Substituent and solvent effects on the fluorescent and photochromic properties of 2-(2-pyridyl)imidazole containing diarylethene derivatives

Article abstract of DOI:10.1039/c3ra43110j

Five diarylethene derivatives with 2-(2-pyridyl)imidazole as the ethene bridges (PI-BTEs) were synthesized and characterized. The structures and performances of the PI-BTEs (L1-L5) could be modified from not only the thiophene unit, but also the substituent on the imidazole nitrogen. Noteworthily, replacing the thiophene with the benzothiophene, the fatigue resistance is significantly improved, the centre absorption bands of the closed forms shift from 572 nm (L3) to 550 nm (L2), and the colours of the solutions change from pale pink in L3 to red-purple in L2. Additionally, the Stokes shift (Δν) values of L3 in various solvents display good linear relationships with the donor number (DN) of the solvents, and the solvent polarity parameter Δf.

Full text of DOI:10.1039/c3ra43110j

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Substituent and solvent effects on the fluorescent and
photochromic properties of 2-(2-pyridyl)imidazole
containing diarylethene derivatives†
Cite this: RSC Adv., 2013, 3, 24146
b
bc
Xuefeng Peng,ab Jian-Guo Deng and Hai-Bing Xu
*
Five diarylethene derivatives with 2-(2-pyridyl)imidazole as the ethene bridges (PI-BTEs) were synthesized
and characterized. The structures and performances of the PI-BTEs (L1–L5) could be modified from not only
the thiophene unit, but also the substituent on the imidazole nitrogen. Noteworthily, replacing the
thiophene with the benzothiophene, the fatigue resistance is significantly improved, the centre
absorption bands of the closed forms shift from 572 nm (L3) to 550 nm (L2), and the colours of the
solutions change from pale pink in L3 to red-purple in L2. Additionally, the Stokes shift (Dn) values of L3
in various solvents display good linear relationships with the donor number (DN) of the solvents, and
the solvent polarity parameter Df.
Received 21st June 2013
Accepted 1st October 2013
DOI: 10.1039/c3ra43110j
www.rsc.org/advances
Diarylethenes with reversibly uorescence intensity variation
induced by photochromic reactions, are particularly useful for the
Introduction
Photochromism has been attracting considerable attention
because of its potential applications to molecular devices, such
as optical memory devices and switches.1 Among the various
photochromic systems, diarylethenes, especially bisthienyle-
thenes (BTEs), are considered as some of the most promising
candidates for applications, due to the superior fatigue resis-
tance and thermal stability.1a,d
Generally, chemical modication of symmetric or asym-
metric BTEs is derived from the thiophene units and the ethene
bridges.2 By contrast with the extensive studies on modication
of the thiophene units,1a,d the central ethene bridges, which are
necessary for versatility of the BTE architecture, have been
mostly limited to the cyclopentene units and its strong electron-
withdrawing analogues, such as peruorocyclopentene, maleic
anhydride, or maleic imide.2,3 However, BTE systems with such
vinyl bridges usually exhibit small uorescence modulation
upon photochromism.3,4
optical memories as well as the molecular switches owing to their
fast response and high sensitivity.5 Therefore, other ethene bridges
instead of the cyclopentene units had been taken into account to
develop novel photoactive molecules with uorescence modula-
tion.3 For instance, Krayushkin,6 Yam,7 Kawai,8 Chen,9 Bielawski,10
and Liu11 et al. had employed the novel photoactive diarylethenes
with an imidazole bridge to demonstrate photochromism.
Although these molecules and their metal complexes exhibit
versatile uorescent and photochromic properties,7,9b,11 the inferior
fatigue resistance of these molecules limits their developments.7a
In order to improve the fatigue resistance and extend the
structural diversity of this system, we report a series of 2-(2-
pyridyl)imidazole containing diarylethene derivatives (PI-BTEs).
On one hand, the structures and performances of these PI-BTEs
are investigated through modifying both the thiophene unit
and the substituent on the imidazole nitrogen. On the other
hand, the fatigue resistance is signicantly improved by
replacing the thiophene with the benzothiophene.
Results and discussion
aState Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou
730000, China
As shown in Scheme 1, L1–L5 were prepared by conventional
Suzuki cross-coupling with the corresponding boronic acids
and 1-aryl-4,5-dibromo-2-(2-pyridyl)imidazoles. The interme-
diate of 3-bromo-2,5-dimethylthiophene can be obtained by two
methods reported in literatures, one is brominating the 2,5-
dimethylthiophene with NBS directly at room temperature,12
the other is a two-step method.13 We found that the former
method is better than the latter one in the yield. The detailed
synthetic procedures and characterizations of these compounds
were described in the Experimental section.
bNew Materials R&D Center, Key Laboratory of Science and Technology on High Energy
Laser, Institute of Chemical Materials, China Academy of Engineering Physics,
Chengdu, Sichuan 610200, China. E-mail: hai_bingxu@163.com; Fax: +86-28-8588-
0792
cState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
† Electronic supplementary information (ESI) available: Lippert–Mataga
equations, absorption and uorescence spectra of L1–L5, absorption and
uorescence spectra of L3 in various solutions, Dn–DN plot, fatigue resistance
of L3 in degassed CH2Cl2, and copies of 1H NMR and 13C NMR of compounds.
See DOI: 10.1039/c3ra43110j
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Scheme 1 Synthetic routes of L1–L5. Reaction conditions: (a) NCS, benzene/CH3COOH ¼ 1 : 1, room temperature; (b) Br2, CHCl3, 0 ꢀC to room temperature; (c) (1)
n-BuLi, THF, ꢁ78 ꢀC. (2) B(n-OBu)3, ꢁ78 ꢀC to room temperature, (3) 2 M HCl, H2O, room temperature; (d) NBS, CH3COOH, room temperature; (e) glyoxal, NH4OH,
ethanol, 0 ꢀC to room temperature; (f) 1-bromo-4-methoxybenzene, Cs2CO3, CuI, 1,10-phenanthroline, DMF, reflux; (g) bromobenzene, Cs2CO3, CuI, 1,10-phenan-
throline, DMF, reflux; (h) 1-iodo-4-nitrobenzene, Cs2CO3, CuI, 1,10-phenanthroline, DMF, reflux; (i) Br2, CHCl3, 0 ꢀC to reflux; (j) the corresponding boronic acid,
Pd(PPh3)4, 2 M Cs2CO3, 1,4-dioxane.
molecular switches, both the electron donor (thiophene group)
and the acceptor (imidazole group) are assembled in one
molecule.21 Finally, in order to enhance the uorescent perfor-
mance, benzene is introduced on the imidazole nitrogen.
As shown in Scheme 2, in order to investigate the electronic
effects on the uorescent and photochromic properties of these
PI-BTEs, the electron-donating methoxyl in L1 and methyl in L4,
and the electron-withdrawing nitro in L5, are studied
Molecular design
The design of the parent compound L3 is based on the following
considerations. First of all, to construct functional photo-
chromic complexes, 2-(2-pyridyl)imidazole with coordination
ability, and 5-chloro-2-methylthiophene with potential coordi-
nation ability for metallic subunits,7a,14–20 are chosen as the
ethene bridge and the thiophene unit, respectively. Secondly, to
afford good Stokes shi and solvent-dependent photochromic
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Fig. 1 Absorption spectra in the photostationary state by UV irradiation (a), the
corresponding colours of the solutions (b), and the emission spectra of the open
forms (lex ¼ 325 nm), and (c) of L1–L5 (5 ꢂ 10ꢁ5 mol Lꢁ1) in CH2Cl2.
Scheme 2 The relationships of L1–L5.
forms of L1 and L4 show slightly bathochromic shis. As for
electron-withdrawing effect of the nitro, slightly hypsochromic
shi is observed in that of L5 (Fig. 1a). Notably, replacing the
thiophene in L3 with the benzothiophene in L2, the centre
absorption bands of the closed forms shi from 572 nm (L3) to
550 nm (L2). Accordingly, the colours of the solutions change
from pale pink in L3 to red-purple in L2. In brief, the central
absorption bands of the closed forms red-shi in this order: L2
< L5 < L3 < L1 < L4, and the colours of the corresponding
solutions are red-purple, yellowish-brown, pale pink, pale
sorrel, and blue, respectively (Fig. 1b).
systematically. To improve the fatigue resistance, the thiophene
groups in L3 are replaced with the benzothiophene groups
in L2.
All of these PI-BTEs have been characterized by 1H NMR, 13
C
NMR, EI-MS, and elemental analysis. Due to a fast intercon-
version of the antiparallel and parallel congurations, probably
resulting from the lack of steric bulk of the imidazole ring,7a
1
only one set of H NMR signals for the methyl groups on the
thiophene moieties appears in the 1H NMR spectroscopy of
L1–L5.
Similarly, uorescent performance of these PI-BTEs also
could be modulated by simply modifying electronic properties
of the substituent. As shown in Fig. S1 (ESI†), upon irradiation
with 302 nm light, due to an efficient energy transfer from the
excited uorophore core to the attached closed ring BTE unit,22
the uorescence of L1–L5 are all quenched gradually, indicating
the formation of the closed forms. On the contrary, the uo-
rescence intensities of these PI-BTEs are gradually restored
with the open forms formation by irradiation with visible light
(l ¼ 520 nm).
Substituent effect
The absorption spectra and colour changes of L1–L5 in
dichloromethane solutions under irradiation with UV and
visible light at appropriate wavelength are shown in Fig. S1
(ESI†). And the photochromic features (including the absorp-
tion maxima, quantum yield, and Stokes shi) of L1–L5 are
summarized in Table 1. The open forms of L1–L5 give colourless
solutions, with an intense absorption band at ca. 282–316 nm,
corresponding to the mixture of p / p* and n / p* transi-
tions of imidazole and thiophene moieties. Upon irradiation
with the UV light (l ¼ 302 nm), two additional absorption bands
at ca. 370–375 and 550–575 nm are observed in these
compounds, indicating the formation of the closed forms.
Compared with L3, due to electron-donating effect of the
methoxyl and the methyl, the maximal absorptions of the closed
The uorescent features of these compounds are summa-
rized in Table 1. Except for L5, which emits weak luminescence
owing to uorescence quenching effect from the electron-
withdrawing nitro and halogen groups, bright luminescence are
observed in L1–L4 (Fig. 1c), and the emission intensities (uo-
rescence quantum yields) increase in this order: L5 (0.003) < L1
(0.027) < L2 (0.041) < L3 (0.063) < L4 (0.12). By comparison with
Table 1 Absorption and fluorescence characteristics of L1–L5
Absorption
Fluorescence
Fc
Compound
lmax/nm (open)a
lmax/nm (PSS)b
FO–C
FC–O
lmaxd/nm
Fe
Stokes shi (Dn/cmꢁ1
)
)
L1
L2
L3
L4
L5
286
304
308
316
282
574
550
572
575
569
0.32
0.68
0.52
0.26
0.30
0.12
0.13
0.11
0.14
0.18
398
397
396
414
412
0.027
0.041
0.063
0.12
9839
7706
7151
7491
11189
0.003
a
Absorption maxima of the open forms. b Absorption maxima of the photostationary states (PSS). c Quantum yields of cyclization reaction (FO–C
and cycloreversion reaction (FC–O), respectively. Emission maxima of uorescence (lex ¼ 325 nm). e Fluorescence quantum yields.
d
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L3, the uorescence maxima of L4 shows a marked red shi could be achieved by introduction of the methoxyl (L1) or the
(Dl ¼ 18 nm) and the uorescence quantum yield climbs up to nitro (L5), and an inferior fatigue resistance (two cycles) is
0.12, when the auxiliary substituent is altered from –Cl (L3) to found in L4. It is pleasant to nd that the fatigue resistance is
–CH3 (L4). Two reasons may account for it. One is ascribed to signicantly improved by replacing the thiophenes in L3 with
the electron-donating effect of –CH3 in L4 and the electron- the benzothiophenes in L2. By comparison with traditional
withdrawing effect of –Cl in L3. The other is the heavy atom photochromic compounds, although the fatigue resistance of
effect of Cl,23 which could increase the rate of intersystem L2 is not so high, L2 still exhibits good photochromism (Fig. 2b)
crossing, attenuating the uorescence of L3. Additionally, with a negligible degradation in CH2Cl2 aer ten cycles. The
compared with other BTEs, L1–L5 show relatively large Stokes remarkable enhancement of the fatigue resistance may attri-
shis (Table 1), suggesting the presence of a twisted intra- bute to the much lower reactivity to singlet oxygen of the ben-
molecular charge transfer (TICT) excited state.21a,24
zothiophene than that of the thiophene.1a As the
benzothiophene can be further functionalized,4,27 it is practi-
cable to modify the benzothiophene units to construct func-
tional photochromic PI-BTEs with superior fatigue resistance.
Fatigue resistance
Fatigue-resistant property, that is how many times photo-
cyclization and cycloreversion reaction cycles could be repeated
without loss of the performance,1a,25 is one of the most impor-
tant factors for practical applications in optical devices.1a
Although the diarylethene compounds with “traditional” per-
uorocyclopentene units withstand several hundreds or even
thousands of cycles. The fatigue resistance of diarylethene
derivatives with potential coordination ability, such as 2-(2-
pyridyl)imidazole or its analogue as the ethene bridges, are
inferior, and the reported N,N-donor ligands containing diary-
lethene derivatives withstand about two cycles in degassed
solutions.7a
It has been demonstrated that high photoreversibility of
traditional photochromic compounds (mostly with the cyclo-
pentene units or its strong electron-withdrawing analogues)
could be achieved, when the active sites on the thiophene in
BTEs are occupied by other non-hydrogen substituent.1a
Inspired by this principle, the photo-fatigue resistance of the
photochromic compounds with 2-(2-pyridyl)imidazole or its
analogue as the ethene bridges may be improved by elaborately
modifying the thiophene units.
Solvent effect
A broad-ranged solubility offers us a good chance to study the
uorescence and photochromism of parent compound L3 in
various organic solvents, such as in n-hexane, toluene, THF,
CH2Cl2, and methanol. As shown in Fig. 3b, upon irradiation
with the UV light, the colourless solutions of L3 turn into pale
pink in THF and CH2Cl2, and blue in n-hexane. Especially, the
colours of L3 in toluene and methanol change into dark blue
and blue-purple, respectively. Whereas, all the solutions of L3
are decolorized by the visible light (l ¼ 520 nm) irradiation. The
absorption and uorescence spectral characteristics of L3 in
various solvents are summarized in Table 2. Similar to the
“negative solvatochromism” phenomenon,28 the absorption
bands of both the open and closed forms shi to shorter
wavelength (Fig. 3a) with the solvent polarity enhancement.
However, the maximal emission bands of the open forms tend
to longer wavelength (Fig. 3c) with the solvent polarity accre-
tion, in which the highest uorescence quantum yield of L3 in
CH2Cl2 (0.063) is beyond four times than that of the lowest one
(0.014) in n-hexane. Similar to the previous report,21a the ring-
closure quantum yields of L3 gradually decrease with the
solvent polarity accretion, while the quantum yields for ring-
opening are solvent-independent.
The fatigue resistance is tested in CH2Cl2 by alternatively
irradiating with the UV and the visible light. As shown in Fig. 2a
and S3 (ESI†), although L3 is obviously destroyed aer three
repeated cycles in both ambient atmosphere and degassed
CH2Cl2, the degradation of the absorbance intensity is much
slower in degassed CH2Cl2 than that in ambient atmosphere,
likely resulting from the by-product formation.26 Compared
with L3, no obvious improvement on the fatigue resistance
For a better understanding of the solvent-dependent, we
have studied the solvent effect on the uorescent and photo-
chromic properties of L3 from the donor number (DN) of
solvents,29 and the solvent polarity parameter Df, respectively.
Interestingly, by increasing the DN, the absorption peak of
L3 shows a hypsochromic shi. Moreover, a good linear
Fig. 2 Fatigue resistance of L3 (a) and L2 (b) was tested in CH2Cl2 with 5 ꢂ 10ꢁ5
mol Lꢁ1 by alternatively irradiating with the UV (l ¼ 302 nm) and the visible light
(l ¼ 520 nm) in ambient atmosphere (open (B) and closed ( ) form), monitored
by the absorbance at 572 nm and 550 nm, respectively.
Fig. 3 Absorption spectra in the photostationary state by UV irradiation (a), the
corresponding colours of the solutions (b), and emission spectra in the open forms
(lex ¼ 325 nm), and (c) of L3 (5 ꢂ 10ꢁ5 mol Lꢁ1) in various solutions.
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Table 2 Absorption and fluorescence characteristics of L3 in various solvents
Absorption
Fluorescence
Fc
Solvents
lmax/nm (open)a
lmax/nm (PSS)b
FO–C
FC–O
lmaxd/nm
Fe
DNf
Df
Stokes shi (Dn/cmꢁ1
)
)
n-C6H14
C6H5CH3
THF
CH2Cl2
CH3OH
318
317
309
308
301
579
578
576
572
568
0.89
0.68
0.54
0.52
0.48
0.10
0.11
0.12
0.11
0.12
377
380
393
396
400
0.014
0.041
0.029
0.063
0.03
0
0.001
0.013
0.209
0.217
0.309
4921
5230
6917
7215
8223
0.1
20
—
41.3
a
Absorption maxima of the open forms. b Absorption maxima of the photostationary states (PSS). c Quantum yields of cyclization reaction (FO–C
d
e
f
and cycloreversion reaction (FC–O), respectively. Emission maxima of uorescence (lex ¼ 325 nm). Fluorescence quantum yields. The donor
number of the solvents.
substituent groups and solvents. Noteworthily, the fatigue
resistance can be signicantly improved by replacing the thio-
phene groups (L3) with the benzothiophene groups (L2). In
addition, L3 shows a solvent dependence, resulting the linear
relationships between the Stokes shi (Dn) values of L3 in
various solvents vs DN and Df, respectively. And the studies on
the performances of their metal complexes are in progress.
Experimental section
Physical measurements
Elemental analyses (C, H, N) were carried out on elemental
1
analyzer. H and 13C NMR spectra were recorded on 400 MHz
and 600 MHz spectrometers in CDCl3 or d6-DMSO with tetra-
methylsilane (TMS) as an internal standard. Mass spectra were
obtained using EI. The UV-visible absorption and uorescence
emission spectra were measured on a Perkin-Elmer Lambda 35
UV-Vis and Edinburgh Instruments FLSP 920 spectrophotom-
eter, respectively. A low-pressure mercury lamp (16 W, 302 nm)
and a Philips U-shaped energy-saving light (70 W) with a lter
(l ¼ 520 nm) were used as light sources for photocoloration and
photobleaching, respectively. The UV (302 nm) irradiation times
for each cycle are 150 seconds for L1, 110 seconds for L2, 60
seconds for L3, 40 seconds for L4, and 180 seconds for L5,
respectively. The visible light (520 nm) irradiation times for
each cycle are 4 minutes for L2, and 30 minutes for other
samples, respectively. The photoluminescent quantum yields of
the open forms of L1–L5 are determined with excitation at
325 nm by the standard methodology32 with quinine sulfate in 1
N H2SO4 as reference (F ¼ 0.546 (ref. 33)). The relative cycli-
zation and cycloreversion quantum yields of L1–L5 were calcu-
lated with 1,2-bis(2-methylbenzothiophene-3-yl)peruorocy-
clopentene as the reference34 by the method described
previously.35
Fig. 4 Lippert–Mataga plot.
relationship is established between the Stokes shi (Dn) values
of L3 in various solvents and the DN (Fig. S3†). Regarding the
DN as a measurement of the electron-donating ability of the
solvent molecule,30 the good linear relationship suggests that
there are donor–acceptor interactions between L3 and solvent.
The effect of the solvents on the emission features can
further be evaluated from a Lippert–Mataga plot.31 As shown in
Fig. 4, a good linear relationship is also observed between the
Dn values of L3 and Df, which is a function of the dielectric
constant 3 and the refractive index n.28 The positive linearity of
the Lippert–Mataga plot of L3 indicates that the dipole moment
in the excited state is higher than that in the ground state.
According to the previous suggestion for explaining the solvent-
dependent photochromism of related donor–acceptor BTEs,21a
such phenomena may attributed to the generation of a
twisted intramolecular charge transfer (TICT) state upon
excitation of L3.
Conclusions
General procedures
In summary, a series of PI-BTEs (L1–L5) with 2-(2-pyridyl)- All chemicals for synthesis were purchased from commercial
imidazole or its analogue as the ethene bridges have been suppliers without further purication. And solvents were puri-
prepared. It is demonstrated that the uorescent and photo- ed according to standard procedures. Compounds 1–8 were
chromic properties of these PI-BTEs can be modulated by the prepared according to the literature procedures.12,13,17,36
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124.3, 124.2, 123.9, 122.5, 119.5, 117.7, 113.8, 108.7; anal. calcd
for C14H8Br2N4O2: C, 39.65; H, 1.90; N, 13.21. Found: C, 39.58;
H, 1.90; N, 13.19%.
2-Methylbenzo[b]thiophen-3-ylboronic acid (5). White solid,
800 mg, 29%; 1H NMR (400 MHz, d6-DMSO): d 8.14 (s, 1H), 7.91
(d, J ¼ 8.0 Hz, 1H), 7.82 (d, J ¼ 8.0 Hz, 1H), 7.92 (td, J ¼ 8.4, 1.2
Hz, 1H), 7.22 (td, J ¼ 8.4, 1.2 Hz, 1H), 2.64 (s, 3H).
L1–L5 were prepared by the cross-coupling reaction of the
corresponding boronic acid with 1-aryl-4,5-dibromo-2-(2-pyr-
idyl)imidazole according to the standard Suzuki coupling
procedure under an inert atmosphere of argon. Aqueous
Cs2CO3 solution (2 M, 5 mL, 10 mmol) was added to a vigorously
stirred solution of the corresponding boronic acid (2.5 mmol),
1-aryl-4,5-dibromo-2-(2-pyridyl)imidazole (1 mmol), Pd(PPh3)4
(0.1 mmol) in dioxane (20 mL) under reux. And the course of
the reaction was monitored by TLC. Aer the reaction nished,
the mixture was extracted with ethyl acetate. The combined
extracts were dried over anhydrous magnesium sulfate. Then
ltration and removal of the solvent in vacuum, the crude
product was puried by silica gel column chromatography to
give the corresponding product.
Compounds 9–11 were synthesized by a slight modication
of the literature procedures.37 2-(2-Pyridyl)imidazole (30 mmol),
halogenobenzene (50 mmol), CuI (10 mmol), 1,10-phenan-
throline (20 mmol) and Cs2CO3 (45 mmol) were dissolved in
anhydrous DMF (120 mL) under reux in an inert atmosphere
of argon for 30 h. Then the solvent was removed in vacuum, and
the crude product was puried by silica gel column chroma-
tography to give the corresponding product.
1-(4-Methoxyphenyl)-2-(2-pyridyl)imidazole (9). White solid,
7.3 g, 97%, mp 94–95 ꢀC; 1H NMR (400 MHz, CDCl3): d 7.66–7.68
(m, 1H), 7.43–7.45 (m, 1H), 7.35–7.38 (m, 2H), 7.25 (t, J ¼ 2.0 Hz,
1H), 7.12–7.21 (m, 2H), 6.98–7.00 (m, 2H), 6.63–6.65 (m, 1H),
3.83 (s, 3H); 13C NMR (100 MHz, CDCl3): d 158.2, 136.3, 128.9,
128.2, 125.9, 122.1, 121.0, 120.0, 114.7, 110.3, 102.8, 55.5; anal.
calcd for C15H13N3O: C, 71.70; H, 5.21; N, 16.72. Found: C,
71.65; H, 5.21; N, 16.70%.
1-(4-Methoxyphenyl)-4,5-bis-[5-chloro-2-methyl-3-thienyl]-2-
(2-pyridyl)imidazole (L1). White solid, 400 mg, 78%, mp 114–
1
ꢀ
115 C; H NMR (400 MHz, CDCl3): d 8.35 (d, J ¼ 4.0 Hz, 1H),
7.94 (d, J ¼ 8.0 Hz, 1H), 7.67–7.72 (m, 1H), 7.14–7.17 (m, 1H),
7.03–7.05 (m, 1H), 6.84–6.92 (m, 2H), 6.73 (d, J ¼ 1.6 Hz, 2H),
6.34 (s, 1H), 3.83 (s, 3H), 2.28 (s, 3H), 1.95 (s, 3H); 13C NMR (100
MHz, CDCl3): d 156.3, 148.7, 146.0, 138.3, 136.3, 134.9, 134.7,
133.1, 130.6, 129.9, 128.5, 127.7, 127.3, 126.2, 126.0, 125.2,
124.8, 124.6, 123.8, 122.8, 110.7, 55.7, 14.3, 13.9; MS (EI): m/z
(%): 515, 513, 511 (M+). Anal. calcd for C25H19Cl2N3OS2: C,
58.59; H, 3.74; N, 8.20. Found: C, 58.37; H, 3.75; N, 8.17%.
1-Phenyl-4,5-bis-[2-methylbenzo[b]thiophen-3-yl]-2-(2-pyr-
idyl)imidazole (L2). White solid, 200 mg, 39%, mp 201–202 ꢀC;
1H NMR (400 MHz, CDCl3): d 8.35–8.36 (m, 1H), 8.03 (d, J ¼ 8.0
Hz, 1H), 7.84–7.86 (m, 1H), 7.72 (td, J ¼ 8.0, 2.0 Hz, 1H), 7.65–
7.67 (m, 1H), 7.56 (d, J ¼ 8.0 Hz, 1H), 6.98–7.25 (m, 11H), 2.10 (s,
3H), 2.04 (s, 3H); 13C NMR (100 MHz, CDCl3): d 149.8, 148.7,
141.7, 140.4, 139.3, 138.6, 138.1, 137.6, 136.3, 128.3, 127.6,
127.1, 124.1, 124.0, 123.9, 123.6, 123.4, 123.1, 122.8, 122.2,
122.0, 121.6, 121.5, 14.9; MS (EI): m/z (%): 513 (M+). Anal. calcd
for C32H23N3S2: C, 74.82; H, 4.51; N, 8.18. Found: C, 74.69; H,
4.52; N, 8.15%.
1-Phenyl-2-(2-pyridyl)imidazole (10). White solid, 6.3 g, 94%
[lit.37 68%], mp 65–66 C.
ꢀ
1-(4-Nitrophenyl)-2-(2-pyridyl)imidazole (11). Yellow solid,
6.1 g, 76%, mp 122–123 ꢀC; 1H NMR (400 MHz, CDCl3): d 8.25–
8.29 (m, 2H), 8.22–8.24 (m, 1H), 8.11 (d, J ¼ 8.0 Hz, 1H), 7.77 (td,
J ¼ 8.0, 2.0 Hz, 1H), 7.41–7.45 (m, 2H), 7.32 (d, J ¼ 1.2 Hz, 1H),
7.17–7.20 (m, 2H); 13C NMR (150 MHz, CDCl3): d 148.9, 148.3,
146.7, 145.3, 144.7, 136.7, 129.8, 126.6, 124.2, 123.6, 123.2,
123.0; anal. calcd for C14H10N4O2: C, 63.15; H, 3.79; N, 21.04.
Found: C, 63.08; H, 3.80; N, 21.01%.
Compounds 12–14 were prepared by modication of a
literature method.38 To a solution of 1-aryl-2-(2-pyridyl)imid-
azole (20 mmol) in CHC13 (350 mL), Br2 (50 mmol) in CHC13 (15
mL) was added dropwise with stirring at room temperature.
Aer reaction for 2 h under reux, the resulting solid was
ltered and washed with aqueous NaHSO3 solution. The
residue was puried by column chromatography on silica gel to
afford the corresponding product.
1-(4-Methoxyphenyl)-4,5-dibromo-2-(2-pyridyl)imidazole (12).
1
ꢀ
1-Phenyl-4,5-bis-[5-chloro-2-methyl-3-thienyl]-2-(2-pyridyl)-
White solid, 8.0 g, 98%, mp 122–123 C; H NMR (400 MHz, d6-
DMSO): d 8.25–8.26 (m, 1H), 7.81–7.87 (m, 2H), 7.24–7.31 (m, 3H),
6.99–7.02 (m, 2H), 3.81 (s, 3H).
1
ꢀ
imidazole (L3). White solid, 338 mg, 70%, mp 123–124 C; H
NMR (400 MHz, CDCl3): d 8.30 (d, J ¼ 4.4 Hz, 1H), 7.90 (d, J ¼ 7.6
Hz, 1H), 7.67 (td, J ¼ 8.0, 2.0 Hz, 1H), 7.28–7.32 (m, 3H), 7.12–7.15
(m, 1H), 7.05–7.07 (m, 2H), 6.74 (s, 1H), 6.27 (s, 1H), 2.28 (s, 3H),
1.93 (s, 3H); 13C NMR (100 MHz, CDCl3): d 149.4, 148.7, 145.8,
138.2, 137.4, 136.2, 135.9, 134.9, 130.5, 128.7, 128.1, 127.6, 127.4,
126.3, 126.0, 125.8, 125.1, 123.7, 122.8, 14.3, 13.9; MS (EI): m/z (%):
485, 483, 481 (M+). Anal. calcd for C24H17Cl2N3S2: C, 59.75; H, 3.55;
N, 8.71. Found: C, 59.66; H, 3.54; N, 8.69%.
1-Phenyl-4,5-dibromo-2-(2-pyridyl)imidazole (13). White
solid, 5.5 g, 72%, mp 115–116 ꢀC; 1H NMR (400 MHz, d6-DMSO):
d 8.18–8.19 (m, 1H), 7.82–7.91 (m, 2H), 7.48–7.50 (m, 3H), 7.32–
7.37 (m, 2H), 7.26–7.30 (m, 1H); 13C NMR (150 MHz, d6-DMSO):
d 148.4, 147.6, 146.1, 144.6, 137.1, 136.9, 136.0, 129.1, 129.0,
128.9, 128.0, 127.8, 123.6, 122.8, 119.6, 117.0, 113.2, 108.9; anal.
calcd for C14H9Br2N3: C, 44.36; H, 2.39; N, 11.09. Found: C,
44.29; H, 2.39; N, 11.07%.
1-(4-Nitrophenyl)-4,5-dibromo-2-(2-pyridyl)imidazole (14).
Yellow solid, 6.9 g, 81%, mp 151–152 ꢀC; 1H NMR (400 MHz, d6-
DMSO): d 8.35–8.38 (m, 2H), 8.16 (d, J ¼ 4.8 Hz, 1H), 8.01 (d, J ¼
8.0 Hz, 1H), 7.89 (td, J ¼ 7.6, 1.6 Hz, 1H), 7.74–7.78 (m, 2H),
7.29–7.32 (m, 1H); 13C NMR (150 MHz, d6-DMSO): d 148.3,
147.5, 147.1, 145.9, 144.4, 143.0, 141.8, 137.2, 129.7, 129.5,
1-Phenyl-4,5-bis-[2,5-dimethyl-3-thienyl]-2-(2-pyridyl)imid-
azole (L4). White solid, 327 mg, 74%, mp 119–120 ꢀC; 1H NMR
(400 MHz, CDCl3): d 8.29 (d, J ¼ 4.8 Hz, 1H), 7.94 (d, J ¼ 8.0 Hz,
1H), 7.66 (td, J ¼ 7.6, 1.6 Hz, 1H), 7.25–7.27 (m, 3H), 7.05–7.12
(m, 3H), 6.62 (s, 1H), 6.08 (s, 1H), 2.36 (s, 3H), 2.24 (s, 3H), 2.21
(s, 3H), 1.89 (s, 3H); 13C NMR (100 MHz, CDCl3): d 149.9, 148.6,
145.1, 138.0, 136.9, 136.6, 135.3, 134.8, 133.5, 130.9, 128.4,
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127.7, 127.6, 127.4, 127.1, 126.9, 126.5, 123.7, 122.4, 15.2, 15.1, 15 D. Sud, R. McDonald and N. R. Branda, Inorg. Chem., 2005,
14.2, 13.8; MS (EI): m/z (%): 441 (M+). Anal. calcd for C26H23N3S2:
C, 70.71; H, 5.25; N, 9.52. Found: C, 70.53; H, 5.27; N, 9.50%.
1-(4-Nitrophenyl)-4,5-bis-[5-chloro-2-methyl-3-thienyl]-2-(2-
44, 5960.
16 T. Fukaminato, T. Doi, N. Tamaoki, K. Okuno, Y. Ishibashi,
H. Miyasaka and M. Irie, J. Am. Chem. Soc., 2011, 133, 4984.
pyridyl)imidazole (L5). Yellow solid, 258 mg, 49%, mp 162–163 17 L. N. Lucas, J. J. D. D. Jong, J. H. Esch, R. M. Kellogg and
ꢀC; 1H NMR (400 MHz, CDCl3): d 8.16–8.21 (m, 4H), 7.76 (td, J ¼
B. L. Feringa, Eur. J. Org. Chem., 2003, 155.
7.6, 1.6 Hz, 1H), 7.23–7.25 (m, 2H), 7.15–7.19 (m, 1H), 6.67 (s, 18 (a) Y. Chen, D. X. Zeng, N. Xie and Y. Z. Dang, J. Org. Chem.,
1H), 6.30 (s, 1H), 2.35 (s, 3H), 1.97 (s, 3H); 13C NMR (100 MHz,
CDCl3): d 148.8, 148.3, 146.9, 145.4, 143.5, 138.9, 136.7, 135.4,
129.7, 128.7, 127.2, 126.9, 126.0, 125.5, 125.2, 124.0, 123.4,
123.2, 14.4, 13.9; MS (EI): m/z (%): 530, 528, 526 (M+). Anal. calcd
for C24H16C12N4O2S2: C, 54.65; H, 3.06; N, 10.62. Found: C,
54.48; H, 3.07; N, 10.59%.
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Acknowledgements
This work was nancially supported by the CAEP (2012B0302039),
KJCX-201204, QNRC-201207, and NSF of Fujian Province
(2011J01065).
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