Photophysical and computational studies of novel heterocyclic imidazole derivatives containing trifluoromethyl group substituent

Article abstract of DOI:10.1016/j.saa.2012.04.033

Phenanthrimidazole ligands with potential electron withdrawing trifluoromethyl substituent have been synthesized and characterized using IR, mass and NMR spectral studies. The effect of solvent on their absorption and emission have been analysed in detail by Kamlet-Taft Catalan approach. The introduction of trifluoromethyl substituent results in a significant blue shift in emission when compared to its parent compound. This is due to the presence of the electron withdrawing trifluoromethyl moiety which changes the π-π conjugation of the cyclometalated ligand and lowers the triplet energy level, thus lowering the lowest unoccupied molecular orbital (LUMO) level. The optimized bond lengths, bond angles and dihedral angles calculated by DFT/B3LYP/6-31G (d,p) method are found to be slightly higher than that of XRD values of its parent compound. NBO, NLO, MEP and HOMO-LUMO energy analysis have been carried out by DFT method. This shows that the synthesized imidazole derivatives possess non-linear optical (NLO) behavior. In addition to these, calculations regarding the chemical potential, hardness and electrophilicity index have also been done.

Full text of DOI:10.1016/j.saa.2012.04.033

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical and computational studies of novel heterocyclic imidazole
derivatives containing trifluoromethyl group substituent
⇑
Jayaraman Jayabharathi , Marimuthu Venkatesh Perumal, Venugopal Thanikachalam
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Phenanthrimidazole ligands with potential electron withdrawing trifluoromethyl substituent have been
synthesized and characterized using IR, mass and NMR spectral studies. The effect of solvent on their
absorption and emission have been analysed in detail by Kamlet–Taft catalan approach. The introduction
of trifluoromethyl substituent results in a significant blue shift in emission when compared to its parent
compound. This is due to the presence of the electron withdrawing trifluoromethyl moiety which
Received 1 February 2012
Received in revised form 28 March 2012
Accepted 5 April 2012
Available online 23 April 2012
changes the p–p conjugation of the cyclometalated ligand and lowers the triplet energy level, thus low-
Keywords:
Phenanthrimidazole
NMR
Blue shift
NBO
ering the lowest unoccupied molecular orbital (LUMO) level. The optimized bond lengths, bond angles
and dihedral angles calculated by DFT/B3LYP/6–31G (d,p) method are found to be slightly higher than
that of XRD values of its parent compound. NBO, NLO, MEP and HOMO–LUMO energy analysis have been
carried out by DFT method. This shows that the synthesized imidazole derivatives possess non-linear
optical (NLO) behavior. In addition to these, calculations regarding the chemical potential, hardness
and electrophilicity index have also been done.
MEP
NLO
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
optical computing, optical storage, and optical information process-
ing [13].
Imidazoles are important heterocycles in medicinal chemistry
and exhibits a broad spectrum of biological activity such as antivi-
ral, antiulcer, antihypertension, and anticancer properties [1]. They
have also found application as a chromophore with readily tune-
able absorption wavelength, fluorophoric properties and synthetic
building block in supramolecular chemistry [2]. 1,10-Phenanthro-
line-5,6-dione and its imidazole derivatives are very versatile li-
gands for the assembly of metal organic materials [3] and can be
used directly as a bis-chelating ligand in organometallic chemistry.
The phenanthrolines and their derivatives have remarkable physi-
ological and pharmacological activities such as anticancer (cop-
per(II)) [4], antiinflammatory (copper(II)) [5], antitumor (Pt(II))
[6], antimicrobial (copper(II)) [7], antibacterial activity (Y(III)) [8]
and metabolic oxidation [9]. In addition, it was reported that phe-
nanthroline derivatives possess commendable NLO activity [10].
These phenanthroline nuclei act as an acceptor group due to the
deficiency of electron density on the ring carbon atoms. These phen-
anthrimidazole derivatives exhibit high thermal stabilities making
them interesting for several applications in materials chemistry
[11,12]. Hence there is considerable interest in the synthesis of
new materials with large optical nonlinearities by virtue of their po-
tential use in device applications related to telecommunications,
In the present work, the phenanthrimidazole ligand was modi-
fied by incorporating a trifluoromethyl group into the imidazole
ring to produce a new ligand. The effects of the trifluoromethyl
substituent and the importance of its position are described. Intro-
duction of trifluoromethyl group could lower the LUMO and HOMO
energy states. The electron-withdrawing property of the fluorine
atom is expected to shift photoluminescence spectra [14,15]. The
emission colour can be tuned by changing p-conjugation strength
of the ligands and adding electron-withdrawing/electron-donating
groups on the ligand [16].
Experimental
Optical measurements and composition analysis
NMR spectra were 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 (Perkin El-
mer, Lambda 35) and corrected for background due to 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 spectra were recorded by using KBr pellets on a Avatar
⇑
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.2012.04.033

498
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
330-Thermo Nicolet FT-IR spectrometer. Mass spectrum was re-
corded using Thermo Fischer LC-Mass spectrometer.
spectra, the emission band was centered around 380 nm (1) and
400 nm (2).
The absorption bands are red shifted whereas the emission
maxima are blue shifted when compared to the parent imidazole
derivative [21]. This may be due to presence of the electron with-
drawing trifluoromethyl substituent at p-position of the aldehydic
phenyl ring i.e., the chromophore needs more energy to get ex-
cited. Because of the distorted geometry in the ground state, the
fluorescence band is hypsochromically shifted to a higher extent.
The blue shift in emission maxima indicates that the polarity of
the ground state is higher than that of the excited state for the
imidazole derivatives reported here.
Computational details
All the calculations have been performed with Gaussian 03 [17]
package. The geometry of all involved structures is fully optimized
with the DFT methodology, using B3LYP/6–31G (d,p) basis set.
Hyperpolarizability calculation
The density functional theory has been used to calculate the di-
pole moment (l), mean polarizability (a) and the total first static
hyperpolarizability (b) [18] of these two imidazole derivatives in
terms of x, y, z components using the following equations:
A multi-parameter correlation analysis [24–27] is employed in
which a physicochemical property is linearly correlated with sev-
eral solvent parameters. Solvent properties mainly affect the
photophysical properties of compounds namely polarity, H-bond
donor capacity and electron donor ability. Different scales for such
parameters can be found in the literature, Kamlet and Taft [28]
1=2
l
a
¼ ðl2x
þ
ly2
þ
lz2
Þ
ð1Þ
ð2Þ
¼ 1=3ðaxx
þ
ayy
þ
azz
Þ
proposed the p⁄
, a and b scales, Catalan et al. [29] suggested the
SPPN, SA and SB scales to describe the polarity/polarizability, the
acidity and basicity of the solvents as shown in the following
equations:
1=2
btot ¼ ðb2x þ b2y þ bz2Þ
(or)
ꢃ
ꢃ
2
2
y ¼ y0 þ aa
a
þ bbb þ cp p ðKamlet ꢁ TaftÞ
ð5Þ
btot ¼ ½ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ byzz þ byxx
Þ
2
1=2
þ ðbzzz þ bzxx þ bzyyÞ ꢂ
ð3Þ
y ¼ y0 þ aSASA þ bSBSB þ cSPPSPP ðCatalanÞ
The correlation between the absorption and fluorescence wave-
numbers calculated by the multi-component linear regression
ð6Þ
The b components of Gaussian output are reported in atomic
units and the calculated values are converted into e.s.u. units
(1 a.u. = 8.3693 ꢀ 10ꢁ33 e.s.u.).
employing the Taft-proposed solvent parameters are shown in
⁄
Fig. 2. The polarity/polarizability of the solvent (Table 2), C or
p
Natural Bond Orbital (NBO) analysis
CNSPP having a positive value shows the correlation between the sol-
vatochromic shifts and the solvent polarity. The electron releasing
capacity or basicity of the solvent, Cb or CSB, does not play an
important role in absorption and fluorescence displacements. The
coefficient representing the electron withdrawing capacity or acid-
The second order Fock matrix was carried out to evaluate the
donor–acceptor interactions in the NBO analysis [19]. For each do-
nor (i) and acceptor (j), the stabilization energy E(2) associated
with the delocalization i ? j is estimated as,
2
ity of the solvent, C or CSA has a negative value, suggesting, the
a
Fði; jÞ
Eð2Þ ¼
D
Eij ¼ q
ð4Þ
absorption and fluorescence bands shift to lower energies with
increasing acidity of the solvent. This effect can be interpreted in
terms of stabilization of resonance structures of the chromophore
(Fig. S1). The resonance structure ‘‘b’’ will be stabilized in acidic
solvents because this resonance structure is predominant in the
S1 state, and the stabilization of the S1 state with the solvent acid-
ity would be more important than that of the S0 state.
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.
Synthesis of the imidazole derivatives 1 and 2
Steric hinderance: a-twist
The imidazole derivatives were synthesized [20–23] from an
unusual four components assembling of a mixture of 1,10-Phenan-
throline-5,6-dione, ammonium acetate, aniline or 4-methoxyani-
line and 4-(Trifluoromethyl)benzaldehyde in distilled ethanol
medium (Scheme-S1). The reaction was monitored by thin layer
chromatography for completion of the reaction. The reaction mix-
ture was then extracted with dichloromethane and the resultant
resinous material was purified by column chromatography using
benzene:ethyl acetate (9:1) as eluent.
In order to discuss the geometry of phenanthrimidazole deriva-
tives, the X-ray crystal data of its analogous compound [30] is ta-
ken for comparision (Table 3). DFT calculation reveals that the
imidazole ring is essentially planar and makes dihedral angle of
87.60° (1), 87.61° (2) and 179.77° (1), 179.76° (2) with the anilinic
phenyl ring and trifluoromethylated phenyl ring for the imidazole
derivatives (1) and (2) (Fig. S2), respectively. The dihedral angle
around 87.60° with the anilinic phenyl ring and the imidazole ring
matches well with the XRD data of its parent compound (C5–N1–
C18–C19 is 82.65°). Variation of dihedral angle with imidazole and
aldehydic phenyl ring is due to the presence of substituents on
Results and discussion
Photophysical studies of the imidazole derivative
both the phenyl rings. The key twist ‘
of imidazole ring from the aryl ring at C(2), (Fig. S3). The
originates from the interaction of substituent at N(1) of the imidaz-
ole with aryl substituent (aniline ring). Due to the existence of
twist, the fluorescence quantum yield (0.30) gets lowered when
compared with its analogue compound [21]. Such a clear correla-
tion indicates the importance of coplanarity between imidazole
and aryl ring at C(2).
a
’ is used to indicate the twist
a
twist
UV–vis absorption and fluorescence spectral studies have been
carried out in different solvents for these phenanthrimidazole
derivatives (Fig. 1). In UV spectral data (Table 1), the main absorp-
tion band was centered around 270 nm (1) and 275 nm (2) with a
high molar absorption coefficient; in addition to this somewhat
weaker bands are observed around 330 nm. In fluorescence
a

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
499
Fig. 1a. Absorption spectra of the imidazole derivatives 1 and 2 in various solvents.
Fig. 1b. Emission spectra of the imidazole derivatives 1 and 2 in various solvents.
Table 1
Photophysical datas for the imidazole derivatives 1 and 2.
Imidazole derivative (1)
Imidazole derivative (2)
Solvents
kabs (nm)
kemi (nm)
Stokes shift (cmꢁ1
)
kabs (nm)
kemi (nm)
Stokes shift (cmꢁ1
)
Hexane
Dioxane
Benzene
267
266
278
272
261
266
268
269
271
266
267
373
338
377
416
384
361
396
406
400
358
336
10,714
8008
9446
12,794
12,273
9964
12,131
12,613
11,969
9732
271
275
278
276
271
274
275
277
274
274
270
374
385
382
388
415
413
410
407
399
404
427
10,162
10,390
9742
Chloroform
Ethyl acetate
Dichloromethane
1-Butanol
Ethanol
Methanol
Acetonitrile
DMSO
10,498
12,777
12,230
11,973
11,544
11,394
11,744
13,645
7691
Potential Energy Surface (PES) scan studies
conformation corresponds to the one in which p-anisidine ring at-
tached to the nitrogen atom (N1) is tilted to an angle of 87.61°
(about C5–N1–C18–C19) which is in good agreement with the
XRD results, whereas the trifluoromethylated phenyl ring attached
to the carbon atom (C2) is in the angle of 0.25° (about N3–C2–C24–
C29). The XRD report for its analogue compound shows that the
angle is around 55° (about N3–C2–C24–C29) [30]. The contraction
in theoretical value may be due to the stronger interaction of triflu-
oromethylated phenyl ring with the adjacent p-anisidine ring at-
tached to the nitrogen atom (N1) and changes the electronic
spectral properties.
The potential energy surface scan about C20–C21–O34–C35 is
performed by using B3LYP/6–31G (d,p) level for the imidazole
derivative (2). The torsional angle about C20–C21–O34–C35 for
the imidazole derivative (2) (39.87°) is also relevant coordinate
for conformation flexibility within the molecule. During the calcu-
lation all the geometrical parameters were simultaneously relaxed
while the C20–C21–O34–C35 torsional angles were varied in steps
of 0°, 10°, 20°, 30°. . .. . ...360°. The potential energy surface diagram
(Fig. 3) shows that the minimum energy (63.23 kcal/mol at 180°)

500
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
Fig. 2. Correlation between the experimental absorption (a) and fluorescence (b) wavenumber with the predicted values obtained by a multicomponent linear regression
using the p⁄
, a and b-scale (Taft) solvent parameters for the imidazole derivatives 1 and 2.
Table 2
Adjusted coefficients ((
t
x)0, ca, cb and cc) for the multilinear regression analysis of the absorption tab and fluorescence tfl wavenumbers and Stokes shift (
D
t
ss) of the imidazole
derivative 1 and 2 with the solvent polarity/polarizability, and the acid and base capacity using the Taft (p⁄
, a
and b) and the Catalan (SPPN, SA and SB) scales.
Imidazole derivative (1)
(tx)
(
tx
)
0 cmꢁ1
(
p⁄
)
c
cb
a
kab
kfl
(3.69 0.029) ꢀ 104
(2.63 0.010) ꢀ 104
(1.06 0.010) ꢀ 104
(6.86 5.309) ꢀ 103
(15.33 19.089) ꢀ 103
ꢁ(8.46 18.750) ꢀ 103
ꢁ(16.34 17.088) ꢀ 103
ꢁ(39.12 61.441) ꢀ 103
(22.77 60.349) ꢀ 103
(10.28 13.383) ꢀ 103
(24.36 48.117) ꢀ 103
ꢁ(14.08 47.262) ꢀ 103
D
tss
=
tab
ꢁ
tfl
tfl
cmꢁ1
cSA
cSB
cNSPP
(tx
)
(
tx
)
0
kabkfl
D
(3.73 0.033) ꢀ 104
(2.71 0.091) ꢀ 104
(1.02 0.087) ꢀ 104
(0.093 11.613) ꢀ 103
(11.26 31.500) ꢀ 103
ꢁ(11.165 30.082) ꢀ 103
ꢁ(0.39 48.840) ꢀ 103
ꢁ(80.26 132.473) ꢀ 103
(79.86 126.508) ꢀ 103
ꢁ(1.57 51.327) ꢀ 103
(91.95 139.218) ꢀ 103
ꢁ(93.53 132.950) ꢀ 103
tss = tab
ꢁ
Imidazole derivative (2)
(tx)
(
tx
)
0 cmꢁ1
(
p⁄
)
c
cb
a
kab
kfl
(3.63 0.019) ꢀ 104
(2.57 0.047) ꢀ 104
(1.06 0.057) ꢀ 104
ꢁ(0.42 3.480) ꢀ 103
ꢁ(0.28 85.65) ꢀ 103
ꢁ(0.14 10.27) ꢀ 103
(3.589 34.544) ꢀ 103
ꢁ(5.94 27.567) ꢀ 103
(9.54 33.057) ꢀ 103
ꢁ(3.623 8.771) ꢀ 103
(4.943 21.189) ꢀ 103
ꢁ(8.567 25.889) ꢀ 103
D
tss
=
=
tab
ꢁ
tfl
N
(tx
)
(
tx)
0 cmꢁ1
cSPP
cSA
cSB
kab
kfl
D
(3.65 0.018) ꢀ 104
(2.57 0.046) ꢀ 104
(1.07 0. 057) ꢀ 104
(34.33 6.189) ꢀ 103
ꢁ(29.96 15.983) ꢀ 103
(33.34 19.778) ꢀ 103
ꢁ(19.72 26.028) ꢀ 103
(109.02 67.215) ꢀ 103
ꢁ(128.74 83.177) ꢀ 103
(23. 90 27.383) ꢀ 103
ꢁ(101.32 70.637) ꢀ 103
(124.42 87.412) ꢀ 103
tss
tabꢁ
tfl
Comparison of
l
bo
correlated with UV–visible data in order to understand the future
optimization of microscopic NLO properties. The band around
330 nm exhibits a solvatochromic shift, characteristic of a large di-
pole moment and frequently suggestive of a large hyperpolariz-
ability due to effective charge separation. These compounds
show red shift in absorption with increasing solvent polarity,
accompanied with non-zero values in the b-components.
The imidazole derivatives shows larger
lgbo values (Table 4),
which is attributed to the positive contribution of their conjuga-
tion. These chromophore exhibits larger non-linearity and its
absorption maxima (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. It is
important to make NLO chromophores as transparent as possible
without compromising the molecule’s non-linearity.
Octupolar and dipolar components
The absorption spectra of these imidazole derivatives were
shown in the UV region around 275 nm. The increased transpar-
ency in the visible region might enable the microscopic NLO
behavior with non-zero values [34–36]. All the absorption bands
These imidazole derivatives (1 and 2) possess more appropriate
ratio of off-diagonal versus diagonal
(r = bxyy/bxxx) which reflects the inplane non-linearity anisotropy
(r = ꢁ0.16 (1) and r = ꢁ0.38 (2)) and the largest gbo values.
b tensorial component
are due to
p
?
p⁄ transitions. The computed b values (Table 4)
l

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
501
Table 3
Selected bond lengths, bond angles and torsional angles for the imidazole derivative (1).
Bond connectivity
Bond length (Å)a
Bond connectivity
Bond angle (°)
Bond connectivity
Torsional angle (°)
N1–C2
N1–C5
N1–C18
C2–N3
C2–C24
N3–C4
C4–C17
C4–C5
C5–C6
N13–C14
C27–C30
C30–F31
1.4824 (1.3752)
1.4632 (1.3840)
1.4699 (1.4402)
1.3530 (1.3168)
1.5399 (1.4782)
1.4660 (1.3789)
1.3974 (1.4330)
1.3774 (1.3770)
1.3974 (1.4321)
1.3443 (1.3230)
1.5400 (–)
N1–C5–C6
127.89 (131.74)
119.99 (119.73)
124.06 (122.78)
124.05 (124.59)
105.33 (104.52)
109.24 (105.14)
101.00 (106.40)
113.40 (128.82)
120. 60 (117.80)
122.85 (123.04)
119.99 (–)
N3–C4–C5–C6
N1–C5–C4–N3
ꢁ172.80 (176.28)
7.16 (ꢁ0.78)
ꢁ136.51 (174.37)
87. 60 (82.65)
166. 66 (179.25)
0.25 (55.54)
165.78 (ꢁ175.82)
17.34 (ꢁ0.73)
ꢁ40. 13 (6.4)
150.00 (–)
N1–C18–C19
N1–C2–C24
N3–C2–C24
C4–N3–C2
C4–C5–N1
C5–N1–C2
C5–N1–C18
C9–N10–C11
C6–C5–C4
C26–C27–C30
C27–C30–F31
C4–C5–N1–C18
C5–N1–C18–C19
C4–N3–C2–C24
N3–C2–C24–C29
C6–C5–N1–C2
C5–N1–C2–N3
C18–N1–C2–C24
C28–C27–C30–F31
C28–C27–C30–F32
C28–C27–C30–F33
ꢁ89.99 (–)
1.3500 (–)
109.47 (–)
30.00 (–)
R[F2 > 2
r
(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 its parent compound [30].
Fig. 3. The potential energy surface (PES) diagram of the imidazole derivatives 1 and 2.
Table 4
two components strongly depends on their ‘r’ ratios. The zone for
Electric dipole moment (D), polarizability (
2.
a
) and hyperpolarisability (btotal) of 1 and
r > r2 and r < r1 corresponds to a molecule of octupolar and dipolar
p
respectively. The critical values for r1 and r2 are (1ꢁ 3)/
p p
p
p p
Parameter
1
2
3
3 + 1) = ꢁ0.16 and
(
3 + 1)/ 3( 3–1) = 2.15, respectively.
Dipole moment (D)
lx
ly
lz
Complying with the Pythagorean theory and the projection closure
condition, the octupolar and dipolar components of the b tensor
can be described as:
1.6832
2.9967
0.3603
3.4558
0.8339
3.3555
ꢁ0.537
3.4991
ltotal
2
2
2D
kJ¼1 bk ¼ ð3=4Þ½ðbxxx þ bxyyÞ þ ½ðbyyy þ byxxÞ ꢂ
ð7Þ
ð8Þ
Polarizability (a)
axx
axy
ayy
axz
ayz
azz
ꢁ228.34
1.83
ꢁ155.83
ꢁ238.03
2
2
2D
ꢁ3.17
ꢁ159.86
0.92
kJ¼3 bk ¼ ð1=4Þ½ðbxxx ꢁ bxyyÞ þ ½ðbyyy ꢁ byxxÞ ꢂ
i
jjj¼32Dbjj
4.24
The parameter
q
2D; ½q2Dꢂ ¼
is convenient to compare the
ꢁ3.57
ꢁ11.97
ꢁ201.62
ꢁ27.38
jjj¼12Dbjj
ꢁ189.74
ꢁ28.31
relative magnitudes of the octupolar and dipolar components of b.
The observed positive small q2D values (1.57 (1) and 0.79 (2)) re-
veals that the biii component cannot be zero and these are dipolar
component. Since most of the practical applications for second or-
der NLO chromophores are based on their dipolar components, this
strategy is more appropriate for designing highly efficient NLO
chromophores.
a
ꢀ 10ꢁ24 (esu)
Hyperpolarisability
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
byyz
145.3
5.18
ꢁ23.59
148.65
9.23
ꢁ52.44
2.48
71.3
ꢁ7.34
13.72
10.99
50
ꢁ6.35
6.86
3.91
Donor–acceptor interactions by NBO analysis
bxzz
byzz
44.53
7.55
8.55
bzzz
9.84
149.73
517.45
9.11
141.78
496.12
Several donor–acceptor interactions [38] observed for these
imidazole derivatives (1 and 2) and the results were tabulated in
Tables 5 and 6. The important contributions to the delocalization
corresponds to the donor–acceptor interactions for the imidazole
derivative (1) and (2) are C9–N10 ? C7–C8, C12–C17 ? C15–C16,
N13–C14 ? C12–C17, C18–C19 ? C20–C21, C24–C25 ? C28–C29
& C26–C27 ? C28–C29 and for the imidazole derivative (2) are
b ꢀ 10ꢁ32 (esu)
l
b0 ꢀ 10ꢁ32 (esu)
The b tensor [37] can be decomposed in a sum of dipolar ðj¼12DbÞ
and octupolar ðj¼32DbÞ tensorial components and the ratio of these

502
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
C6–C11 ? C7–C8,
C18–C19 ? C22–C23, C20–C21 ? C22–C23
C6–C11 ? C12–C17,
N13–C14 ? C15–C16,
C26–C27 ? C28–
Electrophilicity index
&
C29. The charge distribution of these imidazole derivatives (1
and 2) (Fig. 4) shows that, among the nitrogen atoms N1 and N3,
N3 is more basic site [35]. When compared to nitrogen atoms
(N1, N3, N10 and N13), oxygen and fluorine atoms (F31, F32 and
F33) are less electronegative [39].
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, oxygen and fluorine atoms of the imidazole derivatives,
around 70% of p-character and 30% s-character have been
observed.
At the Hatree-Fock level, Koopaman’s theorm 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-
ment is the most obvious quantity to describe the polarity of a
molecule. Electronegativity of imidazole derivative is calculated
by,
l
¼ ꢁ
v
¼ ꢁðdE=dNÞVðrÞ
ð9Þ
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 Iczkowski and Margrave [44], assuming a quadrate rela-
tionship between E and N and in a finite difference approximation,
the equation can be rewritten as,
Molecular electrostatic potential map (MEP) and electronic properties
The MEP map (Fig. S4) of the imidazole derivatives (1 and 2)
shows that the nitrogen, fluorine and oxygen atoms represent
the most negative potential region. The hydrogen atoms bear the
positive charge and the predominance of green region in the MEP
surfaces corresponds to a potential halfway between the two ex-
tremes red and dark blue colour.
The 3D plots of the frontier orbitals HOMO and LUMO is shown
in Fig. S5. In compound 1, the HOMO is located on the imidazole
ring, partly on the aldehydic phenyl ring, phenanthroline ring
whereas the LUMO is located partly on the imidazole ring, phenan-
throline ring and on the aldehydic phenyl ring. In the case of com-
pound 2, the HOMO is located on the imidazole ring and partly on
the aldehydic phenyl ring, phenanthroline ring and the LUMO is lo-
cated partly on the imidazole ring, phenanthroline ring, partly on
the aldehydic phenyl ring and also on the carbon atoms (C2) of
the aniline phenyl ring. The HOMO ? LUMO transition implies that
intramolecular charge transfer takes place [41] within the mole-
cule. The energy gap (Eg) of these imidazole derivatives have been
calculated from the HOMO and LUMO levels. The energy gap ex-
plains the probable charge transfer (CD) inside the chromophore.
v
¼ ꢁl
¼ ðI þ AÞ=2
ð10Þ
0
vkoopaman s ¼ ðEHOMO þ ELUMOÞ=2
ð11Þ
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,
l2=2
This electrophilicity index measures the capacity of electrophile
x
¼
g
ð12Þ
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
l
= ꢁ0.281,
ative (1) and
imidazole derivative (2).
g
= ꢁ0.0498 and
x
= ꢁ0.7930 for the imidazole deriv-
l
= ꢁ0.281,
g
= ꢁ0.0499 and = ꢁ0.7906 for the
x
Table 5
Significant donor–acceptor interactions and their second-order perturbation energies (kcal/mol) of the imidazole derivatives 1 and 2.
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)
C7–C8
C7–C8
C9–N10
C4–C5
C5–C6
C14–N13
C18–C19
C20–C21
C20–C21
C24–C25
C24–C25
C28–C29
C28–C29
C26–C27
LpC6
1.7091
1.7091
1.7431
1.7678
1.7069
1.7460
1.6647
1.6527
1.6527
1.6400
1.6400
1.6395
1.6395
1.6635
1.0375
1.0375
0.9486
0.9486
1.7431
1.6001
1.7460
1.7460
1.6647
1.6647
1.6400
1.6635
LpC6
C9–N10
LpC11
1.0375
0.3153
0.9486
1.0375
0.3137
0.3822
0.3440
0.3560
0.3440
0.2963
0.3557
0.3634
0.3557
0.2963
0.2488
0.2888
0.3153
0.3822
0.2488
0.2443
0.3822
0.2443
0.2888
0.3211
0.2963
0.2963
68.46
46.71
84.35
54.25
47.09
44.77
41.83
44.96
43.06
40.18
41.80
42.16
45.36
38.85
91.11
80.59
105.83
55.23
151.96
294.04
361.41
141.42
46.40
464.06
549.37
564.45
0.26
0.50
0.31
0.27
0.50
0.55
0.49
0.47
0.48
0.49
0.48
0.47
0.48
0.50
0.27
0.27
0.22
0.24
0.04
0.02
0.02
0.04
0.04
0.01
0.01
0.01
0.150
0.137
0.180
0.141
0.137
0.145
0.129
0.130
0.128
0.128
0.127
0.126
0.131
0.126
0.175
0.161
0.171
0.126
0.124
0.120
0.124
0.124
0.077
0.126
0.123
0.123
C7–C8
1.7166
1.6068
1.7558
1.7151
1.7656
1.7120
1.6975
1.6561
1.6561
1.6503
1.6503
1.6484
1.6484
1.6741
1.6741
1.6494
1.6494
1.9690
1.6068
1.6068
1.6068
1.7656
1.8684
1.7120
1.6561
1.6741
C9–N10
C7–C8
0.2991
0.2574
0.3842
0.2915
0.3646
0.2964
0.3502
0.3611
0.2964
0.2898
0.3547
0.3453
0.3547
0.3453
0.2898
0.3397
0.3180
0.3502
0.2574
0.3397
0.3646
0.2387
0.3397
0.2964
0.2964
0.2898
46.71
40.19
44.99
46.94
42.26
44.79
45.97
53.88
30.98
38.06
44.98
42.89
44.05
39.06
39.76
50.34
81.25
42.19
431.41
254.52
304.11
500.67
159.74
351.38
511.06
467.67
0.52
0.50
0.56
0.52
0.57
0.52
0.50
0.49
0.51
0.50
0.49
0.49
0.49
0.51
0.52
0.57
0.54
0.66
0.01
0.02
0.02
0.01
0.02
0.02
0.01
0.01
0.140
0.130
0.147
0.139
0.144
0.137
0.137
0.146
0.114
0.126
0.133
0.130
0.131
0.126
0.129
0.151
0.189
0.159
0.117
0.106
0.107
0.124
0.092
0.127
0.124
0.126
C6 –C11
C9–N10
C15–C16
C14–N13
C18–C19
C22–C23
C20–C21
C20–C21
C24–C25
C24–C25
C28–C29
C28–C29
C26–C27
C26–C27
LpN1
C6–C11
C14–N13
C12–C17
C22–C23
C20–C21
C18–C19
C22–C23
C28–C29
C26–C27
C24–C25
C26–C27
C24–C25
C28–C29
C4–C5
LpC6
C14–N13
C12–C17
C22–C23
C18–C19
C22–C23
C28–C29
C26–C27
C24–C25
C26–C27
C28–C29
C7–C8
LpC6
LpC11
LpC11
C4–C5
C9–N10
C12–C17
C7–C8
C15–C16
C12–C17
C15–C16
C4–C5
C20–C21
C28–C29
C28–C29
LpN1
LpO34
N3–C2
C20–C21
C7–C8
C9–N10
C12–C17
C14–N13
C14–N13
N3–C2
C18–C19
C24–C25
C24–C25
C6 –C11
C6 –C11
C6 –C11
C14–N13
N3–C2
C18–C19
C20–C21
C26–C27
C4–C5
C12–C17
C15–C16
C4–C5
C22–C23
C22–C23
C28–C29
Donor type (p
); Acceptor type (p⁄).

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 497–504
503
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)
C4–C5
ED/Energy (a.u.)
EDA%
EDB%
s%
p%
Bond (A–B)
C4–C5
ED/Energy (a.u.)
EDA%
EDB%
s%
p%
0.7107
0.7035
0.6149
0.7886
0.6473
0.7623
0.7952
0.6064
0.7691
0.6392
0.7871
0.6169
0.7012
0.7130
0.7086
0.7056
0.5262
0.8504
0.5259
0.8506
0.5258
0.8506
–
50.51
–
37.81
–
41.89
–
63.23
–
59.15
–
61.95
–
49.16
–
50.21
–
49.49
–
62.19
–
58.11
–
36.77
–
40.85
–
38.05
–
50.84
–
49.79
–
36.22
34.55
25.33
27.13
25.90
27.58
26.37
28.25
35.65
32.39
28.27
20.92
38.91
29.01
27.10
34.06
21.90
26.38
21.95
26.60
21.95
26.62
–
63.71
65.39
74.51
72.82
73.98
72.34
73.58
71.57
64.27
67.46
71.69
73.96
61.02
70.96
72.84
65.88
77.68
73.53
77.63
73.31
77.63
73.29
–
0.7134
0.7227
0.6074
0.7944
0.6445
0.7646
0.7997
0.6440
0.7668
0.6419
0.7949
0.6068
0.7046
0.7096
0.7225
0.6914
0.5004
0.8658
0.5000
0.8660
0.4997
0.8662
0.5683
0.8228
0.8334
0.5526
50.90
–
36.89
–
41.53
–
63.96
–
58.79
–
63.18
–
49.65
–
52.20
–
25.04
–
25.00
–
24.97
–
32.30
–
49.10
–
63.11
–
58.47
–
36.04
–
41.21
–
36.82
–
50.35
–
47.80
–
74.96
–
75.00
–
75.03
–
67.70
–
35.05
34.83
27.39
32.59
29.34
33.39
33.11
30.12
37.56
33.55
34.20
26.77
36.24
30.56
28.67
35.11
21.56
32.32
21.64
32.77
21.64
32.89
25.56
35.22
30.97
21.10
64.86
65.08
72.43
67.34
70.57
66.27
66.82
69.68
62.08
66.37
65.74
73.05
63.67
69.34
71.19
64.74
77.72
67.55
77.65
67.11
77.65
66.98
74.10
64.67
68.92
78.49
N1–C5
N3–C4
N1–C2
C2–N3
N1–C18
C2–C24
C27–C30
C30–F31
C30–F32
C30–F33
–
N1–C5
N3–C4
N1–C2
C2–N3
N1–C18
C2–C24
C27–C30
C30–F31
C30–F32
C30–F33
C21–O34
O34–C35
27.69
–
27.65
–
72.31
–
72.35
–
27.64
72.36
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
69.46
–
30.54
–
Fig. 4. Bar diagram representing the charge distribution of 1 and 2 using NLO and NBO methods.
Conclusion
Acknowledgment
Trifluoromethyl substituted phenanthrimidazole ligands have
been synthesized and characterized using IR, mass and NMR
spectral studies. The introduction of trifluoromethyl substituent
in imidazole derivatives resulted in blue shift in emission when
compared to its parent compound. This is due to the presence
of the electron withdrawing trifluoromethyl moiety which
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.
changes the
p–p conjugation of the imidazole ligand and lowers
Appendix A. Supplementary data
the triplet energy level and lowering the lowest unoccupied
molecular orbital (LUMO) level. The optimized bond lengths,
bond angles and dihedral angles calculated by DFT/B3LYP/6–
31G (d,p) method are found to be slightly higher than that of
XRD values of its parent compound. NBO, NLO, MEP,
HOMO–LUMO energies, chemical potential, hardness and elec-
trophilicity index have been calculated theoretically and dis-
cussed in detail.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.04.033.
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