An investigation on the key features of a D-π-A type novel chalcone derivative for opto-electronic applications

Article abstract of DOI:10.1039/c5ra13494c

The current study is focused on the donor-bridge-acceptor (D-π-A) type of novel organic charge transport and non-linear optical material, 1-(4-bromophenyl)-3-(2,4,5-trimethoxyphenyl) prop-2-en-1-one (2,4,5-TMBC) to spotlight its various important properti

Full text of DOI:10.1039/c5ra13494c

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Investigation on the key features of DꢀπꢀA type novel chalcone derivative for optoꢀ
electronic applications
Mohd. Shkira,d*, Shabbir Muhammada,d*, Salem AlFaifya,d, Ahmad Irfanc,d, Parutagouda
Shankaragouda Patilb, Manju Arorae, Hamid Algarnia, Zhang Jingpingf
aDepartment of Physics, Faculty of Science, King Khalid University, P.O. Box. 9004 Abha 61413, Saudi Arabia
bDepartment of Physics, K. L. E. Institute of Technology, Opposite Airport, Gokul, Hubli 580 030, India
cDepartment of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box 9004, Saudi Arabia
dResearch Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413,
Saudi Arabia.
eCSIRꢀNational Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India
fInstitute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Renmin Road 5268,
Changchun, Jilin 130024, P.R. China
Abstract
The current work is focused on donorꢀbridgeꢀacceptor (DꢀπꢀA) type novel organic charge
transport and nonlinear optical material 1ꢀ(4ꢀbromophenyl)ꢀ3ꢀ(2,4,5ꢀtrimethoxyphenyl) propꢀ2ꢀ
enꢀ1ꢀone (2,4,5ꢀTMBC) to spotlight its various important properties through experimental and
quantum chemical approaches. The compound 2,4,5ꢀTMBC was synthesized by ClaisenꢀSchmidt
condensation reaction and its single crystal was grown by slow evaporation solution growth
technique. FTꢀIR and FTꢀRaman spectra of 2,4,5ꢀTMBC were recorded and investigated. The
molecular geometry of 2,4,5ꢀTMBC was optimized by HF, B3LYP, CAMꢀB3LYP, wb97xd and
LCꢀBLYP methods using 6ꢀ31G* basis set. The calculated geometrical parameters and
vibrational spectra are in good agreement with the experimental ones. The time dependent
density functional theory (TDꢀDFT) has been applied to investigate the optical properties of the
titled compound. The absorption wavelength calculated at TDꢀB3LYP/6ꢀ31G* level of theory in
gas phase was found in good covenant with the experimental value (~400nm) as compared with
other methods. The HOMOꢀLUMO energy gap was calculated at all the applied levels of theory.
The total dipole moment, polarizability, anisotropy of polarizability and static first and total
hyperpolarizability values of 2,4,5ꢀTMBC molecule were calculated at different levels of theory.
The dipole moment and first hyperpolarizability values are found to be many folds (2 and 56
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times calculated at B3LYP) higher than urea molecule. It is also expected that 2,4,5ꢀTMBC
would be electron transport material due to the smaller electron reorganization energy value. The
study of nonlinear optical (NLO) properties shows that the 2,4,5ꢀTMBC molecule would be an
outstanding candidate for NLO device applications.
Keywords: Chalcones; FTꢀRaman spectroscopy; Optical properties; nonlinear optical material;
Density functional theory
1. Introduction
The chalcones (1, 3ꢀdiarylꢀ2ꢀpropenꢀ1ꢀones) are open chain flavonoids that are extensively
biosynthesized in plants. In recent years an extended scientific research on chalcone pageant
fascinating biological properties such as antiꢀulcer, antiꢀcancer, antimitotic, antiꢀinflammatory,
antiꢀmalarial, antiꢀfungal, antiꢀHIV and antioxidant activities 1ꢀ8. Moreover, the chalcone
molecules with a πꢀconjugated system that provides a large charge transfer axis. The apposite
groups of molecules on two aromatic rings act as donors and acceptors 9, 10. In such molecules
the charges can be transferred from donor to acceptor through the charge transfer axis. The
resilient intermolecular interactions in any molecule, giving upswing to second harmonic
11,
generation (SHG) efficiency
12. Chalcones are having exceptional optical possessions
including high extinction coefficients for absorption in the ultra violet (UV), and noteworthy
nonlinear optical (NLO) responses13ꢀ15
.
Because of the substantial importance of these compounds, the NLO properties of a series of
chalcones have recently been examined experimentally and shown encouraging outcomes in the
future application like optical limiters and nonlinear frequency conversion 16ꢀ21. To develop such
systems for future device applications, it is important to fathom the fundamental characteristics
about their structural and electronic basis of the optical properties.
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To explain the structural and electronic properties in an immense class of materials, ranging from
atoms and molecules to crystals and complex prolonged structures the density functional theory
(DFT) has been proved to be tremendously efficacious and it is also computationally modest.
Due to such reasons, DFT has been developed as mutual contrivance in firstꢀprinciples
calculations directed to predict and describe the characteristic properties of molecular and
condensed matter schemes. In recognizing the ingredients and providing information about the
composition, structure, conformation, functional groups and intraꢀmolecular interactions in new
as well as existing complexes; the vibrational spectroscopy has become an excellent tool. DFT
provides an irreplaceable spectrum for every definite compound. Many studies have been
performed by HartreeꢀFock (HF) as well as DFT on various organic, semiorganic, inorganic,
chalcones, and their complexes and correlated with experimental reports 22ꢀ40
.
Recently, Patil et al., had synthesized and grown the bulk single crystals of a new NLO chalcone
derivative 1ꢀ(4ꢀbromophenyl)ꢀ3ꢀ(2,4,5ꢀtrimethoxyphenyl) propꢀ2ꢀenꢀ1ꢀone (2,4,5ꢀTMBC),
41, 42
which explores its possible application for NLO devices
.
This chalcone derivative
possesses soꢀcalled donorꢀbridgeꢀacceptor (DꢀπꢀA) type structure. The 2,4,5ꢀtrimethoxy acts as a
donor at one end and 1ꢀ(4ꢀbromophenyl)ethanone with C=O bond and the bromo group is an
acceptor at the other terminal end of above titled molecule (Fig. 1).
OCH3
O
H3CO
Br
OCH3
Fig. 1. Chemical structure of 2,4,5ꢀTMBC.
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As per the current available literature, no theoretical study has been performed on 2,4,5ꢀTMBC
molecule so far, which is necessary to have complete understanding about the compound from
various applications point of view.
Therefore, in the current investigation, our aim is to limelight the molecular structural, IR and
Raman spectroscopy (experimental and theoretical), electronic, charge transport and nonlinear
optical properties of DꢀπꢀA type molecule 2,4,5ꢀTMBC. The obtained results have been
compared with the experimental data where available.
2. Experimental details
Initially the synthesis of 2,4,5ꢀTMBC achieved by the condensation of 2,4,5ꢀ
trimethxybenzaldehyde (0.01 mol) with 4ꢀbromoacetophenone (0.01 mol) in 60 ml ethanol in the
company of a catalytic amount of sodium hydroxide solution (5 mL, 20%). The continuous
stirring was done for more than 2 h and further the substances of the flask were transferred into
500 mL iceꢀcold water and left to stand for 12 h. The synthesized crude solid was filtered,
successively washed with the solution of dilute HCl and distilled water, and then recrystallized it
more than twice from acetone to get the pure chalcone. The synthesized 2,4,5ꢀTMBC was further
used to grow the single crystals by solution growth technique with slow evaporation at constant
temperature. The more details are available in previous report 42.
Fourier Transform Infrared spectra were measured in transmission mode in the range 4000–400
cmꢀ1 using a NICOLET 6700 FTꢀIR spectrometer at ambient temperature. The samples were
taken in KBr pellet form. Each spectrum is an average of 32 number of scans at 4 cmꢀ1
resolution.
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The measurement of Fourier Transform Raman spectra of 2,4,5ꢀTMBC was done on a Nicolet
NXR – FTꢀRAMAN spectrometer in the region 4000–50 cmꢀ1 having 1064 nm Nd: YVO4 laser
as the excitation source.
3. Computational methodology
The optimized and stable geometry of 2,4,5ꢀTMBC has been achieved by DFT using B3LYP
(Becke’s three parameter exchange functional B3 combined with LeeꢀYangꢀParr correlation
43, 44
functional LYP)
and HartreeꢀFock (HF) at 6ꢀ31G* basis set. The visualization of obtained
results was performed by Gauss view 5 visualization program 45. Additional, optimizations of
geometries were also done by CAMꢀB3LYP, wb97xd and LCꢀBLYP levels of theory. The
stability of the optimized geometry was additional confirmed by computing its analytical
frequencies. The rangeꢀseparated functionals are highly dependent on the rangeꢀseparation
parameter. Thus one way to eliminate these ambiguities is to tune the rangeꢀseparation parameter
(especially for donorꢀacceptor complexes) 46, 47. Moreover, these techniques could make the
theoretical calculations match even closer to experiment. Furthermore, To calculate the structural
parameters in the periodic boundary conditions, we have imported the crystal information file
(CIF) and optimized it by using generalized gradient approximation GGA/PBE 48 functional and
49
50
DNP basis set . The DMol3 code which is implemented in the Accelrys package Materials
Studio 51 has been used to compute the geometries.
The total first hyperpolarizability (
) and its components for 2,4,5ꢀTMBC molecule were
βtot
evaluated using finite field (FF) approach at all above levels of theory. The FF approach has
been widely used to calculate the first hyperpolarizability of numerous chemical systems and it
has provided very steady results with experimental data.52, 53 Here, we have applied various
methods including B3LYP, HF, range separated hybrid functional such as CAMꢀB3LYP and
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wb97xd and long range corrected LCꢀBLYP at 6ꢀ31G* basis set for calculating the dipole
moment and first hyperpolarizability and compared them with each other. In FF approach,
generally a static electric field (F) is applied and the energy (E) is expressed by the following
equation:
1
1
1
ꢀ = ꢀ(ꢁ) − ꢂꢃꢄꢃ − ꢅꢆꢇꢄ ꢄ − ꢈꢆꢇꢉꢄ ꢄ ꢄꢉ −
ꢊꢆꢇꢉꢋꢄ ꢄ ꢄꢉꢄ − ⋯
(1)
ꢆ ꢇ
ꢆ ꢇ
ꢆ ꢇ
ꢋ
2
6
24
Here E(0) represents the total energy of molecule in the absence of an electronic field,
ꢀ
is the
α
vector component of the dipole moment, is the linear polarizability,
γ
β and are the second
and thirdꢀorder polarizabilities respectively, while x, y and z label the i, j and k components,
respectively. It can be seen from Eq.1 that differentiating E with respect to F obtains the ꢁ, α, β,
and γ values.
The electronic dipole moment, molecular polarizability, anisotropy of polarizability and
molecular static and first total hyperpolarizability were calculated from the following equations.
For a molecule, its dipole moment (ꢁ) is defined as follows:
ꢃ
ꢏ
ꢏ
ꢏ
ꢏ
ꢂꢌꢍꢌ = (ꢂꢎ + ꢂꢐ + ꢂꢑ)
(2)
(3)
Total polarizability was calculated by:
1
ꢅꢌꢍꢌ = (ꢅꢎꢎ + ꢅꢐꢐ + ꢅꢑꢑ)
3
Anisotropy of polarizability was calculated by:
1
ꢏ
ꢏ
ꢒ
(
)
∆ꢅ =
ꢓꢔꢅꢎꢎ − ꢅꢐꢐꢕꢏ + ꢔꢅꢐꢐ − ꢅꢑꢑꢕ + ꢅꢑꢑ − ꢅꢎꢎ ꢏ + 6ꢅꢎꢑꢖ
(4)
(5)
2
√
First hyperpolarizability components of can be evaluated by expression:
ꢔꢈꢆꢇꢇ + 2ꢈꢇꢆꢆꢕ
ꢈꢆ = ꢈꢆꢆꢆ + ꢗ ꢘ
ꢙ
3
ꢆꢚꢇ
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Using x, y, z components the resultant of total first hyperpolarizability (βtot) can be calculated by:
ꢏ
ꢏ
ꢏ
ꢒ
ꢈꢌꢍꢌ = ꢔꢈꢎ + ꢈꢐ + ꢈꢑ ꢕ
(6)
Where βx, βy and βz are:
βx = (βxxx + βxxy + βxyy)
βy = (βyyy+ βxxz+ βyyz)
βz = (βxzz+ βyzz+βzzz)
Hence, the magnitude of total first hyperpolarizability is given by
ꢓꢔꢈꢎꢎꢎ + ꢈꢎꢎꢐ + ꢈꢎꢐꢐꢕꢏ + ꢔꢈꢐꢐꢐ + ꢈꢎꢎꢑ + ꢈꢐꢐꢑꢕꢏ + ꢔꢈꢎꢑꢑ + ꢈꢐꢑꢑ + ꢈꢑꢑꢑꢕꢏꢖ (7)
ꢒ
ꢈꢌꢍꢌ
=
Secondꢀorder polarizability (β) known as a third rank tensor that can be described by a 3 × 3 × 3
matrix. Due to Kleinman symmetry (βxyy = βyxy = βyyx, βyyz = βyzy = βzyy,… likewise other
permutations also take same value), the 27 components of the 3D matrix can be reduced to 10
components 54. These components have been calculated using GAUSSIAN 09 55. Similarly, time
dependent (TD) with B3LYP, HF, range separated hybrid functional CAMꢀB3LYP, wb97xd and
long range corrected LCꢀBLYP has been used to estimate the transition energies of titled
molecule.
The charge transfer rate can be described by Marcus theory via the following equation 56.
½
W=V2/h( π /λkBT) exp(−λ/4kBT )
(8)
where transfer integral (V) and reorganization energy (λ) are the main parameters which
determine the selfꢀexchange electronꢀtransfer rates and finally charge mobility. The smaller λ
and larger V would lead to the efficient charge transport properties.
There are two types of λ, i.e., inner λ (the molecular geometry relaxation when an electron is
added or removed from a molecule) and outer λ (the variations in the surrounding medium due to
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polarization effects). Here, we focused on the inner λ which shows the geometric relaxation in
(1)
rel
(1)
rel
(2)
rel
the molecules. The inner λ can be divided into two parts:
λ
and λr(e2l) , where
λ
and λ
correspond to the geometry relaxation energy of molecule from neutral to charged state, and
from charged to neutral state, respectively 57.
(2)
rel
λ = λ(1)
+
λ
(9)
rel
These terms were calculated from the adiabatic potential energy surfaces 58.
λ = λ(1)
+
λ
= [E(1)(L+/ꢀ) ꢀ E(0)(L+/ꢀ)] + [E(1)(L) ꢀ E(0)(L)]
(2)
rel
(10)
rel
where E(0)(L), E(0)(L+/ꢀ) are the groundꢀstate energies of the neutral and charged states, E(1)(L) is
the energy of the neutral molecule at the optimized charged geometry and E(1)(L+/ꢀ) is the energy
of the charged state at the geometry of the optimized neutral molecule.
4. Results and discussion
4.1. Molecular Geometry
The optimized stable molecular geometry of 2,4,5ꢀTMBC by B3LYP using 6ꢀ31G* basis set has
been shown in Fig. 2 (a), further optimization of its geometry was also done by HF, CAMꢀ
B3LYP, wb97xd and LCꢀBLYP levels of theory (see supplementary data). From Fig. 2 (a) and
Fig. S1 (bꢀe) [see supplementary data]. Additionally, we have studied the geometrical parameters
of of 2,4,5ꢀTMBC applying the periodic boundary conditions in which calculated geometrical
parameters were compared with the experimental crystal structure in solidꢀstate 59, 60. It can be
seen that there is intraꢀmolecular bonding between O(3) and H(20) with bond length 2.332 Å and
O(2) and H(20) with bond length 2.434 Å obtained by B3LYP, while these values calculated by
HF are found to be 2.363 Å and 2.448 Å, respectively. These are in close covenant with the
experimental values 2.360 Å and 2.450 Å, respectively (presented in Table 1) 41. These
geometrical parameters calculated by periodic boundary conditions at PBE/DNP are also found
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in reasonable agreement with experimentally reported values (see Table 1 and Fig. S2). The three
methoxy group are attached at C(22), C(25) and C(26) in the molecule which are almost coplanar
with C(21) and C(27) of benzene ring. Experimentally, its crystal structure is stabilized by
intermolecular C12—H12A
⋯
O1i and C16— H16B
⋯
O1i interactions (code of symmetry is given
O4
in Table 1) and these interactions generate a ring of graphꢀset motif R12 (7). The short Br1
⋯
(−3/ 2 + x, 3/2 − y, 1 −z) contact [3.264 (2)
Å] also backs to the crystal structure stabilization.
The experimentally observed geometry is shown in Fig. 2(b) 41.
All other bond lengths between CꢀC, CꢀH, CꢀBr, CꢀO etc. are in well agreement with the
16,
available literature
61ꢀ64. Table S1 of supporting information shows the values of key
geometrical factors including bond lengths, bond angles and dihedral angles of optimized 2,4,5ꢀ
TMBC molecule at different levels of theory.
ο
Table 1 Experimental and theoretical hydrogenꢀbond geometry (Å, ) parameters of 2,4,5ꢀ
TMBC.
Theoretically calculated (present work)
Exp. 41
B3LYP
HF
LCꢀBLYP
wb97xd
PBE (PBC) b
H⋯A Bond
Angle
Bond
Angle
Bond
Angle
Bond
Angle
D−H⋯A
H⋯A D⋯A
Bond H⋯A
Angle
H⋯A
H⋯A
Bond H⋯A
Angle
2.434 98.70
2.332 100.35
2.448 97.27
2.363 98.36
2.378 98.73
2.313 99.68
2.444 97.789 2.461 97.94
2.354 98.431 2.315 100.20
C9−H9A⋯O1
2.45
2.36
2.57
2.57
2.799(5)
2.720(5)
3.470(4)
102
103
164
C9−H9A⋯O2
C12−H12A⋯O1a
C16−H16B⋯O1a
3.303 (5) 133
a Symmetry code: (i) −x +1, y +1 /2, −z 3/ 2, b With Periodic boundary conditions (PBC)
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Fig. 2. Optimized molecular geometry of 2,4,5ꢀTMBC molecule (a) B3LYP using 6ꢀ31G* basis
set and (b) experimentally observed geometry 41.
4.2. Vibrational analysis
Infrared (IR) and Raman spectroscopy techniques have extensively used by the organic chemists
for the identification of functional groups, bonding to different molecular conformations and
reaction mechanisms by tentatively assigning their observed fundamental modes65ꢀ72. In
vibrational spectroscopy, the shift in peak position is discussed in terms of the change in
crystalline field effect induced electronic effects, hydrogen bonding and Fermi resonance
Electronic effects arise from back donation and induction of bonded groups present in a material.
Interꢀmolecular and intraꢀmolecular hydrogen bonding appears when H atom and N, O or F
atoms are present between two molecules or within a molecule. The vibrational modes in the two
phenyl rings will differ in wave number and the magnitude of splitting will depend on the
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strength of interactions between different parts (internal coordinates) of the two rings. For some
modes, this splitting is too small that they may be considered as quasiꢀdegenerate and for the
other modes a remarkable splitting is observed.
1ꢀ(4ꢀBromophenyl)ꢀ3ꢀ(2,4,5ꢀtrimethoxyphenyl)
propꢀ2ꢀenꢀ1ꢀone
(C18H17BrO4)
possess
orthorhombic structure with P212121 space group and four molecules per unit cell. The unit cell
dimensions are a = 7.3031 (3) Å, b = 10.3422 (4) Å, c = 21.3355 (9) Å, V = 1611.47 (11) Å3 41,
42
.
4
The orthorhombic crystal system with the nonꢀcentrosymmetric space group P212121 (D2 ) the
Bravais cell consists of 4 molecules and occupy C1(4) site symmetry. A single molecule of 2,4,5
TMBC contains 40 atoms and 160 atoms in a unit cell. The irreducible representation of the
point group where all atoms acquire C1 site symmetry as:
Γtotal = 120 A+120 B1 +120 B2 +120 B3
which includes the three acoustic modes corresponding to the block translations of the crystal
Γvib, acoustic = B1 +B2 + B3. The formal classification of fundamental mode predicts that the
456 internal vibrations can be distributed as (114A + 114B
modes such as (3A + 3B + 3B + 3B ) translational and (3A+3B
vibrational modes. Phonons belonging to B1, and B are both Raman and infrared active. The
1
+ 114B
2
+ 114B
3
) and 24 external
1
2
3
1
+3B
2
+3B ) rotational
3
B2
3
phonons belonging to A symmetry are Raman active and infrared inactive.
The results of factor group analysis and summary are presented in Tables 2 and 3 respectively.
Table 2. Factor group analysis of 2,4,5 TMBC
S. No.
1.
Factor Group Symmetry
(D2 )
External Modes
(i) Translational Modes
(ii) Rotational Modes
A
B1
B2
B3
4
6
3
3
6
3
3
6
3
3
6
3
3
2.
Internal Modes
114
114
114
114
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Table 3. Factor group analysis – summary for 2, 4, 5 TMBC
Factor
Group
Site Symmetry
Optical Acoustic
C
H
O
4
Br Modes
Modes
Symmetry External
Internal
18 17
4
(D2 )
A
3T
3T
3T
3T
3R
3R
3R
3R
114
114
114
114
54 51 12
54 51 12
54 51 12
54 51 12
3
3
3
3
120
120
120
120
480
0
1
1
1
3
B1
B2
B3
Total
216 204 48 12
Internal vibrations
2, 4, 5 TMBC molecule does not have any symmetry and so the internal vibrations exhibited are
both IR and Raman active exclusive of acoustic mode. The internal vibrations may be classified
as those arising from the CH , CH3, C6H5, CH3O, enꢀ1ꢀone and CꢀBr functional groups. These
2
vibrations are obtained as a strong bands coupled between themselves.
External vibrations
The external vibrations are generally observed below 500 cmꢀ1 due to the rotational and
translational modes of vibrations of 2,4,5 TMBC ionic functional groups. The rotational modes
are obtained at higher frequency and intensity than translational modes in the Raman spectra.
While, the translational modes are more intense in IR spectra.
The theoretical spectra were derived from B3LYP/6ꢀ31G* and HF/6ꢀ31G* levels of theory. The
experimentally recorded and theoretically calculated spectra of 2,4,5ꢀTMBC are shown in Fig. 3.
The theoretically prophesied vibrational frequencies have no imaginary frequency, which
indicates that the optimized molecular geometry of 2,4,5ꢀTMBC molecule is located at the local
minimum point of the potential energy surface. The wellꢀknown fact about density functional
theory potentials is the overestimation of vibrational wavenumbers. For well comparison with
experimental values, the scaling was used for theoretically calculated spectra [for B3LYP it was
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0.9613 and for HF it was 0.8929]. The recorded (IR transmittance and Raman) spectra and
calculated peak positions of vibrational modes in wavenumber at both B3LYP and HF and their
corresponding tentative assignments are listed in Table S2.
IR transmittance and Raman experimental as well as theoretical spectra analysis of 2,4,5ꢀTMBC
crystal have been made on the basis of the characteristic vibrations of the methoxy group,
carbonyl group, methyl group, phenyl ring with orthoꢀ and paraꢀsubstitution, bromoꢀphenyl
group and propꢀ2ꢀenꢀ1ꢀone bridge. The calculated vibrational wavenumber and the atomic
displacement corresponding to the different normal modes are used for identifying the ambiguity
in vibrational modes. The computed and experimental IR and Raman wavenumbers
corresponding to different modes are listed in Table S2 along with detailed assignments. For
visual assessment, the experimental and calculated IR transmittance and Raman spectra are
presented in Fig. 3. We have tentatively assigned all vibrational bands. The frequencies of the
calculated and observed bands are provided in Table S2.
4.2.1. Methoxy (OꢀCH3) Group
In methoxy group, CH3 group is directly attached to oxygen atom, the CꢀH stretching and
bending peak positions can shift either to higher or lower wavenumber depending on the position
73
and electronic effects of groups to which it is bonded in a material . In the present case, three
methoxy groups are bonded to phenyl ring at orthoꢀ and paraꢀpositions. The asymmetric and
symmetric methyl stretching bands are observed as very weak intensity components in the region
2960 – 2850 cmꢀ1 at 2952, 2927 cmꢀ1 and 2852 cmꢀ1 respectively in IR experimental spectrum
and at 2952, 2933 and 2844 cmꢀ1 in Raman (Exp.) spectrum of this sample 65, 66, 70. The
calculated values of asymmetric and symmetric stretching modes are found at 2929 and 2855 cmꢀ
1
(B3LYP), 2867 (HF) in IR spectra and at 2965, 2910 (B3LYP), 2925 (HF) cmꢀ1 in Raman
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spectra respectively. As the calculations were carried out on a free molecule and the
experimental spectra were recorded for solid sample, due to intermolecular interactions and
crystalline field effects in the sample, the minor variation in peak positions have been observed.
This lowering of stretching frequency from the computed values with the decrease in intensity
66
has been explained in terms of the electronic effects caused by backꢀdonation and induced by
the oxygen atom 66, 67, 70, 74ꢀ76. The strong asymmetric bending vibration of methyl in methoxy
groups appear at 1455, 1558, 1470 cmꢀ1 in IR (B3LYP), (HF) and (Exp.) spectra respectively.
While, as weak and very weak intensity components in Raman spectra at 1472, 1475 and 1482
cmꢀ1 by (B3LYP), (HF) calculated and (Exp.) respectively 77. The medium intensity band at 1470
cmꢀ1 in the IR spectrum has been tentatively assigned to asymmetric bending mode of CH3
group. The symmetric methyl deformation mode in IR transmittance spectra are at 1381
(B3LYP), 1346 (HF) and 1398 cmꢀ1 (Exp.) and in Raman spectra at 1357 (B3LYP), 1346 (HF)
and 1394 cmꢀ1. This deformation asymmetric and symmetric modes shifts to higher wavenumber
by about 25 cmꢀ1 to 50 cmꢀ1 as compared to the calculated results (Table S2) is may be due to the
electronic effects and strengthening of bonding. The CH3 rocking vibrations are mixed with CꢀO
stretch and the CꢀH bending mode, which results a strong peak with a weak band component at
1144 and 1122 cmꢀ1 in IR (BL3YP) and as medium intensity band with weak component at 1144
and 1128 cmꢀ1 respectively.
4.2.2. Vibrations of phenyl ring
In this 2,4,5ꢀTMBC molecule, the two phenyl rings are bonded to 1 and 3 position of propꢀ2ꢀenꢀ
1ꢀone as 4ꢀBromophenyl (ring1) and 2,4,5ꢀtrimethoxyphenyl (ring2) groups. The benzene ring
belongs to D6h symmetry with effective symmetry is C2v or lower due this all twenty vibrations
are allowed either in IR or Raman spectroscopy. In these investigations, Herzberg’s numbering
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system consistent with logical scheme has been adopted as compared to first reported Wilson
numbering system applicable for benzene molecule only 71, 72. The 16 mode of e2g mode of CꢀC
asymmetric stretching vibration modes of two phenyl rings are observed at 1603 (ring1), 1522
(ring2) cmꢀ1 (B3LYP), 1617 (ring1), 1532 (ring2) cmꢀ1 (HF), 1610 (ring1), 1560 (ring2) cmꢀ1
(exp.) in IR transmission spectra and at 1617 (ring1), 1532 (ring2) cmꢀ1 (HF) and 1617 (ring1),
1560 (ring2) cmꢀ1 (Exp.). While CꢀC symmetric stretching vibration 13 mode of e1u vibration ~
1450 cmꢀ1 and 9 mode of b2u vibration ~ 1300 cmꢀ1 of these two phenyl rings are observed at
1418 (ring1), 1306 (ring2) cmꢀ1 (B3LYP), 1403 (ring1), 1303 (ring2) cmꢀ1 (HF), 1442 (ring1),
1295 (ring2) cmꢀ1 (exp.) in IR transmission spectra and at 1395 (ring1), 1280 (ring2) cmꢀ1
(B3LYP), 1403 (ring1), 1276 (ring2) cmꢀ1 (HF) and 1455(ring1), 1278 (ring2) cmꢀ1 (Exp.) from
vibrations of these two modes. The large intensity difference between the two phenyl rings CꢀC
stretching vibrational modes has been attributed to the electronic effects induced by substituting
three methoxy and bromo groups in rings. IR transmission spectra exhibits CꢀH ring stretching
modes at 1585 (ring1) (HF), 1587 (ring1) (Exp.) and 1499 (ring2) (B3LYP), 1485 (ring2) (HF),
1511 (ring2) (Exp.) cmꢀ1 respectively. While Raman CꢀH ring stretching modes of ring1 are only
observed at 1572 (B3LYP), 1582 (HF), 1575 (Exp.) cmꢀ1 and ring2 do not show these modes.
The calculated peak values are quite near to the experimental peak positions. The 9 mode of b2u
vibration is observed in the IR spectrum at 1327 cmꢀ1 and in the Raman spectrum at 1329 cmꢀ1 as
strong bands. The simultaneous IR and Raman activation of the phenyl ring modes of 16 (e2g), 13
(e1u) and 9 (b2u) provide evidence for the charge transfer interaction between the donor and the
acceptor group through the πꢀsystem. In tetraꢀsubstituted benzene, the CꢀCꢀH inꢀplane bending
modes 3(a2g), 17(e2g)a, 14(e1u)b and 14 (e1u)b are found in 1300–1000 cmꢀ1 70 region. The a and b
represents higher and lower wavenumber value of e vibrations respectively. The strong band in
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IR at 1144 cmꢀ1 corresponds to ring mode 14 (e1u)b. Mode 17(e2g)a is observed as a weak band in
Raman spectrum at 1188 cmꢀ1. The increase in intensity of these modes in Exp. spectra has been
attributed to the presence of strong electron donor substituent (OCH3 groups) 65, 66, 70. The CꢀCꢀH
outꢀofꢀplane bending vibrations is expected to occur in the region around 1000–675 cmꢀ1 66, 70
.
The 19(e2u)a CꢀCꢀH outꢀofꢀplane bending vibration is observed as a weak band at 856 cmꢀ1 in IR
(Exp.) spectrum and 19(e2u)b mode is observed at 830 cmꢀ1 in the Raman (Exp.) spectrum. The
presence of this peak confirms the paraꢀsubstitution of phenyl ring. The modes corresponding to
11(e1g)a are observed as medium intensity and weak band at 742 cmꢀ1 and 746 cmꢀ1 in IR
transmittance (Exp.) and Raman spectrum respectively, which is also in agreement with the
theoretical results. In all the aromatic compounds, the carbon–hydrogen stretching vibrations
occur in the region 3100–3000 cmꢀ1. The CꢀH stretching vibrations in the benzene derivatives
arise from two nonꢀdegenerate modes 1(a1g) at 3081 cmꢀ1 IR (exp.), 3083 cmꢀ1 Raman (Exp.),
5(b1u) at 3133 cmꢀ1 Raman (exp.) and two degenerate vibrations of e2g (3033, 3021 cmꢀ1) in
Raman (exp.) spectra, e1u 3041 cmꢀ1 (calc.), i.e., the vibrations 1, 5, 15 and 12 modes
respectively. A weak band at 3041 cmꢀ1 in the Raman spectrum corresponds to the CꢀH
stretching mode 12a. The weak band observed in the IR (Exp.) spectrum at 3081 cmꢀ1
corresponds to mode 1(a1u vibration). In benzene, the fundamentals vibrations of a1g (990 cmꢀ1)
and b1u (1006 cmꢀ1) are the ring breathing and trigonal bending of 1 and 6 modes, respectively.
The trigonal ring bending mode is obtained as a medium intensity band in the IR (exp.) spectrum
at 1029 cmꢀ1. The strong IR active ring breathing modes of substituted phenyl rings are observed
in the region 795–950 cmꢀ1 and tentatively assigned these peaks are listed in Table S2.
4.2.3. Ethylenic bridge vibrations
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The vibrations pertaining to ethylenic bridge are very sensitive to the degree of charge
transfer between the donor and the acceptor groups, so such stretching modes are important to
explore 77. In 2,4,5 TMBC the C8=C9 stretching mode is observed as a strong band at 1587 cmꢀ1
in the IR (Exp.) spectrum and as a strong band at 1575 cmꢀ1 in the Raman (Exp.) spectrum, and
the corresponding calculated mode is at 1585 cmꢀ1 in IR (HF) and at 1572 (B3LYP), 1582 (HF)
cmꢀ1 in Raman spectra. The strong bands in the Raman spectrum and weak in IR spectrum or
vice versa have been attributed to presence or even absence of the inversion symmetry. The
intraꢀmolecular charge transfer from the donor to acceptor group through the singleꢀdouble bond
conjugation may induce large variations in the molecular dipole moment and molecular
polarizability, hence causing activity in both IR and Raman spectra 74. So, the simultaneous
activation of C8=C9 stretching modes in both IR and Raman spectra confirms the charge transfer
interaction between the >C=O group and phenyl ring through the ethylenic bridge. It can be seen
from Fig. 3 that the calculated modes at 1585 and 1582 cmꢀ1 have strong peaks in both Raman
and IR spectrum.
4.2.4. Carbonyl Group Vibration
The wavenumber of the C=O stretch due to carbonyl group mainly depends on the bond strength,
which in turn depends upon inductive, conjugative, steric effects and lone pair of electron on
oxygen. C=O stretching vibration is expected in the range 1740–1640 cmꢀ1. In the present case
very strong C=O experimental band is observed at 1735 cmꢀ1 in the IR (Exp.) spectrum and the
theoretical value is 1739 cmꢀ1 IR (HF) and its conjugation with C=C stretching mode is obtained
at 1654 cmꢀ1 in IR (Exp.) and 1652 cmꢀ1 (Raman (Exp.) spectra. These values are in agreement
with reported values 65, 66, 68, 74. The inꢀplane and outꢀofꢀplane C=O deformations modes are
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assigned at 626, 538 cmꢀ1 in the IR (Exp.) spectrum and at 537, 500 cmꢀ1 in Raman (Exp.)
spectrum.
4.2.5. CꢀBr vibrations
CꢀBr stretch vibrations are observed below 700 cmꢀ1. In 2,4,5ꢀTMBC crystal, CꢀBr asymmetric
and symmetric stretching modes appear at 707(B3LYP), 667,646 (HF) and 647 (Exp.) cmꢀ1 and
448 (B3LYP), 417 (HF) and 437 (Exp.) cmꢀ1, respectively in IR spectra. While in Raman spectra
these modes have been observed at 668 (B3LYP), 689 (HF), 703 (Exp.) cmꢀ1 and 445 (B3LYP),
453 (HF) and 437 (Exp.) cmꢀ1, respectively. These values are in agreement with the reported
values 66, 67, 69ꢀ72
.
4.2.6. Understanding NLO activity by Vibrational Spectroscopy
The vibrational contribution to NonꢀLinear Optical (NLO) activity has been characterized by
from πꢀconjugated systems present in a molecule. Such systems have large valued second order
molecular polarizabilities which encourages activity of some modes in both IR and Raman
spectra. In this case, the simultaneous activity of phenyl ring 16 and 9 modes in IR and Raman
spectra have been observed which play crucial role in NLO activity of the crystal. These modes
split into 16a, 16b and 9a, 9b peaks. The bands observed in IR (Exp.) at 1587, 1442, and 1324
cmꢀ1 are active in the Raman (Exp.) spectrum at 1575, 1455, and 1358 cmꢀ1 respectively. The
relative intensities of both IR and Raman peaks are comparable due to the electron cloud
movement through
group.
πꢀconjugated skeletal structure i.e. from electron donor to electron acceptor
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Fig. 3. Calculated and measured (a) IR and (b) Raman spectra of 2,4,5ꢀTMBC
4.3. Electroꢀoptical properties
.
4.3.1. Frontier Molecular Orbital (FMO) Analysis
In the reactivity of any molecule, frontier molecular orbitals (FMOs) play a vital role. Among
FMOs, the interaction of HOMO and LUMO between reacting species is very significant for the
progress of chemical reactions in the compound formation through the stabilization of the
transition structure of any compound. The HOMO and LUMO energy values were determined at
different levels of theory such as B3LYP, HF, CAMꢀB3LYP, wb97xd and LCꢀBLYP etc. The
values calculated at B3LYP are presented in Table 2. These energy gap values describe the
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chemical reactivity, kinetic stability, optical polarizability and chemical softness of any
molecule. The molecules with larger HOMOꢀLUMO energy gaps are known as harder molecules
and also possess higher thermal and kinetic stabilities according to softnessꢀhardness rule. The
energy gap between HOMO and LUMO of 2,4,5ꢀTMBC molecule is found to be 3.374 eV at
B3LYP/6ꢀ31G* level of theory with its chemical hardness value of 1.687 eV, indicates that
2,4,5ꢀTMBC molecule has good kinetic stability.
Fig. 4. HOMO and LUMO representation of 2,4,5ꢀTMBC molecule obtained at B3LYP/6ꢀ31G*
levels of theory.
TDꢀDFT calculations predict the electronic transition at 3.103 eV (see Table S3 of supporting
information) at B3LYP levels of theory corresponding to the transition from ground state to first
excited state and show charge transfer from HOMO to LUMO. The 3ꢀD plots of frontier
molecular orbitals (FMOs) are shown in Fig. 4 and their respective energy values in eV are given
in Table 4.
It can be seen from Fig. 4 that the HOMO are mainly delocalized on one side of the molecule
whereas LUMO almost on whole molecule. The electronic density distribution is present on the
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whole molecule as indicated by HOMOꢀLUMO transition and also this transition shows intraꢀ
molecular charge transfer in 2,4,5ꢀTMBC molecule. The titled molecule has an advantage of
16, 42
noteworthy transparency and low absorbance as observed experimentally
and it is also
confirmed by the theoretically calculated value of transition energy which is relatively larger and
belongs to violet region of the spectrum. The excitation energy calculated by TDDFT method is
found in good correlation with HOMOꢀLUMO energy gap at same levels of theory i.e. B3LYP.
Table 4 Calculated energy values of frontier molecular orbitals (FMOs) and energy difference of
HOMOs and LUMOs in eV at B3LYP/6ꢀ31G* levels of theory.
Orbitals
EHOMO
EHOMOꢀ1
ELUMO
ELUMO+1
EHOMO ꢀ ELUMO
EHOMOꢀ1 ꢀ ELUMO+1
Energies
ꢀ5.306
ꢀ6.395
ꢀ1.932
ꢀ0.598
3.374
5.796
4.3.2. Time DependentꢀDensity Functional Theory (TDꢀDFT) analysis
For excited state electronic structure calculations in quantum chemistry and solid state physics,
Time Dependent Density Functional Theory (TDꢀDFT) establishes itself as one of the most
popular theoretical methods. The contemporary density functional approaches show a
satisfactory balance between accuracy and computational efficiency when it is compared with
the traditional ab initio and semiꢀempirical approaches. We have applied TD approach at
B3LYP, HF, range separated CAMꢀB3LYP, wb97xd and long range corrected LCꢀBLYP levels
of theory at 6ꢀ31G* basis set to examine the nature of electronic transition in 2,4,5ꢀTMBC
molecule in gas phase. As per the available literature a precise absorption wavelength at
moderately small computing time can be detected easily by such study on the basis of optimized
ground state geometry, which corresponds to the vertical electronic transitions in the molecule 78ꢀ
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80. The experimental UVꢀvisꢀNIR spectra of 2,4,5ꢀTMBC is already reported in the literature
42. The absorption wavelength, excitation energies, oscillator strengths, major contributions were
calculated in gas phase at the ground state of the optimized geometry and presented in Table S3.
The calculated UVꢀvis spectra (from B3LYP, HF, CAMꢀB3LYP, wb97xd and LCꢀBLYP levels
of theory) of 2,4,5ꢀTMBC molecule in gas phase are shown in Fig. 5. As clear from Fig. 5 that
there is great difference in the value of absorption wavelengths calculated from different
methods. We have compared our theoretically calculated absorption wavelength with
experimental value, i.e., about 400 nm (recorded spectra is shown in Fig. S3 of supporting
16,
42
information) and found that the value calculated at TDꢀB3LYP/6ꢀ31G* levels of theory (400
nm) is in good agreement in comparison of other applied methods (Table S3). The optimized
geometry of 2,4,5ꢀTMBC molecule specifies that the visible absorption maxima of this molecule
corresponds to the electron transition from HOMO to LUMO.
Fig. 5. UVꢀVisible spectra of 2,4,5ꢀTMBC molecule calculated at different levels of theory using
6ꢀ31G* basis set.
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4.4. Polarizability and first hyperpolarizability
In nonlinear optical processes of technological importance in device fabrications the
hyperpolarizability plays a significant role and vindicates the growing exertion ardent to the
83ꢀ85
precise determination of this property 81, 82. Several recent studies shows that
besides an
appropriate treatment of electron correlation and a cautious choice of the function basis set, the
inclusion of the contributions arising from the nuclear motion is of fundamental importance for
the calculations of molecular electrical properties like polarizability and hyperpolarizability.
Therefore, we have calculated the electronic total dipole moment (ꢁtot), molecular total and
anisotropy of polarizability (αtot, ꢂα), static and total first hyperpolarizability (β0, βtot) using
different methods such as B3LYP, HF, range separated CAMꢀB3LYP, wb97xd and long range
correction LCꢀBLYP with 6ꢀ31G* basis set. The calculated values at B3LYP levels of theory are
presented in Table 5 along with 10 components. The values calculated at HF, CAMꢀB3LYP,
wb97xd and LCꢀBLYP are also presented for comparison in Table S4 (1ꢀ4) (See supplementary
data). Polarizability and hyperpolarizability values are dominated by their diagonal components
i.e. components along dipole moment axis of αxx and βxxx. The value of total dipole moment (ꢁ)
is 2.343D at B3LYP levels of theory. The variation of hyperpolarizability value at all the applied
levels of theory is shown in Fig. S4 along with urea. From figure it is clear that the
hyperpolarizability value is found to be higher at B3LYP levels of theory than other methods.
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Table 5 Calculated values of polarizability, hyperpolarizability and dipole moment along their
individual tensor components of 2,4,5ꢀTMBC molecule at B3LYP/6ꢀ31G* levels of theory.
Polarizability and dipole moment
Hyperpolarizability
a. u.
esu
Components
a. u.
esu (×10−24
)
Component
(×10−30
)
αxx
αxy
αyy
460.421
ꢀ0.808
29.34
4.30
βxxx
βxxy
βxyy
ꢀ4463.056
2214.039
ꢀ38.565
19.132
242.662
ꢀ6.288
7.913
ꢀ193.007
35.940
ꢀ1.668
0.311
0.833
ꢀ0.176
36.75
8.00
αxz
αyz
βyyy
βxxz
96.447
ꢀ4.74
99.245
ꢀ20.389
αzz
αtot
ꢂα
ꢁx
ꢁy
ꢁz
βxyz
βyyz
βxzz
βyzz
βzzz
β0
32.90
315.177
267.443
2.257
ꢀ0.622
ꢀ0.078
2.343
46.699
39.626
5.738D
ꢀ1.582D
ꢀ0.197D
5.955D
2.185
39.080
8.380
ꢀ5.127
1467.657
2446.095
0.019
0.338
0.072
ꢀ0.044
12.682
21.137
ꢁtot
βtot
For α, 1 a. u. = 0.1482×10−24 esu, for β, 1 a. u. = 0.008629×10−30 esu, ꢁUrea= 1.3732 D and βUrea
0.3728×10−30 esu 86
=
The highest value for ꢁx of dipole moment components has been found, which, is a major
contributor to the total dipole moment in any molecule. In similar way, the average polarizability
(αtot), anisotropy of polarizability (ꢂα) and total first hyperpolarizability (βtot) of 2,4,5ꢀTMBC
molecule have nonꢀzero values of 46.699×10−24, 39.626×10−24 and 21.137×10−30 esu.,
respectively. The nonꢀzero value of βtot shows that the titled molecule possesses microscopic first
static hyperpolarizability acquired by the algebric secondꢀderivative of the electric dipole
moment. The first hyperpolarizability value of 2,4,5ꢀTMBC molecule is 56 times larger than that
of urea [β for urea is 0.3728×10−30 esu]. The experimental second harmonic generation (SHG)
value of 2,4,5ꢀTMBC molecule has been measured and found to be 1.8 times higher than urea 42.
The calculated nonꢀzero βtot and SHG values are many fold higher than organic as well as other
reported materials 37ꢀ40, 86ꢀ93
.
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4.5. Ionization potentials, electron affinities and reorganization energies
To shed light on the charge transport behavior the ionization potential (IP), electron affinity
(EA), λ (h), and λ (e) are significant factors. The smaller/higher IP/EA values means that
materials would be efficient hole/electron transporters. The smaller λ (h) and λ (e) revealed
proficient hole and electron transport materials, respectively. We have tabulated the IPa, IPv,
EAa, EAv, λ (h) and λ (e) in Table 6. The smaller IPa/IPv values of our studied chalcone
derivative than 4,6ꢀdi(thiophenꢀ2ꢀyl)pyrimidine, i.e., 0.99/0.88 eV, respectively illuminating that
prior compound would be proficient hole transport material than later one. 57, 94ꢀ96. The larger
EAa/EAv values of chalcone derivative than that of 4,6ꢀdi(thiophenꢀ2ꢀyl)pyrimidine, i.e.,
0.11/0.7 eV displaying that former would be superior electron transporter as well than the later
one. The smaller computed λ (e) of chalcone derivative than the λ (h) illuminating that it would
be better electron transport material.
The major alteration from neutral geometry to cation and anion has been found in the C15ꢀC16ꢀ
C17 and C17ꢀC19ꢀC21 bond angles. The neutral, cation and anion bond angles in C15ꢀC16ꢀC17 (C17ꢀ
C19ꢀC21) have been observed 118.970, 119.070 and 119.700 (127.900, 127.170 and 126.880),
respectively. The key alteration in the geometry has been noticed from neutral to cation, i.e.,
0.730 and 1.020. Thus the geometry relaxation between neutral and cation is greater than anion
ensuing more polarization in former one consequently the λ (h) is larger than the λ (e).
Table 6 The vertical and adiabatic ionization potentials (IPv/IPa), vertical and adiabatic
electronic affinities (EAv/EAa), hole/electron reorganization energies λ (h)/λ (e) of studied
compound at the B3LYP/6ꢀ31G* level of theory.
Parameters IPv
IPa
EAv
EAa
λ (h)
λ (e)
Units
eV
eV
eV
eV
eV
eV
Values
6.80
6.59
0.50
0.65
0.447
0.305
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4.6. Global chemical reactivity descriptors (GCRD)
The Global chemical reactivity descriptors (GCRD) parameters are very important to know the
connection between structure, stability and global chemical reactivity of molecules and they are
based on conceptual DFT. These descriptors are used in the expansion of quantitative structureꢀ
activity, structureꢀproperty, and structureꢀtoxicity relationships. Hardness of any molecule is
related to aromaticity 97. DFT provides the explanations of principal universal perceptions of
98
molecular structure stability and reactivity . In the current work, we have calculated various
GCRD by using EHOMO as ionization potential (I) and ELUMO as electron affinity (A), respectively
(Details can be found in supporting information). The computed values of GCRD such as η, µ, S,
χ and ω are 1.687, ꢀ3.619, 0.733, 3.619 and 1.809 eV, respectively at B3LYP/6ꢀ31G* levels of
theory. The thermodynamic parameters such as thermal energy (E), specific heat (Cv) and
entropy (S) were calculated at HF, B3LYP, CAMꢀB3LYP, wb97xd and LCꢀBLYP methods with
6ꢀ31G* basis set which have been tabulated in Table S5 (supporting information). The variation
of these thermodynamic parameters at different levels of theory is shown in Fig. S5 (supporting
information) respectively.
4.7. Molecular Electrostatic Potential (MEP)
The 3ꢀD plot of molecular electrostatic potential (MEP) of 2,4,5ꢀTMBC is theoretically predicted
to have better understanding at molecular levels and is shown in Fig. 6. It is the measurement of
electrostatic potential on constant electron density surface. The 3ꢀD plots of MEP overlap on the
top of total energy density surface. It is a very supportive property to investigate the reactivity of
molecular species by predicting that either the approaching nucleophile is attracted towards a
positive region of the molecule or the approaching electrophile is attracted towards a negatively
charged surface of the molecule. In MEP plot the maximum positive potential (Blue) and
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negative potential (red) regions are preferred site for nucleophilic and electrophilic attacks,
respectively 38, 39, 99, 100. The MEP plot of 2,4,5ꢀTMBC is drawn as shown in Fig. 6 to acquire the
simultaneous information about its molecular size, shape along with its positive, negative and
neutral electrostatic potential regions in terms of color grading.
Fig. 6. Molecular electrostatic potential plot of 2,4,5ꢀTMBC molecule with iso value of ±0.02a.u.
5. Conclusion
The ground state molecular geometry of 1ꢀ(4ꢀBromophenyl)ꢀ3ꢀ(2,4,5ꢀtrimethoxyphenyl) propꢀ2ꢀ
enꢀ1ꢀone was obtained at DFT and HF methods using 6ꢀ31G* basis set. The calculated
geometrical parameters at different levels of theory were compared to experimental values and
found in good agreement. Furthermore, these geometrical parameters calculated by periodic
boundary conditions at PBE/DNP are also found in good agreement with experimentally reported
values. Spectral characterization has been done through FTꢀIR and FTꢀRaman analysis and
found in respectable agreement with the experimental results. The lowering of carbonyl
stretching wavenumber indicates the presence of conjugation. The electronic effects of
hyperconjugation and backꢀdonation have also been established. The simultaneous IR and
Raman activation of the phenyl ring modes of 16, 13 and 9 also provides evidence for the charge
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transfer interaction. Hydrogen bonding causes downward wavenumber shifts of peaks. The
intermolecular hydrogen bonds give broad bands, while intraꢀmolecular hydrogen bonds produce
sharp and well resolved peaks. The comprehensible intraꢀmolecular charge transport has been
observed from trimethoxyphenyl moiety (HOMO) to the bromophenyl unit (LUMO). The
absorption wavelength determined at TDꢀB3LYP/6ꢀ31G* levels of theory in gas phase was
observed at ~ 400 nm which has been found in good agreement with the experimental value of
400 nm. The dipole moment and first hyperpolarizability values are found to be many fold (2 and
56 times calculated at B3LYP) higher than urea molecule. The smaller geometry relaxation from
neutral to anion would lead to the smaller electron reorganization energy which ultimately would
increase the electron charge transport. The smaller electron reorganization energy than the hole
showed that the studied compound might be efficient electron transport material. The calculated
MEP plot shows the negative potential sites in the compound by red color, which is favorable for
electrophilic attack while the positive potential sites with blue color favorable for nucleophilic
attack. The higher value of first hyperpolarizability makes it excellent candidate for fabrication
of nonlinear optical (NLO) devices.
Acknowledgements
The authors are thankful to King Khalid University, Abha, Saudi Arabia for providing research
infrastructure.
28

Page 29 of 32
RSC Advances
View Article Online
DOI: 10.1039/C5RA13494C
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