Efficient π-electron conjugated push-pull nonlinear optical chromophore 1-(4-methoxyphenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-one: A vibrational spectral study

Article abstract of DOI:10.1016/j.molstruc.2008.06.031

As part of the efforts for the design new organic nonlinear optical (NLO) materials with high efficiency for present day technological requirements, a comprehensive investigation on the intramolecular charge transfer (ICT) of an efficient π-conjugated pot

Full text of DOI:10.1016/j.molstruc.2008.06.031

Journal of Molecular Structure 917 (2009) 27–36
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Efficient
p-electron conjugated push–pull nonlinear optical chromophore
1-(4-methoxyphenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-one:
A vibrational spectral study
a
a
b
b
c
Jose P. Abraham , D. Sajan , Venkataraya Shettigar , S.M. Dharmaprakash , I. N eˇ mec ,
a
a,
*
I. Hubert Joe , V.S. Jayakumar
Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram 695 015, Kerala, India
Department of Studies in Physics, Mangalore University, Mangalagangotri, Mangalore 574 199, India
a
b
c
Charles University in Prague, Faculty of Science, Department of Inorganic Chemistry, Hlavova 8, 128 40 Prague 2, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2008
Received in revised form 24 June 2008
Accepted 24 June 2008
Available online 6 July 2008
As part of the efforts for the design new organic nonlinear optical (NLO) materials with high efficiency for
present day technological requirements, a comprehensive investigation on the intramolecular charge
transfer (ICT) of an efficient
phenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-one to a strong electron acceptor group through the
-conjugated bridge has been carried out from their vibrational spectra. The NIR FT-Raman and FT-IR
p-conjugated potential push–pull NLO chromophore, 1-(4-methoxy-
p
spectra supported by the density functional theory (DFT) quantum chemical computations have been
employed to analyze the effects of intramolecular charge transfer on the geometries and the vibrational
Keywords:
NIR FT-Raman
FT-IR
Nonlinear optics
Second harmonic generation
Density functional theory
Intramolecular charge transfer (ICT)
modes contributing to the linear electro-optic effect of the organic NLO material. The calculated first
À30
hyperpolarizability of DMMC is 6.650 Â 10
esu, which is 25 times that of urea. The simultaneous IR
(C@C/C–C) mode, ring C@C stretching modes, in-plane
and Raman activation of the phenyl ring modes of
m
deformation modes and the umbrella mode of methyl groups also provide evidences for the charge trans-
fer interaction between the donors and the acceptor group through the -system. Vibrational analysis
p
indicates the electronic effects such as induction and back-donation on the methyl hydrogen atoms caus-
ing the lowering of stretching wavenumbers have also been analyzed in detail. The planar conformations
would give an enhanced NLO activity where as any deviations from planarity would decrease the mobil-
ity of electrons within the
p
-conjugated molecular system, resulting a reduction in NLO activity.
Ó 2008 Elsevier B.V. All rights reserved.
1
. Introduction
In recent years conjugated organic nonlinear optical (NLO)
polarized, giving rise to a relevant molecular dipole moment
which defines a charge transfer axis roughly coincident with
the chain axis of the conjugated system. These molecules are
known as push–pull molecules [3,4]. The basic strategy of using
electron-donor and electron-acceptor substituents to polarize
materials have been attracting attention because of their second-
or third-order hyperpolorizablities compared to those of inor-
ganic NLO materials [1]. Many investigations are being done to
synthesize new organic materials with large second-order optical
nonlinearities in order to satisfy day-to-day technological
requirements [2]. They have innumerable potential applications
including telecommunications, optical computing, optical data
storage, etc. The conjugated molecules consist of a skeleton con-
the
p-electron system of organic materials has been illustrious
for developing the NLO chromophores possessing large molecu-
lar nonlinearity, good thermal stability, improved solubility and
processability [5,6]. Recently, much effort is being devoted to
understand the origin of nonlinearity in large systems and to re-
late the nonlinear optical (NLO) responses to electronic structure
and molecular geometry for designing and fabricating the NLO
materials of large molecular hyperpolarizability [7,8]. In this ser-
ies, 1-(4-methoxyphenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-
one(DMMC) is one of the recently discovered potential organic
NLO materials. It belongs to the monoclinic system with high
electro optic and nonlinear optical coefficients [9]. Its low dielec-
tric constant suggests that the materials can be used for optical
data storage applications.
taining conjugated
to two end groups with electron donor (D) and electron acceptor
A) character, respectively. The electron acceptor group with-
draws electronic charge from the donor through the conjugated
bridge: as a consequence the -electrons of the skeleton become
p-electrons; the conjugated bridge is linked
(
p
*
Corresponding author. Tel.: +91 471 25307887.
E-mail address: vsjk@vsnl.net (V.S. Jayakumar).
0
022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.06.031

2
8
J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
The vibrational spectroscopy has been extensively used to
sion 28 mm  7 mm  3mm within a period of 20 days by slow
evaporation technique.
understand the factors contributing to the linear electro-optic ef-
fect from the vibrational modes in organic materials and to provide
deeper knowledge regarding the intermolecular interactions, and
the relationship between molecular architecture, nonlinear re-
sponse and hyperpolarizability [10]. NIR FT-Raman spectra com-
bined with quantum chemical computations have recently been
successfully employed in the vibrational analysis of drug mole-
cules, biological compounds, natural products and NLO active com-
pounds [11–14], since fluorescence free Raman spectra and
computed results help unambiguous identification of vibrational
modes and provide deeper insight into the bonding and structural
features of complex organic molecular systems. The present work
describes the vibrational spectral studies of 1-(4-methoxy-
phenyl)-3-(3,4-dimethoxyphenyl)-2-propen-1-one (DMMC), to
understand the relationship between the molecular structural fea-
tures and NLO properties, nonlinear optical response and hyperpo-
larizability of this push–pull prospective NLO chromophore with
special emphasis on the role of intramolecular charge transfer
2.2. IR and Raman measurements
The Raman spectra of polycrystalline samples (Fig. 2) and satu-
rated acetone solution (Fig. 3) were measured using a Nicolet Mag-
na 760 FT-IR spectrometer equipped with Nicolet Nexus FT-Raman
module. The measurements were carried out in the range of 100–
À1
À1
3700 cm (Happ–Genzel apodization, 2 cm resolution, 1064 nm
Nd:YVO laser excitation, 450 mW power at the sample). The spec-
4
tra were processed using the OMNIC software [16]. FT-IR spectrum
(Fig. 4) of the synthesized material was recorded in the wavenum-
À1
ber range 400–4000 cm by KBr pellet technique (Thermo Nicolet
AVATAR 370 DTGS FT-IR spectrophotometer).
3
. Computational details
The geometry optimization and calculation of the harmonic
(
ICT) mechanism in such organic materials.
vibrational wavenumbers were performed using the Gaussian 98
program [17], within the density functional theory (DFT) approach,
in order to properly account for the electron correlation effects
2
. Experimental
(
particularly important in this kind of conjugated systems) [18].
2
.1. Synthesis of the compound
The widely employed hybrid method denoted by B3LYP, which in-
cludes a mixture of HF and DFT exchange terms and the gradient
corrected correlation functional of Lee, Yang and Parr, as proposed
and parameterised by Becke [18], was used, along with the double-
zeta split valence basis set 6-311G(d,p) [19]. The calculations are
performed at B3LYP level using 6-311G(d,p) basis set and the
vibrational wavenumbers are scaled by 0.9614 to account for the
error caused by harmonic approximation [20]. Molecular geome-
tries were fully optimized by the Berny algorithm, using redundant
internal coordinates [19]. The bond lengths to within ca. 0.1 pm
and the bond angles to within ca. 0.1°. The final root-mean-square
Commercially available AR grade 3,4-dimethoxybenzaldehyde,
-methoxy acetophenone and ethanol (99%) were used without
4
further purification to synthesize the DMMC by Claisen–Schmidt
condensation method [15]. This is the reaction of substituted ace-
tophenone with substituted benzaldehyde in the presence of an al-
kali. Solution of para methoxyacetophenone (0.01 mol) in ethanol
(
30 ml) was mixed with (0.01 mol). 3,4-dimethoxybenzaldehyde
in ethanol (30 ml) in the presence of NaOH (5 ml, 30%). After stir-
ring for 3 h, the contents of the flask were poured into ice-cold
water (250 ml) and kept aside for 24 h. The resulting crude solid
was collected by filtration, dried and purified by repeated recrys-
tallization. Purity of the compound was checked by thin layer chro-
matography. The melting point of the synthesized material was
found to be 93 °C. A supersaturated solution of DMMC was ob-
tained by dissolving the sample in the acetone at ambient temper-
ature. After about 5 days good quality seed crystals appeared and
these seed crystals were used to grow larger size crystals of dimen-
À4
À1
(rms) gradients were always less than 3 Â 10 hartree bohr or
À1
hartree radian . No geometrical constraints were imposed on
the molecules under study. The first hyperpolarizability (b
0
) of this
) of
of novel molecular system, and related properties (b, and Da
a
0
DMMC are calculated using B3LYP/6-311G(d,p) basis set, based on
the finite-field approach. The calculated first hyperpolarizability of
À30
DMMC is 6.650 Â 10
esu, which is 25 times the corresponding
value for urea.
Fig. 1. Molecular structure of DMMC.

J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
29
1588
4500
4000
3500
3000
2500
2000
1500
1000
1172
500
3078
861
0
3
500
3000
2500
2000
1500
1000
500
-1
Wavenumbers(cm )
Fig. 2. NIR FT-Raman spectrum (solid) of DMMC and the simulated (top) Raman spectrum B3LYP/6-311G(d,p).
3
.1. Optimized geometries
methoxygroupandskeletalmodes. Thecomputedvibrationalwave-
numbers, their IR and Raman activities and the atomic displace-
ments corresponding to the different normal modes are used to
identify the vibrational modes unambiguously (Fig. 6). The calcu-
lated vibrational wavenumbers, measured infrared and Raman band
positions and their tentative assignments are presented in Table 2.
The high efficient push–pull organic NLO chromophore DMMC
The optimized structural parameters of DMMC are listed in
the Table 1 and molecular structure is presented in Fig. 1. The
optimized geometry shows that C@O and OCH groups (Me1
and Me3) substituted in para-position of phenyl ring are co-pla-
nar with Ph1 and Ph2 which predicts maximum conjugation of
molecule with donor and acceptor groups. The density functional
3
is characterized unambiguously by the most intense vibrational
À1
calculation gives shortening of angles C
24–C25–O32 and C –C –O11 and increase of angle C26–C25–O32
and C –C –O11 from 120° at C and C25 position and this asym-
1
–C
6
–C
5
, C26–C25–C24,
mode in the spectrum, between ca 1600–1560 cm
in Raman
C
1
6
and IR spectra. The DFT based eigenvector analysis reveals that
the relevant mode can be described as an in-phase symmetric
5
6
6
metry of angles reveals the conjugation with the phenyl ring is
maximum resulting from the charge transfer interaction of the
phenyl ring and propen-1-one group through C20@C18 double
bond. The evidence of such interaction can be inferred from
XRD data [9]. The optimized geometry of DMMC reveals that
the Me2 methoxy carbon atom lies out of the ring plane by
stretching vibrations of the C@C and C–C bonds,
m(C@C/C–C). This
vibration spreads over the whole -conjugated path with relevant
p
amplitudes from almost all the constituents parts of the molecule.
This vibration involves the intramolecular charge transfer from the
donor to the acceptor and gives rise to a large variation of the di-
pole moment, thus gaining a strong infrared activity [21]. Thus in
DMMC, simultaneous IR and Raman activation of this mode
3
u = 112.47° indicating that O–CH possesses almost perpendicu-
lar confirmation (u = 90°).
m
(C@C/C–C) clearly explains the charge transfer interaction be-
tween the methoxy (–OCH )) group and carbonyl (–C@O) group
through the ethylinic bridge -conjugated system. The -electron
3
4
. Vibrational spectral analysis
p
p
cloud movement from donor to acceptor can make the molecule
highly polarized through the single-double path when it changes
from the ground state into the first excited state.
The vibrational spectral analysis is performed based on the char-
acteristic vibrations of the phenyl ring, carbonyl group, vinyl group,

3
0
J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
4.1. Vibrations of the methoxy groups
The wavenumbers of the vibrational modes of methoxy groups
in DMMC are known to be influenced by a variety of interesting
interactions such as electronic effects, intermolecular hydrogen
bonding in the crystalline network [9] (Fig. 5) and Fermi resonance.
Electronic effects such as back-donation and induction, mainly
caused by the presence of oxygen atom adjacent to CH
can shift the position of CH stretching and bending modes [22–
5]. Of the three methoxy groups Me1, Me2 and Me3 of DMMC,
3
group,
2
1
593
X-ray diffraction studies revealed that there is weak intermolecular
hydrogen bonding of Me1 with the carbonyl oxygen of the neigh-
5
4
3
2
1
000
000
000
000
000
0
ÁÁÁÁ
bouring molecule and this C–H O interaction plays a key role in
the formation of the crystalline network. The asymmetric stretch-
ing modes of CH
3
group are observed in IR spectrum at 2998, 2971
À1
and 2948 cm for Me1, Me3 and Me2, respectively. The more in-
tense symmetric stretching vibrations are observed in IR at 2939,
À1
2
839 and 2830 cm for Me1, Me3 and Me2, respectively. The Ra-
À1
man bands observed at 2988, 2972 and 2948 cm correspond to
the asymmetric and the bands at 2939, 2844 and 2835 cm to
À1
3
the symmetric CH -stretching modes.
Vibrational spectral studies on esters have shown that asym-
metric and symmetric methyl stretching bands can be observed
À1
around 2960 and 2846 cm , respectively [22,26,27]. The asym-
metric and symmetric stretching modes of Me3 are calculated to
1
266
À1
be 2994 and 2914 cm , respectively. The lowering of this mode
for Me3 is probably due to its proximity to the phenyl ring. The
corresponding experimental values for Me3 are lowered by 19
À1
3
500
3000
2500
2000
1500
1000
500
0
and 75 cm , respectively. The asymmetric and symmetric stretch-
-1
ing modes of methoxy groups of the ester part DMMC are observed
Wavenumber/(cm )
as a medium bands in the IR and Raman spectra around 2971 and
À1
Fig. 3. NIR FT-Raman spectrum (acetone solution) of DMMC and the simulated
top) Raman spectrum B3LYP/6-311G(d,p).
2
839 cm . This lowering may be attributed to the electronic
(
Fig. 4. FT-IR spectrum of DMMC and the simulated (top) IR spectrum B3LYP/6-311G(d,p).

J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
31
Table 1
Optimized geometric data for DMMC using B3LYP/6-311G(d,p)
Bond length (Å)
Values
Bond angle (°)
Values
Torsion angle (°)
Values
B3LYP/6-311G(d,p)
B3LYP/6-311G(d,p)
B3LYP/6-311G(d,p)
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
1
2
3
4
5
6
6
3
–C
–C
–C
–C
–C
–C
2
3
4
5
6
1
1.39
1.41
1.40
1.39
1.40
1.40
1.36
1.49
1.48
1.35
1.46
1.41
1.38
1.41
1.36
1.39
1.40
1.42
1.37
1.42
1.36
1.42
1.08
1.37
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
1
2
3
4
5
2
3
–C
–C
–C
–C
–C
–C
2
3
4
5
6
3
–C
–C
–C
–C
–C
3
4
5
6
1
121.09
117.87
121.79
119.45
119.61
124.54
119.29
120.07
128.17
123.43
121.73
119.85
119.08
120.20
117.79
121.60
117.53
115.69
118.48
118.90
115.82
116.03
118.26
120.83
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
1
2
3
4
5
6
1
2
3
–C
–C
–C
–C
–C
–C
–C
–C
2
3
4
5
6
1
2
3
–C
–C
–C
–C
–C
–C
–C
3
4
5
6
1
2
3
–C
–C
–C
–C
–C
–C
4
5
6
1
2
3
0.16
À0.41
0.32
0.03
À0.27
–C16
0.18
–O11
–C16
–C16–C18
–C16
179.5
7.1
À177.6
À179.4
0.87
16–C18–C20
18–C20–C22
20–C22–C23
22–C23–C24
23–C24–C25
24–C25–C26
25–C26–C27
26–C27–C22
22–C27–C26
23–C22–C27
–C16–C18
16–O17
18–C20
20–C22
22–C23
23–C24
24–C25
25–C26
26–C27
27–C22
–C16–C18–C20
16–C18–C20–C22
18–C20–C22–C23
20–C22–C23–C24
22–C23–C24–C25
23–C24–C25–C26
24–C25–C26–C27
22–C23–C24–O31
23–C24–O31–C33
23–C24–C25–O32
30–C27–C26–C25
29–C26–C25–C24
28–C23–C24–C25
21–C20–C18–C16
À179.8
0.15
À0.73
0.66
176.6
112.4
178.1
179.7
À179.8
À179.2
0.49
O
C
O
C
11–C12
24–O31
31–C33
25–O32
32–C37
1 6
–C –C11
6
–O11–C12
H
H
H
H
H
H
23–C24–O31
24–O31–C33
24–C25–O32
25–O32–C37
O
C
C
18–H19
20–H21
19–C18–C16–C
–C –C –C
3
3.16
179.2
H
19–C18–C20
8
2
1
6
effects simultaneously caused by back-donation and induction due
to the presence of the oxygen atom [27].
The X-ray diffraction results show that the methoxy group
Me1 has an important role in forming the crystalline network
through C–H. . .O interactions (Fig. 5). The Me1 asymmetric vibra-
tions are observed as medium band in Raman spectrum at
The methoxy group vibrations are observed as intense bands in
both IR and Raman, but with large variation from the normal val-
ues of methyl groups. This must be due to the electronic effect
caused by the presence of oxygen atoms, which causes a deviation
from the expected values, as reported earlier [11]. For methoxy
substituted phenyl groups, the conjugation of the lone pair of oxy-
À1
À1
2988 cm and medium band in IR at 2998 cm . The symmetric
À1
stretching also observed as medium band in Raman at 2939 cm
À1
and medium band in IR at 2939 cm . In solution Raman spec-
trum, Me1 symmetric stretching band can be observed at
gen with the p
z
orbital of the aryl ring and the back-donation are
2942 m which correlates with the normal Raman spectrum band
À1
two competitive effects whose relative weights determine the con-
formation dependent molecular properties [13]. During the meth-
oxy rotation, the conjugation of the oxygen lone pair with the aryl
position, at 2939 cm
.
The classical intermolecular hydrogen bonding [Y–H. . .X] plays
a key role in the aggregation of molecules resulting in the stabiliza-
tion of molecular network and is accompanied by red shift of Y–H
z
p orbital changes, reaching a maximum when the carbon atom of
the methoxy group lies in the plane of the ring. In DMMC, the
stretching wavenumber and elongation of Y–H bond due to the
*
charge transfer from X to the anti bonding r orbital of the Y–H
bond [29]. The weak C–H. . .O interactions in certain crystals of
organic molecules aroused tremendous interest recently, owing
to their anomalous behaviour [30]. The spectroscopic investiga-
tions of such interactions pointed out a blue shift in the Y–H
stretching wavenumber [31] named ‘‘Improper, blue shifting
methoxy carbon atom lies out of the ring plane by angles u
o
o
o
À0.29 , 1.5 and 112.47 , for Me1, Me3 and Me2, respectively, indi-
cating that only Me2 possesses a almost perpendicular conforma-
tion (u = 90 ). The heavy atoms of the other methoxy groups lie
o
nearly in the plane of the ring as observed in many unhindered
methoxy substituted aromatic molecules. The asymmetric and
symmetric stretching wavenumbers of the methoxy groups are
correlated to values of u. Hence the asymmetric and symmetric
stretching vibration of Me2 possesses the lowest wavenumber.
hydrogen bonds” [29,30]. This is followed by the C–H bond short-
*
ening and the lack of population increase in the
r orbital of the
contracting C–H bond [32], contrary to the conventional red shift-
ing hydrogen bond. Similar unusual behaviour, though reported in
structure data, the C–H. . .O interaction is negligible and do not
show any appreciable change in vibrational bands, since C–H. . .O
distance is 3.21 Å [9].
The C–H stretching wavenumber of the CH
3
group is lowered to
À1
À1
2
839 cm
in IR spectrum and 2844 cm
in Raman spectrum,
which may be attributed to the electronic effects. The first over-
À1
tone of the symmetric bend (ꢀ2850 cm ) falling on the symmetric
C–H stretching band positions produces two bands of comparable
intensities, equally displaced on either side of this frequency (2835
The CH
1468 and 1448 cm
3
umbrella modes are also found in the expected range:
À1
can be assigned to Me3, Me1, and Me2,
À1
and 2918 cm ) resulting from Fermi resonance with one or more
respectively, supported by the computation. The rocking vibrations
of the CH group in DMMC are appears as mixed vibrations. The
mixing of C–O stretch and CH
C–H stretching levels [11,13,28]. The anharmonic interactions,
which couple these levels, have to be strong and probably associ-
3
3
rock, as predicted by computations,
À1
ated with the existence of the oxygen atom adjacent to a
p-system.
result in the medium 1174 and 1155 cm in IR spectrum and the
À1
Thus the possibility of Fermi resonance doublet at 2918 and
corresponding Raman bands at 1174 and 1164 cm . The torsional
À1
2
835 cm in Raman spectrum arising from Me2 makes unambig-
modes of Me1, Me2 and Me3 which are strongly coupled with each
À1
uous identification of symmetric stretching bands of Me1, Me2 and
Me3 difficult. A full quantum mechanical treatment of DMMC and
vibrational analysis of a larger class of similar compounds are
being taken up separately for a detailed investigation of such inter-
actions involved.
other are observed around 200 cm which is in agreement with
the calculated results also. The medium band in Raman spectrum
À1
at 524 and 526 cm in IR spectrum is probably an overtone of
Me1, Me2 and Me3 torsional mode, which is supported by
computed results.

3
2
J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
Table 2
À1
Calculated vibrational wavenumbers, measured IR and Raman band positions (cm ) and assignments for DMMC
Experimental
Computational B3LYP/6-311G(d,p)
Assignment
À1
À1
À1
À1
mIR (cm
)
D
m
Raman (cm
)
D
mRaman Solution (cm
)
mcal (cm
)
Relative intensity
IR
Raman
3
078 m
3078 m
3014 m
3089
3078
3071
3050
3033
2994
2975
2960
7
7
7
1
25
38
38
64
43
37
69
25
114
58
74
2, C–H arom. stretch
20a, C–H arom. stretch
7 b, C–H arom. Stretch + C18–H19 stretch
20b, C–H arom. stretch
Me1 asym. stretch
Me3 asym. stretch
Me2 asym. stretch
Me1 sym. stretch
Me2 sym. stretch overtone
Me3 sym. stretch
Me2 sym. stretch
C16@O171 stretch + C18@C20 stretch
8a ring stretch Ph1
8b ring stretch Ph2
C18@C20 stretch
19a ring stretch Ph1 and Ph2
Me1 + Me2 + Me3 asym. bend
Me1 + Me2 + Me3 asym. bend
19b ring stretch Ph1 and Ph2
Me1 + Me2 + Me3 asym. bend
3056 w
3045 w
3006 m
2998 m
2971 m
2948 sh
2939 m
3053 m
3009 m
2988 m
2972 m
2948 m
2939 m
2956 m
2942 m
108
*
*
2
918 sh
2
2
1
1
1
1
1
1
1
1
1
839 s
2844 s
2835 sh
1651 m
2843 m
2916
2914
1670
1602
1597
1588
1502
1463
1457
1438
1338
1318
1307
1297
1281
1266
1259
1216
1196
1173
1156
1148
1035
1015
1012
997
951
935
914
869
842
794
782
742
715
669
623
614
595
566
521
502
68
83
120
212
349
300
18
6
14
31
134
10
53
119
34
515
285
104
262
7
6
103
6
74
136
20
0.2
14
1
151
124
66
139
257
7784
111
35
27
19
317
66
15
14
115
216
17
139
82
3
21
20
18
68
200
5
2
7
2
3
830 sh
658 vs
593 s
588 vs
573 s
1658 w
1593 vvs
1588 vvs
1569 vvs
1507 m
1573 vs
1512 m
510 s
468 m
448 m
423 m
359 m
1448 m
1353 m
1454 m
1356 m
1330 m
1311 m
1299 sh
1
319 m
H19–C18–C20–H21 i.p bend 3
C–H arom. i.p bend
1
310 s
1311 s
1
1
1263 m
1241 w
1214 m
297 m
281 w
14, ring stretch Ph1 and Ph2
C–O stretch + C–H arom. i.p bend
C–O stretch + C18–H19 i.p bend
C–H i.p bend + C20–H21 i.p bend
15, C–H i.p bend
Me1 + Me2 and Me3 rocking + C–O stretch
9a, C–H i.p bend
Me1 + Me2 and Me3 rocking + C–O stretch
18b, C–H i.p bend
12 ring i.p bend Ph2
12 ring i.p bend Ph1
1 ring breath Ph1
1 ring breath Ph2
17a, C–H arom. o.p bend
5, C–H arom. i.p bend
O17@C16@C18 bend
17b, C–H arom. o.p bend
10a, C–H arom. o.p bend
C–H arom o.p bend
C–H arom. o.p bend + C–O–C bending modes.
11C–H arom. o.p bend
4C–H arom. o.p bend
16b, CCC bend Ph1
16b, CCC bend Ph2
Ring puckering
16a C–C–C bend Ph1
1262 m
1240 m
1212 m
1266 m
1242 w
1214 w
1197 w
1172 w
1161 w
1142 m
1036 m
1024 sh
1006 m
984 w
1
195 w
1174 m
155 w
1142 m
040 m
1019 m
006 m
981 m
1
1
1
9
174 m
142 w
024 s
80 m
1
1
1
9
9
68 m
49 w
920 w
854 w
832 m
796 m
763 m
745 m
920 w
861 m
835 w
796 m
763 m
746 m
869 w
7
17
0.1
3
10
9
4
9
9
6
41
0.4
22
32
3
10
4
25
31
8
796 m
767 m
750 m
718 w
688 w
636 w
618 w
7
15 m
688 m
35 m
6
80 w
2
4
6
619 w
604 m
557 m
526 w
620 w
13
10
4
26
1
554 w
524 w
565 w
530 w
510 w
492 w
16a C–C–C bend Ph2
Me2 + Me1 + Me3 torsion
Me2 + Me1 + Me3 torsion + H19–C18–C20–H21 bend
C–H arom. o.p bend
5
4
02 w
85 w
496
473
1
6
4
60 w
456 w
16a C–C–C bend
4
3
3
3
2
2
1
1
13 w
99 w
80 w
03 m
81 w
21 w
77 w
46 w
408 sh
393 w
407
394
379
305
297
221
164
144
0.1
1
6
1
0.2
4
1
2
30
6
1
1
1
1
Me2 + Me1 + Me3 torsion
Me2 + Me1 + Me3 torsion
Me2 + Me1 + Me3 torsion + H19–C18–C20–H21 bend
10a, C–H o.p bend
Skeletal mode
Skeletal mode
306 w
221 w
177 w
1
1
Skeletal mode
Skeletal mode
*
Fermi resonances.
4.2. Carbonyl and ethylinic bridge vibrations
carbonyl stretching vibrations in saturated esters are expected in
À1
the region 1750–1735 cm
. In DMMC, the conjugation of
Carbonyl group vibrations give rise to characteristic bands in
C16@O17 bond with C18@C20, may increase its single bond character
vibrational spectra and for this reason, such bands have been sub-
ject of extensive studies [34–37]. The intensity of these bands can
increase due to conjugation or formation of hydrogen bonds. The
resulting in lowered values of carbonyl stretching wave numbers.
The sharp intense band in infrared spectrum at 1658 cm is as-
signed to C16@O17 stretching mode, which is also observed in Ra-
À1

J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
33
À1
3
045 cm in the IR spectrum is attributed to the C18–H19 stretch-
ing mode, which is mixed with 7b stretching modes of phenyl ring.
The bands corresponding to in-plane and out-of-plane deforma-
À1
tions are observed, as expected, in the regions 1510–1000 cm
and 1000–750 cm , respectively.
À1
4.3. Phenyl group vibrations
The C–H stretching vibrations are observed around 3100–
À1
3
000 cm in aromatic molecules. They are not appreciably af-
fected by the nature of the substituents [26,27,38,40]. In benzene
derivatives the C–H stretching wavenumber arise from the modes
À1
À1
À1
a
(
3
1g (3062 cm ),
e
2g (3047 cm ),
b1u (3060 cm
) and e
1u
À1
3080 cm ). Hence the bands observed in the IR spectrum at
056, 3045 and 3006 cm and those observed in the Raman spec-
trum at 3078, 3053 and 3009 cm are assigned to C–H stretching
vibrations of DMMC.
À1
À1
The aromatic ring vibrational modes of DMMC have been ana-
lyzed based on the vibrational spectra of previously published
the vibrations of the benzene molecule are helpful in the identifi-
cation of the phenyl ring modes [26,27]. The phenyl ring Ph1 is
p-disubstituted and Ph2 possesses an asymmetric trisubstitution.
The ring stretching vibrations are very prominent, as the double
bond is in conjugation with the ring, in the vibrational spectra of
benzene and its derivatives [40]. There are five normal modes 8a,
Fig. 5. Crystal packing when viewed down the a-axis [9].
8
b, 19a, 19b and 14 allowed to p-disubstituted and asymmetrically
tri substituted phenyl ring for tangential C–C stretch. The charac-
teristic skeletal stretching modes of carbon–carbon bond in ben-
zene give rise to two doubly degenerate vibrations e2g (8a, 8b)
and e2u (19a, 19b) of benzene are not perturbed upon substitution.
À1
man spectrum at 1651 cm as a medium intense band and solu-
À1
tion Raman at1658 cm . The results of computations give the
wavenumber of this mode as 1670 cm , which is coupled with
18@C20 stretching mode. The deviation from the calculated wave-
number for this mode can be attributed to the under estimation of
-electron delocalization due to conjugation and hydrogen bond-
À1
The two resulting vibrational wavenumbers in the Raman and IR
À1
C
spectra lie close to the benzene vibrations ca. 1600 cm
and
À1
1490 cm , respectively, resulting from tangential C–C stretching
vibrations [38]. The doubly degenerate benzene vibration e2g con-
sists of lateral dilation and contraction of the ring, produced mainly
by stretching and compressing of the bonds. On the removal of
degeneracy, the components of this vibration appear separately
(8a, 8b). The doubly degenerate e1u mode of benzene is basically
a ring deformation, since it involves both stretching and bending
of the carbon bonds. Under reduced symmetry, the two compo-
nents appear separately, which are active in both IR and Raman
spectra. It is usually observed that one of the components of these
wavenumbers is lowered while the other component retains the
energy of the degenerate vibration in benzene. DFT computation
gives vibrational modes 8a and 8b at nearly equal wavenumbers,
p
ing network inside the crystal.
The vibrations of the ethylenic bridge are highly sensitive to the
degree of charge transfer between the donor and the acceptor
groups, hence such stretching modes are of particular interest for
spectroscopists [34–36]. The absorption bands arising from C–C
stretching vibrations are usually observed around 1640 cm for
cinnamic acid derivatives [37]. Conjugation of an alkene double
bond with an aromatic ring produces enhanced alkene absorption
near 1625 cm [38]. The sharp intense band in infrared spectrum
at 1573 cm is assigned to C18@C20 stretching mode, which is also
observed in Raman spectrum at 1569 cm as a medium intense
band and solution Raman at 1573 cm . The results of computa-
tions give the C18@C20 stretching mode to be 1588 cm . The
attachment of an electron donor and an electron acceptor groups
at the para-positions of the p-conjugated system induces the low-
ering of this mode [39]. One is mostly localized on the donor-sub-
unit and the other on the acceptor-subunit.
The weak intense band at 1281 cm in Raman spectrum corre-
sponding to the computed wavenumber 1281 cm is assigned to
C–O stretching mode which is coupled with C–H in-plane bending
modes. The medium band at 1262 cm in infrared spectrum and
the corresponding Raman band at 1262 cm have been assigned
to C–O stretch. But these vibrational modes are coupled with
18–H19 in-plane band, so that they cannot be separated and iden-
tified in an unambiguous manner. The weak intense band at
20 cm in IR spectrum and the band at 920 cm in Raman have
been assigned to O16@C17–C18 bend which is supported by com-
À1
À1
À1
À1
À1
À1
1602 and 1597 cm and such a splitting can not be observed
À1
experimentally which is contrary to the selection rule allowed
for p-disubstituted and asymmetrically trisubstituted ring. The
degeneracy of mode 8 remains unperturbed and the mode 8 ap-
pears in IR spetra with equal and high strength at 1593 and
À1
1588 cm , respectively, for Ph1 and Ph2. This can be interpreted
À1
as the result of intermolecular charge transfer between p-electron
À1
donor (OCH group) and acceptor (C@O group) via conjugated path
3
which contributes to the hyperpolarizability enhancement respon-
À1
sible for NLO activity (Fig. 6). The appearance of ring mode 8 with
equal intensity in IR and Raman can be considered as a strong
vibrational spectral evidence of intramolecular charge transfer
and subsequent hyperpolarizability enhancement as reported ear-
lier [13,33].
À1
C
À1
À1
9
With strong donor and acceptor substituents, the 19a mode in
substituted benzene can be expected in the region 1460–
À1
À1
puted results also. The medium IR band at 763 cm and the corre-
sponding Raman band at 763 cm can be attributed to C–O–C
1530 cm with larger intensity and 19b appears as medium band
À1
À1
between 1370 and 1470 cm [26,27,38]. The mode 19b is ex-
bending mode.
pected to interact a little with C–H in-plane bending and can be
generally stated that the band belonging to 19b is weaker in the
IR spectrum. The vibrational modes 19a, 19b and 14 are computed
The aliphatic CH stretching bands are expected between 3050
À1
and 3000 cm
for stilbene derivatives [38]. The band at

3
4
J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
Fig. 6. Atomic displacements corresponding to selected vibrational modes of DMMC at B3LYP/6-311G(d,p).
À1
at 1502, 1438 and 1297 cm , respectively. The mode 19a is found
the in-plane deformation modes 9a. The medium IR and Raman
bands at 1212 and 1214 cm can be attributed to the in-plane
À1
À1
as a very strong band at 1510 cm in IR and as a medium band at
À1
1
507 cm in Raman while for 19b medium bands can be observed
deformation modes 15. Further, the 18b mode generally observes
À1
À1
in IR at 1423 cm . Vibration 14 can be observed as a medium band
in Raman at 1298 cm and the ring mode is coupled with C–O
between 1100 and 1150 cm . The 18b mode is correlated to the
À1
À1
medium bands at 1142 cm
1142 cm
in Raman and weak band at
À1
stretch.
in IR. The intensity enhancement of this particular
groups, the strong electron
In substituted benzene, the phenyl modes 3, 9a, 18a and 18b
have been reported to have C–H in-plane bending character and
mode is due to the presence of OCH
3
donor substituents [26,27]. The in-plane deformation modes 3,
9a and 18b are found to be strongly and simultaneously active in
both IR and Raman spectra because of the symmetry lowering of
the molecule. Furthermore, such deformations change the bond
length between the carbon atoms, which build the charge transfer
axis, and the polarizability (and therefore the refractive index of
À1
can be expected in the region 1300–1000 cm . The C–H in-plane
À1
bending vibrations are derived from
a
2g (1340 cm ),
b
2u
À1
À1
À1
(
[
1200 cm ), e2g (1178 cm ), e1u (1037 cm ) modes of benzene
40]. The mode 3 of the in-plane deformation vibrations usually ap-
pears in the region 1264–1300 cm . The in-plane deformation
À1
mode 3 is observed as strong bands in IR and Raman spectrum at
the macroscopic material) of such
molecules.
p
-conjugated charge transfer
À1
1
310 cm corresponding to the ring Ph1 and Ph2. The range in
À1
À1
which vibration 9a normally appears is 1164–1200 cm [38,40].
In benzene, the fundamentals
a
1g (997 cm
) and b
1u
À1
À1
The medium IR and Raman bands at 1174 cm corresponds to
(1010 cm ) represent the ring breathing and trigonal bending

J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
Table 3
35
modes, respectively, gives rise to combined modes in molecules
Comparison of static first hyperpolarizability, HOMO–LUMO energy gap and SCF
energy for the constituents of DMMC
belonging to the reduced symmetry. As the energies of these vibra-
tions are very close, there is an appreciable interaction between
these vibrations and consequently their energies will be modified.
Further these modes are drastically affected in magnitude upon
substitution. The ring breathing modes for substituted phenyl ring
with entirely different substituents [38,40] have been reported to
Molecules
First hyperpolarizability
btot10À30 (esu)
HOMO–LUMO
energy gap (eV)
SCF energy
(a.u.)
DMMC (planar)
DMMC (nonplanar)
6.650
4.872
3.919
9.517
À997.60
À877.62
be strongly IR active with typical bands in the interval 780–
À1
9
60 cm . The phenyl ring modes 12 can be observed as medium
À1
bands in Raman spectrum at 1040 and 1019 cm for Ph1 and
Ph2, respectively. The phenyl ring mode 1 is observed in Raman
À1
Besides optimized donor–acceptor groups and conjugation
length the planarity of the molecules is another important aspect
to be considered, which is especially important for two- or mul-
ti-ring systems. The extent of the planarity also influences the size
spectrum at 1006 and 981 cm
for Ph1 and Ph2 respectively,
which is in good agreement with computed results.
The absorption bands arising from C–H out-of-plane bending
À1
vibrations are usually observed in the region 1000–675 cm
of the p-electron system and the mobility of the electrons. The ef-
[
b
38,40]. The out-of-plane vibrations in the benzene arises from
À1
À1
À1
À1
fect of twist angles between rings can therefore considerably re-
duce the charge transfer contributions, as the first
hyperpolarizability is strongly dependent on the planarity between
2g (995 cm ), e2u (975 cm ), e1g (849 cm ) and a2u (671 cm
)
modes of benzene. The 17a C–H out-of-plane vibrations are ob-
À1
served as medium band at 968 cm (Raman) while the Raman
À1
the benzene rings of the
planar conformations would give an enhanced NLO activity (Table
) where as any deviations from planarity would decrease the
mobility of electrons within the -conjugated molecular system,
p-conjugated system [4,5]. In DMMC the
band 861 cm corresponds to mode 17b. The ring mode 5 is iden-
À1
tified as weak Raman band at 949 cm , which is in good agree-
3
ment with computational results. The modes 10a appear at 835
À1
p
and 832 cm in Raman and IR spectrum, respectively. The carbon
resulting in a reduction in NLO activity [42,43]. It is also inferred
that there is an inverse relationship between first hyperpolarizabil-
ity and HOMO–LUMO gap, allowing the molecular orbitals to over-
lap to have a proper electronic communication conjugation, which
is a marker of the intramolecular charge transfer (ICT) from the
out-of-plane vibrations are represented by the nondegenerate b2g
À1
À1
(
703 cm ) and degenerate e2u (404 cm ) modes of benzene.
The phenyl ring modes 4, 6a and 16b can be observed in expected
region.
electron donating group through the
electron accepting group [44,45].
p-conjugation system to the
4.4. Lattice modes
À. 1
The lattice vibrations usually appear in the region 0–300 cm
6
. Conclusions
The intramolecular charge transfer and its related processes
About 5 bands are observed in the region, which is much fewer than
predicted by theory. These modes are due to the rotational and
translational vibrations of the molecules and vibrations involving
hydrogen bonds. The low wavenumber bands of the hydrogen bond
vibrations are generally found to be weak, broad and asymmetric in
have been comprehensively investigated for the potential NLO can-
didate 1-(4-methoxyphenyl)-3-(3,4-dimethoxyphenyl)-2-propen-
À1
1
-one from the vibrational spectra and the DFT computations. Con-
siderable changes in the endocyclic and exocyclic angles at both
the donor and acceptor side of the -conjugated system resulting
the Raman spectrum. The modes in the region 160–265 cm could
be associated with hydrogen bond vibrations. The lattice vibrations
of rotatory type are generally stronger in intensity than the transla-
tory type [41]. The lattice modes in DMMC are found to be very in-
tense in Raman compared with other modes in the high frequency
region.
p
from the charge transfer and the hyperconjugative interactions be-
tween the phenyl ring and the methoxy group in the electron do-
nor side have been observed. The calculated first
À30
hyperpolarizability of DMMC is 6.650 Â 10
esu, which is 25
times that of urea. Vibrational analysis indicates the lowering of
the stretching modes of Me1, Me2 and Me3 due to the electronic
effects simultaneously caused by back-donation and induction
arising from the presence of the oxygen atom. The occurrence of
electronic interactions and Fermi resonance could be inferred from
5
. Influence of planarity on the first hyperpolarizability
The large value of second-order polarizability, b, which is the
measure of the nonlinear optical activity of the molecular system,
is associated with the intramolecular charge transfer (ICT), result-
the CH stretching bands in the IR and Raman spectra of the CH
3
ing from the electron cloud movement through
p
-conjugated
group. Analysis of phenyl ring modes shows that C–C stretching
mode 8 is found equally active as strong bands in both IR and Ra-
man which can be interpreted as the evidence of intramolecular
framework from electron donor to electron acceptor groups. The
analysis of the wave function indicates that the electronic absorp-
tion corresponds to the transition from the ground to the first ex-
cited state and is mainly described by one-electron excitation from
the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO). The values of LUMO and
HOMO levels are computed to be À1.754 and À5.673 eV, respec-
tively, and the energy difference is 3.919 eV. Consequently, the
HOMO ? LUMO transition implies an electron density transfer
3
charge transfer between C@O and OCH groups via conjugated ring
path which is responsible for hyperpolarizability enhancement
leading to NLO activity. The overlooking of vibrational selection
rules and complementarity are found as the after effects of intra-
molecular charge transfer. The simultaneous IR and Raman activa-
tion of the phenyl ring modes of
m(C@C/C–C) mode, ring C@C
stretching modes, in-plane deformation modes and the umbrella
mode of methyl groups also provide evidences for the charge trans-
fer interaction between the donors and the acceptor group through
from the more aromatic part of the p-conjugated system including
the electron donor group and mainly to the electron-withdrawing
group. Moreover, these two orbitals significantly overlap in the
the
activity whereas any deviation from planarity would decrease
the mobility of electrons within the -conjugated molecular sys-
p-system. The planar conformations give an enhanced NLO
middle region of the
p-conjugated system, which is an inherent
condition to achieve an efficient charge transfer transition and,
consequently the nonlinear optical response of a push–pull NLO
chromophore.
p
tem, resulting in a reduction in NLO activity. The inverse relation-
ship between first hyperpolarizability and HOMO–LUMO gap has

3
6
J.P. Abraham et al. / Journal of Molecular Structure 917 (2009) 27–36
been observed which allows the molecular orbitals to overlap to
have a proper electronic communication conjugation.
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