Photochromism of new unsymmetrical isomeric diarylethenes bearing a methoxyl group

Article abstract of DOI:10.1002/poc.1545

Three new unsymmetrical isomeric diarylethenes having a methoxyl substituent at ortho-, meta-, and para-position of the terminal benzene ring, namely 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(2-methoxylphenyl)-3-thienyl] perfluorocyclopentene (1o), 1-(2,5

Full text of DOI:10.1002/poc.1545

Research Article
Received: 31 August 2008,
Revised: 11 January 2009,
Accepted: 30 January 2009,
Published online in Wiley InterScience: 23 March 2009
(www.interscience.wiley.com) DOI 10.1002/poc.1545
Photochromism of new unsymmetrical
isomeric diarylethenes bearing a methoxyl
group
a
a
a
*
Shouzhi Pu , Weijun Liu and Wenjuan Miao
Three new unsymmetrical isomeric diarylethenes having a methoxyl substituent at ortho-, meta-, and para-position of the
terminal benzene ring, namely 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(2-methoxylphenyl)-3-thienyl]perfluorocyclopentene
(1o), 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(3-methoxylphenyl)-3-thienyl]perfluorocyclopentene (2o), and 1-(2,5-dimethyl-3-
thienyl)-2-[2-methyl-5-(4-methoxylphenyl)-3-thienyl]perfluorocyclopentene (3o), have been synthesized. The substituent
position effect of methoxyl group on their properties, including photochromism and fluorescence both in hexane solution
and in PMMA film, and their electrochemical properties, were investigated in detail. These diarylethenes showed good
photochromism both in solution and in PMMA film. For the same photochromic diarylethene backbone, the electron-
donating methoxyl substituent can effectively depress the cyclization quantum yields and increase the cycloreversion
quantum yields compared to those of diarylethenes bearing chlorine atoms reported previously. Diarylethenes 1o–3o showed
clear fluorescent switches by photoirradiation both in hexane and in PMMA film. In addition, cyclic voltammetry tests showed
that the electron-donating methoxyl group at different position on the terminal benzene ring had a significant effect on the
electrochemical properties of these isomeric diarylethenes. Copyright ß 2009 John Wiley & Sons, Ltd.
Keywords: photochromism; diarylethene; methoxyl substituent position effect; property
phene rings would increase the cycloreversion quantum
Introduction
yields.[19] Morimitsu et al. and Takami and Irie reported that
bulky alkoxy substituents at 2- and 20-position of thiophene rings
would extraordinarily decrease the cycloreversion quantum yield
and the thermal stability of the colored closed-ring isomers at
high temperature.[20,21] In previous publications, we demon-
strated that different substituents in different photochromic
systems had a significant effect on the properties of diarylethene
derivatives, including photochromism, fluorescence and electro-
chemical features.[22–24] The majority of these works has been
devoted to the development of these photochromic molecules
and investigative studies of their fundamental properties, and
they have contributed to a broad understanding of the
substituent effects on the photochromic characteristics of
diarylethene derivatives.
Photochromism is defined as
a photoinduced reversible
photocoloration in a single chemical species between two forms
having different absorption spectra, which is brought about by
the action of electromagnetic radiation in one direction at least.[1]
Among various photochromic molecules, diarylethenes with
heterocyclic rings have been developed as a new type of
thermally stable and fatigue-resistant photochromic com-
pound.[2–4] Nowadays, these diarylethene derivatives bearing
two thiophene/benzothiophene rings are regarded as the best
candidates for photonics applications, such as optical mem-
ories[5–8] and photoswitches,[9,10] because of their excellent
thermal stability, remarkable fatigue resistance, and rapid
response.[11–13]
It is well known that the photochromic property of
diarylethenes depends on several factors, such as the nature
of the heteroaryl moieties, the conformation of the open-ring
isomer, electron donor/acceptor substituents, p-conjugation
length of the heteroaryl groups, and so forth.[14] Among these
factors, the effect of electron donor/acceptor substituent is the
most important one that is useful for the design efficient
photoactive diarylethene derivatives with tunable properties. For
instance, some reports elucidated that electron-donating sub-
stituents attached bis(3-thienyl)ethane diarylethenes could be
effective to increase the absorption coefficient of the closed-ring
forms and to decrease the cycloreversion quantum yield. But,
those attached bis(2-thienyl)ethane diarylethenes could be an
effective way to increase the maxima absorption of the open-ring
forms and to reduce the cyclization quantum yield.[15–18]
Morimitsu et al. suggested that the introduction of the ethynyl
groups at 2- and 20-position of both thiophene and benzothio-
Although there are many reports concerning the substituent
effect on the properties of diarylethenes, studies on the
substituent position effect are extremely rare. As far as we are
aware, there are only a few publications concerning the
substituent position effect on the optoelectronic properties of
diarylethenes, all reported by our research group.[25–29] Pre-
viously, we mainly focused on the effect of electron-withdrawing
groups, such as fluorine atom and chlorine atom, and we found
that the fluorine/chlorine atoms and their substituted positions
*
Correspondence to: S. Pu, Jiangxi Key Laboratory of Organic Chemistry, Jiangxi
Science and Technology Normal University, Nanchang 330013, P.R. China.
E-mail: pushouzhi@tsinghua.org.cn
a
S. Pu, W. Liu, W. Miao
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology
Normal University, Nanchang, P.R. China
J. Phys. Org. Chem. 2009, 22 954–963
Copyright ß 2009 John Wiley & Sons, Ltd.

NEW UNSYMMETRICAL ISOMERIC DIARYLETHENES
irradiation with UV and visible light with appropriate wavelength
are shown in Fig. 1. As shown in Fig. 1A, the colorless solution
containing the open-ring isomer 1o showed a sharp absorption
peak at 273 nm (e¼ 1.98 ꢀ 104 Lmolꢁ1 cmꢁ1) in hexane, which was
arisen from p! p* transition.[35] Upon irradiation with 297 nm UV
light, the colorless solution turned purple and a new visible
absorption band centered at 551 nm (e¼ 7.5 ꢀ 103 L molꢁ1 cmꢁ1
)
emerged while the original peak at 297 nm decreased, indicating
the formation of the closed-ring isomer 1c. The conversion from 1o
to 1c was 93% in the photostationary state, separated and detected
by LC chromatography. Alternatively, the purple colored solution
returned to their original colorless upon irradiation with visible light
(l > 450 nm), indicating that 1c returned to the initial state 1o.
Similarly, compounds 2o and 3o also showed good photochromism
in hexane, and their absorption spectral changes are shown in Fig.
1B and C. Upon irradiation with 297 nm UV light, absorption bands
in the visible region appeared and the solutions of 2o and 3o turned
purple as a result of the cyclization reaction to produce 2c and 3c,
which the absorption maxima of them were observed at 539 and
543 nm, respectively. The color changes of diarylethenes 1–3 by
photoirradiation in hexane are shown in Fig. 1D. Both the solutions
of 2c and 3c can be decolorized upon irradiation with visible light
(l > 450 nm) attributable to reproduce the open-ring isomers 2o
and 3o. The coloration–decoloration cycles can be repeated more
than 100 times and no observable degradation can be observed. In
the photostationary state, the conversion from the open-ring isomer
to the closed-ring isomer was 91% for 2 and 85% for 3, respectively.
The photochromic properties of diarylethenes 1–3 in hexane
are summarized in Table 1. It could be seen from these data that
the methoxyl substituent position had a significant effect on the
photochromic properties (including absorption maxima, molar
absorption coefficients, and quantum yields) of diarylethenes
1–3. Among diarylethenes 1–3, the absorption maximum of the
open-ring isomer of the para-substituted derivative (compound 3)
(l3o,max ¼ 293 nm) is the biggest and its molar absorption
coefficient (e3o ¼ 1.87 ꢀ 104 L molꢁ1 cmꢁ1) is the smallest, while
the absorption maximum of the open-ring isomer of the
ortho-substituted derivative (compound 1) (l1o,max ¼ 273 nm) is
the smallest and the molar absorption coefficient of the
meta-substituted derivatives (compound 2) (e2o ¼ 2.15 ꢀ
104 L molꢁ1 cmꢁ1) is the biggest. The result indicated that
absorption maxima of the open-ring isomers 1o–3o had a red
shift trend along with the methoxyl group substituting hydrogen
atom of the benzene ring from at ortho- to at para-position, but
their molar absorption coefficients changed irregularly. For the
closed-ring isomers 1c–3c, both the absorption maximum
Scheme 1. Photochromism of diarylethenes 1–3
had a significant effect on the properties of these diarylethene
compounds.[25–28] The results are very important and interesting,
and they also give us some good suggestions and valuable
insights. For the methoxyl group, it is a strong electron-donating
substituent and has
a main role on the photochromic
diarylethene system. The introduction of methoxyl groups to
the reactive positions renders the cycloreversion reaction quantum
yield extraordinarily small.[20,30] To the best of our knowledge, the
electron-donating methoxyl substituent position effect on the
properties of photochromic dithienylethene derivatives has not
hitherto been reported. We considered that dithienylethene
derivatives bearing a methoxyl group at the different position of
the terminal benzene ring would have some new distinguishable
properties. In this work, in order to better understand the
methoxyl substituent position effect on the photochromic
properties of diarylethene derivatives, we have synthesized
three new unsymmetrical isomeric diarylethene derivatives,
namely 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(2-methoxylphenyl)-
3- thienyl]perfluorocyclopentene (1o), 1-(2,5-dimethyl-3-thienyl)-
2-[2-methyl-5-(3-methoxylphenyl)-3-thienyl]perfluorocyclopentene
(2o), and 1-(2,5-dimethyl-3-thien-yl)-2-[2-methyl-5-(4-methoxylphenyl)-
3-thienyl]perfluorocyclopentene (3o), which have a methoxyl
group at ortho-, meta-, and para-positioin of the terminal phenyl
ring. The molecular structures and photochromic schemes of
diarylethenes 1–3, which are discussed in this work, are shown in
Scheme 1.
RESULTS AND DISCUSSION
Synthesis of diarylethenes
The synthetic route for diarylethenes 1o–3o is shown in Scheme
2. At first, three bromobenzene derivatives coupled with
thiophene boronic acid (4)[31,32] by Suzuki reaction to give
methoxylphenylthiophene derivatives (5a–5c). Then, 2,5-
dimethyl-3-thienylperfluorocyclpentene (7) was prepared
according to the same procedure described previously.[33,34]
Finally, compounds 5a–5c was separately lithiated and then
coupled with compound 7 to give diarylethenes 1o–3o,
respectively. The structures of 1o–3o were confirmed by mass
spectrometry, elemental analysis, NMR, and IR (Section ‘Exper-
imental’).
and the molar absorption coefficient of diarylethene
1
(l1c,max ¼ 551 nm, e1c ¼ 7.50 ꢀ 103 L molꢁ1 cmꢁ1) are the biggest,
and those of diarylethene 2 (l2c,max ¼ 539 nm, e2c ¼ 5.85 ꢀ
103 L molꢁ1 cmꢁ1) are the smallest. As shown in Table 1, all the
cyclization quantum yields diarylethenes 1–3 are greatly higher
than their respective cycloreversion quantum yields. This is
differing from unsymmetrical diarylethenes bearing a pyrazole
unit whose cyclization quantum yields are lower than their
respective cycloreversion quantum yields.[29,31] Among diary-
lethenes 1–3, the cyclization quantum yield of diarylethene 1
(Fo-c ¼ 0.45) is the biggest and that of diarylethene
2
Photochromism of diarylethenes
(Fo-c ¼ 0.33) is the smallest, while the cycloreversion quantum
yield of diarylethene 2 (Fc-o ¼ 0.041) is the biggest and that of
diarylethene 3 (Fc-o ¼ 0.015) is the smallest. The cycloreversion
quantum yield of diarylethene 1 (Fc-o ¼ 0.015) is almost equal to
that of diarylethene 3. That is to say, diarylethene 1 has a large
The photochromic properties of diarylethenes 1–3 were measured
at room temperature both in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and in
PMMA medium (10%, w/w). In hexane, the absorption spectral and
color changes of diarylethenes 1–3 induced by alternating
J. Phys. Org. Chem. 2009, 22 954–963
Copyright ß 2009 John Wiley & Sons, Ltd.
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S. PU, W. LIU AND W. MIAO
Figure 1. Absorption spectral and color changes of diarylethenes 1–3 upon alternating irradiation with UV and vis light in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) at
room temperature: (A) spectral changes for 1; (B) spectral changes for 2; (C) spectral changes for 3; (D) color changes for 1–3.
cyclization quantum yield and a small cycloreversion quantum
yield, indicating that it is convenient to achieve high conversion
to the closed-ring isomer at the photostationary state,[36] which is
well in agreement with the experimental result. The result is
remarkably distinguished from those of diarylethenes having a
chlorine atom reported in a previous paper.[28] When replacing
the methoxyl group with chlorine atom in the same diarylethene
system, the cyclization quantum yields increased and the
cycloreversion quantum yields decreased remarkably, and
above all, the photochromic behaviors of these compounds
were also significantly different.[28] In addition, compared to
the non-methoxyl group derivative (1-(2,5-dimethyl-3-thienyl)-2-
(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, D4), the
closed-ring isomers’ molar absorption coefficients and the
Table 1. Absorption characteristics and photochromic reactivity of diarylethenes 1–3 in hexane at 2.0 ꢀ 10ꢁ5 mol Lꢁ1, and in PMMA
film (10% w/w) at room temperature
lo,max (nm)a(e/L molꢁ1 cmꢁ1
)
lc,max (nm)b(e/L molꢁ1 cmꢁ1
)
Fc
Compound
Hexane
PMMA film
Hexane
PMMA film
Fo-c
Fc-o
d
1
2
3
273 (1.98 ꢀ 104)
286 (2.15 ꢀ 104)
293 (1.87 ꢀ 104)
—
—
—
551 (7.50 ꢀ 103)
539 (5.85 ꢀ 103)
543 (6.38 ꢀ 103)
557
555
556
0.454
0.334
0.343
0.017
0.041
0.015
a Absorption maxima of open-ring isomers.
bAbsorption maxima of closed-ring isomers.
cQuantum yields of open-ring (Fo-c) and closed-ring isomers (Fc-o), respectively.
dNo absorption peak because of being absorbed UV light by glass substrate.
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NEW UNSYMMETRICAL ISOMERIC DIARYLETHENES
cycloreversion quantum yields of isomeric diarylethenes 1–3
bearing a electron-donating methoxyl group are much bigger
6 nm for 1c, 16 nm for 2c and 13 nm for 3c, respectively. The red
shift phenomena may be ascribed to the polar effect of the
polymer matrix and the stabilization of molecular arrangement in
solid state.[39]
than those of compound D4 (eD4,c ¼ 3.68ꢀ 103 L molꢁ1 cmꢁ1
,
FD4,c-o ¼ 0.119).[28] The possible reason is that electron-
donating methoxyl group changes effectively the charge
distribution and can be effective to increase the absorption
coefficient and to decrease the cycloreversion quantum
yield.[16] Compared to symmetric analogue, 1,2-bis[2-methyl-5-
(4-methoxylphenyl)-3-thienyl] perfluorocyclopentene (D5),[37]
the cycloreversion quantum yields of these isomeric
derivatives increase remarkably, but their cyclization
quantum yields and molar absorption coefficients decrease
significantly.
Fluorescence of diarylethenes
Fluorescent properties can be useful in molecular scale
optoelectronics,[40] digital fluorescence photoswitches,[41,42]
ion-sensors,[43–46] and so on. In the present work, the
fluorescence properties of diarylethenes 1–3 both in solution
and in PMMA film were measured using a Hitachi F-4500
spectrophotometer. The fluorescence emission spectra of
diarylethenes 1o–3o both in hexane (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and
in PMMA film (10%, w/w) at room temperature are shown in Fig. 3.
When excited at 300 nm, diarylethenes 1o–3o showed an
obvious fluorescent emission bands peaked at 350, 346, and
348 nm, respectively, which were attributed to the p ! p* transfer
of the phenyl-thiophene or thienyl-cyclopentene moiety.
However, a remarkable bathochromic shift of the fluorescent
emission were observed for 1o–3o when doped in PMMA matrix.
The maximal emission peaks of 1o–3o were measured to be 408,
401, and 399 nm, that shifts to the longer wavelength direction,
with 58 nm for 1o, 55 nm for 2o, and 51 nm for 3o, respectively,
compared to those in hexane solution. For diarylethenes 1–3, the
emission intensity and emission peak are distinguishable
different both in hexane and in PMMA film, indicating that
the electron-donating the methoxyl group attached at different
position on the terminal benzene ring had a significant effect
For practical applications in optical devices, it is very important
that photochromic materials can keep good photochromism in a
polymer film, such as the PMMA film.[6,23] Dissolved ultrasonically
10 mg diarylethene sample and 100 mg PMMA into 1.0 mL
chloroform, the films were prepared by spin-coating method. In
PMMA film, diarylethenes 1–3 also showed good photochromism
(Fig. 2) as similar to those in hexane. The photochromic features
of diarylethenes 1–3 are also summarized in Table 1. The
distortion of spectra in UV region is ascribed to absorbing UV light
of the glass substrate.[38] The coloration–decoloration cycles
could be also repeated more than 100 times, and the result
showed that no clear photodegradation for these compounds
was detected. As described above, the maximum absorption
peaks of the closed-ring isomers of diarylethenes 1–3 in PMMA
film are much longer than those in hexane solution. The red shift
values of the absorption maxima of the closed-ring isomers are
Figure 2. Absorption spectral and color changes of diarylethenes 1–3 upon alternating irradiation with UV and vis light in PMMA film (10%, w/w) at
room temperature; (A) spectral changes for 1; (B) spectral changes for 2; (C) spectral changes for 3; (D) color changes for 1–3.
J. Phys. Org. Chem. 2009, 22 954–963
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S. PU, W. LIU AND W. MIAO
Figure 3. Fluorescence emission spectra of diarylethenes 1–3 both in
hexane solution (2.0 ꢀ 10ꢁ5 mol Lꢁ1) and in PMMA film (10%, w/w) at
room temperature; (A) emission spectra in hexane, excited at 300 nm;
(B) emission spectra in PMMA film, excited at 300 nm. This figure is
available in color online at www.interscience.wiley.com/journal/poc
not only on the emission peak but also on the emission
intensity.
As has been observed for most of the reported diary-
lethenes,[22,47–52] diarylethenes 1–3 exhibited a clear fluorescent
switches along with the photochromism from open-ring isomers
to closed-ring isomers by photoirradiation both in hexane and in
PMMA film. Upon irradiated with UV light, the photocyclization
reaction was carried out and the non-fluorescent closed-ring
isomers 1c–3c were produced. The back irradiation by appro-
priate wavelength visible light regenerated the open-ring
isomers 1o–3o and recovered the original emission spectra.
During the process of photoisomerization, the three isomeric
compounds exhibited changes in their fluorescence in hexane as
shown in Fig. 4. Upon irradiation with 297 nm UV light, the
emission intensities of diarylethenes 1o–3o in hexane were
clearly decreased by photocyclization. When arrived at the
photostationary state, the emission intensities of diarylethenes
1–3 were quenched to ca. 58, 48, and 67%, respectively. The back
irradiation with visible light (l > 450 nm) regenerated the
open-ring forms and recovered the original emission spectra.
The phenomena are useful for potential application as
Figure 4. Emission intensity changes of diarylethenes 1–3 in hexane
(C ¼ 2.0 ꢀ 10ꢁ5 mol Lꢁ1) upon irradiation with 297 nm UV light at room
temperature, excited at 300 nm; (A) 1; (B) 2; (C) 3. This figure is available in
color online at www.interscience.wiley.com/journal/poc
fluorescent switches.[53,54] The fluorescence switch property of
diarylethenes 1–3 in PMMA film was similar to that of in hexane
solution, and their emission intensity changes in PMMA film
during the process of photoisomerization are shown in Fig. 5.
Upon irradiation with 313 nm UV light, the emission intensities of
diarylethenes 1–3 decreased remarkably along with the
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J. Phys. Org. Chem. 2009, 22 954–963

NEW UNSYMMETRICAL ISOMERIC DIARYLETHENES
The incomplete cyclization reaction and the existence of parallel
conformation of diarylethenes 1o–3o may be the main cause for
the moderate change in fluorescence induced by photoirradia-
tion.[25–29] In addition, the average times of ‘‘on’’ and ‘‘off’’ state
shortened in proportion to the reciprocal power of radiated light
by changing the power of the UV and visible light, indicating that
the switching effect is indeed photochemical.[47]
Electrochemistry of diarylethenes
It is well known that the ring opening and closing transformation
of some diarylethenes can be initiated not only by UV or visible
light irradiation, but also by electrochemical or chemical
oxidation, such as electrochromism.[55,56] Therefore, besides
their excellent photochromic performance, the electrochemical
behaviors of diarylethenes were also attracted much atten-
tion.[57–60] In present study, the electrochemical examinations
were performed by cyclic voltammograms (CV) method under
the same experimental conditions using diarylethenes 1–3,
respectively.
The CV curves of diarylethenes 1–3 with the scanning rate of
50 mV sꢁ1 are shown in Fig. 6. From this figure, it can be easily
calculated that the oxidation onsets of the three diarylethenes
were initiated at 1.30 and 1.23 V for 1o and 1c, 1.19 and 1.04 V for
2o and 2c, and 1.07 and 0.90 V for 3o and 3c, respectively. The
result indicated that the oxidation potential onsets of the
open-ring isomers of diarylethenes 1–3 were much higher than
those of the corresponding closed-ring isomers. This phenom-
enon is consonant with the theory that longer conjugation length
generally leads to less positive potentials due to the addition of
each heterocyclic ring.[18] After cyclization reaction, the
Figure 5. Emission intensity changes of diarylethenes 1–3 in PMMA film
(10%, w/w) upon irradiation with 313 nm UV light at room temperature,
excited at 300 nm; (A) 1; (B) 2; (C) 3. This figure is available in color online
at www.interscience.wiley.com/journal/poc
photoisomerization from open-ring isomers to closed-ring
isomers when excited at 300 nm. When arrived at the
photostationary state, the emission intensities of diarylethenes
1–3 were quenched to ca. 50, 61, and 49%, respectively. The back
irradiation with visible light (l > 450 nm) regenerated the
open-ring forms and recovered the original emission spectra.
Figure 6. Cyclic voltammetry (second scan) of diarylethenes 1–3 in
acetonitrile at a scan rate of 50 mV sꢁ1; (A) 1; (B) 2; (C) 3. This figure is
available in color online at www.interscience.wiley.com/journal/poc
J. Phys. Org. Chem. 2009, 22 954–963
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S. PU, W. LIU AND W. MIAO
Table 2. Electrochemical properties of diarylethenes 1–3
Oxidation
Reduction
Compound
Band gap (Eg)
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
1o
1c
2o
2c
3o
3c
þ1.30
þ1.23
þ1.19
þ1.04
þ1.07
þ0.90
ꢁ6.10
ꢁ6.03
ꢁ5.99
ꢁ5.84
ꢁ5.87
ꢁ5.70
ꢁ0.84
ꢁ0.95
ꢁ0.91
ꢁ0.96
ꢁ0.91
ꢁ0.99
ꢁ3.96
ꢁ3.85
ꢁ3.89
ꢁ3.84
ꢁ3.89
ꢁ3.81
2.14
2.18
2.10
2.00
1.98
1.89
The acetonitrile used as solvent was spectrograde and was purified by distillation before use.
Scheme 2. Synthetic route for diarylethenes 1o–3o
p-conjugation lengths of the closed-ring isomers of diarylethenes
1-3 were much longer than those of open-ring isomers, inducing
to the lower oxidation potential onset. Furthermore, the
oxidation onsets of the open- and closed-ring isomers of
diarylethenes 1–3 decreased along with the methoxyl group
substituting hydrogen atom of the benzene ring from at ortho- to
at para-position, which may be attributed to the different
electron-donating ability of the methoxyl group at different
substituted-position. From Fig. 6, we can easily see that there are
great differences of the electronic current and polarization curve
shapes between the open- and closed-ring isomers of diary-
lethenes 1–3. The values of oxidation waves are 1.40 and 1.35 V
for 1o and 1c, 1.36 and 1.47 V for 2o and 2c, and 1.46 and 1.23 V
for 3o and 3c, respectively. The result suggested that the
methoxyl group and its substituted position have a significant
effect on the electrochemical properties of the three isomeric
diarylethene derivatives.
where the units of potentials are volt, and those of IP and EA are
electronvolts; 4.8 eV is the constant of the energy level of the
ferrocene/ferrocenium (Fe) below the vacuum level.
With regard to the energy level of the ferrocene reference
(4.8 eV below the vacuum level), the HOMO and LUMO energy
levels can be estimated. As shown in Fig. 6, the onset potentials
(Eonset) of oxidation and reduction of 1o were observed at þ1.30
and ꢁ0.84 V, respectively. So the values of IP and EA were
calculated to be ꢁ6.10 and ꢁ3.96 eV according to Eqn (1) and (2).
Based on the HOMO and LUMO energy levels, the band gap (Eg,
Eg ¼ LUMO–HOMO) of diarylethene 1o can be determined as
2.10 eV. Similarly, the Eg of 1c can be calculated as 2.18 eV.
Corresponding values for diarylethenes 2 and 3 are summarized
in Table 2. Among these diarylethene derivatives, the Eg was the
smallest in 3c, implying that the charge transfer must be faster in
3c compared to that in others.[59] The result showed that band
gap Eg of the open- and closed-ring isomers of diarylethenes 1–3
gradually decreased along with the methoxyl group substituting
hydrogen atom of the benzene ring from at ortho- to at
para-position. All these data in Table 2 suggested that the
methoxyl group and its substituted position have a great effect
on the electrochemical characteristics of these isomeric
diarylethenes. It should be noted here that calculation absolute
HOMO and LUMO levels from electrochemical data in combi-
nation with the energy gap is still in debate.[62]
According to Eqn (1) and (2),[24,61,62] the energy parameters EA
and IP were calculated.
ox
HOMO : IP ¼ ꢁf½Eon
ꢂ
þ 4:8g
þ 4:8g
(1)
(2)
red
LUMO : EA ¼ ꢁf½Eon
ꢂ
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J. Phys. Org. Chem. 2009, 22 954–963

NEW UNSYMMETRICAL ISOMERIC DIARYLETHENES
Pd(PPh3)4 (0.8g) and Na2CO3 (6.40 g, 60 mmol) in tetrahydrofuran
(THF) (80 mL containing 10% water) for 15 h at 70 8C. The product
5a was purified by column chromatography on SiO2 using
hexane as the eluent and 4.48 g obtained as buff solid in 70.2%
yield. 1H NMR (400 MHz, CDCl3): d 2.41 (s, 3H, —CH3), 3.91 (s, 3H,
—CH3), 6.95–6.99 (m, 2H, phenyl-H), 7.27 (s, 1H, thienyl-H), 7.30 (s,
1H, phenyl-H), 7.54 (d, 1H, J ¼ 8.0 Hz, phenyl-H).
Conclusions
Three new isomeric diarylethenes having a methoxyl group were
prepared to study substituent position effect on optoelectronic
properties. All of these compounds showed good photochro-
mism and fluorescent switches both in solution and in PMMA
film. The results suggested that the optoelectronic performances
of the three isomeric diarylethenes, including absorption spectra,
photochromic reactivity, fluorescent property, and electroche-
mical feature, were clearly dependent on the effect of substituent
position. Compared to diarylethenes bearing chlorine atoms
reported previously, diarylethenes with an electron-donating
methoxyl group showed some new characteristics and changing
trends, which would be helpful for tuning the optoelectronic
properties of photochromic diarylethenes for further appli-
cations.
3-Bromo-2-methyl-5-(3-methoxylphenyl)thiophene (5b)
Compound 5b was prepared by a method similar to that used for
1
5a and obtained as buff solid in 75.3% yield. H NMR (400 MHz,
CDCl3): d 2.41 (s, 3H, —CH3), 3.83 (s, 3H, —CH3), 6.83 (d, 1H,
J ¼ 8.0 Hz, phenyl-H), 7.03 (s, 1H, thienyl-H), 7.09 (t, 2H, J ¼ 4.0 Hz,
phenyl-H), 7.27 (t, 1H, J ¼ 8.0 Hz, phenyl-H).
3-Bromo-2-methyl-5-(4-methoxylphenyl)thiophene (5c)
Compound 5c was prepared by a method similar to that used for
1
5a and obtained as buff solid in 78.3% yield. H NMR (400 MHz,
EXPERIMENTAL
CDCl3): d 2.40 (s, 3H, —CH3), 3.83 (s, 3H, —CH3), 6.90 (d, 2H,
J ¼ 8.8 Hz, phenyl-H), 6.99 (s, 1H, thienyl-H), 7.43 (d, 2H, J ¼ 8.8 Hz,
phenyl-H).
General
The solvents were purified by distillation before use. Mass spectra
were measured with an Agilent MS Trap VL spectrometer. The
elemental analysis was measured with a PE CHN 2400 analyzer.
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 a Bruker
Vertex-70 spectrometer. The absorption spectra were measured
using an Agilent 8453 UV/VIS spectrometer. Photoirradiation was
carried out using SHG-200 UV lamp, CX-21 ultraviolet fluor-
escence analysis cabinet, and BMH-250 visible lamp. Light of
appropriate wavelengths was isolated by different light filters.
The quantum yields were determined by comparing the reaction
yields of these dithienylethene compounds in hexane against
(2,5-Dimethyl-3-thienyl)perfluorocyclopentene (7)
To a stirred solution of 3-iodo-2,5-dimethylthiophene (6) (18.2 g,
76.5mmol) in THF was added dropwise a 1.6 mol Lꢁ1 n-BuLi/
hexane solution (50 mL, 80 mmol) at ꢁ78 8C under argon
atmosphere. Stirring was continued for 30 min at this low
temperature. Perfluorocyclopentene (10.3 mL, 76.5 mmol) was
added to the reaction mixture at ꢁ78 8C, and the mixture was
stirred for another 2 h at the temperature, then the reaction
was warned to the room temperature spontaneously. The
reaction was stopped by the addition of methanol (5 mL). The
product was extracted with ether. The organic layer was washed
with 1 M HCl aqueous solution and water, respectively. 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 15.3 g of 7 obtained as
yellow oil in 65.8% yield. 19F NMR (376 MHz, CDCl3): d 108.53 (2F),
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene
in
hexane (with their respective absorption maxima visible light
irradiation).[63] Fluorescence spectra were measured using a
Hitachi F-4500 spectrophotometer. Electrochemical examinations
were performed in a one-compartment cell by using a Model 263
potentiostat–galvanostat (EG&G Princeton Applied Research)
under computer control at room temperature. Platinum-
electrodes (diameter 0.5 mm) served as working electrode and
counter electrode. Platinum wire served as a quasireference
electrode. It was calibrated using the ferrocene (Fc/Fcþ) redox
couple which has a formal potential E1/2 ¼ þ0.35 V versus
platinum wire. The typical electrolyte was acetonitrile (5 mL)
containing 0.1 mol Lꢁ1 tetrabutylammonium tetrafluoroborate
((TBA)BF4) and 4.0 ꢀ 10ꢁ3 mol Lꢁ1 dithienylethene. All solutions
were deaerated by a dry argon stream and maintained at a slight
argon overpressure during electrochemical experiments.
1
117.84 (2F), 128.22 (1F), 129.90 (2F). H NMR (400 MHz, CDCl3):
d 2.281 (s, 3H, —CH3), 2.23(s, 3H, —CH3), 6.54 (s, 1H, thienyl-H).
1-(2,5-Dimethyl-3-thienyl)-2-[2-methyl-5-(2-methoxylphenyl)-3-thi-
enyl]perfluorocyclopentene (1o)
To a stirred anhydrous THF containing 5a (2.0 g, 7.06 mmol) was
added dropwise a 2.5 mol Lꢁ1 n-BuLi solution (2.8 mL) at ꢁ78 8C
under argon atmosphere. After the mixture has been stirred for
30 min at ꢁ78 8C, compound 7 (2.15 g, 7.06 mmol) in solvent of
anhydrous THF was added. The reaction was further stirred at
ꢁ78 8C for 1 h, and the reaction was allowed to slowly warn to the
room temperature and stirred there for 1 h. The reaction was
quenched with distilled water. The product was extracted with
ether, dried with MgSO4, and concentrated under reduced
pressure. The crude product was purified by column chroma-
tography using petroleum ether as the eluent to yield 1.52 g
(44%) of 1a as white solid. MS m/z (Mþ) 487.09; Calcd for
C23H18F6OS2 (%): Calcd C, 56.55; H, 3.71. Found C, 57.07; H, 3.81; 1H
NMR (400 MHz, CDCl3): d 1.78 (s, 3H, —CH3), 1.86 (s, 3H, —CH3),
2.33 (s, 3H, —CH3), 3.83 (s, 3H, —CH3), 6.66 (s, 1H, thienyl-H), 6.90
(t, 2H, J ¼ 7.6 Hz, phenyl-H), 7.17 (s, 1H, phenyl-H), 7.35 (s, 1H,
Synthesis
The synthesis method of three diarylethenes 1o–3o is shown
in Scheme 2 and experimental details were carried out as
following.
3-Bromo-2-methyl-5-(2-methoxylphenyl)thiophene (5a)
Compound 5a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid (4)[35] (5.0 g; 22.6 mmol) with 1-bromo-
2-methoxylbenzene (4.23 g, 22.6 mmol) in the presence of
J. Phys. Org. Chem. 2009, 22 954–963
Copyright ß 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

S. PU, W. LIU AND W. MIAO
thienyl-H), 6.89 (d, 1H, J ¼ 7.6 Hz, phenyl-H); 13C NMR (400 MHz,
CDCl3): d 14.53, 15.00, 55.59, 111.8, 121.0, 121.8, 124.8, 127.9,
128.7, 130.1, 131.5, 136.6, 137.5, 139.7, 141.4, 155.6; IR (n, KBr,
cmꢁ1): 739, 764, 825, 855, 893, 983, 1049, 1115, 1191, 1273, 1336,
1442, 1487, 1630, 2919.
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1-(2,5-Dimethyl-3-thienyl)-2-[2-methyl-5-(3-methoxylphenyl)-3-thi-
enyl]perfluorocyclopentene (2o)
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Compound 2o was prepared by a method similar to that used for
1o and obtained as solid in 48% yield. MS m/z (Mþ) 487.05; Calcd
for C23H18F6OS2 (%): Calcd C, 56.55; H, 3.71. Found C, 56.99; H,
1
3.74; H NMR (400 MHz, CDCl3): d 1.80 (s, 3H, —CH3), 1.86 (s, 3H,
—CH3), 2.36 (s, 3H, —CH3), 3.77 (s, 3H, —CH3), 6.65 (s, 1H,
thienyl-H), 6.77 (d, 2H, J ¼ 7.8 Hz, phenyl-H), 6.97 (s, 1H, thienyl-H),
7.05 (d, 1H, J ¼ 7.6 Hz, phenyl-H), 7.19 (t, 1H, J ¼ 7.6 Hz, phenyl-H);
13C NMR (400 MHz, CDCl3): d 14.20, 14.35, 15.02, 55.26, 111.3,
113.3, 118.2, 122.8, 124.7, 126.0, 129.8, 134.7, 137.7, 139.7, 141.2,
141.9, 160.1; IR (n, KBr, cmꢁ1): 745, 776, 823, 836, 892, 977, 1051,
1108, 1189, 1273, 1336, 1445, 1492, 1571, 1594, 2924.
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197, 415–425.
1-(2,5-Dimethyl-3-thienyl)-2-[2-methyl-5-(4-methoxylphenyl)-3-thi-
enyl]perfluorocyclopentene (3o)
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Compound 3o was prepared by a method similar to that used for
1o and obtained as solid in 51% yield. MS m/z (Mþ) 487.03; Calcd
for C23H18F6OS2 (%): Calcd C, 56.55; H, 3.71. Found C, 57.17; H,
1
3.85; H NMR (400 MHz, CDCl3): d 1.80 (s, 3H, —CH3), 1.83 (s, 3H,
—CH3), 2.34 (s, 3H, —CH3), 3.75(s, 3H, —CH3), 6.65 (s, 1H,
thienyl-H), 6.83 (d, 2H, J ¼ 7.6 Hz, phenyl-H), 7.05 (s, 1H, thienyl-H),
7.37 (d, 2H, J ¼ 8.0 Hz, phenyl-H); 13C NMR (400 MHz, CDCl3): d
14.21, 14.27, 15.02, 55.37, 114.3, 121.5, 121.8, 124.6, 124.7, 125.9,
126.3, 127.9, 137.6, 139.7, 140.1, 142.0, 159.5; IR (n, KBr, cmꢁ1): 727,
819, 888, 984, 1049, 1103, 1187, 1265, 1336, 1440, 1489, 1550,
1622, 2922.
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11–15.
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Acknowledgements
This work was supported by the Project of National Natural
Science Foundation of China (20564001), the Key Scientific Pro-
ject from Education Ministry of China (208069), the Project of
Jiangxi Youth Scientist, and the Science Funds of the Education
Office of Jiangxi, China (GJJ08365).
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Copyright ß 2009 John Wiley & Sons, Ltd.
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