Synthesis and photochromic properties of imidazole-based diarylethenes

Article abstract of DOI:10.1039/c0pp00337a

A series of symmetrical imidazole- and N-methylated imidazole-based diarylethenes were synthesized, and the structures have been well characterized by NMR spectroscopy and mass spectrometry. Their photochromism were investigated upon UV/vis light irradiation in DMF solution. A significant red-shift of UV absorption was observed after irradiation with UV light when substituents on the benzene ring (such as methoxyl group) were introduced. But N-methylation on the two imidazole rings will result in a blue-shift of UV absorption. The photophysical properties can be adjusted by changing the substituents, which provides a new strategy for the design of diarylethenes with excellent properties. The Royal Society of Chemistry and Owner Societies 2011.

Full text of DOI:10.1039/c0pp00337a

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PAPER
Synthesis and photochromic properties of imidazole-based diarylethenes
Jingjing Yuan, Ziyong Li, Ming Hu, Sisi Li, Shuyuan Huang, Jun Yin* and Sheng Hua Liu*
Received 11th November 2010, Accepted 8th December 2010
DOI: 10.1039/c0pp00337a
A series of symmetrical imidazole- and N-methylated imidazole-based diarylethenes were synthesized,
and the structures have been well characterized by NMR spectroscopy and mass spectrometry. Their
photochromism were investigated upon UV/vis light irradiation in DMF solution. A significant
red-shift of UV absorption was observed after irradiation with UV light when substituents on the
benzene ring (such as methoxyl group) were introduced. But N-methylation on the two imidazole rings
will result in a blue-shift of UV absorption. The photophysical properties can be adjusted by changing
the substituents, which provides a new strategy for the design of diarylethenes with excellent properties.
Introduction
Property-switchable compounds, such as photoresponsive mate-
rials, are widely used in optoelectronic materials and biological
applications due to their sensitive response times and simple
operation. Therefore, in the past decade, a large number of pho-
tochromic compounds with various photoresponsive units have
been synthesized and their properties have been investigated.1,2
Scheme 1 Reversible photocyclization reactions of diarylethene.
Among the various photochromic systems, diarylethenes with
heterocyclic rings are considered to be among the most promising
the imidazole ring on the photochromic properties are not clear.
In the present study, we have designed and synthesized several
dithienylethenes containing imidazole and N-methylated imida-
zole units, and have investigated their photochromic properties
and the effects of substituent groups thereon. The results indicate
that they can be easily isomerized upon irradiation with UV/Vis
light, with each compound showing a very broad absorption
band between 500 and 800 nm. The 4-methoxyphenyl-substituted
imidazole diarylethene 5 displays an absorption maximum at
682 nm after irradiation with UV light in DMF solution.
because of their good thermal stability, excellent fatigue resistance,
and rapid response times.3–5 In particular, diarylethenes containing
two thiophene or benzothiophene rings have been widely applied
in optical memories6–9 and photoswitches.10,11 The most commonly
used diarylethenes are diarylperfluorocyclopentenes.
As shown in Scheme 1, the ring-closed isomer has a more
extensive p-conjugated system than the ring-open isomer, and
the introduction of fluorine atoms on the cyclopentene is a very
useful means of enhancing the stability of the ring-closed isomer
and hence improving the photochromic properties. Therefore,
studies on thiophene-substituted perfluorocyclopentenes have
attracted increasing attention, and current efforts are focused
on the function of the R groups in order to identify novel
structures with excellent photochromism.12–25 For example, the
N-heterocyclic imidazole unit is an important building block
that has been widely used in many fields, such as medicine,26
ionic liquids,27 anion sensors,28,29 as well as electronic and optical
materials.30–32 Therefore, by incorporating an imidazole unit into
a diarylethene, it is anticipated that diarylethenes with excellent
photochromic properties will be obtained. For example, Yi
and Huang reported imidazo[4,5-f ][1,10]phenanthroline-based
diarylethenes, which were found to be sensitive to both light
and metal ions/protons.33 However, the effects of substituents on
Experimental
General
All manipulations were carried out under an argon atmosphere by
using standard Schlenk techniques. DMF was dried with magne-
sum sulfate then distilled under vacuum. 4,4¢-(perfluorocyclopent-
1-ene-1,2-diyl)bis(5-methylthiophene-2-carbaldehyde) (1) was
prepared by literature methods.34,35 Different benzene-substituted
diketones (2) were prepared by modified procedures of reported
methods.36 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 yields with the known yields of the compound 1,2-bis(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene.37
College of Chemistry, Central China Normal University, Wuhan, HuBei,
430079, China. E-mail: yinj@mail.ccnu.edu.cn, chshliu@mail.ccnu.edu.cn;
Fax: +86-27-6786-7725; Tel: +86-27-6786-7725
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Scheme 2 Synthesis of imidazole-based dithienylethenes 3–8.
Synthesis of dithienylethenes with two imidazole units 3–5
MS: m/z = 924.22 (M), calculated exact mass: 924.22. Anal. calcd
for C49H38F6N4O4S2: C, 63.63; H, 4.14; N, 6.06; S, 6.93%. Found:
C, 63.76; H, 4.05; N, 6.12; S, 7.05%.
Target compounds 3–5 were prepared according to the synthetic
route presented in Scheme 2 by modified procedures of reported
methods.38
Synthesis of N-methylated derivatives 6–8
General synthetic proceduring. To a solution of ammonium
acetate (8.0 mmol) in refluxing glacial acetic acid (15 mL) was
added compound 1 (0.50 mmol) under an argon atmosphere at
General synthetic proceduring. To
a solution of 3–5
(0.25 mmol) in N,N-dimethylformamide (10 mL) in the presence
of potassium carbomate (1.0 mmol) was added methyl iodide
(1.0 mmol) and stirred for 12 h under dark conditions at
room temperature. The reaction mixture was poured into water
(100 mL). The formed precipitate was collected and the crude
product was washed with water. The solid was dissolved in DCM.
After drying over sodium sulfate, and removing the solvent under
reduced pressure, the products were recrystallized from hexane to
give a grey solid.
◦
120 C, and the mixture was refluxed for 2 h. Benzil (1.0 mmol)
was added and further stirred for overnight. The reaction mixture
was allowed to cool to room temperature, and then poured into
ice water (200 mL), carefully neutralized with a 10% sodium
carbonate solution and adjusted the pH at 6.5–7.0. The formed
precipitate was collected, washed with water and recrystallized
from methylene chloride to give an off-white solid.
1
Compound 3. 80% yield, H NMR (400 MHz, DMSO-d6):
Compound 6. 42% yield 1H NMR (600 MHz, CDCl3): d 2.07
(s, 6H, CH3), 3.56 (s, 6H, N–CH3), 7.14 (t, J = 7.2 Hz, 2H, Ar–
H), 7.20 (t, J = 7.2 Hz, 4H, Ar–H), 7.32 (s, 2H, thiophene-H),
7.36 (d, J = 7.2 Hz, 4H, Ar–H).7.45–7.48 (m, 10H, Ar–H). 13C
NMR (150 MHz, CDCl3): d 14.51 (s, CH3), 32.91 (s, N–CH3),
124.98, 125.20, 126.53, 126.94, 128.06, 128.83, 129.09, 130.41,
130.81, 131.01, 131.82, 134.06, 136.10, 138.12, 140.81, 143.03 (s,
thiophene, ethene, Ar). EI MS: m/z = 832.23 (M), calculated exact
mass: 832.21. Anal. calcd for C47H34F6N4S2: C, 67.77; H, 4.11; N,
6.73; S, 7.70%. Found: C, 67.58; H, 4.01; N, 6.93; S, 7.89%.
d 1.94 (s, 6H, CH3), 7.23–7.51 (m, 20H, Ar–H), 7.82 (s, 2H,
thiophene-H), 12.97 (s, 2H, N–H). EI MS: m/z = 804.25 (M+),
calculated exact mass: 804.18. Anal. calcd for C45H30F6N4S2: C,
67.15; H, 3.76; N, 6.96; S, 7.97%. Found: C, 67.38; H, 3.68; N,
6.93; S, 8.12%.
1
Compound 4. 65% yield, H NMR (600 MHz, DMSO-d6):
d 1.92 (s, 6H, CH3), 2.31 (s, 12H, CH3),7.11–7.37 (m, 16H,
Ar–H), 7.79 (s, 2H, thiophene-H), 12.89 (s, 2H, N–H). EI MS:
m/z = 860.15 (M), calculated exact mass: 860.24. Anal. calcd for
C49H38F6N4S2: C, 68.36; H, 4.45; N, 6.51; S, 7.45%. Found: C,
68.58; H, 4.38; N, 6.50; S, 7.59%.
Compound 7. 64% yield.1H NMR (600 MHz, CDCl3): d 2.05
(s, 6H, CH3), 2.28 (s, 6H, CH3), 2.42 (s, 6H, CH3), 3.54 (s, 6H,
N–CH3), 7.01 (d, J = 7.8 Hz, 4H, Ar–H), 7.22–7.26 (m, 8H, Ar–
H), 7.29 (s, 2H, thiophene-H), 7.38 (d, J = 7.8 Hz, 4H, Ar–H).
13C NMR (150 MHz, CDCl3): d 14.37 (s, CH3), 21.12 (s, CH3),
21.36 (s, CH3), 32.82 (s, N–CH3), 124.80, 125.17, 126.76, 127.50,
1
Compound 5. 85% yield, H NMR (600 MHz, DMSO-d6): d
1.91 (s, 6H, CH3), 3.74 (s, 6H, OCH3), 3.79 (s, 6H, OCH3), 6.87 (d,
J = 7.2 Hz, 4H, Ar–H), 7.00 (d, J = 6.6 Hz, 4H, Ar–H), 7.37–7.41
(m, 8H, Ar–H), 7.76 (s, 2H, thiophene-H), 12.78 (s, 2H, N–H). EI
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128.75, 129.76, 130.67, 131.34, 131.99, 136.02, 138.06, 138.63,
140.53, 142.86 (s, thiophene, ethene, Ar). EI MS: m/z = 888.25
(M), calculated exact mass: 888.28. Anal. calcd for C51H42F6N4S2:
C, 68.90; H, 4.76; N, 6.30; S, 7.21%. Found: C, 69.06; H, 4.63; N,
6.37; S, 7.34%.
irradiation with UV light in DMF solution, which indicated that
the substituent groups can affect the photochromic properties.
Similar spectral changes were observed when solutions of the
other diarylethenes 4–8 in DMF were irradiated with UV/Vis
light (Fig. 2). For the diarylethenes 6–8, obvious blue-shifts were
observed upon irradiation with UV light, which may be attributed
to the formation of nonplanar structures after N-methylation
of the imidazole rings. Furthermore, according to the UV/Vis
data, we found that all of the compounds displayed very broad
absorption from 500 to 800 nm after irradiation with UV light
in DMF solution, which indicated that such a structure might
be used as a precursor of near-infrared dyes by appropriate
functionalization.
Compound 8. 65% yield.1H NMR (600 MHz, CDCl3): d 2.07
(s, 6H, CH3), 3.53 (s, 6H, N–CH3), 3.74 (s, 6H, OCH3), 3.85 (s, 6H,
OCH3),6.75 (d, J = 8.4 Hz, 4H, Ar–H), 6.98 (d, J = 8.4 Hz, 4H,
Ar–H), 7.26 (d, J = 8.4 Hz, 4H, Ar–H), 7.29 (s, 2H, thiophene-H),
7.42 (d, J = 9.0 Hz, 4H, Ar–H). 13C NMR (150 MHz, CDCl3):
d 14.37 (s, CH3), 32.69 (s, N–CH3), 55.03 (s, OCH3), 55.18 (s,
OCH3), 113.44, 114.46, 122.53, 124.67, 125.10, 126.94, 127.94,
129.90, 131.99, 132.08, 137.75, 140.28, 142.75, 158.22, 159.78 (s,
thiophene, ethene, Ar). EI MS: m/z = 952.27 (M), calculated exact
mass: 952.26. Anal. calcd for C51H42F6N4O4S2: C, 64.27; H, 4.44;
N, 5.88; S, 6.73%. Found: C, 64.45; H, 4.35; N, 5.85; S, 6.87%.
Results and discussion
Scheme 2 outlines the synthesis of imidazole-based diarylethenes
3–5 in good yields, according to a previous literature report.33
However, these compounds showed poor solubility in organic
solvents. After N-methylation of the imidazole ring in the presence
of CH3I and base, compounds 6–8 with different substituent
groups showed good solubility.
The photochromic properties of diarylethenes 3–8 were mea-
sured at room temperature in DMF (2.0 ¥ 10-5 mol L-1). As
depicted in Scheme 1, photoisomerization occurred between 3o–
8o (ring-open isomers) and 3c–8c (ring-closed isomers) upon
alternating irradiation with UV light (l = 302 nm) and visible
light (l > 402 nm). As shown in Fig. 1, the absorption maximum
of compound 3 in DMF was observed at 332 nm (e = 2.92 ¥
104 L mol-1 cm-1), which was considered to be due to a p–p*
transition.39 When the solution was irradiated with 302 nm UV
light, it changed from colorless to blue and a new absorption
band centered at 664 nm (e = 5.30 ¥ 104 L mol-1 cm-1) appeared
as a result of the cyclization reaction to form the ring-closed
isomer 3c. Upon irradiation with visible light (l > 402 nm),
the colored 3c returned to the initial colorless form (ring-opened
isomer 3o). Compared with those of the diarylethenes 3 and
4, the absorption of diarylethene 5 bearing methoxy groups
showed an obvious red-shift, with a maximum at 682 nm, after
Fig. 2 Absorption spectral changes of diarylethene 8 by photoirradiation
in DMF (2.0 ¥ 10-5 mol L-1) at room temperature.
Table 1 shows the absorption maxima of each of the compounds.
According to these data, we found that the diarylethenes 5 and 8
displayed red-shifts of 18–31 nm and 6–10 nm, respectively, com-
pared with the unsubstituted and methyl-substituted diarylethenes
3, 4, 6, and 7. Furthermore, the substituents on the benzene ring
and N-methylation of the imidazole ring had remarkable effects
on the photochromic properties of these diarylethenes, such as the
quantum yields of the cyclization and cycloreversion reactions.
After the introduction of the substituent groups, the cyclization
quantum yields (jo–c) showed an obvious decrease, while the
cycloreversion quantum yields showed a slight increase (except
for 4c). In addition, the quantum yields of the cyclization and
cycloreversion reactions of diarylethenes 3–5 are much lower than
those of diarylethenes 6–8, thus indicating that N-methylation of
the two imidazole rings improves these quantum yields.
Conclusions
Six symmetrical diarylethenes containing two imidazole bridging
units have been synthesized as novel N-heterocycle-substituted
dithienylperfluorocyclopentene photoswitches for use as photore-
sponsive materials. The results suggest that both the introduction
of substituents on the benzene ring and methylation of the N–H
units on the two imidazole rings can significantly affect the pho-
tochromic properties, such as the UV/Vis absorption spectra and
quantum yields, of these diarylethenes. In particular, each of the
compounds shows a very broad absorption band between 500 and
800 nm. The 4-methoxyphenyl-substituted imidazole diarylethene
Fig. 1 Absorption spectral changes of diarylethene 3 by Photoirradiation
in DMF (2.0 ¥ 10-5 mol L-1) at room temperature.
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Table 1 Absorption characteristics and photochromic quantum yields of 3–8 in DMF (2.0 ¥ 10-5 mol L-1)
lmaxAbs/nma (e/¥10-4 L mol-1 cm-1)
lmaxAbs/nmb (e/¥10-4 L mol-1 cm-1)
Uc
Compound
(Open)
(PSS)
jo–c (l/nm)
jc–o (l/nm)
3
4
5
6
7
8
332 (2.92)
337 (3.75)
343 (2.74)
318 (3.16)
322 (2.67)
328 (2.64)
664 (5.30)
651 (6.24)
682 (3.96)
630 (4.86)
632 (3.94)
638 (3.78)
0.531 (664)
0.302 (651)
0.109 (682)
0.592 (630)
0.572 (632)
0.224 (638)
0.0017 (332)
0.0015 (337))
0.0022 (343)
0.0040 (318)
0.0056 (322)
0.0054 (328)
a Absorption maxima of open-ring isomers. b Absorption maxima of closed-ring isomers. c Quantum yields of open-ring (jc–o) and closed-ring isomers
(jo–c), respectively.
5 displayed an absorption maximum at 682 nm after irradiation
with UV light in DMF solution, which holds promise for the
synthesis of novel, photocontrollable near-infrared dye molecules.
Furthermore, our research may also prompt the development and
application of other heterocycles in photochromism.
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Acknowledgements
The authors acknowledge financial support from National Nat-
ural Science Foundation of China (No. 2072039, 20931006,
21072070) and Program for Changjiang Scholars and Innovative
Research Team in University (No. IRT0953).
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The Royal Society of Chemistry and Owner Societies 2011
Photochem. Photobiol. Sci., 2011, 10, 587–591 | 591
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