Synthesis, characterization, optical properties and theoretical calculations of blue light-emitting 1-alkyl-2-substituted benzimidazoles

Article abstract of DOI:10.14233/ajchem.2013.14025

A series of novel blue light-emitting 1-alkyl-2-substituted benzimidazoles (4a-4g) were synthesized by reactions of the 2-substituted benzimidazoles (3a-3e) with ethyl bromide or n-butyl bromide as electrophilic reagent. Compounds 3a, 3c, 3e were synthesized by the condensation of 1a-1c with 1,2-diamino benzene o-phenylene diamine using air as oxidizing agent and compounds 3b, 3d were synthesized by the condensation of 2d-2e with 1,2-diamino benzene o-phenylene diamine using phosphoric acid and polyphosphoric acid as catalyst. Their optical properties were investigated by UV-visible spectroscopy and photoluminescence techniques in solution. They emit violetblue light (lEm max = 380-440 nm) with fluorescence quantum yields of 0.43 to 0.92 while diluted in acetonitrile solution. In order to find the relationship between the structures and properties of compounds 4a-4g, theoretical calculations using density functional theory at the B3LYP/6-31g*level were also studied.

Full text of DOI:10.14233/ajchem.2013.14025

Asian Journal of Chemistry; Vol. 25, No. 8 (2013), 4481-4486
http://dx.doi.org/10.14233/ajchem.2013.14025
Synthesis, Characterization, Optical Properties and Theoretical
Calculations of Blue Light-Emitting 1-Alkyl-2-substituted Benzimidazoles
1,2
1
1
1
1,*
HONG SHI , TAO WU , PENG JIANG , XIAO-DONG JIN and HONG-JUN ZHU
1Department of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, P.R. China
2Teaching and Research Department, Jiangsu College of Information Technology, Wuxi 214153, P.R. China
*Corresponding author: Fax: +86 25 83587428; Tel: +86 25 83172358; E-mail: zhuhjnjut@hotmail.com; shihong@njut.edu.cn
(Received: 7 May 2012;
Accepted: 15 February 2013)
AJC-12993
A series of novel blue light-emitting 1-alkyl-2-substituted benzimidazoles (4a-4g) were synthesized by reactions of the 2-substituted
benzimidazoles (3a-3e) with ethyl bromide or n-butyl bromide as electrophilic reagent. Compounds 3a, 3c, 3e were synthesized by the
condensation of 1a-1c with 1,2-diamino benzene o-phenylene diamine using air as oxidizing agent and compounds 3b, 3d were synthesized
by the condensation of 2d-2e with 1,2-diamino benzene o-phenylene diamine using phosphoric acid and polyphosphoric acid as catalyst.
Their optical properties were investigated by UV-visible spectroscopy and photoluminescence techniques in solution. They emit violet-
blue light (λEmmax = 380-440 nm) with fluorescence quantum yields of 0.43 to 0.92 while diluted in acetonitrile solution. In order to find
the relationship between the structures and properties of compounds 4a-4g, theoretical calculations using density functional theory at the
B3LYP/6-31g* level were also studied.
Key Words: Benzimidazole, Synthesis, Optical property, Fluorescence, Theoretical calculation.
Many efforts have been taken to improve the character-
istic of the benzimidazole-based compounds. Chih-Hsin et al.13
designed benzimidazole compounds by introduction of
benzene ring on N atom and the chromophores or auxochromes
on 2 position of imidazole ring. Nomura et al.14 reported novel
substituted 2-(4-styrylphenyl)-1H-benzimidazole that could be
used as multifunctional fluorescent materials. However, these
compounds have the disadvantage of poor solubility because
of the larger π-conjugated system. Our research group15
reported several terphenyl-bridged bisbenzimidazoles which
obviously improved the solubility and exhibited satisfactory
fluorescence properties. In this study, a series of novel small
molecular benzimidazoles (4a-4g) were designed and synthe-
sized (Scheme-I and II). For 4f and 4g, triphenylamine group
and carbazole group were introduced because of their blue
electroluminescent properties16, high fluorescence quantum
efficiency17 and good hole-transporting abilities18. The optical
properties of these compounds and theory calculation were
also investigated.
INTRODUCTION
In recent years organic light-emitting diodes have attracted
much interest due to their potential application in flat panel
displays1,2. The blue light-emitting material is one of the
important research subjects.Although great deals of blue light-
emitting materials have been described in literature, the
performance of their device leaves much to improve in colour
purity, luminance efficiency and device stability3,4. The
design and synthesis of blue emitters suitable for fabrication
of stable organic light-emitting diode devices, however, remain
major challenges for researchers.
Benzimidazole-based compounds were found to have
intensive fluorescence and higher fluorescence efficiency and
are widely used as fluorescence probes5-7 fluorescent bright-
ening agents8 as components in organic light-emitting diodes9
and in organic field effect transistors10. They also bear electron-
withdrawing imine nitrogen (C=N) moieties, which behave
as electron-accepting molecules that can facilitate electro-
chemical reduction, a feature that renders them suitable electron
carriers11. In addition, for organic light-emitting diodes, the
purity of materials directly affects the emission efficiency and
operational stability. Small molecular organic materials have
definite structures and can be purified by proper methods12,
which can solve the problems.
EXPERIMENTAL
Melting points were measured on an X-4 microscope
electrothermal apparatus (Taike) and remained uncorrected.
1H NMR and 13C NMR spectra were recorded on either a
Bruker AV-500 spectrometer at 500 MHz or a Bruker AV-300

4482 Shi et al.
Asian J. Chem.
H
N
NH2
NH2
DMF,NH4Cl
70-85 ºC
R1
CHO
R1
N
1a-1b
2:OPDA
3a-
3b
1a: R1=H,
1b: R1=CH3
3a: R1=H,
3b: R1=CH3
H
N
PA,PPA
NH2
NH2
Br
CN
Br
180
8h
ºC,
N
2:OPDA
3c
1c
TBAB
K2CO3,DMA,Cu,140 ºC, 48 h
N
CN
Br
CN
NH
2d
1d
OPDA, PPA,PA
H
N
N
N
185 ºC
24 h
3d
H
N
OPDA, NH4Cl, DMF
80ºC, 5 h
DMF, POCl3
0-45ºC, 6 h
N
N
N
CHO
N
1e
2e
Scheme-I: Synthesis and molecular structure of 2-substituted benzimidazoles
3e
spectrometer at 300 MHz, using DMSO-d6 and CDCl3 as
the solvents, with tetramethylsilane (TMS) as the internal
standard. Electrospray ionization-mass spectroscopy (ESI-
MS) measurements were carried out with an Agilent 1100
series LC/MSD Trap SL mass spectrometer. Elemental analyses
were performed with aVario EL III elemental analyzer. Optical
absorption spectra were obtained using an HP-8453 UV/VIS/
near-IR spectrophotometer (Agilent). Photoluminescence
spectra were carried out on an LS-55 spectrofluorometer
(PerkineElmer).
formamide (DMF) and POCl3 according to the reported
method20.
Synthesis of 2-substituted-1H-benzimidazoles (3a-3e):
2-Substituted-1H-benzimidazoles (3a-3e) were synthesized by
condensation of o-phenylenediamine with aromatic carboxylic
aldehydes (2a-2e) in the presence of N,N-dimethylformamide
(DMF) or in the presence of phosphoric acid and polyphosphoric
acid according to the reported methods21-23
.
2-Phenyl-1H-benzimidazole (3a):Yield: 83.4 %; yellow
crystals; m.p. 289-290 ºC [lit24]. 285-287 ºC).
All the chemicals for synthesis were purchased either from
Sinopharm Chemical Reagent Co. Ltd. or from Shanghai
Lingfeng Chemical Company and solvents were purified
according to standard procedures. 4-(Carbazole-9-yl)benzo-
nitrile (2d) was synthesized from carbazole and 4-bromo-
benzonitrile according to methods described in the literature19.
4-(N,N-diphenylamino)benzaldehyde (2e) was synthesized by
formylation of p-phthalic acid (TPA) using N,N-dimethyl
2-(4-Methylphenyl)-1H-benzimidazole (3b):Yield: 77.6
%; blue crystals; m.p. 295-296 ºC [lit24].
2-(4-Bromophenyl)-1H-benzimidazole (3c):Yield: 87.7
%; white crystals; m.p. 293-295 ºC [lit25]. 296-298 ºC).
2-{4-(9H-carbazole-9-yl)phenyl}-1H-benzimidazole
(3d): Yield: 58.9 %; light yellow crystals; m.p. > 300 ºC; 1H
NMR (DMSO-d6, 300 MHz): δ 7.25-7.18 (m, 2H, Ar-H), 7.33
(t, J = 7.32, 2H, Ar-H), 7.61-7.45 (m, 4H,Ar-H), 7.71-7.66

Vol. 25, No. 8 (2013)
Synthesis and Properties of Blue Light-Emitting 1-Alkyl-2-substituted Benzimidazoles 4483
(m, 2H, Ar-H), 7.84 (d, J = 8.32 Hz, 2H, Ar-H ), 8.27 (d, J =
7.76Hz, 2H, Ar-H), 8.47(d, J = 8.32 Hz, 2H, Ar-H), 13.1 (s,
(M+2H+); Anal. calcd. for C18H20N2: C, 81.78; H, 7.63; N,
10.60; Found C, 81.75; H, 7.61; N, 10.63.
1
1H, Ar-NH); C NMR (DMSO-d6, 300 MHz): δ 151.05,
1-Ethyl-2-(4-bromophenyl)benzimidazole (4d): Com-
pound 4d was obtained from 2-(4-bromophenyl)-1H-benzimi-
dazole (3c) and ethyl bromide.Yield: 94.8 %; yellow crystals;
m.p.137-138 ºC; 1H NMR (CDCl3, 500 MHz, ppm): δ 1.44 (t,
J = 7.2 Hz,3H, CH3), 4.25(q, J = 7.2 Hz, 2H, CH2), 7.33-7.29
(m, 2H, Ar-H), 7.42-7.40 (m, 1H, Ar-H), 7.59 (d, J = 8.15 Hz,
2H, Ar-H), 7.65 (d, J = 8.4 Hz, 2H, Ar-H), 7.83-7.81 (m, 1H,
Ar-H); 1C NMR (CDCl3, 300 MHz): δ 152.15, 143.10, 135.36,
131.91, 130.66, 129.47, 124.20, 122.88, 122.47, 119.99,
109.88, 39.55, 15.18; MS: m/z 301.0 (M+H+), 304.0 (M+4H+),
323.0 (M+Na+);Anal. calcd for C15H13BrN2: C, 59.82; H, 4.35;
N, 9.30; Found C, 59.86; H, 4.31; N, 9.35.
140.36, 138.60, 129.57, 129.48, 122.68, 127.46, 126.90,
123.46, 121.09, 120.85, 110.31; Anal. calcd. for C25H17N3: C,
83.54; H, 4.77; N, 11.69; Found: C, 83.57; H, 4.76; N, 11.60.
4-(1H-benzimidazol-2-yl)-N,N-diphenylbenzenamine
(3e): Yield: 69.3 %; yellow crystals; m.p. > 300 ºC; 1H NMR
(DMSO-d6, 300 MHz): δ 7.05 (d, 2H, J = 8.61 Hz), 7.18-7.10
(m, 8H, Ar-H), 7.37 (t, 4H, J = 7.38 Hz, Ar-NH), 7.74-7.56
(m, 2H, Ar-H), 8.05 (d, 2H, J = 8.61 Hz, Ar-NH), 12.71 (s,
1
1H, Ar-NH); C NMR (DMSO-d6, 300 MHz): δ 151.14,
148.65, 146.57, 139.65, 127.57, 124.82, 123.83, 123.31,
121.66, 121.57; Anal. calcd for C25H19N3: C, 83.08; H, 5.30;
N, 11.63; Found: C, 83.11; H, 5.28; N, 11.61.
1-Butyl-2-(4-bromophenyl)benzimidazole (4e): Com-
pound 4e was obtained from 2-(4-bromophenyl)-1H-benzimi-
dazole (3c) and n-butyl bromide.Yield: 93.3 %; yellow crystals;
m.p. 81-83 ºC; 1H NMR (CDCl3, 500 MHz, ppm): δ 0.88 (t,
J = 7.35 Hz, 3H, CH3), 1.31-1.24 (m, 2H, CH2), 1.82-1.76 (m,
2H, CH2), 4.21 (t, J= 7.65 Hz, 2H, CH2), 7.33-7.29 (m, 2H,
Ar-H), 7.47-7.40 (m, 1H, Ar-H), 7.58 (d, J = 8.3 Hz, 2H, Ar-
H), 7.65 (d, J = 8.2 Hz, 2H, Ar-H), 7.82-7.80 (m, 1H, Ar-H);
1C NMR (CDCl3, 500 MHz): δ 152.50, 143.05, 135.64, 131.96,
130.80, 129.65, 128.27, 124.23, 122.79, 120.01, 114.97,
110.14, 44.58, 31.86, 19.93, 13.51; MS: m/z 329.1 (M+H+),
332.1 (M+4H+), 351.0 (M+Na+); Anal. calcd. for C17H17BrN2:
C, 62.02; H, 5.20; N, 8.51; Found C, 62.04; H, 5.19; Br, 24.23;
N, 8.54.
Synthesis of (4a-4g): 1-Ethyl-2-phenylbenzimidazole
(4a): 2-Phenyl-1H-benzimidazole (3a) (1.94 g, 10 mmol) and
DMF (30 mL) were put into a 50 mL four-necked flask
equipped with a temperature probe and magnetic stirrer and
stirred to dissolve. The whole mixture was cooled to 0 ºC using
ice bath. After adding NaH (1.44 g, 60 mmol), the mixture
was warmed to room temperature and stirred for 1.5 h. Then
ethyl bromide (1.31 g, 12 mmol) was added and the mixture
was heat to 45 ºC.After reacting for 6-10 h at that temperature,
saturated salt water (10 mL) was added dropwise and the
reaction stopped. The mixture was extracted with ethyl
acetate (3*10 mL), washed with saturated salt water and
separated. The organic layer was dried over MgSO4, filtered
and concentrated under reduced pressure to give 4a with
following properties: white crystal; m.p. 89-91 ºC (reference26):
88-88.5 ºC);Yield: 93.1 %. 1H NMR (CDCl3, 300 MHz, ppm):
δ 1.46 (t, J = 7.6 Hz, 3H, CH3), 4.26 (q, J = 7.6 Hz, 2H, CH2),
7.42-7.48 (m, 3H, Ar-H), 7.50-7.56 (m, 3 H, Ar-H), 7.71-7.81
(m, 3 H, Ar-H); Anal. calcd. for C15H14N2: C 81.05, H 6.35, N
12.60; found C 81.09, H 6.33, N 12.58.
1-Ethyl-2-{(4-(9H-carbazole-9-yl)phenyl}benzimid-
azole (4f): Compound 4f was obtained from 2-{4-(9H-carba-
zole-9-yl)phenyl}-1H-benzimidazole (3d) and ethyl bromide.
1
Yield: 88.7 %; yellow crystals; m.p. 166-168 ºC; H NMR
(CDCl3, 500 MHz, ppm): δ 1.58 (t, J = 7.25 Hz, 3H, CH3),
4.45-4.40 (q, J = 7.25 Hz, 2H, CH2), 7.38-7.30 (m, 4H, Ar-H),
7.51-7.43 (m, 5H, Ar-H), 7.76 (d, J = 8.35 Hz, 2H, Ar-H),
7.89-7.87 (m, 1H, Ar-H), 8.00 (d, J = 8.35 Hz, 2H, Ar-H),
8.16 (d, J = 7.7 Hz, 2H, Ar-H); 1C NMR (CDCl3, 300 MHz): δ
152.48, 140.57, 139.25, 135.47, 130.82, 129.35, 127.16,
126.13, 123.68, 123.02, 122.65, 120.41, 110.02, 109.73, 39.81,
15.40; MS: m/z 388.2 (M+H+), 389.2 (M+2H+), 410.2
(M+Na+); Anal. calcd. for C27H21N3: C, 83.69; H, 5.46; N,
10.84; Found C, 83.72; H, 5.44; N, 10.84.
1-Ethyl-2-(4-methylphenyl)benzimidazole (4b): Follo-
wing the same procedure described for the synthesis of 4a,
2-(4-methylphenyl)-1H-benzimidazole (3b) and ethyl bromide
were used to obtained product 4b.Yield: 92 %; white crystals;
m.p. 104-106 ºC; 1H NMR (CDCl3, 500 MHz, ppm): δ 1.46 (t,
J = 7.25 Hz, 3H, CH3), 2.44 (s, 3H, CH3), 4.28 (q, J = 7.25
Hz, 2H, CH2), 7.33-7.29 (m, 4H, Ar-H), 7.43-7.40 (m, 1H,
Ar-H), 7.62(d, J = 8.05 Hz, 2H, Ar-H), 7.84-7.81 (m, 1H, Ar-
4-(1-Ethyl-1H-benzimidazol-2-yl)-N,N-diphenyl-
benzenamin (4g): Compound 4g was obtained from 4-(N,N-
diphenylaminophenyl)-1H-benzimidazole (3e) and ethyl
bromide. Yield: 90.2 %; light yellow crystals; m.p. 170-172
ºC; 1H NMR (CDCl3, 300 MHz, ppm): δ 1.48 (t, J = 7.17 Hz,
3H, CH3), 4.30 (q, J = 7.17 Hz, 2H, CH2), 7.10-7.05 (m, 2H,
Ar-H), 7.24-7.15 (m, 6H, Ar-H), 7.32-7.26 (m, 6H, Ar-H),
7.41-7.38 (m, 1H, Ar-H), 7.59 (d, J = 8.46 Hz, 2H, Ar-H),
1
H); C NMR (CDCl3, 300 MHz): δ 152.60, 143.19, 139.83,
135.38, 129.40, 129.13, 127.65, 122.53, 122.26, 119.89,
109.81, 39.56, 21.40, 15.22; MS: m/z 237.1 (M+H+), 238.1
(M+2H+), 359.1 (M+Na+);Anal. calcd. for C16H16N2: C, 81.32;
H, 6.82; N, 11.85; Found : C, 81.35; H, 6.78; N, 11.87.
1-Butyl-2-(4-methylphenyl)benzimidazole (4c): Comp-
ound 4c was obtained from 2-(4-methylphenyl)-1H-benzimi-
dazole (3b) and n-butyl bromide.Yield: 91.4 %; white crystals;
m.p. 62-63 ºC; 1H NMR (CDCl3, 500 MHz, ppm): δ 0.87 (t,
J = 7.35 Hz, 3H, CH3), 1.31-1.25 (m, 2H, CH2), 1.83-1.77 (m,
2H, CH2), 2.44 (s, 3H, CH3), 4.22 (t, J = 7.65 Hz, 2H, CH2),
7.30-7.27 (m, 4H, Ar-H), 7.41-7.39 (m, 1H, Ar-H), 7.61 (d,
1
7.81-7.78 (m, 1H, Ar-H); C NMR (CDCl3, 300 MHz): δ
130.02, 129.39, 125.10, 123.67, 123.35, 122.35, 122.14,
119.69, 109.71, 39.59, 15.23; MS: m/z 390.3 (M+H+), 391.3
(M+2H+), 412.3 (M+Na+);Anal. calcd. for C27H23N3: C, 83.26;
H, 5.95; N, 10.79; Found C, 83.28; H, 5.90; N, 10.82.
1
J = 8.1 Hz, 2H, Ar-H), 7.83-7.81 (m, 1H, Ar-H); C NMR
Theory and computational details: Computations were
performed using the Gaussian 03 simulation program27 through
the use of DFT based on Kohn-Sham approximations28. This
method can determine molecular properties as a function of
(CDCl3, 300 MHz): δ 153.77, 139.82, 135.56, 129.38, 129.29,
129.21, 127.61, 122.53, 122.28, 119.99, 119.79, 110.02, 44.53,
31.83, 21.41, 19.94, 13.52; MS: m/z 265.2 (M+H+), 266.2

4484 Shi et al.
Asian J. Chem.
electronic density when the molecules are in their ground state.
Gaussian 03 has several functional options and basis sets which
are available in its DFT module. The equilibrium geometry
for each of the eight neutral molecules was found by using the
hybrid B3LYP functional, which is a hybrid DFT functional
that combines Becke's three parameter exchanges functional29
with Lee-Yang-Parr's correlation functional30. 6-31g* basis set
was also used. Equilibrium geometry was then calculated by
using these sets.
lowest energy absorption bands are from the π-π* transitions
by virtue of their large molar extinction coefficients (ε = 104
M-1 cm-1). All eight compounds showed strong absorptions,
with maximum wavelengths in the range of 290-334 nm. The
increase in conjugation length and the increased electron
density associated with carbazoleyl groups in 4f and
triphenylamine group in 4g lead to a large bathochromic shift
of absorption maximum in 4f to 310 nm and in 4g to 334 nm
Ethyl and n-butyl had no influence on absorption maxima
(λAbsmax). Compared to 4a and 4d, 4d was red-shifted by 3 nm.
This was due to the electron-withdrawing group bromide atom,
which caused to produce intermolecular hydrogen bond
(C-H···Br) with adjacent molecule and increased the coplanar
degree of benzene ring and the benzimidazole ring. Optical
band gaps (Egopt) determined from the absorption edge of the
solution spectra33 are also given in Table-2.
RESULTS AND DISCUSSION
Synthesis and characterization: The synthesis of 1-alkyl-
2-substituted benzimidazoles (4a-4g) is outlined in Scheme-
II. Compounds 3a, 3c, 3e were synthesized by the condensation
of 1a-1c with 1,2-diamino benzene o-phenylene diamine using
air as oxidizing agent and compounds 3b, 3d were synthesized
by the condensation of 2d-2e with 1,2-diamino benzene o-
phenylene diamine using phosphoric acid and polyphosphoric
acid as activator (Scheme-I). By alkylation reactions of the
2-substituted benzimidazoles (3a-3e) with ethyl bromide or
n-butyl bromide as electrophilic reagent31. Compounds 4a-4g
were obtained. The yields were all 90 % above. 1H NMR spectra,
13C NMR spectra, mass spectra and elemental analyses of 4a-
4g confirmed the proposed structures and their purity. The
crystal structure of 4g monohydrate was revealed by X-ray
crystallography32.
TABLE-1
INFLUENCE OF REACTION TEMPERATURE ON
YIELD (TAKE 4a AS AN EXAMPLE)
Reacting temperature (ºC)
(after adding C2H5Br)
Yield (%)
25
35
45
55
70
80
92.4 93.1 93.1 92.2 89.6 84.8
R1
H
N
DMF,NaH
R1-Br
N
R2
R2
N
N
4a-4g
3a-3e
CH3
Br
4b: R1= C2H5
4a: R1= C2H5
4c: R1=n-Bu
4e: R1=n-Bu
R2=
R2=
R2=
R2=
CH3
4d: R1= C2H5
R2=
R2=
Br
4f: R1= C2H5
N
Fig. 1. UV-visible absorption spectra of the 4a-4g in 10-5 acetonitrile solution
4g: R1= C2H5
R2=
N
TABLE-2
PHYSICAL PROPERTIES OF 4a-4g
ε
Stokes
shift
(nm)
λAbs
(nm)
λEm
(nm)
Egopt
(ev)b
Scheme-II: Synthesis and molecular structure of 1-alkyl-2-substituted
benzimidazoles
φFa
max
max
(104 mol-1
cm-1)
Compound
4.55
4.71
4.74
3.12
3.52
4.88
5.36
290
290
290
293
293
310
334
392
391
389
401
393
412
436
102
101
99
0.81
0.85
0.86
0.43
0.46
0.89
0.92
4.94
4.88
4.87
4.92
4.90
4.77
4.68
4a
4b
4c
4d
4e
4f
Attention to the reaction of synthesis 4a-4g, different
reacting temperature of system (after adding C2H5Br) led to
different yields.As seen from Table-1 (take 4a as an example),
when the reacting temperature was below 50 ºC, the yields
changed little. However, when the temperature was above 50
ºC, the yields decreased. The higher the temperature, the lower
the yields were. Therefore, considering this factor, 45 ºC of
reacting temperature was chosen and the yields were all 90 %
above.
108
100
102
102
4g
aDetermined in CH3CN solution (A<0.05) at room temperature using
quinine sulfate (φF = 0.55 in 0.1 M H2SO4) as standard;bEstimated from
the onset of absorption spectra (Egopt = 1240/λonset
)
Optical properties: The UV-visible absorption spectra
of the 1-alkyl-2-substituted benzimidazoles in acetonitrile
solution (10-5 mol L-1) are displayed in Fig. 1 and their
photophysical properties are summarized in Table-2. The
The emission spectra of the eight 1-alkyl-2-substituted
benzimidazoles (10-8 mol L-1) in acetonitrile solution (λex =
λ
UV-max) are shown in Fig. 2. The fluorescence properties of

Vol. 25, No. 8 (2013)
Synthesis and Properties of Blue Light-Emitting 1-Alkyl-2-substituted Benzimidazoles 4485
the compounds are also summarized in Table-2. All eight
compounds have structured emission bands with the emission
maximum in the 380-440 nm ranges. The excitation of these
compounds at their respective absorption band maximum
results in weak violet to blue luminescence, corresponding to
a Stokes shift of 99-108 nm. Compared to 4c and 4e, 4c was
blue-shifted 3 nm while 4e was red-shifted 1 nm. This may be
due to the intermolecular hydrogen bond (C-H···Br) which
increased the coplanar degree of the molecule. Compared to
4d and 4e, 4d was red-shifted 9 nm while 4e was red-shifted
1 nm. This is due to the steric effect of the N-alkyl chains. The
effect will be more apparent with the increase of carbon atoms
in the N-alkyl chains side chains.
optical and chemical properties of these complexes rely mainly
on the changes in the ground state electronic structures37. The
HOMO-LUMO energy differences (energy band gaps, calcu-
lated Egcal) at DFT/B3YLP level of theory are presented in
Table-3.
TABLE-3
ENERGY VALUES THEORETICAL CALCULATED
FOR MOLECULES 4a-4g IN GAS PHASE
Energy (eV)
Molecule
HOMO
-5.74
-5.96
-5.94
-6.15
-6.14
-5.67
-5.31
LUMO
-0.91
-1.17
-1.16
-1.50
-1.49
-1.51
-1.28
Egcal
4.83
4.79
4.79
4.66
4.66
4.17
4.04
4a
4b
4c
4d
4e
4f
4a (exc @ 290nm)
4b (exc @ 290nm)
4c (exc @ 290nm)
4d (exc @ 293nm)
4e (exc @ 293nm)
4f (exc @ 310nm)
4g (exc @ 334nm)
1.0
0.8
0.6
0.4
0.2
0.0
4g
cal: Energies theoretical calculated for gas phase by means of DFT
B3LYP/6-31g*
The optimized structures of all the compounds showed
the decrease of Eg when introduced conjugated groups, which
is accord with the results obtained on the optical properties.
Compared to 4c and 4d, the Eg both resulted in 4.79 eV, which
indicated that N-alkyl chains have no influence on gap energy.
For 4d and 4f, Eg resulted in 4.79 eV of 4d and 4.65 eV of 4f,
respectively, which indicates that the electron acceptor groups
make the Eg decreasing. For molecules 4f and 4g, HOMO
resulted in -5.67 eV of 4f and in-5.31 eV of 4g, suggesting the
electron-transporting properties in 4g have been greatly
improved. Compared to 4f and 4g, when phenylcarbazole
group is replaced with triphenylamine group, HOMO is elevated,
but LUMO is lowered, which facilitate both the hole and electron-
transporting ability38.
300
400
500
600
Wavelength (nm)
Fig. 2. Emission spectra of the 4a-4g in acetonitrile solution (λex = λUV-max
)
The fluorescence quantum yields (φF) of these compounds
in acetonitrile solution were determined by the standard
method, using quinine sulfate (φF = 0.55 in 0.1 M H2SO4) as
standard34. The fluorescence quantum yields were in the range
from 0.43 for 4d to 0.92 for 4g. As can be seen from Table-2,
with the increase in conjugation length, the fluorescence
maximum shifted to longer wavelength and the fluorescence
quantum yields also higher. It is noted that large p conjugated
system can play important role in enhancing the quantum
efficiency and decreasing the band-gap of chromophores. The
fluorescence quantum yields of 4d and 4e were lower than
other compounds, which is likely the results of competing
S1→T1 intersystem crossing because of the internal heavy-
atom effect35.
Charge-transporting abilities: The electrical and optical
activity of electroluminescent materials relies on the ability
of the materials to transport electrical charges. The molecules
consisting of defined hole-transporting (donor) or electron-
transporting (acceptor) segments are interesting because the
electron and hole affinities can be enhanced simultaneously.
Applications of such donor-acceptor copolymers to organic
light-emitting diodes and to bulk heterojunction photovoltaic
devices have recently been studied39.
As can be seen from Fig. 3, the HOMO orbital of 4f is
localized mainly on the carbazole unit of the molecular,
whereas the LUMO orbital is localized mainly on 2-phenyl-
benzimidazole unit, which is different from 4g and other
compounds. This charge transfer implies that carbazole unit
is a good electron-donating charge carrier and the 2-phenyl-
benzimidazole unit serves as electron-accepting functionalities
by the presence of the electron-withdrawing imine (C=N)
nitrogen. This result will contribute to design the donor-
acceptor conjugated copolymers for the future work.
Theoretical calculation: To gain insight into the electron
properties and the geometries of the 1-alkyl-2-substituted
benzimidazoles, quantum chemical calculations were perfor-
med. In the calculations, the ground state geometries of 4a-4g
were fully optimized using density functional theory (DFT)
at the B3LYP/6-31g* level, as implement in Gaussian 03. DFT/
B3LYP calculation of lowest excitation energies was performed
at the optimized geometries of the ground states.
Frontier molecular orbitals and band energies: Because
the relative ordering of the occupied and virtual orbitals
provide a reasonable qualitative indication of the excitation
properties36, the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) for these
compounds are examined. The observed differences in the
Conclusion
A series of novel blue light-emitting 1-alkyl-2-substituted
benzimidazoles were synthesized and characterized by UV-
vis absorption and fluorescence emission spectroscopy. Fluore-
scence measurements showed that the compound emitted

4486 Shi et al.
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