Synthesis and optoelectronic properties of unsymmetrical isomeric diarylethene derivatives having a fluorine atom

Article abstract of DOI:10.1016/j.molstruc.2009.07.012

Three unsymmetrical diarylethene compounds bearing a fluorine atom at either ortho-, meta-, or para-positions of the terminal phenyl ring, namely 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(2-fluorophenyl)-3-thienyl]perf luorocyclopentene (1o), 1-(2,5-dimet

Full text of DOI:10.1016/j.molstruc.2009.07.012

Journal of Molecular Structure 936 (2009) 29–36
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and optoelectronic properties of unsymmetrical isomeric
diarylethene derivatives having a fluorine atom
*
Weijun Liu, Shouzhi Pu , Gang Liu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2009
Received in revised form 16 July 2009
Accepted 16 July 2009
Available online 22 July 2009
Three unsymmetrical diarylethene compounds bearing a fluorine atom at either ortho-, meta-, or para-
positions of the terminal phenyl ring, namely 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(2-fluoro-
phenyl)-3-thienyl]perfluorocyclopentene (1o), 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(3-fluoro-
phenyl)-3-thienyl]perfluorocyclopentene (2o), and 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(4-
fluorophenyl)-3-thienyl]perfluorocyclopentene (3o), have been synthesized. Their structures were deter-
mined by single-crystal X-ray diffraction analysis. The fluorine atom position effect on their properties,
such as photochromism, fluorescence, and their electrochemical properties, were investigated in detail.
The results elucidated that the fluorine atom and its substituted position had a significant effect on
the optoelectronic properties of these compounds. For diarylethenes 1–3, the ortho-substituted derivative
1 had the biggest cyclization and cycloreversion quantum yields; while the meta-substituted derivative 2
showed the biggest extinction coefficient and the absorption maxima both in hexane and in PMMA film.
The emission peaks of diarylethenes 1o–3o displayed an increasing trend along with the fluorine atom
from the ortho- to meta- to the para-position of substitution both in hexane and in PMMA film, but their
intensities showed a reverse changing trend. The oxidation waves of diarylethenes 1c–3c were clearly
observed at 1.44, 1.27 and 1.42 V by performing the cyclic voltammograms, and those of 1o–3o were
observed at 1.29, 1.12 and 1.41, respectively.
Keywords:
Photochromism
Diarylethene
Crystal
Fluorine atom position effect
Optical and electrochemical property
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
20]. The majority of the research work reported to date has been de-
voted to the development of these molecules and investigative stud-
Photochromism is a reversible transformation in a chemical
species between two forms by means of photoexcitation [1,2].
The quick property change arising from the cyclization–cyclorever-
sion reaction via photoexcitation has been attracting much atten-
tion not only from the viewpoint of the fundamental chemical
reaction processes but also from the viewpoint of the application
to optoelectronic apparatuses such as rewritable optical recording
and photo-switches [3]. Various types of photochromic com-
pounds, such as spirobenzopyrans [4], furylfulgides [5], azobenz-
enes [6,7] and diarylethenes [8,9], etc., have been developed.
Among these compounds, diarylethenes with heterocyclic aryl
rings are regarded as the most promising candidates for optical
memory and switch because of their excellent thermally irrevers-
ibility, high fatigue resistance, and rapid response [8–15].
ies of their fundamental properties, and the results obtained have
contributed to a broad understanding of the substituent effect on
the photochromic property of diarylethene compounds. Although
many publications concerning the substituent effect on the property
of diarylethene are reported, reports on the substituent position ef-
fect are extremely rare. To the best of our knowledge, only a few
publications concerning the substituent position effect on the opto-
electronic properties of diarylethenes are hitherto reported by our
research group [21–25]. Previously, we mainly focused on the effect
of electron-withdrawing groups such as cyano group and chlorine
atom, and we found that these substituents and their substituted
positions had a significant effect on the properties of these diary-
lethene compounds [21–24]. The results are very important and
interesting, and they also give us valuable insight.
In general, different substituents linked to photochromic diary-
lethene system can inevitably influence the optoelectronic proper-
ties of corresponding diarylethene derivatives. By far, many
publications concerning substituent effects on the photochromic
properties of diarylethene compounds have been reported [16–
In previous publications, we reported the electron-donating
methoxyl group position effect on the properties of diarylethenes
with a pyrazole unit and found that introduction of methoxyl
group at different positions of the terminal phenyl ring could sig-
nificantly influence the optical and electrochemical properties
[26,27]. We also reported the substituent position effect on the
properties of symmetrical diarylethenes with fluorine atoms of
both terminal phenyl rings [25]. As far as we are aware, this is a un-
* Corresponding author. Tel./fax: +86 791 3831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.07.012

30
W. Liu et al. / Journal of Molecular Structure 936 (2009) 29–36
ique example of the fluorine atom position effect on the properties
of diarylethenes hitherto reported. However, the fluorine atom po-
sition effects on the properties of unsymmetrical dithienylethene
derivatives have not been reported. In this work, in order to eluci-
date the fluorine atom position effect on the optoelectronic charac-
teristics of unsymmetrical diarylethenes, we have synthesized
three unsymmetrical isomeric diarylethenes, namely 1-(2,5-di-
methyl-3-thienyl)-2-[2-methyl-5-(2-fluorophenyl)-3-thienyl]per-
fluorocyclopentene (1o), 1-(2,5-dimethyl-3-thienyl)-2-[2-methyl-
5-(3-fluorophenyl)-3-thienyl]perfluorocyclopentene (2o), and 1-
(2,5-dimethyl-3-thienyl)-2-[2-methyl-5-(4-fluorophenyl)-3-thie-
nyl]perfluorocyclopentene (3o), which bearing a fluorine at either
the ortho-, meta-, or para-positions on the terminal phenyl ring.
The three compounds showed good photochromism in solution,
in PMMA film, as well as in the single crystalline phase. The photo-
chromic scheme of diarylethenes 1–3 is shown in Scheme 1.
Among these diarylethene derivatives, 1o and 2o are new com-
pounds. Although the crystal structure of diarylethene 3o was re-
ported by us [28], its property is not reported. We present its
property data here for comparison with those of other two diary-
lethene compounds.
were deaerated by a dry argon stream and maintained at a slight
argon overpressure during electrochemical experiments.
2.2. Synthesis of diarylethenes 1o–3o
The synthesis method of diarylethenes 1o–3o is shown in
Scheme 2 and experimental details are carried out as following.
2.2.1. 3-Bromo-2-methyl-5-(2-fluorophenyl)thiophene (5a)
Compound 5a was prepared by reacting 3-bromo-2-methyl-5-
thienylboronic acid (4) [27] (3.0 g; 13.6 mmol) with 1-bromo-2-
fluorobenzene (2.38 g; 13.6 mmol) in the presence of Pd(PPh3)4
(0.4 g) and Na2CO3 (6.40 g; 60 mmol) in tetrahydrofuran (THF)
(80 mL containing 10% water) for 15 h at 70 °C. The product 5a
was purified by column chromatography on SiO2 using hexane as
the eluent and 4.48 g obtained as baby yellow solid in 57.0% yield.
1H NMR (400 MHz, CDCl3) d 2.43 (s, 3H, –CH3), 7.14 (d, 2H, J =
8.8 Hz, phenyl-H), 7.25 (s, 1H, thienyl-H), 7.27 (s, 1H, phenyl-H),
7.53 (d, 1H, J = 8.0 Hz, phenyl-H).
2.2.2. 3-Bromo-2-methyl-5-(3-fluorophenyl)thiophene (5b)
Compound 5b was prepared by a method similar to that used
for 5a and obtained as buff solid in 62.4% yield. 1H NMR
(400 MHz, CDCl3) d 2.42 (s, 3H, –CH3), 6.98 (t, 1H, J = 8.0 Hz, phe-
nyl-H), 7.12(s,1H), 7.20 (d, H, J = 5.6 Hz, phenyl-H), 7.29 (s, 1H,
phenyl-H), 7.32 (t,1H, J = 6.8 Hz, phenyl-H).
2. Experimental
2.1. General
All solvents used were spectrograde and were purified by
distillation before use. NMR spectra were recorded on Bruker
AV400 (400 MHz) spectrometer with CDCl3 as the solvent and tet-
ramethylsilane as an internal standard. IR spectra were recorded
on Bruker Vertex-70 spectrometer. The elemental analysis was
measured with PE CHN 2400. Melting point was determined by
WRS-1B melting point determination apparatus. The absorption
spectra were measured using a Agilent 8453 UV/VIS spectrometer.
Photo-irradiation was carried out using SHG-200 UV lamp, CX-21
ultraviolet fluorescence analysis cabinet, and BMH-250 visi-
ble lamp. Light of appropriate wavelengths was isolated by differ-
ent light filters. The quantum yields were determined by
comparing the reaction yields of the diarylethenes in hexane
against 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopen-
tene in hexane [14]. Fluorescence spectra were measured using a
Hitachi F-4500 spectrophotometer. Crystal data of diarylethenes
1o–3o were collected by Bruker SMART APEX2 CCD area-detector.
Electrochemical examinations were performed in a one-compart-
ment cell by using a Model 263 potentiostat–galvanostat (EG&G
Princeton Applied Research) under computer control at room tem-
perature. Platinum-electrodes (diameter 0.5 mm) served as work-
ing electrode and counter electrode. Platinum wire served as a
quasireference electrode. It was calibrated using the ferrocene
(Fc/Fc+) redox couple which has a formal potential E1/2 = + 0.35 V
versus platinum wire. The typical electrolyte was acetonitrile
(5 mL) containing 0.1 mol/L tetrabutylammonium tetrafuoroborate
((TBA)BF4) and 4.0 Â 10À3 mol/L dithienylethene. All solutions
2.2.3. 3-Bromo-2-methyl-5-(4-fluorophenyl)thiophene (5c)
Compound 5c was prepared by a method similar to that used
for 5a and obtained as buff solid in 57.0% yield. 1H NMR
(400 MHz, CDCl3) d 2.41 (s, 3H, –CH3), 7.03 (s, 1H, thienyl-H),
7.04–7.08 (t, 2H, J = 8.0 Hz, phenyl-H), 7.45–7.48 (t, 2H, J =
6.2 Hz, phenyl-H).
2.2.4. (2,5-Dimethyl-3-thienyl)perfluorocyclopentene (7)
The compound was prepared by the same method as that re-
ported in Ref. [21].
2.2.5. 1-(2,5-Dimethyl-3-thienyl)-2-[2-methyl-5-(2-fluorophenyl)-3-
thienyl]perfluorocyclopentene (1o)
To a stirred anhydrous THF containing 5a (2.1 g, 7.7 mmol) was
added dropwise a 2.5 mol/L n-BuLi solution (3.1 mL) at À78 °C un-
der argon atmosphere. After the mixture has been stirred for 30 min
at À78 °C, compound 7 (2.36 g, 7.7 mmol) in solvent of anhydrous
THF was added. The reaction was further stirred at À78 °C for 1 h,
and the reaction was allowed to slowly warn to the room tempera-
ture and stirred there for 1 h. The reaction was quenched with dis-
tilled water. The product was extracted with ether, dried with
MgSO4, and concentrated under reduced pressure. The crude prod-
uct was purified by column chromatography petroleum ether to af-
ford to 1.57 g (43%) of 1a as white solid. M.p. 83.3–84.0 °C; 1H NMR
(400 MHz, CDCl3): d 1.90 (s, 3H, –CH3), 1.98 (s, 3H, –CH3), 2.46 (s,
3H, –CH3), 6.75 (s, 1H, thienyl-H), 7.15 (d, 1H, J = 8.0, phenyl-H),
7.19 (t, 1H, J = 8.0, phenyl-H), 7.29 (d, 1H, J = 5.6, phenyl-H), 7.43
(s, 1H, thienyl-H), 7.58 (t, 1H, J = 7.6, 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 (KBr, cmÀ1): 753, 826, 840, 897, 938, 987, 1050, 1113,
1190, 1272, 1336, 1439, 1498, 1519, 1628, 2925; Calcd for
C22H15F7S2 (%): Calcd C, 55.46; H, 3.17. Found C, 55.49; H, 3.16.
F
F
F
F
F
F
F
F
UV
Vis
F
F
F
F
S
S
S
S
R
R
2.2.6. 1-(2,5-Dimethyl-3-thienyl)-2-[2-methyl-5-(3-fluorophenyl)-3-
thienyl]perfluorocyclopentene (2o)
Compound 2o was prepared by a method similar to that used
for 1o and obtained as solid in 51% yield. M.p. 88.1–89.0 °C; 1H
NMR (400 MHz, CDCl3): d 1.90 (s, 3H, –CH3), 1.97 (s, 3H, –CH3),
1o: R=ortho-F
2o: R=meta-F
3o: R=para-F
1c: R=ortho-F
2c: R=meta-F
3c: R=para-F
Scheme 1. Photochromism of diarylethenes 1–3.

W. Liu et al. / Journal of Molecular Structure 936 (2009) 29–36
31
Br
R
Br
Br
S
B(OH)2
S
4
R
F
F
Pd(Pph3)4
Na2CO3
F
F
5a: R=ortho-
F
F
F
5b: R=meta-
F
n-BuLi,-78 o
C
5c: R=para-
F
S
S
F
F
I
F
F
1 ) n-BuLi,-78 o
2 ) C5F8
C
F
F
R
1o: R=ortho-
2o: R=meta-
3o: R=para-
F
S
6
F
F
F
S
7
Scheme 2. Synthetic rout for diarylethenes 1o–3o.
2.45 (s, 3H, –CH3), 6.75 (s, 1H, thienyl-H), 7.01 (t, 2H, J = 8.0 Hz,
phenyl-H), 7.24 (d, 1H, J = 8.0, phenyl-H), 7.28 (s, 1H, thienyl-H),
7.31–7.37(m, 2H, phenyl-H); 13C NMR (400 MHz, CDCl3): d 14.25,
14.41, 15.07, 112.4, 114.5, 121.2, 123.4, 124.5, 126.1, 130.5,
135.5, 137.9, 139.8, 140.5, 141.8, 161.9, 164.4; IR (KBr, cmÀ1):
734, 774, 821, 855, 866, 901, 983, 1053, 1117, 1189, 1272, 1336,
1442, 1494, 1586, 1612, 2926; Calcd for C22H15F7S2 (%): Calcd C,
55.46; H, 3.17. Found C, 55.51; H, 3.20.
their color changes induced by photoirradiation at room tempera-
ture are shown in Fig. 1. The absorption maximum of diarylethene
1o was observed at 272 nm in hexane, which was arisen from
p?
p* transition [29]. Upon irradiation with 297 nm UV light, the
colorless open-ring isomer 1o induced a ring-closure reaction
and evolved into the purple color of the closed-ring isomer 1c, in
which the absorption maximum was observed at 539 nm. Alterna-
tively, the purple solution returned colorless upon irradiation with
visible light (k > 450 nm) because 1c reverted to the initial state 1o.
Just as diarylethene 1, diarylethenes 2 and 3 also showed photo-
chromism in hexane (Fig. 1). Upon irradiation with 297 nm light,
absorption bands in visible region appeared and both solutions
containing 2o and 3o, respectively, turned purple as a result of
the cyclization reactions to produce the closed-ring isomers 2c
and 3c, which their absorption maxima were observed at 542
and 538 nm, respectively. The two solutions can be decolorized
upon irradiation with visible light attributable to reproducing their
respect open-ring isomer. The color changes of diarylethenes
1o–3o upon photoirradiation in hexane are shown in Fig. 1D. The
photochromic parameters of diarylethenes 1–3 are summarized
in Table 1. From these data, it can be seen that the fluorine atom
position effect on the molar absorption coefficients and the cycliza-
tion/cycloreversion quantum yields are remarkable, but the effect
on the absorption maxima of both open-ring and closed-ring iso-
mers are not significant. In hexane, the molar absorption coeffi-
cients of both the open-ring and the closed-ring isomers of the
meta-substituted derivative (compound 2) are the biggest; while
those of diarylethenes 1 and 3 are relatively smaller and they are
not significantly different each other. For diarylethenes 1–3, the
cyclization and cycloreversion quantum yields of ortho-substituted
derivative (compound 1) are the biggest; while those of diaryleth-
enes 2 and 3 are relatively smaller and they are almost equal each
other. Compared to those symmetric analogue diarylethenes bear-
ing two fluorine atoms [25], the results are remarkably different.
For those analogues, the cyclization and cycloreversion quantum
yields of para-substituted diarylethene are the biggest; while those
of meta-substituted diarylethene are the smallest [25].
2.2.7. 1-(2,5-Dimethyl-3-thienyl)-2-[2-methyl-5-(4-fluorophenyl)-3-
thienyl]perfluorocyclopentene (3o)
Compound 3o was prepared by a method similar to that used for
1o and obtained as solid in 52% yield. M.p. 92.3–93.1 °C; 1H NMR
(400 MHz, CDCl3): d 1.80 (s, 3H, –CH3), 1.85 (s, 3H, –CH3), 2.35 (s,
3H, –CH3), 6.65 (s, 1H, thienyl-H), 7.00 (d, 2H, J = 8.0, phenyl-H),
7.10(s, 1H, thienyl-H), 7.40–7.43 (m, 2H, phenyl-H); 13C NMR
(400 MHz, CDCl3): d 14.25, 14.35, 15.07, 115.8, 116.1, 122.5, 124.5,
126.1, 127.3, 129.7, 137.8, 139.7, 140.9, 141.1, 161.3, 163.7; IR
(KBr, cmÀ1): 743, 830, 843, 893, 984, 1049, 1049, 1118, 1139,
1186, 1232, 1265, 1335, 1439, 1511, 1555, 1626, 1739, 2919; Calcd
for C22H15F7S2 (%): Calcd C, 55.46; H, 3.17. Found C, 55.41; H, 3.15.
2.3. Determination of the crystal structures of diarylethenes
Crystal data of diarylethenes 1o–3o were collected by a Bruker
SMART APEX2 CCD area-detector equipped with graphite mono-
chromatized Mo
( = 0.71073 Å). The linear absorption coefficients,
K
a
radiation at room temperature
, of the three
l
diarylethenes for Mo K radiation were 3.21, 3.17 and 3.14 cmÀ1. Di-
rect phase determination yielded the positions of all non-hydrogen
atoms. All non-hydrogen atoms were subjected to anisotropic
refinement. All hydrogen atoms were generated geometrically
with C–H bond distances of 0.93–0.96 Å according to criteria de-
scribed in the SHELXTL manual. They were included in the refine-
ment with Uiso (H) = 1.2 Ueq (C) or 1.5 Ueq (methyl C). Further
details on the crystal structure investigation have been deposited
with The Cambridge Crystallographic Data Centre as supplemen-
tary publication number CCDC 615591, 615592 and 6615593 for
diarylethenes 1o–3o, respectively. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax:+44 1223–336033.
In PMMA amorphous film (10%, w/w), diarylethenes 1–3 also
showed good photochromic properties. Their absorption spectra
and color changes are shown in Fig. 2. Upon irradiation with
313 nm light, the colors of the three diarylethene/PMMA films
changed from colorless to violet with the appearance of a new
broad absorption band at 554, 558 and 553 nm, respectively, which
was assigned to the formation of the closed-ring isomers 1c–3c. All
colored diarylethene/PMMA films can invert to colorless upon irra-
diation with visible light of wavelength longer than 450 nm. The
distortion of spectra in UV region is ascribed to absorbing UV light
of the glass substrate in the PMMA film with glass substrate [30].
The photochromic properties of diarylethenes 1–3 are also dis-
played in Table 1. Compared to those in hexane, the maximum
3. Results and discussion
3.1. Photochromism of diarylethenes 1–3
The photochromic reactivity of diarylethenes 1–3 was exam-
ined in hexane (2.0 Â 10À5 mol/L). Their absorption spectra and

32
W. Liu et al. / Journal of Molecular Structure 936 (2009) 29–36
0.4
0.3
0.2
0.1
0.0
0.4
(A)
(C)
(B)
UV
Vis
0.3
UV
Vis
0.2
UV
Vis
UV
Vis
0.1
0.0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
2o 3o
(D) 1o
0.4
0.3
0.2
0.1
0.0
UV
Vis
UV
Vis
Vis
UV
300
400
500
600
700
800
Wavelength (nm)
1c 2c 3c
Fig. 1. Absorption spectra and color changes of diarylethenes 1–3 by photoirradiation in hexane (2.0 Â 10À5 mol/L) at room temperature: (A) spectral changes for 1, (B)
spectral changes for 2, (C) spectral changes for 3, (D) color changes for 1–3. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
Table 1
within the range where crystalline-state photochromism is ex-
pected to occur [32–36]. Therefore, diarylethenes 1o–3o can un-
dergo photochromism in the single crystalline phase. In fact,
Absorption characteristics and photochromic reactivity of diarylethenes 1–3 in
hexane (2.0 Â 10À5 mol/L) and in PMMA film (10%, w/w).
Compound
ko,max (nm)a
kc,max (nm)b
Uc
these crystals showed photochromism in the crystalline state, in
accordance with the expected ring closure, to form their respect
closed-ring isomers. Their color changes upon photo-irradiation
are shown in Fig. 3D. Upon irradiation with 313 nm light, the col-
orless crystals 1o–3o turned to purple quickly. When the purple
crystals were dissolved in hexane, the solutions quickly turned to
purple, and the absorption maxima were the same as those of
the closed-ring isomers 1c–3c, respectively. Alternatively, these
colored crystals reverted to the colorless one upon irradiation with
appropriated wavelength visible light (k > 450 nm). Furthermore,
the diarylethene crystal exhibited remarkable fatigue resistance
(larger than 100 cyclization/cycloreversion repeat cycles) and its
closed-ring isomer remained stable for more than one year in the
dark at room temperature. So, this crystal will be a promising can-
didate for optoelectronic applications, such as high density three-
dimensional optical recording medium and optical switch material
[37,38].
(e
/L mol-1 cmÀ1
)
(
e
/L molÀ1 cmÀ1
)
Hexane
Hexane
PMMA film Uo–c Uc–o
1
2
3
272 (1.27 Â 104)
274 (1.74 Â 104)
274 (1.43 Â 104)
539 (5.33 Â 103)
542 (8.01 Â 103)
538 (5.08 Â 103)
554
558
553
0.63
0.52
0.51
0.072
0.040
0.041
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring
b
c
(Uo–c
)
and closed-ring isomers
(
Uc–o),
respectively.
absorption peaks of the closed-ring isomers of diarylethenes 1–3 in
PMMA film are much longer than those in hexane. The red shift
values of the absorption maxima of the closed-ring isomers are
15 nm for 1c, 16 nm for 2c and 15 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 [31].
Moreover, X-ray crystallographic studies have been carried out
for diarylethenes 1o–3o. These single crystals were obtained by the
slow evaporation in hexane. The X-ray crystallographic analysis
data are listed in Table 2. The ORTEP drawings of diarylethenes
1o–3o and their color changes by photo-irradiation in the single
crystalline phase are shown in Fig. 3. All molecules of diarylethenes
1o–3o adopt the anti-parallel conformation, and the distances be-
tween the two reacting carbon atoms (d(C5C13)) are 3.568(4) Å for
1o, 3.791(5) Å for 2o and 3.572(5) Å for 3o, respectively, which is
3.2. Fluorescence of diarylethenes 1–3
Fluorescent properties can be useful in molecular-scale opto-
electronics [39], digital fluorescence photo-switches [40–42], as
well as ion-sensors [43–47]. In this work, the fluorescence proper-
ties of diarylethenes 1o–3o were measured both in solution and in
PMMA film using a Hitachi F-4500 spectrophotometer. Fig. 4
shows the fluorescence emission spectra of diarylethenes 1o–3o
both in hexane solution (2.0 Â 10À5 mol/L) and in PMMA film

W. Liu et al. / Journal of Molecular Structure 936 (2009) 29–36
33
(A) 1.2
(B)
1.6
0.9
1.2
0.6
0.3
0.0
UV
Vis
0.8
0.4
0.0
UV
Vis
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
(D)
(C) 1.8
1.5
1.2
0.9
UV
Vis
0.6
0.3
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Absorption spectra and color changes of diarylethenes 1–3 by photoirradiation 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. (For interpretation of the references to color in this figure legend, the reader is referred to the web version
of this article.)
PMMA film, and the red shift values are 50 nm for 1o, 50 nm for
2o, and 52 nm for 3o, respectively. For diarylethenes 1o–3o, the
changing trend of emission intensity is reverse to that of emission
peak, which was accompanied by appreciably decreasing of emis-
sion intensity when the fluorine atom was joined into the benzene
ring from the ortho- to meta- to the para-positions both in hexane
and in PMMA film.
Table 2
Crystal data for diarylethenes 1–3.
1o
2o
3o
Formula
C22H25F7S2
476.46
294(2)
Monoclinic
P2(1)/c
C22H25F7S2
476.46
294(2)
Monoclinic
C2/c
C22H25F7S2
476.46
294(2)
Triclinic
P-1
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
Upon exposure to 313 nm UV light, diarylethenes 1o–3o were
photocyclized to the closed-ring isomers 1c–3c, along with a sig-
nificant decrease in the fluorescence intensities in PMMA film, as
shown in Fig. 5. When reaching the photostationary state, the fluo-
rescence efficiency of diarylethenes 1–3 was distinctly quenched
to ca. 47, 40 and 41%, respectively. This is resulted from the -elec-
tron delocalization enlargement of the closed-ring isomers as a
fluorescence quencher in the possible channel of Föster resonance
energy transfer [48,49]. Upon irradiation, each of the generated
closed-ring isomers 1c–3c has a broad absorption which covers
the majority of the visible area (Fig. 2), thus resulting in an efficient
overlap between the absorption (400–650 nm) and emission band
(350–550 nm). Upon irradiation with visible light, the emissions
were completely converted into their respective open-ring isomers
according to the spectral change, showing the classical switchable
property of diarylethene. In PMMA solid medium, 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 photoirradiation. Unexpectedly,
diarylethenes 1–3 showed almost no fluorescence modulation in
hexane, which may be attributed to the solvent polar effect.
13.693(2)
18.547(3)
8.6042(14)
90.00
106.099(2)
90.00
2099.4(6)
4
1.507
23.916(7)
11.889(3)
15.826(4)
90.00
109.073(5)
90.00
4253(2)
8
1.488
8.602(2)
11.709(3)
11.984(3)
105.472(4)
94.977(4)
109.737(4)
1074.1(5)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Density (calcd.) (g/cm3)
1.473
1.074
Goodness-of-fit on F2
1.045
1.022
Final R indices [I/2r(I)]
R1
wR2
R indices (all data)
R1
wR2
0.0385
0.1006
0.0726
0.1744
0.0459
0.1300
0.0616
0.1167
0.1617
0.2379
0.0706
0.1469
(10%, w/w) at room temperature. The emission peaks of diaryleth-
enes 1o–3o were observed at 341, 343 and 348 nm when excited
at 300 nm in hexane; while those of the three compounds were ob-
served at 391, 393 and 400 nm when excited at 300 nm in PMMA
film. The result showed that the emission peaks of diarylethenes
1o–3o displayed a minor increasing trend along with the fluorine
atom substituting hydrogen atom of the benzene ring from the
ortho- to meta- to the para-positions both in hexane and in PMMA
film. Compared to those in hexane, the emission peaks of the three
isomeric diarylethene derivatives show a remarkable red shift in
3.3. Electrochemistry of diarylethenes 1–3
It was well known that the ring opening and closing transfor-
mation of some diarylethenes can be initiated not only by UV or

34
W. Liu et al. / Journal of Molecular Structure 936 (2009) 29–36
Fig. 3. ORTEP drawings of crystals 1o–3o and their color changes by photoirradiation in the single crystalline phase: (A) ORTEP drawing of 1o, (B) ORTEP drawing of 2o, (C)
ORTEP drawing of 3o, (D) color changes for 1–3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
behaviors of diarylethenes were also attracted much attention
[52,53]. The electrochemical properties of diarylethenes can be
used potentially for molecular switches and molecular-scale elec-
(A) 800
1o
2o
tronic switches [54,55]. In this study, the electrochemical examin-
ations 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/s were shown in Fig. 6. From the figure, it can be seen that
the oxidation potential onsets of the closed-ring isomers of diary-
lethenes 1–3 were higher than those of the corresponding open-
600
400
200
0
3o
350
400
450
ring isomers. After cyclization reaction, the p-conjugation lengths
Wavelength (nm)
of the closed-ring isomers 1c–3c were much longer than those of
the open-ring isomers 1o–3o. The initiation values of their oxida-
tion are 1.17 and 1.31 V for 1o and 1c, 0.50 and 0.60 V for 2o
and 2c, 1.26 and 1.30 V for 3o and 3c, respectively. The results is
reverse to that of symmetrical analogues bearing two fluorine
atoms at both terminal phenyl rings [25], which their oxidation po-
tential onsets of the open-ring isomers were higher than those of
the corresponding closed-ring isomers. Furthermore, there are
great differences of the electronic current and polarization curve
shapes between the open- and closed-ring isomers of diaryleth-
enes 1 and 2, with the exception of diarylethene 3. All of these
diarylethene derivatives showed the distinct oxidation waves,
and the values are observed at 1.29 and 1.44 V for 1o and 1c,
1.12 and 1.27 V for 2o and 2c, and 1.41 and 1.42 V for 3o and 3c,
respectively. This is very different from those of symmetrical and
unsymmetrical diarylethene analogues reported previously [21–
23]. For those analogues, only the closed-ring isomers showed a
distinct oxidation wave at one voltage, but all of their open-ring
isomers did not [21–23].
(B)
1200
900
600
300
0
1o
2o
3o
350
400
450
500
550
Wavelength (nm)
Fig. 4. Emission spectra of diarylethenes 1–3 both in hexane (2.0 Â 10À5 mol/L) and
in PMMA film (10%, w/w) at room temperature, excited at 300 nm: (A) in hexane,
(B) in PMMA film.
visible light irradiation, but also by electrochemical or chemical
oxidation, such as electrochromism [50,51]. Therefore, besides
their excellent photochromic performance, the electrochemical
According to the same method described in previous works
[56–58], the HOMO and LUMO energy levels can be estimated by
using the energy level of the ferrocene as reference. As shown in

W. Liu et al. / Journal of Molecular Structure 936 (2009) 29–36
35
(A)1400
(A)
1o
1c
UV Vis
1120
840
560
280
0
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
(B)
2o
2c
350
400
450
500
550
Wavelength (nm)
(B)1300
1040
780
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
UV Vis
(C)
3o
3c
520
260
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Potential/Vvs. SCE
0
350
400
450
500
550
Fig. 6. Cyclic voltammetry (second scan) of diarylethenes 1–3 in acetonitrile at a
scan rate of 50 mV/s: (A) 1, (B) 2, and (C) 3.
Wavelength (nm)
(C) 1400
1120
840
Table 3
UV Vis
Electrochemical properties of diarylethenes 1–3.
Compound
Oxidation
Reduction
Eonset (V)
Band gap
Eonset (V)
IP (eV)
EA (eV)
Eg
560
1o
1c
2o
2c
3o
3c
+1.17
+1.31
+0.50
+0.60
+1.26
+1.30
À5.97
À6.11
À5.3
À1.25
À1.34
À0.91
À0.76
À1.36
À1.27
À3.55
À3.46
À3.89
À4.04
À3.44
À3.53
2.42
2.65
1.41
1.36
2.62
2.57
280
À5.4
À6.06
0
À6.1
350
400
450
500
550
*The acetonitrile used as solvent was spectrograde and was purified by distillation
before use.
Wavelength (nm)
Fig. 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, and (C) 3.
erties were studied. These unsymmetrical compounds showed
good photochromism in solution, in PMMA film and in the single
crystalline phase. Their fluorescence and electrochemical behav-
iors are remarkably dependent on the fluorine atom position effect,
which may be attributed to the different electron-withdrawing
ability and steric effect when the fluorine atom was substituted
on the different position of the terminal phenyl ring.
Fig. 6A, the onset potentials (Eonset) of oxidation and reduction of
1o were observed at +1.17 and À1.25 V, respectively. So the values
of IP and EA were calculated to be À5.97 and À3.55 eV. Based on
the the HOMO and LUMO energy levels, the band gap (Eg, Eg = LU-
MO–HOMO) of 1o can be determined as 2.42 eV. Similarly, the Eg of
1c can be calculated as 2.65 eV. Corresponding values for diary-
lethenes 2 and 3 are summarized in Table 3. From these data, it
can be clearly seen that the band gaps of the closed-ring isomers
2c and 3c are higher than those of their corresponding open-ring
isomers 2o and 3o, but that of 1c are smaller than that of 1o.
The result indicated that the fluorine atom and its substituent po-
sition have a great effect on the electrochemical behaviors of these
diarylethene derivatives.
Acknowledgements
We thank the Program for New Century Excellent Talents 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) for financial support.
4. Conclusions
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