Optical properties of organic nonlinear optical crystal - A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.saa.2011.09.067

The optical properties of the synthesized imidazole derivative, 1-(4-methoxyphenyl)-4,5-diphenyl-2-styryl-1H-imidazole, has been studied both experimentally and theoretically. Fluorescence enhancement have been found in the presence of transition metal io

Full text of DOI:10.1016/j.saa.2011.09.067

Spectrochimica Acta Part A 86 (2012) 69–75
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Optical properties of organic nonlinear optical crystal—A combined experimental
and theoretical study
Jayaraman Jayabharathi∗, Venugopal Thanikachalam, Kumar Brindha Devi,
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:
The optical properties of the synthesized imidazole derivative, 1-(4-methoxyphenyl)-4,5-diphenyl-2-
styryl-1H-imidazole, has been studied both experimentally and theoretically. Fluorescence enhancement
have been found in the presence of transition metal ions and this may result from the suppression of radi-
ationless transitions from the n-ꢀ* state in the chemosensors. Quantum chemical calculations of heat of
formation, optimized geometry, NLO, HOMO–LUMO, MEP and NBO analysis of 1-(4-methoxyphenyl)-
4,5-diphenyl-2-styryl-1H-imidazole (mpdsi) have been carried out by using density functional theory
(DFT/B3LYP) method with 6-31G(d,p) as basis set. This chromophore possess more appropriate ratio of
off-diagonal versus diagonal ˇ tensorial component (r = ˇxyy/ˇxxx = −0.002) which reflects the inplane
non-linearity anisotropy. Since they have largest ꢁˇ0 value, the reported imidazole can be used as
potential NLO material. The solvent effect on the absorption and fluorescence has been analyzed simul-
taneously.
Received 26 August 2011
Received in revised form
23 September 2011
Accepted 30 September 2011
Keywords:
Emission kinetics
Chemosensor
NLO
DFT
HOMO–LUMO
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
for potential NLO applications. In this paper we have focused the
synthesized imidazole derivative has non-linear optical (NLO)
Research on heterocyclic imidazole derivatives is wide because
of their optical properties [1]. These compounds play an important
role in chemistry as mediators for synthetic reactions, primarily for
preparing functionalized materials [2]. Imidazole nucleus present
in the human organisms namely the amino acid histidine, Vitamin-
B12, a component of DNA base structure, purines, histamine and
biotin and present in many natural and synthetic drug molecules,
i.e., azomycin, cimetidine and metronidazole. They have significant
analytical applications due to their fluorescence and chemilumi-
nescence properties [3].
behaviour. This chromophore possesses more appropriate ratio of
off-diagonal versus diagonal ˇ tensorial component (r = ˇxyy/ˇxxx
which reflects the inplane non-linearity anisotropy since they
have largest ꢁˇ0 value, the reported imidazole can be used as
potential NLO material and their photophysical properties have
been investigated in a wide variety of solvents.
)
2. Experimental
2.1. Optical measurements and composition analysis
Searching organic materials with non-linear optical (NLO) prop-
erties are 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 hyperpo-
larizability of the substances [4]. Currently there is an insufficient
understanding for designing optimal NLO materials, even certain
classes of D-ꢀ-A compounds have been theoretically studied [5].
Not only the push–pull effect in D-ꢀ-A compounds quantified,
but also a linear dependence of the push–pull quotient (ꢀ*/ꢀ)
on molar hyperpolarizability were detected [6]. Thus, ꢀ*/ꢀ is a
sensitive parameter of the donor–acceptor quality of compounds
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.
2.2. Non-linear optical measurements
The second harmonic generation efficiency was assessed by
Kurtz powder technique [7] 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
fundamental wavelength of 1064 nm, generating about 4.1 mJ and
∗
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.2011.09.067

70
J. Jayabharathi et al. / Spectrochimica Acta Part A 86 (2012) 69–75
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.
2.3. Computational details
Quantum mechanical calculations have been carried out using
Gaussian-03 program [8]. As the first step of our DFT calculation,
the geometry taken from the starting structure was optimized.
2.3.1. Hyperpolarizability calculation
The density functional theory has been used to calculate the
dipole moment (ꢁ), mean polarizability (˛) and the total first static
hyperpolarizability (ˇ) [9] for mpdsi in terms of x, y, z components
and is given by following equations.
Fig. 1. Absorption solvatochromism of mpdsi in various solvents.
ꢁ = (ꢁx2 + ꢁ2 + ꢁz2)1/2
(1)
(2)
v
3. Results and discussion
1
3
˛ = (˛xx + ˛ + ˛zz
)
vv
The absorption and emission spectra have been recorded in
dichloromethane in the region of 200–500 nm. The band appeared
around 276 nm (ꢃabs) and 334 nm (ꢃem) is due to the ꢀ–ꢀ* tran-
sition. Since, n–ꢀ* and ꢀ–ꢀ* transitions are in close proximity,
the low intensity n–ꢀ* transition is often completely overlaid by
the intensive ꢀ–ꢀ* transition [24]. The UV–visible spectrum was
recorded in different solvents (Fig. 1 and Table 1). The observed
solvatochromic shifts may be due to existence of non-coplanarity
ˇtot = (ˇx2 + ˇy2 + ˇz2)1/2
(or)
ˇtot = [(ˇxxx + ˇxvv + ˇxzz)2] + (ˇ + ˇ + ˇ
)
vxx
2
vvv
vzz
+ (ˇzzz + ˇzxx + ˇzxx)2]1/2
(3)
of the imidazole derivative [25,26]. In order to understand the
coplanarity, optimization of mpdsi, DFT at B3LYP/6-31G(d,p) have
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.).
been performed using Gaussian-03. Since this compound possesses
poor crystalline nature, XRD data of its analogue derivative [15]
have been taken for comparision. The observed dihedral angle
from XRD [N3–C2–C21–C22 (−137.56◦); N3–C2–C21–C26 (40.16◦)
and C5–N1–C11–C12 (84.05◦); C5–N1–C11–C16 (−94.68◦)] and
DFT [N6–C3–C4–C5 (150.46◦); and C1–N7–C43–C44 (98.21◦);
C1–N7–C43–C45 (−81.21◦)] results strongly evidenced the exis-
tence of non co-planarity of imidazole nucleus (Fig. 2). The observed
fluorescence spectrum (Fig. 3) is broad and more sensitive to chang-
ing the polarity of the solvents. This is in argument with the fact that
greater charge transfer takes place from aromatic ring to imidazole
nucleus in S1 state than S0 state which is evidenced by the decrease
in the dihedral angle of N6–C3–C4–C5 and C1–N7–C43–C44 from
150.46◦ to 130.7◦ and 98.21◦ to 50.0◦, respectively. Moreover, the
2.3.2. Natural bond orbital (NBO) analysis
The second order Fock matrix have been carried out to evaluate
the donor-acceptor interactions in the NBO analysis [10]. For each
donor (i) and acceptor (j), the stabilization energy E(2) associated
with the delocalization i → j is estimated as,
F(i, j)2
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.
˚
˚
reduction in the bond distances of C2–C21 from 1.45 A to 1.32 A on
excitation from S0 to S1 state (Table 2).
2.4. Synthesis of
1-(4-methoxyphenyl)-4,5-diphenyl-2-styryl-1H-imidazole
(mpdsi)
Table 1
Absorption and emission datas of mpdsi.
Solvents
mpdsi
The experimental procedure used is the same as described in our
recent research works [11–23]. The imidazole derivative has been
synthesized from an unusual four components assembling of Ben-
zil (40 mmol), ammonium acetate (30 mmol), p-methoxyaniline
(30 mmol) and cinnamaldehyde (30 mmol). These four components
have been refluxed in ethanol for 24 h at 80 ◦C. The reaction mix-
ture was extracted with dichloromethane and purified by column
chromatography using hexane: ethyl acetate (9:1) as the eluent.
Yield: 55%. mp = 192 ◦C, Anal. calcd. for C30H24N2O: C, 84.08; H,
5.65; N, 6.54. Found: C, 83.00; H, 4.60; N, 7.13. 1H NMR (500 MHz,
CDCl3): ı3.85(s,3H), 6.69 (d, 1H), 7.56 (d, 1H), 6.98–7.77 (m, 19H).
13C (100 MHz, CDCl3): ı 55.42, 114.46, 126.32–132.58 (Aromatic
carbons), 134.60, 136.82, 138.16, 146.07, 147.07, 159.29. MS: m/e
428.19, calcd. 429.19 [M+1].
ꢃabs (nm)
ꢃemi (nm)
Stokes shift (cm−1
)
Hexane
1,4-dioxan
Benzene
278
297
291
288
281
282
284
276
269
263
261
271
272
281
334
363
368
371
374
389
389
334
390
396
373
418
378
406
6006
6186
7257
7775
8844
9736
9472
6345
11,615
12,796
11,534
13,051
10,392
10,912
Toluene
Chloroform
Ethyl acetate
Diethyl ether
Dichloromethane
DMSO
Butanol
Ethanol
Methanol
Acetonitrile
Ethyl methyl ketone

J. Jayabharathi et al. / Spectrochimica Acta Part A 86 (2012) 69–75
71
Fig. 3. Emission solvatochromism of mpdsi in various solvents.
3.1. Quantum yield and radiation kinetics
The fluorescence quantum yield (0.52) for mpdsi have been
measured in acetonitrile, using coumarin 47 in ethanol as a stan-
dard according to the equation,
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
Áunk
Ástd
˚
= ˚std
(5)
unk
Aunk
where Фunk, Фstd, Iunk, Istd, Aunk, Фunk and Фstd are the fluores-
Fig. 2. Optimized structure of mpdsi.
cence quantum yields, the integration of the emission intensities,
Table 2
Selected bond lengths (Å), bond angles (◦) and torsional angles (◦) of mpdsi.
Bond lengths (Å)
Experimental XRD (Å)
Bond angles (◦)
Experimental XRD (◦)
Torsional angles (◦)
Experimental XRD (◦)
C1–C2
C1–N7
C1–C20
C2–N6
C2–C14
C3–C4
C3–C6
C4–C5
C5–C8
C7–C43
C8–C9
C8–C10
C9–C11
C10–C12
C11–C13
C12–C13
C14–C16
C15–C17
C16–C18
C17–C19
C18–C19
C20–C21
C20–C22
C21–C23
C22–C24
C23–C25
C24–C25
C44–C46
C45–C48
C46–C50
C48–C50
C50–O53
O53–C54
1.4208
1.4063
1.4528
1.3966
1.4541
1.4575
1.3601
1.3413
1.4542
1.4152
1.4047
1.4023
1.3922
1.3937
1.3954
1.3942
1.4030
1.3931
1.3937
1.3947
1.3945
1.4026
1.4018
1.3934
1.3936
1.3950
1.3946
1.3866
1.3923
1.4080
1.3993
1.3790
1.4239
C2–C1–C20
C7–C1–C20
C1–C2–C14
C6–C2–C14
C4–C3–N6
C3–C4–C5
C4–C5–C8
C2–N6–C3
105.9867
124.1956
127.6070
122.6351
123.6221
124.8386
124.0757
106.7168
107.0277
125.5590
127.3749
127.3749
121.9673
118.9761
120.4327
120.3785
120.1796
120.2483
119.7823
120.4044
120.2684
120.2481
120.1014
120.2113
120.3171
119.8005
119.8355
119.4862
120.1426
120.2490
119.8957
120.3291
120.4391
119.2294
120.4751
119.6789
119.6814
114.8751
124.6157
116.2880
N7–C1–C2–N6
−0.0235
N7–C1–C2–C14
C20–C1–C2–N6
C2–C1–N7–C3
C2–C1–N7–C43
C20–C1–C7–C3
C2–C1–C20–C21
C2–C1–C20–C22
N7–C1–C20–C21
C1–C2–N6–C3
C14–C2–N6–C3
C1–C2–C14–C15
C1–C2–C14–C15
C1–C2–C14–C16
N6–C2–C14–C15
N6–C3–C4–C5
179.0615
179.2649
0.047
−177.8346−179.2921
−55.1174
124.5622
124.0555
−0.0105
C1–N7–C3
C1–N7–C43
C1–N7–C43
C5–C8–C9
C5–C8–C10
C9–C8–C10
−179.1497
−179.1497
−35.7003 145.3052
143.2771
150.4634
C8–C9–C11
−29.7142
−179.8983
−2.067
C8–C10–C12
C9–C11–C13
C10–C12–C13
C11–C13–C12
C2–C14–C15
C2–C14–C16
C14–C15–C17
C14–C16–C18
C15–C17–C19
C16–C18–C19
C17–C19–C18
C1–C20–C21
C21–C20–C22
C20–C22–C24
C21–C23–C25
C23–C25–C24
N7–C43–C44
N7–C43–C45
C44–C43–C45
C43–C45–C48
C44–C46–C50
C45–C48–C50
C46–C50–O53
C48–C50–O53
C50–O53–C54
N7–C3–C4–C5
C4–C3–N7–C1
C4–C3–N7–C43
N6–C3–N7–C1
C3–C4–C5–C8
−0.0562
−178.9323−156.7997
23.8653
98.2171
C4–C5–C8–C9
−81.2064
−79.2338
−179.9501
−0.0595
C4–C5–C8–C10
C1–N7–C43–C44
C1–N7–C43–C45
C3–N7–C43–C44
C5–C8–C9–C11
C10–C8–C9–C11
C9–C8–C10–C12
C9–C11–C13–C12
C2–C14–C15–C17
C2–C14–C16–C18
C2–C14–C15–C17
C15–C14–C16–C18
C14–C15–C17–C19
N7–C43–C44–C46
N7–C43–C44–C46
N7–C43–C44–C48
C47–C44–C46–C50
C44–C46–C50–O53
179.7550
0.4195
0.3665
0.0450
−0.4759
0.4893
0.1866
−0.2143
0.0935
−0.0795
0.3490
−179.8831
0.0975
−0.0126
−178.4401

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J. Jayabharathi et al. / Spectrochimica Acta Part A 86 (2012) 69–75
Fig. 4. Emission spectra of mpdsi for various metal ions.
the absorbance’s at the excitation wavelength and the refractive
indexes of the mpdsi and the standard, respectively. The radia-
tive (kr) and non-radiative (knr) rate constants have been calculated
from the following equations by using life time (ꢄ):
˚
p
kr =
(6)
ꢄ
˚
1
ꢄ
p
knr
=
−
(7)
(8)
ꢄ
−1
ꢄ = (kr + knr
)
Fig. 5. Potential energy surface diagram of mpdsi.
The calculated radiative, non-radiative and life time of S1 excited
state are 0.22, 0.21 and 2.3 ns, respectively.
corresponds to the one in which p-methoxyphenyl ring attached
to the nitrogen atom (N7) is tilted to an angle of 150.46◦ (about
[N6–C3–C4–C5) and the styryl ring attached to the carbon atom
(C3) is in the angle of 98.21◦ (about C1–N7–C43–C44) which are in
good agreement with the XRD results.
3.2. mpdsi as a fluorescent chemosensor
Imidazole derivatives have been used to construct highly sensi-
tive fluorescent chemosensors for sensing and imaging of metal
ions and its chelates in particular those with Ir3+ are major
components for organic light emitting diodes and are promising
candidates for fluorescent chemosensors for metal ions, if their
radiationless channel could be blocked by metal binding. In the
presence of metal ions such as Mn2+, Pb2+, Co2+, Zn2+, Hg2+ and
Cu2+. The observed enhancement of fluorescence was observed to a
different extent which shows that the metal ions binding blocks the
radiationless decay channel in mpdsi (Fig. 4). The radiative (kr) and
non-radiative (knr) rate constants have been calculated for mpdsi
3.4. Steric hindrance in imidazoles: X-ray analysis
DFT calculation reveal that the imidazole ring is essentially
planar and makes dihedral angle of 150.46◦ and 98.21◦ with the
p-methoxyphenyl and styryl rings, respectively. The key twist, des-
ignated as ˛ have been examined and it is used to indicate the twist
of imidazole ring from the styryl ring at C(3). The ˛ twist originates
from the interaction of substituent at N(7) of the imidazole with
the substituent at C(3). 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 more drops in the
fluorescence quantum yield. Such a clear correlation indicates the
importance of coplanarity between imidazole and the styryl ring at
C(3) and this correlation can be ascribed to the conjugation rigidity.
When the two adjacent aromatic species are in a coplanar geom-
etry, the p-orbitals from the C–C 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. The
in Cu2+ in acetonitrile. The observed non-radiative emission (knr
)
in mpdsi may be due to n–ꢀ* transition [27]. It was found that the
substantial increase in the quantum yield in presence of Cu2+ is due
to dramatic decrease in non-radiative transition whereas the radia-
tion constant remains unchanged within the experimental error i.e.,
the radiationless decay due to n–ꢀ* transitive in mpdsi is blocked
upon metal binding and the emission of chelate [Cu2+-mpdsi] orig-
inates only from ꢀ–ꢀ* transition.
3.3. Potential energy surface (PES) scan studies of mpdsi (1)
The potential energy surface scan about C46–C50–O53–C54 is
performed by using B3LYP/6-31G (d,p) level. During the calculation
all the geometrical parameters have been simultaneously relaxed
while the C46–C50–O53–C54 torsional angles have been varied in
steps of 0◦, 10◦, 20◦, 30◦. . .360◦. The potential energy surface dia-
gram (Fig. 5) clearly reveals that the minimum energy conformation
˚
present bond distance of C3–C4 is 1.45 A (Table 2) is shorter than
˚
the regular single bond distance between two sp2 carbons (1.48 A),
because of delocalization. When the two rings are deviated from
each other, the p-orbital overlapping will be reduced. However,
from the theoretical values it was found that most of the optimized

J. Jayabharathi et al. / Spectrochimica Acta Part A 86 (2012) 69–75
73
Table 3
Electric dipole moment (D), polarizability (˛) and hyperpolarizability (ˇtotal) of
mpdsi.
Parameter
mpdsi (1)
Dipole moment (D)
ꢁx
0.2044
ꢁy
ꢁz
ꢁtot
−0.4776
−2.0152
−2.2884
Polarizability (˛)
˛
508.6269
−14.4608
195.7711
−40.6890
−15.3935
396.8649
5.4402
xx
Fig. 6. Orientation of dipole moments.
˛
xy
˛
yy
˛
xz
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 have been aimed at the iso-
lated molecule in the gaseous phase whereas the XRD results were
aimed at the molecule in the solid state.
˛
yz
˛
˛
zz
tot × 10−23 esu
Hyperpolarizability (ˇ)
ˇxxx
ˇxxy
ˇxyy
ˇyyy
ˇxxz
3559.6137
−258.4129
−6.0330
3.5. Second harmonic generation (SHG) studies of mpdsi
−9.2657
19.5005
ˇxyz
ˇyyz
ˇxzz
ˇyzz
−19.7016
−52.0586
−770.6952
5.6209
135.0122
241.6480
552.9873
Second harmonic signal of 40 mV was obtained for mpdsi 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 [28]. Higher efficiencies can be achieved by opti-
mizing the phase matching [29]. 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.
ˇzzz
ˇtot × 10−31 esu
ꢁˇ0 × 10−31 esu
of a future optimization of the microscopic NLO properties. The
band at around 290 nm exhibits a solvatochromic shift, character-
istic 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.
3.6. Comparison of ꢁˇ0
The overall polarity of the synthesized imidazole derivative
was small when their dipole moment aligned in a parallel fashion
(Fig. 6). When the electric field is removed, the parallel alignment
of the molecular dipole moments begins to deteriorate and even-
tually 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
[30].
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 pack-
age [8]. From Table 3, it is found that the imidazole chromophore
show larger ꢁˇ0 values, which is attributed to the positive con-
tribution of their conjugation. This chromophore exhibits larger
non-linearity and its ꢃabs is red-shifted when compared with
unsubstituted imidazole. Therefore, it is clear that the hyperpolar-
izability 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 possible without compromising the molecule’s
non-linearity.
To determine the transference region and hence to know the
suitability for microscopic non-linear optical applications, the
UV–visible spectra have been recorded by using the spectrometer
in the range of 200–780 nm. The absorption spectrum was shown
in the UV region between 260 and 296 nm. The increased trans-
parency in the visible region might enable the microscopic NLO
behaviour with non-zero values [31–33]. All the absorption bands
are due to ꢀ → ꢀ* transitions. The computed ˇ values (Table 3)
have been correlated with UV–visible spectroscopic data in order to
understand the molecular-structure and NLO relationship in view
3.7. Octupolar and dipolar components of mpdsi
The imidazole possesses more appropriate ratio of off-diagonal
versus diagonal ˇ tensorial component (r = ˇxyy/ˇxxx) which reflects
the inplane non-linearity anisotropy and the largest ꢁˇ0 val-
ues. The difference of the ˇxyy/ˇxxx ratios can be well understood
by analyzing their relative molecular orbital properties. Theoreti-
cally investigated electrostatic first hyperpolarizabilities (ˇ0) and
dipole moment (ꢁ) of the chromophores explained by the reduced
planarity in such chromophores caused by the steric interaction
between the two styryl and p-methoxyphenyl rings at C(3) and
N(7) atoms. Hence, the steric interaction must be reduced in order
to obtain larger ˇ0 values.
The ˇ tensor [34] can be decomposed in a sum of dipolar
(J = 12Dˇ) and octupolar (J = 32Dˇ) 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
√
√
√
√
3
√
√
and r2 are (1 − 3)/
3
3 + 1) = −0.16 and ( 3 + 1)/
3 − 1) =
−2.15, respectively. Complying with the pythagorean theory and
the projection closure condition, the octupolar and dipolar compo-
nents of the ˇ tensor can be described as:
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢀ ꢁ
2D
J = 1ˇ
3
=
=
[(ˇxxx + ˇxvv)2] + [(ˇ + ˇvxx)2]
vvv
(9)
4
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢀ ꢁ
2D
J = 3ˇ
1
[(ˇxxx−3 + ˇxvv)2] + [(ˇ − ˇvxx)2]
vvv
(10)
4

74
J. Jayabharathi et al. / Spectrochimica Acta Part A 86 (2012) 69–75
Table 4
Second order perturbation theory analysis of Fock matrix in NBO for mpdsi.
Donor (i)
Type
ED/e
Acceptor (j)
Type
ED/e
E(2)
E(j) − E(i)
F(i,j)
C1–C2(2)
C3–N6(2)
C8–C9(2)
C8–C9(2)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
1.9726
1.9832
1.9753
1.6366
1.9810
1.6691
1.9814
1.6595
1.9732
1.6410
1.9810
1.6651
1.9813
1.6651
1.9730
1.6548
1.9808
1.6609
1.9814
1.6614
1.9756
1.6960
1.9779
1.6988
1.9805
1.6618
1.6332
1.6332
C3–N6(2)
C1–C2(2)
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
ꢀ*
0.3585
0.3063
0.3167
0.3348
0.3651
0.3348
0.3651
0.0130
0.3204
0.0143
0.0208
0.3418
0.3586
0.0126
0.0133
0.0142
0.0241
0.3328
0.3616
0.3162
0.0108
0.0243
0.0240
0.3517
0.3536
0.3005
0.3063
0.3585
28.05
39.00
39.06
43.60
41.91
41.53
41.79
41.65
40.22
43.90
41.41
42.61
41.61
41.13
40.36
41.85
41.60
42.92
43.04
40.61
44.19
31.53
33.68
44.74
52.29
30.96
51.67
75.64
0.48
0.57
0.48
0.48
0.49
0.49
0.49
0.49
0.48
0.48
0.49
0.48
0.49
0.49
0.49
0.48
0.49
0.49
0.49
0.49
0.50
0.49
0.49
0.49
0.49
0.51
0.54
0.50
0.108
0.139
0.124
0.130
0.129
0.127
0.128
0.128
0.126
0.130
0.128
0.128
0.129
0.128
0.126
0.127
0.128
0.129
0.130
0.127
0.133
0.113
0.116
0.134
0.143
0.113
0.152
0.175
C10–C12(2)
C11–C13(2)
C8–C9(2)
C11–C13(2)
C8–C9(2)
C10–C12(2)
C10–C12(2)
C11–C13(2)
C11–C13(2)
C14–C15(2)
C14–C15(2)
C16–C18(2)
C16–C18(2)
C17–C19(2)
C17–C19(2)
C20–C22(2)
C20–C22(2)
C21–C23(2)
C21–C 23(2)
C24–C25(2)
C24–C25(2)
C43–C45(2)
C 43–C45(2)
C44–C46(2)
C44–C46(2)
C48–C50(2)
C48–C50(2)
LP N7(1)
C10–C12(2)
C16–C18(2)
C17–C19(2)
C14–C15(2)
C17–C19(2)
C14–C15(2)
C16–C18(2)
C21–C23(2)
C24–C25(2)
C20–C22(2)
C24–C25(2)
C20–C22(2)
C21–C23(2)
C44–C46(2)
C48–C50(2)
C43–C45(2)
C48–C50(2)
C43–C45(2)
C44–C46(2)
C1–C2(2)
LP N7(1)
C3–N6(2)
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
2D
2D
nitrogen atoms (N7 and N6), oxygen (O53) is more electronegative
in mpdsi [36].
2D 2D
The parameter
[
=
J = 3ˇ/J = 1ˇ ] is convenient to com-
ꢂ
pare the relative magnitudes of the octupolar and dipolar
components of ˇ. The observed positive small
2D
value (0.096)
3.9. Molecular electrostatic potential map (MEP)and electronic
properties
reveals that the ˇiii component cannot be zero and these are dipo-
lar component. 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.
The molecular electrostatic potential surface (MEP) (Fig. 8) is a
plot of electrostatic potential mapped onto the iso-electron density
surface simultaneously displays molecular shape, size and elec-
trostatic potential values of the imidazole derivative. MEP surface
diagram is used to understand the reactive behaviour 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 mpdsi clearly suggests that the nitrogen and oxygen
atoms represent the most negative potential region. The hydrogen
atoms bear the maximum brunt of positive charge. The predomi-
nance of green region in the MEP surfaces corresponds to a potential
halfway between the two extremes red and dark blue colour.
3.8. Natural bond orbital (NBO) analysis
NBO analysis have been performed for mpdsi 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 [35]
were tabulated in Table 4. Several donor–acceptor interactions
have been observed for the imidazole derivative and among
the strongly occupied NBOs, the most important delocalization
sites are in the ꢀ system and in the lone pairs (n) of the oxygen
and nitrogen atoms. The ꢅ system shows some contribution
to the delocalization, and the important contributions to the
delocalization corresponds to the donor–acceptor interactions
are C8–C9 → C11–C13, C10–C12 → C8–C9, C10–C12 → C11–C13,
C11–C13 → C8–C9, C11–C13 → C10–C12, C14–C15 → C16–C18,
C14–C15 → C17–C19, C16–C18 → C14–C15, C16–C18 → C17–C19,
C17–C19 → C14–C15, C20–C22 → C21–C23, C21–C23 → C24–C25,
C43–C45 → C48–C50, C48–C50 → C43–C45 and LPN7 → C3–N6.
The charge distribution was calculated from the atomic charges
by NLO and NBO analysis (Fig. 7). These two methods predict the
same trend i.e., among the nitrogen atoms N6 and N7, N6 is consid-
ered as more basic site [32]. The charge distribution shows that the
more negative charge is concentrated on N15 atom whereas the
partial positive charge resides at hydrogens. When compared to
Fig. 7. Bar diagram representing the charge distribution of mpdsi.

J. Jayabharathi et al. / Spectrochimica Acta Part A 86 (2012) 69–75
75
Fig. 8. HOMO–LUMO orbital picture of mpdsi.
The HOMO is the orbital that primarily acts as an electron donor
and the LUMO is the orbital that largely acts as the electron accep-
tor. HOMO is located on imidazole ring, partly styryl ring, two
phenyl rings at C1 and C2 carbons and oxygen of the methoxy
group whereas LUMO is located imidazole ring and styryl ring
The HOMO → LUMO transition implies that intramolecular charge
transfer takes place [31] within the molecule. The energy gap (Eg) of
mpdsi has been calculated from the HOMO and LUMO levels (Fig. 8).
The decrease in the HOMO and LUMO energy gap explains the
probable charge transfer (CD) taking place inside the chromophore.
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The presence of ˛ twist in this imidazole drops the fluorescence
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ability can be explained by the reduced planarity caused by the
steric interaction between the two styryl and p-methoxyphenyl
rings at C(3) and N(7) atoms. Hence, the steric interaction must
be reduced in order to obtain larger ˇ0 values. From the physic-
ochemical studies on imidazoles it was concluded that molecules
of higher hyperpolarizability have larger dipole moments used as
potential NLO molecules.
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Acknowledgments
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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-
ing funds to this research work.
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