Physicochemical and solvatochromic analysis of an imidazole derivative as NLO material

Article abstract of DOI:10.1016/j.saa.2011.08.060

Bioactive imidazole derivative, 2-(2,4-difluorophenyl)-1-phenyl-1H- imidazo[4,5-f][1,10]phenanthroline, has been synthesized and characterized by IR, UV-vis, NMR and elemental (CHN) analysis. The electric dipole moment (μ) and the hyperpolarizability (β)

Full text of DOI:10.1016/j.saa.2011.08.060

Spectrochimica Acta Part A 85 (2012) 31–37
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Physicochemical and solvatochromic analysis of an imidazole derivative
as NLO material
Jayaraman Jayabharathi∗, Venugopal Thanikachalam, Marimuthu Venkatesh Perumal
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2011
Received in revised form 17 August 2011
Accepted 24 August 2011
Bioactive imidazole derivative, 2-(2,4-difluorophenyl)-1-phenyl-1H-imidazo[4,5-f][1,10]phenanthro-
line, has been synthesized and characterized by IR, UV–vis, NMR and elemental (CHN) analysis. The
electric dipole moment (ꢀ) and the hyperpolarizability (ˇ) have been studied both experimentally and
theoretically, which reveals that the synthesized imidazole derivative possesses non-linear optical (NLO)
behavior. This chromophore possess more appropriate ratio of off-diagonal versus diagonal ˇ tenso-
rial component (r = ˇxyy/ˇxxx = −0.19) which reflects the in plane nonlinearity anisotropy. Since they
have largest ꢀ␤0 value, the reported imidazole can be used as potential NLO material. Within this con-
text, reasonable conclusions concerning the steric hindrance in the chromospheres, push–pull character,
hyperpolarizability of the imidazole and their application as NLO materials will be drawn. The solvent
effect on the absorption and fluorescence bands was analyzed by a multi-component linear regression
in which several solvent parameters were analyzed simultaneously.
Keywords:
DFT
NLO
NBO
Multi-component linear regression
Octupolar and dipolar components
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
the solid state, their interaction and other physicochemical prop-
erties like strong intramolecular charge-transfer absorptions and
Heterocyclic imidazole derivatives have attracted considerable
attention because of their unique optical properties [1]. These com-
pounds play very important role in chemistry as mediators for
synthetic reactions, primarily for preparing functionalized mate-
rials [2]. Imidazole nucleus forms the main structure of human
organisms, i.e., the amino acid histidine, Vitamin B12, a component
of DNA base structure, purines, histamine and biotin and present
in structure of many natural or synthetic drug molecules, i.e.,
azomycin, cimetidine and metronidazole and also have significant
analytical applications due to their fluorescence and chemilumi-
nescence properties [3].
Searching organic materials with non-linear optical (NLO) prop-
erties is usually concentrated on molecules with donor–acceptor
␲-conjugation (D-␲-A) and deals with the substituent effects on
the degree of ␲-conjugation, steric hindrance and the hyperpolar-
izability of the substances. [4]. Nowadays there is an insufficient
understanding for designing optimal NLO materials, even certain
classes of D-␲-A compounds were theoretically studied [5]. Search-
ing for organic materials with NLO properties is usually focused
on molecules with donor–acceptor ␲-conjugation and deals with
the systematic investigation of substituent effects on the degree of
␲-conjugation, steric hindrance and the hyperpolarizability of the
molecules. Besides, geometrical arrangement of the molecules in
engineering possibilities are also important [6]. At present, there
is an insufficient understanding of all influences for designing
optimal NLO materials, even if the influencing factors in certain
classes of D-␲-A compounds were theoretically studied [7]. To
quantify the push-pull effect in D-␲-A compounds, bond length
alternation (BLA) and out-of-plane distortions of the polarized
carbon-carbon double bonds, available from X-ray studies, have
been employed for a long time [8]. Alternatively, dipole moment
measurements [9], bond lengths [10], barriers to rotation about
the partial carbon-carbon double bonds [11] (from dynamic NMR
studies), and the occupation quotients (ꢁ/ꢁ*) [12] of the bond-
ing (to quantify the acceptor activity) and anti-bonding orbital
(to quantify the donor activities) of these carbon-carbon double
bonds were adopted. Not only the push–pull effect in D-␲-A com-
pounds quantified, but also a linear dependence of the push–pull
quotient (ꢁ*/ꢁ) on molar hyperpolarizability were detected [10].
Thus, ꢁ*/ꢁ is a sensitive parameter of the donor–acceptor quality
of compounds for potential NLO applications. The electric dipole
moment (ꢀ) and the first-hyperpolarizability (ˇ) revealed that,
the synthesized imidazole derivative have non-linear optical (NLO)
behavior. This chromophore possesses more appropriate ratio of
off-diagonal versus diagonal ˇ tensorial component (r = ˇxyy/ˇxxx
)
which reflects the inplane nonlinearity anisotropy since they have
largest ꢀˇ0 value, the reported imidazole can be used as potential
NLO material and their photophysical properties were investigated
in a wide variety of solvents. The solvent effects on the absorp-
tion and fluorescence bands were analyzed by a multi-component
∗
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.060

32
J. Jayabharathi et al. / Spectrochimica Acta Part A 85 (2012) 31–37
linear regression in which several solvent parameters were simul-
taneously analyzed.
2.4. Synthesis of 2-(2,4-difluorophenyl)-1-phenyl-1H-
imidazo[4,5-f][1,10]phenanthroline
The experimental procedure was used as same as described
in our recent papers [15–17]. The imidazole derivative, dfppip
was synthesized from an unusual four components assembling of
a mixture of 1,10-Phenanthroline-5,6-dione, ammonium acetate,
aniline and 2,4-difluorobenzaldehyde in distilled ethanol medium.
The reaction was monitored by thin layer chromatography for
the completion of the reaction. The reaction mixture was then
extracted with dichloromethane and the resultant resinous mate-
rial was purified by column chromatography using benzene:ethyl
acetate (9:1) as the eluent. Yield: 60%. mp. 254 ◦C, Anal. calcd. for
C25H14F2N4: C, 73.5,; H, 3.46; N, 13.72. Found: C, 72.8; H, 3.23; N,
13.17. 1H NMR (500 MHz, CDCl3): ı 7.01–7.10 (m, 3H) [aldehydic
phenyl ring], 7.68–7.73 (m, 3H), 8.27–8.33 (m, 2 ortho protons of
aniline ring), 8.57 (d, 2H, J = 10.4 Hz), 8.85 (d, 2H, J = 8.0 Hz), 9.18
(d, 2H, J = 4.0 Hz). 13C (100 MHz, CDCl3): ı 105.65, 112.37, 117.88,
122.82, 123.45, 123.65, 128.41, 128.77, 130.83, 131.80, 134.43,
143.42, 144.95, 149.26, 159.44, 159.49, 159.81, 159.94, 162.42,
163.63, 166.17. MS: m/z calcd for [M+H]+ 408.9. IR (cm−1): 3062
(ꢃ C–H), 1614 (ꢃC N),1112–1062 (polyfluorinated compounds).
2. Experimental
2.1. Optical measurements and composition analysis
NMR spectra were recorded on a Bruker 400 MHz NMR instru-
ment. The ultraviolet–visible (UV–vis) spectra were measured
on a UV–vis spectrophotometer (Perkin Elmer, Lambda 35) and
corrected for background due to solvent absorption. Photolumi-
nescence (PL) spectra were recorded on a (Perkin Elmer LS55)
fluorescence spectrometer. The infrared spectra were recorded on
a Avatar 330-Thermo Nicolet FT-IR spectrometer. Mass spectrum
was recorded using Agilant 1100 Mass spectrometer.
2.2. Non-linear Optical measurements
The second harmonic generation efficiency was assessed by
Kurtz powder technique [11] at IISc., Bangalore, India. It is a well
established tool to evaluate the conversion efficiency of non-linear
optical materials. A Q-switched Nd:YAG laser operating at the fun-
damental wavelength of 1064 nm, generating about 4.1 mJ and
pulse width of 8 ns was used for the present experimental study.
The input laser beam was passed through an IR reflector and then
incident on the powder form of the specimen, which was packed
in a glass capillary tube. The output energy was detected by a pho-
todiode detector integrated with oscilloscope assembly.
3. Results and discussion
To analyze the solvatochromic effects, we have checked
several methods [18,19]. Neither the absorption nor the fluores-
cence wavenumbers linearly correlate with the Lippert parameter
ꢂf(ε,n2) [20], which consider the solvent polarity/polarizability
or with the Reichardt parameter ETN(30) [19,21], which takes
into account several solvent properties (polarity and H-bond
donor capacity) in a common parameter. For these reasons, a
multi-parameter correlation analysis is employed in which a
physicochemical property is linearly correlated with several sol-
vent parameters by means of Eq. (1):
2.3. Computational details
Quantum mechanical calculations were carried out using
Gaussian-03 program [12]. As the first step of our DFT calculation,
the geometry taken from the starting structure was optimized.
2.3.1. Hyperpolarizability calculation
(XYZ) = (XYZ)0 + CaA + CbB + CcC + . . .
(5)
The density functional theory has been used to calculate the
dipole moment (ꢀ), mean polarizability (˛) and the total first static
hyperpolarizability (ˇ) [13] for dfppip in terms of x, y, z components
and is given by following equations:
where (XYZ)0 is the physicochemical property in an inert solvent
and Ca, Cb, Cc and so forth are the adjusted coefficients that reflect
the dependence of the physicochemical property (XYZ) on several
solvent properties. Solvent properties that mainly affect the pho-
tophysical properties of compounds are polarity, H-bond donor
capacity and electron donor ability. Different scales for such param-
eters can be found in the literature, Taft et al. [22] proposed the ꢁ*,
˛ and ˇ scales, whereas more recently Catalan et al. [23] suggested
the SPPN, SA and SB scales to describe the polarity/polarizability,
the acidity and basicity of the solvents, respectively.
ꢀ = (ꢀ2x + ꢀ2y + ꢀs2)1/2
(1)
(2)
1
3
˛ = (˛xx + ˛yy + ˛zz
)
ˇtot = (ˇx2 + ˇy2 + ˇz2)1/2 (or)
2
2 + (ˇyyy + ˇyzz + ˇyxx
)
(3)
Fig. 1 shows the obtained correlation between the absorp-
tion and fluorescence wavenumbers calculated by the multi-
component linear regression employing the Taft-proposed solvent
parameters and the experimental values listed in Table 1. The dom-
inant coefficient affecting the absorption and fluorescence bands
of this imidazole derivative (dfppip) is that describing the polar-
ˇtot = [(ˇxxx + ˇxyy + ˇxss
)
The ˇ components of Gaussian output are reported in atomic units
and therefore the calculated values are converted into e.s.u. units
(1 a.u. = 8.3693 × 10−33 e.s.u.).
ity/polarizability of the solvent, Cꢁ* or CN having a positive value,
2.3.2. Natural bond orbital (NBO) analysis
SPP
The second order Fock matrix was carried out to evaluate the
donor–acceptor interactions in the NBO analysis [14]. For each
donor (i) and acceptor (j), the stabilization energy E(2) associated
with the delocalization i → j is estimated as,
corroborating the solvatochromic shifts with the solvent polarity.
The coefficient controlling the electron releasing capacity or basic-
ity of the solvent, Cˇ or CSB, is the lowest coefficient (Table 1), hence,
the solvent basicity does not play an important role in absorption
and fluorescence displacements. The adjusted coefficient repre-
senting the acidity of the solvent, C˛ or CSA has a negative value,
suggesting that the absorption and fluorescence bands shift to
lower energies with the increasing acidity of the solvent. This effect
can be interpreted in terms of the stabilization of the resonance
structures of the chromophore (Fig. 2). In resonance structure “a”
all the nitrogens are having lone pair of electrons and that structure
F(i, j)
E(2) = ꢂEij = q
(4)
i εj − εi
where qi is the donor orbital occupancy, εi and εj are diagonal ele-
ments and F(i,j) is the off diagonal NBO Fock matrix element. The
larger the E(2) value, the more intensive is the interaction between
electron donors and electron acceptors.

J. Jayabharathi et al. / Spectrochimica Acta Part A 85 (2012) 31–37
33
Fig. 1. Correlation between the experimental absorption and fluorescence wavenumber with the predicted values obtained by a multicomponent linear regression using the
ꢁ*, ˛ and ˇ-scale (Taft) solvent parameters for dfppip.
Table 1
Adjusted coefficients ((ꢃx)0, Ca, Cb and Cc) and correlation coefficients (r) for the multilinear regression analysis of the absorption ꢃab and fluorescence ꢃfl wavenumbers and
Stokes shift (ꢂꢃss) of the imidazole derivative dfppip with the solvent polarity/polarizability, and the acid and base capacity using the Taft (ꢁ*, ˛ and ˇ) and the Catalan
(SPPN, SA and SB) scales.
(ꢃx)
(ꢃx)0 cm−1
(ꢁ*)
C˛
Cˇ
r
ꢄab
ꢄfl
(3.63 0.002) × 104
(2.58 0.011) × 104
(1.04 0.011) × 104
(6.08 0.574) × 103
(2.16 2.199) × 103
(3.91 2.107) × 103
−(14.81 1.919) × 103
−(10.71 7.349) × 103
−(4.09 7.044) × 103
(9.24 1.541) × 103
(12.838 5.904) × 103
−(3.59 5.658) × 103
0.97
0.93
0.95
ꢂꢃss = ꢃab − ꢃfl
(ꢃx)
(ꢃx)0 cm−1
CSNPP
CSA
CSB
r
ꢄab
ꢄfl
(3.66 0.015) × 104
(2.58 0.018) × 104
(1.07 0.024) × 104
−(3.11 6.837) × 103
−(0.14 8.075) × 103
−(2.97 10.773) × 103
(9.86 42.167) × 103
(27.82 37.198) × 103
−(17.95 44.672) × 103
−(8.08 29.114) × 103
−(43.56 34.387) × 103
(35.48 45.875) × 103
0.67
0.77
0.70
ꢂꢃss = ꢃab − ꢃfl
will be stabilized in acidic solvents because this resonance struc-
ture is predominant in the S1 state, and the stabilization of the S1
state with the solvent acidity would be more important than that
of the S0 state.
more is the drop in the fluorescence quantum yield. Such a clear cor-
relation indicates the importance of coplanarity between imidazole
and the aromatic ring at C(23) and this correlation can be ascribed
to the conjugation rigidity. When the two adjacent aromatic species
are in a coplanar geometry, the p-orbitals from the C–C bond con-
necting 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 of (dpip). Since X-ray crystal study of the dfppip is cur-
rently under progress; its analogous data [24] is taken for compar-
3.1. Steric hindrance in imidazoles: X-ray analysis
DFT calculation reveals that the imidazole ring is essentially pla-
nar and makes dihedral angle of 91.93◦ and 88.08◦ with the phenyl
and difluorophenyl rings, respectively. The key twist, designated as
˛ have been examined. ˛ is used to indicate the twist of imidazole
ring from the aromatic six-membered ring at C(23), (Fig. 3). The
˛ twist originates from the interaction of substituent at N(15) of
the imidazole with the substituent at C(23). The present structural
information allows us to further explore the correlation between
structural features and fluorescent property. It reveals that ˛ twist
is correlated with fluorescent property, the larger the ˛ twist, the
˚
ision. The present bond distance of C23–C24 is 1.475 (4) A (Table 2)
is shorter than the regular single bond distance between two sp2
˚
carbons (1.48 A) [18], because of delocalization. When the two rings
are deviated from each other, the p-orbital overlapping will be
reduced. All these XRD data are in good agreement with the the-
oretical values. However, from the theoretical values it was found
that most of the optimized bond lengths, bond angles and dihedral
Fig. 2. Resonance structures (a and b) of the imidazole chromophore dfppip.
Fig. 3. The key ˛ twist of imidazole ring at C(23).

34
J. Jayabharathi et al. / Spectrochimica Acta Part A 85 (2012) 31–37
Table 2
Selected bond lengths, bond angles and torsional angles for the imidazole derivative dfppip.
Bond connectivity
Bond length (Å)a
Bond connectivity
Bond angle (◦)
Bond connectivity
Torsional angle (◦)
N1–C5
N1–C18
N1–C2
C2–C24
N3–C4
N3–C2
C4–C5
C4–C17
C5–C6
N10–C11
C18–C19
C18–C23
C24–C29
C24–C25
C27–F27
C29–F29
1.4106 (1.3840)
1.4361 (1.4402)
1.4159 (1.3752)
1.4757 (1.4782)
1.3908 (1.3789)
1.3403 (1.3168)
1.4190 (1.3770)
1.4475 (1.4330)
1.4506 (1.4321)
1.3675 (1.3506)
1.4169 (1.3840)
1.4170 (1.3745)
1.4322 (1.3860)
1.4153 (1.3890)
1.3203 (0.9500)
1.3201 (0.9500)
N1–C2–N3
N1–C5–C6
N1–C5–C4
N1–C18–C19
N1–C18–C23
C2–N1–C5
C2–C24–C25
C2–C24–C29
C2–C3–C4
111.25707 (112.63)
132.3278 (131.74)
105.0439 (105.14)
119.9259 (119.73)
119.7224 (118.93)
106.7315 (106.40)
122.1452 (121.16)
119.7692 (119.87)
106.3648 (104.52)
110.0620 (111.31)
123.5253 (124.59)
126.7377 (127.30)
122.1671 (123.04)
122.6599 (121.38)
128.5619 (128.80)
121.1143 (120.44)
121.9126 (120.30)
C4–C5–C6–C7
N1–C5–C6–C7
N3–C4–C5–C6
C17–C4–C5–C6
C2–N1–C5–C6
C5–N1–C18–C19
C5–N1–C18–C23
C5–N1–C2–N3
C5–N1–C2–C24
C2–N3–C4–C17
C4–N3–C2–C24
N1–C18–C19–C20
N1–C18–C23–C22
N1–C2–C24–C29
C18–N1–C2–N3
N3–C2–C24–C29
N1–C2–C24–C25
C2–C24–C29–C28
C2–N1–C18–C23
C7–C6–C11–N10
C9–N10–C11–C6
C14–N13–C12–C17
−179.2650 (−175.39)
0.5413 (0.8)
179.8021 (176.28)
−0.3744 (−3.5)
−179.9210 (−175.82)
87.8925 (82.65)
−93.3349 (−98.62)
0.2113 (−0.73)
179.5378 (−179.76)
−179.6357 (−179.93)
179.5785 (179.25)
178.3979 (177.39)
−178.3995 (−176.22)
−91.8453 (−125.55)
178.8895 (−174.58)
−88.9081 (55.54)
−91.3145 (56.56)
177.8084 (−177.29)
88.2840 (73.81)
N3–C4–C5
N3–C2–C24
N3–C4–C17
C4–C5–C6
C5–C4–C17
C5–N1–C18
C24–C25–C26
C24–C29–C28
0.0443 (0.6)
0.1303 (−0.1)
−0.0955 (−3.2)
a
values given in the brackets are corresponding to XRD values of dpip.
Fig. 4. Orientation of dipole moments.
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 whereas the
XRD results were aimed at the molecule in the solid state.
Theoretical investigation plays an important role in under-
standing the structure-property relationship, which is able to
assist in designing novel NLO chormophores. The electrostatic first
hyperpolarizability (ˇ) and dipole moment (ꢀ) of the imidazole
chromophore have been calculated by using Gaussian 03 package
[12]. From Table 3, it is found that the imidazole chromophore show
larger ꢀˇ0 values, which is attributed to the positive contribution of
their conjugation. This chromophore exhibits larger non-linearity
and its ꢄmax is red-shifted when compared with unsubstituted imi-
dazole. Therefore, it is clear that the hyperpolarizability is a strong
function of the absorption maximum. Since even a small absorption
at the operating wavelength of optic devices can be detrimental, it
is important to make NLO chromophores as transparent as pos-
sible without compromising the molecule’s nonlinearity. Thermal
stability of the imidazole chromophore was estimated by thermo
gravimetric analysis (TGA).
3.2. Second harmonic generation (SHG) studies of dfppip
Second harmonic signal of 50 mV was obtained for dfppip 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 energy. The second
order non-linear efficiency will vary with the particle size of the
powder sample [25]. Higher efficiencies are achieved by optimizing
the phase matching [26]. On a molecular scale, the extent of charge
transfer (CT) across the NLO chromophore determines the level of
SHG output, the greater the CT and the larger the SHG output.
To determine the transference region and hence to know the
suitability of dfppip for microscopic non-linear optical applica-
tions, the UV–visible spectra have been recorded by using the
spectrometer in the range of 200–750 nm. The absorption spectrum
of dfppip was shown in the UV region between 270 and 320 nm.
The increased transparency in the visible region might enable the
microscopic NLO behavior with non-zero values [28,29,14]. All
the absorption bands are due to ␲ → ␲* transitions. The ˇ values
(Table 3) computed here might be correlated with UV–visible
spectroscopic data in order to understand the molecular-structure
3.3. Comparison of ꢀˇ0
The overall polarity of the synthesized imidazole derivative was
small when their dipole moments were aligned in a parallel fashion
(Fig. 4). When the electric field is removed, the parallel alignment of
the molecular dipole moments begins to deteriorate and eventually
the imidazole derivative loses its NLO activity. The ultimate goal in
the design of polar materials is to prepare compounds which have
their molecular dipole moments aligned in the same direction [27].

J. Jayabharathi et al. / Spectrochimica Acta Part A 85 (2012) 31–37
35
Table 3
can be well understood by analyzing their relative molecular
orbital properties. The electrostatic first hyperpolarizabilities (ˇ0)
and dipole moment (ꢀ) of the chromophores have been investi-
gated theoretically. These observed results can be explained by the
reduced planarity in such chromophores caused by the steric inter-
action between the two phenyl rings at C(23) and N(15) atoms.
Hence, the steric interaction must be reduced in order to obtain
larger ˇ0 values.
Electric dipole moment (D), polarizability (˛) and hyperpolarizability (ˇtotal) of
dfppip.
Parameter
(1)
Dipole moment (D)
ꢀx
ꢀy
ꢀz
−4.8837
3.8654
−1.1755
6.3383
ꢀtotal
The ˇ tensor [30] can be decomposed in a sum of dipo-
lar (2J=D1ˇ) and octupolar (J2=D3ˇ) tensorial components, and the
ratio of these two components strongly depends on their ‘r’
ratios. The zone for r > r2 and r < r1 corresponds to a molecule
of octupolar and dipolar, respectively. The critical values for r1
Polarizability (˛)
˛
xx
−195.8733
9.3770
−151.0243
−1.6154
1.2928
˛
xy
˛
yy
˛
xz
˛
√
√ √
√
√ √
yz
and r2 are (1 − 3)/
3
3 + 1) = −0.16 and
(
3 + 1)/ 3( 3 −
˛
−167.2760
zz
˛ × 10−24 (esu)
−25.4002
1) = 2.15, respectively. Complying with the Pythagorean theory
and the projection closure condition, the octupolar and dipolar
components of the ˇ tensor can be described as:
ꢁ ꢂ
Hyperpolarizability
ˇxxx
ˇxxy
ˇxyy
ˇyyy
ˇxxz
ˇxyz
ˇyyz
ˇxzz
139.4198
4.8411
−26.7741
25.1737
10.8520
−1.5723
5.6214
59.6695
5.8253
−1.0489
54.7580
347.0726
ꢀ
ꢀ
ꢀ
ꢀ
3
4
2 + (ˇyyy + ˇyxx)2]
(6)
(7)
2D
J=1ˇ
=
=
[(ˇxxx + ˇyyy
)
ꢁ ꢂ
ꢀ
ꢀ
ꢀ
ꢀ
1
2D
J=3ˇ
[(ˇxxx−3ˇxyy)
2 + (ˇyyy−ˇyxx)2]
4
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
ˇyzz
ˇzzz
2D
2D
The parameter ꢅ2D[ꢅ2D
=
J=3ˇ
/
J=1ˇ ] is convenient to
ꢀ
ꢀ ꢀ
ˇ × 10−33 (esu)
compare the relative magnitudes of the octupolar and dipolar com-
ponents of ˇ. The observed positive small ꢅ2D value (0.096) reveals
that the ˇiii component cannot be zero and these are dipolar com-
ponent. Since most of the practical applications for second order
NLO chromophores are based on their dipolar components, this
strategy is more appropriate for designing highly efficient NLO
chromophores.
␮␤ × 10−31 (esu)
and NLO relationship in view of a future optimization of the
microscopic NLO properties. The band at around 320 nm exhibits
a solvatochromic shift, characteristic of a large dipole moment
and frequently suggestive of a large hyperpolarizability. This
compound show red shift in absorption with increasing solvent
polarity, accompanied with the upward shifts non-zero values in
the ˇ-components.
3.5. Natural bond orbital (NBO) analysis
NBO analysis have been performed for dfppip at the
DFT/B3LYP/6-31++G (d,p) level in order to elucidate the intramolec-
ular, hybridization and delocalization of electron density within
the molecule. The importance of hyperconjugative interaction and
electron density transfer (EDT) from lone pair electrons to the
antibonding orbital has been analyzed and the results [31] were
tabulated in Tables 4 and 5. Several donor–acceptor interactions
3.4. Octupolar and dipolar components of dfppip
The imidazole derivative (dfppip) possess a more appropri-
ate ratio of off-diagonal versus diagonal ˇ tensorial component
(r =ˇxyy/ˇxxx) which reflects the inplane non-linearity anisotropy
and the largest ꢀˇ0 values. The difference of the ˇxyy/ˇxxx ratios
Table 4
Significant donor–acceptor interactions of dfppip and their second-order perturbation energies (kcal/mol).
Donor (i)
Type
ED/e
Acceptor (j)
Type
ED/e
E(2) a.u.
E(j) − E(i)
F(i,j)
C1–C2
C3–C4
C5–N14
C5–C14
C6–C9
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
1.7096
1.9720
1.9879
1.9879
1.9713
1.9735
1.9814
1.9878
1.9878
1.9824
1.6487
1.6487
1.9414
1.9414
1.9720
1.9720
1.9879
1.9713
1.9735
1.9878
1.9824
1.9760
1.9760
1.9760
1.9813
C5–N14
C5–N14
C1–C2
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
ꢁ*
0.0102
0.0131
0.0103
0.0305
0.0167
0.0101
0.0101
0.0296
0.0126
0.0249
0.0249
0.0167
0.0220
0.0352
0.0131
0.0296
0.0132
0.0296
0.0125
0.0125
0.0249
0.0132
0.0147
0.0119
0.0119
45.99
31.79
26.46
43.04
31.13
32.58
46.60
40.47
25.63
35.07
52.72
72.94
27.96
0.48
0.46
0.53
0.52
0.47
0.46
0.48
0.53
0.54
0.56
0.53
0.51
0.77
0.78
0.02
0.02
0.01
0.01
0.02
0.02
0.02
0.01
0.02
0.02
0.02
0.134
0.111
0.106
0.138
0.109
0.112
0.134
0.135
0.105
0.134
0.149
0.174
0.142
0.143
0.114
0.105
0.121
0.105
0.115
0.119
0.091
0.121
0.125
0.123
0.120
C3–C4
N16–C23
C12–N13
C12–N13
C7–C8
C10–C11
C6–C9
C7–C8
C10–C11
C12–N13
C12–N13
N16–C23
Lp N15
Lp N15
Lp F30
Lp F31
C3–C4
C3–C4
C5–N14
C6–C9
C6–C9
N16–C23
C27–C29
C24–C25
C1–C2
C7–C8
C1–C2
27.89
305.20
348.68
468.41
461.61
340.11
271.22
190.85
507.95
417.92
348.82
253.24
C7–C8
C7–C8
C10–C11
C10–C11
C6–C9
C19–C21
C20–C22
C26–C28
C26–C28
C12–N13
N16–C23
C17–C18
C17–C18
C24–C25
C27–C29

36
J. Jayabharathi et al. / Spectrochimica Acta Part A 85 (2012) 31–37
Table 5
Percentage of s and p-character on each natural atomic hybrid of the natural bond
orbital.
Bond (A–B)
C4–N14
ED/Energy (a.u.)
EDA
%
EDB%
s%
p%
0.6459
0.7635
0.6341
0.7732
0.6064
0.7951
0.6464
0.7630
0.6464
0.7630
0.6340
0.7733
0.7966
0.6045
0.8000
0.6000
0.7634
0.6459
0.5180
0.8554
0.5181
0.8553
41.71
–
40.21
–
36.78
–
41.79
–
41.79
–
40.19
–
63.46
–
64.00
–
58.26
–
59.79
–
63.22
–
58.21
–
58.21
–
59.81
–
36.54
–
36.00
–
30.20
35.64
30.99
36.98
27.33
33.10
30.26
35.54
28.98
32.99
31.08
37.00
35.02
26.67
31.86
29.07
35.36
32.91
23.41
32.53
23.60
32.53
69.80
64.36
69.01
63.02
72.67
66.9
C5–N14
C6–N15
C7–N13
C9–N16
C12–N13
N15–C17
N15–C23
N16–C23
C25–F31
C29–F30
69.74
64.46
71.02
67.01
68.92
63.00
64.98
73.33
68.14
70.93
64.64
67.09
76.59
67.47
76.40
67.47
Fig. 5. Bar diagram representing the charge distribution of dfppip using NLO and
NBO methods.
58.28
–
26.83
–
26.85
–
41.72
–
73.17
–
73.15
–
fluorine atoms (F30 and F31) are less electronegative in dfppip and
among the nitrogen atoms N15 is considered as more basic site
[32].
The percentage of s and p-character [33] in each NBO natural
atomic hybrid orbital are displayed in Table 5. For all the carbon,
nitrogen and fluorine atoms, around 60% of p-character and 30%
s-character have been observed.
are observed for the imidazole derivative (dfppip) and among
the strongly occupied NBOs, the most important delocalization
sites are in the ꢁ system and in the lone pairs (n) of the oxygen,
fluorine and nitrogen atoms. The
system shows some contri-
3.6. Molecular electrostatic potential map (MEP) and electronic
properties
bution to the delocalization, and the important contributions to
the delocalization corresponds to the donor-acceptor interac-
tions are C3–C4 → C1–C2, C3–C4 → C7–C8, C5–N14 → C1–C2,
The molecular electrostatic potential surface MEP (Fig. 6) which
is a plot of electrostatic potential mapped onto the iso-electron
density surface simultaneously displays molecular shape, size and
electrostatic potential values of the imidazole derivative (dfppip).
MEP surface diagram 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 dfppip 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 region in the MEP surfaces corresponds to
C6–C9 → C7–C8,
C7–C8 → C10–C11,
C12–N13 → C10–C11,
N16–C23 → C6–C9, C17–C18 → C19–C21, C17–C18 → C20–C22,
C24–C25 → C26–C28 and C27–C29 → C26–C28.
The charge distribution of dfppip was calculated from the atomic
charges by NLO and NBO analysis (Fig. 5). These two methods
predict the same trend i.e., among the nitrogen atoms N15 and
N16, N15 is considered as more basic site [29]. The charge distri-
bution shows that the more negative charge is concentrated on
N15 atom whereas the partial positive charge resides at hydro-
gens. When compared to nitrogen atoms (N13, N14, N15 and N16),
Fig. 6. HOMO–LUMO and MEP surface diagram of dfppip.

J. Jayabharathi et al. / Spectrochimica Acta Part A 85 (2012) 31–37
37
a potential halfway between the two extremes red and dark blue
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We have reported a new, simple and an efficient route to the
synthesis of biologically active heterocyclic substituted imidazole.
The presence of ˛ twist in this imidazole drops the fluorescence
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One of the authors Dr. J. Jayabharathi, Associate professor,
Department of Chemistry, Annamalai University is thankful to
Department of Science and Technology [No. SR/S1/IC-07/2007] and
University Grants commission (F. No. 36-21/2008 (SR)) for provid-
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