Synthesis of benzimidazoles containing pyrazole group and quantum chemistry calculation of their spectroscopic properties and electronic structure

Article abstract of DOI:10.1007/s10895-011-0947-7

Five benzimidazole compounds containing pyrazole group were synthesized via one-step reaction of ophenylenediamine and 1-arylpyrazole-4-carbaldehyde in ethanol under mild conditions. The composition and structure of resultant benzimidazole compounds were

Full text of DOI:10.1007/s10895-011-0947-7

J Fluoresc (2012) 22:201–212
DOI 10.1007/s10895-011-0947-7
ORIGINAL PAPER
Synthesis of Benzimidazoles Containing Pyrazole Group
and Quantum Chemistry Calculation of Their Spectroscopic
Properties and Electronic Structure
Ren Tie-gang & Cheng Hong-bin & Zhang Jing-lai &
Li Wei-jie & Guo Jia & Yang Li-rong
Received: 16 June 2011 /Accepted: 2 August 2011 /Published online: 9 August 2011
#
Springer Science+Business Media, LLC 2011
.
.
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Abstract Five benzimidazole compounds containing pyra-
zole group were synthesized via one-step reaction of o-
phenylenediamine and 1-arylpyrazole-4-carbaldehyde in eth-
anol under mild conditions. The composition and structure of
resultant benzimidazole compounds were analyzed by means
Keywords Benzimidazole Pyrazole Synthesis
.
.
Spectroscopic properties Electronic structure Quantum
chemistry calculation
1
of elemental analysis, mass spectrometry, H-nuclear mag-
Introduction
netic resonance spectroscopy and X-ray single crystal
diffraction. The ultraviolet–visible light spectra and fluores-
cent spectra of the products were measured. Their ground-
state (S0) equilibrium geometries and vibrational frequencies
were determined based on B3LYP method, and their first
excited-state (S1) geometries were fully optimized based on
6–31G (d, p) basis set of TD-B3LYP method. Besides, the
spectroscopic properties of the products were computed
based on cc-pVTZ basis set of TD-B3LYP method and
compared with corresponding experimental data. It has been
found that benzimidazole compounds containing pyrazole
group can be readily synthesized in a high yield via one-step
reaction of o-phenylenediamine and 1-arylpyrazole-4-carbal-
dehyde in ethanol solvent. The fluorescence properties of the
five synthesized compounds are closely related to their
molecular structure; and their computed fluorescence spectra
well correspond to their experimental values. Moreover, they
have stable structure and strong fluorescence, showing
potential application in time-resolved fluoroimmunoassay
and DNA probe.
Nitrogen-containing heterocyclic compounds like benzimid-
azole and pyrazole have been attracting extensive
attention in pharmaceutics[1, 2] and organic synthesis[3, 4].
The derivatives of benzimidazole and pyrazole have found
applications in diverse physiological and pharmacological
areas such as anti-virals, anti-cancers, anti-fungals, anti-
inflammatories, anti-oxidation, and treatment of hypoglyce-
mia and physiological disorders [5–11]. In the meantime,
benzimidazole and pyrazole possess excellent photoelectric
properties and are widely used in photoelectric functional
materials[12–15] and display device like organic light-
emitting diode (OLED) [16, 17].
Usually, two traditional methods can be used to
synthesize benzimidazoles and their derivatives[18, 19].
One is the coupling of phenylenediamines with carboxylic
acids or their derivatives, which often needs harsh
dehydrating condition. The other is a two-step procedure
involving the oxidative cyclo-dehydrogenation of aniline Schiff
base which is often generated in situ from the condensation of
phenylenediamine and aldehyde. 2.3-dichloro-5,6-dicyanoben-
zoquinone (DDQ)[20, 21], MnO2[22], Pb(OAc)4[23],
Na2S2O5[24] and so on are the commonly used oxidants for
the two-step process, but they bring about potential hazards
and environmentally problematic by-products. To overcome
those drawbacks, some researchers have made efforts to
establish modified synthetic routes to synthesizing benzimi-
dazoles and their derivatives[25–28]. Unfortunately, the
modified synthetic methods still have disadvantages such as
:
:
:
:
:
R. Tie-gang (*) C. Hong-bin Z. Jing-lai L. Wei-jie G. Jia
Y. Li-rong
Fine Chemistry and Engineering Research Institute, College of
Chemistry and Chemical Engineering, Henan University,
Kaifeng 475004, People’s Republic of China
e-mail: rtg@henu.edu.cn

202
J Fluoresc (2012) 22:201–212
often requiretadious work-up procedures and purifications or
poor selectivity.
were verified by comparing their data with those reported in
the literature.
To our pleasure and surprise, when we intended to prepare a
series of bis-Schiff base from o-phenylenediamine and 1-
arylpyrazole-4-carbaldehyde, we obtained benzimidazole
compounds containing pyrazole group by accident. Being
advantageous over previous methods, the present procedure
can be used to synthesize benzimidazole compounds in
excellent yield under mild conditions and without use of any
catalysts or promoting and dehydrating reagents. In this
article we report the simple procedure for synthesizing
benzimidazole compounds containing pyrazole group as
well as characterization of their structure and evaluation of
their fluorescent properties. The ground-state (S0) equilibrium
geometries and vibrational frequencies of the five com-
pounds (determined by Becke’s three parameter functional
and the Lee-Yang-Parr functional method, B3LYP in short)
[29, 30], their first excited-state (S1) geometry (fully
optimized based on the 6–31G (d, p) basis set of TD-
B3LYP method), and their spectroscopic properties in
relation to absorption and emission obtained based on cc-
pVTZ basis set of TD- B3LYP method are also reported.
1-phenylpyrazole-4-carbaldehyde: white solid, yield
57%. H NMR (CDCl3) δ: 2.41 (s, 3H. CH3), 7.29(d, 2H,
1
J=8 Hz, Ar-H), 7.59 (d, 2H, J=8.4 Hz, Ar-H), 8.15 (s, 1H,
pyrazole-H), 8.40 (s, 1H, pyrazole-H), 9.95(s, 1H, CHO).
1-(p-tolyl)-pyrazole-4-carbaldehyde: pale-yellow solid,
1
yield 57%. H NMR (CDCl3) δ: 2.41 (s, 3H, CH3), 7.28–
7.30 (d, 2H, J=8 Hz, Ar-H), 7.57–7.59 (d, 2H, J=8.4 Hz,
Ar-H), 8.14 (s, 1H, pyrazole-H), 8.39 (s, 1H, pyrazole-H),
9.95 (s, 1H, CHO).
1-(4-chlorophenyl)-pyrazole-4-carbaldehyde: pale-yellow
needle-like solid, yield 64%. 1H NMR (CDCl3) δ: 7.48
(d, 2H, J=8.8 Hz, Ar-H), 7.67 (d, 2H, J=8.8 Hz, Ar-H),
8.17 (s, 1H, pyrazole-H), 8.41 (s, 1H, pyrazole-H), 9.97
(s, 1H, CHO).
3,5-dimethyl-1-phenyl-pyrazole-4-carbaldehyde: pale-
1
yellow solid, yield 54%. H NMR (CDCl3) δ: 2.53 (s, 3H,
CH3), 2.55 (s, 3H, CH3), 7.41 (m, lH, Ar-H), 7.45 (m, 2H,
Ar-H), 7.50 (m, 2H, Ar-H), 10.03 (s, lH, CHO).
5-chloro-3-methyl-1-phenylpyrazole-4-carbaldehyde:
pale-yellow needle-like solid, yield 63%. 1H NMR (CDCl3)
δ: 2.37 (s, 3H, CH3), 7.28 (d, lH, J=7.2 Hz, Ar-H), 7.44
(t, 2H, J=7.4 Hz, Ar-H), 7.89 (d, 2H, J=7.6 Hz, Ar-H),
9.52 (s, lH, CHO).
Experimental
General Procedure
General Procedure for Preparing Benzimidazoles
Mass spectra were determined on an Agilent 1100LC-MS
mass spectrometer. Nuclear magnetic resonance (1H NMR)
spectra were recorded on an INOVA-400 spectrometer in
CDCl3 or dimethyl sulphoxide (DMSO) in the presence of
tetramethylsilane (TMS) as an internal standard. Elemental
analysis was conducted with a PE2400 elemental analysis
apparatus. The fluorescent properties of the products were
evaluated using a Hitachi F-7000 apparatus (light source:
Xe arc lamp; room temperature). Ultraviolet–visible light
(UV–vis) absorption spectra of the products at room
temperature were recorded using a Hitachi U-4100 apparatus.
The equilibrium geometries and vibrational frequencies of the
five compounds in their ground-states (S0) were determined
based on the 6–31G (d, p) basis set of B3LYP method. The
first excited-state (S1) structure of the five compounds was
fully optimized also based on 6–31G (d, p) basis set of TD-
B3LYP method. The spectroscopic properties of the five
compounds in relation to their absorption and emission were
investigated based on cc-pVTZ basis set of TD-B3LYP
method. All calculations were implemented with Gaussian
09 program packages.
A certain amount of 1-phenylpyrazole-4-carbaldehydes
(2 mmol) was dissolved in C2H5OH (EtOH, 20 mL),
followed by dropwise addition of EtOH solution containing
o-phenylenediamine (0.22 g, 2 mmol). Resultant mixture
was refluxed in a water bath for 4 h before rotary
evaporating to remove solvent and cooling to room
temperature. As-separated crystalline precipitates were
collected by filtration and recrystallized with EtOH.
2-(1-phenylpyrazol-4-yl)-benzimidazole (1): pale-yellow
solid, yield 87%. APCI-MS (m/z) Calcd (M+) 261.3, found:
1
261.1. H NMR (DMSO-d6) δ: 7.19 (m, 2H, Ar-H), 7.39
(t, 1H, J=7.4 Hz, Ar-H), 7.57 (t, 4H, J=7.6 Hz, Ar-H),
7.92 (d, 2H, J=8 Hz, Ar-H), 8.38 (s, 1H, pyrazole-H), 9.13
(s, 1H, pyrazole-H), 12.74 (s, 1H, NH). Anal. Calcd for
C16H12N4: C, 73.83; H, 4.65; N, 21.52. Found: C, 73.84; H,
4.65; N, 21.51.
2-(1-(p-tolyl)pyrazol-4-yl)-benzimidazole (2): pale-
yellow solid, yield 91%. APCI-MS (m/z) Calcd (M+)
1
275.3, found: 275.1. H NMR (DMSO-d6) δ: 2.37 (s, 3H,
CH3), 7.17–7.20 (m, 2H, Ar-H), 7.35 (d, 2H, J=8.4 Hz,
Ar-H), 7.57 (m, 2H, Ar-H), 7.79 (d, 2H, J=8.4 Hz, Ar-H),
7.92 (d, 2H, J=8 Hz, Ar-H), 8.34 (s, 1H, pyrazole-H),
9.06 (s, 1H, pyrazole-H), 12.72 (s, 1H, NH). Anal. Calcd
for C17H14N4: C, 74.43; H, 5.14; N, 20.42. Found: C,
74.47; H, 5.13; N, 20.40.
All chemicals and solvents are of commercial reagent grade
and used without further purification. 1-phenylpyrazole-4-
carbaldehydes were prepared according to published proce-
dure [31, 32]. Known structures of synthesized products

J Fluoresc (2012) 22:201–212
203
2-(1-(p-chlorophenyl)pyrazol-4-yl)-benzimidazole (3):
yellowish brown solid, yield: 89%. APCI-MS (m/z) Calcd
(M+) 295.7, found: 295.1. 1H NMR (DMSO-d6) δ: 7.20 (m,
2H, Ar-H), 7.57 (dd, 2H, J=3.2, 3.2 Hz, Ar-H), 7.62 (d,
2H, J=8.8 Hz, Ar-H), 7.96 (d, 2H, J=8.8 Hz, Ar-H), 8.40
(s, 1H, pyrazole-H), 9.16 (s, 1H, pyrazole-H), 12.72 (s, 1H,
NH). Anal. Calcd for C16H11N4Cl: C, 65.20; H, 3.76; N,
19.01. Found: C, 65.24; H, 3.78; N, 18.95.
still obtained in an isolated yield of 56%. This means that
the reaction has a high selectivity, and the formation of the
mono-Schiff base intermediate is the key step for preparing
benzimidazoles, the products of thermodynamic control
reaction with a better stability than Schiff base.
By adopting the simple synthetic route, we successfully
prepared benzimidazole containing pyrazole group from equal
molar amount of o-phenylenediamine and 1-phenylpyrazole-
4-carbaldehyde under mild conditions. To extend the scope
of the method, we had also examined a number of differently
substituted 1-arylpyrazole-4-carbaldehydes. We were pleased
to find that heteroaryl aldehydes bearing electron-donating
and electron-withdrawing substituents gave desired benzimi-
dazoles in excellent yields, and the substitutional groups in
the pyrazole group had little impact on the formation of
benzimidazoles. Besides, no 1,2-disubstituted benzimida-
zoles were separated at the end of the reaction, conforming
to the finding that no bis-Schiff base intermediate was
detected by MS during the course of the reaction.
2-(3,5-dimethyl-1-phenylpyrazol-4-yl)-benzimidazole
(4): white solid, yield 85%. APCI-MS (m/z) Calcd (M+)
1
289.4, found: 289.1. H NMR (DMSO-d6) δ: 2.50 (s, 3H,
CH3), 2.57 (s, 3H, CH3), 7.17–7.21 (m, 2H, Ar-H), 7.45–
7.49 (m, 1H, Ar-H), 7.57 (d, 6H, J=4.4 Hz, Ar-H), 12.13
(s, 1H, NH). Anal. Calcd for C18H16N4: C, 74.98; H, 5.59;
N, 19.43. Found: C, 74.94; H, 5.64; N, 19.43.
2-(5-chloro-3-methyl-1-phenylpyrazol-4-yl)-benzimidazole
(5): pale-yellow solid, yield 86%. APCI-MS (m/z) Calcd
1
(M+) 309.8, found: 309.1. H NMR (DMSO-d6) δ: 2.57
(s, 3H, CH3), 7.18 (d, 2H, J=4.8 Hz, Ar-H), 7.45–7.50
(m, 1H, Ar-H), 7.57 (t, 5H, J=7.6 Hz, Ar-H), 7.64 (s, 1H,
Ar-H), 12.14 (s, 1H, NH). Anal. Calcd for C17H13N4Cl: C,
66.13; H, 4.24; N, 18.15. Found: C, 66.14; H, 4.25; N,
18.13.
Crystal Structure Description of the Products with a Bruker
APEX-II CCD Detector
Suitable single crystal of the synthesized compounds with a
dimension of 0.37 mm×0.26 mm×0.23 mm was mounted
on a Bruker APEX-II CCD detector to identify the crystal
structure (graphite monochromated Mo-Kα radiation, l=
0.71073Å; 293(2) K; in a range of 1.81–25.50°). The
crystal structure was solved by direct methods and refined
using full-matrix least-square calculations in relation to
anisotropic thermal parameters for all non-hydrogen atoms.
All calculations were performed by using the SHELXTL-97
software package [33]. A summary of representative
crystallographic information and selected bond lengths
and angles are given in Tables 1 and 2.
The structure of the synthesized compounds is schemati-
cally shown in Fig. 2. One molecule of ethanol exists in the
structure; and the bond distances and bond angles of the
compound are within normal ranges reported in references
[34–36]. The centroid to centroid distance between adjacent
Results and Discussion
Synthesis
The route to synthesizing 2-(1-arylpyrazol-4-yl)-benzimi-
dazoles (1–5) is outlined in Fig. 1.
Usually, o-phenylenediamine yields a bis-Schiff base in
the presence of aldehyde under normal circumstances. To our
surprise, when we intended to synthesize a series of bis-
Schiff base from 1-phenylpyrazole-4-carbaldehyde (4 mmol)
and o-phenylenediamine (2 mmol), only mono-Schiff base
intermediate was detected by MS during the course of the
reaction, and a series of benzimidazoles were obtained as the
final products. Even when reactant molar ratio was changed,
a series of 2-(1-phenylpyrazol-4-yl)-benzimidazoles were
Fig. 1 Route to synthesizing 2-(1-
arylpyrazol-4-yl)-benzimidazoles
O
ligand
R1
H
R2
H
R3
H
R1
1
2
3
4
5
NH
N
N
N
H
H
CH3
Cl
H
R1
R2
N
reflux
EtOH
H
H
+
N
R2
H2N
NH2
CH3
CH3
CH3
Cl
R3
H
R3

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J Fluoresc (2012) 22:201–212
Table 1 Summary of crystallo-
graphic data for benzimidazole 1
Item
Data
Empirical formula
Formula weight
Temperature/K
Wavelength/Å
Crystal system
space group
C18H17N4O
305.360
293(2)
0.71073
orthorhombic
P2(1)2(1)2(1)
a/Å
7.6078(8)
b/Å
14.2029(15)
c/Å
15.0102(15)
α/(°)
90.00
β/(°)
90.00
γ/(°)
90.00
Z
4
Density (calculated)/g·cm−3
2.101
F(000)
644
Crystal size/mm3
θ range for data collection/°
Limiting indices
Reflections collected/unique
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Volume/Å3
0.37×0.26×0.23
2.87 to 23.64
−8 ≤ h ≤ 9, −16 ≤ k ≤16, −15 ≤ l ≤17
10350/2845 [R(int) = 0.0213]
Full-matrix least-squares on F2
2845/2/211
1.064
1621.89
R1 indices [I >2σ (I)]
wR2 indices (all data)
Largest diff. peak and hole/(e·Å−3
R1 = 0.0471
wR2 = 0.1400
)
0.30 and −0.26
parallel aryl rings is 7.6078Å, which indicates that no
π-π stacking occurs in the framework of the title
compounds. The three dimensional (3D) framework of
the compounds is generated and stabilized through hydrogen
bonds, as shown in Fig. 3; and corresponding data are listed
in Table 2.
Fluorescent Spectra
The fluorescent spectra of five benzimidazole com-
pounds (1–5) measured at room temperature are shown
in Fig. 5; and corresponding data are listed in Table 3. In
terms of the chemical structure, organic fluorescent
compounds generally contain an aromatic ring, fused
aromatic rings, or other conjugated ring systems along
with plane rigid groups such as C = N, N = N in
benzimidazoles [37, 38]. Besides, compound 1 has a good
coplanarity, as evidenced by relevant crystal structure, so
it possesses strong fluorescence and shows the maximum
emission peak at 347.8 nm under excitation at 305 nm.
Compounds 2 and 3 show the maximum emission peaks at
348.2 nm and 352.0 nm under the same condition.
Moreover, compound 2 has much stronger fluorescence
than 1, but compound 3 has much weaker fluorescence
than 1, which is attributed to the introduction of the
electron donor group and electron-withdrawing group,
respectively. In addition, when compounds 4 and 5 were
excited at 294 nm and 295 nm, they showed the maximum
emission peaks at 345.2 nm and 345.8 nm, respectively.
UV–Vis Spectra
Figure 4 shows the UV–vis spectra of benzimidazoles 1–
5 in DMF. Within the tested wavelength range of 200–
800 nm, they all had a remarkable absorption peak of π-
π* transition and n-π* transition at 270–340 nm. Besides,
the lmax of 3 shows a bathochromic shift (2 nm) as
compared to that of 1, so does the lmax of 5 as compared
to that of 4. The reason lies in that the introduction of the
auxochrome group, Cl atom, enhances the operation
scope of electrons, leading to shift of the absorption
peaks towards long wavelength. However, compounds 1
and 2 have the same lmax, which implies that the
hyperconjugation effect of the electron donor group,
methyl, is faint.

J Fluoresc (2012) 22:201–212
205
Table 2 Selected bond lengths
(Å) and angles (°) for
benzimidazole 1
Bond
Length
Bond
Length
N(1)—C(7)
1.362(3)
1.384(3)
1.317(3)
1.393(3)
1.325(4)
1.352(3)
1.347(3)
1.426(3)
1.391(4)
1.372(4)
1.380(4)
angle
C(1)—C(2)
1.388(4)
1.392(3)
1.368(4)
1.391(5)
1.361(4)
1.388(4)
1.457(4)
1.394(4)
1.365(4)
1.374(4)
1.371(4)
angle
N(1)—C(1)
C(1)—C(6)
N(2)—C(7)
C(2)—C(3)
N(2)—C(6)
C(3)—C(4)
N(3)—C(9)
C(4)—C(5)
N(3)—N(4)
C(5)—C(6)
N(4)—C(10)
C(7)—C(8)
N(4)—C(11)
C(8)—C(9)
C(11)—C(12)
C(12)—C(13)
C(13)—C(14)
C(14)—C(19)
C(16)—C(19)
C(11)—C(16)
Bond
Bond
C(7)—N(1)—C(1)
C(9)—N(3)—N(4)
C(10)—N(4)—C(11)
N(1)—C(1)—C(2)
C(2)—C(1)—C(6)
C(2)—C(3)—C(4)
C(4)—C(5)—C(6)
C(5)—C(4)—N(1)
N(2)—C(7)—N(1)
N(1)—C(7)—C(8)
C(10)—C(8)—C(7)
N(3)—C(9)—C(8)
C(16)—C(11)—C(12)
C(12)—C(11)—N(4)
C(12)—C(13)—C(14)
C(11)—C(16)—C(19)
106.38(19)
104.3(2)
128.6(2)
132.6(2)
121.5(3)
121.3(3)
118.6(3)
130.6(2)
113.3(2)
121.9(2)
127.5(2)
112.8(2)
119.2(3)
120.0(2)
121.2(3)
120.3(3)
C(7)—N(2)—C(6)
C(10)—N(4)—N(3)
N(3)—N(4)—C(11)
N(1)—C(1)—C(6)
C(3)—C(2)—C(1)
C(5)—C(4)—C(3)
C(5)—C(6)—C(1)
C(1)—C(6)—N(2)
N(2)—C(7)—C(8)
C(10)—C(8)—C(9)
C(9)—C(8)—C(7)
N(4)—C(10)—C(8)
C(16)—C(11)—N(4)
C(13)—C(12)—C(11)
C(13)—C(14)—C(19)
C(14)—C(19)—C(16)
104.9(2)
111.3(2)
120.1(2)
105.9(2)
117.5(3)
121.3(3)
119.9(3)
109.5(2)
124.8(2)
103.5(2)
129.0(2)
108.2(20)
120.6(2)
119.8(3)
119.0(3)
120.5(3)
Hydrogen bonds (Å) and angles (°)
D-H···A
d (D-H)
0.8200
d (H···A)
d (D···A)
∠(D-H···A)
172.60
O(1)-H(1)···N(2)
1.9700
2.788(3)
Thus it can be concluded that the subsitituents on phenyl
group and pyrazolyl group have different impacts on the
maximum emission peak and stoke shift of the synthesized
compounds.
Theoretical Study of Electronic Structure and Spectroscopic
Properties
Ground State Geometries and Electronic Structure
Calculated at the B3LYP Level
Ground-State Geometries and Stabilities The molecular
structures of the five compounds are depicted in Fig. 6,
and their primary geometric parameters optimized with
B3LYP and TD-B3LYP (bond lengths, bond angles, and
dihedral angles) are summarized in Table 4. As shown in
Fig. 6, the five benzimidazole compounds have the same
matrix, i.e., compound 1. In combination with the data
listed in Table 4, it can be seen that their ground-state (S0)
geometric parameters, bond lengths and bond angles, are
Fig. 2 Structure of the synthesized compounds

206
J Fluoresc (2012) 22:201–212
3500
3000
2500
2000
1500
1000
500
1
2
3
4
5
0
250
300
350
400
450
500
550
600
650
Wavelength / (nm)
Fig. 5 The emission spectra of benzimidazoles 1–5 in DMF solution
(1×10−6 mol/L)
1.318(1.317)a Å, 1.384(1.362)a Å, 1.384(1.393)a Å and
1.384(1.384)a Å, respectively, shorter than that of conven-
tional N–C single bond (1.47Å) but longer than that of
conventional N = C double bond (1.28Å). This indicates
that the chemical bond linking imino and aromatic ring is
partially a double bond in nature, due to π-conjugation
involving the participation of N lone electron pair. For the
same reason, bond lengths of N(3)-C(9) and N(4)-C(10) in
pyrazole ring are 1.326(1.325)a Å and 1.360(1.347)a Å,
respectively; and bond length of N(3)-N(4) (1.361(1.352)a Å)
is shorter than that of N–N single bond. Furthermore, the
bond length of the benzene ring connected with pyrazole N
(4)-C(11) is 1.422(1.426)a Å, showing weaker double bond
component.
Fig. 3 Packing diagram of the compounds with hydrogen bonds
shown in dashed line
very similar. In the meantime, compounds 1, 2 and 3 with
dihedral angles of 1.698°, 1.499°, and 2.031° in N(1)-C(7)-
C(8)-C(10) as well as 158.0°, 156.5°, and 159.8° in C(10)-
N(4)-C(11)-C(16) have better coplanarities than compounds
4 and 5 (4.914° and 13.99° in N(1)-C(7)-C(8)-C(10);
137.8° and 134.7° in C(10)-N(4)-C(11)-C(16)).
The calculated results reveal that all of the complexes
have X1A ground state. By comparing with the data shown
in Table 1, we can see that the theoretically calculated
structural parameters of 1 are in accordance with the
experimental ones. Moreover, bond lengths of N(1)-C(7),
N(2)-C(7), N(1)-C(1) and N(2)-C(6) in compound 1 are
In addition, frequency analyses based upon B3LYP/6-31G
(d, p) vibrational calculations reveal that all the five
synthesized compounds have stable structures on the
potential energy curves, showing no imaginary frequency.
.6
.5
Electronic Structure The plots of the frontier molecular
orbitals, the highest-occupied molecular orbital (HOMO)
and lowest-unoccupied molecular orbital (LUMO), of
compounds 1–5 in relation to their B3LYP-optimized
geometries are shown in Fig. 7. Compounds 1–5 have the
1
2
3
.4
4
5
.3
Table 3 Fluorescent data of synthesized compounds
.2
.1
Compounds
lUV. max/nm
l
FL. max/nm
Stokes shift/nm
1
2
3
4
5
305
305
307
294
295
347.8
348.2
352.0
345.2
345.8
42.8
43.2
45.0
51.2
50.8
0.0
260
280
300
320
340
360
380
400
420
Wavelength / (nm)
Fig. 4 UV–vis absorption spectra of benzimidazoles 1–5 in DMF
solution (1×10−5 mol/L)

J Fluoresc (2012) 22:201–212
207
Fig. 6 Molecular structure of
compounds 1, 2, 3, 4 and 5
same electron density of LUMO. In the meantime, the
electron density of HOMO in compound 1 is distributed
over the whole molecules and has some delocalization
along N(1)-C(7)-N(2), C(9)-C(8)-C(10), N(3)-N(4) and C
(12)-C(11)-C(16), which are also observed in compounds 2
and 3. However, in compounds 4 and 5, the delocalization
is distributed over C(11)-C(16) but not C(12)-C(11)-C(16).
The energies of frontier molecular orbitals of compound
1 are EHOMO = −5.576 eV and ELUMO = −1.047 eV,
respectively, corresponding to ε-value of 4.529 eV which is
a little bit larger than that of 2 and 3 (4.427 eV and
4.415 eV) but a little bit smaller than that of 4 and 5
(4.610 eV and 4.596 eV). For compounds 1–5, conjugated
π-electron systems, the smaller the energy gap e-value is,
Table 4 B3LYP-optimized primary geometric parameters of ground-states (S0) of 1–5 and first excited state (S1) of 1–4: bond lengths (in Å), bond
angles and dihedral angles (in °)
Atom numbers
Primary geometric parameters
1
2
3
4
5
S0
S1
S0
S1
S0
S1
S0
S1
S0
S1
N(1)-C(7)
1.318(1.317)a
1.384(1.362)a
1.384(1.393)a
1.384(1.384)a
1.326(1.325)a
1.360(1.347)a
1.361(1.352)a
1.422(1.426)a
1.373
1.384
1.345
1.383
1.318
1.360
1.409
1.393
1.318
1.385
1.384
1.383
1.324
1.364
1.362
1.420
1.510
1.359
1.397
1.350
1.376
1.316
1.373
1.406
1.384
1.505
1.317
1.384
1.384
1.383
1.325
1.361
1.362
1.419
1.756
1.371
1.382
1.345
1.383
1.320
1.356
1.407
1.394
1.761
1.322
1.387
1.384
1.383
1.325
1.369
1.366
1.424
1.333
1.396
1.381
1.379
1.338
1.363
1.371
1.426
1.318
1.388
1.382
1.382
1.327
1.365
1.363
1.427
N(2)-C(7)
N(1)-C(1)
N(2)-C(6)
N(3)-C(9)
N(4)-C(10)
N(3)-N(4)
N(4)-C(11)
C(14)-R1
C(9)-R2
1.499
1.494
105.6
107.6
111.9
112.5
105.5
4.914
137.8
1.500
1.493
105.8
108.0
111.2
112.3
105.8
4.940
137.8
1.499
1.718
105.4
107.3
112.3
110.9
106.0
13.99
134.7
C(10)-R3
C(1)-N(1)-C(7)
C(6)-N(2)-C(7)
N(1)-C(7)-N(2)
C(10)-N(4)-N(3)
N(4)-N(3)-C(9)
N(1)-C(7)-C(8)-C(10)
C(10)-N(4)-C(11)-C(16)
105.2(104.9)a
107.2(106.4)a
112.6(113.3)a
112.0(111.3)a
104.8(104.3)a
1.698
105.5
108.0
111.0
111.5
105.1
0.019
179.9
105.3
107.3
112.5
111.8
105.1
1.499
156.5
105.5
107.7
111.2
112.0
104.8
0.679
172.1
105.2
107.2
112.6
112.0
104.8
2.031
159.8
105.5
107.9
111.1
111.6
105.1
0.020
179.9
158.0
a Experimental values

208
J Fluoresc (2012) 22:201–212
Fig. 7 Schematic diagrams
showing the frontier molecular
orbitals of compounds 1–5:
(a and b) 1; (c and d) 2;
(e and f) 3; (g and h) 4; and
(i and j) 5
(a) HOMO, -5.576 eV
(b) LUMO, -1.047 eV
(c) HOMO, -5.540 eV
(d) LUMO, -1.068 eV
(e) HOMO, -5.681 eV
(f) LUMO, -1.266 eV
(g) HOMO, -5.432 eV
(h) LUMO, -0.822 eV
(i) HOMO, -5.604 eV
(j) LUMO, -1.008 eV
the easier the delocalized π-electron is to be excited and the
longer corresponding strongest absorption wavelength is.
Therefore, the strongest absorption wavelength of com-
pound 3 is the longest among the five compounds, as
confirmed by relevant theoretical and experimental results
(see Table 5).

J Fluoresc (2012) 22:201–212
209
Table 5 Calculated absorption
wavelength of 1–5 as well as
the emission wavelength of 1–4
at TD-B3LYP/cc-pVTZ level
Compounds
1
State
Transition
Absorption
Emission
l/nm
f
l/nm
f
X1A
21A
51A
191A
X1A
21A
(67a)2(68a)2(69a)0
68a→69a
308.69(305)a
264.79
0.6537
0.1649
0.2203
359.02(347.8)a
362.63(348.2)
0.6362
0.9840
68a→71a
64a→70a
(71a)2(72a)2(73a)0
204.17
2
3
a
a
a
a
72a→73a
310.65(305)
265.34
0.9374
0.1857
0.2103
0.2884
51A
72a→75a
131A
201A
X1A
21A
72a→76a
223.89
66a→73a
(75a)2(76a)2(77a)0
203.55
a
76a→77a
316.77(307)
269.43
0.6601
0.3677
0.1916
0.1859
366.96(352.0)
352.68(345.2)
0.6326
0.6524
51A
76a→79a
131A
211A
X1A
21A
76a→80a
223.36
70a→77a
(75a)2(76a)2(77a)0
205.70
4
5
a
a
76a→77a
301.90(294)
271.18
0.6526
0.1707
0.3099
41A
76a→79a
131A
X1A
21A
41A
131A
76a→80a
(79a)2(80a)2(81a)0
223.75
80a→81a
301.81(295)
266.34
0.6381
0.1810
0.3562
80a→82a
78a→82a
224.22
TD-B3LYP-Optimized Geometries of S1 States (except for 5)
coplanarity than in S0 state; and in particular, compounds 1
and 3 are nearly co-planar. But the coplanarity of compound
4 keeps almost unchanged with varying state from S1 to S0.
The geometries of the first excited state S1 are optimized at
TD-B3LYP/6-31G(d, p) level, and the primary geometrical
parameters for compounds 1–4 are given in Table 4.
Absorption Spectra
Since the lowest singlet transition of each compound
refers to the transition from the HOMO and LUMO
orbitals, the relaxation of bond length can be interpreted
by analyzing the nodal patterns of the HOMO and LUMO
orbitals (see Fig. 6). Taking compound 1 as an example, its
HOMO orbitals involve N(1)-C(7) (1.318Å) and N(3)-N(4)
(1.361Å) bonds, while its LUMO orbitals (1.373Å and
1.409Å, respectively) has nodes across these bonds,
resulting in an extension of the N(1)-C(7) and N(3)-N(4)
bonds (+0.055Å and +0.048Å, respectively). In addition,
its HOMO has nodes in N(1)-C(1) (1.384Å) and N(4)-C
(11) (1.422Å) bonds, but its LUMO (1.345Å and 1.393Å,
respectively) has bonding at these regions. As a result, N
(1)-C(1) and N(4)-C(11) bonds are shortened by −0.039 and
−0.029Å, respectively. A similar behavior has also been
found for compounds 2, 3 and 4. Furthermore, by comparing
the dihedral angles of S0 and S1 (see Table 4), we can infer
that compounds 1, 2 and 3 in S1 state have a better
The absorption wavelengths (λ in nm) and oscillator
strengths (f) of compounds 1–5 computed based on cc-
pVTZ basis set of TD-B3LYP method are listed in Table 5
and shown in Fig. 8 for a direct comparison with relevant
experimental results (see Table 3 and Fig. 4). Electron
promotion from HOMO to LUMO gives rise to the most
intense absorption in the UV region for every compound
(see Table 5). By comparing with the experimental values in
DMF solution, we can see that the theoretical absorption
spectra values well correspond to the experimental ones.
In terms of the theoretical absorption spectra, all these
observations agree well with relevant experimental
results. As shown in Fig. 4, the UV–vis absorption
spectra of benzimidazoles 1–5 in DMF solution, all
emerging as single peaks, are located within 260–
400 nm. However, as illustrated in Table 5 and Fig. 8,
the theoretical calculations indicate that they have absorptions

210
J Fluoresc (2012) 22:201–212
Fig. 8 Theoretical absorption spectra of 1–5
below 260 nm. For example, compound 1 has an X1A singlet
ground-state with an electronic configuration of
(67a)2(68a)2(69a)0 (see Table 2) and 21A second excited
state derived from HOMO 68a to LUMO 69a orbitals; the
absorption wavelength 308.69 nm corresponds to the largest
f=0.6537, in good agreement with the experimental value of
305 nm. Nevertheless, with respect to the calculated electron
excitations of 68a→71a leading to 51A state at 264.79 nm
with f=0.1649 and of 64a→70a leading to 191A state at
204.17 nm with f=0.2203, no absorption was observed in the
experiment.
Emission Spectra
The emission wavelengths (λ in nm) and oscillator strengths
(f) of compounds 1–4 computed using the same TD-B3LYP
method with the cc-pVTZ basis set, under the optimized
geometries of S1 state, are summarized in Table 5, where the
experimental results at room temperature are also presented
for a comparison. As it can be seen in Table 5, the calculated
emission wavelengths of compounds 1–4 are 359.02 nm,
362.63 nm, 366.96 nm and 352.68 nm, respectively, in good
agreement with corresponding experimental data of

J Fluoresc (2012) 22:201–212
211
5. Wang HY, Chen G, Xu XP et al (2010) Synthesis and properties of
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347.8 nm, 348.2 nm, 352.0 nm and 345.2 nm. Besides, the
emission spectra show noticeable red-shift as compared with
corresponding absorption spectra, possibly due to relaxation
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The B3LYP calculation shows that all the synthesized
benzimidazole compounds (1–5) have stable S0 and S1
geometric configurations (excluding 5) and CS symmetry.
Besides, compounds 1, 2 and 3 have better coplanarities
than 4 and 5 in S0 state. In terms of the B3LYP-optimized
geometries, the plots of the frontier molecular orbitals of
compounds 1–5, all being conjugated delocalized π-
electron systems, have a sequence of ε3 < ε2 < ε1 < ε5 ≈ ε4,
which implies that their UV–vis absorption spectra have a
sequence of l3 > l2 > l1 > l5 ≈ l4, as confirmed by relevant
experimental observation and theoretical computation.
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Summary
An efficient and facile method has been established for
preparing 2-(1-arylpyrazol- 4-yl)-benzimidazoles. The
method is dominated by simple procedure, mild condition,
easy purification, and high efficiency and does not need any
commercial oxidants/additives. Spectroscopic investigation
indicates that the fluorescence spectra of the five synthe-
sized compounds are closely related to their molecular
structure. The data of quantum chemistry calculation of
benzimidazole compounds correspond to the experimental
values very well. Five benzimidazole compounds (1–5)
have stable structure and strong fluorescence, showing
potential applications in time-resolved fluoroimmunoassay
and DNA probe.
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and optical behaviors of 2-(9-phenanthrenyl)-, 2-(9-anthryl)-, and
2-(1-pyrenyl)-1-alkylimidazole homologues. Spectrochim Acta
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13. Yan Y, Pan W, Song H (2010) The synthesis and optical properties
of novel 1,3,4-oxadiazole derivatives containing an imidazole
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Acknowledgements The authors gratefully thank the Natural Science
Foundation of the Education Department of Henan Province, China
(grant No. 2009B150005) and the National Natural Science Fund of
China (grant No. 20971036) for financial support.
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Supplementary material
The X-ray crystallographic file No. 769532 for compound 1, in CIF
format, is provided by the Cambridge Crystallographic Data Centre
(CCDC). Copies of the file can be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.
ac.uk).