Growth and physicochemical properties of l-phenylalaninium maleate: A novel nonlinear optical crystal

Article abstract of DOI:10.1016/j.saa.2012.03.088

Single crystals of novel organic nonlinear optical material, l-phenylalaninium maleate (LPM) having dimensions up to 10 × 6 × 6 mm³ were grown by slow evaporation solution growth technique at room temperature. The grown crystals were confirmed by single crystal X-ray diffraction studies and the various functional groups were identified by FT-IR spectroscopic analysis. The optical properties were studied by optical absorption, second harmonic generation (SHG), photoconductivity and photoluminescence (PL) studies and reported for the first time. The optical band gap energy and SHG efficiency were found to be 4.85 eV and 1.5 times higher than KDP crystals. Negative photoconducting nature was confirmed by photoconductivity studies. Photoluminescence studies confirm the suitability of the sample for blue and green radiations. The title compound was also subjected to TG/DTA analysis and dielectric studies. Layer like growth pattern was analyzed by SEM micrograph.

Full text of DOI:10.1016/j.saa.2012.03.088

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 369–373
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Growth and physicochemical properties of L-phenylalaninium maleate:
A novel nonlinear optical crystal
b,
c
d
e
f
F. Yogam a, , I. Vetha Potheher , M. Vimalan , R. Jeyasekaran , T. Rajesh Kumar , P. Sagayaraj
⇑
⇑
a Department of Physics, Anand Institute of Higher Technology, Chennai 603 103, India
b Department of Physics, Anna University of Technology Tiruchirappalli, Tiruchirappalli 620 024, India
c Department of Physics, S.T. Hindu College, Nagercoil 629 002, India
d Department of Physics, Saveetha Engineering College, Chennai 602 105, India
e Department of Physics, N.P.R. College of Engineering and Technology, Natham 624 401, India
f Department of Physics, Loyola College, Chennai 600 034, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Single crystals of novel organic nonlinear optical material,
L
-phenylalaninium maleate (LPM) having
dimensions up to 10 ꢀ 6 ꢀ 6 mm3 were grown by slow evaporation solution growth technique at room
temperature. The grown crystals were confirmed by single crystal X-ray diffraction studies and the var-
ious functional groups were identified by FT-IR spectroscopic analysis. The optical properties were stud-
ied by optical absorption, second harmonic generation (SHG), photoconductivity and photoluminescence
(PL) studies and reported for the first time. The optical band gap energy and SHG efficiency were found to
be 4.85 eV and 1.5 times higher than KDP crystals. Negative photoconducting nature was confirmed by
photoconductivity studies. Photoluminescence studies confirm the suitability of the sample for blue
and green radiations. The title compound was also subjected to TG/DTA analysis and dielectric studies.
Layer like growth pattern was analyzed by SEM micrograph.
Received 23 December 2011
Received in revised form 15 February 2012
Accepted 26 March 2012
Available online 6 April 2012
Keywords:
X-ray diffraction
Solution growth technique
Nonlinear optical material
Scanning electron microscope
Photoconductivity
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
etc. [2]. Organic materials with delocalized p electrons usually dis-
play a large NLO response which makes it most resourceful for var-
ious applications including optical communication, optical
computing, optical information processing, optical disk data stor-
age, laser fusion reactions and laser remote sensing [3]. In solid
state amino acid contains a protonated amino group ðNHþ3 Þ and
deprotonated carboxylic acid group (COOꢁ). The dipolar nature
exhibits peculiar physical and chemical properties in amino acids
which make them ideal candidates for NLO applications [4–6].
Organic nonlinear optical (NLO) materials are attracting a great
deal of attention due to their potentially high nonlinearity and ra-
pid response in the electro-optic effect compared to inorganics.
New molecular organic compounds with one or more aromatic sys-
tems in conjugated positions, leading to highly efficient charge
transfer systems have been actively studied. Most of the organic
crystals are composed of aromatic molecules that are substituted
with
p
electron donors and acceptors which exhibit intermolecular
Maleic acid with relatively large p conjugation is yet another ami-
charge transfer resulting in high SHG efficiency. These compounds
must crystallize in a non-centrosymmetric class in view of applica-
tions making use of quadratic optically nonlinear effects. Organic
compounds are formed by weak Van der Waal’s and hydrogen
bonds and possess high degree of delocalization [1]. Hence they
are optically more nonlinear than inorganic materials. Some of
the advantages of organic materials include flexibility in the meth-
ods of synthesis, scope for altering the properties by functional
substitution, inherently high nonlinearity, high damage resistance,
no acid NLO material. Efforts have been taken to combine amino
acids with interesting organic acid (Maleic acid), to produce out-
standing materials to challenge the existing prospective materials.
These materials exhibit promising structural background in view of
their zwitterionic and protonated forms and structural stabiliza-
tion with hydrogen bonding. Maleate complexes of a-amino acids
are reported for their potential second harmonic generation [7–
10]. Earlier, synthesis, SHG and thermal properties of LPM were re-
ported by Anbuchezhiyan et al. [11]. In the present investigation,
the size of the crystal was improved compared to the reported size
[11]. The grown crystals were characterized by single crystal X-ray
diffractometry (XRD), FT-IR, UV–Vis–NIR, thermal and SHG analy-
ses. Also the grown crystals were subjected to dielectric, photocon-
ductivity, photoluminescence studies and SEM analysis and
reported for the first time.
⇑
Corresponding authors. Mobile: +91 99655 59905 (F. Yogam), +91 99429 94274
(I.V. Potheher).
E-mail addresses: yogamveda@gmail.com (F. Yogam), potheher11@gmail.com
(I. Vetha Potheher).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.03.088

370
F. Yogam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 369–373
the sample was kept unexposed from any radiation. After measur-
Experimental procedure
Synthesis and crystal growth
ing the dark current, the photocurrent was measured by illuminat-
ing the sample with a halogen lamp of 100 W power. A spot of light
on the sample was focused with the help of a convex lens. The
resulting photocurrent was measured by varying the applied field
for the same range. Surface morphology of the grown crystal was
analyzed by Scanning Electron Microscopy (SEM) using JEOL/EO-
JSM-5610 Scanning Electron Microscope.
High purity L-phenylalanine (Merck 99%) and maleic acid (Ana-
lar grade) are taken in 1:1 M ratio and dissolved in deionized
water. The reaction is as follows,
C6H5CH2CHðNH2ÞþC4H4O4 ! C6H5CH2CHðNH3ÞþCOOH C4H3Oꢁ
4
Based on the solubility data, the supersaturated solution was
prepared and the solution was stirred for 3–4 h. Then the solution
was covered by Teflon sheet and kept for nucleation. A number of
small holes were made on the top of the Teflon sheet for evapora-
tion of the solution. Due to spontaneous nucleation, optically
quality transparent single crystals were grown in a period of
25–30 days with dimensions up to 16 ꢀ 2 ꢀ 2 mm3 by slow solvent
evaporation technique. The size of the crystal was improved by
successful recrystallization. The dimension of the grown crystal
was improved compared to the reported dimensions [11]. The pho-
tograph of as grown single crystal of LPM crystal is shown in Fig. 1.
From the photograph, it is evident that the grown crystals exhibit
needle shaped morphology.
Results and discussion
Single crystal XRD
The single crystal XRD analysis data of LPM indicate that it crys-
tallizes in the monoclinic system with P21 space group and the cell
parameters are a = 11.0564 Å, b = 5.3321 Å and c = 11.4710 Å. The
single crystal XRD data of LPM are presented in Table 1. The single
crystal XRD data determined in the present work are in good
agreement with the reported data [13].
FT-IR analysis
Fig. 2 shows FT-IR spectrum of LPM. From the spectrum, it is ob-
served that the peak at 3423 cmꢁ1 is assigned to OH stretch of
water. It is supported by its bending mode at 1625 cmꢁ1. The OH
stretch of maleate should also occur close to 3423 cmꢁ1. But it is
not clearly resolved. The peak at 3177 cmꢁ1 is due to N–H vibration
of NHþ3 . The aromatic C–H vibration gives its peak at 3032 cmꢁ1. The
aliphatic C–H vibration occurs at 2971 and 2948 cmꢁ1. Hydrogen
bonding of maleate OH water and NH of NHþ3 give peaks at 2714
Characterization
In order to confirm the grown crystal, single crystal X-ray dif-
fraction studies were carried out using ENRAF NONIUS CAD4-F sin-
gle X-ray diffractometer. The various functional groups present in
LASP crystal were identified and conformed by the FT-IR study.
The spectrum was recorded in the range 4000–400 cmꢁ1 using
BRUKER IFS-66V spectrometer by KBr pellet technique. The UV–
Vis–NIR analysis of LASP crystal was carried out between 200 and
2000 nm covering the entire near ultra violet, visible and infrared
regions using the VARIAN CARY 5E model spectrophotometer. The
thermal behavior of the grown crystal was characterized using
SDT Q600 thermal analyzer in nitrogen atmosphere at a heating
rate of 20 °C/min in the temperature range of 20–1000 °C. The mea-
surement of dielectric constant and dielectric loss as a function of
frequency and temperature were calculated using HIOKI 3532-50
LCR HITESTER in the frequency range 100 Hz–5 MHz. The SHG effi-
ciency was measured by employing Kurtz powder technique [12].
The photoluminescence of the sample was recorded by VARIAN
spectrometer with 2.5 kV fluorescence lamp. The measurements
of photocurrent (Ip) and dark current (Id) were carried out by a pico
Ammeter (Keithley 485). A well polished sample was attached to
the microscopic slide on which two copper electrodes were fixed
with silver paint at known distance of 1 mm. The applied field
was varied from 150 to 2650 V/cm. For measuring the dark current,
and 2606 cmꢁ1
. The C@O vibration of maleate occurs at
1723 cmꢁ1. The asymmetric and symmetric vibrations of NHþ3 occur
at 1610 and 1498 cmꢁ1. The asymmetric and symmetric vibrations
of CO2 occur at 1573 and 1386 cmꢁ1. The bending modes of CH2
give peaks at 1358 and 1458 cmꢁ1. The C–COO vibration give peaks
at 1270 and 1230 cmꢁ1. The C–H bending mode of maleate gives a
shoulder in the lower energy region of the peak at 700 cmꢁ1. The
aromatic C–H bend modes are due to peaks at 700 and 739 cmꢁ1
.
The torsional oscillation of NHþ3 gives
a peak at 576 and
520 cmꢁ1. Hence from the IR spectral analysis the presence of male-
ate in association with phenylalanine is clearly evident.
Optical absorption studies
The UV–Vis–NIR response curve of LPM (Fig. 3) shows very low
absorption in the visible and NIR regions. The UV cut-off wave-
length of LPM is around 240 nm which is lower than some of the
amino acid single crystals such as
L-arginine tetrafluoroborate
(LAFB) (270 nm) and -arginine maleate (LARM) (300 nm) [14,7].
L
The optical energy gap of LPM single crystal has been calculated
Table 1
Single crystal XRD data of LPM crystal.
Crystal data
L-Phenylalaninium maleate
Empirical formula
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C13H15NO6
Monoclinic
P21
11.057
5.330
11.472
90
a
(°)
b (°)
101
c
(°)
90
676
Volume (Å3)
Z
2
Fig. 1. Photograph of grown single crystal of LPM.

F. Yogam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 369–373
371
Fig. 2. FT-IR spectrum of LPM.
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Eg = 4.85 eV
Fig. 3. UV–Vis–NIR spectrum of LPM crystal.
0.0
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
2
hν (eV)
using Tauc’s plot [15]. A plot is drawn between h
m and (ahm)
(Fig. 4). By extrapolating the straight line portion of the curve,
the optical band gap energy was determined and it was found to
be 4.85 eV.
Fig. 4. Tauc’s plot of LPM.
LPM. During the next stages, maleic acid in the crystal decomposes
and becomes anhydride [9] and results in further release of CO2
and CO molecules at 197.52 and 278.75 °C at the third stage. The
reactions of simplest amino acids induced by heating include the
condensation reactions of carboxyl and amino groups leading to
the formation of peptide bonds. More volatile substances such as
CO and CH4 are liberated around 396.91 °C. Further heating results
in the liberation of NH3 molecule at 523.69 °C.
NLO studies
The powdered sample of LPM was illuminated using the funda-
mental beam of 1064 nm from Q-switched Nd:YAG laser with
pulse energy 6 mJ/pulse and pulse width of 10 ns at a repetition
rate of 10 Hz. Emission of green radiation from the sample con-
firms the presence of NLO property. Second harmonic signals
(532 nm) of 55 and 85 mV were obtained through KDP and LPM
samples, respectively. Thus the SHG efficiency of LPM is 1.5 times
higher than that of KDP. It is seen that the SHG efficiency of LPM is
greater when compared with some of the amino acid analogs such
as LAFB (0.8) [13] and LARPCL (1.2) [16].
The endothermic peak observed at 127 °C in DTA trace corre-
sponds to the melting point of the sample. This is attributed to uti-
lization of the thermal energy to overcome the valence bonding
between the L-phenylalaninium cation and maleate anion, which
happens in the initial stage of decomposition. This endotherm
and other endotherms that follow at high temperatures coincide
with different stages of weight loss in TGA.
Thermal analysis
Thermograms shown in Fig. 5 are illustrating the simultaneous
record of TGA and DTA of LPM. As indicated by the TGA curve the
initial decomposition of the sample starts at about 140.90 °C. Since
it is beyond 100 °C, there is no water of crystallization present in
Dielectric studies
Studies on the frequency and temperature dependence of
dielectric properties unveil useful information about structural

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F. Yogam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 369–373
Fig. 5. TG/DTA thermogram of LPM.
changes, defect behavior and transport phenomena. Figs. 6 and 7
show the variation of dielectric constant and dielectric loss with
log frequency at different temperatures. The dielectric constant
of the materials is due to the contribution of electronic, ionic, ori-
entational and space charge polarizations, which depend on the
frequencies. The influence of space charge polarization is notice-
able in the low frequency region. Materials used for device fabrica-
tion in photonics industry demand the low value of dielectric
constant [17].
From Fig. 7 it is observed that the dielectric loss is strongly
dependent on the frequency of the applied field as in the case of
dielectric constant. In the low frequency region, dielectric loss is
more due to the loss associated with ionic mobility [17]. However,
the low value of dielectric loss at high frequency implies that this
parameter is of vital importance for NLO materials in their applica-
tion [18].
Photoluminescence studies
PL spectrum of the sample was recorded between 300 and
800 nm. A graph was plotted against PL intensity and wavelength.
Fig. 8 shows the variation of PL intensity with wavelength. From
the figure the intense emission peak was appeared close to 400
and 550 nm, which may be due to the emission of blue radiation.
This is because of aromatic ring and their contribution to molecular
association through Van der Waal’s force in the crystal. The broad-
ening of emission illustrates much interaction of carboxyl group
with the lattice. The sharp emission peaks observed between 500
900
3.0
308 K
328 K
348 K
800
700
600
500
400
300
200
100
0
308 K
328 K
2.5
2.0
1.5
1.0
0.5
0.0
348 K
1
2
3
4
5
6
7
2
3
4
5
6
7
log f
log f
Fig. 6. Variation of dielectric constant with log frequency for LPM.
Fig. 7. Variation of dielectric loss with log frequency for LPM.

F. Yogam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 369–373
373
Photoconductivity studies
Fig. 9 shows the variation of field dependent dark current and
photo current of LPM crystal. It is observed that both the dark
and photo currents increase linearly with the applied electric field,
but the photocurrent is less compared to the dark current which is
termed as negative photoconductivity. The negative photoconduc-
tivity exhibited by the sample may be due to the reduction of the
number of charge carriers in the presence of radiation [19].
SEM analysis
The SEM micrographs of LPM were taken at room temperature
with magnification factor 75 in the operating voltage of 20 kV and
are shown in Fig. 10. The figure indicates the layer like growth pat-
tern. Also few micro particles are seen on the surface of the crystal.
Conclusion
Fig. 8. Photoluminescence spectrum of LPM crystal.
Optically good quality single crystals of LPM were grown by
slow solvent evaporation technique. Single crystal XRD confirms
that the grown crystal belongs to monoclinic crystal system with
P21 space group. The optical absorption spectrum confirms that
the crystal has very low absorption in the entire visible and infra-
red regions, with lower UV cut-off wavelength around 240 nm,
which is an essential consideration for NLO crystals. Thermogravi-
metric analysis implies that the grown crystal was thermally stable
up to 127 °C. The photoconductivity study confirms the negative
photoconductivity nature of the grown crystal. The dielectric stud-
ies indicate that the dielectric constant and dielectric loss de-
creases with increasing frequency. The SEM results revealed the
surface morphology of the as grown crystal. The suitability of the
material in the blue and green emission regions was confirmed
by photoluminescence studies.
150
100
50
Photocurrent
Dark current
0
Acknowledgment
0
200
400
600
800
1000
The authors acknowledge Dr. M. Palanichamy (Department of
Chemistry, Anna University, Chennai 600 025) for useful discus-
sions and constant encouragement.
Electric field (V/cm)
Fig. 9. Field dependent photoconductivity of LPM.
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