Synthesis of novel diarylethene compounds containing two imidazole bridge units and tuning of their optical properties

Article abstract of DOI:10.1016/j.dyepig.2010.09.015

A novel series of symmetrical diarylethenes incorporating two imidazole bridge units and their N-methylated derivatives have been synthesized, and the products have been characterized by means of NMR and MS. Each of the compounds displays excellent photochromism and both "turn-off" and "turn-on" fluorescence properties upon UV/vis light irradiation in solution. It has been found that the electronic properties of substituents on the benzene ring, methylation of the N-H units on the two imidazole rings and Cu²⁺ have great effects on both the photochromic property and fluorescence property of these diarylethenes. The photophysical properties of these compounds can be easily tuned by varying the electronic properties of the substituents or by simple modification of the molecular structure, which provides a new strategy for the design of novel fluorescent diarylethene systems to be used as switches.

Full text of DOI:10.1016/j.dyepig.2010.09.015

Dyes and Pigments 90 (2011) 245e252
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of novel diarylethene compounds containing two imidazole
bridge units and tuning of their optical properties
Ziyong Li, Yan Lin, Jian-Long Xia, Honglin Zhang, Fangying Fan, Qingbin Zeng,
*
*
Dan Feng, Jun Yin , Sheng Hua Liu
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, 152 Luoyu Road, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel series of symmetrical diarylethenes incorporating two imidazole bridge units and their
N-methylated derivatives have been synthesized, and the products have been characterized by means of
NMR and MS. Each of the compounds displays excellent photochromism and both “turn-off” and “turn-
on” fluorescence properties upon UV/vis light irradiation in solution. It has been found that the electronic
properties of substituents on the benzene ring, methylation of the NeH units on the two imidazole rings
and Cu2þ have great effects on both the photochromic property and fluorescence property of these
diarylethenes. The photophysical properties of these compounds can be easily tuned by varying the
electronic properties of the substituents or by simple modification of the molecular structure, which
provides a new strategy for the design of novel fluorescent diarylethene systems to be used as switches.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 28 June 2010
Received in revised form
24 September 2010
Accepted 29 September 2010
Available online 12 January 2011
Keywords:
Diarylethene
Imidazole
N-Methylation
Synthesis
Photochromism
Fluorescence
1. Introduction
As a current key area of research for photochromic dithienyle-
thenes is to develop novel structures with unique properties, an
Photochromic diarylethene compounds have attracted much
attention in recent years since the first report by Kellogg et al. [1]
and the subsequent extensive studies by Irie and co-workers
[2e6] on the photochromic properties of these systems. For prac-
tical applications, several attempts to modulate the photochromic
properties of diarylethenes have recently been reported. Tian and
co-workers [7e12] enhanced the photochromism of these mole-
cules through their coordination to various metal cations. Irie et al.
[13e15] modulated the photochromic properties of diarylethenes
using intramolecular hydrogen bonds. Huang’s group [16,17] gated
the spectral properties of diarylethenes by means of Lewis acide
base interaction between B and Fꢀ. Among the diarylethene
derivatives hitherto reported, diarylethenes with heterocyclic aryl
groups, especially those bearing two thiophene or benzothiophene
rings, are the most promising candidates by virtue of their excellent
thermally irreversible property, remarkable fatigue resistance, and
high sensitivity [3,7e22].
effective method is to modify the structure by functionalizing the
group R in Fig. 1. Many studies on the photochromic properties of
diarylethene derivatives have been focused on the substituent
on the thiophene moiety [23e36]. Imidazole is a common scaffold
and imidazole derivatives are important five-membered nitrogen-
containing heterocyclic compounds that are widely used in
many fields, such as medicine [37], ionic liquids [38], anion
sensors [39,40], as well as electrical and optical materials [41e43].
Against this background, two substituted imidazole units were
attached to the dithienylethene backbone in the hope of achieving
novel structures with excellent photochromic properties, with
a view to developing novel optical materials having potential
applications. In the work described herein, several dithienylethenes
bearing two imidazole units and their N-methylated derivatives
have been synthesized and their photochromic and fluorescence
properties have been investigated. It is demonstrated that the
photophysical properties of these compounds can be easily tuned
by introducing different substituents or by methylation of the NeH
moieties on the two imidazole rings. Simultaneously, due to
imidazole is an outstanding ligand, the effect of various metal ions
on optical properties of all the diarylethene compounds will also be
researched.
* Corresponding authors. Tel./fax: þ86 27 67867725.
E-mail addresses: yinj@mail.ccnu.edu.cn (J. Yin), chshliu@mail.ccnu.edu.cn
(S.H. Liu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.09.015

246
Z. Li et al. / Dyes and Pigments 90 (2011) 245e252
Synthesis of 3: To a solution of ammonium acetate (1.2 g,
15.2 mmol) in refluxing glacial acetic acid (15 mL) was added 1
(0.30 g, 0.95 mmol) under an argon atmosphere at 120 ꢁC, and it
was refluxed for 2 h. Benzil (0.40 g, 0.95 mmol) was added to the
reaction solution above and continue to reacted further for over-
night. The reaction mixture was allowed to cool to room temper-
ature, and then transferred to ice water (200 mL), carefully
neutralized with a 10% sodium carbonate solution to a pH of
6.5e7.0. The formed precipitate was collected and the crude
product was washed with water, the dried solid was recrystallized
from methylene chloride to give an ivory-white solid (0.48 g) in
uv
vis
S
S
S
S
R
R
R
R
ring-open isomer
ring-closed isomer
Fig. 1. Ring-opening and ring-closing photoisomerization of diarylethene.
2. Materials and methods
a yield of 73%. 1H NMR (400 MHz, DMSO-d6):
d 1.89 (s, 6H, CH3),
2.04e2.08 (m, 2H, CH2), 2.82 (t, J ¼ 7.2 Hz, 4H, CH2), 7.26e7.48 (m,
2.1. Experimental
22H, AreH), 12.7 (s, 2H, NeH). MS (m/z): 696 [M]þ.
Synthesis of 4: Compound 4 was prepared by an analogous
method similar to that used for to 3 and was obtained as an ivory-
white solid (0.46 g) in a yield 64%. 1H NMR (600 MHz, DMSO-d6):
General: All manipulations were carried out under an argon
atmosphere by using standard Schlenk techniques, unless otherwise
stated. DMF was dried with magnesum sulfate then distilled under
vacuum. 1,2-Bis(5-formyl-2-methylthien-3-yl)cyclopentene (1) was
prepared by literature methods [44]. Different substitutional benzil
(2) except benzil were prepared by modified procedures of reported
methods [45]. All other starting materials were obtained commer-
cially as analytical-grade and used without further purification. The
relative quantum yields were determined by comparing the reaction
yield with the known yield of the compound 1,2-bis(2-methyl-5-
phenyl-3-thienyl)perfluorocyclopentene [46]. 1H and 13C NMR
spectra were collected on American Varian Mercury Plus 400 spec-
trometer (400 MHz) or 600 MHz.1H and 13C NMR chemical shifts are
relative to TMS. UVeVis spectra were obtained on U-3310 UV Spec-
trophotometer. Fluorescence spectra were taken on a Fluoromax-P
luminescence spectrometer (HORIBA JOBIN YVON INC.).
d
1.89 (s, 6H, CH3), 2.05e2.09 (m, 2H, CH2), 2.27 (s, 6H, CH3), 2.33
(s, 6H, CH3), 2.83 (t, J ¼ 6.6 Hz, 4H, CH2), 7.09 (d, J ¼ 7.8 Hz, 4H,
AreH), 7.22 (d, J ¼ 7.2 Hz, 4H, AreH), 7.33e7.37 (m, 8H, AreH), 7.49
(s, 2H, thiophene-H), 12.56 (s, 2H, NeH). MS (m/z): 753 [M]þ.
Synthesis of 5: Compound 5 was prepared by an analogous
method to 3 and obtained as an ivory-white solid (0.71 g) in a yield
91%. 1H NMR (400 MHz, DMSO-d6): 1H NMR (400 MHz, DMSO-d6):
d
1.89 (s, 6H, CH3), 2.05e2.09 (m, 2H, CH2), 2.83 (t, J ¼ 7.2 Hz, 4H,
CH2), 3.34 (s, 6H, OCH3), 3.36 (s, 6H, OCH3), 6.86 (d, J ¼ 8.4 Hz, 4H,
AreH), 6.99 (d, J ¼ 8.4 Hz, 4H, AreH), 7.38 (t, 8H, AreH), 7.47 (s, 2H,
thiophene-H), 12.50 (s, 2H, NeH). MS (m/z): 816 [M]þ.
Synthesis of 6: Compound 6 was prepared by an analogous
method to 3 and purified on a silica gel column with ethyl acetate/
petroleum ether (1:1) to obtain a yellow-blue solid (0.52 g) in
a yield 63%. 1H NMR (400 MHz, DMSO-d6):
d 1.88 (s, 6H, CH3),
2.04e2.08 (m, 2H, CH2), 2.82 (t, J ¼ 7.2 Hz, 4H, CH2), 6.69e7.45 (m,
2.2. Synthesis
18H, AreH), 12.30 (s, 2H, NeH). MS (m/z): 869 [M]þ.
2.2.1. Synthesis of dithienylethene with two imidazole units 3e6
Target compounds 3e6 were prepared according to the synthetic
route presented in Fig. 2 by modified procedures of reported
methods [47].
2.2.2. Synthesis of N-methylation derivatives 7e9
Target compounds 7e9 were prepared according to the
synthetic route presented in Fig. 2.
R
R
N
N
OHC
R
CHO
R
S
S
S
S
NH
HN
1
NH4OAc
+
HOAc
refluxing
R
R
O O
C C
3
4
5
6
2
R=H, CH3, OCH3, N(CH3)2
R
R
N
N
CH3I
S
S
N
N
K2CO3,DMF
r.t.
R
R
7
8
9
R=H, CH3, OCH3
Fig. 2. Synthesis of dithienylethene with two imidazole units 3e6 and their derivatives 7e9.

Z. Li et al. / Dyes and Pigments 90 (2011) 245e252
247
Table 1
(t, J ¼ 7.2 Hz, 4H, CH2), 3.47 (s, 6H, NeCH3), 7.09 (s, 2H, thiophene-
H), 7.03e7.47 (m, 20H, AreH). MS (m/z): 724 [M]þ.
Crystal data and structure refinement parameters for 7.
Synthesis of 8: Compound 8 was prepared by an analogous
7
method to 7 and obtained as a grey solid (0.25 g) in a yield 64%. 1H
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
a (Å)
C47H40N4S2
724.95
298 (2) K
0.71073 Å
Monoclinic
C2/c
9.6777 (13)
16.041 (2)
24.854 (3)
NMR (400 MHz, CDCl3): d 2.12 (s, 6H, CH3), 2.00e2.12 (m, 2H, CH2),
2.27 (s, 6H, CH3), 2.40 (s, 6H, CH3), 2.86 (t, J ¼ 6.4 Hz, 4H, CH2), 3.44
(s, 6H, NeCH3), 6.99 (d, J ¼ 7.2 Hz, 4H, AreH), 7.06 (s, 2H, thio-
phene-H), 7.23 (d, J ¼ 13.6 Hz, 4H, AreH), 7.37 (d, J ¼ 7.2 Hz, 4H,
AreH). 13C NMR (100 MHz, CDCl3):
d 14.32 (s, CH3), 21.13 (s, CH3),
b (Å)
c (Å)
21.35 (s, CH3), 23.03 (s, CH2), 32.78 (s, CH2), 38.17 (s, NeCH3),
126.76, 127.30, 127.84, 128.66, 128.88, 129.64, 130.02, 130.69, 131.64,
134.78,135.69, 136.02,137.60, 138.29, 141.77 (s, thiophene, ethene,
Ar). MS (m/z): 780 [M]þ.
a
b
g
(deg)
(deg)
(deg)
90
99.304 (3)
90
3807.5 (9)
Volume (Åꢀ3
)
Synthesis of 9: Compound 9 was prepared by an analogous
method to 7 and obtained as a grey solid (0.27 g) in a yield 65%. 1H
Z
4
Density (calculated)
Absorption coefficient
F (000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta ¼ 26.00
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
1.265 Mg/m3
NMR (400 MHz, CDCl3):
d 2.12 (s, 6H, CH3), 2.10e2.14 (m, 2H, CH2),
0.179 mmꢀ1
2.85 (t, J ¼ 7.2 Hz, 4H, CH2), 3.43 (s, 6H, OCH3), 3.74 (s, 6H, NeCH3),
3.84 (s, 6H, NeCH3), 6.74 (d, J ¼ 8.8 Hz, 4H, AreH), 6.95 (d, J ¼ 8.8 Hz,
4H, AreH), 7.05 (s, 2H, thiophene-H), 7.24 (d, J ¼ 8.8 Hz, 4H, AreH),
1528
0.12 ꢂ 0.10 ꢂ 0.02 mm3
2.48e26.00
7.41 (d, J ¼ 8.8 Hz, 4H, AreH). 13C NMR (100 MHz, CDCl3):
d 14.29
ꢀ11 ꢃ h ꢃ 11, ꢀ19 ꢃ k ꢃ 19, ꢀ30 ꢃ l ꢃ 30
19896
(s, CH3), 23.01 (s, CH2), 32.68 (s, CH2), 38.12 (s, OCH3), 55.05
(s, NeCH3), 55.20 (s, NeCH3), 113.36, 114.35, 122.94, 127.18, 127.29,
127.95, 128.93, 129.23, 132.13, 134.77, 135.93, 136.00, 137.33, 141.55,
158.03, 159.58 (s, thiophene, ethene, Ar). MS (m/z): 844 [M]þ.
3755 [R (int) ¼ 0.0928]
100.0%
None
0.9971 and 0.9788
Full-matrix least-squares on F2
3755/0/242
2.3. Source of metal ions and preparation of mixtures containing
metal ions and target ligands in DMF
1.045
R1 ¼ 0.0616, wR2 ¼ 0.1289
R1 ¼ 0.1084, wR2 ¼ 0.1425
0.279 and ꢀ0.225 eꢀ3
Largest diff. peak and hole
All metal ions for binding experiments used acetate salts as
sources except for Ca2þ, Kþ, Fe3þ and Cd2þ, which were used as
CaCl2, KCl, FeCl3, and CdCl2 as sources. Metal ions in water were
obtained by dissolution of the metal salts in water. Metal ions in
water were added to the ligand solution in DMF by syringe, and
then mixed round under dark conditions for overnight at room
temperature.
Synthesis of 7: To a solution of 3 (0.35 g, 0.5 mmol) in N,N-
dimethylformamide (10 mL) in the presence of potassium carbo-
mate (0.28 g, 2 mmol) was added methyl iodide(0.12 mL, 2 mmol)
under dark conditions for 12 h at room temperature. The reaction
mixture was allowed to cool, transferred to about 100 mL of water.
The formed precipitate was collected and the crude product was
washed with water, the dried solid was redissolved in DCM. The
solution was dried over sodium sulfate, then removed solvent
under reduced pressure and purified on a silica gel column using
petroleum ether / ethyl acetate(1:1) as the eluent to obtain the
target compound as a grey solid (0.5g) in a yield of 42%. 1H NMR
2.4. Crystallographic details
Crystals suitable for X-ray diffraction were grown from
a dichloromethane of solution 7 layered with hexane. A crystal with
approximate dimensions of 0.12 ꢂ 0.10 ꢂ 0.02 mm3 for 7 was
mounted on a glass fiber for diffraction experiment. Intensity data
(400 MHz, CDCl3):
d
2.13 (s, 6H, CH3), 2.05e2.13 (m, 2H, CH2), 2.87
were collected on a Nonius Kappa CCD diffractometer with Mo Ka
Fig. 3. Molecular structures of 7. The H atoms have been omited for clarity.

248
Z. Li et al. / Dyes and Pigments 90 (2011) 245e252
R1
R1
R1
R1
N
N
N
N
N
N
N
N
S
uv
S
S
S
R2
R2
R2
R2
vis
R1
R1
R1
R1
R2=H, CH3
R2=H, CH3
Fig. 4. Photochromism of diarylethenes 3e9.
radiation (0.71073 Å) at 298 K. The structures were solved by
a combination of direct methods (SHELXS-97) and Fourier differ-
ence techniques and refined by full-matrix least squares (SHELXL-
97). All non-H atoms were refined anisotropically. The hydrogen
atoms were placed in the ideal positions and refined as riding
atoms. Crystallographic data for the structure in this paper have
been deposited with the Cambridge Crystallographic Data Centre as
supplemental publication CCDC 781133. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: 544 0 1223 336 033 or e-mail:
deposit@ccdc.cam.ac.uk).
The molecular structures of 7 is depicted in Fig. 3. The diarylethene
7 crystallizes with an approximate C2 symmetry with the photo-
active antiparallel conformation in the crystalline phase, which can
undergo photocyclization [48]. The molecule includes five kinds of
planar rings. The dihedral angle between the cyclopentene ring and
the thiophene ring is 46.02ꢁ for S1/C4eC7, and that between the
thiophene ring and the linked imidazole ring is 19.97ꢁ, and those
between the imidazole ring and two linked benzene rings are
69.92ꢁ for C13eC18 and 33.58ꢁ for C19eC24. The intramolecular
distance between the two reactive carbons (C5eC5a) is 3.559 Å,
which is short enough for the cyclization reaction to take place as
photochromic reactivity usually appears when the distance
between the reactive carbon atoms is less than 4.2 Å [49].
3. Results and discussion
3.1. Discussion of the synthetic strategy
3.3. Photochromic behavior
The target dithienylethenes with two imidazole units 3e6 were
obtained by condensation reactions of 1,2-bis(5-formyl-2-methyl-
thien-3-yl)cyclopentene (1) and differently substituted benzils (2) in
the presence of excess ammonium acetate in moderate to good yields
of 63e91% (Fig. 2). N-Methylation was achieved by treatment of 3e5
with iodomethane in the presence of potassium carbonate to give
7e9 in moderate yields of 42e64%. It was found that a 2:1 ratio of
iodomethane to imidazoles 3e5 was needed because a greaterexcess
of MeI could have led to the formation of salts, thereby reducing the
yield of methylation products. Unfortunately, N-methylation of 6
was not successful, presumably because N-methylation of the N,
N-dimethyl moiety is much easier than that of imidazole, resulting in
the formation of a water-soluble quaternary ammonium salt. All of
the target compounds were characterized by NMR and MS.
The photochromic behavior of diarylethenes 3e9 induced by
photoirradiation in DMF was measured at room temperature. The
synthesized diarylethenes undergo photoisomerization between
3oe9o (ring-open isomer) and 3ce9c (ring-closed isomer) upon
alternating irradiation with UV light (
l
¼ 302 nm) and visible light
(l
>402 nm), as illustrated in Fig. 4. As shown in Fig. 5A, the
absorption maximum of compound 4 in DMF was observed at
336 nm (
3
¼ 5.23 ꢂ104 L molꢀ1 cmꢀ1) as a result of a
pep* transition
[50]. This colorless solution turned purple and a new absorption
band centered at 552 nm (
when it was irradiated with 302 nm UV light, as a result of a ring-
closure reaction to give the ring-closed isomer 4c. In addition,
a well-defined isosbestic point was observed at 361 nm. Upon
3
¼ 3.27 ꢂ 104 L molꢀ1 cmꢀ1) appeared
irradiation with visible light (l >402 nm), the colored 4c underwent
a cycloreversion reaction to the initial colorless ring-opened isomer
4o. Similar results were obtained when solutions of the other dia-
rylethenes 3, 5e9 in DMF were irradiated with UV/vis light.
The photochromic parameters of diarylethenes 3e9 are
summarized in Table 2. These data show that different substituent
3.2. X-ray structures of 7
The molecular structures of 7 was determined by X-ray crys-
tallography. The crystallographic details are given in Table 1.
A
B
UV
VIS
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
vis
uv
uv
vis
vis
uv
300
350
400
450
500
550
600
650
700
750
800
300
350
400
450
500
550
600
650
700
750
800
Wavelength/nm
Wavelength/nm
Fig. 5. Absorption spectral changes of diarylethene 4 and 8 by photoirradiation in DMF (2.0 ꢂ 10ꢀ5 mol/L) at room temperature. (A) spectral changes for 4; (B) spectral changes for 8.

Z. Li et al. / Dyes and Pigments 90 (2011) 245e252
249
Table 2
Table 3
Absorption characteristics and photochromic quantum yields of 3e9 in DMF
1H NMR chemical shifts of opened and closed-ring isomers for 3e6.
(2.0 ꢂ 10ꢀ5 mol/L).
Compounds
dMeo
dMec
dTho
dThc
Conversion
Compound
lAmbasx/nma
lAmbasx/nmb
Fc
3
4
5
6
1.91
1.90
1.86
1.89
1.96
2.02
2.03
1.99
7.51
7.48
7.46
7.45
6.83
6.83
6.81
6.78
65%
72%
72%
45%
(3
ꢂ 10ꢀ4
)
(
3
ꢂ 10ꢀ4
)
(Open)
(PSS)
foec
(
l/nm)
fceo (l/nm)
3
4
5
6
7
8
9
334 (4.86)
336 (5.23)
340 (4.99)
320 (5.39)
296 (2.93)
296 (2.90)
292 (3.86)
550 (2.71)
552 (3.27)
554 (2.79)
568 (3.43)
542 (1.56)
542 (2.26)
544 (2.21)
0.559 (550)
0.375 (552)
0.510 (554)
0.411 (568)
0.580 (542)
0.675 (542)
0.726 (544)
0.0014 (334)
0.0011 (336)
0.00073 (256)
0.00034 (276)
0.0076 (296)
0.0055 (296)
0.0042 (292)
signal of the protons attached to the thiophene rings appeared
at ¼ 6.83 ppm for 4c. Thus the chemical
¼ 7.48 ppm for 4o and
d
d
shift of the protons on the thiophene rings appeared at higher field
compared to that in the case of 4o, while the chemical shift of the
methyl protons appeared at lower field. Similar shift changes have
also been observed in the 1H NMR spectra of other diarylethene
compounds 3, 5, and 6. The photocyclization yields of diarylethenes
3e6 was from 45% to 72% according to 1H NMR analysis. Unfortu-
nately, 1H NMR Chemical Shifts and photocyclization yields of
Opened and Closed-Ring Isomers for their N-methylated deriva-
tives 7e9 cannot be obtained.
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring (fceo) and closed-ring isomers (foec), respectively.
b
c
on the benzene ring and N-methylation of the imidazole ring have
a remarkable effect on the photochromic properties of these dia-
rylethenes, including the absorption maxima and quantum yields
of the cyclization and cycloreversion reactions. For diarylethenes 6,
the absorption maxima of in the visible region of the ring-closed
isomers are red-shifted compared with those of 3e5. However, the
absorption maxima of the ring-closed isomers of 7e9 are blue-
shifted compared with those of 3e5, which may be attributed to
a reduction in the degree of conjugation after N-methylation of the
imidazole rings. As shown in Table 2, the cyclization quantum
yields of diarylethenes 3e9 are much higher than their respective
cycloreversion quantum yields. The quantum yields of cyclization
and cycloreversion reactions of diarylethenes 3e5 are much lower
than those of diarylethenes 7e9. This indicates that N-methylation
of the two imidazole rings improves the quantum yields of cycli-
zation and cycloreversion.
3.4. Fluorescence of the diarylethenes
The fluorescence change of diarylethenes 3e9 induced by
photoirradiation in DMF was investigated at room temperature. As
shown in Fig. 7A, upon excitation of 5 in DMF solution at an exci-
tation wavelength of 335 nm, it exhibited emission at 413 nm, and
a small fluorescence quantum yield (ff ¼ 0.027) was measured
using quinoline sulfate (ff ¼ 0.55, in 0.1 M aqueous H2SO4) as
a reference. The emission intensity of diarylethene 5 was rapidly
decreased upon irradiation with 302 nm UV light. On reaching the
photostationary state, the emission intensity of 5 was quenched by
The photochromic behavior of the diarylethenes 3e6 was
examined by means of NMR spectrometry. The ratios between the
closed and open isomers at the photostationary state were
measured from the 1H NMR spectra. The characteristic shifts of
their Me and thiophene groups were observed in the 1H NMR
spectra. Irradiation with UV light caused the appearance of a new
set of singlets with the concomitant disappearance of the singlets of
the original state. As shown in Fig. 6 and Table 3, the 1H NMR signal
of the methyl groups attached to the thiophene rings appeared
ca. 91%. Irradiation with visible light (l >402 nm) regenerated the
ring-opened isomer and restored the original emission spectrum.
Similar results were obtained when solutions of the other diary-
lethenes 3, 4, and 6e9 in DMF were irradiated with UV/vis light.
The corresponding fluorescence data of the other compounds are
summarized in Table 4.
As shown in Table 4, Fig. 7C and D, different substituent on the
benzene ring and N-methylation of the imidazole ring obviously
affect the fluorescence property of these diarylethenes. As shown in
Fig. 7C and Table 4, the emission bands of the ring-open isomers
at
d
¼ 1.90 ppm for 4o and
d
¼ 2.02 ppm for 4c, and the 1H NMR
Fig. 6. 1H NMR spectral changes of diarylethene 4 upon 302 nm light irradiation (observed in DMSO-d6). (methyl and thiophene signal).

250
Z. Li et al. / Dyes and Pigments 90 (2011) 245e252
uv
vis
500
700
600
500
400
300
200
100
0
B
A
uv
vis
400
300
200
100
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength/nm
Wavelength/nm
(3)
(4)
(5)
H
CH3
OCH3
700
600
500
400
300
200
100
0
D
C
800
700
600
500
400
300
200
100
0
N(CH3)2 (6)
5
(N-methylation)
9
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength/nm
Wavelength/nm
Fig. 7. Emission intensity changes of partial diarylethenes in DMF (2.0 ꢂ 10ꢀ5 mol/L) with UV/vis light irradiation (lex ¼ 335 nm). (A) spectral changes for 5; (B) spectral changes for
9; (C) fluorescence intensity of 3e6 before UV irradiation; (D) fluorescence intensity of 5 and 9 before UV irradiation.
exhibit significant red-shifts with increasing electron-donating
ability of the substituents, and it was found that the fluorescence
intensity was enhanced so that the fluorescence quantum yields of
diarylethenes 3e6 gradually increased. However, the fluorescence
of 9 was dramatically reduced upon N-methylation (Fig. 7D), which
is in agreement with a previous literature report [51]. This may be
mainly because the introduction of a methyl group at the N-posi-
tion of the imidazole ring distorts the relative orientation of the
thiophene and imidazole rings. The same effect was observed for
the other N-methylated products.
at room temperature. It was found that Cu2þ has an obvious effect
on the optical properties of diarylethenes 3e6, but no, or a very
small, change in absorbance and emission was detected by binding
with other metal ions (Kþ, Agþ, Ca2þ, Cu2þ, Mn2þ, Zn2þ, Co2þ, Ni2þ
Cd2þ, Hg2þ and Fe3þ) under the same conditions.
,
For diarylethenes 4, as shown in Figs. 5A and 8, the absorption
maxima of the ring-open isomer of diarylethene 4 in the presence
of Cu2þ (4 eq) was blue-shifted (26 nm) compared with that of
free 4, while its absorbance maxima of ring-closed isomer was
3.5. Efect of metal ions on the optical properties
1.0
0.8
Both absorbance and fluorescence change of diarylethenes 3e9
induced by addition of various metal ions in DMF was investigated
Vis
UV
0.6
0.4
0.2
0.0
Table 4
Fluorescence data of diarylethene 3e9 in DMF (lex ¼ 335 nm).
a
Compound
lem (nm)
ff
Rate of quenching (%)
3
4
5
6
7
8
9
400
403
413
462
399
401
412
0.018
0.019
0.027
0.032
0.011
0.011
0.020
89
90
91
93
74
90
94
300
400
500
600
700
800
Wavelength / nm
a
Fluorescence quantum yields of diarylethene 3e9 before uv irradiation.
Fig. 8. Absorption changes of 4 in DMF binding with Cu2þ (4 eq).

Z. Li et al. / Dyes and Pigments 90 (2011) 245e252
251
derivatives 7e9. It may be because NeH of imidazole participated
in coordinating to Cu2þ
.
0.8
0.6
0.4
0.2
0.0
0-10eq
4. Conclusions
Seven symmetrical diarylethenes incorporating two imidazole
bridge units have been synthesized. Their photochromic and fluo-
rescence properties have been investigated. It has been demon-
strated that the electronic properties of the substituent on the
benzene ring, methylation of the NeH units on the two imidazole
rings and Cu2þ have great effects on the photochromic property and
fluorescence property of these diarylethenes.
Acknowledgments
300
400
500
600
700
800
900
Wavelength / nm
The authors acknowledge financial support from National
Natural Science Foundation of China (Nos. 2072039, 20931006,
21072070) and Program for Changjiang Scholars and Innovative
Research Team in University (No. IRT0953).
Fig. 9. UVeVis absorption spectra of compound 4 (2 ꢂ 10ꢀ5 M) in DMF upon the
addition of various amounts of Cu2þ (0, 0.5, 1, 2, 4, 10 eq) in PSS by light of 302 nm.
Insert figure: relationship between the changes of the absorption maxima in the visible
region and time of photocyclization by irradiation with 302 nm.
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