Studies on the growth aspects of organic L-alanine maleate: a promising nonlinear optical crystal

doi:10.4236/ns.2009.13029

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Vol.1, No.3, 216-221 (2009)

doi:10.4236/ns.2009.13029

SciRes

Natural Science

Studies on the growth aspects of organic L-alanine

maleate: a promising nonlinear optical crystal

D. Balasubramanian1,3*, R. Jayavel2, P. Murugakoothan3

1Department of Physics, Municipal School, Tiruvannamalai, India; dr.d.balu@gmail.com

2Crystal Growth Centre, Anna University, Chennai, India

3PG & Research Department of Physics, Pachaiyappa’s College, Chennai, India

Received 17 September 2009; revised 10 October 2009; accepted 12 October 2009.

ABSTRACT

A organic nonlinear optical material, L-alanine

maleate (LALM) was synthesized. Bulk Single

crystals of LALM have been grown by slow

cooling method with a solution pH of 5. The

solubility of L-alanine maleate has been deter-

mined for various temperatures. Large size sin-

gle crystal of 2.0 x 1.2 x 0.8 cm3 has been grown

with reasonable growth rate along the three

crystallographic directions by optimizing the

growth parameters. The structure of LALM crys-

tal was studied by single-crystal X-ray diffrac-

tion analysis. The presence of functional groups

was confirmed by Fourier transform infrared

spectroscopy. The LALM crystal was analysed

for its thermal and mechanical behaviours. The

grown crystals have also been subjected to

linear and non-linear optical property studies.

From these studies, it is inferred that the LALM

crystals exhibit better thermal and mechanical

stabilities with improved optical properties.

Thus satisfies the essential requirements for

optical device fabrication.

Keywords: Organic; LALM; Crystal Growth; Bulk

Single Crystals; Non-Linear Optical Crystal

1. INTRODUCTION

Recently, there is considerable interest in the synthesis

of new nonlinear optical (NLO) material, both organic

and inorganic, with large second-order optical nonlin-

earities, as these materials have a significant impact on

laser technology, optical communication and optical

storage technology etc. Over the years many organic and

inorganic materials have been developed [1-4] to cover

the potential applications in ultra-violet, near-and far-

infrared wavelength regions. Amino acid nonlinear opti-

cal materials are often formed by weak van der Waals

and hydrogen bonds and hence possess high degree of

delocalization. The basic structure of organic NLO ma-

terials is based on the  bond system. Due to the over-

lapping of  orbital, the delocalization of electronic

charge distribution leads to a high mobility of electrons.

Functionalization of both ends of the  bond system with

appropriate electron donor and acceptor groups can en-

hance the asymmetric electronic distribution in either or

both ground and excited states, this leads to an increased

optical nonlinearity. All these favourable properties

paved the way for the development of amino acid crys-

tals like L-arginine phosphate (LAP) [5], L-histidine

dihydrogen phosphate (LHP) [6], L-arginine tetrafluo-

roborate (L-AFB) [7], L-alanine tetrafluoroborate

(L-AlFB) [8], L-alanine [9], L-arginine acetate [10], and

L-alanine acetate [11]. LAP crystal was reported to have

promising NLO properties comparable to that of the

well-known inorganic crystal of KDP. L-alanine crystal

shows Type II phase matching, for doubling the Nd:YAG

fundamental, by propagating the pump beam nearly

normal to the {12 0} and {011} faces.

The growth of single crystals of L-alanine which is

the simplest acentric member of the amino acid family

has already been little investigated. In order to widen the

properties of L-alanine and to develop new crystals with

better NLO properties, L-alanine complexes with car-

boxylic acids have been tried. Maleic acid, a dicarbox-

ylic acid with relatively large pi-conjugation has at-

tracted much attention. Though the L-alanine maleate

crystal was already grown by slow evaporation method

[12], in the present study, bulk single crystals of

L-alanine maleate (LALM) single crystals have been

grown by the slow cooling method with optimized

growth conditions first time. The grown crystals were

subjected to various characterization studies. The im-

D. Balasubramanian et al. / Natural Science 1 (2009) 216-221

217

proved optical transmittance, NLO efficiency and the

mechanical stability of the grown crystals were realized.

2. EXPERIMENTAL PROCEDURE

2.1. Crystal Growth

High purity (99%) L-alanine and analar grade maleic

acid were taken in equimolar ratio and dissolved in

de-ionized water. The solution was slightly heated and

kept in undisturbed conditions. Three days later, trans-

parent seed crystals were obtained. The synthesized salt

was used to prepare the growth solution according to the

following reaction.



CHCHNHCOOH COOH CHCH COOH  

L-alanine + Maleic acid

()CHCHNHCOOH OOCCHCHCOOH



 

L-alanine maleate (LALM)

The amount of L-alanine maleate dissolved in 10 ml

of water at 30 ºC was estimated from the saturated solu-

tion. The solubility was estimated for different tempera-

tures and the solubility of L-alanine maleate at 40 ºC is

estimated to be 33 g/100 ml. Crystals were grown from

aqueous solution prepared from the recrystallized salt of

LALM saturated at 40 ºC. In order to reproduce the su-

persaturation conditions, the solution was tested by

checking the dissolution of a probe crystal over a period

of one week. Then the solution was cooled down at a

rate of 0.1 C/day over a period of 25 days. Optical

quality single crystal elongated in c-axis was obtained.

2.2. Characterization

The structure of the crystals was examined by sin-

gle-crystal X-ray diffraction analysis and the lattice pa-

rameter values were determined. Powder X-ray diffrac-

tion analysis was also carried out using a Rich Seifert

diffractometer with CuK ( = 1.5418 Å) radiation to

verify the correctness of lattice parameter values. FTIR

spectrum was recorded by the KBr pellet technique us-

ing a Perkin–Elmer 783 spectrophotometer in order to

confirm the presence of functional groups in the crystal

lattice. Optical transmittance spectrum was recorded at

room temperature using Shimadzu 1601 (UV-VIS) spec-

trophotometer. Optical second-harmonic generation was

measured for the grown crystalline sample using Kurtz

and Perry technique. Thermo gravimetric (TG) and Dif-

ferential thermal analysis (DTA) for L-alanine maleate

dihydrate crystals were carried out by ZETZSCH–Ger-

atebau GmbH Thermal Analyzer. Etching studies were

carried out on the {011} face of L-alanine maleate crys-

tal using different etchants like water, methanol and

ethanol, in order to investigate the growth mechanism

and surface features. Microhardness studies were carried

out on the {011} face of the L-alanine maleate crystals.

3. RESULTS AND DISCUSSIONS

As-grown crystal of L-alanine maleate (LALM) is

shown in Figure 1. The crystals possess well defined

morphology with reasonable growth rate along all the

three crystallographic directions. The molecular struc-

ture with the numbering scheme is shown in the Figure

2. The cationic alanine molecule exists with a positively

charged amino group and an uncharged carboxylic acid

group. The maleic acid molecule exists in the monoion-

ized state (i.e. as a semimaleate). The semimaleate ion is

essentially planar and the intramolecular hydrogen bond

between atoms O3 and O5 is found to be asymmetric, as

in the crystal structure of maleic acid [13]. The single

crystal X- ray diffraction studies confirm the orthorhom-

bic structure with space group P212121. The lattice pa-

rameter values of LALM were calculated as given in the

Table 1.

Figure 1. Bulk single crystal of LALM grown in opti-

mizedgrowth condition.

Table 1. The single crystal X-ray data for LALM single crystal.

Molecular formula C3H8O2N+. C4H3O4-

Molecular weight 205.17

System Orthorhompic

Space group P212121

Lattice parameter a = 5.5873 Å,

b = 7.3864 Å,

c = 23.688 Å

Volume (V) 977.6 Å3

Number of atom in unit cell (Z) 4

Density 1.394 Mgm-3

D. Balasubramanian et al. / Natural Science 1 (2009) 216-221

218

The FTIR absorption spectrum of LALM is shown in

Figure 3. A broad, strong absorption in the 3300–2300

cm-1 range, including the absorptions at 3205 cm-1 cor-

responds to the stretching bonds of the NH3 + ion of the

amino acid. This region is due to superimposing of O–H

and NH3 + stretching bonds. Absorption in this region

was also characterized by multiple fine structures on the

lower wave number side of the bond and the weak ab-

sorptions due to COO- ions. The prominent absorption

band is relatively strong due to symmetric NH3+ bending

bond at 1569 cm-1. A strong band arising from C–COO -

stretching is observed at 1219 cm-1. Further strong car-

bonyl absorption at 1722 cm-1 confirms the COOH and

COO- groups of the compound. The C=O stretch of car-

boxylic acid was observed to produce its peak in the

same region of asymmetrical NH3+ bending vibration.

The CH3 bending modes were assigned to the peak at

1374 cm-1. O–H bending of the COOH group was ob-

served at 1333 cm-1. The peaks between 918 and 1106

cm-1 were assigned to asymmetrical coupled vibration of

maleic acid and alanine.

This analysis also indicates that the protonation of

carboxyl group in alanine takes place by maleic acid.

The absorptions of LALM have been compared with

those of the L-alanine [14] in Table 2. The shifts in the

positions of the characteristic peaks confirm the forma-

tion of the compound.

Figure 2. The molecular structure of LALM with atom-

numbering scheme and 50%probability displacement el-

lipsoids.

Figure 3. FT-IR spectrum of LALM crystal.

Table 2. FT-IR spectral band assignments of LALM.

L-alanine LALM Assignments

1506

1361

1114

850

772

649

3205

2930

1722

1504

1374

1333

1262

1219

1106

862

762

661

567

NH3+ asymmetric stretching

C-H stretching

COO- stretching

NH3+ symmetric bending

C-H deformation in CH3

O-H plane deformation in COOH

=C-H deformation (over tone)

C-COO- stretching

C-O stretching, NH3 rocking

O–H out-of-plane deformation

CH2 rocking

O-C = O in plane deformation

COO- wagging

The transmission spectrum of LALM crystal was re-

corded in the range 200–1200 nm is shown in Figure 4.

A sample of thickness 2mm was used to record the

transmission spectrum. The crystal possesses 75 % tran-

smittance and the lower cutoff is found to be as low as

320 nm, allowing for frequency conversion down to

UV-region which account for the suitability of this mate-

rial for optoelectronics applications and the second and

third harmonic generation of Nd:YAG fundamental.

The powder second harmonic generation (SHG) test

was carried out for LALM using Kurtz and Perry tech-

nique. Powdered sample of LALM was tightly packed in

the micro capillary tubes of uniform diameter (1.5 mm)

and irradiated by an incident laser radiation 1064 nm of

pulse width 8 ns and pulse energy of 10–800 mJ from a

Q-switched quanta ray of Nd:YAG laser. KDP was used

for calibrating the SHG intensity. The second harmonic

nonlinearity of LALM was confirmed by the emission of

green radiation (532 nm) by the crystal. The powder

SHG efficiency of LALM was found to be 1.2 times that

of the standard KDP. This confirms that the LALM has

higher SHG efficiency than the relative efficiencies of

L-alanine (0.2) and L-alanine acetate (0.3) with respect

to KDP [15].

D. Balasubramanian et al. / Natural Science 1 (2009) 216-221

219

Figure 4. Optical transmittance spectrum of LALM crystal.

(a)

(b)

Figure 5. (a) TG / DTG curves of LALM; (b) DTA/

TG curves of LALM.

(a)

(b)

(c)

Figure 6. Etch patterns obtained on (011) of LALM-

crystal for different etchants. (a) Water; (b) Methanol ;

Figure 7. Load (P) Vs Hardness number (Hv) of

LALM crystal.

400 600

8001000 1200

Waveleng

Transmittance%

th (nm)

10 20 30 40 50

Hardness number HV (kg/mm2)

Load P (g )

demodem

demodemodemodemodemo

odemodemodemo

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220

Simultaneous thermo-gravimetric analysis (TG) and

differential thermal analysis (DTA) were carried out for

the as-grown LALM crystals to study the thermal stabil-

ity. The characteristic curves are shown in Figure 5a and

5b. Finely powdered crystal was used for the TG/DTA

analysis in the temperature range of 26 to 500 C with a

heating rate of 5 C/min. The alumina (Al2O3) crucible

was used as a reference for the sample. A weight loss of

21% occur at 162 C in TGA corresponds to the decom-

position range of LALM. An endothermic peak observed

at 160 C in DTA is attributed to the utilization of ther-

mal energy to overcome the valence bonding between

the alaninium cation and the maleate anion, which hap-

pens during the initial stage of decomposition. As the

temperature is increased further, maleic acid decomposes

and becomes anhydride and results in the further release

of CO2 and CO molecules at 205 and 258 C, which is

evident from the DTG. The reactions of simplest amino

acids induced by heating include the condensation reac-

tions of carboxyl and amino groups leading to the for-

mation of peptide bonds. In the dehydration at the initial

stage, H2O molecule is not liberated immediately; in-

stead, it is absorbed by alumina, which acts as a catalyst,

and then is released along with another water molecule

obtained from the decomposition of alanine at 320C.

Because of this, an endothermic effect is noted in DTA.

CO and CH4 molecules are liberated at around 450C.

The NLO efficiency of the grown crystals mainly de-

pends on their optical quality, because the segregated

impurities and dislocations occurring during the growth

results in the distortion of the optical beam to be proc-

essed. In the present investigation, grown crystal of

LALM was subjected to chemical etching to study the

microstructural imperfection or crystal defects in the

grown crystal. A thin plates of 3 mm thickness parallel to

{0 1 1} face were cut from the as grown crystal of

LALM with the help of a wet thread. Polishing of the

surfaces was carried out using soft felt-cloth wetted with

ethanol and tertiary butanol mixture (3 : 1). Polished

plates of 2 mm in thickness, free from visible inclusions

or cracks were selected for etch pit study. Etchants em-

ployed to reveal dislocations are taken in homologous

series of alcohols i.e. water, methanol and ethanol.

Etching of the surfaces was carried out by dipping the

plates in etchants for few seconds to few minutes at

room temperature and then wiping them with dry filter

paper. Etch patterns were observed and photographed

under an optical (Carl-Zesis Jenavert) microscope in the

reflected light. Elongated circular etch pits were ob-

served when LALM single crystal was etched with water

for five seconds as shown in Figure 6a. There was no

change in etch pit dimension and density with varying

etching time (10–20 s). Trapezoid etch pits were ob-

served as shown in Figure 6b when LALM single crys-

tal was etched with methanol for five seconds. Rectan-

gular etch pits were observed when the crystal was

etched with ethanol for fifteen seconds as shown in Fig-

ure 6c. From the results of etching behavior of different

etchants on LALM crystals, it is inferred that all the or-

ganic solvents used in this experiment have successfully

revealed the presence of dislocation in the crystal. The

observed etch pits, due to layer growth, confirmed the

two dimensional (2D) nucleation mechanism with less

dislocations. Fast dissolving etchant like water produces

better contrasting dislocation etch pits of all surfaces and

hence it is intensive to surfaces orientation.

Hardness value on the (011) of LALM crystal was es-

timated for different loads. The relation between hard-

ness number (Hv) vs load (P) for LALM is shown in

Figure 7. At lower loads, hardness is relatively lower

and it increases for higher loads and remains constant up

to 40g. Above 40g, a significant cracking occurred due

to the release of internal stress generated locally by in-

dentation. It has been observed that the hardness values

of LALM are comparable with pure L-alanine.

4. CONCLUSIONS

A new organic optical material for second order NLO

applications, L-alanine maleate (LALM) was synthe-

sized. Single crystals were grown and characterized by

X-ray diffraction (single crystal XRD) to confirm the

formation of the crystalline phase. FT-IR spectroscopic

analysis confirms the presence of all the functional

groups in the crystal lattice. Etching studies were carried

out for LALM crystal using various etchants. Mechani-

cal behavior of grown crystal was studied on (011) using

micro hardness measurement and the hardness values are

found to be comparable with pure L-alanine. TG-DTA

studies reveal that the material starts decomposing at

162.2 oC. The UV-Vis spectrum establishes the good

transmittance window and the lower cutoff are found to

be as low as 320 nm, allowing for frequency conversion

down to UV-region. From the Kurtz-Perry powder tech-

nique, the second harmonic generation efficiency of the

grown LALM crystal was found to be 1.2 times that of

KDP crystal.

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