Optical properties of 1,2-diaryl benzimidazole derivatives - A combined experimental and theoretical studies

Article abstract of DOI:10.1016/j.saa.2013.06.001

Some novel 1,2-diaryl benzimidazole derivatives have been designed, synthesized and characterized by mass, ¹H, ¹³C-NMR spectral studies and single crystal XRD. The charge distribution has been calculated from the atomic charges by no

Full text of DOI:10.1016/j.saa.2013.06.001

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 74–78
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Optical properties of 1,2-diaryl benzimidazole derivatives – A combined
experimental and theoretical studies
⇑
J. Jayabharathi , V. Thanikachalam, K. Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
DFT calculation is in good agreement
with single crystal XRD data.
Steric interaction must be reduced in
order to obtain larger b values.
0
NBO analysis elucidates the
delocalization within the molecule.
Benzimidazoles can be used as
potential NLO materials.
Charge distribution was calculated by
NBO and Mulliken methods.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 April 2013
Received in revised form 29 May 2013
Accepted 4 June 2013
Available online 17 June 2013
Some novel 1,2-diaryl benzimidazole derivatives have been designed, synthesized and characterized by
mass, H, C-NMR spectral studies and single crystal XRD. The charge distribution has been calculated
from the atomic charges by non-linear optical (NLO) and natural bond orbital (NBO) analyses have been
1
13
calculated by abinitio method. Synthesized 1,2-diaryl benzimidazole derivatives have the largest
value and can be used as potential NLO materials. Analysis of the molecular electrostatic potential
MEP) energy surface exploited the region for non-covalent interactions in the molecule. Calculated bond
g 0
l b
(
Keywords:
DFT/B3LYP/6-31G(d,p)
NLO
NBO
XRD
MEP
lengths, bond angles and dihedral angles are found to be slightly higher than that of X-ray diffraction
values of its experimental datas.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
materials with non-linear optical (NLO) properties is usually con-
centrated on molecules with donor–acceptor
p
-conjugation (D-
p-
Benzimidazole based chromophores have received increasing
attention due to their distinctive linear, non-linear optical proper-
ties and also due to their excellent thermal stability in guest–host
systems [1]. The imidazole ring can be easily tailored to accommo-
date functional groups, which allows the covalent incorporation of
the NLO chromophores into polyamides leading to NLO side chain
A) and deals with the substituent effects on the degree of
gation, steric hindrance and the hyperpolarisability of the sub-
stances [4]. Nowadays there is an insufficient understanding for
designing optimal NLO materials, even certain classes of D-p-A
compounds were theoretically studied [5]. Searching for organic
materials with nonlinear optical (NLO) properties is usually con-
p
-conju-
polymers [2]. Most
determining second-order NLO response [3]. Searching organic
p
-conjugated systems play a major role in
centrated on molecules with donor–acceptor
A) and deals with the systematic investigation of substituent ef-
fects on the degree of -conjugation, steric hindrance and the
p-conjugation (D-p-
p
hyperpolarisability of the substances. Besides, geometrical
arrangement of the molecules in the solid state, their interaction
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1
386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.06.001

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 74–78
75
and other physicochemical properties (e.g. strong intramolecular
Computational details
charge-transfer absorptions) and engineering possibilities are also
important [4]. At present, there is an insufficient understanding of
all influences for designing optimal NLO materials, even if the
Quantum mechanical calculations were used to carry out the
optimized geometry, NLO, NBO and HOMO–LUMO analysis with
Gaussian-03 program using the Becke3-Lee-Yang-Parr (B3LYP)
functional supplemented with the standard 6-31G(d,p) basis set
[11]. As the first step of our DFT calculation for NLO, NBO and
HOMO–LUMO analysis, the geometry taken from the starting
structures were optimized and then, the electric dipole moment
influencing factors in certain classes of D-
p-A compounds were
theoretically studied [5]. To quantify the push–pull effect in D-
p
-
A compounds, bond length alternation (BLA) and out-of-plane dis-
tortions of the polarized C@C double bonds, available from X-ray
studies, have been employed for a long time [6]. Alternatively, di-
pole moment measurements [7], bond lengths [8], barriers to rota-
l
and b tensor components of the studied compounds were calcu-
tion about the partial C@C double bonds [9] (from dynamic NMR
lated, which has been found to be more than adequate for obtain-
ing reliable trends in the first hyperpolarizability values.
We have reported the btot (total first hyperpolarizability) for the
investigated molecules and the components of the first hyperpo-
larizability can be calculated using equation:
ꢁ
studies), and the occupation quotients (
p/p
) [10] of the bonding
(
to quantify the acceptor activity) and anti-bonding orbital (to
quantify the donor activities) of these C@C double bonds were
adopted. Not only the push–pull effect in D- -A compounds could
p
be quantified, but also a linear dependence of the push–pull quo-
X
ꢁ
bi ¼ biii þ 1=3 ðbijj þ bjij þ bjjiÞ
ð1Þ
tient (
p
/
p
) on molar hyperpolarisability of these compounds were
ꢁ
i–j
detected. Thus,
p
/p
proves to be an easily accessible, general and
sensitive parameter of the donor–acceptor quality of compounds
for potential NLO applications. Furthermore, we have reported
the Quantum chemical analysis ((DFT/B3LYP) method with 6-
Using the x, y and z components, the magnitude of the first
hyperpolarizability tensor can be calculated by
1=2
2
x
2
y
2
z
3
11G++(d,p) as basis set) of benzimidazole derivatives 1–6 (heat
b
totðb þ b þ b Þ
ð2Þ
of formation, geometrical structure, vibration wave numbers,
NLO and NBO analysis) and the optimized geometrical parameters
obtained by DFT calculation is in good agreement with single crys-
tal XRD data. The Mulliken and NBO charge analysis were also cal-
culated and discussed about the more basic nature of the nitrogen
The complete equation for calculating the magnitude of first
hyperpolarizability from Gaussian-03 output is given as follows:
h
i
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
1=2
2
2
2
btot
¼
bxxx þ bxyy þ bxzz
þ
byyy þ byzz þ byxx
þ
bzzz þ bzxx þ bzyy
ð3Þ
atom of the imidazole derivatives. The electric dipole moment (l)
All the electric dipole moment and the first hyperpolarizabili-
and the first-hyperpolarisability (b) value of the investigated mol-
ecules have been studied by both experimentally and theoretically
which reveal that they have non-linear optical (NLO) behavior with
non-zero values. These chromophores possess a more appropriate
ratio of off – diagonal versus diagonal b tensorial component (r = b
xyy/b xxx) which reflects the inplane nonlinearity anisotropy since
ties are calculated by taking the Cartesian coordinate system (x,
y, z) = (0, 0, 0) at own center of mass of the compounds.
Natural bond orbital (NBO) analysis
NBO analysis have been performed on the molecule at the DFT/
B3LYP/6-31G(d,p) level in order to elucidate the intramolecular,
rehybridization and delocalization of electron density within the
molecule. The second order Fock matrix was carried out to evalu-
ate the donor–acceptor interactions in the NBO analysis [12]. The
interactions result is a loss of occupancy from the localized NBO
of the idealized Lewis structure into an empty non-Lewis orbital.
For each donor (i) and acceptor (j), the stabilization energy E(2)
associated with the delocalization i ? j is estimated as
they have largest
0
lb values, the reported benzimidazoles can be
used as potential NLO materials.
Experimental
Spectral measurements
The proton spectra at 400 MHz were obtained at room temper-
ature using a Bruker 400 MHz NMR spectrometer. Proton decou-
pled 1 C NMR spectra were also recorded at room temperature
employing a Bruker 400 MHz NMR spectrometer operating at
2
Fði; jÞ
3
Eð2Þ ¼
D
E
ij ¼ qi
ð4Þ
e
j ꢂ
ei
1
00 MHz. The mass spectra of the samples were obtained using a
Thermo Fischer LC-Mass spectrometer in fast atom bombardment
FAB) mode. Single crystal XRD has been recorded in Agilent Xcal-
where q is the donor orbital occupancy, ei and ej are diagonal ele-
ments and F (i, j) is the off diagonal NBO Fock matrix element
i
(
[13]. The larger the E(2)value, the more intensive is the interaction
between electron donors and electron acceptors, i.e., the more
donating tendency from electron donors to electron acceptors and
the greater the extent of charge transfer or conjugation of the whole
system.
ibur Ruby Gemini diffractometer (For 7, 8 and 12). Data collection:
APEX2 (Bruker, 2008); cell refinement: APEX2 and SAINT (Bruker,
2
SHELXS97 (Sheldrick, 2008); program(s) used to refine structure:
SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek,
008); data reduction: SAINT; program(s) used to solve structure:
2
009), Bruker Kappa APEXII diffractometer (for 9–11).
Results and discussion
Non-linear optical measurements
Single crystal XRD analysis
The non-linear optical conversion efficiencies were parted using
a modified set up of Kurtz and Perry. A Q-switched Nd: YAG laser
beam of wavelength of 1064 nm was used with an input power of
Crystal structure of 1,2-diphenyl-1H-benzo[d]imidazole (1)
1,2-Diphenyl-1H-benzo[d]imidazole is monoclinic crystal and
crystallizes in the space group C2/c. The cell dimensions are
a = 10.1878 (3) Å, b = 16.6399 (4) Å, c = 17.4959 (5) Å. ORTEP dia-
gram of 1 presented in Fig. S1a, shows that the benzimidazole unit
is close to being planar (maximum deviation = 0.0102 (6) Å) and
forms dihedral angles of 55.80 (2) and 40.67 (3)° with the adjacent
phenyl rings; the dihedral angle between the phenyl rings is 62.37
4
.1 mJ/pulse width of 10 ns, scattering geometry 90°, the repetition
rate being 10 Hz, monochromater Jobin Youon Triax 550, slit width
.5 mm, focal length of focusing lens 20 cm, PMT model number
XP2262B used in Philips photonics, power supply for PMT is
.81 KU/mA with oscilloscope Jektronix TDS 3052B.
0
1

7
6
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 74–78
(
3)°. Fig. S1b displays the crystal packing diagram of 1 [14]. In the
benzene rings; the dihedral angle between the latter two planes
is 64.30 (7)°. An intramolecular OAHꢃ ꢃ ꢃN hydrogen bond generates
an S(6) ring motif. In the crystal, molecules are linked by CAHꢃ ꢃ ꢃN
and CAHꢃ ꢃ ꢃO hydrogen bonds, and consolidated into a three-
crystal, one CAHꢃ ꢃ ꢃN hydrogen bond and three weak CAHꢃ ꢃ ꢃ
p
interactions involving the fused benzene ring and the imidazole
ring are observed, leading to a three-dimensional architecture.
dimensional architecture by p–p stacking interactions, with a cen-
Crystal structure of 2-(4-fluorophenyl)-1-phenyl-1H-
benzo[d]imidazole (2)
troid–centroid distance of 3.8428 (12) A. Fig. S6b displays the crys-
tal packing diagram of 6 [19]. Intramolecular hydrogen bonding
from the ORTEP initiates to study the ESIPT.
2
-(4-Fluorophenyl)-1-phenyl-1H-benzo[d]imidazole is mono-
clinic crystal and crystallizes in the space group P2 /n. The cell
1
Optimization have been performed by DFT at B3LYP/6-31G(d,p)
using Gaussian-03. All these XRD data are in good agreement with
the theoretical values (Tables S1–S6). However, from the theoreti-
cal values it can be found that most of the optimized bond lengths,
bond angles and dihedral angles are slightly higher than that of
XRD values. These deviations can be attributed to the fact that
the theoretical calculations were aimed at the isolated molecule
in the gaseous phase and the XRD results are on at the molecule
in the solid state.
dimensions are a = 8.7527 Å, b = 10.1342 Å, c = 17.0211 Å. ORTEP
diagram of 2 presented in Fig. S2a, shows that the benzimidazole
unit is almost planar [maximum deviation = 0.0342 (9) A for C6].
Fig. S2b displays the crystal packing diagram of 2 [15]. The dihedral
angles between the planes of the benzimidazole and the phenyl
and the fluorobenzene groups are 58.94 (3) and 51.43 (3)°, respec-
tively. The dihedral angle between the planes of the phenyl and the
fluorobenzene rings is 60.17 (6)°. Intermolecular C4AH4ꢃ ꢃ ꢃF4,
C7AH7ꢃ ꢃ ꢃF4 and C26AH26ꢃ ꢃ ꢃF4 hydrogen bonds and weak
C16AH16ꢃ ꢃ ꢃ
p
and C22AH22ꢃ ꢃ ꢃ
p interactions involving the fused
a
key twist
benzene ring are found in the crystal structure.
The key twist, designated as
a have been examined. a is used to
Crystal structure of 1-phenyl-2-p-tolyl-1H-benzo[d]imidazole (3)
indicate the twist of benzimidazole ring from the aromatic six-
membered ring at C-2. The twist originates from the interaction
of substituent at benzyl rig attached nitrogen of the benzimidazole
with the substituent at junction of aldehydic ring. The present
structural information allows us to further explore the correlation
between structural features and fluorescent property. When the
two adjacent aromatic species are in a coplanar geometry, the p-
orbitals from the CAC bond connecting the two species will have
maximal overlapping and the two rings will have a rigid and partial
delocalized conjugation, as the result, the bond is no longer a pure
single bond, as evident from the X-ray data.
1
-Phenyl-2-p-tolyl-1H-benzo[d]imidazole is orthorhombic
crystal and crystallizes in the space group Pbca. The cell dimen-
sions are a = 15.6755 (4) Å, b = 9.3509 (6) Å, c = 21.1976 (8) Å. OR-
TEP diagram of
3 presented in Fig. S3a, shows that the
benzimidazole ring system forms dihedral angles of 28.50 (7)
and 72.44 (7)° with the tolyl and phenyl rings, respectively.
Fig. S3b displays the crystal packing diagram of 3 [16]. In the crys-
tal, molecules are linked into chains along the a-axis direction by
weak CAHꢃ ꢃ ꢃN interactions. The crystal structure also features
CAHꢃ ꢃ ꢃ
p interactions.
Crystal structure of 2-(4-methoxyphenyl)-1-phenyl-1H-
benzo[d]imidazole (4)
Second harmonic generation (SHG) studies of 1,2-diaryl benzimidazole
derivatives
2
-(4-Methoxyphenyl)-1-phenyl-1H-benzo[d]imidazole
is
monoclinic crystal and crystallizes in the space group P21/c. The
cell dimensions are a = 12.3220 (3) Å, b = 7.3030 (2) Å,
c = 18.2450 (3) Å. ORTEP diagram of 4 presented in Fig. S4a, shows
that the 1H-benzimidazole ring forms dihedral angles of 48.00 (6)
and 64.48 (6)°, respectively with the benzene and phenyl rings,
which are inclined to one another by 58.51 (7)°. Fig. S4b displays
Second harmonic signals of 51 (1), 57 (2), 54 (3), 49 (4), 58 (5)
and 55 (6) mV was obtained for 1,2-diaryl benzimidazole deriva-
tives by an input energy of 4.1 mJ/pulse. But the standard KDP
crystal gave a SHG signal of 110 mV/pulse for the same input en-
ergy. The second order non-linear efficiency will vary with the par-
ticle size of the powder sample [20]. Higher efficiencies are
achieved by optimizing the phase matching [21]. On a molecular
scale, the extent of charge transfer (CT) across the NLO chromo-
phore determines the level of SHG output, the greater the CT and
the larger the SHG output.
the crystal packing diagram of 4 [17]. In the crystal, weak CAHꢃ ꢃ ꢃ
p
interactions are the only intermolecular interactions present.
Crystal structure of 2-(4-(trifluoromethyl)phenyl)-1-phenyl-1H-
benzo[d]imidazole (5)
2
-(4-(Trifluoromethyl)phenyl)-1-phenyl-1H-benzo[d]imidazole
is triclinic crystal and crystallizes in the space group Pꢂ1. The cell
dimensions are a = 8.7179 (4) Å, b = 9.6796 (5) Å, c = 11.3612 (6) Å.
ORTEP diagram of 5 presented in Fig. S5a, shows that the benz-
imidazole unit is close to being planar [maximum devia-
tion = 0.012 (1) Å] and forms dihedral angles of 31.43 (7) and
Comparison of lb
0
When their dipole moment aligned in a parallel fashion the
overall polarity of the synthesized 1,2-diaryl benzimidazole deriv-
atives was small (Fig. S7). When the electric field is removed, the
parallel alignment of the molecular dipole moments begins to
deteriorate and eventually the benzimidazoles loses its NLO activ-
ity. The ultimate goal in the design of polar materials is to prepare
compounds which have their molecular dipole moments aligned in
the same direction [22].
6
1.45 (9)° with the 4-(trifluoromethyl)phenyl and 1-phenyl rings,
respectively; the dihedral angle between these rings is 60.94
10)°. Fig. S5b displays the crystal packing diagram of 5 [18]. In
(
the crystal, CAHꢃ ꢃ ꢃF hydrogen bonds link the molecules into chains
along the c-axis direction. The CF3 group is rotationally disordered
with an occupancy ratio of 0.557 (8):0.443 (8) for the F atoms.
Theoretical investigation plays an important role in under-
standing the structure–property relationship, which is able to as-
sist in designing novel NLO chromophores. The electrostatic first
Crystal structure of 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenol (6)
2
-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenol is triclinic crys-
hyperpolarizability (b) and dipole moment (l) of the imidazole
tal and crystallizes in the space group Pꢂ1. ORTEP diagram of 6
was presented in Fig. S6a, the benzimidazole unit is close to being
planar [maximum deviation = 0.0253 (11) Å] and forms dihedral
angles of 68.98 (6) and 20.38 (7)° with the adjacent phenyl and
chromophore have been calculated by using Gaussian 03 package
[23]. Table 1 shows the synthesized 1,2-diaryl benzimidazole
derivatives have larger
g 0
l b values, which is attributed to the posi-
tive contribution of their conjugation.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 74–78
77
Table 1
Electric dipole moment (
l), polarizability (
a) and hyperpolarisability (b) of 1–6.
Parameter
1
2
3
4
5
6
l
l
l
l
x
ꢂ0.4087
0.8032
1.0823
0.2820
0.4944
1.0454
ꢂ0.4741
0.7439
0.9931
2.2110
0.2376
0.0939
0.9872
1.3187
0.9087
0.1249
1.4069
ꢂ0.7940
0.5535
0.7135
2.0610
y
z
tot
1.4768
1.8218
2.4405
a
a
a
a
a
a
a
xx
xy
yy
xz
yz
253.7866
ꢂ49.9949
119.0665
ꢂ8.5772
12.7956
227.6581
2.9665
ꢂ158.3280
41.8472
ꢂ27.9548
42.2292
65.2160
ꢂ32.5731
58.1896
ꢂ64.8049
55.0181
53.5076
29.1300
43.0192
290.5813
ꢂ57.2139
125.1516
ꢂ5.0785
14.4184
238.8667
3.2337
ꢂ47.4430
ꢂ8.1897
20.0538
26.4870
8.4763
8.7341
6.2045
22.4067
9.9729
4.5970
314.3461
ꢂ62.1915
134.8593
ꢂ3.5873
13.9635
246.3638
3.4361
17.7405
ꢂ17.1877
4.2778
33.9766
11.6881
8.9065
5.1520
8.1889
327.8061
ꢂ55.0390
134.3488
ꢂ2.4792
14.6297
247.4461
3.5054
22.0357
ꢂ31.4917
8.9495
32.7747
ꢂ41.1196
13.2622
6.7521
16.1520
12.1062
4.0579
331.7883
ꢂ17.6190
124.1276
ꢂ12.3539
ꢂ20.1380
238.9194
3.4325
ꢂ1359.7655
ꢂ38.6190
ꢂ13.6394
ꢂ0.03635
ꢂ119.1107
ꢂ19.0536
20.1559
290.2289
ꢂ56.1661
125.9119
ꢂ4.2619
15.5970
250.5170
3.2933
30.4546
ꢂ38.4757
31.0037
ꢂ13.4541
11.4575
10.1066
0.4994
zz
tot ꢅ 10ꢂ23
b
b
b
b
b
b
b
b
b
b
b
xxx
xxy
xyy
yyy
xxz
xyz
yyz
xzz
yzz
ꢂ130.7981
6.2343
8.3431
6.2315
7.1309
12.4308
4.2884
4.0638
17.9414
zzz
231.8710
130.4710
318.4145
ꢂ31
ꢅ 10ꢂ31
tot ꢅ 10
2.9873
5.4422
4.9778
6.5642
l
ꢅ b
0
8.9850
14.6968
Octupolar and dipolar components of 1,2-diaryl benzimidazole
derivatives
hyperconjugative interaction and electron density transfer from
lone pair electrons to the antibonding orbital has been analyzed
[
25]. Several donor–acceptor interactions are observed for the
The 1,2-diaryl benzimidazole derivatives possess a more appro-
priate ratio of off-diagonal versus diagonal b tensorial component
1,2-diaryl benzimidazole derivatives and among the strongly
occupied NBOs, the most important delocalization sites are in the
(r = bxyy/bxxx) which reflects the inplane non-linearity anisotropy
p
system and in the lone pairs (n) of the oxygen, fluorine and nitro-
and the largest values. The difference of the bxyy/bxxx ratios
l
b
0
gen atoms. The system shows some contribution to the
r
can be well understood by analyzing their relative molecular orbi-
tal properties. The r values of 1,2-diaryl benzimidazole derivatives
are ꢂ0.2643 (1), ꢂ0.4227 (2), ꢂ0.2411 (3), 0.4061 (4), ꢂ0.0100 (5),
delocalization, and the important contributions to the delocaliza-
tion corresponds to the donor–acceptor interactions are
C27AC30 ? C25AC26, C27AC30 ? C25AC26, C8AC10 ? C9AC11,
C9AC11 ? C12AC13, C9AC11 ? C12AC13, C12AC13 ? C8AC10,
C27AC31 ? C24AC25, C27AC31 ? C26AC29, C8AC9 ? C10AC12,
C26AC30 ? C25AC28. The charge distribution of 1,2-diaryl benz-
imidazole derivatives was calculated from the atomic charges by
NLO and NBO analysis (Fig. S8). These two methods predict the
same trend i.e., among the two nitrogen atoms, azomethine nitro-
gen is considered as more basic site [26]. When compared to nitro-
gen fluorine atom are less electronegative in 1,2-diaryl
benzimidazole derivatives [27].
ꢂ1.0180 (6). The electrostatic first hyperpolarizabilities (b
0
) and
dipole moment (l) of the chromophores have been investigated
theoretically. These observed results can be explained by the re-
duced planarity in such chromophores caused by the steric interac-
tion azomethine nitrogen atom. Hence, the steric interaction must
0
be reduced in order to obtain larger b values.
2
D
The b tensor [24] can be decomposed in a sum of dipolar ð bÞ
J¼1
2
D
and octupolar ð bÞ tensorial components, and the ratio of these
J¼3
two components strongly depends on their ‘r’ ratios. Complying
with the Pythagorean theory and the projection closure condition,
the octupolar and dipolar components of the b tensor can be
described as:
Molecular electrostatic potential map (MEP) and electronic properties
h
i
h
i
2
j¼1
D
2
2
MEP surface diagram (Fig. S9) is used to understand the reactive
behavior of a molecule, in that negative regions can be regarded as
nucleophilic centers, whereas the positive regions are potential
electrophilic sites. The MEP map of synthesized benzimidazoles
clearly suggests that the nitrogen and fluorine atoms represent
the most negative potential region. The hydrogen atoms bear the
maximum brunt of positive charge. The predominance of green re-
gion in the MEP surfaces corresponds to a potential halfway be-
tween the two extremes red and dark blue colour.
k
bk ¼ ð3=4Þ ðb þ bxyy
Þ
þ ðb þ byxx
Þ
ð5Þ
xxx
yyy
h
i
h
i
2
j¼3
D
2
2
k
bk ¼ ð1=4Þ ðb ꢂ 3bxyy
Þ
þ ðb þ byxx
Þ
ð6Þ
xxx
yyy
2
D
k
k
bk
2D
q2D
J¼3
The parameter
q
½
¼
2D ꢄ is convenient to compare the
bk
J¼1
relative magnitudes of the octupolar and dipolar components of
b. The observed positive small
ponent cannot be zero and these are dipolar component. Since
most of the practical applications for second order NLO chromoph-
ores are based on their dipolar components, this strategy is more
appropriate for designing highly efficient NLO chromophores.
2D
q
value reveals that the biii com-
Conclusion
In this article we have reported benzimidazole based chro-
mophores as potential NLO materials. All the compounds have
been characterized by single crystal XRD. Comparison between
Natural bond orbital (NBO) analysis
NBO analysis have been performed for 1,2-diaryl benzimidazole
derivatives at the DFT/B3LYP/6-31++G(d,p) level in order to
elucidate the intramolecular, hybridization and delocalization of
electron density within the molecule. The importance of
theoretical and experimental have been done. The presence of a
twist in these benzimidazoles drops the fluorescence quantum
yield. The observed dipole moment and hyperpolarisability can
be explained by the reduced planarity caused by the steric interac-

7
8
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 74–78
tion in nitrogen atom attached to benzyl ring. Hence, the steric
interaction must be reduced in order to obtain larger b values.
DFT calculations show that molecules of higher hyperpolarizability
have larger dipole moments used as potential NLO materials.
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Acknowledgments
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(
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One of the authors Prof. J. Jayabharathi is thankful to DST [No.
SR/S1/IC-73/2010] and DRDO (NRB-213/MAT/10-11) for providing
funds to this research study. Mr. K. Jayamoorthy is thankful to DST
(
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Butcher, Acta Cryst. E69 (2013) o62.
[
No. SR/S1/IC-73/2010] for providing fellowship.
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