Novel disubstituted phenylene-linked bis-imidazole derivatives: Facile synthesis and optical properties

Article abstract of DOI:10.1002/hlca.201000014

Six novel disubstituted phenylene-linked bis-imidazole derivatives, 3a-3f, were prepared by a one-pot, microwave-assisted method under solvent-free conditions, in yields ranging from 61.6 to 85.6%. The new compounds were characterized by ¹H- and ¹³C-NMR, UV/VIS, and fluorescence spectroscopy, and mass spectrometry, as well as by elemental analyses. The influence of substituents and solvents on the optical properties of 3a-3f was investigated. It was found that there is little influence on absorption and excitation spectra in contrast to emission spectra. Compounds 3a-3f exhibit strong fluorescence in solution, their fluorescence quantum yields ranging from 0.27 to 0.96.

Full text of DOI:10.1002/hlca.201000014

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Novel Disubstituted Phenylene-Linked Bis-imidazole Derivatives:
Facile Synthesis and Optical Properties
by Guang-Liang Song, Hong-Jun Zhu*, Lei Chen, Shan Liu, and Zhong-Hua Luo
Department of Applied Chemistry, College of Science, Nanjing University of Technology,
Nanjing 210009, P. R. China
(phone: þ 86-25-83172358; fax: þ 86-25-83172358; e-mail: zhuhjnjut@hotmail.com)
Six novel disubstituted phenylene-linked bis-imidazole derivatives, 3a – 3f, were prepared by a one-
pot, microwave-assisted method under solvent-free conditions, in yields ranging from 61.6 to 85.6%. The
new compounds were characterized by 1H- and 13C-NMR, UV/VIS, and fluorescence spectroscopy, and
mass spectrometry, as well as by elemental analyses. The influence of substituents and solvents on the
optical properties of 3a – 3f was investigated. It was found that there is little influence on absorption and
excitation spectra in contrast to emission spectra. Compounds 3a – 3f exhibit strong fluorescence in
solution, their fluorescence quantum yields ranging from 0.27 to 0.96.
Introduction. – New molecular fluorophores, which play an important role in
fluorescence sensors [1 – 3], fluorescence imaging [4][5], and fluorescence switching
[6 – 8], have recently attracted great interest in the analysis of biological systems [9 –
13] and optical applications [14 – 18]. Among these molecular fluorophores, phenyl-
ene-linked bis-imidazole derivatives have been used due to their unique optical
properties [19 – 21]. In addition, they can be used in two-photon fluorescence and as pH
probe [22][23]. To the best of our knowledge, several reports on the synthesis and on
the physical properties of tetrasubstituted phenylene-linked bis-imidazole compounds
have been published [24 – 29], and the synthesis of phenylene-linked bis-imidazole
compounds was only accomplished by condensation of 1,2-dicarbonyl compounds and
aldehydes with NH3. However, to our surprise, studies on the synthesis and optical
properties of disubstituted phenylene-linked bis-imidazole compounds were limited. In
1975, Mahadik and Sunthankar [30] reported the synthesis of some N,N’-disubstituted
phenylene-linked bis-imidazoles with yields of 50 – 60%. Since then, there were no
reports on these compounds, except for the one of Jadhav and co-workers [24] in 2008,
who described the synthesis of 2,2’-(1,4-phenylene)bis[5-phenyl-1H-imidazole] only,
without giving any optical properties. Considering the above reports, the synthesis of
novel disubstituted phenylene-linked bis-imidazoles and the study of the underlying
structure – property relationships was a challenge.
In this article, a microwave-assisted and solvent-free method for the synthesis of 5-
aryl-substituted phenylene-linked bis-imidazoles is reported, involving condensation of
benzene-1,4-dicarboximidamidium dichloride with 1-aryl-2-bromoethanones in the
presence of using KHCO3 as acid-binding agent. The six novel disubstituted phenylene-
linked bis-imidazole derivatives 3a – 3f were synthesized, characterized, and their
optical properties investigated.
ꢀ 2010 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Results and Discussion. – Synthesis and Characterization. Monoimidazoles have
already been prepared with an efficient procedure by using benzamidine hydrochloride
monohydrate and 1-aryl-2-bromoalkan-1-ones as substrates [31 – 36]. According to
these method, disubstituted phenylene-linked bis-imidazole derivatives were obtained
with quite low yields after refluxing for 72 h and purification by silica-gel column
chromatography (e.g., for compound 3a only 30% yield). Considering that the
microwave-assisted synthesis method has been efficient in some condensation reactions
[37 – 39], an improved method for synthesis of 3a – 3f was elaborated successfully
under solvent-free conditions by microwave irradiation. Under the microwave-assisted
conditions, the reaction time to yield, e.g., 3a was dramatically reduced from the 72 h in
the conventional heating to just a few minutes. More important, the starting materials
and by-products in the crude products were easily removed by washing with hot brine
followed by recrystallization from MeOH/THF 1:1. The desired products 3a – 3f were
thus obtained in 61.6 – 85.6% yields (Scheme). The improved method is an environ-
mentally friendly and effective procedure for the synthesis of disubstituted phenylene-
linked bis-imidazoles 3, starting from benzene-1,4-dicarboximidamidium dichloride (1)
and 1-aryl-2-bromoethanones 2. All the compounds were obtained as white solids, and
1
their structure was confirmed by H- and 13C-NMR spectra, EI-MS, and elemental
analyses. Compounds 3a – 3f were soluble in CHCl3, toluene, THF, MeOH, and
HCO2H in varying degrees.
Scheme. Synthesis of Bis-imidazole Derivatives 3
Photophysical Properties of 3a – 3f in Solution. The absorption spectra of the bis-
imidazoles 3a – 3f in MeCN solution (10ꢀ5 m) are shown in Fig. 1, and the absorption
properties are given in Table 1. There are two absorption bands at ca. 275 and 340 nm,
and they all can be assigned to the p – p* electron transitions by virtue of their large
epsilon (e ꢁ 104 mꢀ1 cmꢀ1). All disubstituted phenylene-linked bis-imidazoles 3a – 3f
Abs
have an almost identical absorption maximum (l ), ranging from 339 to 348 nm,
max
Abs
which suggests that the l
of these compounds mainly depends on the length of the
max
main conjugated structure with little effect of the substituents.
The dilute-solution (10ꢀ5 m) fluorescence spectra of 3a – 3f in MeCN are shown in
Fig. 2, and the fluorescence properties and corresponding Ff are given in Table 1. All
compounds 3a – 3f emit blue light with an emission maximum in the range of 389.5 –
419.5 nm. Compared to 3a, the alkyl-substituted 3b and 3e and chloro-substituted 3c
show blue-shifted emission spectra, while the thiophene-substituted 3d and the MeO-
substituted 3f show a red shift. The largest red shift (17.5 nm) of 3d is a result of

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Fig. 1. UV/VIS Absorption spectra of 3a – 3f in MeCN solution (c ¼ 10ꢀ5 m)
Table 1. UV/VIS Absorption and Fluorescence Data for 3a – 3f in MeCN (c ¼ 10ꢀ5 m)
b
e [10ꢀ4 mꢀ1 cmꢀ1
]
l
[nm]
l
[nm]
l
[nm] Stokes shift [nm] EOg pt a) [eV] Ff
)
Abs
Ex
Em
max
max
max
3a 3.80
345
340
340
348
339
348
347
402.0
392.5
389.5
419.5
396.5
414.0
55.0
54.0
49.0
70.5
64.5
65.5
3.19
3.25
3.25
3.06
3.28
3.16
0.74
3b 4.73
3c 2.35
3d 3.23
3e 4.15
3f 3.12
338.5
340.5
349
332
348.5
0.88
0.64
0.27
0.96
0.77
a) Estimated from the onset of the absorption spectra (EgOpt ¼ 1240/lonset). b) Determined in MeCN
solution (Abs < 0.1) at room temperature, with 9,10-diphenylanthracene (Ff ¼ 0.90 in hexane) as
standard.
increased electron density associated with the thiophene group and the weak
intramolecular donor– acceptor interaction inducing a polarization in the electronically
excited state [40]. The fluorescence quantum yield (Ff) of compounds 3a – 3f in MeCN
solution is relatively high, ranging from 0.27 for 3d to 0.96 for 3e by using 9,10-
diphenylanthracene (Ff ¼ 0.90 in hexane) [41] as a reference. Comparison of
compound 3a with 3b and 3e shows that replacing the H-atoms in para- or ortho-
position by Me groups leads to an increase of Ff by more than 15%. Compared to 3a,
the Ff of 3d is significantly lower. The low emission quantum yield of 3d is largely due
to the donor– acceptor– donor nature of this bis-imidazole, and consequently to
intramolecular charge transfer, which is known as an important luminescence-
quenching mechanism in such donor– acceptor systems [42]. Comparison of compound

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Fig. 2. Fluorescence spectra of 3a – 3f in MeCN solution (c ¼ 10ꢀ5 m)
3a with 3f indicates that the substitution of the para H-atoms of the phenyl substituents
by MeO groups hardly changes the fluorescence quantum yield. Comparison of
compound 3a with 3c indicates that the substitution of the para H-atoms by Cl
substituents leads to a slight decrease of the fluorescence quantum yield 3c, the low
value of Ff being the result of competing S1 ! T1 intersystem crossing because of the
ꢂinternal heavy-atom effectꢃ [22]. Optical band gaps (EOg pt ) determined by the
absorption edge of the solution spectra are also given in Table 1, which vary from
3.06 eV in 3d to 3.28 eV in 3e.
Solvent Effects on the Absorption and Fluorescence Properties. The absorption and
emission spectra of 3d in various solvents, i.e., in several protic (MeOH and HCO2H)
and aprotic (MeCN, acetone, CDCl3, AcOEt, Et2O, toluene, and THF) solvents, are
shown in Figs. 3 and 4. Compound 3d shows a pronounced change in position and
intensity of the absorption or emission band accompanying a change in the polarity of
the solvent. This phenomenon can also be explained by the effects of charge transfer
which cause the change of color [25]. The UV/VIS absorption and fluorescence
properties of 3d in various solvents are given in Table 2, 3d has a nearly identical
Abs
Ex
absorption maximum (l ) (from 348 to 351 nm) and excitation maximum (l
)
max
max
(from 348.5 to 354 nm) in the above nine solvents. A red shift of the emission maximum
Em
(l ), which ranges from 397 nm in nonpolar solvents such as toluene, to 419.5 nm in
max
medium-polar solvents such as MeCN, and then to 438 nm in strongly polar solvents
such as MeOH. The common feature of these latter solvents is the presence of a lone
pair of electrons which can form an H-bond with the NꢀH group of 3d. In HCO2H, the
red shift of the emission may be the result of the excited-state protonation [26].
Phenomena of color change in different solvents of these disubstituted phenylene-
linked bis-imidazoles such as 3d were more obvious than those of the tetrasubstituted

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Fig. 3. UV/VIS Absorption spectra of 3d in various solvents (c ¼ 10ꢀ5 m)
Fig. 4. Fluorescence spectra of 3d in various solutions (c ¼ 10ꢀ5 m)
phenylene-linked bis-imidazoles [25]. This might be due to the steric hindrance. The
disubstituted phenylene-linked bis-imidazoles can easier form H-bonds with solvents
than the tetrasubstituted phenylene-linked bis-imidazoles.

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Table 2. UV/VIS Absorption and Fluorescence Properties of 3d in Various Solvents (c ¼ 10ꢀ5 m)
Abs
Ex
Em
e [10ꢀ4 mꢀ1 cmꢀ1
]
l
[nm]
l
[nm]
l
[nm]
Stokes shift [nm]
max
max
max
Toluene
AcOEt
Et2O
3.27
2.53
3.28
3.67
3.20
3.15
3.46
3.18
1.27
352
348
349
351
348
348
348
351
336
353
348.5
350
351.5
350
354
349
350
353
397
397
44
48.5
42.5
48
59.5
57.5
70.5
88
392.5
399.5
409.5
411.5
419.5
438
THF
Acetone
CHCl3
MeCN
MeOH
HCO2H
455.5
102.5
Conclusions. – In summary, a practical and effective microwave-assisted synthetic
method under solvent-free conditions was successfully applied to the synthesis of the
disubstituted phenylene-linked bis-imidazole derivatives 3a – 3f in 61.6 – 85.6% yields.
The optical properties of 3a – 3f were investigated, showing that this series of
disubstituted phenylene-linked bis-imidazole derivatives can serve as electron-trans-
porting electro-luminescent materials, blue-light-emitting materials, and nonlinear
optical materials.
This work was supported in part by the Academic Fund of Nanjing University of Technology (No.
39704026).
Experimental Part
General. All starting materials and solvents were obtained from commercial suppliers and used as
obtained. Compounds 2a [43], 2b [44], 2c [45], 2d [46], 2e [47], and 2f [48] were prepared according to
the published procedures. Microwave synthesis: WBFY-201 apparatus. M.p.: X-4 microscope electro-
thermal apparatus; uncorrected. UV and Fluorescence spectra: HP-8453-UV/VIS/near-IR spectropho-
Abs
Em
tometer and LS-55 spectrofluorometer, resp.; l
(e /104 mꢀ1 cmꢀ1) and l
in nm; Stokes shift in nm;
max
max
EOg pt in eV; optical pathlength of cells, 1 cm under all conditions; excitation wavelength ¼ l
of the
Em
max
concerned compound. H- and 13C-NMR Spectra: Bruker spectrometer at 500 and 300 MHz, resp.; in
CDCl3 or (D6)DMSO; d in ppm rel. to Me4Si as internal standard, J in Hz. EI-MS: Agilent 1100 LC/
DAD/MSD; in m/z. Elemental analyses: Vario-EL-III elemental analyzer.
1
Benzene-1,4-dicarboximidamide Hydrochloride (1 :2) (1). Benzene-1,4-dicarbonitrile (7.4 g,
58 mmol) was added in small portions over 30 min to a well-stirred soln. of (Me3Si)2NLi in THF
(240 ml, 240 mmol). The mixture was stirred at r.t. for another 3 h and then cooled to 08 in an ice bath.
The reaction was quenched by careful addition of 6m HCl/EtOH (200 ml), and the mixture set aside for
several hours. The precipitate was then filtered, washed with Et2O, and recrystallized from EtOH/H2O
1:1: 1 (10.4 g, 96%). White powder. M.p. > 3008 ([49]: > 3008). 1H-NMR ((D6)DMSO, 300 MHz): 9.62
(s, 2 þNH2¼); 8.37 (s, 2 NH2); 8.03 (s, 4 arom. H).
2-Bromo-1-phenylethanone (2a). Yield 85.0%. White powder. M.p. 50 – 518 ([43]: 49 – 518).
1H-NMR (CDCl3, 300 MHz): 7.98 (d, J ¼ 4.4, HꢀC(2’), HꢀC(6’)); 7.61 (dd, J1 ¼ J2 ¼ 4.41, HꢀC(4’));
7.48 (dd, J ¼ 4.53, 4.86, HꢀC(3’), HꢀC(5’)); 4.45 (s, CH2Br).
2-Bromo-1-(4-methylphenyl)ethanone (2b). Yield 85.5%. White powder. M.p. 51 – 538 ([44]: 51 –
528). 1H-NMR (CDCl3, 300 MHz): 7.88 (d, J ¼ 5.0, HꢀC(2’), HꢀC(6’)); 7.28 (d, J ¼ 4.8, HꢀC(3’),
HꢀC(5’)); 4.43 (s, CH2Br); 2.43 (s, Me).
2-Bromo-1-(4-chlorophenyl)ethanone (2c). Yield 90.2%. White plates. M.p. 95 – 968 ([45]: 95 – 968).
1H-NMR (CDCl3, 300 MHz): 7.93 (d, J ¼ 5.1, HꢀC(2’), HꢀC(6’)); 7.47 (d, J ¼ 5.2, HꢀC(3’), HꢀC(5’));
4.40 (s, CH2Br).

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2-Bromo-1-(thiophen-2-yl)ethanone (2d). Yield 88.0%. White plates. M.p. 31 – 338 ([46]: 33 – 358).
1H-NMR (CDCl3, 300 MHz): 7.71 (m, HꢀC(3’)); 7.62 (m, HꢀC(5’)); 7.17 (m, HꢀC(4’)); 4.37 (s, CH2Br).
2-Bromo-1-(2,4-dimethylphenyl)ethanone (2e). Yield 80.0%. White plates. M.p. 41 – 428 ([47]: 418).
1H-NMR (CDCl3, 300 MHz): 7.60 (d, J ¼ 4.7, HꢀC(2’)); 7.08 (d-like, 2 arom. H); 4.40 (s, CH2Br); 2.51 (s,
Me); 2.37 (s, Me).
2-Bromo-1-(4-methoxyphenyl)ethanone (2f). Yield 88.0%. White plates. M.p. 69 – 718 ([48]: 68 –
708). 1H-NMR (CDCl3, 300 MHz): 8.07 (d, J ¼ 5.4, HꢀC(2’), HꢀC(6’)); 6.95 (d, J ¼ 5.4, HꢀC(3’),
HꢀC(5’)); 4.39 (s, CH2Br); 3.89 (s, MeO).
Phenylene-Linked Bis-imidazole Derivatives 3: General Procedure. Hydrochloride 1 (1.18 g,
5 mmol), 1-aryl-2-bromoethanone 2 (10 – 12 mmol), KHCO3 (2 g, 20 mmol), and NaCl (5.85 g,
100 mmol) were thoroughly mixed in a mortar. The mixture was transferred into a beaker and irradiated
with microwaves for 2 min (300 W). Then, the mixture was cooled to r.t. and irradiated for 2 min again.
This procedure was repeated until the completion of the reaction (TLC (AcOEt)) monitoring. The crude
product was washed with hot brine and recrystallized from MeOH/THF 1:1.
2,2’-(1,4-Phenylene)bis[5-phenyl-1H-imidazole] (3a): Yield 75.2%. White solid. M.p. 283 – 2868
([24]: 187 – 2898). 1H-NMR ((D6)DMSO, 300 MHz): 12.71 (br., 2 NH); 8.11 (s, 4 arom. H); 7.87 (AA’ of
AA’BB’C, J ¼ 7.1, 4 arom. H); 7.79 (s, 2 arom. H); 7.39 (BB’ of AA’BB’C, J ¼ 7.3, 4 arom. H); 7.24 (C of
AA’BB’C, J ¼ 7.5, 2 arom. H). 13C-NMR ((D6)DMSO, 300 MHz): 145.6; 129.9; 128.9; 128.4; 127.9; 126.2;
125.1; 124.3. EI-MS: 363.2 ([M þ H]þ, C24H18N4þ ; calc. 362.15). Anal. calc. for C24H18N4 · 2 H2O (398.47):
C 72.34, H 5.57, N 14.06; found: C 72.25, H 5.60, N 13.99.
2,2’-(1,4-Phenylene)bis[5-(4-methylphenyl)-1H-imidazole] (3b): Yield 70.4%. White solid. M.p.
> 3108. 1H-NMR ((D6)DMSO, 300 MHz): 12.64 (br., 2 NH); 8.10 (s, 4 arom. H); 7.76 (AA’ of AA’BB’,
J ¼ 7.10, 4 arom. H); 7.68 (s, 2 arom. H); 7.21 (BB’ of AA’BB’, J ¼ 6.9, 4 arom. H); 2.33 (s, 2 Me).
13C-NMR ((D6)DMSO, 300 MHz): 145.5; 135.4; 129.9; 129.0; 125.1; 124.3; 114.0; 20.7. EI-MS: 391.2
([M þ H]þ, C26H22N4þ ; calc. 390.18), 389.1 ([M ꢀ H]þ), 779.0 ([2M ꢀ H]þ), 781.2 ([2M þ H]þ). Anal.
calc. for C26H22N4 · 2 H2O (426.51): C 73.22, H 6.14, N 13.14; found: C 73.26, H 6.18, N 13.19.
2,2’-(1,4-Phenylene)bis[5-(4-chlorophenyl)-1H-imidazole] (3c): Yield 85.6%. White solid. M.p.
> 3108. 1H-NMR ((D6)DMSO, 300 MHz): 12.67 (br., 2 NH); 8.10 (s, 4 arom. H); 7.89 (AA’ of AA’BB’,
J ¼ 8.3, 4 arom. H); 7.76 (s, 2 arom. H); 7.43 (BB’ of AA’BB’, J ¼ 8.3, 4 arom. H). 13C-NMR ((D6)DMSO,
300 MHz): 145.8; 133.5; 132.7; 130.5; 129.9; 128.5; 128.0; 126.0; 125.2; 124.5. EI-MS: 429.1 ([M ꢀ H]þ,
C24H16N4Clþ2 , calc. 430.08). Anal. calc. for C24H16Cl2N4 · 2 H2O (467.35): C 61.68, H 4.31, N 11.99; found: C
61.59, H 4.27, N 12.12.
2,2’-(1,4-Phenylene)bis[5-(thiophen-2-yl)-1H-imidazole] (3d): Yield 61.6%. White solid. M.p.
> 3108. 1H-NMR ((D6)DMSO, 300 MHz): 12.77 (br., 2 NH); 8.04 (s, 4 arom. H); 7.68 (s, 2 H); 7.35
(AB of ABC, 4 arom. H); 7.06 (C of ABC, J ¼ 4.6, 2 arom. H). 13C-NMR ((D6)DMSO, 300 MHz): 145.2;
138.5; 136.8; 129.7; 128.5; 127.6; 125.2; 124.6; 123.2; 121.4; 113.8. EI-MS: 373.1 ([M ꢀ H]þ, C20H14N4S2þ ;
calc. 374.07). Anal. calc. for C20H14N4S2 · 2 H2O (410.51): C 58.52, H 4.42, N 13.65; found: C 58.59, H 4.37,
N 13.52.
2,2’-(1,4-Phenylene)bis[5-(2,4-dimethylphenyl)-1H-imidazole] (3e): Yield 65.6%. White solid. M.p.
> 3108. 1H-NMR ((D6)DMSO, 300 MHz): 12.64 (br., 2 NH); 8.10 (s, 4 arom. H); 7.73 (s, 2 arom. H); 7.38
(s, 2 arom. H); 7.07 (BC of ABC, 4 arom. H); 2.30 (s, 2 Me); 2.17 (s, 2 Me). 13C-NMR ((D6)DMSO,
300 MHz): 144.8; 135.4; 134.2; 131.3; 130.0; 128.0; 126.2; 125.1; 21.4; 20.6. EI-MS: 419.3 ([M þ H]þ,
C28H26Nþ4 ; calc. 418.22), 417.3 ([M ꢀ H]þ). Anal. calc. for C28H26N4 · 2 H2O (454.56): C 73.98, H 6.65, N
12.33; found: C 73.79, H 6.57, N 12.48.
2,2’-(1,4-Phenylene)bis[5-(4-methoxyphenyl)-1H-imidazole] (3f): Yield 64.8%. White solid. M.p.
1
> 3108. H-NMR ((D6)DMSO, 300 MHz): 12.59 (br., 2 NH); 8.42 (s, 4 arom. H); 7.80 (m, 6 arom. H);
6.91 (s, 4 arom. H); 3.78 (s, 2 MeO). 13C-NMR ((D6)DMSO, 300 MHz): 158.0; 157.2; 145.3; 141.2; 130.0;
125.7; 125.1; 113.9; 113.2; 55.6. EI-MS: 423.2 ([M þ H]þ, C26H22N4O2þ ; calc. 422.17), 421.1 ([M ꢀ H]þ).
Anal. calc. for C26H22N4O2 · 2 H2O (458.51): C 68.11, H 5.72, N 12.22; found: C 68.29, H 5.57, N 12.16.

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Received January 11, 2010