Synthesis, crystal structure and its optical and electrochemical properties of a new unsymmetrical diarylethene

Article abstract of DOI:10.1016/j.molstruc.2006.08.027

A new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl),2-(2-methyl-5-(3-methylphenyl)-3-thienyl)-3,3,4,4,5,5-hexafluorocyclopent-1-ene (MPMTH), was synthesized and its structure was determined by single-crystal X-ray diffraction analysis. Its optical and electrochemical properties, including photochromic reactivity both in solution and in the crystalline phase, fluorescence and electrochemical property were investigated. The diarylethene underwent reversible photochromism, changing from colorless to blue after irradiation with UV light or visible light. The maximum absorption of the closed-ring isomer of MPMTH was observed at 577 nm in hexane. In hexane, the open-ring isomer of MPMTH exhibited relatively strong fluorescence at 310 nm when excited at 280 nm. The irreversible anodic oxidation of MPMTH was observed by performing linear sweep experiment.

Full text of DOI:10.1016/j.molstruc.2006.08.027

Journal of Molecular Structure 833 (2007) 23–29
www.elsevier.com/locate/molstruc
Synthesis, crystal structure and its optical and electrochemical
properties of a new unsymmetrical diarylethene
Shouzhi Pu a,¤, Gang Liu a, Guizhen Li a, Ruji Wang b, Tianshe Yang a
a Jiangxi Key Lab of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, China
b Department of Chemistry, Tsinghua University, Beijing 100084, China
Received 25 June 2006; received in revised form 20 August 2006; accepted 25 August 2006
Available online 12 October 2006
Abstract
new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl),2-(2-methyl-5-(3-methylphenyl)-3-thienyl)-
A
3,3,4,4,5,5-hexaXuorocyclopent-1-ene (MPMTH), was synthesized and its structure was determined by single-crystal X-ray diVraction
analysis. Its optical and electrochemical properties, including photochromic reactivity both in solution and in the crystalline phase, Xuo-
rescence and electrochemical property were investigated. The diarylethene underwent reversible photochromism, changing from colorless
to blue after irradiation with UV light or visible light. The maximum absorption of the closed-ring isomer of MPMTH was observed at
577 nm in hexane. In hexane, the open-ring isomer of MPMTH exhibited relatively strong Xuorescence at 310 nm when excited at 280 nm.
The irreversible anodic oxidation of MPMTH was observed by performing linear sweep experiment.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Photochromism; Diarylethene; Crystal structure; Optical and electrochemical properties
1. Introduction
optical recording, photo-switches, and full-color displays
and photodriven actuators, because of the good ther-
mally irreversible properties of the two isomers and their
high sensitivity, fast response and remarkable fatigue
resistance [3,11–16].
The development of photochromic compounds is an
ever-growing Weld due to some new potential applica-
tions of this class of materials [1,2]. Generally, the revers-
ible photochemical isomerization of photochromic
molecules can lead to global changes of the bulk materi-
als characteristics such as UV–vis absorption spectra,
Xuorescence spectra, oxidation/reduction potentials and
refractive indices, etc. [3,4], or trigger a mechanical defor-
mation, from reorientation to a matter transfer that
could be used for various purposes from data technology
(data storage) to astronomy (active Wlters) [5–7]. To date,
some thermally irreversible photochromic compounds,
such as diarylethenes, furylfulgides and phenoxynaph-
thacene–quinones [8–10], have been developed. Of all
these compounds, diarylethene derivatives are the most
promising materials for applications to high-density
Although many photochromic diarylethene derivatives
with two thiophene-derived groups have been reported to
date, compounds which show strong photochromic reac-
tivity in the crystalline phase are very rare [17–20]. How-
ever, they are very useful for practical applications, such
as rewritable three-dimentional (3D) optical storage as
well as nonlinear optical devices with switching properties
[21,22]. Diarylethene crystals belong to a unique class of
photochromic crystals, which exhibit thermally stable and
fatigue resistant photochromic performance, and they are
very promising for practical applications because they can
not only reversibly turn various colors (yellow, red, blue
or green) from colorless, depending on their molecular
structure, upon irradiation with UV and appropriate
wavelength visible light, but also exhibit good thermal sta-
bility and remarkable fatigue resistance [19,23]. Therefore,
*
Corresponding author. Tel.: +86 791 3805183; fax: +86 791 3826894.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0022-2860/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.08.027

24
S. Pu et al. / Journal of Molecular Structure 833 (2007) 23–29
2. Experimental
F
F
F
F
F
F
F
F
F
F
F
F
UV
Vis
2.1. General methods
1H NMR and 19F NMR spectra were recorded on Bru-
ker AV400 (400 MHz) spectrometer with CDCl3 as the sol-
vent and tetramethylsilane as an internal standard. Fourier
transform infrared (FT-IR) spectra were measured from
KBr pellets using a Bruke (Vertex 70) system. The absorp-
tion spectra were measured using a Perkin-Elmer Lambda-
900 UV/VIS/NIR spectrometer. Photo-irradiation was
carried out using SHG-200 UV lamp, CX-21 ultraviolet
Xuorescence analysis cabinet, and BMH-250 Visible lamp.
Light of appropriate wavelengths was isolated by diVerent
light Wlters. Fluorescence spectra were measured using a
Hitachi F-4500 spectrophotometer. Elemental analysis was
performed on a Yanaco CHN C ORDER MT-3 apparatus.
The X-ray experiment of the single-crystal was performed
on a Bruker P4 diVractometer equipped with graphite
monochromatized Mo Kꢀ radiation at room temperature
(295§2 K).
S
S
S
S
MPMTH-c
Scheme 1. Photochromism of diarylethene MPMTH.
MPMTH-o
investigation of diarylethene crystals have attracted much
attention today, and a larger number of diarylethene crys-
tal structures and their properties have already been
reported [17,19,23,24].
Among these diarylethene compounds reported, many
of them are symmetrical diarylethenes; reports on unsym-
metrical derivatives are very rare [25]. In a previous publi-
cation, Irie et al., reported photochromism of
a
diarylethene compound [1,2-bis(2,4-dimethyl-5-phenyl-3-
thienyl)perXuorocyclopentene (1a)] [26], and they found
that the single crystal of this compound could reversibly
shrink and expand by alternate irradiation with UV and
visible light. The results showed that the shrinkage of
the surface was digital, and each step corresponded to
the thickness of one molecular layer as short as 1 nm. The
reversible surface morphological changes could be
applied to photodriven, nanoscale actuators that revers-
ibly change thickness stepwise by alternate irradiation
with UV and visible light [16]. In 2000, Irie et al., revealed
the photochromic properties of 1,2-bis(2-metheyl-5-
phenyl-3-thienyl)perXuorocyclopentene (2a) both in solu-
tion and in a single-crystalline phase [11]. In that paper,
they carried out kinetic study of photochromism of this
compound and its derivatives in the single-crystalline
phase and examined the reactivities in crystals at various
temperatures. In addition, they had also reported their
remarkably diVerent fatigue resistance of compounds 1a
and 2a [27]. Compared with these publications, we found
that the methyl groups at 4 and 4Ј-positions of thiophene
rings can aVect signiWcantly not only on their crystal
structures but also on their various photochemical prop-
erties, such as absorption maxima, ease of cyclization
quantum yield and fatigue resistant properties. These
results are very interesting and important, and they also
give us some good suggestions. We supposed that diary-
lethene had some new properties when the methyl groups
at 4 and 4Ј-positions of thiophene rings were transferred
to the benzene rings. In this study, in order to investigate
one methyl group at the meta-position of the benzene of
diarylethene eVect on crystal structure and photochemical
properties, we have now synthesized a new unsymmetrical
diarylethene, 1-(2-methyl-5-phenyl-3-thienyl),2-(2-methyl-5-
(3-methylphenyl)-3-thienyl)-3,3,4,4,5,5-hexaXuorocyclopent-
1-ene (MPMTH, shown in Scheme 1). We also investi-
gated systematically its X-ray crystal structure, photo-
chromism, and optical and electrochemical properties.
2.2. Synthesis of MPMTH
The new diarylethene MPMTH was derived originally
from 2-methylthiophene. The synthesis method of diary-
lethene MPMTH was described in Scheme 2 and experi-
mental details were carried out as following.
2.2.1. Synthesis of 3,5-dibromo-2-methylthiophene 2
To a stirred solution of 2-methylthiophene (24.0g,
244.8mmol) in acetic acid (100mL) at 0°C was slowly added
Br2/acetic acid (v/vD29/40) solution. The reaction mixture
was stirred overnight at this temperature. The reaction was
stopped by the addition of water. The mixture was neutral-
ized by Na2CO3 and extracted with ether. The ether extract
was dried, Wltrated, and concentrated. The residue was puri-
Wed by distillation in vacuo. Compound 2 was obtained as
1
colorless oil of 50.7g in 81.2% yield. H NMR (400MHz,
CDCl3, TMS): ꢀ 2.94 (s, 3H), 7.61 (s, 1H); IR (ꢁ, cm¡1): 780,
810, 950, 1010, 1139, 1302, 1451, 1536, 2786, and 3081.
2.2.2. Synthesis of 3-bromo-2-methyl-5-thienylboronic acid 3
To a stirred solution of 2 (16.3 g, 63.7 mmol) in dry ether
(150mL) was added a 1.6 mol/L n-BuLi/hexane solution
(40.6mL, 65.0 mmol) at ¡78 °C under nitrogen atmosphere.
Stirring was continued for 30 min, boric acid tri-butyl ester
(18.8 mL) was quickly added to the reaction mixture. After
increasing to room temperature, the reaction was stopped
by the addition of 4% HCl (10 mL). The mixture was
extracted with 4% NaOH (100 mL), and the aqueous
NaOH solution was neutralized by 10% HCl. The residue
was washed, Wltrated, and dried. The product 3 was
1
obtained as yellowish solid of 12.0 g in 85.5% yield. H
NMR (400 MHz, CDCl3, TMS): ꢀ 2.51 (s, 3H), 4.59 (s, 2H),
7.33 (s,1H); IR (ꢁ, cm¡1): 690, 790, 832, 1010, 1132, 1340,
1474, 1528, 1291, and 3202.

S. Pu et al. / Journal of Molecular Structure 833 (2007) 23–29
25
Br
Br
Br
BuLi,-78
B(OBu)3
Br2
Pd(PPh3)4,
Na2CO3,aq.
B(OH)2
S
S
1
Br
S
3
2
Br
S
F
F
4
F
F
F
F
F
F
BuLi,-78
C5F8
F
F
F
F
S
S
F
S
MPMTH
5
Scheme 2. Synthetic route for diarylethene MPMTH.
2.2.3. Synthesis of 3-bromo-2-methyl-5-
(3-methylphenyl)thiophene 4
(s, 1H, thiophene-H), 7.29 (s, 1H, thiophene-H), 7.26–7.27
(m, 4H, J D7.6 Hz, phenyl-H), 7.34–7.56 (m, 5H, J D7.8Hz,
phenyl-H). 13C NMR (100MHz, CDCl3, TMS): ꢀ 14.56,
21.43, 29.72, 122.20, 122.70, 125.60, 125.80, 127.90, 128.70,
129.00, 133.20, 138.70, and 142.20. IR (ꢁ, cm¡1): 536, 692,
740, 757, 779, 837, 899, 986, 1054, 1110, 1137, 1192, 1265,
1336, 1440, 1605, and 2915.
Compound 4 was prepared by reacting compound 3
(2.4 g, 10.7mmol) with 1-bromo-3-methylbenzene (1.8g,
10.7 mmol) in the presence of Pd(PPh3)4 (0.3 g) and Na2CO3
(4.2 g, 40 mmol) in THF (100 mL containing 10% water),
for 16h at 70 °C. The mixture was cooled at room tempera-
ture and then extracted with ether. The ether extract was
dried over MgSO4, Wltered and concentrated. The residue
was puriWed by silica gel column chromatography using
petroleum ether as the eluent. Compound 4 was obtained as
2.3. Determination of the crystal structure of diarylethene
MPMTH
1
yellowish solid powder of 1.9 g in 65.0% yield. H NMR
Crystal data were collected by a Bruker P4 diVractometer
equipped with graphite monochromatized Mo Kꢀ radiation
at room temperature. Unit cell was obtained and reWned by
53 well centered reXections with 1.8°<ꢂ<17.4°. Data collec-
tion was monitored by three standards every 100 reXection
collected. No decay was observed except the statistic Xuctua-
tion in the range of §4.8%. Raw intensities were corrected
for Lorentz and polarization eVect. Direct phase determina-
tion yielded the positions of all non-hydrogen atoms. All
non-hydrogen atoms were subjected to anisotropic reWne-
ment. All hydrogen atoms were generated geometrically
with C–H bond distances of 0.93–0.96Å according to crite-
ria described in the SHELXTL manual. They were included
in the reWnement with Uiso(H)D1.2Ueq(C) or 1.5Ueq(methyl
C). The Wnal full-matrix least-square reWnement on F2 con-
verged with R1 D0.0645 and wR2 D0.1225 for 2079 observed
reXections [I72ꢃ(I)]. The Wnal diVerence electron density
map shows no features. The crystallographic data obtained
and experimental details employed were summarized in
Table 1. Selected bond lengths (Å) and angles (°) were listed
in Table 2. Further details on the crystal structure investiga-
tion have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication number
CCDC 608148 for this paper. These data can be obtained
free of charge via www.ccdc.can.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Centre, 12 Union
(400 MHz, CDCl3, TMS): ꢀ 2.41(s, 3H, –CH3), 2.38 (s, 3H,
phenyl-H), 6.92 (s, 1H, thiophene-H), 7.09–7.37 (m, 4H,
J D8.4 Hz, phenyl-H).
2.2.4. Synthesis 1-(2-methyl-5-phenyl-3-thienyl),
2-(2-methyl-5-(3-methylphenyl)-3-thienyl)-3,3,4,4,5,
5-hexaXuorocyclopent-1-ene MPMTH
To a stirred THF solution (100mL) of compound 4
(1.8 g, 6.6 mmol) was slowly added a 1.6 mol/L n-BuLi/hex-
ane solution (4.2 mL, 6.7mmol) at ¡78°C under a nitrogen
atmosphere. After 30 min, 2-methyl-5-phenyl-3-thienyl-per-
Xuorocyclopentene 5 [26] (2.4 g, 6.6 mmol) was added and
the mixture was stirred for 2 h at this temperature. The
reaction mixture was extracted with diethyl ether and evap-
orated in vacuo, then puriWed by column chromatography
(petroleum ether) to give MPMTH (2.1 g, 3.9 mmol), in
59.5% yield. The compound was recrystallized from chloro-
form/ hexane (v/v D 1/5) at room temperature and pro-
duced the crystals suitable for X-ray analysis. The structure
of MPMTH was also conWrmed by the melting point,
elemental analysis, and NMR spectroscopy and IR. Mp:
114.6–115.7°C. Anal. Calcd. for C28H20F6S2 (%): C, 62.91; H,
3.77; found: C, 63.03; H, 3.91. 19F NMR (376MHz, CDCl3,
1
TMS): ꢀ 109.97 (4F), 131.80 (2F). H NMR (400MHz,
CDCl3, TMS): ꢀ 1.96 (s, 6H, –CH3), 2.39 (s, 3H, –CH3), 7.13

26
S. Pu et al. / Journal of Molecular Structure 833 (2007) 23–29
Table 1
0.8
0.7
0.6
0.5
Crystal data and structure reWnement
0s
10s
30s
50s
Empirical formula
Formula weight
Temperature
C28H20F6S2
534.56
295(2) K
0.71073 Å
80s
Wavelength
140s
260s
440s
680s
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/n (No. 14)
a D 11.956(2) Å D 90.00°
b D 9.2230(19) Å ꢇ D 92.04(22)°
c D 22.630(9) Å ꢈ D 90.00°
2493.9(12) Å3
0.4
0.3
0.2
0.1
0.0
UV
Volume
Z
Density (calculated)
Absorption coeYcient
F (000)
4
1.424 g/cm3
300
400
500
600
700
800
0.274 mm¡1
Wavelength(nm)
1096
Crystal size
ꢂ range for data collection
Limiting indices
0.2 £ 0.3 £ 0.4 mm
Fig. 1. Absorption spectral change of diarylethene MPMTH in hexane
solution (2.0 £ 10¡5 mol/L) upon irradiation with 313 nm UV light.
1.8–17.4°
¡1 < h < 14, ¡10 < k < 4, ¡26 < l < 26
5573
4256 [R (int) D 0.0405]
Full-matric least-squares on F2
4256/10/366
ReXections collected
Unique reXections
ReWnement method
Data/restraints/parameters
Goodness-of-Wt on F2
Final R indices [I > 2ꢀ(I)]
R indices (all data)
Largest diV. peak and hole
(MPMTH-c). The conversion from MPMTH-o to
MPMTH-c in the photostationary state under irradiation
with 313 nm light was 89%. The absorption coeYcient of
MPMTH-c at 577nm was 1.9£104 L mol¡1 cm¡1. The blue
color disappeared by irradiation with visible light
(ꢅ > 450nm). The coloration–decoloration cycle could be
repeated more than 100 times and a clear isosbestic point
was observed at 306 nm even after 100 times. All these
results described above indicated that it owned good pho-
tochromic properties. The quantum yields of cyclization
and cycloreversion reactions of MPMTH were determined
by comparing the reaction yields of the diarylethene in hex-
ane against diarylethene 1a in hexane at room temperature
[11], and their values were 0.51 (278nm) and 0.032 (577nm),
respectively. Compared with those of diarylethenes 1a and
2a, the cyclization quantum of MPMTH was appreciably
larger than that of 1a (ꢆD 0.46) and less than that of 2a
(ꢆD0.59), while the cycloreversion quantum yield of the
diarylethene was remarkably larger than those of diaryleth-
enes 1a and 2a (ꢆ D0.015 and 0.013, respectively) [11]. The
molar absorption coeYcients, both open-ring and closed-
ring forms, of diarylethene MPMTH were much higher
than those of diarylethenes 1a and 2a [11,26]. The methyl
group at meta-position of the benzene of diarylethene
increased signiWcantly the cycloreversion quantum yield
and the molar absorption coeYcient.
1.028
R1 D 0.0645, ꢉR2 D 0.1225
R1 D 0.1430, ꢉR2 D 0.1693
0.461 and ¡0.280 e Å¡3
Table 2
Selected bond length (Å) and angles (°)
S(1)–C(6)
S(1)–C(9)
1.717(2)
1.725(2)
1.710(2)
1.731(2)
1.526(3)
1.527(3)
1.517(4)
1.522(4)
1.353(3)
1.494(3)
1.495(3)
1.371(3)
1.500(3)
1.477(3)
1.496(4)
1.501(3)
C(6)–S(1)–C(9)
93.25(10)
107.8(2)
103.52(18)
104.2(2)
F(4A)–C(4A)–F(3A)
C(5)–C(4A)–C(3)
C(3)–C(4B)–C(5)
C(2)–C(1)–C(7)
C(2)–C(1)–C(5)
C(1)–C(2)–C(19)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4B)
C(2)–C(3)–C(4A)
C(1)–C(5)–C(4B)
C(1)–C(5)–C(4A)
C(6)–C(7)–C(1)
C(7)–C(6)–C(16)
C(8)–C(9)–C(10)
C(11)–C(12)–C(17)
S(2)–C(18)
S(2)–C(21)
C(4A)–C(5)
C (4A)–C(3)
C(4B)–C(3)
C(4B)–C(5)
C(1)–C(2)
C(1)–C(5)
C(2)–C(3)
C(6)–C(7)
C(6)–C(16)
C(9)–C(10)
C(12)–C(17)
C(18)–C(28)
128.99(19)
111.15(17)
131.17(18)
109.52(17)
107.21(19)
105.79(16)
105.67(19)
105.72(16)
124.67(18)
130.61(18)
128.66(19)
120.1(3)
Road, Cambridge CB21EZ, UK [fax: (+44) 1223-336033;
e-mail: deposit@ccdc.cam.ac.uk]).
3.2. Relationship between crystal structure and photochromic
properties
3. Results and discussion
3.1. Photochromic properties of MPMTH in solution
Final structural conWrmation of diarylethene MPMTH
was provided by X-ray crystallographic analysis. Its struc-
ture features was shown in Fig. 2 and a packing diagram
was shown in Fig. 3. As shown in Fig. 2, the molecule
occupied approximately in C2 symmetry, and it was packed
in a photoactive anti-parallel conformation in the crystal-
line phase, which can undergo photocryclization reaction
[28]. It was found that the methyl group substitutes ran-
domly to the benzene rings of both wings of each molecule.
They were named as C17 and C29 and their occupancies
Photochromic MPMTH underwent photochromism in
solution. The absorption spectral change of MPMTH in
hexane (CD2.0 £ 10¡5 mol/L) by irradiation with 313 nm
light was shown in Fig. 1. The absorption maximum of its
colorless open-ring isomer (MPMTH-o) was observed at
278nm (ꢄ, 3.9 £104 L mol¡1 cm¡1). Upon irradiation with
313nm UV light, the colorless solution turned to blue with
a new broad absorption band centered at 577 nm. The blue
color is due to the formation of the closed-ring isomer

S. Pu et al. / Journal of Molecular Structure 833 (2007) 23–29
27
Fig. 2. ORTEP drawings of crystal MPMTH. The ellipsoids were drawn at 35% probability level.
phene moieties were linked by the C1BC2 double bond, with
both of them attached to the ethylene group via the 3-posi-
tion. The two methyl groups located on diVerent sides of the
double bond, reXected in the torsion angles C1–C2–C19–C18
[42.0(3)°] and C2–C1–C7–C6 [43.5(3)°], and thus trans with
respect to the double bond. Such a conformation is crucial to
the photochromic and photo-induced properties of such
materials [30]. The intramolecular distance between the two
reactive C atoms (C6,C18) is 3.537(3)Å. The value is in
between those of diarylethenes 1a and 2a (they are 3.96Å for
1a and 3.51Å for 2a) [16,31]. The crystal can be expected to
undergo photochromism to generate the closed isomer of
diarylethene MPMTH (MPMTH-c) (see Scheme 1), because
photochromic reactivity usually appears when the distance
between the reactive C atoms is less than 4.2Å and the mole-
cule is Wxed in an anti-parallel mode [32–34]. It is well known
that the absorption spectrum and color are dependent on the
substituent eVects and the ꢁ-conjugation length in molecule.
The arrangement described above was very beneWcial to
form the extended ꢁ-conjugation. After irradiation with UV
light, the ꢁ-conjugation extended throughout the whole mol-
ecule, and the UV–vis absorption spectra displayed drastic
changes resulting in displaying remarkable diVerent color.
In fact, crystal of MPTHP showed photochromic
reaction coincident with the theoretical analysis. Upon
irradiation with 313nm light, the colorless crystal of
MPMTH-o turned to blue quickly. When the blue crystal
was dissolved in hexane, the solution turned to blue, and
the absorption maximum was observed at 577nm, which
was the same as that of the closed-ring isomer MPMTH-c.
The blue color disappeared upon irradiation with appropri-
ate wavelength visible light and the absorption spectrum of
the solution containing the colorless crystal was the same as
that of the open-ring isomer MPMTH-o.
Fig. 3. A packing views along the b direction.
were assigned to be 0.5 in the Wnal reWnement according to
the reWnement of occupancy before [29]. The conformation
of perXuorocyclopentene ring is an envelope and the C4
atom is disordered which was named as C4A and C4B.
Therefore, the two Xuorine atoms of C4 are split to two sets
named as F3A, F3B and F4A, F4B. The occupancies for
the diVerent conformations were reWned to be 0.720(2):
0.280(2). The dihedral angles between the least-squares
plane of the central perXuorocyclopentene ring and the two
thiophene rings, S1/C6–C9 and S2/C18–C21, are 44.5(1)°
and 42.0(1)°, respectively. The dihedral angle between the
thiophene ring and adjacent benzene ring is 3.6(3)° for the
C10–C15 benzene ring and 26.0(3)° for the C22–C27 ben-
zene ring.
3.3. Fluorescence properties of MPMTH
In the perXuorocyclopentene ring of MPMTH, distances
of C1–C2, C2–C3, C3–C4A, C4A–C5 and C1–C5 are
1.353(3)Å, 1.495(3)Å, 1.527(3)Å, 1.526(3)Å and 1.494(3)Å,
respectively. These data clearly indicate that the C1–C2 bond
is a double bond, being signiWcantly shorter than the other
carbon-carbon single bonds. Two justly symmetrical thio-
The Xuorescence properties in the solution of the com-
pound were measured using a Hitachi F-4500 spectropho-
tometer, and the breadths of excitation and emission slit
were selected 5.0nm. Fig. 4 shows the Xuorescence spectral

28
S. Pu et al. / Journal of Molecular Structure 833 (2007) 23–29
1800
14
12
-4
2100
1800
1500
1200
900
600
300
0
c = 1x 10 mol/L
1600
1400
1200
1000
800
600
400
200
0
0
30
90
210
390
690
990 s
10
8
a
6
4
b
2
-6
300 350 400 450 500
a c = 5 x 10 mol/L
-5
b c = 1 x 10 mol/L
-5
c c = 2 x 10 mol/L
-4
d c = 1 x 10 mol/L
-3
e c = 1 x 10 mol/L
c
d
e
300
320
340
360
380
400
300
320
340
360
380
400
Wavelength/nm
Wavelength/nm
Fig. 4. Fluorescence spectra changes of MPMTH in hexane at
c D 1.0 £ 10¡5 mol/L and room temperature, excited at 280 nm.
Fig. 5. Fluorescence emission spectra of MPMTH in various concentra-
tions in hexane solution at room temperature, monitored at 290 nm.
7
changes of MPMTH-o in hexane (cD1.0£10¡5 mol/L) at
room temperature upon irradiation with 313nm light. As
shown in this Wgure, it can be clearly seen that the hexane of
MPMTH-o exhibits relatively strong Xuorescence at 310nm
when excited at 280nm. Solution Xuorescence spectroscopy
at diVerent irradiation times shows that the Xuorescence
intensity in the range of 295–400nm decreases with increas-
ing the exposal time upon irradiation with 313nm UV light
(Fig. 4). This phenomenon veriWed that MPMTH has good
photochromic properties in solution. Along with the photo-
chromism from open-ring isomers MPMTH-o to closed-
ring isomers MPMTH-c upon irradiation with 313nm light,
the Xuorescence intensity decreased dramatically, and the
photostationary state showed relatively weak Xuorescence.
The closed-ring isomers MPMTH-c showed almost no Xuo-
rescence. It had been reported that this kind of diarylethenes
which showed Xuorescence in the open-ring isomer and
showed no Xuorescence or weak Xuorescence in the closed-
ring isomer could be potentially applied to optical memory
with Xuorescence readout method [35].
The concentration dependence of Xuorescence emission
spectra was measured in hexane solution at room tempera-
ture, as shown in Fig. 5. When the concentration of diaryleth-
ene MPMTH in hexane increased from 5.0£10¡6 to
1.0£10¡4 mol/L, the maximum emission arose from 329 to
360nm when excited at 290nm, and the relative Xuorescence
intensity decreased dramatically. The hexane solution
showed no Xuorescence when the concentration increased to
1.0£10¡3 mol/L. Therefore, the Xuorescence spectra of
MPMTH-o showed remarkable concentration dependence.
It showed a systematic red shift and the Xuorescence intensity
decreased rapidly with the concentration increased. The
result indicated that molecular aggregation and Xuorescence
quench take place due to concentration increasing [36].
MPMTH-o
MPMTH-c
6
5
4
3
2
1
0
0.0
0.3
0.6
0.9
1.2
1.5
Potential / V s. Pt
ν
Fig. 6. The anodic polarization curves of diarylethene MPMTH.
applied to molecular-scale electronic switches [37]. The oxi-
dative cyclization and cycloreversion of diarylethenes have
been reported [38–40]. In order to investigate the electro-
chemical properties of diarylethene MPMTH, we per-
formed electrochemical examinations by linear sweep
method. Experimental method and condition were
described in Ref. [37]. The typical electrolyte was acetoni-
trile
(5 ml) containing 0.10mol/L LiClO4 and
4.0 £10¡3 mol/L diarylethene MPMTH. The solution was
deaerated by a dry argon stream and maintained at a slight
argon overpressure during electrochemical experiments.
Fig. 6 showed the anodic polarization curves of MPMTH-o
and MPMTH-c on Platinum electrode. It can be clearly
seen from Fig. 6 that the oxidation of MPMTH-o and
MPMTH-c was initiated at 0.52 and 0.38V, respectively.
The oxidation potential onset of closed-ring from
(MPMTH-c) is lower than that of open-ring form
(MPMTH-o), and the oxidation potential diVerence
between MPMTH-o and MPMTH-c was 0.14 V. This is in
accordance with the theory that longer conjugation length
generally leads to less positive potentials, with the addition
of each heterocycle [41]. During the cyclization reaction, the
ꢁ-conjugation length of MPMTH-c was much longer than
3.4. Electrochemical properties of MPMTH
The electrochemical properties of diarylethene are being
used for molecular switching and also can be potentially

S. Pu et al. / Journal of Molecular Structure 833 (2007) 23–29
29
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that of MPMTH-o. Thus the oxidation onset of MPMTH-
c was much lowered than that of MPMTH-o. In addition,
three oxidation processes can be easily seen during anodic
oxidation of MPMTH-o and MPMTH-c, indicating that
the anodic oxidation processes may involve the oxidation
of thiophene ring, benzene ring, together with the methyl-
benzene ring of this compound.
4. Conclusions
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(2002) 759.
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In conclusion, a new diarylethene presented here were
synthesized and its structure was established by X-ray crys-
tallographic analysis. The compound exhibited reversible
photochromism both in solution and in the crystalline
phase and exhibited relatively strong Xuorescence at
310 nm when excited at 280 nm. The molecules of MPMTH
adopt a photoactive anti-parallel in the single crystalline
phase, and the distance between the two reactive C atoms is
3.537(3) Å. The irreversible anodic oxidation of MPMTH
in acetonitrile containing LiClO4 as the supporting electro-
cyte was initialed at 0.52 V.
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Acknowledgements
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This work was supported by the Projects of National
Natural Science Foundation of China (Grant No.
20564001), the Natural Science Foundation of Jiangxi,
China (Grant No. 050017) and the Science Funds of the
Education OYce of Jiangxi, China (Grant No. [2005] 140).
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