematically to elucidate the substitu-
ent effects of the terminal phenyl group on photochromism. Computational studies were performed to
provide further insight into their frontier molecular orbitals and spatial distributions of electronic den-
sity. Each compound exhibited favorable photochromism in hexane and functioned as notable fluorescent
photo-switches with fluorescence quenching efficiency of ꢁ96% induced by the intensity change of UV/
vis light. Among the three diarylethenes, the cyano group on the terminal phenyl group significantly
decreased the distance of the two reactive carbon atoms and strengthened the intermolecular hydrogen
bond stacking interactions, resulting in improved stability in crystal state. Furthermore, it exhibited a
red-shifted absorption maximum, an enhanced molar absorption coefficient, cyclization quantum yield,
and fluorescence quantum yield in solution.
Received 26 February 2014
Received in revised form 10 April 2014
Accepted 14 April 2014
Available online 30 April 2014
Keywords:
Photochromism
Diarylethene
Crystal structure
Substituent effect
Fluorescence switch
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
memory [15], and solar cells [16]. Diarylethene derivatives are the
most promising candidates for such applications because of their
In the past several decades, various types of photochromic com-
pounds have been synthesized for applications in multifunctional
optoelectronic fields [1,2], such as optical switch devices, optical
data storage [3–9], fluorescence sensing of specific ions [10–13],
photo-switchable probe for imaging living cells [14], organic
excellent photochromic characteristics [17]. In addition, the photo-
chromic properties of diarylethene can be predictably improved by
structure variation to meet a special functional requirement.
Two structural features strongly influence the photochromic
properties of diarylethene, including the heteroaryl moieties and
the terminal aryl moieties. There are a number of reports on the
effect of the heteroaryl moieties [17,18–22]. For example, diary-
lethenes bearing thiophene/benzothiophene moieties exhibited
⇑
Corresponding author. Tel./fax: +86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
http://dx.doi.org/10.1016/j.saa.2014.04.131
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

236
C. Fan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
notable thermal stability and remarkable fatigue resistance [23–
25], whereas diarylethenes bearing two pyrrole groups were ther-
mally unstable and returned to the open-ring isomers even in the
dark [26]. It was found by our group that diarylethenes bearing both
thiazole and benzene moieties had poor fatigue resistance with 88%
of closed ring isomers destroyed after 10 repeat cycles [27]. This
result was in agreement with the explanation that the heteraryl
groups of aromatic stabilization energy (ASE) level determined
the thermal stability of the closed-ring isomers [17,22]. However,
diarylethene with thienyl group, which has low ASE, and the other
aromatic heterocyclic moieties also exhibited good photochromic
properties [28–34]. For instance, diarylethenes bearing thiophene
and isoxazole moieties exhibited remarkable photochromism
which functioned as a fluorescent photoswitch in solution. Diary-
lethenes bearing both thiophene and benzofuran showed good
thermal stability as well [31,32]. Another example is diarylethene
containing thiophene moiety and six-membered pyridine unit,
which exhibited good photochromism both in solution and in the
crystalline phase by permitting the pyridine unit to participate in
the photoinduced cyclization reaction [33]. In our previous
research, it was found that diarylethenes based on thiophene moi-
ety and a hybrid structure of six-membered naphthalene exhibited
good photochromism and notable fatigue resistance, which was
applied as fluorescent photoswitches in solution [34]. Based on
these results, it is postulated that unsymmetrical diarylethenes
containing thiophene moiety of low ASE should be able to effec-
tively improve the properties of photochromism.
The effect of the other structural feature, i.e. the electron-donat-
ing/withdrawing substituents at the terminal groups in the aryl
moieties, has also been studied, including its impact on the diary-
lethene absorption peaks, cyclization and cycloreversion quantum
yields, and the fluorescence emission peaks [35,36]. Jeong et al.
reported that the introduction of acetyl groups at the 6,60-positions
and heptyl groups at the 2,20-positions in disulfonyl diarylethene
gave rise to an increase in the fluorescence quantum yield and fluo-
rescence lifetime [37]. In the study of a series of diarylethenes con-
taining different electron-donating/withdrawing groups, it was
found that the electron-donating groups significantly increased
the cyclization quantum yield, depressed the cycloreversion quan-
tum yield, red-shifted the emission peak, and decreased the fluores-
cence quantum yield in solution; while the electron-withdrawing
groups functioned as an inverse action for these diarylethene deriv-
atives [28–32].
i.e.,
{1-[2-methyl-5-(4-cyanophenyl)-3-thienyl]-2-(2-methoxyl-
phenyl)}perfluorocyclopentene (1o), {1-[2-methyl-5-(4-fluoro-
phenyl)-3-thienyl]-2-(2-methoxylphenyl)}perfluorocyclopentene
(2o), and {1-[2-methyl-5-(4-methylphenyl)-3-thienyl]-2-(2-meth-
oxylphenyl)}perfluorocyclopentene (3o), all of which showed good
reversible photochromism in hexane solution. The scheme show-
ing the photochromism of 1–3 is presented in Scheme 1.
Results and discussions
Photochromism in solution
The photochromic behaviors of diarylethenes 1–3 were exam-
ined in solution (c = 3.0 ꢂ 10ꢃ5 mol/L) at room temperature. The
changes in the absorption spectra of 1–3 induced by alternating
irradiation with UV and visible light in hexane are shown in Fig. 1.
In hexane, the colorless solution of diarylethene 1o exhibits a
sharp absorption peak at 313 nm corresponding to a
p ?
p⁄ transi-
tion [39]. Upon irradiation with 313 nm light, a new absorption
band in the visible region centered at 591 nm emerged with a color
change from colorless to blue due to the formation of the closed-
ring isomer 1c for S0–S1 transition [40]. Alternatively, the blue solu-
tion could bleach to colorless upon irradiation with visible light
(k > 450 nm), and the absorption spectrum return to the initial state
1o (Fig. 1A and D). The coloration-decoloration cycles could be
repeated many times and a clear isosbestic point was observed at
337 nm. Just like diarylethene 1, diarylethenes 2 and 3 also show
good photochromism in hexane (Fig. 1B and C). Upon irradiation
with 297 nm light, absorption bands in the visible region appeared
and the solutions of 2o and 3o turned blue due to the cyclization
reactions leading to the closed-ring isomers 2c and 3c, respectively.
These compounds had absorption maxima in the visible region at
584 and 589 nm, respectively. The blue solutions of 2c and 3c could
be decolorized upon irradiation with visible light (k > 450 nm),
indicating the return to initial state of 2o and 3o.
To further investigate the optical properties of 1o–3o, molecu-
lar orbital calculations at the B3LYP/6-31G(d) level of density func-
tional theory (DFT) were used [41,42]. Fig. 2 shows the calculated
spatial distributions of the HOMO and LUMO levels of diaryleth-
enes 1o–3o. It could be easily seen that the HOMO levels of 1o–
3o are
p orbitals concentrated on the central thiophene–phenyl
moiety. For LUMO levels, 1o with a strong electron-withdrawing
cyano group is not concentrated on the central active part, but
2o and 3o are still concentrated on the central active part. The
result is well consistent with that of the reported diarylethenes
[43].
The DFT results indicate that the features of the frontier molec-
ular orbital change significantly with different substituents. The
calculated HOMO–LUMO gaps of 1o–3o follow the decending order
of 2o (397.78 kJ/mol) > 1o (395.35 kJ/mol) > 3o (390.83 kJ/mol),
indicating that the HOMO–LUMO gap of 3o is the smallest among
the three diarylethene derivatives. The possible reason is that the
electron-withdrawing groups as in compound 1o and 2o tend to
induce broader energy gaps, thus can stabilize the molecule signif-
icantly. On the contrary, the electron-donating groups as in 3o
lower the HOMO–LUMO energy gap resulting in weakened chem-
ical stability.
The absorption spectral parameters of diarylethenes 1–3 in hex-
ane solution are summarized in Table 1. The cyclization and cyclo-
reversion quantum yields of 1–3 were measured by comparison of
them to 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopen-
tene [44] under identical conditions (Table 1). As shown in Table 1,
cyano terminated diarylethene 1 in the open-ring isomer state has
the largest maximum absorption wavelength of 313 nm among the
three diarylethenes. The molar absorption coefficient of the
So far, there have been a few examples of photochromic diary-
lethenes bearing thiophene moieties and aryl moieties [38]. To the
best of our knowledge, for these diarylethenes bearing thiophene
moieties and aryl moieties, the effects of the electro-donating or
electro-withdrawing groups on their photochromism and crystal
packing properties have not been reported.
Herein, we have designed and synthesized a new class of pho-
tochromic diarylethene derivatives, with different groups at the
terminal of diarylethenes and a benzene ring as an aryl moiety,
F
F
F
F
F
F
F
F
F
F
F
F
UV
Vis
S
O
S
O
R
R
1o:R=-CN
2o:R=-F
1c:R=-CN
2c:R=-F
3o:R=-CH3
3c:R=-CH3
Scheme 1. Photochromism of diarylethenes 1–3.

C. Fan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
237
0.8
0.6
0.4
0.2
0.0
Vis
UV
0.6
0.4
0.2
0.0
Vis
UV
Vis
UV
Vis
UV
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
(A)
(B)
0.6
1c
2c
3c
1o
2o
3o
Vis
UV
0.5
0.4
0.3
0.2
0.1
0.0
UV
Vis
Vis
UV
300
400
500
600
700
800
Wavelenth (nm)
(C)
(D)
Fig. 1. Absorption spectral and color changes of diarylethenes 1–3 by photoirradiation in hexane solution (C = 3.0 ꢂ 10ꢃ5 mol Lꢃ1) at room temperature: (A) absorption
spectral of 1, (B) absorption spectral of 2, (C) absorption spectral of 3, and (D) color changes of 1–3. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
F
F
F
F
F
F
F
F
F
F
F
Br
F
F
Br
R
Br
O
S
Pd(PPh3)4,
Na2CO3,aq.
B(OH)2
S
n-BuLi,195 K
S
O
R
R
1a:R= -CN
2a:R= -F
1o:R= -CN
2o:R= -F
3a:R= -CH3
3o:R= -CH3
Scheme 2. Synthetic route for diarylethenes 1o–3o.
open-ring isomers follows a descending order of 1o > 2o > 3o. For
the closed-ring isomers, diarylethene with a strong electron-
donating group (3) has stronger molar absorption coefficient than
diarylethenes with an electron-withdrawing group (1 and 2).
Cyano terminated diarylethene 1 has the largest cyclization quan-
tum yield (Uo–c = 0.46). The results show that the electron-with-
drawing/donating substituent group has a significant effect on
the photochromic features of diarylethenes, including the absorp-
tion maxima, molar absorption coefficients, and quantum yields.
of 2o was monoclinic with space group P21/n, which may be attrib-
uted to the coordinating effects of the fluorine atom with hexaflu-
orocyclopentene moiety. The ORTEP drawings of the single crystals
1o–3o showing 35% probability displacement ellipsoids are shown
in Fig. 3. In Fig. 3A, the molecule of 1o is a photoactive anti-parallel
conformation in the crystalline phase, which can be expected to
undergo photocyclization reaction [45]. The methyl and methoxyl
groups are located on different sides of the perfluorocyclopentene
plane and trans-direction of the thiophene and benzene ring
planes, respectively. The intramolecular distance between the
two active carbon atoms (C1AC13) of 1o is 3.622 Å. The corre-
sponding dihedral angles and the distances between the reactive
carbon atoms of 1o–3o are summarized in Table 3.
Diarylethenes in the single-crystal state
Single crystals of 1o–3o suitable for X-ray crystallographic anal-
ysis were obtained by evaporating a hexane solution of the respec-
tive compound at room temperature. The X-ray crystallographic
analysis data are listed in Table 2. It was found that the crystal
system of 1o or 3o was triclinic with space group P ꢃ 1, while that
For the crystal of 1o, molecules are arranged by the intermolec-
ular CAHꢄ ꢄ ꢄF hydrogen bonds with an average Hꢄ ꢄ ꢄF distance of
2.61 Å, and CAHꢄ ꢄ ꢄF angle of 140.85°. Interestingly, weak CAHꢄ ꢄ ꢄN
interactions exist between methyl, benzene ring units and N atom,

238
C. Fan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
Fig. 2. Calculated spatial distributions of the HOMO and LUMO levels of diarylethenes 1o–3o.
Table 1
Table 2
Absorption characteristics and photochromic reactivity of diarylethenes 1–3 in
Crystal data for diarylethenes 1o–3o.
hexane at 3.0 ꢂ 10ꢃ5 mol/L.
1oa
2ob
3oc
a
b
Compound
kmax, (nm (
e
/dm3
kmax, (nm (e
/dm3
U c
molꢃ1o cmꢃ1))
molꢃ1c cmꢃ1))
Formula
C24H15F6NOS
C23H15F7OS
472.41
291(2)
Monoclinic
P21/n
C24H18F6OS
468.44
273(2)
Triclinic
P ꢃ 1
Uo–c Uc–o
Formula weight
Temperature
Crystal system
Space group
479.43
293(2)
Triclinic
P – 1
1
2
3
313 nm (2.43 ꢂ 10ꢃ4
273 nm (2.36 ꢂ 10ꢃ4
276 nm (2.00 ꢂ 10ꢃ4
)
)
)
591 nm (3.67 ꢂ 10ꢃ3
584 nm (4.00 ꢂ 10ꢃ3
589 nm (5.00 ꢂ 10ꢃ3
)
)
)
0.46
0.21
0.28
0.098
0.11
0.069
Unit cell dimensions
a
b
c
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization (Uo–c) and cycloreversion (Uc–o), respectively.
a (Å)
b (Å)
c (Å)
7.5057(9)
9.1626(11)
16.685(2)
82.0360(10)
80.9930(10)
77.9380(10)
1101.6(2)
2
1.445
8039
3990
1.016
7.8240(7)
20.8712(17)
12.8403(11)
90.00
90.9330(10)
90.00
2096.5(3)
4
1.497
9.4505(11)
10.4163(12)
11.4299(14)
87.3230(10)
76.3910(10)
82.005(2)
1082.8(2)
2
1.437
7400
3914
1.031
a
(o)
b (o)
c
(°)
Volume (Å3)
with an average H(7A)ꢄ ꢄ ꢄN1 and H(20)ꢄ ꢄ ꢄN1 distances of 2.64 Å and
2.72 Å, respectively (Fig. 4A). The special CAHꢄ ꢄ ꢄN interactions are
responsible for the improved stability of diarylethene as shown by
the relatively high melt points (417.5–418 K for 1o, 403.1–403.7 K
for 2o and 395.2–395.5 K for 3o). The inherent forces of the
CAHꢄ ꢄ ꢄN interactions might be electrostatic interactions between
the electron-rich N atom and the electron-deficient carbon atom.
For the crystals of 2o and 3o, the intermolecular hydrogen bonds
between CAH and F connected molecular each other (Table 4
and Fig. 4). These molecular interactions together with hydrogen
bonding may strengthen the stability of these crystals.
Z
Density (calcd.) (g/cm3)
Reflections collected
Unique reflections
Goodness-of-fit on F2
15777
3898
1.059
Final R indices [I/2r(I)]
R1
wR2
0.0377
0.0977
0.0503
0.1276
0.0472
0.1166
R indices (all data)
R1
wR2
0.0453
0.1047
0.0626
0.1385
0.0627
0.1289
ꢀ
ꢁ
ꢀ
ꢁ
2
w ¼ 1=½r2 Fo2 þ ð0:0522PÞ þ 0:3308Pꢅ where P ¼ Fo2 þ 2F2c =3.
a
b
c
ꢀ
ꢁ
ꢀ
ꢁ
2
w ¼ 1=½r2 Fo2 þ ð0:055PÞ þ 1:3232Pꢅ where P ¼ Fo2 þ 2F2c =3.
Non-photochromism in the crystalline phase
ꢀ
ꢁ
ꢀ
ꢁ.
2
w ¼ 1=½r2 Fo2 þ ð0:0559PÞ þ 0:5061Pꢅ where P ¼ Fo2 þ 2F2c 3.
As mentioned above, for the crystals of 1o–3o, all the distances
of the two reactive carbon atoms are less than 4.2 Å and they
packed in photoactive anti-parallel conformations in their
respective crystalline phase. Therefore, 1o–3o may be expected
to undergo photochromism in the single crystalline phase

C. Fan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
239
Fig. 4. The crystal packing fragments linked with
and (C) 3o.
H
bonds: (A) 1o, (B) 2o,
Fig. 3. ORTEP drawings of 1o–3o, showing 35% probability displacement ellipsoids:
(A) 1o, (B) 2o, and (C) 3o.
[46,47]. But, in fact, the color changes of crystals 1o–3o are not vis-
ible by the naked eye upon photoirradiation. This non-photochro-
mism behavior may be attributed to the relatively high aromatic
energy and the rigidity of the benzene ring in the crystal structure.
However, when the crystals of 1o–3o were dissolved in hexane, the
solutions turned blue upon irradiation with UV light. The absorp-
tion maxima were observed at 591, 584, and 589 nm, respectively,
which were the same as those of the closed-ring isomers 1c–3c.
The colored solutions were bleached completely and their
absorption spectra recovered to those of the open-ring isomers
1o–3o upon irradiation with appropriate wavelength visible light
(k > 450 nm).
Table 3
Distances between the reacting carbon atoms (d, Å) and dihedral angles (h, °)
of 1o–3o.
Compound
d (Å)
h (°)a
h1
h2
h3
1o
2o
3o
C1ꢄ ꢄ ꢄC13
C1ꢄ ꢄ ꢄC13
C1ꢄ ꢄ ꢄC13
3.622
3.654
3.751
48.5
126.8
51.3
57.1
54.5
50.0
23.5
29.7
2.1
a
h1, dihedral angle between the cyclopentene ring and cyclopentene ring linked
up with the benzene ring; h2, dihedral angle between the cyclopentene ring and the
thiophene ring; h3, dihedral angle between the thiophene ring and the adjacent
benzene ring.

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C. Fan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
Table 4
(A)
Hydrogen bond lengths (Å) and bond angles (°) of 1o–3o.
6000
4000
2000
0
Compounds
DAHꢄ ꢄ ꢄA
DAH
Hꢄ ꢄ ꢄA
2.61
2.64
2.72
Dꢄ ꢄ ꢄA
3.38
3.39
3.57
DAHꢄ ꢄ ꢄA
140.85
135.16
151.67
C(3)AH(3)ꢄ ꢄ ꢄF(3)a
C(7)AH(7A)ꢄ ꢄ ꢄN(1)b
C(20)AH(20)ꢄ ꢄ ꢄN(1)c
0.93
0.96
0.93
UV
1o
Vis
d
2o
3o
C(3)AH(3)ꢄ ꢄ ꢄF(7)
0.93
0.93
0.93
2.58
2.65
2.56
3.44
3.20
3.48
153.84
119.03
166.78
e
C(3)AH(3)ꢄ ꢄ ꢄF(2)
f
C(2)AH(2)ꢄ ꢄ ꢄF(4)
g
C(7)AH(7B)ꢄ ꢄ ꢄF(3)
0.96
2.57
3.32
134.70
Symmetry operation code.
a
ꢃ1 + x, 1 + y, z.
b
420
480
540
2 ꢃ x, 2 ꢃ y, ꢃz.
c
Wavelength (nm)
3 ꢃ x, 2 ꢃ y, ꢃz.
d
1.5 + x, 0.5 – y, ꢃ0.5 + z.
e
0.5 + x, 0.5 ꢃ y, 0.5 + z.
f
(B)
0.5 + x, 0.5 ꢃ y, 0.5 + z.
g
1 + x, y, z.
Vis UV
800
400
0
Fluorescence of diarylethenes
The fluorescence emission spectra of the diarylethenes 1–3 are
measured in hexane (1.0 ꢂ 10ꢃ3 mol/L) (Fig. 5). In hexane, the
emission peaks of 2o and 3o were observed at 414 and 417 nm
when excited at 346 and 354 nm, respectively. Double peaks at
413 and 434 nm were observed when the hexane solution of 1o
was excited at 360 nm. This may attribute to the strong electron-
withdrawing cyano group that affected on the fluorescence emis-
sion peak in hexane solution at room temperature. Moreover, 1o
shows the strongest fluorescence emission among the three diary-
lethenes, which may be attributed to the strong electron-with-
drawing nature of the cyano group. In hexane, the fluorescence
quantum yields of 1o–3o are 0.053, 0.015, and 0.017, respectively,
using anthracene (0.27) as a reference.
Compared to most of the reported diarylethenes [48–50], diary-
lethenes 1–3 in hexane exhibited relatively large fluorescence
changes in the photoisomerization process as shown in Fig. 5.
When irradiated with 297 nm UV light, the fluorescence of the
samples was effectively quenched. Irradiation by visible light of
appropriate wavelength regenerated the open-ring isomers 1o–
3o and recovered the original emission spectra. Upon irradiation
with 297 nm UV light, the emission intensity of 1–3 could be
quenched to ca. 2.2%, 4.2%, and 4.4%, respectively. In addition, the
‘‘on’’ and ‘‘off’’ state of the fluorescence was measured by varying
the power of the UV and visible light. The average ‘‘on’’ and ‘‘off’’
times were shortened with increasing power, indicating that the
switching effect is indeed photochemical [39]. Therefore, these
diarylethene compounds could be potentially applied as optical
memory with fluorescence readout or as a fluorescence-modulated
switch [51,52]. Furthermore, the electron-withdrawing/donating
terminal group can be varied to select specific fluorescence proper-
ties for desired applications.
420
490
Wavelength (nm)
(C)
Vis
UV
800
400
0
420
480
540
Wavelength (nm)
Fig. 5. Emission intensity changes of diarylethene 1–3 in hexane
(c = 1.0 ꢂ 10ꢃ3 mol Lꢃ1
) upon irradiation with 297 nm UV light at room
temperature: (A) 1, (B) 2, and (C) 3.
substituent effects on the crystal structure and photochromic
properties of diarylethenes with thiophene and benzene moieties.
Materials and methods
Conclusions
Experimental section
A new class of unsymmetrical photochromic diarylethenes
based on a skeleton of thiophene and six-membered benzene were
synthesized. These compounds exhibited notable photochromism
and functioned as fluorescence-modulated switches. The different
electron-donating/withdrawing substituents at the para-position
of the terminal phenyl group had a significant effect on their pho-
tochromic and fluorescence properties. Furthermore, the electron-
donating/withdrawing group affected the hydrogen bonding in
crystal-packing to strengthen the stability of the framework
structure. These results are helpful for the understanding of the
General: All solvents used were spectrograde and were purified
by distillation before use. NMR spectra were recorded on a Bruker
AV400 (400 MHz) spectrometer with CDCl3 as the solvent and tet-
ramethylsilane as an internal standard. IR spectra were performed
by a Bruker Vertex-70 spectrometer. Melting point was determined
by a WRS-1B melting point determination apparatus. The elemental
analyses were measured with a PE CHN 2400 analyzer. The absorp-
tion spectra were measured using an Agilent 8453 UV/vis spec-
trometer. Photo-irradiation was carried out using an SHG-200 UV

C. Fan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 235–242
241
lamp, a CX-21 ultraviolet fluorescence analysis cabinet and a BMH-
250 Visible lamp. Light of appropriate wavelength was isolated by
different light filters. The quantum yields were determined by com-
paring the reaction yields of the diarylethenes in hexane against
138.3, 141.6, 156.5. IR (m
, KBr, cmꢃ1): 760 (1,2-disubstituted ben-
zene), 819 (1,4-disubstituted benzene), 1269 (CAO), 1602 and
1642 (benzene), 2227 (ACBN), 2847 (AOCH3), 2982 (ACH3).
m.p.: 417.5–418.0 K.
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene
in
hexane [44]. Fluorescent spectra were measured using a Hitachi
F-4500 spectrophotometer. Suitable crystals of 1o–3o were
obtained by slow evaporation of a hexane solution. All the measure-
ments were made on a Bruker SMART APEX II CCD diffractometer
{1-[2-Methyl-5-(4-fluorophenyl)-3-thienyl]-2-(2-methoxylphenyl)}
perfluorocyclopentene (2o)
The compound 2o was synthesized from the reaction of (2-
methoxylpheny)perfluorocyclopentene (1.20 g, 4.0 mmol) [54]
and 3-bromo-2-methyl-5-(4-fluorophenyl)thiophene (2a) (1.08 g,
4.00 mmol) [55] with 2.5 M n-butyl lithium (1.60 mL, 4.00 mmol)
at 195 K under a nitrogen atmosphere. After an hour, the reaction
was quenched by the addition of water. The solid product was
purified by column chromatography on silica with petroleum ether
as the eluent to give 2o 0.62 g (1.31 mmol) in 32.8% yield. Anal.
Calcd. for C23H15F7OS (%): C, 58.47; H, 3.20; Found C, 58.45; H,
3.19; 1H NMR (400 MHz, CDCl3, TMS): d 1.90 (s, 3H, ACH3), 3.51
(s, 3H, AOCH3), 6.84 (d, J = 8.0 Hz, 1H, benzene-H), 6.99 (t,
J = 8.0 Hz, 1H, benzene-H), 7.20 (s, 1H, thiophene-H), 7.35 (t,
J = 9.4 Hz, 4H, benzene-H), 7.44 (t, J = 8.4 Hz, 2H, benzene-H); 13C
NMR (100 MHz, CDCl3): d 14.0, 55.2, 111.5, 115.8, 116.1, 117.4,
120.9, 123.0, 126.7, 127.2, 127.2, 130.0, 131.8, 131.8, 139.8,
using a MULTI scan technique and Mo Ka radiation. The structures
were solved by direct methods and refined by full-matrix least-
squares procedures on F2 by full-matric least-aquares techniques
using SHELXTL-97 program. All nonhydrogen atoms were refined
anisotropically. Crystallographic data for the structure 1o–3o have
been deposited with the Cambridge Crystallographic Data Centre as
supplemental publication CCDC 729699, 729600, and 729697,
respectively. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 0 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk)].
Synthesis
140.1, 157.1. 161.2, 163.6. IR (m
, KBr, cmꢃ1): 762 (1,2-disubstituted
benzene), 822 (1,4-disubstituted benzene), 1269 (CAO), 1341
(ACAF), 1601 and 1652 (benzene), 2838 (AOCH3), 2924 (ACH3).
m.p.: 403.1–403.7 K.
The synthesis route for diarylethenes 1o–3o is shown in
Scheme 2.
3-Bromo-5-(4-cyanophenyl)-2-methylthiophene (1a)
Compound
3-bromo-5-(4-cyanophenyl)-2-methylthiophene
was prepared by reacting 3-bromo-2-methyl-5-thienylboronic
acid [53] (2.21 g, 10.00 mmol) with 1-bromo-4-cyanobenzene
(1.82 g, 10.00 mmol) in the presence of Pd(PPh3)4 (0.27 g,
0.23 mmol) and Na2CO3 (6.36 g, 60.00 mmol) in tetrahydrofuran
(THF) (80.0 mL containing 10% water). After refluxing for 15 h,
the product was extracted with ether, dried over MgSO4, filtrated,
and evaporated. The crude product was purified by column chro-
matography on silica gel using hexane as the eluent to yield a
white solid 2.32 g, in 83.50% yield. 1H NMR (400 MHz, CDCl3,
TMS): d 2.44 (s, 3H, ACH3), 7. 22 (s, 1H, thiophene-H), 7.58 (d,
J = 8.0 Hz, 2H, benzene-H), 7.65 (d, J = 8.0 Hz, 2H, benzene-H).
{1-[2-Methyl-5-(4-methylphenyl)-3-thienyl]-2-(2-methoxylphenyl)}
perfluorocyclopentene (3o)
Compound 3o was prepared by a method similar to that used
for 1o and used (2-methoxylpheny)perfluorocyclopentene
(1.20 g, 4.00 mmol) [54] and 3-bromo-5-(4-methylphenyl)-2-
methylthiophene (3a) (1.07 g, 4.00 mmol) with 2.5 M n-butyl
lithium (1.60 mL, 4.00 mmol), then 0.59 g (1.26 mmol) of 3o was
obtained as solid in 31.5% yield. Anal. Calcd. for C24H18F6OS (%):
C, 61.53; H, 3.87; Found C, 61.49; H, 3.90; 1H NMR (400 MHz,
CDCl3, TMS): d 1.89 (s, 3H, ACH3), 2.36 (s, 3H, ACH3), 3.50 (s, 3H,
AOCH3), 6.83, (d, J = 8.0 Hz, 1H, benzene-H), 6.99 (t, J = 8.0 Hz,
1H, benzene-H), 7.18 (d, J = 8.0 Hz, 2H, benzene-H), 7.18 (s, 1H, thi-
ophene-H), 7.37 (t, J = 8.0 Hz, 2H, benzene-H), 7.41 (d, J = 8.0 Hz,
2H, benzene-H); 13C NMR (100 MHz, CDCl3): d 14.1, 21.2, 55.2,
111.4, 117.4, 120.8, 122.4, 125.4, 126.6, 129.6, 129.9, 130.9,
3-Bromo-5-(4-methylphenyl)-2-methylthiophene (3a)
3-Bromo-5-(4-methylphenyl)-2-methylthiophene was prepared
by the method similar procedure to 3-bromo-5-(4-cyanophenyl)-2-
methylthiophene by Suzuki coupling between 3-bromo-2-methyl-
5-thienylboronic acid and 1-bromo-4-methylbenzene (1.71 g,
10.00 mmol) (1.82 g yellow solid, yield: 68.20%). 1H NMR
(400 MHz, CDCl3, TMS): d 2.29 (s, 3H, ACH3), 2.34 (s, 3H, ACH3),
6.99 (s, H, thiophene-H), 7.10 (d, J = 8.0 Hz, 2H, benzene-H), 7.33
(d, J = 8.0 Hz, 2H, benzene-H).
131.7, 137.6, 139.18, 141.3, 157.1. IR (m
, KBr, cmꢃ1): 757 (1,2-
disubstituted benzene), 809 (1,4-disubstituted benzene), 1262
(CAO), 1599 and 1634 (benzene), 2843 (AOCH3), 2915 (ACH3).
m.p.: 395.2–395.5 K.
Acknowledgments
1-[2-Methyl-5-(4-cyanophenyl)-3-thienyl]-2-(2-methoxylphenyl)}
perfluorocyclopentene (1o)
This work was supported by the National Natural Science Foun-
dation of China (21262015, 21362013, 21363009), the Science
Funds of Natural Science Foundation of Jiangxi Province
(20132BAB203005), and the Project of the Science Funds of Jiangxi
Education Office (KJLD12035, GJJ13577).
Used
(2-methoxylpheny)perfluorocyclopentene
(1.35 g,
4.50 mmol) [54] and solution of 3-bromo-2-methyl-5-(4-cyano-
phenyl)thiophene (1a) (1.25 g, 4.50 mmol) [55] with n-butyl lith-
ium (2.5 M, 1.80 mL, 4.50 mmol), at 195 K under a nitrogen
atmosphere. After an hour, the reaction was quenched by the addi-
tion of water. The solid product was purified by column chroma-
tography on silica with petroleum ether as the eluent to obtain
0.92 g (1.90 mmol) of solid 1o in 42.2% yield. Anal. Calcd. for C24-
H15F6NOS (%): C, 60.12; H, 3.13; Found C, 61.15; H, 3.15; 1H NMR
(400 MHz, CDCl3, TMS): z 1.94 (s, 3H, ACH3), 3.51 (s, 3H, AOCH3),
6.85 (d, J = 8.0 Hz, 1H, benzene-H), 7.01 (t, J = 8.0 Hz, 1H, benzene-
H), 7.34 (s, 1H, thiophene-H), 7.36–7.41 (m, 2H, benzene-H), 7.61
(d, J = 8.0 Hz, 2H, benzene-H), 7.66 (d, J = 8.0 Hz, 2H, benzene-H);
13C NMR (100 MHz, CDCl3): d 13.7, 54.7, 110.3, 111.0, 116.6,
118.2, 120.4, 124.7, 125.1, 126.8, 129.4, 131.5, 132.3, 137.4,
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