Synthesis, crystal structure and optoelectronic properties of a new unsymmetrical photochromic diarylethene

Article abstract of DOI:10.1016/j.saa.2007.10.026

A new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (1a), 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 solid state (PMMA film and the single-crystalline phase), fluorescence and electrochemical properties were investigated in detail. The compound showed excellent photochromism even in the single-crystalline phase by photo-irradiation. In acetonitrile, the open-ring isomer of diarylethene 1 exhibited relatively strong fluorescence at 470 nm when excited at 300 nm, and its emission intensity decreased along with the photochromism upon irradiation with 313 nm light. Its closed-ring isomer showed almost no fluorescence. The electrochemical properties of diarylethene were investigated by performing cyclic voltammetry experiment and its HOMO and LUMO energy level were calculated.

Full text of DOI:10.1016/j.saa.2007.10.026

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Spectrochimica Acta Part A 70 (2008) 1065–1072
Synthesis, crystal structure and optoelectronic properties of a new
unsymmetrical photochromic diarylethene
Tianshe Yang, Shouzhi Pu∗, Congbin Fan, Gang Liu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China
Received 6 August 2007; received in revised form 29 September 2007; accepted 15 October 2007
Abstract
new unsymmetrical photochromic diarylethene, 1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-trifluoromethylphenyl)-3-
A
thienyl]perfluorocyclopentene (1a), 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 solid state (PMMA film and the single-
crystalline phase), fluorescence and electrochemical properties were investigated in detail. The compound showed excellent photochromism even
in the single-crystalline phase by photo-irradiation. In acetonitrile, the open-ring isomer of diarylethene 1 exhibited relatively strong fluorescence
at 470 nm when excited at 300 nm, and its emission intensity decreased along with the photochromism upon irradiation with 313 nm light.
Its closed-ring isomer showed almost no fluorescence. The electrochemical properties of diarylethene were investigated by performing cyclic
voltammetry experiment and its HOMO and LUMO energy level were calculated.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Photochromism; Diarylethene; Crystal structure; Fluorescence; Cyclic voltammetry
1. Introduction
inevitably influence the optoelectronic properties of correspond-
ing diarylethenes.
Photochromism has attracted considerable attention because
of its potential application to molecular devices, such as optical
memories and switches [1]. Among various thermally irre-
versible photochromic compounds, diarylethene derivatives are
the most promising compounds because of their fatigue resis-
tance and thermally irreversible properties [2,3]. In the past 20
years, the family of the 1,2-diarylethenes have gained much
attention with respect to their good thermally irreversible pho-
tochromic performance, remarkable fatigue-resistance, rapid
response and high reactivity in solid state [3,4]. Diarylethene
derivatives so far synthesized have hetero-aryl moieties such
as thiophene [5], benzothiophene [6], thiazole [7], crysoth-
iophene [8] and indole rings [9]. Above all, dithienylethene
derivatives bearing phenyl groups on the end are the most
promising, because the end group can be substituted by electron
donating group or electron withdrawing group. These groups
It is well known that diarylethene derivatives can undergo
cyclization/cycloreversion photochromic reactions in solution
upon photo-irradiation. Generally, the open-ring isomers of
diarylethenes in solution have two conformations, namely, par-
allel conformation with the two hetero-aryl rings in mirror
symmetry and anti-parallel conformation with them in C2 sym-
metry [10], and the photoinduced cyclization and cycloreversion
reactions can proceed only in a conrotatory mode by alternate
irradiation with UV and visible light only from the anti-parallel
conformation [11]. The absorption maxima of closed-ring iso-
mers are known to depend on the ␲-conjugate length of the
aryl groups. In fact, the longer ␲-conjugated systems were
introduced into the molecule of diarylethene, the longer absorp-
tion wavelengths of the closed-ring isomers were observed.
Diarylethene crystals belong to a unique class of photochromic
crystals, which exhibit thermally stable and fatigue resistant
photochromic performance [12]. Diarylethene crystals in the
anti-parallelconformationareverypromisingforpracticalappli-
cations because they can reversibly turn various colors (yellow,
red, blue or green) from colorless, depending on their molecular
structure, upon irradiation with UV and appropriate wavelength
∗
Corresponding author. Tel.: +86 791 3805183; fax: +86 791 3805212.
E-mail addresses: pushouzhi@tsinghua.org.cn, pushouzhi@yahoo.com.cn
(S. Pu).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.026

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T. Yang et al. / Spectrochimica Acta Part A 70 (2008) 1065–1072
Scheme 1. Photochromism of diarylethene 1.
visible light. Therefore, investigation of diarylethene crystals
have attracted much attention today, and a larger number of
diarylethene crystal structures and their properties have already
been reported [12,13].
Among these diarylethene compounds reported, many of
them are symmetrical diarylethenes. In our previous paper, we
reported one unsymmetrical compound bearing methyl group
at the meta-position of the benzene of diarylethene [14]. In the
present contribution, we have synthesized a new unsymmetrical
photochromic diarylethene compound with a trifluoromethyl
group at the meta-position of one terminal benzene ring,
Photo-irradiation was carried out using SHG-200 UV lamp,
CX-21 ultraviolet fluorescence analysis cabinet, and BMH-250
visible lamp. Light of appropriate wavelengths was isolated by
different light filters. Fluorescence spectra were measured using
a Hitachi F-4500 spectrophotometer. Mass spectrometry was
measured using an Agilent Ion Trip MSD. Elemental analy-
sis was performed on a PE CHN 2400 apparatus. The X-ray
experiment of the single-crystal was performed on a Bruker
P4 diffractometer equipped with graphite monochromatized Mo
K␣ radiation at room temperature (294 1 K). Electrochemi-
cal 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 tempera-
ture. Solvents were purified according to standard procedures,
and all chemicals for synthesis were purchased from commercial
suppliers.
namely,
1-(2-methyl-5-phenyl-3-thienyl)-2-[2-methyl-5-(3-
trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (1a).
Its photochromic reactivity in solution and in PMMA film,
crystal structure, fluorescence and electrochemical properties
were also presented here. The photochromism of diarylethene
1a is shown in Scheme 1, and its detailed synthetic procedures
are described in Section 2.
2.2. Synthesis of diarylethene 1a
The new diarylethene 1a was derived originally from 2-
methylthiophene, and the synthesis method of 1a was described
in Scheme 2 and experimental details were carried out as fol-
lowing.
2. Experimental
2.1. General methods
1H NMR, 19F NMR and 13C NMR spectra were recorded
on Bruker AV400 (400 MHz) spectrometer with CDCl3 as the
solvent and tetramethylsilane as an internal standard. Fourier
transform infrared (FT-IR) spectra were measured from KBr
pellets using a Bruker (Vertex 70) system. The absorption spec-
tra were measured using an Agilent 8453 UV/VIS spectrometer.
2.2.1. Synthesis of
3-bromo-2-methyl-5-(3-trifluoromethylphenyl)thiophene (3)
3-Bromo-2-methyl-5-(3-trifluoromethylphenyl)thiophene
(3) was prepared by reacting 3-bromo-2-methyl-5-thienyl-
boronic acid (2) [15] (4.4 g, 20.0 mmol) with 1-bromo-3-
trifluoromethylbenzene (4.5 g, 20.0 mmol) in the presence of
Scheme 2. Synthetic route for diarylethene 1a.

T. Yang et al. / Spectrochimica Acta Part A 70 (2008) 1065–1072
1067
Pd(PPh3)4 (0.7 g) and Na2CO3 (5.3 g, 50.0 mmol) in THF
(100 mL containing 10% water) for 16 h at 70 ◦C. The mixture
was cooled at room temperature and then extracted with
ether. The ether extract was dried over MgSO4, filtered and
concentrated. The residue was purified by silica gel column
chromatography using petroleum ether as the eluent. Compound
3 was obtained as yellowish solid powder of 5.4 g in 84.0%
1
yield. H NMR (400 MHz, CDCl3): 2.43 (s, 3H, –CH3), 7.17
(s, 1H, thienyl-H), 7.47–7.53 (dd, 2H, J = 7.60 Hz, phenyl-H),
7.65 (d, 1H, J = 7.8 Hz, phenyl-H), 7.73 (s, 1H, phenyl-H).
2.2.2. Synthesis of object diarylethene 1a
To a stirred THF solution (100 mL) of compound 3
(2.1 g, 6.5 mmol) was slowly added a 1.6 mol/L n-BuLi/
hexane solution (4.1 mL, 6.5 mmol) at −78 ◦C under
a nitrogen atmosphere. After 30min, 2-methyl-5-phenyl-
3-thienyl-perfluorocyclopentene (4) [16] (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 evaporated
in vacuo, then purified by column chromatography (petroleum
ether) to give 1a (2.2 g, 3.7 mmol) in 57% yield. The compound
was recrystallized from chloroform at room temperature and
produced the crystals suitable for X-ray analysis. The structure
of 1a was also confirmed by the melting point, mass spectrom-
etry and elemental analysis and NMR spectroscopy and IR.
m.p. 113 ◦C (uncorrected); MS m/z (M+) 587.2; Anal. Calcd. for
C28H17F9S2 (%): C, 57.14; H, 2.91; found: C, 57.09; H, 3.01;
1H NMR (400 MHz, CDCl3): δ 2.01 (s, 6H, –CH3), 7.28 (s, 1H,
benzene-H), 7.30(s, 1H, thienyl-H), 7.36(s, 1H, thienyl-H), 7.39
(t, 2H, J = 7.6 Hz, phenyl-H), 7.51–7.57 (m, 4H, phenyl-H), 7.73
(d, 1H, J = 7.34 Hz, phenyl-H), 7.78 (s, 1H, phenyl-H); 19F NMR
(376 MHz, CDCl3): δ 62.79 (3F), 110.06 (4F), 131.81 (2F);
13C NMR (100 MHz, CDCl3): 14.53, 122.30, 123.60, 124.40,
125.60, 126.20, 127.90, 128.80, 129.00, 129.50, 131.40, 131.70,
Fig. 1. Absorption spectral changes of diarylethene 1 in hexane and in PMMA
film. (A) In hexane solution (C = 2.0 × 10−5 mol/L) and (B) in PMMA amor-
phous film (10%, w/w).
belonging to ␲–␲* transition, and its maximum absorption
observed at 276 nm (ε = 3.58 × 104 dm3 mol−1 cm−1). Upon
irradiation 313 nm UV light, the colorless solution turned
blue with a new broad absorption band centered at 579 nm
(ε = 1.46 × 104 dm3 mol−1 cm−1) (Fig. 1(A)). This may be
ascribed to the formation of 1b. Upon irradiation with appropri-
ate wavelength visible light (λ > 450 nm), 1b could be bleached
completely to revert to 1a, accompanying the color change
of the solution from blue to colorless. This indicates that the
diarylethene compound underwent an efficient photocyclization
reactioninhexane [17]. Thecoloration–decolorationcyclecould
be repeated more than 100 times and a clear isosbestic point
was observed at 309 nm. The results illustrated that it owned
good photochromic behavior in hexane solution. The quantum
yields of cyclization and cycloreversion reactions of the target
diarylethene were determined by comparing the reaction yields
of this diarylethene compound in hexane against 1,2-bis(2-
methyl-5phenyl-3-thienyl)perfluorocyclopentene (2a) [18] in
hexane at room temperature, and their values were 0.42 (313 nm)
and 0.013 (579 nm), respectively. Similarly, diarylethene 1 also
showed good photochromism in the PMMA amorphous film,
as shown in Fig. 1(B). Upon irradiation with 313 nm UV
light, the color of 1a/PMMA film (λmax = 314 nm) changed
from colorless to blue with the appearance of a new broad
absorption band at λmax = 587 nm, which was assigned to the
133.30, 134.10, 140.50, 141.20, 142.30, 142.5; IR (ν, cm−1
,
KBr): 754, 796, 890, 971, 989, 1055, 1109, 1130, 1159, 1191,
1273, 1336, 1445, 1638.
3. Results and discussion
3.1. Photochromism in solution and in PMMA film
The diarylethene/PMMA film was prepared by solubilizing
ultrasonically 10 mg diarylethene 1a and 100 mg PMMA in
chloroform (1 mL), then spin-coating the homogeneous solu-
tion on a glass substrate (20 mm × 20 mm × 1 mm) with a spin
rotation speed of 1500 rpm. The thickness of the film was about
15 ␮m. The film was dried in air and kept in darkness at room
temperature.
The photochromism of diarylethene 1 is described in
Scheme 1. The two isomers 1a (open-ring isomer) and 1b
(closed-ring isomer) underwent photoisomerization upon irra-
diation with UV–vis light. Fig. 1 shows absorption spectral
changes of diarylethene 1 by photo-irradiation in hexane
(C = 2.0 × 10−5 mol/L) and in PMMA film (10%, w/w), respec-
tively. In hexane, the absorption band of colorless open-ring
isomer 1a shows absorption band only in the ultra violet region

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T. Yang et al. / Spectrochimica Acta Part A 70 (2008) 1065–1072
Table 1
Crystal data and structure refinement
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C28H17F9S2
588.54
294(1) K
˚
0.71073 A
Triclinic
¯
P1(no. 2)
◦
˚
Unit cell dimensions
a = 9.857(3) A, α = 79.81(4)
◦
˚
˚
b = 10.709(5) A, β = 72.36(4)◦
c = 12.916(4) A, γ = 82.88(4)
3
˚
Volume
1275.3(9) A
Z
2
Density (calculated)
Absorption coefficient
F(0 0 0)
1.533 g/cm3
0.292 mm−1
596
Crystal size
θ range for data collection
Limiting indices
Reflections collected
Unique reflections
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2δ(I)]
R indices (all data)
0.2 mm × 0.4 mm × 0.4 mm
θmax = 25.0◦
Fig. 2. ORTEP drawings of crystal 1a. The ellipsoids were drawn at 35% prob-
ability level, showing the atomic numbering scheme.
0 < h < 11, −12 < k < 12, −14 < l < 15
4756
4477 observed [I ≥ 2σ(I)]: 1356
Full-matrix least-squares on F2
4477/0/354
formation of the closed-ring isomer 1b. The colored PMMA
film could return to colorless upon irradiation of appropriate vis-
ible light (λ > 450 nm). The distortion of spectra in UV region
is ascribed to absorbing UV light of the glass substrate. The
coloration–decoloration cycle could be also repeated more than
100 times and it indicated no remarkable photo-degradation,
however, isosbestic point was not observed in PMMA film. From
the above description, we can easily draw a conclusion that the
maximum absorption wavelengths of both the open-ring isomer
and the closed-ring isomer in PMMA film are longer than those
in hexane solution. The red shift values are 38 nm for the open-
ring isomer and 8 nm for the closed-ring isomer. The red shift
phenomena in PMMA film in comparison with those in solution
can be ascribed to the stabilization of molecular arrangement in
solid state [19]. In addition, it was found that both open-ring iso-
mer and closed-ring isomer of the diarylethene compound were
stable both in solution and in PMMA film in darkness.
1.012
R1 = 0.1123, ωR2 = 0.1937
R1 = 0.2921, ωR2 = 0.2321
−3
˚
Largest diff. peak and hole
−0.335 to 0.330e A
shows no features. The crystal structure is shown in Fig. 2
and a packing diagram is shown in Fig. 3. The ORTEP draw-
ing of 1a showed that the molecule is packed in a photoactive
anti-parallel conformation in the crystalline phase, which can
undergo a photocyclization reaction [20]. The crystallographic
data obtained and experimental details employed were sum-
marized in Table 1. Further details on the crystal structure
investigation have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication number
CCDC 274837 for this paper. These data can be obtained free
of charge via http://www.ccdc.can.ac.uk/conts/retrieving.html
3.2. Relationship between crystal structure and
photochromic properties
Final structural conformation of diarylethene 1a was pro-
vided by X-ray crystallographic analysis on a Bruker P4
diffractometer equipped with graphite monochromatized Mo
K␣ radiation at room temperature (294 1 K). Unit cell was
obtained and refined by 70 well centered reflections with
3.3◦ < θ < 12.4◦. Data collection was monitored by three stan-
dards every 100 reflection collected. No decay was observed
except the statistic fluctuation in the range of 3.9%. Raw inten-
sities were corrected for Lorentz and polarization effects. Direct
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 A according to crite-
ria described in the SHELXTL manual. They were included
in the refinement with Uiso(H) = 1.2Ueq(C) or 1.5Ueq(methyl
C). The final full-matrix least-square refinement on F2 con-
verged with R1 = 0.1123 and ωR2 = 0.1937 for 1356 observed
reflections [I ≥ 2σ(I)]. The final difference electron density map
Fig. 3. A packing view along the a direction.

T. Yang et al. / Spectrochimica Acta Part A 70 (2008) 1065–1072
1069
(or from the Cambridge Crystallographic Centre, 12 Union
Road, Cambridge CB21EZ, UK [fax: +44 1223 336033; e-mail:
http://deposit@ccdc.cam.ac.uk]).
turned blue, and the absorption maximum was observed at
579 nm, which was the same as that of 1b. The blue color dis-
appeared upon irradiation with appropriate wavelength visible
light and the absorption spectrum of the solution containing the
colorless crystal was the same as that of 1a. This phenomenon
suggests that the target compound can undergo a photochromic
reaction and produce its closed-ring molecule in the single-
crystalline phase. We have not, so far, been able to determine the
crystal structure of 1b because of the limitations of the experi-
mental conditions: the structure of the closed-ring isomer should
be determined at a reduced temperature [22]. Comparing the
two photographs shown in Fig. 4, it clearly showed that the
photochromic process had taken place in the single-crystalline
phase upon irradiation with UV and visible light. Furthermore,
the diarylethene crystals exhibited more than 200 times of col-
oration/decoloration cycles by alternate irradiation with UV and
visible light. So, the crystals of diarylethene 1a will be good
candidates for optoelectronic applications [12].
The molecule has five planar rings, including a perfluorocy-
clopentene ring, two thiophene rings and two benzene rings. So,
they can form four dihedral angles between every two adjacent
rings. The dihedral angles between the planes of perfluorocy-
clopentene and thiophene rings are 43.8(1)◦ for S1/C6–C9 and
58.2(2)◦ for S2/C18–C21, while those between the thiophene
rings and adjacent benzene rings are 4.1(2)◦ for C10–C15 and
19.1(2)◦ for C22–C27. In the central cyclopentene ring of 1a,
distances of C1–C2, C2–C3, C3–C4, C4–C5 and C1–C5 are
˚
1.351(4), 1.528(5), 1.557(5), 1.548(5) and 1.483(5) A, respec-
tively. These data indicate that the C1–C2 bond is a double bond,
beingsignificantlyshorterthantheothersinglebonds. Twojustly
symmetrical thiophene moieties were linked by the C1 C2 dou-
ble bond, with both of them attached to the ethylene group via the
3-position. The two methyl groups located on different sides of
the double bond, reflected in the torsion angles C1–C7–C6–C17
[−4.6(6)◦] and C2–C19–C18–C28 [−10.4(6)◦], and thus trans
with respect to the double bond. Such a conformation is crucial
to the photochromic and photoinduced properties of such mate-
rials [11]. The intramolecular distance between the two reactive
3.3. Fluorescence of diarylethene 1
The fluorescence emission of 1a was carried out using a
fluorescence spectrometer F-4500, and the breadths of exci-
tation and emission slit were selected 10.0 nm. Generally, the
open-ring isomer of diarylethene emits fluorescence, while the
fluorescence of closed-ring isomer is inactive [23]. In the study,
diarylethene 1a exhibited fluorescent changes during the process
of photoisomerization as most of the diarylethene derivatives
did [24]. Fig. 5 shows the fluorescence spectral changes of
diarylethene 1a in acetonitrile (C = 2.0 × 10−5 mol/L) and in
PMMA film (10%, w/w) at room temperature upon irradia-
tion with 313 nm light. The result showed that diarylethene 1a
exhibited good fluorescence both in acetonitrile and in PMMA
film. As shown in Fig. 5(A), it can be clearly seen that the ace-
tonitrile of 1a exhibits fluorescence at 470 nm when excited at
300 nm. Solution fluorescence spectroscopy at different irradi-
ation times shows that the fluorescence intensity in the range
of 400–570 nm decreases dramatically with increasing exposal
time upon irradiation with 313 nm UV light. This phenomenon
verified that 1a has good photochromic properties in solution.
Along with the photochromism from 1a to 1b upon irradi-
ation with 313 nm light, the fluorescence intensity decreased
dramatically. The photostationary state showed almost no fluo-
rescence, indicating that the diarylethene compound underwent
an efficient photocyclization reaction in acetonitrile, as well as
revealing that the closed-ring isomers showed no fluorescence.
The original emission spectrum was recovered when the solu-
tion of the photostationary state was bleached upon irradiation
with appropriate wavelength visible light (λ > 450 nm) irradia-
tion. Similarly, diarylethene 1a exhibits fluorescence at 422 nm
when excited at 300 nm in PMMA film (10%, w/w), and its
emission intensity changes during the process of photoisomer-
ization was shown in Fig. 5(B). Upon irradiation with 313 nm
light, the emission intensity of 1a decreased remarkably along
with the photocyclization reaction from the open-ring isomer 1a
to the closed-ring isomer 1b. When arrived at photostationary
state, the emission intensity of diarylethene 1 was quenched to
˚
C atoms (C6. . .C18) is 3.614(5) A. If the molecule is fixed in an
anti-parallel mode and the distance between reacting C atoms on
˚
the aryl rings is less than 4.2 A, the molecule undergoes a photo-
cyclization reaction [21]. Therefore, the crystal can be expected
to undergo photochromism to generate 1b in the crystalline
phase. It is well known that the absorption spectrum and color
are dependent on the substituent effects and the ␲-conjugation
length in molecule. The arrangement described above was very
beneficial to form the extended ␲-conjugation. After irradiation
with UV light, the ␲-conjugation extended throughout the whole
molecule, and the UV–vis absorption spectra displayed drastic
changes resulting in displaying remarkable different color.
In fact, crystal of 1a showed photochromic reaction coin-
cident with the theoretical analysis. Their color changes upon
photo-irradiation are shown in Fig. 4. Upon irradiation with
313 nm light, the colorless crystal of 1a turned blue quickly.
When the blue crystal was dissolved in hexane, the solution
Fig. 4. Photographs of photochromic process of diarylethene 1 in the crystalline
phase.

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T. Yang et al. / Spectrochimica Acta Part A 70 (2008) 1065–1072
Fig. 6. Fluorescence emission spectra of diarylethene 1a in vari-
ous concentrations in acetonitrile and room temperature, excited at
300 nm (a: 5.0 × 10−6 mol/L; b: 1.0 × 10−5 mol/L; c: 2.0 × 10−5 mol/L;
d: 5.0 × 10−5 mol/L; e: 1.0 × 10−4 mol/L; f: 2.0 × 10−4 mol/L; g:
1.0 × 10−3 mol/L).
significantly changes and the fluorescence intensity showed a
remarkable initial increase with subsequent dramatic decrease
with increasing concentration. The result indicated that molec-
ular aggregation and fluorescence quench take place due to
concentration increasing [26].
3.4. Electrochemistry of diarylethene 1
Fig. 5. Fluorescence spectra changes of diarylethene 1a in acetonitrile and in
PMMA film at room temperature. (A) In acetonitrile (C = 2.0 × 10−5 mol/L),
excited at 300 nm and (B) in PMMA film (10%, w/w), excited at 300 nm.
Electrochemical examinations were performed in a one-
compartment cell by using a Model 263 potentiostat–galvanostat
(EG & GP Princeton Applied Research) under computer con-
trol at room temperature. The typical electrolyte was acetonitrile
(5 mL) containing 0.10 mol/L tetrabutylammonium tetraflu-
oroborate ((TBA)BF4) and 4.0 × 10−3 mol/L diarylethene.
Platinum wire (0.5 mm) and steel electrodes were used as work-
ing and counter electrodes, respectively. Platinum wire (0.5 mm)
in the supporting electrolyte solutions was used as a quasi-
reference electrode, which was calibrated through by the cyclic
voltammetry of ferrocene in similar electrolyte. Regarding the
energyleveloftheferrocene/ferrocenium(Fc)reference, onecan
calculate the LUMO and HOMO energies with the assumption
that the energy level of Fc is 4.8 eV below vacuum [27], that is,
ca. 58%, which was higher than that in acetonitrile (ca. 20%).
The incomplete cyclization reaction and the existence of parallel
conformations may be the main cause for the moderate change in
fluorescence induced by photo-irradiation in PMMA amorphous
film. It was also found that the fluorescence switching between
open-ring isomer and photostationary state performed well and
no significant fluorescence intensity change was detected after
100 cycles both in solution and in PMMA film. This charac-
teristic could be potentially applied to fluorescence switches or
optical memory with fluorescence readout method [25].
The concentration dependence of fluorescence emission
spectra was measured in hexane at room temperature, as
shown in Fig. 6. When the concentration of diarylethene 1a
in acetonitrile increased from 5.0 × 10−6 to 2.0 × 10−5 mol/L,
the maximum emission almost arose at 470 nm when excited
at 300 nm, and the relative fluorescence intensity increased
dramatically. However, when the concentration increased
from 2.0 × 10−5 to 1.0 × 10−3 mol/L, the maximum emission
appeared ca. 475 nm, and the relative fluorescence intensity
decreased remarkably. The acetonitrile solution showed no
fluorescence excited at 300 nm when the concentration was
increased to 1.0 × 10−3 mol/L. The biggest value of fluores-
cence intensity were obtained at 2.0 × 10−5 mol/L. Therefore,
the fluorescence spectra of 1a showed remarkable concentra-
tion dependence. It indicated that the fluorescence peaks had no
IP = −{[Eonset
]
ox + 4.8}, EA = −{[Eonset
]
red + 4.8}, where the
onset potentials are in volts and IP and EA are in electronvolts.
The electrochemical properties of diarylethenes are being
used for molecular switching and can also be potentially
applied to molecular-scale electronic switches [28]. In order to
investigate the electrochemical property of diarylethene 1, we
performed electrochemical examinations by cyclic voltamme-
try (CV) with the scanning rate of 50 mV/s, as shown in Fig. 7.
This compound undergoes a reversible electrochemical oxida-
tion with the potential onset at 1.21 V for 1b and 1.23 V for 1a,
and their potential onset of electrochemical reduction is −1.09
and −1.20 V, respectively. With regard to the energy level of the
ferrocene reference (4.8 V below the vacuum level), the HOMO
and LUMO energy level can be estimated. Based on the HOMO

T. Yang et al. / Spectrochimica Acta Part A 70 (2008) 1065–1072
1071
cence stwiching between open-ring isomer and photostationary
state both in acetonitrile and in PMMA film. The cyclic voltam-
metrytestsindicatedthattheelectrochemicalpropertiesbetween
the open-ring isomer and the closed-ring isomer of diarylethene
1 were significantly different from each other. In addition, the
HOMO and LUMO energy level of diarylethene 1 were calcu-
lated by the potential onset of electrochemical oxidation and
reduction.
Acknowledgements
This work was supported by the Projects of National Natural
Science Foundation of China (20564001), the Natural Science
Foundation of Jiangxi, China (0620012) and the Science Funds
of the Education Office of Jiangxi, China ([2007] 287).
Fig. 7. Cyclic voltammetry (second scan) of diarylethene 1 on a platinum wire
(diameter 5 mm) electrode in an electrolyte solution of TBA(BF4) in CH3CN
with the scanning rate of 50 mV/s.
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Table 2
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In conclusion, we have synthesized a new unsymmetrical
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crystal structure, fluorescence and electrochemistry. The com-
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