Characterization, photophysical and DFT calculation study on 2-(2,4-difluorophenyl)-1-(4-methoxyphenyl)-1H-imidazo[4,5-f][1,10] phenanthroline ligand

Article abstract of DOI:10.1016/j.saa.2012.04.059

The synthesized imidazole derivative 2-(2,4-difluorophenyl)-1-(4- methoxyphenyl)-1H-imidazo[4,5-f][1,10] phenanthroline (dfpmpip) has been characterized using IR, mass, ¹H, ¹³C NMR and elemental analysis. The photophysical properties of dfpmpip have been studied using UV-visible and fluorescence spectroscopy in different solvents. The solvent effect on the absorption and fluorescence bands has been analyzed by a multi-component linear regression. Theoretically calculated bond lengths, bond angles and dihedral angles are found to be slightly higher than that of X-ray Diffraction (XRD) values of its parent compound. The charge distribution has been calculated from the atomic charges by non-linear optical (NLO) and natural bond orbital (NBO) analysis. Since the synthesized imidazole derivative has the largest μ_gβ₀ value, the reported imidazole can be used as potential NLO material. The energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and the molecular electrostatic potential (MEP) energy surface studies evidenced the existence of intramolecular charge transfer (ICT) within the molecule. Theoretical calculations regarding the chemical potential (μ), hardness (η) and electrophilicity index (ω) have also been calculated.

Full text of DOI:10.1016/j.saa.2012.04.059

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Characterization, photophysical and DFT calculation study
on 2-(2,4-difluorophenyl)-1-(4-methoxyphenyl)-1H-imidazo
[4,5-f][1,10]phenanthroline ligand
⇑
Jayaraman Jayabharathi , Venugopal Thanikachalam, Marimuthu Venkatesh Perumal
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesized imidazole derivative 2-(2,4-difluorophenyl)-1-(4-methoxyphenyl)-1H-imidazo
[4,5-f][1,10] phenanthroline (dfpmpip) has been characterized using IR, mass, 1H, 13C NMR and elemental
analysis. The photophysical properties of dfpmpip have been studied using UV–visible and fluorescence
spectroscopy in different solvents. The solvent effect on the absorption and fluorescence bands has been
analyzed by a multi-component linear regression. Theoretically calculated bond lengths, bond angles and
dihedral angles are found to be slightly higher than that of X-ray Diffraction (XRD) values of its parent
compound. The charge distribution has been calculated from the atomic charges by non-linear optical
(NLO) and natural bond orbital (NBO) analysis. Since the synthesized imidazole derivative has the largest
Received 6 February 2012
Received in revised form 4 April 2012
Accepted 10 April 2012
Available online 24 April 2012
Keywords:
Photophysical studies
Multi-component linear regression
DFT calculation
l
gb0 value, the reported imidazole can be used as potential NLO material. The energies of the highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and the
molecular electrostatic potential (MEP) energy surface studies evidenced the existence of intramolecular
NLO behavior
PES
Electrophilicity index
charge transfer (ICT) within the molecule. Theoretical calculations regarding the chemical potential (
hardness ( ) and electrophilicity index ( ) have also been calculated.
Ó 2012 Elsevier B.V. All rights reserved.
l),
g
x
Introduction
synthesis of new materials with optical non-linearity by virtue of
their potential use in device applications related to telecommuni-
cations, optical computing, optical storage, and optical information
processing [13–16]. Herein we report the synthesis, photophysical
and theoretical studies of 2-(2,4-difluorophenyl)-1-(4-methoxy-
phenyl)-1H-imidazo[4,5-f][1,10] phenanthroline (dfpmpip).
Phenanthroline derivatives form structural elements of impor-
tant pharmaceutical drugs such as antimalarial, antifungal, antitu-
moral, antiallergic, anti-inflammatory and antiviral drugs [1–3]
and imidazo[4,5-f][1,10]phenanthroline derivatives have shown
interesting proton induced on–off emission switching characteris-
tics [4,5]. 2,9, the 4,7 or the 5,6-di-substituted 1,10-Phenanthroline
are used as chelating agents [6,7].
Experimental
Most
p-conjugated systems play a major role in determining
Computational details
second-order NLO response [8]. Benzimidazole based chromoph-
ores have received increasing attention due to their distinctive
linear, non-linear optical properties and also due to their excellent
thermal stability in guest–host systems [9]. The imidazole ring can
be easily tailored to accommodate functional groups, which
allows the covalent incorporation of the NLO chromophores into
polyamides leading to NLO side chain polymers [10].
All the calculations are performed with Gaussian 03 [17] pack-
age of programs. The geometry of all involved structures is fully
optimized with the DFT methodology, using B3LYP/6-31G(d,p)
basis set. The molecular electrostatic potential (MEP) of the
optimized structures and the representation of HOMO and LUMO
orbital density calculations were done at the same level of theory
and plotted using Gaussview program.
Our approach is to design a newly p-conjugated phenanthroline
derivative to be used as materials in material chemistry [11,12].
The planarity and extension of conjugation of the phenanthroline
moiety with imidazole and aryl units lead to an increase of the
overall conjugation. Hence there is considerable interest in the
Spectroscopy
NMR spectrum was recorded on a Bruker 400 MHz NMR instru-
ment using CDCl3 as NMR solvent. The ultraviolet–visible
(UV–vis) spectra were measured on a UV–vis spectrophotometer
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.059

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
615
2
2
(Perkin Elmer, Lambda 35) and corrected for background due to
btot ¼ ½ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ byzz þ byxx
Þ
solvent absorption at 25.0 0.1 °C in the 200–600 nm spectral
range by employing a 1 cm quartz cell, a 1 ꢀ 10ꢁ5 M solution is
used for acquisition. All the fluorescence spectra were recorded
on a (Perkin Elmer LS55) fluorescence spectrometer equipped with
1.0 cm quartz cell and the excitation wavelength set was 320 nm.
The infrared spectrum was recorded by using KBr pellets on a Ava-
tar 330-Thermo Nicolet FT-IR spectrometer in the 4000–600 cmꢁ1
spectral range. Mass spectrum was recorded using Thermo Fischer
LC–Mass spectrometer (Plate 1).
2
1=2
þ ðbzzz þ bzxx þ bzyyÞ ꢂ
ð3Þ
The b 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 ꢀ 10ꢁ33 e.s.u.).
Natural bond orbital (NBO) analysis
The second order Fock matrix was carried out to evaluate the
donor–acceptor interactions in the NBO analysis [20]. For each do-
nor (i) and acceptor (j), the stabilization energy E(2) associated
with the delocalization i ? j is estimated as,
Non-linear optical measurements
The second harmonic generation efficiency was assessed by
Kurtz powder technique [18] 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 photo-
diode detector integrated with oscilloscope assembly.
2
Fði; jÞ
Eð2Þ ¼
D
Eij ¼ q
ð4Þ
i ej ꢁ
ei
where qi is the donor orbital occupancy, ei and ej 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.
Hyperpolarizability calculation
Synthesis of 2-(2,4-difluorophenyl)-1-(4-methoxyphenyl)-1H-imidazo
[4,5-f][1,10] phenanthroline (dfpmpip)
The density functional theory has been used to calculate the di-
pole moment (l), mean polarizability (a) and the total first static
hyperpolarizability (b) [19] for dfpmpip in terms of x, y, z compo-
nents and is given by following equations:
The synthesis of 2-(2,4-difluorophenyl)-1-(4-methoxyphenyl)-
1H-imidazo[4,5-f][1,10] phenanthroline involves a four components
assembling [21–23] of a mixture of 1,10-Phenanthroline-5,6-dione
(2.10 g, 10 mM), ammonium acetate (2.5 g, 30 mM), 4-methoxyan-
iline (1.23 g, 10 mM) and 2,4-difluorobenzaldehyde (1.1 ml,
10 mM) in distilled ethanol medium (20 ml) (Scheme 1). The reac-
tion mixture was refluxed at the boiling point of ethanol (78 °C)
and the completion of the reaction was monitored by thin layer
chromatography (TLC) technique using benzene:ethyl acetate
(9:1) as the eluent. The reaction mixture was then extracted with
dichloromethane and the resultant resinous material was purified
by column chromatography.
ꢀ
ꢁ
þ
1=2
l
a
¼
lx2
þ
ly2
þ
lz2
ð1Þ
ð2Þ
¼ 1=3ðaxx
þ
ayy
azz
Þ
ꢀ
ꢁ
1=2
btot
¼
b2x þ b2y þ b2z
(or)
Plate 1. LC–MS spectrum of the imidazole derivative dfpmpip.

616
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
Scheme 1. Synthesis route of dfpmpip.
Results and discussion
Photophysical studies of the imidazole derivative
UV–vis absorption and fluorescence spectral studies have been
carried out in different solvents for dfpmpip and the results have
been summarized in Table 1. In UV spectral data (Fig. 1), the main
absorption band is centered around 270 nm with a high molar
absorption coefficient, in addition to this somewhat weaker bands
are also observed around 320 nm. The fluorescence spectrum kemi
is centered around 385 nm (Fig. 2).
A multi-parameter correlation analysis [24–27] is employed in
which a physicochemical property is linearly correlated with sev-
eral solvent parameters by means of:
ðXYZÞ ¼ ðXYZÞ0 þ CaA þ CbB þ CcC þ ꢃ ꢃ ꢃ
ð5Þ
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 photo-
physical 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. [28] proposed the pꢄ
,
a
and b scales, whereas more recently Catalan et al. [29] suggested
the SPPN, SA and SB scales to describe the polarity/polarizability, the
acidity and basicity of the solvents respectively.
Fig. 3 shows the correlation between the absorption and fluo-
rescence wavenumbers calculated by the multi-component linear
regression employing the Taft-proposed solvent parameters and
the experimental values. The dominant coefficient affecting the
absorption and fluorescence bands of dfpmpip is the polarity/
polarizability of the solvent, C or CNSPP having a positive value, cor-
roborating the solvatochromic shifts with the solvent polarity. The
coefficient controlling the electron releasing capacity or basicity of
the solvent, Cb or CSB, is the lowest coefficient (Table 2), hence, the
solvent basicity does not play an important role in absorption and
Fig. 1. Absorption spectra of the imidazole derivative in various solvents.
fluorescence displacements. The adjusted coefficient representing
the acidity of the solvent, C or CSA has a negative value, suggesting
a
that the absorption and fluorescence bands shift to lower energies
with the increasing acidity character of the solvent. This effect can
be interpreted in terms of the stabilization of the resonance struc-
tures of the chromophore (Fig. S1). The resonance structure ‘‘b’’
bearing negative charge on the carbon atom of the 4-methoxy phe-
nyl ring, so that the structure will be more predominant in acidic
solvents because this resonance structure 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.
p
ꢄ
Table 1
Photophysical data of dfpmpip in different solvents.
Solvent
Imidazole derivative (1)
kabs ( 0.1 nm)
kemi ( 0.4 nm)
D )
tss (cmꢁ1
Steric hindrance versus fluorescence quantum yield
Hexane
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
Dichloromethane
Butanol
271
270
279
273
268
271
273
270
274
270
269
270
375
380
383
384
385
388
396
408
410
389
339
392
10,234
10,721
9733
DFT calculation reveals that the imidazole ring is essentially
planar and makes dihedral angle of 152.40° and 179.73° with the
4-methoxyphenyl and difluorophenyl rings (Fig. S2), respectively.
10,588
11,339
11,127
11,378
12,527
12,106
11,330
7676
The key twist ‘a’ is used to indicate the twist of imidazole ring from
the aromatic six-membered ring at C(23) (Fig. S3). The
originates from the interaction of substituent at N(15) of the imid-
azole with the substituent at C(23). Emission spectral results reveal
a
twist
Ethanol
Methanol
Acetonitrile
DMSO
that, larger the
a twist, the fluorescence quantum yield (0.30) will
1-Propanol
11,527
decrease when compared with its analog compound [30]. Such a

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
617
the optimized bond lengths, bond angles and dihedral angles are
slightly higher than that of XRD values (Table 3). 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.
Potential energy surface (PES) Scan studies
The potential energy surface scan about C20–C22–O45–C46 is
performed by using B3LYP/6-31G(d,p) level of theoretical approxi-
mation for the compound (dfpmpip). The torsional angle of
C20–C22–O45–C46 is also relevant coordinate for conformation
flexibility within the molecule. During the calculation all the geo-
metrical parameters were simultaneously relaxed while the
C20–C22–O45–C46 torsional angles were varied in steps of
0°,10°,20°,30°,. . .,90°. The potential energy surface diagram
(Fig. 4) clearly reveals that the minimum energy (546.6 kcal/mol
at 91°) conformation corresponds to the one in which p-anisidine
ring attached to the nitrogen atom (N15) is tilted to an angle of
92° (about C23–N15–C17–C18) which is in good agreement with
the XRD results, whereas the difluoro phenyl ring attached to the
carbon atom (C23) is in the angle of 0.27° (about N16–C23–C24–
C26). The XRD report for its analog compound shows that the angle
is around 55° about N16–C23–C24–C26 [31]. The contraction in
theoretical value may be due to the stronger interaction of difluo-
rophenyl ring with the adjacent p-anisidine ring attached to the
nitrogen atom (N15).
Fig. 2. Emission spectra of the imidazole derivative in various solvents.
clear correlation indicates the importance of coplanarity between
imidazole and the aromatic ring at C(23). Since X-ray crystal study
of the dfpmpip is currently under progress, its analogous data [31]
is taken for comparision. 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 theoretical values.
However, from the theoretical values it was found that most of
Second harmonic generation (SHG) studies of dfpmpip
Second harmonic signal of 50 mV was obtained for dfpmpip 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 sec-
ond order non-linear efficiency will vary with the particle size of
the powder sample [32]. Higher efficiencies are achieved by opti-
Fig. 3. Correlation between the experimental absorption (a) and fluorescence (b) wavenumber with the predicted values obtained by a multicomponent linear regression
using the pꢄ
, a and b-scale (Taft) solvent parameters for dfpmpip.
Table 2
Adjusted Coefficients ((
t
x)0, Ca, Cb and Cc) for the multilinear regression analysis of the absorption (
t
ab) and fluorescence (
tfl) wavenumbers and stokes shift (Dtss) of the imidazole
derivative with the solvent polarity/polarizability, and the acid and base capacity using the Taft (pꢄ
, a
and b) and the Catalan (SPPN, SA and SB) scales.
Imidazole derivative (1)
(tx)
(tx
)
0
(cmꢁ1
)
(
pꢄ
)
C
a
Cb
kab
kfl
(3.66 0.021) ꢀ 104
(2.62 0.080) ꢀ 104
(1.04 0.075) ꢀ 104
(2.81 3.717) ꢀ 103
ꢁ(4.28 14.456) ꢀ 103
(7.09 13.503) ꢀ 103
ꢁ(4.69 11.964) ꢀ 103
(12.31 46.530) ꢀ 103
ꢁ(16.99 43.462) ꢀ 103
CSA
(2.02 9.370) ꢀ 103
ꢁ(8.93 36.440) ꢀ 103
(30.95 34.037) ꢀ 103
CSB
D
tss
=
=
tab
ꢁ
ꢁ
tfl
CNSPP
kab
kfl
(3.68 0.023) ꢀ 104
(2.60 0.055) ꢀ 104
(1.08 0.056) ꢀ 104
(2.92 7.797) ꢀ 103
ꢁ(10.39 32.790) ꢀ 103
ꢁ(125.23 79.777) ꢀ 103
(114.83 80.614) ꢀ 103
(8.02 34.459) ꢀ 103
(25.66 18.970) ꢀ 103
ꢁ(22.743 19.169) ꢀ 103
(129.13 83.838) ꢀ 103
ꢁ(121.11 84.718) ꢀ 103
D
tss
tab
tfl

618
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
Table 3
Selected bond lengths, bond angles and torsional angles for dfpmpip.
Bond connectivity
Bond lengtha (Å)
Bond connectivity
Bond anglea (°)
Bond connectivity
Torsional anglea (°)
C6–C9
1.3710 (1.3770)
1.4622 (1.3840)
1.4666 (1.3789)
1.4700 (1.4402)
1.4826 (1.3752)
1.3530 (1.3168)
1.5400 (1.4782)
1.3500 (1.3890)
1.4300 (–)
C3–C6–C9
C3–C6–N15
C6–C9–N16
121.5689 (123.04)
128.7629 (131.74)
109.1597 (111.31)
113.4900 (128.80)
101.2294 (106.40)
105.3174 (104.52)
120.0021 (119.73)
124.0936 (122.78)
119.9975 (121.16)
119.9997 (–)
C3–C6–C9–N16
172.7835 (176.28)
43.5222 (ꢁ2.3)
ꢁ165.8720 (ꢁ175.82)
27.5620 (82.65)
ꢁ162.3655 (ꢁ179.76)
7.0381 (ꢁ0.78)
179.7608 (56.56)
ꢁ179.9960 (ꢁ176.22)
0.2420 (ꢁ122.35)
ꢁ40.2389 (6.4)
180.0000 (–)
C6–N15
C9–N16
N15–C17
N15–C23
N16–C23
C23–C24
C24–C25
C22–O45
C25–F31
C29–F30
O45–C46
C3–C6–N15–C17
C3–C6–N15–C23
C6–N15–C17–C18
C6–N15–C23–C24
N15–C6–C9–N16
N15–C23–C24–C25
N15–C17–C18–C20
N16–C23–C24–C25
C17–N15–C23–C24
C18–C20–C22–O45
C23–C24–C25–F31
C26–C28–C29–F30
C21–C22–O45–C46
C6–N15–C17
C6–N15–C23
C9–N16–C23
N15–C17–C19
N15–C23–C24
C23–C24–C25
C24–C25–F31
C28–C29–F30
C21–C22–O45
C22–O45–C46
1.3500 (–)
1.4300 (–)
1.4014 (–)
120.0004 (–)
120.0024 (–)
109.4725 (–)
ꢁ0.0028 (–)
179.9958 (–)
90.0284 (–)
a
Values given in the brackets are corresponding to XRD values of its parent compound [31].
Fig. 4. The potential energy surface (PES) diagram of the imidazole derivative.
mizing the phase matching [33]. 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 out-
put. The ultimate goal in the design of polar materials is to prepare
compounds which have their molecular dipole moments aligned in
the same direction [34].
using Gaussian 03 package [17]. Theoretical investigation plays
an important role in understanding the structure–property
relationship, which is able to assist in designing novel NLO
chormophores. From Table 4, it is found that the imidazole chro-
mophore show larger lbo value, which is attributed to the positive
contribution of their conjugation. This chromophore exhibits larger
non-linearity and its kabs 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 de-
vices can be detrimental, it is important to make NLO chromoph
Comparison of lbo
The electrostatic first hyperpolarizability (b) and dipole mo-
ment ( ) of the imidazole chromophore have been calculated by
l

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
619
Table 4
Electric dipole moment (D), polarizability (a) and
hyperpolarisability (btotal) of dfpmpip.
Parameter
1
Dipole moment (D)
lx
ly
ꢁ3.1503
3.6150
lz
ltotal
ꢁ0.8743
4.8742
Polarizability (a)
axx
axy
ayy
axz
ayz
azz
ꢁ205.9024
ꢁ12.7827
ꢁ160.3089
9.6128
ꢁ6.7893
ꢁ188.9867
ꢁ27.4268
a
ꢀ 10ꢁ24 (esu)
Hyperpolarisability
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
ꢁ84.6523
ꢁ60.4795
24.5416
88.0493
ꢁ20.9547
5.1599
Fig. 5. Bar diagram representing the charge distribution of dfpmpip using NLO and
NBO methods.
Table 6
byyz
bxzz
byzz
bzzz
10.1611
ꢁ27.4756
ꢁ4.8360
15.2910
78.2723
381.5148
Percentage of s and p-character on each natural atomic hybrid of the natural bond
orbital for dfpmpip.
Bond (A–B)
C6–C9
ED/Energy (a.u.)
EDA (%)
EDB (%)
s (%)
p (%)
b0 ꢀ 10ꢁ32 (esu)
b0 ꢀ 10ꢁ32 (esu)
0.7080
0.7062
0.6183
0.7859
0.6520
0.7582
0.7819
0.6234
0.7912
0.6115
0.7647
0.6444
0.7042
0.7100
0.5291
0.8486
0.5252
0.8510
0.5818
0.8133
0.8216
0.5701
50.12
–
38.23
–
42.51
–
61.14
–
62.60
–
58.47
–
49.58
–
27.99
–
49.88
–
61.77
–
57.49
–
38.86
–
37.40
–
41.53
–
50.42
–
72.01
–
37.26
34.81
27.04
28.50
26.58
27.20
27.23
26.42
25.45
28.72
35.10
33.57
37.60
29.17
23.13
29.49
23.09
30.21
24.54
27.08
25.06
20.71
62.74
65.19
72.96
71.50
73.42
72.80
72.77
73.58
74.55
71.28
64.90
66.43
62.40
70.83
76.87
70.51
76.91
69.79
75.46
72.92
74.74
79.29
l
C6–N15
ores as transparent as possible without compromising the mole-
cule’s non-linearity.
The absorption spectrum of dfpmpip was shown in the UV region
around 270 nm. The increased transparency in the visible region
might enable the microscopic NLO behavior with non-zero values
C9–N16
N15–C17
N15–C23
N16–C23
C25–F31
C26–F49
C29–F30
C22–O45
O45–C46
[35–37]. All the absorption bands are due to
p ?
pꢄ transitions.
The b values (Table 4) might be correlated with UV–visible spectro-
scopic data in order to understand the molecular-structure and NLO
relationship in view of a future optimization of the microscopic NLO
properties. The band at around 340 nm exhibits a solvatochromic
shift, characteristic of a large dipole moment and frequently sugges-
tive of a large hyperpolarizability. This compound show red shift in
27.58
–
33.85
–
67.50
–
72.42
–
66.15
–
32.50
–
Table 5
Significant donor–acceptor interactions of dfpmpip and their second-order perturba-
tion energies (kcal/mol).
Donor (i)
ED/e
Acceptor (j) ED/e
E(2) a.u E(j) ꢁ E(i) F(i,j)
C1–C2
C3–C4
C5–N14
C7–C8
1.9792 C5–N14
0.0125
0.0152
0.0346
0.0239
0.0128
0.0373
0.0143
0.0276
0.0276
0.0253
0.0253
0.0143
0.0145
0.0270
0.0334
0.0093
0.0270
0.0334
24.60
20.35
23.49
20.03
25.37
22.99
20.42
18.33
20.69
18.64
21.75
20.16
22.00
23.66
22.43
21.32
21.20
21.36
0.27
0.28
0.32
0.28
0.26
0.32
0.28
0.29
0.28
0.28
0.27
0.28
0.29
0.27
0.29
0.34
0.42
0.41
0.01
0.02
0.02
0.02
0.04
0.01
0.01
0.073
0.070
0.080
0.069
0.074
0.079
0.069
0.065
0.069
0.065
0.069
0.067
0.072
0.072
0.073
0.076
0.091
0.092
0.083
0.082
0.081
0.083
0.055
0.063
0.081
1.9736 C1–C2
1.9853 C3–C4
1.9706 C6–C9
C10–C11
C12–N13
C17–C18
C17–C18
C19–C21
C19–C21
C20–C22
C20–C22
C24–C25
C26–C28
C27–C29
LpN15
19.811
C12–N13
1.9854 C7–C8
1.9761 C19–C21
1.9761 C20–C22
1.9723 C20–C22
1.9723 C17–C18
1.9815 C17–C18
1.9815 C19–C21
1.9775 C26–C28
1.9716 C27–C29
1.9787 C24–C25
1.7217 N16–C23
1.9884 C27–C29
1.9883 C24–C25
1.9854 C1–C2
1.9854 C3–C4
1.9855 C7–C8
1.9855 C10–C11
1.9867 C6–C9
1.9867 C24–C25
1.9787 C26–C28
LpF30
LpF31
C5–N14
C5–N14
C12–N13
C12–N13
N16–C23
N16–C23
C27–C29
0.0152 189.01
0.0346 139.51
0.0373 153.70
0.0137 153.12
0.0249
0.0334 134.85
0.0145 232.79
27.64
Fig. 6a. MEP surface diagram of dfpmpip.

620
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
Fig. 6b. HOMO–LUMO energy diagram of dfpmpip.
absorption with increasing solvent polarity, accompanied with the
upward shifts non-zero values in the b-components.
methods predict the same trend i.e., among the nitrogen atoms
N15 and N16, N15 is considered as more basic site [36]. The charge
distribution 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 (N15 and N16), oxygen
and fluorine atoms (F30 and F31) are less electronegative and it
can be seen from the bar diagram [39].
b-Tensorial component versus non-linearity
The title compound (dfpmpip) possess a more appropriate ratio
of off-diagonal versus diagonal b tensorial component (r = bxyy/bxxx
)
The percentage of s and p-character [40] in each NBO natural
atomic hybrid orbital are displayed in Table 6. For all the carbon,
nitrogen and fluorine atoms, around 70% of p-character and 30%
s-character have been observed.
which reflects the inplane non-linearity anisotropy and the largest
l
bo values.
The b tensor [38] can be decomposed in a sum of dipolar (j¼12Db)
and octupolar (j¼12Db) 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
Molecular electrostatic potential map (MEP) and electronic properties
p
p p
3 3 +
respectively. The critical values for r1 and r2 are (1 ꢁ 3)/
p
p p
1) = ꢁ0.16 and ( 3 + 1)/ 3( 3 ꢁ 1) = 2.15, respectively. Comply-
ing with the Pythagorean theory and the projection closure condi-
tion, the octupolar and dipolar components of the b tensor can be
described as:
The molecular electrostatic potential surface (MEP) (Fig. 6a) dis-
plays molecular shape, size and electrostatic potential values of the
imidazole derivative (dfpmpip). MEP surface diagram is used to
understand the reactive behavior of a molecule, in that negative re-
gions can be regarded as nucleophilic centers, whereas the positive
regions are potential electrophilic sites. The MEP map of dfpmpip
shows that the nitrogen, fluorine and oxygen atoms represent
the most negative potential region. The hydrogen atoms bear the
maximum region of positive charge. The predominance of green
region in the MEP surfaces corresponds to a potential halfway be-
tween the two extremes red and dark blue color.1
2
2
kj2¼D1 bk ¼ ð3=4Þ½ðbxxx þ bxyyÞ þ ½ðbyyy þ byxxÞ ꢂ
ð6Þ
ð7Þ
2
2
kj2¼D1 bk ¼ ð1=4Þ½ðbxxx ꢁ bxyyÞ þ ½ðbyyy ꢁ byxxÞ ꢂ
kj2¼D3 b k
The parameter q2D
,
q2D
¼
ꢂ is convenient to compare the
kj2¼D1 b k
The 3D plots of the frontier orbitals HOMO and LUMO of dfpm-
pip is shown in Fig. 6b. The HOMO is located on the imidazole ring,
partly on the phenanthroline ring and on the difluoro phenyl ring,
and partly on the carbon atoms of the methoxy phenyl ring. The
LUMO is located on the phenanthroline ring, partly on the imidaz-
ole ring and on the carbon atoms of both the phenyl rings of the
imidazole derivative. The HOMO ? LUMO transition implies that
intramolecular charge transfer takes place [41] within the mole-
cule. The energy gap (Eg) of dfpmpip has been calculated from
the HOMO and LUMO levels. The HOMO–LUMO energy gap sup-
ports the probable charge transfer (CD) inside the chromophore.
relative magnitudes of the octupolar and dipolar components of
b. The observed positive small
q
2D value (0.386) reveals that the biii
component 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.
Intramolecular charge transfer from NBO analysis
Several donor–acceptor interactions observed for the imidazole
derivative (dfpmpip) have been shown by natural bond orbital
(NBO) analysis which have been performed for dfpmpip at the
DFT/B3LYP/6-31++G(d,p) level in order to elucidate the intramolec-
ular, delocalization of electron density within the molecule
(Tables 5). Among the strongly occupied NBOs, the most important
Electrophilicity index
At the Hatree-Fock level, Koopaman’s theorem suggests that the
energy of HOMO is good approximation to the negative experi-
mental ionization potential (-IP) [42]. Similarly, it suggests that
the electron affinity (EA) for an N-electron system is equal to the
negative of the LUMO energy and it is a measure of susceptibility
of molecule towards attack by nucleophiles [43]. The dipole mo-
delocalization sites are in the
p system and in the lone pairs (n) of
the oxygen, fluorine and nitrogen atoms. The important
contributions to the delocalization corresponds to the donor–
acceptor interactions are C5–N14 ? C1–C2, C5–N14 ? C3–C4,
C12–N13 ? C7–C8, C12–N13 ? C10–C11, N16–C23 ? C6–C9,
N16–C23 ? C24–C25 and C27–C29 ? C26–C28.
1
The charge distribution of dfpmpip has been calculated from
the atomic charges by NLO and NBO analysis (Fig. 5). These two
For interpretation of color in Figs. 1–6, the reader is referred to the web version of
this article.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 614–621
621
ment is the most obvious quantity to describe the polarity of a
References
molecule. Electronegativity of imidazole derivative is calculated
by,
[1] J. Pedraza-Chaverri, N. Cardenas-Rodriguez, M. Orozco-Ibarra, J.M. Perez-Rojas,
Food. Chem. Toxicol. 46 (2008) 3227.
[2] M.V. Ignatushchenko, R.W. Winter, M. Riscoe, Am. J. Trop. Med. Hgy. 62 (2000)
77.
l
¼ ꢁv
¼ ꢁðdE=dNÞVðrÞ
ð8Þ
[3] G. Gopalakrishnan, B. Banumathi, G. Suresh, J. Nat. Prod. 60 (1997) 519.
[4] F. Gao, X. Chen, F. Zhou, et al., Inorg. Chim. Acta 362 (2009) 4960.
[5] F.R. Liu, K.Z. Wang, G.Y. Bai, et al., Inorg. Chem. 43 (2004) 1799.
[6] P.G. Sammes, G. Yahioglu, Chem. Soc. Rev. 23 (1994) 327.
[7] A.R. Katritzky, Q.H. Long, N. Malhotra, T.A. Ramanarayanan, H. Vedage,
Synthesis (1992) 911.
[8] C.W. Dirk, H.E. Katz, M.L. Schilling, L.A. King, Chem. Mater. 2 (1990) 700.
[9] E.M. Across, K.M. White, R.S. Moshrefzadeh, C.V. Francis, Macromolecules 28
(1995) 2526.
where E is the energy, N is the number of electrons and V(r) is the
constant external potential. By combining the above equation with
the work of Ickowski and Margrave [44], assuming a quadrate rela-
tionship between E and N and in a finite difference approximation,
the equation can be rewritten as,
v
¼ ꢁl
¼ ðI þ AÞ=2
ð9Þ
[10] X.R. Bu, H. Li, D.V. Derveer, E.A. Mintz, Tetrahedron Lett. 37 (1996) 7331.
[11] C.R. Moylan, R.D. Miller, R.J. Twieg, K.M. Betterton, V.Y. Lee, T.J. Matray, C.
Nguyen, Chem. Mater. 5 (1993) 499.
[12] M. Ammamm, P. Bauerle, Org. Biomol. Chem. 3 (2005) 4143.
[13] S.R. Marder, J.E. Sohn, G.D. Stucky (Eds.), Materials for Non-linear Optics:
Chemical Perspectives. In: ACS Symposium Series, vol. 455, American
Chemical Society, Washington, DC, 1991.
0
vkoopaman s ¼ ðEHOMO þ ELUMOÞ=2
ð10Þ
and the global hardness is defined as,
g
= ½(d2E/dN2)V(r) or ½(EHOMO
ꢁ ELUMO).
Electrophilicity index of a molecule can be calculated by,
[14] J. Messier, F. Kajzar, P. Prasad (Eds.), Organic Molecules for Non-linear Optics
and Photonics, Kluwer Academic, Dordrecht, 1991.
[15] L.R. Dalton, A.W. Harper, R. Ghosn, W.H. Steir, M. Ziari, H. Fetterman, Y. Shi,
R.V. Mustacich, A.K.Y. Jen, K.J. Shea, Chem. Mater. 7 (1995) 1060.
[16] R.G. Benning, J. Mater. Chem. 5 (1995) 365.
x
¼
l
2=2
g
ð11Þ
This electrophilicity index measures the capacity of electrophile
to accept the maximal number of electrons in a neighboring reser-
voir of Electron Sea. By using the above equations, the chemical po-
tential, hardness and electrophicity index have been calculated as
[17] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery, T. Vreven Jr., K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, P.
Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A.
Pople, Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT, 2004.
[18] S.R. Flom, P.F. Barbara, Chem. Phys. Lett. 94 (1983) 488.
[19] M. Wagener, J. Sadowsky, J. Gasteiger, J. Am. Chem. Soc. 117 (1995) 7769.
[20] Y. Yang, W.J. Zhang, X.M. Gao, Int. J. Quantum Chem. 106 (2006) 1199–1207.
[21] .S. Rosepriya, M. Venkatesh Perumal, A. Thiruvalluvar, J. Jayabharathi, R.J.
Butcher, J.P. Jasinski, J.A. Golen, Acta Cryst. E67 (2011) o1965.
[22] .J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal, N. Srinivasan,
Spectrochim. Acta Part A 79 (2011) 236.
[23] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal, K. Saravanan, J.
Fluoresc. (2011), http://dx.doi.org/10.1007/s10895-011-0878-3.
[24] Y. Marcus, Chem. Soc. Rev. 22 (1993) 409.
[25] C. Reichardt, Chem. Rev. 94 (1994) 2319.
[26] E. Lippert, in: J.B. Birks (Ed.), Organic Molecular Photophysics, vol. 2, Wiley-
Interscience, Bristol, England, 1975, p. 1.
[27] K. Dimroth, C. Reichardt, Liebigs Ann. Chem. 727 (1969) 93.
[28] M.J. Kamlet, R.W. Taft, J. Am. Chem. Soc. 98 (1976) 377.
[29] J. Catalan, V. Lopez, P. Perez, J. Fluoresc. 6 (1996) 15.
[30] J. Jayabharathi, V. Thanikachalam, M. Venkatesh Perumal, N. Srinivasan, J.
Fluoresc. (2011), http://dx.doi.org/10.1007/s10895-011-1009-x.
[31] S. Rosepriya, A. Thiruvalluvar, J. Jayabharathi, M. Venkatesh Perumal, R.J.
Butcher, J.P. Jasinski, J.A. Golen, Acta Cryst. E67 (2011) o989.
[32] Y. Porter, K.M. OK, N.S.P. Bhuvanesh, P.S. Halasyamani, Chem. Mater. 13 (2001)
1910.
l
= ꢁ0.273,
g
= ꢁ0.047 and
x
= ꢁ0.7928.
Conclusion
We have synthesized and characterized a new imidazole deriv-
ative using various spectroscopic techniques. The photophysical
properties of the compound have been studied in various solvents
and the solvent effect on the absorption and fluorescence bands
has been analyzed by a multi-component linear regression in
which several solvent parameters were analyzed. Since the acidity
of the solvent, C or CSA has a negative value, which suggesting that
the absorption and fluorescence bands shift to lower energies with
the increasing acidity of the solvent. The presence of
imidazole derivative lowers the fluorescence quantum yield. From
the PES scan study, the minimum energy conformation of the mol-
ecule is drawn. The observed dipole moment and hyperpolarizabil-
ity can be explained by the reduced planarity caused by the steric
interaction between the two phenyl rings at C(23) and N(15)
atoms. Hence, the steric interaction must be reduced in order to
obtain larger b0 values. From the physicochemical studies on imi-
dazoles it was concluded that molecules of higher hyperpolariz-
ability have larger dipole moments used as potential NLO
molecules. The electrophilicity index has been calculated in order
to measure the capacity of the electrophile to accept the maximal
number of electrons in a neighbor reservoir of Electron Sea.
a
a twist of the
[33] M. Narayana Bhat, S.M. Dharmaprakash, J. Cryst. Growth 236 (2002) 376.
[34] D. Steiger, C. Ahlbrandt, R. Glaser, J. Phys. Chem. B 102 (1998) 4257.
[35] V. Crasta, V. Ravindrachary, R.F. Bharantri, R. Gonsalves, J. Cryst. Growth 267
(2004) 129.
[36] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem. Phys. 1 (1999) 3519.
[37] Y. Yang, W.J. Zhang, X.M. Gao, Int. J. Quant. Chem. 106 (2006) 1199.
[38] S.F. Tayyari, S. Laleh, Z.M. Tekyeh, M.Z. Tabrizi, Y.A. Wang, H. Rahemi, Mol.
Struct. 827 (2007) 176.
[39] G. Wang, F. Lian, Z. Xie, G. Su, L. Wang, X. Jing, F. Wang, Synth. Met. 131 (2002)
1.
[40] M. Szafran, A. Komasa, E.B. Adamska, J. Mol. Struct. 827 (2007) 101.
[41] K. Fukui, T. Yonezawa, H. Shingu, J. Chem. Phys. 20 (1952) 722.
[42] L.J. Bellamy, The infrared Spectra of Complex Molecules, vol. 2, Chapman and
Hall, London, 1982.
Acknowledgments
One of the authors Dr. J. Jayabharathi is thankful to Department
of Science and Technology [No. SR/S1/IC-73/2010], University
Grants Commission (F.No. 36-21/2008 (SR)) and Defence Research
and Development Organization (DRDO) (F.No. NRB-213/MAT/10-
11) for providing funds to this research study.
Appendix A. Supplementary material
[43] R.G. Parr, R.A. Donnelly, M. Levy, W.E. Palke, J. Chem. Phys. 68 (1978) 3801.
[44] R.P. Iczkowski, J.L. Margrave, J. Am. Chem. Soc. 83 (1961) 315.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.059.