E. Selvakumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 114–119
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frequency conversion, frequency mixing, optical data storage and
electro optic modulation, etc. [1–8]. Organic nonlinear optical
materials have gained considerable attention than the inorganic
materials, as they have high optical nonlinearity and synthetic flex-
ibility [9]. Organic materials allow one to fine tune their optical
properties through molecular engineering and chemical synthesis
[10]. More than 70% of organic compounds crystallize in centro-
symmetric space groups due to the predominant anti-parallel
fifteen days time. The photograph of the as-grown crystals is de-
picted in Fig. S2.
Characterization
To confirm the molecular structure of CATS crystal, the 1H and
13C NMR spectra were recorded employing a Bruker 500 MHz spec-
trometer in deuetrated solvents using TMS as the internal refer-
ence standard. The crystal structure was determined from the
single-crystal X-ray diffraction data obtained with an X’calibur
CCD area-detector diffractometer (Graphite – monochromated,
p-stacking between the aromatic rings as a consequence of dipolar
interactions [11,12]. As a result, they are unable to exhibit the SHG
activity. To overcome this major problem, various molecular engi-
neering approaches have been suggested to form acentric crystal
structures. Out of the various strategies, the hydrogen bonding
interactions play a pivotal role in creating acentric crystal structure
in organic molecules by orienting the molecular dipoles in a head-
to- tail manner and also enhance the mechanical and thermal sta-
bilities [13–17]. Hydrogen bonding supplemented by a mutual
polarization will also give rise to an efficient second harmonic gen-
eration [18]. Simple donor- acceptor o-, p- and m- disubstituted
benzene derivatives represent a peculiar type of molecules pos-
sessing nonlinear optical properties both in their solutions and in
solid states [19]. This is due to the interaction between the donor
and acceptor group via aromatic ring induces an asymmetric elec-
tronic distribution, leading to an enhanced nonlinear optical
response [20,21]. 3-amino benzoic acid contains an electron with-
drawing ACOOH group and electron releasing ANH2 group con-
Mo Ka1 = 0.713). The data were collected at 25 °C and the structure
was solved by direct methods using the program SHELXS-97 [25]
and refined by full-matrix least-squares method using SHELXL-97
[26]. The H-atoms could all be located in Fourier difference maps.
FT-IR spectrum was recorded using potassium bromide pellet
method employing a Perkin Elmer FT-IR spectrometer in the range
4000–450 cmꢁ1. The UV–Vis–NIR transmittance spectrum of CATS
was recorded employing a VARIAN CARY 5E UV–Vis–NIR spectro-
photometer. The TGA and DTA studies were carried out on a PER-
KIN ELMER DIAMOND instrument with a heating rate of 10 °C/
min in the temperature range from 20 to 400 °C in nitrogen atmo-
sphere. Quantitative estimation of relative SHG efficiency of CATS
crystal with respect to KDP was made by the modified Kurtz and
Perry powder technique. The electronic property and charge trans-
fer interaction of the title complex was studied by B3LYP method.
nected through p-conjugated system of the aromatic ring [22,23].
P-toluene sulfonic acid is a strong acid forms strong hydrogen
bonding complexes with bases like amines. The m-disubstituted
benzene derivatives appear to have the tendency to crystallize in
non-centrosymmetric space groups relative to their ortho and para
analogues [24]. Based on the above said aspects, in the present
investigation we present the synthesis, growth and spectroscopic
characterization of SHG active organic hydrogen bonding complex
salt 3-carboxyl anilinium p-toluene sulfonate (CATS). The title
material was synthesized and the single crystals were grown and
characterized through electronic, vibrational absorptions, nuclear
magnetic resonance spectral studies, and TG–DTA and Nonlinear
optical studies. On the basis of the above studies, the molecular
structure, optical property, proton transfer interaction and thermal
stability of the title complex have been reported.
Results and discussion
NMR spectral studies
The 1H NMR spectrum of CATS crystal is shown in Fig. S3. The
intense singlet signal appearing at d 2.36 ppm is due to the three
methyl protons of p-toluene sulfonate moiety in the salt. The C3
and C5 aromatic protons of the same kind in the same moiety ex-
hibit a doublet centered at d 7.21 ppm. The complex multiplet
appearing from d 7.63 to 7.65 ppm is attributed to the C4 and C6
aromatic protons in the same chemical environment in 3-carboxyl
anilinium moiety. The doublet centered at d 7.70 ppm owes to the
C2 and C6 aromatic protons of the same kind in p-toluene sulfo-
nate moiety in the salt. The C2 aromatic proton of 3-carboxyl ani-
linium moiety stands responsible for the singlet signal at d
8.06 ppm. The multiplet signal appearing from d 8.10 ppm to d
8.13 ppm has been assigned to C5 aromatic proton of the same
moiety. Thus appearance of six distinct proton signals confirms
the molecular structure of the salt crystal.
Experimental procedure
Material synthesis
AR grade 3-amino benzoic acid and p-toluene sulfonic acid were
purchased and used as such without further purification. Equimo-
lar solutions of 3-amino benzoic acid and p-toluene sulfonic acid
were prepared separately in ethanol and Millipore water respec-
tively and henceforth mixed together. The resulting solution was
stirred well for about an hour using a temperature controlled mag-
netic stirrer at room temperature. The obtained product was fil-
tered off, dried and repeatedly recrystallized in ethanol to
improve the purity of the synthesized compound. The reaction in-
volved is represented in Fig. S1.
The 13C NMR spectrum of CATS crystal is depicted in Fig. S4. The
appearance of 12 distinct carbon signals in the spectrum estab-
lishes the molecular structure of the title salt. The carbon signal
appearing in the downfield at d 166 ppm is attributed to the highly
deshielded carboxyl carbon of the 3-carboxyl anilnium moiety in
the salt. The carbon signal due to the methyl carbon is exhibited
at d 19.953 ppm in the upfield. The aromatic carbon atoms of 3-
carboxyl anilinium and P-toluene sulfonate moieties appear in
the range from d 123.95 to 141.86 ppm and their respective assign-
ments are given in Table 1.
Solubility and crystal growth
Single crystal X-ray diffraction analysis
A saturated ethanolic solution of CATS was prepared, stirred
well for about an hour and heated slightly to ensure the complete
dissolution of the material. The solution was then filtered through
a Whatman 41 grade filter paper to remove the suspended impuri-
ties completely. The clear filtrate was kept aside unperturbed in an
atmosphere conducive for the growth of single crystals. Well
grown good optical quality single crystals were harvested in about
Single crystals suitable for X-ray crystallographic analysis were
selected following an examination under a polarizing microscope.
X-ray intensity data of 2912, reflections (of which 2627 were
unique) were collected on a X’calibur CCD area-detector diffrac-
tometer equipped with graphite monochromated Mo K
a radiation
(k = 0.71073 Å). The cell dimensions were determined by least-
Selvakumar
