2
0
S. Tamilselvan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 114 (2013) 19–26
3
plays. In spite of the rapid development of blue laser diodes and
concurrent physical principles like optical upconversion, optical
second-harmonic generation (SHG) is still one of the important
methods to achieve intense coherent blue and green light with
good optical beam quality [1]. It has been generally understood
that the second-order molecular nonlinearity can be enhanced by
10 ꢂ 9 ꢂ 4 mm were grown over a period of 35–40 days by using
slow evaporation technique. The photograph of as grown LPB sin-
gle crystals is shown in Fig. 1.
Results and discussion
large delocalized p-electron systems with strong donor and accep-
Single crystal XRD study
tor groups [2,3]. From symmetry aspects, SHG is only possible in
crystals belonging to the point group that lack a center of symme-
try. Furthermore, efforts have been made to develop new inor-
ganic, organic and semi-organic nonlinear optical (NLO) crystals
An automatic X-ray diffractometer (MESSRS ENRAF NONIUS
CAD4-F, The Netherlands) was employed to collect the single crys-
tal X-ray data of LPB single crystal. The unit cell parameters of LPB
were determined using 25 reflections collected through random
[
4–6]. Organic compounds are formed by weak Van der Waal’s
and hydrogen bonds and possess high degree of delocalization
4]. Hence they are optically more nonlinear than inorganic mate-
search routine with graphite monochromated Mo
Ka
[
(k = 0.71073) radiation and indexed by the method of short vectors
rials. Some of the advantages of organic materials include flexibil-
ity in the methods of synthesis, scope for altering the properties by
functional substitution, inherently high nonlinearity, high damage
resistance, etc. [5]. Many of natural amino acids are individually
exhibiting the nonlinear optical properties because they are char-
acterized by chiral carbons, a proton-donating carboxyl group
followed by the least squares refinement. The study revealed that
LPB crystal belongs to monoclinic crystal system with a non-cen-
trosymmetric space group P2 and the lattice parameters were
1
found to be a = 5.421 Å, b = 7.435 Å, c = 17.816 Å, b = 93°, and the
3
cell volume V = 718 Å . The experimental data has good agreement
with the reported data [14].
and the proton-accepting amino group [6]. Among them,
alanine is an essential protein amino acid, which is used by the
body to build neurotransmitters [7,8]. The crystal of the -phenyl-
alanine–benzoic acid (LPB) compound, C , the ami-
ꢁC
L-phenyl-
FT-IR analysis
L
9
H11NO
2
7 6 2
H O
The FT-IR spectrum of LPB was recorded using BRUKER IFS-66V
spectrometer in the range between 4000 and 400 cm . The middle
no acid molecule exists as a zwitterion and the carboxylic acid
molecule is in a unionized state. There is a strong OAHꢁ ꢁ ꢁO inter-
molecular hydrogen bond between the phenylalanine and benzoic
acid molecules, and these molecular pairs form hydrogen-bonded
double layers. Recently, the crystal structures of complexes of
phenylalanine with maleic acid [9,10] and fumaric acid [11], tri-
chloroacetic acid [12] and malonic acid [13] were elucidated. The
ꢃ1
IR spectrum of LPB is shown in Fig. 2. The broad envelope between
ꢃ1
3
400 and 2500 cm includes absorption of stretching bands due
þ
to NH3 ion of the amino acid. The OAH stretch of COOH produces
ꢃ1
þ
the characteristic peak at 3452 cm . The NH3 asymmetric and
symmetric stretching vibrations are positioned at 3290 and
ꢃ1
2
598 cm respectively. These two bands clearly indicate unproto-
structure of L-phenylalanine–benzoic acid was first reported by
nation of carboxylate ions. In other words,
zwitterionic with respect to its amino acid grouping. Strong
carbonyl absorption at 1725 cm identifies the COOH and COO
L-phenylalanine salt is
Suresh et al. [14]. The present investigation deals with the growth
of LPB single crystal by slow solvent evaporation technique at
room temperature. The grown crystals were characterized by
studying their structural, optical, thermal, mechanical, and electri-
cal and laser damage threshold properties and reported for the first
time. The mechanical and electrical properties were performed
along (001) plane of the grown crystal.
ꢃ1
ꢃ
groups of the compound. Multiple fine structures at the lower
energy mode of the envelope indicates the strong hydrogen bond-
þ
ꢃ
ing interaction of NH group with weak absorptions of COO group
3
ꢃ1
at 1598 and 1417 cm . Further strong band observed at about
ꢃ1
ꢃ
1
217 cm is due to CACOO stretching. The presence of
L-phenyl-
alanine ions in the crystal structure is reflected in the bands of the
ꢃ1
ꢃ
ꢃ1
Experimental details
NH2 rocking (780 cm ), COO wagging (555 cm ), NH2 out-
ꢃ1
of-plane bending (703 cm ) and OH out-of-plane deformation
ꢃ1
Synthesis of LPB
(620 cm ). Moreover, rocking mode, symmetric deformation and
wagging are found from the spectrum and are presented in Table 1.
Stoichiometric amounts of
ken in 1:1 ratio were dissolved in double distilled water to prepare
the aqueous solution of LPB. In deionized water, -phenylalanine
L-phenylalanine and benzoic acid ta-
Optical studies
L
and benzoic acid undergo the following chemical reaction to pro-
duce LPB.
Fig. 3 shows the optical absorption spectrum of LPB along with
Tauc’s plot. The good absorption property of the crystal in the
entire visible region suggests its suitability for second harmonic
C
9
H11NO
2
þ C
7
H
6
O
2
!C
9
H11NO
2
ꢁ C
7 6 2
H O
Synthesized salt of LPB was obtained from the solution by evap-
orating the solvent and collecting the precipitate formed at the
bottom of the container having the solution.
Growth of LPB single crystals
Based on the solubility data, the supersaturated solution was
prepared at room temperature for growth experiments. The sol-
vent of the supersaturated solution was allowed to evaporate
through the perforated lid of the container. Due to spontaneous
nucleation, optically clear tiny crystals were formed in a period
of 5–7 days. The defect free and well shaped ones were chosen
and used as seed crystals for further growth experiments. Good
optical grade and colorless crystals of dimension up to
Fig. 1. Photograph of as grown LPB single crystals.