Physico-chemical studies of fused phenanthrimidazole derivative as sensitive NLO material

Article abstract of DOI:10.1016/j.saa.2012.09.089

Heterocyclic phenanthrimidazole derivative, 2-(4-fluorophenyl)-1-p-tolyl- 1H-imidazo[4,5-f] [1,10] phenanthroline (FPTIP) has been synthesized and characterised by NMR, mass and CHN analysis. The FPTIP was evaluated concerning their solvatochromic propert

Full text of DOI:10.1016/j.saa.2012.09.089

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 249–253
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Physico-chemical studies of fused phenanthrimidazole derivative as sensitive
NLO material
⇑
Jayaraman Jayabharathi , Venugopal Thanikachalam, Ramalingam Sathishkumar,
Karunamoorthy Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar, 608 002 Tamil Nadu, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Catalan–Taft Parameter shows
acidity of the solvent, Cb or CSB has a
negative value.
The steric interaction must be
reduced in order to obtain larger b0
values.
NBO analysis shows the
intramolecular, hybridization and
delocalization within the molecule.
Probable charge transfer (CD) taking
place inside the chromophore.
The charge distribution was
calculated by the NBO and Mulliken
methods.
"
"
a r t i c l e i n f o
a b s t r a c t
Article history:
Heterocyclic phenanthrimidazole derivative, 2-(4-fluorophenyl)-1-p-tolyl-1H-imidazo[4,5-f] [1,10] phe-
nanthroline (FPTIP) has been synthesized and characterised by NMR, mass and CHN analysis. The FPTIP
was evaluated concerning their solvatochromic properties and molecular optical nonlinearities. Their
Received 14 June 2012
Received in revised form 22 September
2012
Accepted 26 September 2012
Available online 5 October 2012
electric dipole moment (
cally and the results indicate that the extension of the
l
), polarizability (
a
) and hyperpolarizability (b) have been calculated theoreti-
-framework of the ligands has an effect on the
p
NLO properties. The energies of the HOMO and LUMO levels and the molecular electrostatic potential
(MEP) energy surface studies have exploited the existence of intramolecular charge transfer (ICT) within
the molecule.
Keywords:
Phenanthrimidazole
NLO
Ó 2012 Elsevier B.V. All rights reserved.
HOMO–LUMO
MEP
ICT
Introduction
have found many applications [1]. Aryl-imidazo-phenanthrolines
play important role in materials science and medicinal chemistry
New material properties can be achieved, when new conjugated
systems are composed by different heterocyclic nuclei, which al-
low the fine tuning of important photophysical properties. As a re-
sult of the optical and conductive properties, conjugated materials
containing thiophene, imidazole and phenanthroline heterocycles
due to their optoelectronic properties [2–4]. They are used as li-
gands for the synthesis of metal complexes of ruthenium(II), cop-
per(II), cobalt(II), nickel(II), manganese(II), iridium(III) and several
lanthanides for nonlinear optical (NLO) applications. They are
important building blocks for the synthesis of proton, anion and
cation sensors. They have diverse biological applications such as
probes of DNA structure and new therapeutic agents due to their
capacity to bind or interact with DNA [4].
⇑
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.2012.09.089

250
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 249–253
Our approach to design new
p-conjugated systems for several
Results and discussion
potential optical applications is based on the use of six membered
aromatics and imidazoles in the conjugation pathway, combined
with electron deficient heterocycles such as phenanthroline. These
nuclei act as an acceptor group due to the deficiency of electron
density on the ring carbon atoms. These imidazole derivatives ex-
hibit high thermal stabilities making them interesting for several
applications in materials chemistry [5,6]. Hence there is consider-
able interest in the synthesis of new materials with large optical
nonlinearities by virtue of their potential use in device applications
related to telecommunications, optical computing, optical storage,
and optical information processing [7–10]. Herein we focussed on
the synthesis, physico-chemical and theoretical studies of aryl imi-
dazophenanthroline derivative and have them shown as potential
candidates for NLO materials by virtue of their high harmonic
generation.
Photophysical studies of FPTIP
Absorption and fluorescence spectra of FPTIP are shown in
Fig. 1. In apolar solvents, the main absorption band is appeared
at 290 nm, whereas the fluorescence spectrum is appeared at
390 nm. The fluorescence quantum yield is around 0.40. The ob-
served red shift absorption maxima (kmax) suggest an increase of
molecular hyperpolarizability, according to the theoretical NLO
studies. The absorption maxima (kmax) have been compared with
the p⁄ values for each solvent, determined by Kamlet et al. The ob-
served results show that the imidazole derivative exhibited posi-
tive solvatochromism with respect to their charge transfer (CT)
absorption band. The position of the absorption maximum shifted
to longer wavelength as the polarity of the solvent increased due to
a greater stabilization of the excited state relative to the ground
state with an increase of solvent polarity.
Experimental section
Taft and Catalan solvatochromism
Materials and methods
A multi-parameter correlation analysis is employed in which a
physico-chemical property is linearly correlated with several sol-
vent parameters by means of the following equation:
1,10-Phenanthroline-5,6-dione has been synthesized and puri-
fied according to the reported literature procedure [11]. 4-Fluoro-
benzaldehyde, 4-methylaniline and all the other reagents have
been purchased from S.D. fine chemicals and used without further
purification.
ðXYZÞ ¼ ðXYZÞ0 þ CaA þ CbB þ CcC þ . . .
ð1Þ
NMR spectra have been recorded on a Bruker 400 MHz NMR
instrument. Absorption and fluorescence spectra have been re-
corded on Perkin Elmer spectrophotometer Lambda 35 and Perkin
Elmer LS55 spectrofluorimeter, respectively. Fluorescence spectra
were corrected from the monochromator wavelength dependence
and the photomultiplier sensibility. Fluorescence quantum yield (/
) have been determined by means of the corrected fluorescence
spectra of a dilute solution of imidazole derivative in dichloro-
methane, using coumarin 47 in ethanol as a reference and by tak-
ing into account the solvent refractive index.
where (XYZ)0 is the physico-chemical property in an inert solvent
and Ca, Cb, Cc and so forth are the adjusted coefficients that reflect
the dependence of the physico-chemical property (XYZ) on several
solvent properties. Solvent properties that mainly affect the photo-
physical properties of aromatic compounds are polarity, H-bond do-
nor capacity and electron donor ability. Different scales for such
parameters can be found in the literature, Taft et al. [13]. propose
the p⁄
, a and b scales, whereas more recently Catalan et al. [14] sug-
gest the SPPN, SA and SB scales to describe the polarity/polarizabil-
ity, the acidity and basicity of the solvents as shown in the following
equations:
þ bbb þ cꢀppꢀ
ðKamlet-TaftÞ
ð2Þ
Computational details
Y ¼ y0 þ aa
a
Quantum mechanical calculations have been used to carry out
the optimized geometry, NBO, NLO, MEP and HOMO–LUMO analy-
sis with Gaussian-03 program using the Becke3–Lee–Yang–Parr
(B3LYP) functional supplemented with the standard 6-31G(d,p) ba-
sis set [12].
Y ¼ y0 þ aSASA þ bSBSB þ cSPPSPP
The correlation (Figs. 2a and 2b) of the absorption as well as
fluorescence wavenumbers obtained by the multi-component lin-
ear regression employing the Taft and Catalan solvent parameters
ðCatalanÞ
ð3Þ
Fig. 1. Absorption and emission spectrum of FPTIP.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 249–253
251
Fig. 2a. Correlation between the experimental absorption wavenumber with the
predicted values obtained by a multicomponent linear regression using the p⁄
and b-scale (Taft) solvent parameters of FPTIP.
, a
Fig. 3. Resonance structure of FPTIP.
Second harmonic generation (SHG) studies
Second harmonic signal of 45 mV were obtained for FPTIP 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 [15,16]. 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.
Comparison of lbo
Theoretical investigation plays an important role in under-
standing the structure–property relationship, which is able to as-
sist in designing novel NLO chormophores. The electrostatic first
Fig. 2b. Correlation between the experimental fluorescence wavenumber with the
predicted values obtained by a multicomponent linear regression using the p⁄
and b-scale (Taft) solvent parameters of FPTIP.
, a
hyperpolarizability (b) and dipole moment (l) of the imidazole
chromophores have been calculated by Gaussian 03 package. From
Table 2, it is found that the imidazole chromophore show larger
with the experimental values given in Table 1. The polarity/polar-
izability of the solvent coefficient, [C or CNSPP] value correlating the
lbo values, which is attributed to the positive contribution of their
p
⁄
conjugation. Therefore, it is clear that the hyperpolarizability is a
strong function of the absorption maximum. The b values com-
puted here might be correlated with UV–visible spectroscopic data
in view of a future optimization of the microscopic NLO properties.
The solvatochromic shift exhibits, characteristic of a large dipole
moment and frequently suggestive of a large hyperpolarizability.
This compound show red shift in absorption with increasing sol-
vent polarity, accompanied with the upward shifts non-zero values
in the b-components [17].
solvatochromic shifts with the solvent polarity. The coefficient
controlling the H-donor capacity or acidity of the solvent [C or
a
CSA] is the lowest coefficient; therefore, the solvent acidity does
not play an important role in absorption and fluorescence displace-
ments. The coefficient representing the electron releasing ability or
basicity of the solvent [Cb or CSB] has a negative value suggesting
that the absorption and fluorescence bands shift to lower energies
with the increasing electron-donating ability of the solvent. This
effect can be interpreted in terms of the stabilization of the reso-
nance structures of the chromophore (Fig. 3). Resonance structure
‘‘II’’ has the positive charge located at the nitrogen atom and it will
be stabilized in basic solvents because this resonance structure is
predominant in the S1 state and the stabilization of the S1 state
with the solvent basicity would be more important than that of
the S0 state. Consequently, the energy gap between the S1 and S0
states decreases and the absorption and fluorescence wavelengths
shift to longer wavelength with increasing solvent basicity.
Octupolar and dipolar components
FPTIP possess a more appropriate ratio of off-diagonal versus
diagonal b tensorial component (r = bxyy/bxxx) [r = ꢁ0.23] which re-
flects the inplane non-linearity anisotropy and the largest lbo val-
ues. The difference of the bxyy/bxxx ratios can be well understood by
analyzing their relative molecular orbital properties. Hence, the
Table 1
Adjusted coefficients ((tx)0, ca, cb and cc) and correlation coefficients (r) for the multilinear regression analysis of the absorption tab and fluorescence tfl wavenumbers and stokes
shift (
D
t
ss) of FPTIP with the solvent polarity/polarizability, and the acid and base capacity using the Taft (p⁄
cmꢁ
,
a
and b) and the Catalan (SPPN, SA and SB) scales.
(tx)
(tx
)
0
(
p⁄
)
c
cb
r
a
kab
kfl
(3.515 0.037) ꢂ 104
(2.498 0.021) ꢂ 104
(1.016 0.052) ꢂ 104
ꢁ(3.546 2.488) ꢂ 103
(2.877 4.323) ꢂ 103
ꢁ(6.423 10.382) ꢂ 103
(14.995 25.027) ꢂ 103
ꢁ(9.111 14.448) ꢂ 103
(24.106 34.700) ꢂ 103
CSA
ꢁ(12.899 20.103) ꢂ 103
(6.680 11.405) ꢂ 103
ꢁ(19.579 27.872) ꢂ 103
CSB
0.74
0.85
0.68
r
D
tss
=
=
tab
–
–
tfl
(tx)
cmꢁ1
0
CNSPP
(tx)
kab
kfl
(3.520 0.032) ꢂ 104
(2.506 0.019) ꢂ 104
(1.013 0.045) ꢂ 104
ꢁ(0.861 11.911) ꢂ 103
ꢁ(1.494 6.925) ꢂ 103
(0.632 16.693) ꢂ 103
(4.806 49.420) ꢂ 103
(0.585 28.735) ꢂ 103
(4.221 69.259) ꢂ 103
ꢁ(2.538 51.523) ꢂ 103
(3.010 29.958) ꢂ 103
ꢁ(5.549 72.207) ꢂ 103
0.57
0.52
0.87
D
tss
tab
tfl

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J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 249–253
Table 2
Electric dipole moment (
l
), polarizability (
a
) and hyperpolarisability (b) of FPTIP.
Parameter
(1)
Dipole moment (l)
lx
ly
lz
ltotal
2.5039
2.5475
0.9719
3.7019
Polarizability (a)
axx
axy
ayy
axz
ayz
azz
ꢁ208.32
14.0136
ꢁ153.68
ꢁ1.1925
ꢁ13.1
ꢁ167.39
ꢁ2.4155
a
ꢂ 10ꢁ24 (esu)
Hyperpolarisability (b)
bxxx
bxxy
bxyy
byyy
11.913
30.1064
ꢁ25.449
1.6654
Fig. 4. Bar diagram representing the charge distribution of FPTIP using NLO and
NBO methods.
bxxz
bxyz
byyz
bxzz
byzz
bzzz
12.767
25.2539
25.6008
18.2333
17.7437
9.5331
b(tot) ꢂ 10ꢁ32 (esu)
59.6571
220.845
l
b(tot) ꢂ 10ꢁ32 (esu)
Table 3
Selected bond lengths, bond angles and torsional angles for FPTIP.
Bond Bond Bond Bond Bond
Torsional
connectivity length
(Å)
connectivity angle (°) connectivity angle (°)
C9–N22
C5–N20
N21–C31
C6–N21
C9–C8
1.4679 C9–N22–
105.2068 C9–N22–
166.1229
C31
1.3442 C6–N21–
C31
C31–C32
101.3175 C6–N21–
C31–C32
ꢁ162.3487
17.2203
ꢁ13.4461
179.7565
ꢁ0.242
1.4777 N22–C9–C6
109.707
C6–N21–
C31–N22
1.4647 N21–C31–
111.2931 N21–C31-
N22–C9
124.3621 N21–C31–
C32–C33
124.3432 N21–C31–
C32–C34
N22
1.3997 N21–C31–
C32
1.3995 N22–C31–
C34
C6–C3
N21–C23
C31–C32
C6–C9
1.47
C31–C32–
C33
C31–C32–
C34
119.9998 C3–C6–
N21–C31
120.0001 C8–C9–
N22–C31
113.4589 N22–C9–
C8–C7
123.1356 N21–C6–
C3–C4
118.3357 C1–C5–
N20–C4
119.9998 C7–N19–
C13–C12
120.0001 N19–C7–
C4–N20
120.5316 C8–C7–C4–
N20
165.6809
ꢁ175.0181
174.9208
175.9635
0.0584
1.54
Fig. 5. MEP surface diagram of FPTIP.
1.3405 C6–N21–
C23
1.54
of the components strongly depends on their ‘r’ ratios. The zone
for r > r2 and r < r1 corresponds to a molecule of octupolar and
dipolar respectively. The critical values for r1 and r2 are ꢁ0.16
and 2.15, respectively. Complying with the Pythagorean theory
and the projection closure condition, the octupolar and dipolar
components of the b tensor can be described as:
C30–C45
N19–C7
N19–C13
N20–C4
N20–C5
C6–C9–C8
1.3393 C4–C3–C6
1.3442 C28–C30–
C45
1.3393 C26–C30–
C45
1.3442 C7–N19–
C13
0.0328
1.5348
ꢀ
ꢀ
2
2
2D
ꢀ
ꢀ
jj¼1 b j ¼ ð3=4Þ½ðbxxx þ bxyyÞ þ ½ðbyyy þ byxxÞ ꢃ
ð4Þ
ꢁ178.2515
179.3393
180
ꢀ
ꢀ
2
2
2D
ꢀ
ꢀ
C4–N20–C5
120.5317 C25–C28–
C30–C45
121.2943 C24–C26–
C30–C45
jj¼1 b j ¼ ð1=4Þ½ðbxxx ꢁ 3bxyyÞ þ ½ðbyyy ꢁ byxxÞ ꢃ
ð5Þ
h
i
j¼3b
j¼1b
2D
N19–C7–C4
The parameter q2D q2D ¼
is convenient to compare the
relative magnitudes of the oc2tDupolar and dipolar components of
b. The observed positive small q2D value for compound (1.342) re-
veals that the biii component cannot be zero and these are dipolar
components. Since most of the practical applications for second or-
der NLO chromophores are based on their dipolar components and
this strategy is more appropriate for designing highly efficient NLO
chromophores.
steric interaction must be reduced in order to obtain larger b0
values.
The b-tensor [15] can be decomposed in a sum of dipolar
ðj¼12DbÞ and octupolar ðj¼32DbÞ tensorial components and the ratio

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 249–253
253
Fig. 6. HOMO–LUMO molecular orbitals pictures of FPTIP.
Natural Bond Orbital (NBO) analysis
Conclusions
NBO analysis [16] have been performed for FPTIP at the DFT/
B3LYP/6-31++G (d,p) level in order to elucidate the intramolecular,
charge transfer within the molecule and the results have been tab-
ulated in Table 3. Several donor–acceptor interactions have been
observed in FPTIP and among the strongly occupied NBOs, the most
The observed emission wavelength reveals that the existence of
twist drops the fluorescence quantum yield which indicate the
a
importance of non-coplanarity between the imidazole and the aryl
ring at C(2). The electron releasing ability or basicity of the solvent,
[Cb or CSB] has a negative value, suggesting that the absorption and
fluorescence bands shift to lower energies with the increasing elec-
tron donating ability of the solvent. The solvatochromic shift
exhibits, characteristic of a large dipole moment and frequently
suggestive of a large hyperpolarizability. The calculated dipolar
and octupolar components supported imidazole derivative is effi-
cient NLO chromophores. The observed positive q2D value for com-
pound (1.342) show that the biii component cannot be zero and it is
dipolar component.
important delocalization sites are in the
pairs (n) of the, fluorine and nitrogen atoms. The
p
system and in the lone
system shows
r
some contribution to the delocalization and the important contri-
butions to the delocalization corresponds to the donor–acceptor
interactions for FPTIP are C1–C2 ? C5–N20, C3–C4 ? C9–C9, C5–
N20 ? C3–C4, C7–C8 ? C10–C12, C10–C12 ? C13–N19, C23–
C24 ? C25–C28,
C26–C30 ? C23–C24,
C32–C34 ? C33–C35,
C36–C38 ? C32–C34, LP1–N21 ? N22–C31, LP3–F44 ? C36–C38.
The charge distribution of FPTIP has been calculated from the
atomic charges by NBO and NLO analyses (Fig. 4). These methods
reveal that among the nitrogen atoms N15, N16 and fluorine atom,
N15 is considered as more basic site. When compared to nitrogen
atoms (N13, N14, N15 and N16), fluorine atoms are less electroneg-
ative [18]. The charge distribution shows that the more negative
charge is concentrated on N15 atom whereas the partial positive
charge resides at hydrogen atoms.
Acknowledgments
One of the authors Prof. J. Jayabharathi is thankful to DST [No.
SR/S1/IC-73/2010], UGC (F. No. 36-21/2008 (SR)) and DRDO (F.
No. NRB-213/MAT/10-11) for providing funds to this research
study. Mr. R. Sathishkumar is thankful to DRDO (F. No. NRB-213/
MAT/10-11) for providing fellowship.
Molecular electrostatic potential map (MEP) and electronic properties
Appendix A. Supplementary material
Molecular electrostatic potential (MEP) maps (Fig. 5) show the
distribution of charges across the surface of a molecule. This dia-
gram is used to understand the reactive behaviour of a molecule,
in that negative regions can be regarded as nucleophilic centres,
whereas the positive regions are potential electrophilic sites. The
MEP map of the imidazole derivative shows that the nitrogen
and fluorine atoms represent the most negative potential region.
The hydrogen atoms bear the positive charge and the predomi-
nance of green region in the MEP surfaces corresponds to a poten-
tial halfway between the two extremes1 red and dark blue colour.
The HOMO orbital acts as an electron donor and the LUMO orbi-
tal that acts as the electron acceptor. The 3D plots of the frontier
orbitals HOMO and LUMO is shown in Fig. 6. In imidazole com-
pound, the HOMO is located on the aldehydic phenyl ring, imidaz-
ole ring and partly on the phenanthroline ring and the LUMO is
located partly on the phenanthroline ring and on the carbon atoms
(C2 and C5) of the imidazole ring. The HOMO ? LUMO transition
implies that intramolecular charge transfer takes place [19] within
the molecule. The energy gap (Eg) of the imidazole derivative has
been calculated from the HOMO and LUMO levels. The energy
gap explains the probable charge transfer (CD) inside the
chromophore.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.09.089.
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