Imidazole-based dithienylethenes: Synthesis, photochromism, and effects of metal ions

Article abstract of DOI:10.1080/15421406.2012.762497

A series of imidazole-bridging dithienylethenes was synthesized by the condensation of thiophene-based diketone and methoxyl-substituted benzaldehyde. Their UV-Vis absorption and fluorescent spectra were investigated before and after irradiation with UV light. It is worth mentioning that the metal ions such as Cd²⁺, Cu²⁺, Fe³⁺ can obviously enhance the intensity of emission spectra.

Full text of DOI:10.1080/15421406.2012.762497

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Imidazole-Based Dithienylethenes:
Synthesis, Photochromism, and Effects
of Metal Ions
C. Zhang a , Z. Chen a , C. Jiang a , Guang-Ao Yu a , S. H. Liu a & J.
Yin a
a Key Laboratory of Pesticide and Chemical Biology, Ministry of
Education, College of Chemistry , Central China Normal University ,
Wuhan , PR China
Published online: 22 Apr 2013.
To cite this article: C. Zhang , Z. Chen , C. Jiang , Guang-Ao Yu , S. H. Liu & J. Yin (2013): Imidazole-
Based Dithienylethenes: Synthesis, Photochromism, and Effects of Metal Ions, Molecular Crystals and
Liquid Crystals, 575:1, 1-7
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Mol. Cryst. Liq. Cryst., Vol. 575: pp. 1–7, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.762497
Imidazole-Based Dithienylethenes: Synthesis,
Photochromism, and Effects of Metal Ions
C. ZHANG, Z. CHEN, C. JIANG, GUANG-AO YU, S. H. LIU∗,
AND J. YIN∗
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education,
College of Chemistry, Central China Normal University, Wuhan, PR China
A series of imidazole-bridging dithienylethenes was synthesized by the condensation
of thiophene-based diketone and methoxyl-substituted benzaldehyde. Their UV–Vis ab-
sorption and fluorescent spectra were investigated before and after irradiation with UV
light. It is worth mentioning that the metal ions such as Cd2+, Cu2+, and Fe3+ can
obviously enhance the intensity of emission spectra.
Keywords Dithienylethene; effects of metal cations; imidazole unit; photochromism
Introduction
Dithienylethene is considered to be among the most promising systems for application in
the field of optical memory materials and devices, because of their excellent switchable
ability, thermal stability, fatigue resistance, and sensitivity [1]. Two isomers (ring-open and
ring-closed isomers) of dithienylethene can reversibly be transformed upon irradiation with
UV and visual light, as shown in Scheme 1. Cyclopentene has been extensively applied
to dithienylethene and derivatives, as a connecting bridge between thiophenes. In order to
explore the novel system of dithienylethene, some other bridge units such as ethane, maleic
anhydride, maleimide, and thiophene have been used in the dithienylethenes [1]. Recently,
chemists realized that the N-hetero imidazole can also be utilized as the connecting bridge
in the dithienylethene system. For example, Huang and co-workers [2], Liu and Chen
[3], Yam et al. [4] have reported imidazole-based dithienylethenes, and have confirmed
that some dithienylethenes containing imidazole units possessed excellent photochromic
properties.
In our previous works imidazole, as building block, was also introduced to the R sites
(Scheme 1) of dithienylethene, and imidazole-based dithienylethenes revealed excellent
photo switchable properties [5]. Furthermore, we also reported a series of imidazole-
bridged dithienylethenes and investigated the effect of substituted groups on the imidazole
ring [6]. Recently, we found that the open-ring and closed-ring isomers of imidazole-based
dithienylethenes displayed high selectivity toward Fe3+, and the addition of Fe3+ obvi-
ously suppressed their fluorescence intensity when imidazole unit as connecting bridge
was introduced to the 2-site of thiophene [7]. In view of the obvious difference of this
∗Address correspondence to S. H. Liu and J. Yin, Key Laboratory of Pesticide and Chemical
Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan
430079, PR China. E-mail: chshliu@mail.ccnu.edu.cn and yinj@mail.ccnu.edu.cn
1

2
C. Zhang et al.
Scheme 1. Photochromic reactions of dithienylethene.
dithienylethene, herein, we further introduce the imidazole bridge to the 3-site of thiophene
and synthesize a series of imidazole-based dithienylethenes, and investigate their UV–Vis
absorption and fluorescence spectra before and after irradiation with UV light. Most inter-
estingly, the metal ions such as Cd2+, Cu2+, Fe3+ can obviously enhance the intensity of
emission spectra.
Experimental
General
All experiments were carried out under an argon atmosphere by using standard Schlenk
techniques, unless otherwise stated. DMF was dried with magnesium sulfate was then
distilled under vacuum. Tetrahydrofuran (THF) was dried with sodium wire and refluxed
with benzophenone as indicator, then distilled until the solution turned blue. The 1,2-bis(5-
cholro-2-methylthiophen-3-yl)ethane-1,2-dione (1) was prepared by literature methods [8].
All other starting materials were obtained commercially as analytical grade and used without
further purification. 1H and 13C NMR spectra were collected on American Varian Mercury
Plus 400 spectrometer (400 MHz) or 600 MHz. 1H and 13C NMR chemical shifts are relative
to tetramethylsilane (TMS). Elemental analysis was performed on an Elementar Vario
MICRO instrument. UV–Vis spectra were obtained on U-3310 UV Spectrophotometer.
Fluorescence spectra were taken on a Fluoromax-P luminescence spectrometer (Horiba
Jobin Yvon, Inc.).
Synthesis of Dithienylethenes 3a–c
Aldehyde 2 (1.0 mmol) was added to a solution of ammonium acetate (0.8 mmol), refluxing
glacial acetic acid (15 mL), under an argon atmosphere at 120◦C, and it was refluxed for 2 h.
The 1,2-bis(5-cholro-2-methylthiophen-3-yl)ethane-1,2-dione (1) (1.0 mmol) was added to
the reaction solution above, which continued to react further overnight. The reaction mixture
was allowed to cool down to room temperature, and then transferred to ice water (50 mL),
which was carefully neutralized with 10% sodium carbonate solution to a pH of 6.5–7.0.
The formed precipitate was collected and the crude product was washed with water; the
dried solid was dissolved in dichloromethene (DCM). Then dried with sodium sulfate, upon
removal of solvent under reduced pressure, the solution was purified on a silica gel column
using petroleum ether/ethyl acetate (4:1) as the eluent to obtain the target compounds as
white powder in yields of 69%–80%.
1
Compound 3b. H NMR (400 MHz, DMSO): δ = 2.02 (s, 3H, CH3), 2.23 (s, 3H, CH3), 3.82
(s, 3H, OCH3), 6.83, 7.12 (s, 1H, thiophene-H), 6.94 (m, 1H, Ar-H), 7.38 (t, J = 8.0 Hz, 1H,
Ar-H), 7.61 (t, J = 8.0 Hz, 1H, Ar-H), 7.61 (s, 1H, Ar-H), 12.72 (s, 1H, -NH ). 13C NMR

Imidazoled-Dithienylethene
3
(100 MHz, DMSO): δ 13.53 (s, CH3), 13.88 (s, CH3), 55.15 (s, O-CH3), 111.14, 114.30,
117.48, 122.83, 123.49, 124.33, 127.55, 128.35, 129.85, 131.44, 131.83, 133.53, 134.13,
135.74, 145.49, 159.55 (s, thiophene, ethene, Ar). Anal. Calcd. for C20H16Cl2N2OS2: C,
55.17; H, 3.70; N, 6.43. Found: C, 55.27; H, 3.61; N, 6.46.
Synthesis of Dithienylethenes 4a–c
Methyl iodide was added (1.8 mmol) to a solution of 3 (0.9 mmol), in N,N-
dimethylformamide (10 mL), in the presence of potassium carbonate (1.8 mmol) under
dark conditions for 12 h at room temperature. The reaction mixture was allowed to cool,
and transferred to about 50 mL of water. The formed precipitate was collected and the crude
product was washed with water; the dried solid was redissolved in DCM. Then dried with
sodium sulfate, upon removal of solvent under reduced pressure, the solution was purified
on a silica gel column using petroleum ether/ethyl acetate (5:1) as the eluent to obtain the
target compound as a white solid in yields of 36%–41%.
1
Compound 4a. H NMR (400 MHz, CDCl3): δ = 2.05 (s, 3H, CH3), 2.25 (s, 3H, CH3), 3.32
(s, 3H, N-CH3), 3.85 (s, 3H, OCH3), 6.70, 6.80 (s, 1H, thiophene-H), 6.99 (d, J = 8.4 Hz,
1H, Ar-H), 7.07 (t, J = 7.2 Hz, 1H, Ar-H), 7.43 (t, J = 7.2 Hz, 1H, Ar-H), 7.57 (d, J =
8.4 Hz, 1H, Ar-H). 13C NMR (100 MHz, CDCl3): δ 13.82 (s, CH3), 14.25(s, CH3), 32.14 (s,
N-CH3), 55.53 (s, O-CH3), 110.97, 119.93, 120.94, 123.85, 124.59, 126.51, 126.72, 127.39,
130.93, 131.14, 132.40, 133.81, 135.09, 138.07, 146.03, 157.27 (s, thiophene, ethene, Ar).
MS (m/z): 448 [M]+. Anal. Calcd. for C21H18Cl2N2OS2: C, 56.12; H, 4.04; N, 6.23. Found:
C, 56.29; H, 3.89; N, 6.25.
1
Compound 4b. H NMR (600 MHz, CDCl3)−δ = 2.04 (s, 3H, CH3), 2.27 (s, 3H, CH3),
3.56 (s, 3H, N-CH3), 3.89 (s, 3H, OCH3), 6.69, 6.79 (s, 1H, thiophene-H), 6.99 (d, J =
8.4 Hz, 1H, Ar-H), 7.28 (d, J = 12.6 Hz, 2H, Ar-H), 7.40 (t, J = 7.8 Hz, 1H, Ar-H). 13C
NMR (150 MHz, CDCl3): δ 13.83 (s, CH3), 14.21(s, CH3), 33.09 (s, N-CH3), 55.32 (s,
O-CH3), 114.25, 114.73, 120.95, 124.76, 124.84, 126.26, 126.78, 127.14, 127.26, 129.49,
130.69, 131.56, 134.21, 135.10, 138.43, 147.78, 159.62 (s, thiophene, ethene, Ar). MS
(m/z): 448 [M]+. Anal. Calcd. for C21H18Cl2N2OS2: C, 56.12; H, 4.04; N, 6.23. Found: C,
56.22; H, 3.88; N, 6.35.
1
Compound 4c. H NMR (400 MHz, CDCl3): δ = 2.03 (s, 3H, CH3), 2.26 (s, 3H, CH3), 3.55
(s, 3H, N-CH3), 3.87 (s, 3H, OCH3), 6.71, 6.79 (s, 1H, thiophene-H), 7.03 (d, J = 8.4 Hz,
2H, Ar-H), 7.69 (d, J = 8.4 Hz, 2H, Ar-H). 13C NMR (100 MHz, CDCl3): δ 13.86 (s,
CH3), 14.24(s, CH3), 33.06 (s, N-CH3), 55.30 (s, O-CH3), 113.98, 122.75, 124.36, 124.84,
126.40, 126.74, 127.20, 127.29, 130.15, 130.76, 134.15, 134.84, 138.33, 147.94, 160.09 (s,
thiophene, ethene, Ar). MS (m/z): 448 [M]+. Anal. Calcd. for C21H18Cl2N2OS2: C, 56.12;
H, 4.04; N, 6.23. Found: C, 56.25; H, 3.91; N, 6.24.
Results and Discussion
Imidazole-based dithienylethenes were synthesized according to the following synthetic
route. As outlined in Scheme 2, compounds 3a–c were obtained in 69%–80% of yields
by the condensation of thiophene-based diketone 1 and substituted benzaldehyde 2 in the
presence of ammonium acetate in acetic acid under nitrogen atmosphere. In this case, it
was worth mentioning that dithienylethenes 3a and 3c have been reported by Liu and

4
C. Zhang et al.
Scheme 2. Synthetic route of imidazole-based dithienylethenes.
Chen [3]. Subsequently, N-methylation of imidazole was performed with CH3I in the
presence of K2CO3 in DMF under nitrogen atmosphere to afford compounds 4a–c. All of
the imidazole-based dithienylethenes were characterized by standard analytical techniques
such as 1H NMR, 13C NMR, and elemental analysis.
These dithienylethenes displayed excellent solubility in the common solvents. Sub-
sequently, the UV–Vis absorption spectra and fluorescent spectra of the dithienylethenes
were measured at room temperature in acetonitrile. As described in Fig. 1(A), a new ab-
sorption band in the region of visual light was observed, which indicated that compound
0.7
(B)
(A)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
3b
4c
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Vis
UV
Vis
UV
Vis
UV
Vis
UV
300
400
500
600
300
400
500
600
700
Wavelength / (nm)
Wavelength / (nm)
90
80
70
60
50
40
30
20
10
0
20
(C)
(D)
UV
UV
4a
3a
18
16
14
12
10
8
6
4
2
0
350
400
450
500
550
600
350
400
450
500
Wavelength / (nm)
Wavelength / (nm)
Figure 1. Absorption spectral changes of dithienylethenes by photoirradiation in acetonitrile (2.0 ×
10−5 mol·L−1) at room temperature. (A) 3b and (B) 4c. Emission intensity changes of partial
dithienylethenes in acetonitrile (2.0 × 10−5 mol·L−1) with UV-light irradiation. (C) 3a and (D)
4a.

Imidazoled-Dithienylethene
5
Table 1. Absorption and fluorescence data of dithienylethenes 2–3 and 8–10 in acetonitrile
(2.0 × 10−5 mol·L−1
)
Abs
max
Abs
em
max
em
Compounds
λ
(nm)
λ
(nm)
λ
(nm)
λ
(nm)
max
max
Open isomer
Closed isomer
Open isomer
Closed isomer
3a
3b
3c
4a
4b
4c
310
298
296
288
290
287
506
520
533
564
566
570
384
385
389
407
389
414
456
451
480
445
470
473
3b underwent a photoisomerization upon irradiation with UV light (λ = 302 nm). Mean-
while, a reversible open-ring reaction was also found upon irradiation with visible light
(λ > 402 nm). Similar properties also existed in compounds 3a and 3b. It has been noted that
compound 3a showed a 12- to 14-nm red-shift compared to compounds 3b and 3c before
irradiation with UV light, as shown in Table 1. It was possibly attributed to the existence of
intramolecular N–H. . . O hydrogen bond (imidazole ring and methoxyl group). Although
the methoxyl groups of compounds 3b and 3c are in 3- and 4-sites of benzene ring, they
displayed similar absorption. According to the data in Table 1, closed-ring isomers of the
dithienylethenes revealed a 14-nm red-shift from 3a to 3c, respectively. The results sug-
gested that the difference in position of substituted group can affect the photoisomerization.
After N-methylation of NH on the imidazole ring, compounds 4a–c showed similar
UV–Vis absorbance. Interestingly, a weak blue shift could be observed in Table 1, possibly
owing to the decrease of the conjugated degree after N-methylation on the imidazole rings,
which was well in agreement with our previous report [5a, 7]. Upon irradiation with UV
light, a new absorbance band at 570 nm was observed for compound 4c in Fig. 1(B). A
similar phenomenon was also found in the solution of compounds 4a and 4b as shown in
Table 1, and these compounds also displayed very good reversibility. It is worth mentioning
that compounds 4a–c revealed very obvious red shifts in comparison to compounds 3a–c.
Subsequently, we investigated fluorescence spectra of the dithienylethenes in acetonitrile
(2.0 × 10−5 mol·L−1). As can be observed in Fig. 1(C) and (D), obvious “turn-on” fluores-
cent changes could be found in the solution of compounds 3a and 4a when they underwent
a UV-light irradiation. Other compounds also possessed similar properties in Table 1.
Further investigation based on metal ions binding was performed because of the coor-
dinative function of imidazole unit, in which different metal ions (such as Cd2+, K+, Cu2+
,
Zn2+, Co2+, Ca2+, Fe3+, Hg2+, Ni2+, and Cu+) were added to the acetonitrile solution of
dithienylethenes 3 and 4. According to Fig. 2(A), the obvious fluorescent enhancement of
compound 3c in acetonitrile (2 × 10−5 M) was observed when the metal ions Cu2+, Fe3+
,
and Cd2+ were added, respectively, while other metal ions displayed little changes. Further-
more, the introduction of Cu2+ showed the largest enhancement of fluorescent intensity,
while the effect of Cd2+ was the weakest. Similar experiments were carried out in the
acetonitrile solution of compounds 3a and 3b, and the same results were easily achieved.
Subsequently, N-methylated compounds 4a–c were subjected to perform the same reac-
tion. According to Fig. 2(B), it is suggested that compound 4a possessed completely the
same binding properties toward Cu2+, Fe3+, and Cd2+. Uniformly, the binding properties

6
C. Zhang et al.
Cu2+
Fe3+
(A)
(B)
Cu2+
160
120
80
40
0
100
Fe3+
80
60
40
20
0
Cd2+
Cd2+
350
400
450
500
550
350
400
450
500
Wavelength / (nm)
Wavelength / (nm)
Figure 2. Fluorescence responses of partial dithienylethenes (2 × 10−5 M) in acetonitrile upon the
addition of Cd2+, K+, Cu2+, Zn2+, Co2+, Ca2+, Fe3+, Hg2+, Ni2+, and Cu+ (10 eq.). (A) 3c and (B)
4a.
of compounds 4b and 4c were well in agreement with compound 4a. From these data, we
found that metal ions Cu2+, Fe3+, and Cd2+ can affect their fluorescence spectra.
Conclusion
In summary, a series of imidazole-bridged dithienylethenes was synthesized in good yields.
UV–Vis absorption and fluorescence spectra of the dithienylethenes were investigated be-
fore and after irradiation with UV light. The results indicated that these compounds revealed
very good reversibility upon irradiation with UV and visual light, and possessed the “turn-
on” fluorescent properties. Meanwhile, we found that metal ions can affect fluorescence
spectra of the dithienylethenes. It is worth mentioning that the metal ions such as Cd2+
,
Cu2+, Fe3+ can obviously enhance the intensity of emission spectra. Further research will
focus on the selective binding toward metal ions.
Acknowledgments
The authors acknowledge financial support from National Natural Science Foundation of
China (nos. 20931006, 21072070, 21072071, 21272088), Program for Changjiang Scholars
and Innovative Research Team in University (no. IRT0953), and the Program for Academic
Leader in Wuhan Municipality (no. 201271130441), and self-determined research funds of
CCNU from the colleges’ basic research and operation of MOE.
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