Syntheses and substituent effects on the properties of unsymmetrical photochromic diarylethenes bearing a benzothiophene unit

Article abstract of DOI:10.1071/CH08289

Photochromic unsymmetrical diarylethenes 1a5a that bear different electron-donating or electron-withdrawing substitutes have been synthesized, and the structures of 2a5a determined by single-crystal X-ray diffraction analysis. Substituent effects on their

Full text of DOI:10.1071/CH08289

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2009, 62, 464–474
www.publish.csiro.au/journals/ajc
Syntheses and Substituent Effects on the Properties
of Unsymmetrical Photochromic Diarylethenes
Bearing a Benzothiophene Unit
Shouzhi Pu,A,B,C Min Li,A,B Gang Liu, and Zhanggao LeB
A
AJiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University,
Nanchang 330013, China.
B
Key Laboratory of Nuclear Resources and Environment of Ministry of Education,
East China Institute of Technology, Fuzhou 344000, China.
Corresponding author. Email: pushouzhi@tsinghua.org.cn
C
Photochromic unsymmetrical diarylethenes 1a–5a that bear different electron-donating or electron-withdrawing sub-
stitutes have been synthesized, and the structures of 2a–5a determined by single-crystal X-ray diffraction analysis.
Substituent effects on their optoelectronic properties, including photochromism, fluorescence, and electrochemical prop-
erties are investigated in detail. The strong electron-donating substituents have effective contributions to the absorption
maxima of the closed-ring isomers and the molar coefficients, whereas the electron-withdrawing substituents can shift
significantly the absorption maxima of the diarylethenes to a longer wavelength and increase their cyclization quantum
yield. Diarylethenes 2, 3, and 4 show good photochromism both in solution and in the single crystalline phase. However,
diarylethenes 5 shows no photochromism in the crystalline phase. Diarylethenes 1, 2, and 3 exhibit good fluorescent
switching upon alternating irradiation with UV and visible light, and their fluorescent conversions in the photostationary
state are all larger than 80% in hexane. In addition, cyclic voltammogram tests show that different electron-donating and
electron-withdrawing substituents have a remarkable effect on the electrochemical behaviours of these diarylethenes.
Manuscript received: 8 July 2008.
Final version: 20 March 2009.
Introduction
diarylethenes could be effective to increase the absorption
coefficient of the closed-ring forms and decrease the cyclo-
reversion quantum yield; whereas electron-donating substituents
ofthe bis(2-thienyl)ethane diarylethenescouldincrease themax-
Photochromic compounds have been extensively studied for
optoelectronic devices, such as for optical data storage, full-
colour displays, and photoswitches.[1–4] Up to date, various
types of photochromic compounds, such as spirobenzopy-
rans, azobenzenes, fulgides, and diarylethenes, have been
ima absorption of the open-ring forms and reduce the cyclization
quantum yield.[
10–14]
Tanifuji et al. elucidated that diarylethene
[
5,6]
developed.
Among such compounds, diarylethenes that bear
derivatives that bear imino nitroxide and nitronyl nitroxide could
reduce the cyclization/cycloreversion quantum yield.[
15]
thiophene/benzothiopheneringshavereceivedthemostattention
because of the good thermal stability of both isomers, excellent
fatigue resistance, high quantum yields, and large changes of the
absorption wavelength between the two isomers.[1,3]
The photochromic properties of the diarylethene deriva-
tives can be improved by modifying the chemical structures
upon introducing different substituents. Therefore, substituent
effects on the photochromic performances of diarylethenes have
As described above, we found that most works focussed
ꢀ
on different substituents at the 2- and 2 -positions of the
thiophene rings or at different positions of the terminal
phenyl groups. Investigations concerning substitution at the
5-position of the thiophene ring and their influence on the
optoelectronic properties are very rare.[ In addition, most
of the reported diarylethene derivatives hitherto are symmetri-
cal bis(3-thienyl)perfluorocyclopentene or bis(3-benzothienyl)
perfluorocyclopentene derivatives, and very little work on the
unsymmetrical hybrid diarylethenes that bear both thiophene
16]
attracted much attention in recent years, and many works con-
cerning substituent effects have been reported.[
7–15]
Irie and
his co-workers reported that the introduction of the bulky iso-
propyl groups in the 2- and 2 -positions of both benzothiophene
rings would increase the cyclization quantum yield. This is
the first guiding principle for the increase of ring-closure quan-
tum yield ever reported. Some reports indicate that the bulky
alkoxy substituents heavily decrease the cycloreversion quan-
ꢀ
and benzothiophene moieties has been attempted with the
[7]
[17–19]
exception of a few reports.
However, the reports con-
cerning this type of hybrid aryl moiety usually focussed on
the applications in some specific fields when incorporating
the hybrid diarylethenes into other functional groups. In this
paper, one of our research goals was to develop a series
of new unsymmetrical diarylethenes bearing both thiophene
and benzothiophene moieties and investigate the substituent
tum yield and the thermal stability of the coloured closed-ring
isomer at high temperature.[
8,9]
Pu et al. and Irie et al. revealed
that electron-donating substituents of the bis(3-thienyl)ethane
©
CSIRO 2009
10.1071/CH08289
0004-9425/09/050464

Properties of Unsymmetrical Photochromic Diarylethenes
465
effects on the optoelectronic properties of diarylethenes
that have various electron-donating or electron-withdrawing
groups at the 5-position of the thiophene ring. Therefore,
we have synthesized five unsymmetrical diarylethenes with
different substituents at the 5-position of the thiophene ring
(Scheme 1), namely 1-(2-ethyl-3-benzothienyl)-2-(2-n-pentyl-
5-hydroxymethyl-3-thienyl)perfluorocyclopentene (1a), 1-(2-
ethyl-3-benzothienyl)-2-{2-n-pentyl-5-[2-(1,3-dioxolane)]-3-
thienyl}perfluorocyclopentene(2a), 1-(2-ethyl-3-benzothienyl)-
2
-{2-n-pentyl-5-[2-(1,3-dithiolpentane)]-3-thienyl}perfluoro-
F
F
F
F
F
F
cyclopentene (3a), 1-(2-ethyl-3-benzothienyl)-2-(2-n-pentyl-5-
formyl-3-thienyl)perfluorocyclopentene (4a), and 1-(2-ethyl-3-
benzothienyl)-2-[2-n-pentyl-5-(2,2 -dicyanovinyl)-3-thienyl]
perfluorocyclopentene (5a). In addition, the crystal structures
of 2a–5a were determined by single-crystal X-ray diffraction
analysis, and their photochromic performances in the single
crystalline phase were also investigated in detail.
F
F
F
F
UV
Vis
F
ꢀ
F
R
R
S
S
S
S
1
2
b
b
R ϭ
R ϭ
CH OH
1
2
a
a
R ϭ
R ϭ
CH OH
2
2
Results and Discussion
O
O
Synthesis of Diarylethenes
O
S
O
S
The synthetic route for diarylethenes 1a–5a is shown in
Scheme2. First, 4-bromo-2-n-pentyl-5-(1,3-dioxolane)thiophene
derivatives (4c) were prepared according to the procedure
described in a previous paper.
was then obtained by reacting benzothiophene with ethylbro-
3
b
R ϭ
3
a
R ϭ
S
S
4
5
b
b
R ϭ
R ϭ
[20]
4
a
a
R ϭ
R ϭ
CHO
CHO
2-Ethylbenzothiophene (1c)
CN
CN
CN
CN
5
mide in the presence of n-BuLi/tetrahydrofuran (THF) solution
◦
at −78 C. Compound 2c was obtained by brominating com-
Scheme 1. Photochromism of diarylethenes 1, 2, 3, 4, and 5.
pound 1c with N-bromosuccinimide. Lithiation of 2c followed
F
F
n-BuLi, Ϫ78
°
C
Br
NBS
THF
C5F8
F
F
F
F
F
Ϫ78°C
Br
S
S
S
c
1
c
2
3c
S
Br
F
F
O
O
F
F
F
S
F
F
F
(
4c)
ϩp-TsOH
F
F
F
F
N
n-BuLi, Ϫ78
°
C
O
O
Acetone/H O
2
S
S
S
S
O
4a
2a
F
F
F
F
F
F
NaBH4
THF
S
S
OH
1
a
F
F
F
F
F
F
F
F
F
F
F
F
O
S
S
p-TsOH/(CH2SH)2
S
S
S
Benzene
S
4
a
3a
F
F
F
F
F
F
CH (CN)2
2
CN
CN
S
S
N
H
5a
Scheme 2. Synthetic route for diarylethenes 1, 2, 3, 4, and 5.

4
66
S. Pu et al.
by the addition of excess octafluorocyclopentene simultaneously
generated compounds 3c, which could be further treated with
the anion generated from 4c to yield compound 2a. Compound
green and the absorption maximum developed at 677 nm in hex-
ane. All solutions of 2b–5b can be decolourized by irradiating
them with visible light of wavelength greater than 450 nm, which
induces the cycloreversion reactions and reproduce 2a–5a.After
repeating 30 colouration–decolouration cycles, the isosbestic
points for diarylethenes 2–5 were observed at 272, 274, 311,
and 383 nm, respectively.
4
a was prepared by hydrolyzing compound 2a in the pres-
ence of pyridine and p-toluenesulfonic acid in acetone/water.
Diarylethene 1a was prepared by the reduction of diarylethene
4
a with sodium borohydride. Finally, diarylethene 3a was syn-
thesized by the condensation reaction of diarylethene 4a with
ethane-1,2-dithiol, and diarylethene 5a was synthesized by the
Knoevenagel condensation reaction of diarylethene 4a with mal-
onodinitrile. The structures of 1a–5a were confirmed by NMR,
IR, and elemental analysis (see the Experimental section). The
crystals of 2a–5a have been determined by single-crystal X-ray
diffraction analysis.
The colour changes of diarylethenes 1–5 upon photoirradi-
ation in hexane are shown in Fig. 2. The absorption spectro-
scopic properties and the cyclization/cycloreversion quantum
yields of these compounds are summarized in Table 1. The
results indicate that different substituents at the 5-position
of the thiophene ring had a significant effect on the photo-
chromic behaviours of diarylethenes 1–5. For compounds 1–3,
the absorption maxima of their open-ring isomers only fluc-
tuate over a very small numerical value range (229–236 nm),
which indicates that the electron-donating substituents do not
significantly influence the absorption maxima of diarylethenes
Photochromism in Solution
The photochromic behaviours of diarylethenes 1–5 were inves-
−
5
−1
tigated in hexane (c 2.0 × 10 mol L ). Diarylethenes 1–5
showed good photochromic properties and could be tog-
gled between their colourless open-ring isomers (1a–5a) and
coloured closed-ring isomers (1b–5b) by alternating irradiation
with UV light and appropriate wavelength visible light, as mon-
itored using UV-Vis absorption. The absorption spectroscopic
change of diarylethene 1 induced by photoirradiation at room
temperature is shown in Fig. 1. The absorption maximum of
the colourless open-ring isomer (1a) was observed at 236 nm
1a–3a. However, the electron-donating substituents have an
efficient contribution to the molar absorption coefficient of
the open-ring isomers of diarylethenes 1–3. For compounds
1a–3a, the molar absorption coefficient increased remark-
ably when replacing the hydroxymethyl group with the strong
electron-donating 1,3-dioxolane group. Meanwhile, the molar
absorption coefficient decreased to some extent when replac-
ing the 1,3-dioxolane group with the stronger electron-donating
1
,3-dithiolpentane group, but it was still higher than that of com-
4
−1
−1
(
2
ε 2.1 × 10 L mol cm ) in hexane. Upon irradiation with
pound 1a. For the closed-ring isomers of diarylethenes 1–3,
the electron-donating substituents have a significant effect on
54 nm light, the colourless solution turned red with a new broad
3
−1
−1
absorption band centred at 529 nm (ε 5.2 × 10 L mol cm ).
The red colour is a result of the formation of the closed-ring iso-
mer (1b). The red coloured solution returned to colourless upon
irradiation with visible light (>450 nm), which indicates that 1b
returned to the initial open-ring isomer (1a). The colouration–
decolouration cycles could be repeated more than 30 times and a
clear isosbestic point was observed at 271 nm.The spectroscopic
changes of diarylethenes 2–5 were similar to that of diarylethene
1a
2a
3a
4a
5a
1
. In hexane, upon irradiation with UV light, diarylethenes 2a
and 3a turned purple, the absorption maxima of which were
observed at 538 and 542 nm, respectively, as the closed-ring iso-
mers 2b and 3b were generated; whereas diarylethene 4a turned
blue and the absorption maximum was observed at 582 nm.
Upon irradiation with 365 nm UV light, diarylethene 5a turned
UV
Vis
0
0
0
0
0
.4
.3
.2
.1
.0
4
1
120
80
4
0
80 s
80
UV
Vis
0
300
400
500
600
700
Wavelength [nm]
1b
2b
3b
4b
5b
Fig. 1. Absorption spectroscopic change of diarylethene 1 in hexane
solution (c 2.0 × 10 mol L ) at room temperature.
Fig. 2. Colour changes of diarylethenes 1, 2, 3, 4, and 5 upon alternating
irradiation with UV and visible light in hexane.
−
5
−1

Properties of Unsymmetrical Photochromic Diarylethenes
467
Table 1. Absorption spectroscopic characteristics and photochromic
substituents are much higher than those of diarylethenes 4 and 5
that have electron-withdrawing substituents. The results may be
ascribed to the substituent position effect on the π-conjugated
system in diarylethene molecules. The cycloreversion quan-
tum yields decreased with an increase in the π-conjugation
reactivity of diarylethenes 1, 2, 3, 4, and 5 in hexane at 2.0 ×
−5
−1
mol L
10
Compound
Absorption spectra
Quantum yieldC
A
B
[1]
λo, max [nm]
λc, max [nm]
−1
ꢀo-c
ꢀc-o
length of the aryl groups. For diarylethenes 4 and 5, the
−
1
−1
−1
(
ε [L mol cm ]) (ε [L mol cm ])
electron-withdrawing formyl and dicyanovinyl groups can par-
ticipate in the conjugation system, which results in a larger
extension of the π-conjugation in the molecules of 4 and 5.
As a result, diarylethenes 4 and 5 exhibit more marked π-
conjugation lengths than 1, 2, and 3, which is consistent with
the red-shifted absorption maxima and the lower cyclorever-
sion quantum yields. For diarylethenes 1–5, all the cyclization
quantum yields are less than 0.5, and are generally lower
4
3
1
2
3
4
5
236 (2.1 × 10 )
529 (5.7 × 10 )
0.22
0.29
0.16
0.33
0.30
0.32
0.31
0.19
0.17
0.11
4
3
229 (3.3 × 10 )
536 (5.1 × 10 )
4
3
231 (2.6 × 10 )
542 (5.0 × 10 )
4
3
257 (2.2 × 10 )
582 (4.7 × 10 )
4
3
358 (1.6 × 10 )
668 (5.5 × 10 )
A
Absorption maxima of the open-ring isomers.
Absorption maxima of the closed-ring isomers.
Quantum yields of cyclization reaction (ꢀo-c) and cycloreversion reaction
B
C
than those of diarylethene derivatives bearing phenyl groups
[10]
on the end.
However, the quantum yields are much higher
(
ꢀc-o), respectively.
than those of photochromic bis(2-thienyl)perfluorocyclopentene
derivatives.[
14]
both the absorption maximum and the molar absorption co-
efficient. The absorption maxima of the closed-ring isomers
The thermal stabilities of diarylethenes 1–5 were examined
in hexane both at room temperature and at 80 C. The results
◦
1
b–3b increased remarkably (from 529 nm to 542 nm) with
show that no changes in their UV-vis spectra were observed, and
no decomposition was detected when these compounds were
exposed to air for more than 20 days at room temperature. At
increasing electron-withdrawing ability. However, the molar
absorption coefficients of the closed-ring isomers 1b–3b
decreased gradually with increasing electron-withdrawing abil-
ity. Among diarylethenes 1–3, the molar absorption coefficient
◦
80 C, diarylethenes 1–4 still showed good thermal stabilities
for more than 24 h, but the green colour of 5b disappeared com-
pletely after 2 min.The thermal instability of 5b is ascribed to the
fact that the photogenerated central carbon–carbon bond in the
closed-ring isomer is weakened by the dicyanovinyl groups.[
3
−1
−1
of the closed-ring isomers 1b (ε 5.7 × 10 L mol cm ) is
3
−1
−1
)
the biggest, whereas that of 3b (ε 5.0 × 10 L mol cm
1]
is the smallest. The result is contrary to that reported by
Irie et al., which indicated that the molar absorption co-
efficient increased gradually with increasing electron-donating
[
12]
Photochromism in the Single Crystalline Phase
ability.
The electron-withdrawing substituents (formyl and
cyano groups) significantly shifted the absorption maxima of
the diarylethenes 4 and 5 to a longer wavelength, and the molar
absorption coefficients of 4b and 5b increased with increasing
electron-withdrawing ability.The result is in agreement with that
reported previously.[
The cyclization and cycloreversion quantum yields of
diarylethenes 1–5 were determined in hexane at room tempera-
ture. Both cyclization and cycloreversion quantum yields were
found to depend on the substituents, as also shown inTable 1.The
cyclization quantum yield of diarylethene 1 was 0.22. When a
strong electron-donating 1,3-dioxolane group was substituted at
the 5-position of the thiophene ring such as in compound 2, the
cyclization quantum yield increased significantly (ꢀo-c 0.29).
However, when the 1,3-dithiolpentane group was substituted at
the same position of the thiophene ring such as in compound 3,
the cyclization quantum yield decreased to some extent (ꢀo-c
Single crystals of 2a–5a were obtained by recrystallization
from a mixture of diethyl ether and hexane. To gain a deeper
understanding of the relation between the conformation and
the photochromic behaviours of diarylethenes 2a–5a in the
crystalline phase, the final structural conformations of these
compounds were provided by X-ray crystallographic analysis.
The X-ray crystallographic analysis data are listed inTable 2.The
ORTEP drawings of the single crystals 2a–5a are shown in Fig. 3
and their packing diagrams are shown in Fig. 4. As shown in
Fig. 3, the molecule of 3a occupied approximately a C2 symme-
try, and it was packed in a photoactive anti-parallel conformation
in the crystalline phase, which can undergo a photocyclization
reaction.[ The molecule has five planar rings, which can form
four dihedral angles between every two adjacent planar rings.
The dihedral angles between the hexafluorocyclopentene ring
10–14,16]
21]
◦
and the two thiophene rings are 59.9(3) for S1/C1–C3/C8 and
◦
0
.16). However, electron-withdrawing formyl and dicyanovinyl
52.2(3) for S2/C16–C19. The dihedral angles between the left
groups increased the cyclization quantum yield, and the cycliza-
tion quantum yield was observed at 0.33 for 4 and 0.30 for 5,
respectively, both of which were then higher than those of com-
pounds 1–3 bearing electron-donating groups. The result is also
in good agreement with that reported by Irie and co-workers.[
The cycloreversion quantum yields of diarylethenes 1, 2, and 3
that have electron-donating substituents are 0.32, 0.31, and 0.19,
respectively. The results indicate that the cycloreversion quan-
tum yield was significantly dependent on the electron-donating
substituents. Furthermore, the cycloreversion quantum yield of
diarylethene 4 and 5 were 0.17 and 0.11, which suggests that
thiophene ring (S1/C1–C3/C8) and the adjacent benzene ring
(C3–C8) is 1.0(3) , and that between the right thiophene ring
◦
(S2/C16–C19) and the 1,3-dithiolpentane ring (C25/S3/C26–
◦
C27/S4) is 89.0(3) . In the perfluorocyclopentene ring of 3a,
12]
the distances of the bond lengths for C11–C12, C12–C13, C13–
C14, C14–C15, and C11–C15 are 1.504(5), 1.523(5), 1.513(5),
1.508(4), and 1.348(4) Å, respectively. These data clearly indi-
cate that the C11–C15 bond is a double bond, being significantly
shorter than the other carbon–carbon single bonds.The two thio-
phene moieties were linked by the C11=C15 double bond, with
both of them attached to the ethylene by the 3-position. The
pentyl and ethyl groups are located on different sides of the dou-
ble bond and are in a trans-direction to the two thiophene planes.
This kind of conformation was crucial to its photochromic
and photoinduced properties.[ The intermolecular distance
the cycloreversion quantum yield decreased upon increasing the
strength of the electron-withdrawing substituents.[
12,16]
From
Table 1, it can be easily seen that the cycloreversion quantum
yields of diarylethenes 1, 2, and 3 that have electron-donating
22]

4
68
S. Pu et al.
Table 2. Crystal data for diarylethenes 2a, 3a, 4a, and 5a
Parameter
2a
3a
4a
5a
Formula
C27H26F6O2S2
560.60
291(2)
Triclinic
P-1
10.2996(11)
10.7924(11)
13.3082(14)
106.6800(10)
100.8440(10)
105.1640(10)
1311.1(2)
2
C27H26F6S4
592.72
294(2)
Triclinic
P-1
10.2169(13)
11.0204(14)
13.4245(17)
67.0280(10)
84.732(2)
89.163(2)
1385.4(3)
2
C25H22F6OS2
516.55
293(2)
Triclinic
P-1
9.8868(12)
10.1076(12)
12.6866(15)
86.8120(10)
75.5780(10)
80.7620(10)
1211.8(3)
2
C28H22F6N2S2
564.60
291(2)
Triclinic
P-1
10.930(5)
11.525(6)
13.715(6)
65.218(6)
67.338(6)
64.912(6)
1374.5(11)
2
Formula weigh
Temperature [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
◦
α [ ]
◦
β [ ]
◦
γ [ ]
3
Volume [Å ]
Z
Density (calcd.) [g cm 3]
−
1.420
1.058
1.421
1.032
1.416
1.057
1.052
1.058
2
Goodness-of-fit on F
Final R indices [I/2σ(I)]
R1
wR2
0.0488
0.1258
0.0555
0.1386
0.0521
0.1365
0.0743
0.2086
R indices (all data)
R1
wR2
0.0615
0.1355
0.0867
0.1623
0.0717
0.1544
0.1220
0.2482
F3
(
a)
(b)
F4
F4
C13
F5
F1
F2
F3
C13 F6
F2
F6
C12
C14
C15
F1
C5
F5
C11
C2
C3
C12
C11
C7
C14
C15
C17
C18
C19
C10
C4
C17
C9
C25
C18
C19
C5
C6
C1
C4
C3
C6
C9
C8
O1
S4
C27
C8 S1
C16
C21
C23
S3
C20
C21
C10 C16
C23
C20
O2
C7
S2
C26
C22
C1
C24
C25
S2
C2 S1
C22
C24
C26
C27
F4
(
c)
(d)
F3
F2
F6
C13
F3
F4
F1
F5
F1
C13
C14
C14
C15
C17
F2
C12
C11
C2
F5
C12
C11
C7
F6
C9
C18
C19
C15
C4
C3
C5
C5
C6
C16
C9
C8 C10
C10
C20
C4
C1
C23
C24
C16
C21
C6
S1
C8
S2
C3
C25
C28
O1
C1 S1
C17
C18
C22
C7
C2
C23
C26
S2
N1
C19
C21
C22
C20
C27
C24
C25
N2
Fig. 3. ORTEP drawings of crystals 2a, 3a, 4a, and 5a, showing 35% probability displacement ellipsoids: (a) 2a, (b) 3a, (c) 4a, (d) 5a.

Properties of Unsymmetrical Photochromic Diarylethenes
469
(
a)
(b)
(
c)
(d)
Fig. 4. Packing views of crystals 2a–5a along the a direction: (a) 2a, (b) 3a, (c) 4a, (d) 5a.
◦
Table 3. Distances between the reacting carbon atoms (d, Å) dihedral angles (θ, ) of 2a, 3a, 4a, and 5a
◦
Compound
d [Å]
θ [ ]
2a
3a
4a
5a
C8· · · C16
3.938(7)
3.766(7)
3.914(3)
3.979(4)
S1/C1/C6–C8
S1/C1–C3/C8
S1/C1–C3/C8
S1/C1/C6–C8
62.9(5)
59.9(3)
65.1(5)
70.6(6)
S2/C16–C19
S2/C16–C19
S2/C16–C19
S2/C16–C17/C23–C24
53.5(5)
52.2(3)
59.9(5)
60.0(6)
C1· · · C16
C1· · · C16
C8· · · C17
between the two active C atoms (C1· · · C16) is 3.766(7) Å. The
corresponding dihedral angles and the distances between the
reacting carbon atoms for 2a, 4a, and 5a were also calculated,
and these data are summarized inTable 3. All molecules of crys-
tals 2a–5a were packed in an anti-parallel mode in the crystalline
phase and the distance between the two reactive carbon atoms
was less than 4.2 Å, which was close enough for the reaction
to take place, which indicates that they could by expected to
colourless crystals of 2a, 3a, and 4a turned purple, purple, and
blue upon irradiation with 254 nm UV light, respectively. When
the coloured crystals were dissolved in hexane, the solution
turned to the corresponding colour and the absorption maximum
was observed at the same wavelength as that of the closed-ring
isomer 2b, 3b, and 4b, respectively.Thecolourdisappearedupon
irradiation with visible light and the absorption spectrum of the
solution that contained the respective colourless crystal was the
same as that of the open-ring isomer 2a, 3a, and 4a. However,
although the crystal of 5a was fixed in an anti-parallel mode,
and the distance of the two reactive carbon atoms was less than
4.2 Å, it cannot undergo photochromic reaction in the single
[23,24]
undergo photochromism in the single crystalline phase.
In fact, crystals of 2a, 3a, and 4a showed a photochromic
reaction coincident with the theoretical analysis. The colour
changes upon photoirradiation are shown in Fig. 5. The

4
70
S. Pu et al.
2
a
3a
4a
5a
2000
1a
2a
1
1
500
000
3a
Vis UV
Vis UV
Vis UV
Vis
UV
500
5a
4a
0
350
400
450
500
Wavelength [nm]
Fig. 6. Fluorescence emission spectra of diarylethenes 1a, 2a, 3a, 4a, and
5
2
b
3b
4b
5b
a in hexane (c 1.0 × 10−4 mol L ) at room temperature, excited at 312 nm.
−1
Fig. 5. Photographs of photochromic processes of diarylethenes 2, 3, 4,
and 5 in the crystalline phase.
fluorescence switch along with the photochromism from open-
ring isomers to closed-ring isomers. When irradiated by 254 nm
UV light, photocyclization reactions occurred and the emission
intensities of diarylethenes 1, 2, and 3 decreased significantly,
which is attributed to the production of the non-fluorescent
closed-ring isomers. The back irradiation by appropriate wave-
length visible light regenerated their open-ring isomers and
recovered the original emission intensity. During the process of
photoisomerization, the three compounds exhibited changes in
crystalline phase. This was verified by the fact that irradiating a
single crystal of 5a with UV light for 5 h resulted in no observ-
able colour change. When the crystal was dissolved in hexane,
the solution remained colourless and the absorption spectrum
was the same as that of diarylethene 5a. The reason for this
is currently not clear and further work is still in progress. To
the best of our knowledge, this is another special diarylethene
example that shows no photochromism in the crystalline phase
although it satisfies the two conditions, with the exception of
those diarylethene crystals with a six-membered unit.[ Fur-
thermore, the diarylethene crystals of 2a, 3a, and 4a exhibited
−
4
−1
their fluorescence in hexane (c 1.0 × 10 mol L ) as shown
in Fig. 7. Upon irradiation with 254 nm light, the emission
intensities of diarylethenes 1a, 2a, and 3a were decreased by
photocyclization. The emission intensities of 1, 2, and 3 in a
photostationary state were quenched to ∼17%, 12%, and 11%,
respectively, which indicates that all of them exhibit a very strong
fluorescent switching in hexane. We also measured the ‘on’ and
the ‘off’ state of the switchable fluorescence by changing the
power of the UV and visible light, respectively. The results show
that the average ‘on’ and ‘off’ times shortened in proportion
to the reciprocal power of the radiated light, which indicates
that the switching effect is indeed photochemical.[ There-
fore, these diarylethene compounds can be potentially applied
to optical memories with a fluorescence readout method and
photomodulation switches.[
25]
2
00 colouration–decolouration cycles by alternating irradiation
with UV and visible light. So, the crystals of diarylethenes 2a–4a
are good candidates for potential optoelectronic applications.[
26]
Fluorescence of Diarylethenes 1a–5a
Fluorescent photochromic materials attract strong interest for
their possible applications in optical memories as well as
in fluorescent probes. In particular, fluorescent diarylethenes,
which show a reversible change in fluorescence intensity with
photochromic reaction, are useful for non-destructive optical
32]
33–35]
[
3]
read-out systems. To date, many diarylethene derivatives and
[17,27,28]
Electrochemical Properties of Diarylethenes 1–5
their fluorescent properties have been reported.
In this
paper, the fluorescence spectra of diarylethenes 1a–5a in hex-
Reversible modulation of electrochemical properties by photo-
irradiation is of basic importance for the development of
molecular electronic devices. To date, many diarylethene
−4
−1
ane (c 1.0 × 10 mol L ) at room temperature are illustrated
in Fig. 6. The results illustrate that 1a, 2a, and 3a show good flu-
orescence in hexane, whereas diarylethenes 4a and 5a showed
very weak fluorescence. Therefore, we only discuss the fluores-
cence properties of 1a–3a in this paper.As shown in Fig. 6, it can
be clearly seen that the emission peaks of diarylethenes 1a–3a
are observed at 405, 407, and 410 nm in hexane. These data indi-
cate that the effect of electron-donating groups on the emission
peak is not significant and the values undergo a minor red-shift
with increasing electron-donating ability. However, the effect
of electron-donating substituents on their emission intensity
are very remarkable. Among diarylethenes 1a–3a, the emission
intensity of diarylethene 3a was the weakest, and that of 1a and
derivatives and their electrochemical properties have been
reported.[
11,20,36–38]
In this work, the electrochemical prop-
erties of 1–5 were investigated using cyclic voltammetry
(CV) methods under the same experimental conditions.
The typical electrolyte was acetonitrile (5 mL) that con-
−
1
tained 0.10 mol L
tetrabutylammonium tetrafluoroborate
−
3
−1
((TBA)BF4) and 4.0 × 10 mol L diarylethenes 1–5. The
experimental methods and conditions are described in a pre-
vious paper.[ Fig. 8 shows the CV curves of diarylethene 1
with a scanning rate of 50 mV s . From the anodic polarization
curves, it can be clearly seen that the oxidation of the ring-open
isomer 1a is initiated at 1.47V and that of the ring-closed isomer
20]
−
1
2
a were almost equal to each other.
As has been observed for most of the reported diaryl-
1b is at 1.39V. According to the same method described in pre-
[
29–31]
[20,39,40]
ethenes,
diarylethenes 1–3 exhibited a relatively strong
vious publications,
the values of the ionization potential

Properties of Unsymmetrical Photochromic Diarylethenes
471
(
a) 2000
1
1
50
00
1a
1b
Vis UV
1
1
500
000
50
0
500
Ϫ50
Ϫ2
Ϫ1
0
1
2
3
3
50
400
450
500
500
500
Potential [V]
Wavelength [nm]
(
b) 2000
Fig. 8. Cyclic voltammetry (second scan) of diarylethene 1 in acetonitrile
with a scanning rate of 50 mV s
−
1
.
Vis UV
clearly higher than those of the closed-ring isomers. This is in
accordance with the theory that longer conjugation length gen-
erally leads towards lower positive potentials, with the addition
of each heterocyclic ring. During the ring-closed reaction, the π-
conjugation lengths of 1b, 2b, 3b, 4b, and 5b was much longer
than those of 1a, 2a, 3a, 4a, and 5a, thus leading to lower oxida-
tion onset. The difference of oxidation onset between the open-
and closed-ring isomers of diarylethenes 1–5 was 0.08V for 1,
1
1
500
000
500
0
.01V for 2, 0.41V for 3, 0.02V for 4, and 0.08V for 5, respec-
tively. For the band-gap of diarylethenes 1–5, all the values of Eg
of the open-ring isomers were higher than those of the closed-
ring isomers, with the exception of diarylethene 2. Among these
compounds, the Eg of 3b was the smallest, which implies that
the charge transfer of 3b must be faster compared with those
of the others. All the data described above suggest that different
electron-donating and electron-withdrawing substituents have a
remarkable effect on the electrochemical behaviours of these
diarylethenes. It should be noted here that the absolute HOMO
and LUMO levels in combination with the energy gap calculated
by electrochemical data is still under debate.[
3
50
400
450
Wavelength [nm]
(
c)
1
1
500
000
Vis UV
39,40]
Conclusions
500
In summary, five new unsymmetrical diarylethenes that have dif-
ferent substituents at the 5-position of the thiophene ring were
synthesized and the substituent effects on their optoelectronic
properties were investigated.The results showed that the absorp-
tion characteristics, photochromic reactivity, fluorescence, and
electrochemical properties were significantly dependent on the
substituents attached at the 5-position of thiophene ring, which
may be attributed to the different electron-donating or electron-
withdrawing substituent effects. The result of this study is useful
for the efficient design and synthesis of photoactive diarylethene
derivatives with excellent characteristics.
0
350
400
450
Wavelength [nm]
Fig. 7. Emission intensity changes of diarylethenes 1, 2, and 3 in hex-
ane (c 1.0 × 10 mol L ) upon irradiation with 254 nm UV light at room
temperature, excited at 312 nm: (a) 1, (b) 2, and (c) 3.
−
4
−1
and electron affinity were calculated to be −6.27 and −3.53,
respectively. Based on the highest occupied molecular orbitals
Experimental
(
HOMO) and lowest unoccupied molecular orbital (LUMO)
General Methods
energy level, the band-gap Eg of 1a and 1b can be determined
to be 2.74 eV and 2.70 eV, respectively. The corresponding val-
ues for diarylethenes 2–5 are summarized in Table 4. As shown
in Table 4, it can be clearly seen that the electrochemical para-
meters of the five compounds are remarkably dependent on the
substituent effects. The oxidation of 1a–5a is initiated at 1.47,
.80, 1.81, 1.46, and 1.75V, and that of 1b–5b is initiated at
.39, 1.79, 1.40, 1.44, and 1.67V, respectively. The results show
that the oxidation potential onsets of the open-ring isomers are
NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer with CDCl3 as the solvent and tetramethylsilane
as an internal standard. IR spectra were recorded on Bruker
Vertex-70 spectrometer and mass spectra were measured with
Agilent MS Trap VL spectrometer. The elemental analysis was
measured with a PE CHN 2400. The absorption spectra were
measured using an Agilent 8453 UV/VIS spectrometer. Photo-
irradiation was carried out using a SHG-200 UV lamp and a
1
1

4
72
S. Pu et al.
Table 4. Electrochemical properties of diarylethenes 1, 2, 3, 4, and 5 in acetonitrile
Compound
Oxidation
Ionization
Reduction
Electron affinity
Band gap
Eg
Eonset
Eonset
[V]
potential [eV]
[V]
[eV]
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
+1.47
+1.39
+1.80
+1.79
+1.81
+ 1.40
+1.46
+1.44
+1.75
+1.67
−6.27
−6.19
−6.60
−6.59
−6.61
−6.20
−6.26
−6.24
−6.55
−6.47
−1.27
−1.31
−1.62
−1.68
−0.91
−1.22
−1.76
−1.71
−1.01
−1.13
−3.53
−3.49
−3.18
−3.12
−3.89
−3.58
−3.04
−3.09
−3.79
−3.67
2.74
2.70
3.42
3.47
2.72
2.62
3.22
3.15
2.76
2.68
BMH-250 Visible lamp. Light of appropriate wavelengths was
isolated by different light filters. Electrochemical examinations
were performed in a one-compartment cell using a Model 263
potentiostat-galvanostat (EG&G Princeton Applied Research)
under computer control at room temperature. All solvents used
were of spectrograde and were purified by distillation before use.
Crystal data of diarylethenes 2a, 3a, 4a, and 5a were collected by
a Bruker SMARTAPEX2 CCD area-detector. Further details on
the crystal structure investigation have been deposited with The
Cambridge Crystallographic Data Centre as supplementary pub-
lication numbers CCDC 661184, 676246, 676245, and 676247
for 2a, 3a, 4a, and 5a. These data can be obtained free of charge
at http://www.ccdc.cam.ac.uk/data_request/cif.
ether) to give 7.5 g of 2c in 89% yield. δH (CDCl3, 400 MHz,
TMS) 1.38 (t, 3H, J 7.4, –CH3), 2.91–2.97 (m, 2H, –CH2), 7.22–
7.28 (m, 1H, phenyl-H), 7.30–7.35 (m, 1H, phenyl-H), 7.74 (t,
2H, phenyl-H). (Found: C 49.85, H 3.81. Calc. for C10H9BrS:
C 49.81, H 3.76.)
1
3
-(3-Bromo-2-n-ethyl)-1-(benzothiophene-
-yl)perflourocyclopentene (3c)
To a stirred THF solution (80 mL) that contained compound
2
c (6.79 g, 26.63 mmol) was slowly added 10.52 mL of n-BuLi
◦
(
2.5 M) at −78 C under a nitrogen atmosphere. After 40 min,
−1
C5F8 (3.64 mL, 0.136 mL mol ) was added quickly to the reac-
◦
tion mixture, and left to stand with stirring at −78 C under
a nitrogen atmosphere for 2 h. The reaction mixture was then
extracted with diethyl ether and evaporated under vacuum. The
residue was purified by column chromatography on silica gel
to give 5.16 g of 3c in 55% yield. δH (CDCl3, 400 MHz, TMS)
Synthesis
The syntheses of diarylethenes 1a–5a is shown in Scheme 2.The
experimental details were carried out as follows.
1
.36 (t, 3H, J 7.5, –CH3), 2.82–2.88 (m, 2H, –CH2), 7.36–7.41
2
-Ethylbenzothiophene (1c)
(m, 2H, phenyl-H), 7.48 (d, 1H, J 7.6, phenyl-H), 7.82 (d, 1H,
J 7.6, phenyl-H). (Found: C 50.91, H 2.61. Calc. for C15H9F7S:
C 50.85, H 2.56.)
To a stirredTHF solution (60 mL) that contained benzothiophene
(
(
5.00 g, 37.26 mmol) was slowly added 15.60 mL of n-BuLi
2.5 mmol) at −78 C under a nitrogen atmosphere, and the solu-
◦
tionwasstirredfor45 min. Ethylbromide(3.05 mL, 40.98 mmol)
was then added slowly to the reaction mixture, and left to stand
1-(2-Ethyl-3-benzothienyl)-2-{2-n-pentyl-5-[2-(1,3-
dioxolane)]-3-thienyl}perfluorocyclopentene (2a)
◦
withstirringat −78 Ctoroomtemperaturefor18 h.Thereaction
To a stirred THF solution (30 mL) that contained 3-bromo-2-n-
pentyl-5-(1,3-dioxolane)thiophene (4c)
was slowly added 3.31 mL of n-BuLi (1.6 M) at −78 C under a
nitrogen atmosphere. After 40 min, a 10 mL THF solution that
contained compound 3c (2.79 g, 8.20 mol) was added slowly to
mixture was then poured into concentrated sodium chloride solu-
tion and extracted with diethyl ether.The organic layer was dried,
filtered, and concentrated. The residue was purified by column
chromatography on silica gel (petroleum ether) to give 5.70 g of
[20]
(2.62 g, 8.20 mmol)
◦
1c in 94% yield. δH (CDCl3, 400 MHz, TMS) 1.38 (t, 3H, J 7.4,
–CH3), 2.91–2.97 (m, 2H, –CH2), 7.01 (s, 1H, thienyl-H), 7.22–
7.28 (m, 1H, phenyl-H), 7.30–7.35 (m, 1H, phenyl-H), 7.74 (t,
2H, phenyl-H). (Found: C 74.11, H 6.23. Calc. for C10H10S:
◦
the reaction mixture, and left to stand with stirring at −78 C
under a nitrogen atmosphere for 2 h. The reaction mixture was
then extracted with diethyl ether and evaporated under vacuum.
Theresiduewaspurifiedbycolumnchromatographyonsilicagel
to give 2.03 g of 2a in 44% yield. δH (CDCl3, 400 MHz, TMS)
C 74.03, H 6.21.)
0
3
.74 (t, 3H, J 7.1, –CH3), 0.98–1.01 (m, 4H, –CH2), 1.07 (t,
H, J 7.5, –CH3), 1.26–1.27 (m, 2H, –CH2), 2.03–2.28 (m, 2H,
3
-Bromo-2-ethyl-benzothiophene (2c)
Although this compound was reported,[13] its synthetic method
was not described. To a stirred THF solution (60 mL) that con-
–CH2), 2.43–2.67 (m, 2H, –CH2), 3.96–4.05 (m, 4H, dioxolane-
H), 5.96 (s, 1H, dioxolane-H), 7.03 (s, 1H, thienyl-H), 7.33–
7.38 (m, 2H, phenyl-H), 7.60 (d, 1H, J 7.1, phenyl-H), 7.75
(d, 1H, J 7.8, phenyl-H). δC (CDCl3, 100 MHz, TMS) 13.70,
15.33, 22.19, 22.73, 29.23, 30.96, 31.04, 65.15, 65.21, 99.87,
118.85, 122.21, 122.40, 122.47, 122.62, 124.42, 124.83, 125.86,
tained compound 1c (5.70 g, 37.26 mmol) was slowly added
◦
6
.88 g of N-bromosuccinimide (38.64 mmol) at 5 C. The reac-
tion mixture was stirred for 22 h, and was then poured into
sodium thiosulfate solution and extracted with diethyl ether. The
organic layer was dried, filtered, and concentrated. The residue
was purified by column chromatography on silica gel (petroleum
−
1
138.22, 138.31, 139.66, 150.10, 150.22. νmax (KBr)/cm 731,
754, 942, 970, 989, 1018, 1073, 1100, 1141, 1190, 1275, 1343,

Properties of Unsymmetrical Photochromic Diarylethenes
473
1
435, 1458, 1640, 2930, 2954. (Found: C 58.01, H 4.48. Calc.
15.43, 22.23, 22.77, 29.36, 31.05, 39.76, 39.92, 50.55, 118.94,
122.22, 122.36, 122.45, 122.53, 124.44, 124.86, 125.07, 138.24,
for C27H26F6O2S2: C 57.85, H 4.67.)
−
1
138.34, 149.34, 150.21. νmax (KBr)/cm 730, 759, 975, 1017,
1
3
-(2-Ethyl-3-benzothienyl)-2-(2-n-pentyl-5-formyl-
-thienyl)perfluorocyclopente (4a)
1071, 1099, 1139, 1191, 1275, 1342, 1384, 1634, 2926. (Found:
C 55.13, H 4.52. Calc. for C27H26F6S4: C 54.71, H 4.42.)
Compound 2a (1.63 g, 2.9 mmol) and p-toluenesulfonic acid
0.55 g) were dissolved in a mixture of water (30 mL) and ace-
ꢀ
1
-(2-Ethyl-3-benzothienyl)-2-[2-n-pentyl-5-(2,2 -
(
dicyanovinyl)-3-thienyl]perfluorocyclopentene (5a)
tone (90 mL). Pyridine (0.23 mL) was added into the mixture and
it was refluxed for 24 h. After stopping the reaction, the mixture
was washed sequentially by aqueous NaHCO3 and water. The
organic layer was dried, filtered, and evaporated. The residue
was purified by column chromatography on silica gel to give
To a stirred solution of compound 1 (0.1 g, 0.19 mmol) and mal-
onodinitrile (0.019 g, 0.19 mmol) in anhydrous ethanol (10 mL),
a very small quantity of piperidine was added dropwise at room
◦
temperature.The reaction mixture was stirred overnight at 50 C.
The mixture was then extracted with chloroform and evaporated
under vacuum. The residue was purified by column chromato-
graphy on silica gel to give 0.08 g of 5a in 75% yield. δH (CDCl3,
00 MHz, TMS) 0.76 (t, 3H, J 7.1, –CH3), 0.98–1.04 (m, 4H,
CH2), 1.07 (t, 3H, J 7.5, –CH3), 1.10–1.14 (m, 2H, –CH2),
.17–2.40 (m, 2H, –CH2), 2.46–2.68 (m, 2H, –CH2), 7.34–7.40
m, 2H, phenyl-H), 7.56 (d, 1H, J 7.5, phenyl-H), 7.60 (s, 1H,
thienyl-H), 7.73 (s, 1H, –CH), 7.78 (d, 1H, J 7.6, phenyl-H).
δ (CDCl , 100 MHz, TMS) 13.73, 15.56, 22.13, 22.90, 30.04,
0.91, 31.01, 112.63, 113.44, 117.99, 122.02, 122.08, 122.53,
24.87, 125.21, 125.52, 133.06, 137.82, 138.30, 150.04, 150.57,
1.23 g of 4a in 83% yield. δH (CDCl3, 400 MHz, TMS) 0.74 (t,
3H, J 7.0, –CH3), 0.96–1.04 (m, 4H, –CH2), 1.08 (t, 3H, J 7.5,
–CH3), 1.26–1.27 (m, 2H, –CH2), 2.07–2.34 (m, 2H, –CH2),
2.43–2.68 (m, 2H, –CH2), 7.33–7.40 (m, 2H, phenyl-H), 7.60
4
–
2
(
d, 1H, J 7.4, phenyl-H), 7.72 (s, 1H, thienyl-H), 7.77 (d, 1H, J
7
1
1
1
1
1
.7, phenyl-H), 9.81 (s, 1H, –CHO). δC (CDCl3, 100 MHz,TMS)
3.74, 15.47, 22.16, 22.83, 29.85, 30.81, 30.89, 118.27, 122.17,
22.24, 122.42, 124.71, 124.79, 125.08, 135.96, 137.92, 138.26,
41.43, 150.46, 159.03, 182.11. νmax (KBr)/cm 733, 757, 993,
071, 1111, 1147, 1204, 1235, 1276, 1343, 1388, 1460, 1545,
677, 2958. (Found: C 58.39, H 4.12. Calc. for C25H22F6OS2:
(
−
1
C
3
3
1
1
1
3
−
1
60.55. νmax (KBr)/cm 731, 757, 948, 990, 1074, 1139, 1187,
275, 1333, 1384, 1437, 1581, 2228, 2928. (Found: C 59.83, H
.95, N 4.87. Calc. for C28H22F6N2S2: C 59.56, H 3.93, N 4.96.)
C 58.13, H 3.89.)
1
3
-(2-Ethyl-3-benzothienyl)-2-(2-n-pentyl-5-hydroxymethyl-
-thienyl)perfluorocyclopentene (1a)
Acknowledgements
To a stirred THF solution (30 mL) that contained 4a
This work was supported by the Program for New Century Excellent Tal-
ents in University (NCET-08–0702), the Project of Jiangxi Youth Scientist,
National Natural Science Foundation of Jiangxi Province (2008GZH0020),
the Key Scientific Project from Education Ministry of China (208069) and
the Science Funds of the Education Office of Jiangxi, China (GJJ08365).
(
(
0.2 g, 0.39 mmol) was added 0.017 g of sodium borohydride
0.47 mmol) at room temperature. The reaction mixture was
stirred for 22 h, and then stopped by addition of a 6% hydrochlo-
ric acid solution. The mixture was extracted with chloroform
and evaporated under vacuum. The residue was purified by col-
umn chromatography on silica gel to give 0.15 g of 1a in 76%
yield. δH (CDCl3, 400 MHz, TMS) 0.73 (t, 3H, J 7.1, –CH3),
References
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0
.88–0.95 (m, 4H, –CH2), 1.06 (t, 3H, J 7.5, –CH3), 1.19–1.24
(
m, 2H, –CH2), 2.02–2.28 (m, 2H, –CH2), 2.43–2.68 (m, 2H,
[
[
–
7
1
1
1
1
1
CH2), 4.68–4.76 (m, 2H, –CH2), 6.92 (s, 1H, thienyl-H), 7.30–
.39 (m, 2H, phenyl-H), 7.61 (d, 1H, J 7.3, phenyl-H), 7.75 (d,
H, J 7.7, phenyl-H). δC (CDCl3, 100 MHz, TMS) 13.78, 15.28,
5.39, 22.25, 22.77, 29.24, 30.98, 31.16, 59.99, 118.93, 122.25,
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-(2-Ethyl-3-benzothienyl)-2-{2-n-pentyl-5-[2-(1,3-
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2
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