Photophysical studies of fused phenanthrimidazole derivatives as versatile π-conjugated systems for potential NLO applications

Article abstract of DOI:10.1016/j.saa.2012.01.066

Two new heterocyclic imidazole derivatives consists of π-conjugated system attached to a phenanthrimidazole moiety have been synthesized in moderate yield by the condensation of 1,10-phenanthroline-5,6-dione with substituted aromatic aldehydes and 4-metho

Full text of DOI:10.1016/j.saa.2012.01.066

Spectrochimica Acta Part A 92 (2012) 113–121
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical studies of fused phenanthrimidazole derivatives as versatile
␲-conjugated systems for potential NLO applications
Jayaraman Jayabharathi∗, Venugopal Thanikachalam, Marimuthu Venkatesh Perumal
Department of Chemistry, Annamalai University, Annamalainagar, Tamilnadu 608002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new heterocyclic imidazole derivatives consists of ␲-conjugated system attached to
a
Received 16 November 2011
Received in revised form
22 December 2011
phenanthrimidazole moiety have been synthesized in moderate yield by the condensation of 1,10-
phenanthroline-5,6-dione with substituted aromatic aldehydes and 4-methoxyaniline in the presence
of ammonium acetate in ethanol medium. The photophysical properties of these imidazole derivatives
were studied in several solvents. These derivatives were evaluated concerning their solvatochromic prop-
erties and molecular optical nonlinearities. Their electric dipole moment (ꢀ) and hyperpolarizability (ˇ)
have been calculated theoretically and the results indicate that the extension of the ␲-framework of the
ligands has an effect on the NLO properties of these imidazole derivatives. The non-zero tensor compo-
nents of these imidazole derivatives reveal that they possess potent non-linear optical (NLO) behavior.
The energies of the HOMO and LUMO levels and the molecular electrostatic potential (MEP) energy sur-
face studies have exploited the existence of intramolecular charge transfer (ICT) within the molecule.
Accepted 16 January 2012
Keywords:
ORTEP
Imidazole
NLO
HOMO–LUMO
MEP
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
density on the ring carbon atoms. These imidazole derivatives
exhibit high thermal stabilities making them interesting for several
New material properties can be achieved, when new conjugated
systems are composed by different heterocyclic nuclei, which allow
the fine tuning of important photophysical properties. As a result
of the optical and conductive properties, conjugated materials
containing thiophene, imidazole and phenanthroline heterocycles
have found many applications [1].
Aryl-imidazo-phenanthrolines play important role in materials
science and medicinal chemistry due to their optoelectronic prop-
erties [2–4]. They are used as ligands for the synthesis of metal
complexes of ruthenium(II), copper(II), cobalt(II), nickel(II), man-
ganese(II), iridium(III) and several lanthanides for nonlinear optical
(NLO) applications. They are important building blocks for the
synthesis of proton, anion and cation sensors. They have diverse
biological applications such as probes of DNA structure and new
therapeutic agents due to their capacity to bind or interact with
DNA [4].
applications in materials chemistry [5,6]. Hence there is consider-
able interest in the synthesis of new materials with large optical
nonlinearities by virtue of their potential use in device applications
related to telecommunications, optical computing, optical stor-
age, and optical information processing [7–10]. Herein we focussed
the synthesis, photophysical and theoretical studies of some novel
arylimidazophenanthroline derivatives (1 and 2) and shown as
potential candidates for NLO materials by virtue of their high har-
monic generation.
2. Experimental
2.1. Materials and methods
1,10-Phenanthroline-5,6-dione has been synthesized and puri-
fied according to the reported literature procedure [11]. Benzalde-
hyde, 4-fluorobenzaldehyde, 4-methoxyaniline and all the other
reagents have been purchased from S.D. fine chemicals and used
without further purification for the synthesis of these imidazole
derivatives reported here (Scheme 1) [12–20].
NMR spectra have been recorded on a Bruker 400 MHz NMR
instrument (Figs. 1 and 2). UV–vis absorption and fluorescence
spectra have been recorded on Perkin Elmer spectrophotometer
Lambda35 and Perkin Elmer LS55 spectrofluorimeter, respectively.
Fluorescence spectra were corrected from the monochromator
Our approach to design new ␲-conjugated systems for several
potential optical applications is based on the use of six membered
aromatics and imidazoles in the conjugation pathway, combined
with electron deficient heterocycles such as phenanthroline. These
nuclei act as an acceptor group due to the deficiency of electron
∗
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.01.066

114
J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
Scheme 1. Synthesis of imidazole derivatives (1) and (2).
Fig. 1. 1H NMR spectrum of imidazole derivative (1).
Fig. 2. 13C NMR spectrum of imidazole derivative (1).

J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
115
Fig. 3. Absorption spectrum of imidazole derivatives (1) and (2) in various solvents.
wavelength dependence and the photomultiplier sensibility. Flu-
orescence quantum yields (␾) have been determined by means of
the corrected fluorescence spectra of a dilute solution of imidazole
derivatives in dichloromethane, using coumarin 47 in ethanol
as a reference and by taking into account the solvent refractive
index. Mass spectrum was recorded using Agilent 1100 Mass
spectrometer.
respectively. In apolar solvents, the main absorption band is cen-
tered around 270 nm with a high molar absorption coefficient,
whereas the fluorescence spectrum is centered around 370 nm with
a fluorescence quantum yield around 0.38.
The observed red shift absorption maxima (ꢁmax) suggest an
increase of molecular hyperpolarizability, according to the the-
oretical NLO studies. The absorption maxima (ꢁmax) have been
compared with the ꢂ* values for each solvent, determined by
Kamlet and Taft [22]. The observed results show that these
imidazole derivatives exhibited positive solvatochromism with
respect to their charge transfer (CT) absorption band. The position
of the absorption maximum shifted to longer wavelengths as the
polarity of the solvent increased due to a greater stabilization of
the excited state relative to the ground state with an increase of
solvent polarity.
2.2. Computational details
Quantum mechanical calculations have been used to carry out
the optimized geometry, NBO, NLO, MEP and HOMO–LUMO anal-
ysis with Guassian-03 program using the Becke3–Lee–Yang–Parr
(B3LYP) functional supplemented with the standard 6-31G(d,p)
basis set [21].
3. Results and discussion
3.2. Twisting structure and XRD analysis
3.1. Photophysical studies of the imidazole derivatives 1 and 2
The fluorescence spectrum of the imidazole derivative 2 is
shifted to higher energies with respect to compound 1 in com-
mon solvents. These spectral shifts were attributed to the higher
UV–vis absorption and fluorescence spectra (Table 1) of these
imidazole derivatives 1 and 2 are shown in Figs. 3 and 4,
Fig. 4. Emission spectrum of imidazole derivatives (1) and (2) in various solvents.

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J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
Table 1
Photophysical data of the imidazole derivatives 1 and 2 in different solvents.
Solvent
Imidazole derivative (1)
Imidazole derivative (2)
ꢁab
(
0.1 nm)
ꢁfl
(
0.4 nm)
ꢃꢄss (cm−1
10,310
)
ꢁab
(
0.1 nm)
ꢁfl
(
0.4 nm)
ꢃꢄss (cm−1
)
Hexane
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
Dichloromethane
Butanol
272
276
274
276
271
274
269
274
275
273
269
273
378
396
386
390
389
390
394
398
404
409
410
396
273
270
281
277
275
275
275
274
275
273
276
274
377
389
388
390
387
388
386
398
375
393
377
384
10,105
11,330
9814
10,460
10,524
10,590
10,457
11,371
9697
10,979
10,590
10,591
11,193
10,855
11,794
11,371
11,611
12,180
12,784
11,378
Ethanol
Methanol
Acetonitrile
DMSO
11,185
9707
10,455
1-Propanol
Table 2
Selected bond lengths, bond angles and torsional angles for the imidazole derivative (2).
a
Bond angle (◦)
Bond connectivity
Torsional angle (◦)
˚
Bond connectivity
Bond length (A)
Bond connectivity
C3 C6
C6 C9
1.4409 (1.4321)
1.3663 (1.3770)
1.3968 (1.3840)
1.3740 (1.3789)
1.3855(1.3752)
1.3053 (1.3168)
1.4284 (1.4402)
1.3661 (–)
C3 C6 C9
C3 C6 N15
C8 C9 N16
121.8614 (123.04)
132.7598 (131.74)
127.0967(127.30)
106.7446 (104.52)
110.9912(112.63)
126.8748 (128.80)
122.3559 (124.59)
120.0807 (119.73)
115.7374 (–)
C3 C6 C9 C8
C2 C3 C6 N15
−1.8322 (−3.5)
1.2692 (0.8)
C6 N15
C9 N16
N15 C23
N16 C23
N15 C17
C22 O31
O31 C32
C23 C24
C29 F30
C3 C6 N15 C23
C3 C6 N15 C17
N15 C6 C9 N16
C6 N15 C23 N16
C6 N15 C17 C18
C6 N15 C23 C24
N15 C23 C24 C25
N16 C23 C24 C25
N16 C23 C24 C26
C25 C27 C29 F30
C6 N15 C17 C18
N15 C17 C19 C21
C20 C22 O31 C32
C9 C6 N15 C17
−178.4425 (−175.82)
−1.4544 (−2.3)
−0.5353 (−0.78)
−0.1705 (−0.73)
81.2192 (82.65)
−177.7521 (−179.76)
37.0943 (56.56)
−145.2018 (−122.35)
32.2310 (55.54)
179.7448 (–)
C9 N16 C23
N15 C23 N16
C6 N15 C17
N16 C23 C24
N15 C17 C18
C20 C22 O31
C22 O31 C32
C23 C24 C25
C28 C29 F30
1.4315 (–)
1.4730 (1.4782)
1.3733 (–)
121.9927 (–)
123.8171 (121.16)
118.7298 (–)
81.2192 (82.65)
179.9334 (177.39)
179.4798 (–)
177.4109 (174.37)
R[F2 > 2ꢅ(F2)] = 0.042; wR(F2) = 0.117; R indices (all data) = 0.0458.
a
Values given in the brackets are corresponding to XRD values of 3.
inductive electron-acceptor character of the fluorine atom located
at C(29) carbon atom. These results suggest an important geomet-
rical rearrangement in the S1 excited state and this finding is in
good agreement with the literature report [23]. To understand the
geometry of the molecule both in ground and excited states, X-ray
data of its analog compound 3 (Fig. 5) reported by Jayabharathi et al.
[24] is taken for discussion. The crystal study of compounds 1 and
2 is currently under progress. X-ray data of 3 (Table 2) reveal that
the fused ring system is essentially planar. The obtained dihedral
angles of 77.41◦ and 56.26◦ with the phenyl ring attached to N(1)
nitrogen and phenyl ring attached to C(2) carbon the imidazole
ring respectively. The dihedral angle between the two phenyl rings
is 65.50◦. The molecular modeling of 1 and 2 has been examined by
density functional theory calculation [25]. Though XRD data is in
good agreement with the theoretical results (Table 2), the theoret-
ical bond lengths, bond angles and dihedral angles differs slightly
from the XRD values. Since 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.
The key twist (˛) is used to indicate the twist of imidazole
ring from the aldehydic phenyl ring at C(2) (Fig. 6). The observed
emission wavelength reveals that existence of ˛ twist drops the
fluorescence quantum yield. Such a clear correlation indicates the
importance of non-coplanarity between the imidazole and the
phenyl ring at C(2). When the two adjacent aromatic species are in
a coplanar geometry, the p-orbitals from the C C bond connecting
the two species will have maximal overlapping. As the result, the
bond is no longer a pure single bond, as evident from the X-ray data
of 3. When the two rings are deviated from each other, the p-orbital
overlapping will be reduced. The partial conjugation will lead to less
Fig. 5. ORTEP diagram of 1,2-Diphenyl-1H-imidazo[4,5-f][1,10]phenanthroline (1).

J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
117
3.3. Taft and Catalan solvatochromism
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):
(XYZ) = (XYZ)0 + CaA + CbB + CcC + · · ·
(1)
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 aromatic compounds are polarity, H-bond
donor capacity and electron donor ability. Different scales for such
parameters can be found in the literature, Taft et al. [22] propose the
ꢂ*, ˛ and ˇ scales, whereas more recently Catalan et al. [26] suggest
the SPPN, SA and SB scales to describe the polarity/polarizability, the
acidity and basicity of the solvents as shown in the Eqs. (2) and (3).
Fig. 6. The key ˛ twist of imidazole ring at C(2).
y = y0 + a˛˛ + bˇˇ + cꢂ · ꢂ∗ (Kamlet–Taft)
y = y0 + aSASA + bSBSB + cSPPSPP (Catalan)
(2)
(3)
∗
The correlation (Fig. 7a and b) of the absorption as well as flu-
orescence wavenumbers obtained by the multi-component linear
regression employing the Taft and Catalan solvent parameters with
the experimental values given in Table 3. The polarity/polarizability
of the solvent coefficient [cꢂ* or cSNPP] value correlating the sol-
vatochromic shifts with the solvent polarity. The coefficient
controlling the H-donor capacity or acidity of the solvent [c˛
or cSA], is the lowest coefficient, therefore, the solvent acidity
does not play an important role in absorption and fluorescence
displacements. The coefficient representing the electron releasing
ability or basicity of the solvent [cˇ or cSB] has a negative value,
suggesting that the absorption and fluorescence bands shift to
lower energies with the increasing electron-donating ability of the
rigid structure. Therefore, radiationless twist motion will deacti-
vate the emitting state, leading to the low quantum yields. Thus, it
is suggested that fluorophores with the substitution at C(2) with a
minimum loss of fluorescence property.
The distortion of the geometry in the excited state implies a
decrease in the resonance energy so that the fluorescence band is
bathochromically shifted to a higher extent than the absorption
band. Moreover, the loss of planarity in the excited state of the
imidazole derivative could explain the lower fluorescence quantum
yield in apolar solvents owing to an increase in the non-radiative
processes.
Fig. 7. (a) Correlation between the experimental absorption wavenumber with the predicted values obtained by a multicomponent linear regression using the ꢂ*, ˛ and ˇ-
scale (Taft) solvent parameters for (1) and (2). (b) Correlation between the experimental fluorescence wavenumber with the predicted values obtained by a multicomponent
linear regression using the ꢂ*, ˛ and ˇ-scale (Taft) solvent parameters for (1) and (2).

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J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
Table 4
Electric dipole moment (D), polarizability (˛) and hyperpolarizability (ˇtotal) of 1
and 2.
Parameter
1
2
Dipole moment (D)
ꢀx
ꢀy
ꢀz
ꢀtotal
−7.4089
−0.4545
−0.9658
7.4854
−2.8610
1.4449
−1.2847
3.4530
Polarizability (˛)
˛
−173.4208
17.2806
−190.6077
−13.8204
−156.8421
4.9408
xx
˛
xy
˛
−162.7023
yy
˛
8.9074
xz
˛
˛
8.5832
−10.6596
−186.5096
−26.3776
yz
−175.1103
zz
˛ × 10−24 (esu)
−25.2549
Fig. 8. Resonance structures (I and II) of the imidazole chromophore.
Hyperpolarizability
ˇxxx
ˇxxy
ˇxyy
ˇyyy
ˇxxz
ˇxyz
ˇyyz
ˇxzz
−145.3533
71.8808
−115.0533
−12.2879
17.0836
37.2364
−9.4232
8.0924
−5.3027
−7.0947
−11.5510
6.5528
solvent. This effect can be interpreted in terms of the stabilization
of the resonance structures of the chromophore (Fig. 8). Resonance
structure “II” has the positive charge located at the nitrogen atom
and it will be stabilized in basic solvents because this resonance
structure is predominant in the S1 state and the stabilization of
the S1 state with the solvent basicity would be more important
than that of the S0 state. Consequently, the energy gap between
the S1 and S0 states decreases and the absorption and fluorescence
wavelengths shift to longer wavelength with increasing solvent
basicity.
−56.6777
−42.9243
−54.4800
−26.6085
11.1701
−39.8778
−8.5337
6.3180
ˇyzz
ˇzzz
ˇ × 10−31 (esu)
21.2155
158.8065
91.7753
316.9001
ꢀˇo × 10−31 (esu)
3.5. Comparison of ꢀˇo
3.4. Second harmonic generation (SHG) studies
Theoretical investigation plays an important role in under-
standing the structure-property relationship, which is able to
assist in designing novel NLO chromophores. The electrostatic first
hyperpolarizability (ˇ) and dipole moment (ꢀ) of the imidazole
chromophores have been calculated by using Gaussian 03 package
[21]. From Table 4, it is found that the imidazole chromophore show
larger ꢀˇo values, which is attributed to the positive contribution of
their conjugation. This chromophore exhibits larger non-linearity
and its ꢁmax is red-shifted when compared with unsubstituted
Second harmonic signal of 40 mV and 42 mV were obtained for
the imidazole derivatives 1 and 2 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 [27,28]. 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.
Table 3
Adjusted coefficients ((ꢄx)0, Ca, Cb and Cc) and correlation coefficients (r) for the multilinear regression analysis of the absorption ꢄab and fluorescence ꢄfl wavenumbers and
Stokes shift (ꢃꢄss) of the imidazole derivative 1 and 2 with the solvent polarity/polarizability, and the acid and base capacity using the Taft (ꢂ*, ˛ and ˇ) and the Catalan
(SPPN, SA and SB) scales.
Imidazole derivative (1)
(ꢄx)
(ꢄx)0 (cm−1
)
(ꢂ*)
c˛
cˇ
ꢁab
ꢁfl
(3.64 0.012) × 104
(2.59 0.025) × 104
(1.05 0.025) × 104
(0.63 2.546) × 103
−(3.59 5.035) × 103
(4.23 5.089) × 103
−(1.24 8.509) × 103
(6.24 16.828) × 103
−(7.49 17.010) × 103
(0.76 5.300) × 103
−(4.08 13.518) × 103
(4.84 13.663) × 103
ꢃꢄss = ꢄab − ꢄfl
Imidazole derivative (1)
(ꢄx)
(ꢄx)0 (cm−1
)
cSNPP
cSA
cSB
ꢁ
abꢁfl
(3.65 0.012) × 104
(2.58 0.021) × 104
(1.07 0.024) × 104
−(2.20 5.451) × 103
−(12.62 9.595) × 103
(10.413 10.780) × 103
(9.58 22.602) × 103
(46.34 39.787) × 103
−(36.76 44.701) × 103
−(9.70 23.211) × 103
−(46.58 40.858) × 103
(36.88 45.905) × 103
ꢃꢄss = ꢄab − ꢄfl
Imidazole derivative (2)
(ꢄx)
(ꢄx)0 (cm−1
)
(ꢂ*)
c˛
cˇ
ꢁab
ꢁfl
(3.62 0.051) × 104
(2.59 0.068) × 104
(1.14 0.107) × 104
(17.67 10.336) × 103
−(7.00 13.84) × 103
(18.41 21.596) × 103
−(49.58 34.544) × 103
(34.73 46.261) × 103
−(73.14 72.177) × 103
(34.69 27.748) × 103
−(32.245 37.160) × 103
(60.627 57.977) × 103
ꢃꢄss = ꢄab − ꢄfl
Imidazole derivative (2)
(ꢄx)
(ꢄx)0 (cm−1
)
cSNPP
cSA
cSB
ꢁab
ꢁfl
(3.71 0.057) × 104
(2.67 0.065) × 104
(1.12 0.109) × 104
−(18.20 55.893) × 103
−(28.94 29.371) × 103
−(12.73 49.131) × 103
(69.37 107.306) × 103
(101.49 121.787) × 103
(55.367 203.720) × 103
−(67.80 110.257) × 103
−(89.16 125.067) × 103
−(62.884 209.206) × 103
ꢃꢄss = ꢄab − ꢄfl

J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
119
Table 5
Significant donor–acceptor interactions of dfppip and their second-order perturbation energies (kcal/mol). Donor type (ꢂ); acceptor type (ꢂ*).
Imidazole derivative (1)
Imidazole derivative (2)
Donor (i)
ED/e
Acceptor (j)
ED/e
E(2) a.u.
E(j) − E(i)
F(i, j)
Donor (i)
ED/e
Acceptor (j)
ED/e
E(2) a.u.
E(j) − E(i)
F(i, j)
C1 C2
C3 C4
1.9603
1.9709
1.9878
1.9810
1.9740
1.9768
1.9794
1.9719
1.9719
1.9796
1.9796
1.9803
1.9803
1.6572
1.6572
1.9738
1.9709
1.9709
1.9878
1.9706
1.9722
1.9877
1.9829
1.9829
1.9740
1.9794
C5 N14
C1 C2
C3 C4
0.0101
0.0142
0.0314
0.0100
0.0119
0.0258
0.0252
0.0143
0.0159
0.0235
0.0159
0.0235
0.0143
0.0260
0.0129
0.0258
0.0142
0.0306
0.0142
0.0306
0.0134
0.0134
0.0260
0.0235
0.0119
0.0119
45.28
38.23
40.14
46.40
43.44
46.16
53.32
41.13
39.71
41.61
44.76
45.59
39.30
52.70
72.18
36.83
293.38
441.17
384.84
371.65
290.89
240.25
163.28
77.41
220.58
302.90
0.49
0.48
0.53
0.49
0.50
0.47
0.47
0.49
0.48
0.47
0.48
0.47
0.48
0.53
0.51
0.64
0.02
0.01
0.01
0.02
0.02
0.02
0.02
0.03
0.03
0.02
0.134
0.125
0.136
0.134
0.132
0.133
0.143
0.127
0.124
0.126
0.131
0.132
0.124
0.150
0.171
0.147
0.115
0.107
0.120
0.109
0.116
0.119
0.087
0.075
0.121
0.121
C1 C2
C3 C4
C5 N14
C7 C8
1.9789
1.9698
1.9869
1.9705
1.9797
1.9869
1.9820
1.9737
1.9745
1.9745
1.9789
1.9737
1.9742
1.9742
1.9810
1.9991
1.9991
1.9999
1.9997
1.9698
1.9698
1.9698
1.9869
1.9820
1.9737
1.9810
C5 N14
C1 C2
C3 C4
C10 C11
C12 N13
C7 C8
0.0115
0.0151
0.0326
0.0145
0.0112
0.0323
0.0263
0.0134
0.0252
0.0265
0.0252
0.0143
0.0230
0.0238
0.0230
0.0263
0.0119
0.0238
0.0265
0.0151
0.0263
0.0323
0.0145
0.0263
0.0134
0.0143
46.62
39.57
43.59
36.53
47.35
41.03
36.05
44.63
34.86
44.83
51.58
45.08
36.71
50.96
46.49
51.21
78.52
20.86
38.43
426.59
377.20
311.25
316.55
129.38
624.64
510.98
0.50
0.49
0.54
0.50
0.50
0.55
0.59
0.51
0.49
0.49
0.49
0.49
0.48
0.48
0.50
0.55
0.53
0.78
0.64
0.01
0.01
0.02
0.02
0.03
0.01
0.01
0.137
0.128
0.142
0.125
0.137
0.139
0.138
0.135
0.118
0.134
0.143
0.134
0.120
0.140
0.137
0.150
0.182
0.124
0.150
0.118
0.108
0.109
0.123
0.091
0.124
0.126
C5 N14
C10 C11
C17 C19
C18 C20
C21 C22
C24 C26
C24 C26
C25 C27
C25 C27
C28 C29
C28 C29
LpN15
LpN15
LpO30
C3 C4
C3 C4
C5 N14
C6 N9
C7 C8
C12 N13
N16 C23
N16 C23
C17 C19
C21 C22
C12 N13
C18 C20
C21 C22
C17 C19
C25 C27
C28 C29
C24 C26
C28 C29
C24 C26
C25 C27
C6 C9
N16 C23
C21 C22
C1 C2
C7 C8
C1 C2
C10 C11
C12 N13
N16 C23
C17 C19
C18 C20
C18 C20
C21 C22
C24 C25
C26 C28
C26 C28
C27 C29
LpN15
LpN15
LpF30
LpO31
C3 C4
C3 C4
C3 C4
C12 N13
N16 C23
C17 C19
C27 C29
C6 C9
C18 C20
C17 C19
C21 C22
C17 C19
C26 C28
C24 C25
C27 C29
C24 C25
C6 C9
N16 C23
C27 C29
C21 C22
C1 C2
C6 C9
C7 C8
C10 C11
C6 C9
C18 C20
C26 C28
C7 C8
C10 C11
C10 C11
C6 C9
C24 C26
C18 C20
C18 C20
imidazole. Therefore, it is clear that the hyperpolarizability is a
strong function of the absorption maximum. The ˇ values (Table 4)
computed here might be correlated with UV–vis spectroscopic data
in view of a future optimization of the microscopic NLO proper-
ties. The band centered around 320 nm exhibits a solvatochromic
shift, characteristic of a large dipole moment and frequently sug-
gestive 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 [29–32].
ˇxyy/ˇxxx ratios can be well understood by analyzing their relative
molecular orbital properties. Hence, the steric interaction must be
reduced in order to obtain larger ˇo values.
The ˇ-tensor [33] 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 and
√
√
√
√
√ √
r2 are (1 − 3)/ 3( 3 + 1) = −0.16 and ( 3 + 1)/ 3( 3 − 1) = 2.15,
respectively. Complying with the Pythagorean theory and the pro-
jection closure condition, the octupolar and dipolar components of
the ˇ tensor can be described as:
3.6. Octupolar and dipolar components
These imidazole derivatives possess a more appropriate ratio of
off-diagonal versus diagonal ˇ tensorial component (r = ˇxyy/ˇxxx
)
ꢁ ꢂ
ꢀ
ꢀ
ꢀ
ꢀ
3
4
J=1ˇ
=
[(ˇxxx + ˇxyy
)
2 + [(ˇyyy + ˇyxx)2]
(4)
[r = 0.39 (1) and −0.15 (2)] which reflects the inplane non-linearity
2D
anisotropy and the largest ꢀˇo values. The difference of the
Table 6
Percentage of s and p-character on each natural atomic hybrid of the Natural Bond Orbital for the imidazole derivatives 1 and 2.
Imidazole derivative (1) Imidazole derivative (2)
Bond (A B)
C6 C9
ED/energy (a.u.)
EDA
%
EDB%
s%
p%
Bond (A B)
C6 C9
ED/energy (a.u.)
EDA
%
EDB%
s%
p%
0.7146
0.6996
0.6073
0.7945
0.6476
0.7619
0.7945
0.6072
0.8017
0.5977
0.7626
0.6469
0.7062
0.7080
0.5732
0.8194
0.8310
0.5563
51.06
–
36.88
–
41.94
–
63.13
–
64.27
–
58.15
–
49.87
–
32.85
–
48.94
–
63.12
–
58.06
–
36.87
–
35.73
–
41.85
–
50.13
–
67.15
–
33.44
32.84
27.62
32.70
29.34
32.74
35.70
27.44
31.55
28.33
34.40
32.63
39.01
30.60
24.93
32.01
29.25
20.37
66.56
67.16
72.38
67.30
70.66
67.26
64.30
72.56
68.45
71.67
65.50
67.37
60.99
69.40
75.07
67.99
70.75
79.63
0.7125
0.7016
0.6082
0.7938
0.6465
0.7629
0.7948
0.6069
0.7987
0.6017
0.7649
0.6442
0.7064
0.7078
0.5169
0.8560
0.5714
0.8207
0.8331
0.5532
50.77
–
36.99
–
41.80
–
63.17
–
63.80
–
58.51
–
49.90
–
26.72
–
49.23
–
63.01
–
58.20
–
36.83
–
36.20
–
41.49
–
50.10
–
73.28
–
34.92
34.68
27.14
32.56
28.74
33.60
34.56
26.75
32.86
29.46
37.35
32.74
37.77
31.01
21.62
29.37
24.93
34.38
30.11
19.99
65.08
65.32
72.86
67.44
71.26
66.40
65.44
73.25
67.14
70.54
62.65
67.26
62.23
68.99
78.38
70.63
75.07
65.62
69.89
80.01
C6 N15
C6 N15
C9 N16
C9 N16
N15 C17
N15 C23
N16 C23
C23 C24
C22 O30
O30 C31
N15 C17
N15 C23
N16 C23
C23 C24
C29 F30
C22 O31
O31 C32
69.06
–
30.94
–
32.65
–
69.40
–
67.35
–
30.60
–

120
J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
Fig. 9. Bar diagram representing the charge distribution of imidazole derivatives (1) and (2) using NLO and NBO methods.
ꢁ ꢂ
ꢀ
ꢀ
ꢀ
1
4
3.7. Natural Bond Orbital (NBO) analysis
2D
ꢀ
J=3ˇ
=
[(ˇxxx
−
3ˇxyy
)
2 + [(ˇyyy − ˇyxx)2]
(5)
NBO analysis [34] have been performed for these imidazole
derivatives 1 and 2 at the DFT/B3LYP/6-31++G (d,p) level in order to
elucidate the intramolecular, charge transfer within the molecule
and the results have been tabulated in Tables 5 and 6. Several
donor–acceptor interactions have been observed in the imidazole
derivatives (1 and 2) and among the strongly occupied NBOs,
the most important delocalization sites are in the ꢂ system and
in the lone pairs (n) of the oxygen, fluorine and nitrogen atoms.
The ꢅ system shows some contribution to the delocalization, and
the important contributions to the delocalization corresponds to
2D
2D
The parameter ꢆ2D [ꢆ2D = ||J=3ˇ||/||J=1ˇ||] is convenient to
compare the relative magnitudes of the octupolar and dipolar com-
ponents of ˇ. The observed positive small ꢆ2D value for compound
1 (5.938) and compound 2 (1.540) reveals that the ˇiii component
cannot be zero and these are dipolar components. Since most of
the practical applications for second order NLO chromophores are
based on their dipolar components and this strategy is more appro-
priate for designing highly efficient NLO chromophores.
Fig. 10. (a) MEP surface diagram of imidazole derivatives (1) and (2). (b) HOMO–LUMO molecular orbitals of imidazole derivatives (1) and (2).

J. Jayabharathi et al. / Spectrochimica Acta Part A 92 (2012) 113–121
121
efficient NLO chromophores. The observed positive small ꢆ2D value
for compound 1 (5.938) and compound 2 (1.540) show that the ˇiii
component cannot be zero and these are dipolar components.
the donor–acceptor interactions for the imidazole derivatives 1
and 2 are LpN15 → N16–C23, C3–C4 → C1–C2, C3–C4 → C7–C8,
C5–N14 → C1–C2,
C6–C9 → C7–C8,
C7–C8 → C10–C11,
C12–N13 → C10–C11, N16–C23 → C6–C9, C17–C19 → C18–C20,
C21–C22 → C18–C20 (for compound 1) and C3–C4 → C1–C2,
Acknowledgments
C3–C4 → C6–C9,
C3–C4 → C7–C8,
C12–N13 → C10–C11,
N16–C23 → C6–C9, C17–C19 → C18–C20, C27–C29 → C26–C28
(for compound 2).
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-
ing funds to this research study.
The charge distribution of the imidazole derivatives 1 and 2 has
been calculated from the atomic charges by NBO and NLO analy-
ses (Fig. 9). These methods reveal that among the nitrogen atoms
N15 and N16, oxygen and fluorine atom, N15 is considered as more
basic site [31]. When compared to nitrogen atoms (N13, N14, N15
and N16), fluorine and oxygen atoms are less electronegative [35].
The charge distribution shows that the more negative charge is
concentrated on N15 atom in both derivatives whereas the par-
tial positive charge resides at hydrogen atoms. The percentage of
s and p-character [36] in each NBO natural atomic hybrid orbital
are displayed in Table 6 for both the imidazole derivatives 1 and 2.
For all the carbon, nitrogen fluorine and oxygen atoms, around 37%
s-character and 63% of p-character have been observed.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2012.01.066.
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The observed emission wavelength reveals that the existence
of ˛ twist drops the fluorescence quantum yield. Such a clear cor-
relation indicates the importance of non-coplanarity between the
imidazole and the aryl ring at C(2). The electron releasing ability or
basicity of the solvent [cˇ or cSB] has a negative value, suggesting
that the absorption and fluorescence bands shift to lower ener-
gies with the increasing electron donating ability of the solvent.
The band centered around 320 nm exhibits a solvatochromic shift,
characteristic of a large dipole moment and frequently suggestive
of a large hyperpolarizability. The calculated dipolar and octupo-
lar components supported that these imidazole derivatives are