Synthesis, Crystal Growth and Characterization of L-ProlineLithium Chloride Monohydrate: A New Semiorganic NonlinearOptical Material

T. Uma Devi,*,† N. Lawrence,‡ R. Ramesh Babu,§ S. Selvanayagam,|

Helen Stoeckli-Evans,⊥ and K. Ramamurthi§

Department of Physics, CauVery College for Women, Tiruchirappalli - 620018, India, Department ofPhysics, St. Joseph’s College (Autonomous), Tiruchirappalli - 620002, India, Crystal Growth and ThinFilm Laboratory, School of Physics, Bharathidasan UniVersity, Tiruchirappalli - 620024, India,Department of Physics, Kalasalingam UniVersity, Krishnan Koil - 626 190, India, and Institute ofPhysics, UniVersity of Neuchatel, Rue Emile-Argand 11, CP 158, CH-2009 Neuchatel

ReceiVed June 7, 2008; ReVised Manuscript ReceiVed December 2, 2008

ABSTRACT: A new semiorganic material, L-proline lithium chloride monohydrate (LPLCM), was synthesized for the first time.Its solubility and metastable zonewidth in double distilled water were estimated. Employing a temperature reduction method, acrystal of size 16 × 6 × 5 mm3 was grown from aqueous solution. The cell dimensions obtained by single crystal X-ray diffractionstudies reveal that the crystal belongs to the monoclinic system. Functional groups of the grown crystal were identified from FTIRspectral analysis. UV-vis-NIR studies show that the crystal is transparent in the wavelength range of 300-1100 nm. Secondharmonic generation conversion efficiency found using the Kurtz and Perry method is about 0.2 times that of KDP. The thermalstability of the compound was determined by TG-DTA analyses of the specimen. The microhardness test was carried out, and theload dependent hardness was measured.

Introduction

Second-order nonlinear optical (SNLO) materials have at-tracted much attention because of their potential applicationsin emerging optoelectronic technologies.1,2 Organic materialshave been of particular interest, because their nonlinear optical(NLO) responses in these materials is microscopic in origin,thus offering an opportunity to use theoretical modeling coupledwith synthetic flexibility to design and produce novel materials.Inorganic materials have excellent mechanical and chemicalproperties but are often of limited use because they possess lownonlinear coefficients when compared with organic counterpartsor it is difficult and expensive to grow these crystals. Becauseof the properties of organic and inorganic materials, semiorganicmaterials have the potential for combining the high opticalnonlinearity and chemical flexibility of organics with thephysical ruggedness of inorganic materials.3 Thus, extensiveinvestigation in this direction has resulted in the discovery of aseries of new semiorganic NLO crystals.4 A close survey ofthe literature shows that the various amino acids offer a widerange of choice to synthesize new semiorganic materialsexhibiting enhanced NLO properties.5-7

Among the amino acids, all except glycine, are characterizedby chiral carbons, a proton donating carboxyl (-COOH) group,and a proton-accepting amino (-NH2) group. Proline is anabundant amino acid in collagen and is exceptional among theamino acids because it is the only one in which the amine groupis part of a pyrrolidine ring, thus making it rigid and directionalin biological systems.8 L-Proline has been exploited for theformation of salts with different organic and inorganic acids.9

L-Prolinium picrate exhibits relative second harmonic generation

(SHG) efficiency 52 times higher than that of KDP.10 Growthand characterization of L-proline cadmium chloride monohy-drate11 single crystal was reported recently. As metal atoms orions occur widely in association with proteins and show a varietyof functions, one can expect that synthesizing the amino acidcomplexes with metal salts and characterizing them would yielduseful and informative results.12-14 Hence, in this work wereport on the synthesis and growth of a new semiorganic NLOcrystal L-proline lithium chloride monohydrate (LPLCM) fromaqueous solution by a temperature reduction method for the firsttime.

Experimental Procedures

Synthesis. LPLCM was synthesized by the reaction between lithiumchloride (Loba Chemie) and L-proline (Loba Chemie) taken in anequimolar ratio. The calculated amount of lithium chloride was firstdissolved in double distilled water. L-Proline was then slowly addedto the solution and stirred well using a temperature controlled magneticstirrer to yield a homogeneous mixture of solution. Then the solutionwas allowed to evaporate at room temperature, which yielded thecrystalline salt of LPLCM. The reaction mechanism involved in thesynthesis of LPLCM is given in Scheme 1.

Solubility. The amount of LPLCM required to saturate the aquasolution at 30 °C was estimated from a gravimetric method, and thisprocess was repeated for different temperatures. The solubility dataobtained in this work was used to estimate the metastable zonewidth.One hundred milliliters of solution was preheated to 5 °C above the

* Corresponding author. E-mail: kavin_shri@yahoo.co.in. Phone: 91-431-2751232. Fax: +91-431-2407045.

† Cauvery College for Women.‡ St. Joseph’s College.§ Bharathidasan University.| Kalasalingam University.⊥ University of Neuchatel.

Scheme 1. The Reaction Mechanism Involved in theSynthesis of LPLCM

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saturation temperature. Metastable zonewidth was estimated by theconventional polythermal method,15 where the equilibrium saturatedsolution is cooled from the superheated temperature to a temperatureat which the first speck is observed. This corresponds to metastablezonewidth at that particular temperature. The solubility curve alongwith the metastable zonewidth is represented in Figure 1. One canobserve that the metastable zonewidth decreases with increasingtemperatures.

Crystal Growth. Saturated aqua solution of LPLCM was preparedat 35 °C from recrystallized salt, and this solution was filtered withmicrofilters. About 200 mL of this solution was taken in a beaker andplaced in a constant temperature bath having an accuracy of (0.01°C. One of the better quality crystals obtained from slow evaporationof the solvent at room temperature was used as a seed crystal (Figure2a). Single crystal of LPLCM was grown by reducing the temperaturefrom 35 to 33.5 °C at the rate of 0.1 °C per day. Optically clear andwell-shaped crystal of size 16 × 6 × 5 mm3, harvested in a growthperiod of 15 days, is shown in Figure 2b.

Results and Discussion

The three-dimensional crystal structure of LPLCM wasdetermined by single crystal X-ray diffraction analysis. Suitablecrystals of LPLCM were obtained as colorless blocks by theslow evaporation technique. Data set was obtained using aStoe Image Plate Diffraction System16 using Mo KR graphitemonochromated radiation at 193 K. The structure was solvedby Direct methods using the program SHELXS-9717 and refinedby full-matrix least-squares method using SHELXL-97.17 TheR-value of the full-matrix least-squares refinement is given inTable 1. The H-atoms could all be located in Fourier differencemaps. The water H-atoms were freely refined, while theremainder of the H-atoms were included in calculated positionsand treated as riding atoms using SHELXL default parameters.

The X-ray study confirmed the molecular structure and atomicconnectivity for the title compound as illustrated in thePLATON18 drawing (Figure 3). All the C-C and C-N bondlengths in the five-membered ring are comparable to the relatedliterature values. The molecular structure is influenced by strongintramolecular N-H · · ·O hydrogen bond involving atoms

N1-H1N · · ·O1. Chlorine atom plays a major role for molecularpacking in the form of three intermolecular hydrogen bonds.The water molecule hydrogen atoms is involved in an O-H · · ·Clintermolecular hydrogen bond with two neighboring moleculesin the equivalent positions 1 + x, y, z and 1 + x, 1 + y, z. Inaddition to this, atom N1 is involved in two intermolecularhydrogen bond with the chlorine atom at (1 + x, y, z and 1 -x, -1/2 + y, 1 - z) through the hydrogen atom H2N. One ofthese hydrogen bonds creates a C(9) chain motif along the “bc”plane in the unit cell (Figure 4). The molecules are furtherstabilized by weak C-H · · ·O intermolecular interaction involv-ing atoms C2-H2 · · ·O1.

The FTIR spectrum of the grown LPLCM crystal wasrecorded using KBr pellet technique in the frequency region400-4000 cm-1 (Figure 5) employing Perkin-Elmer spec-trometer. In LPLCM, the peak at 3401 cm-1 is assigned to theOH stretching vibration of H2O (O-H). The peak at 3191 cm-1

corresponds to the NH stretching vibration. The absence of anystrong IR band at 1700 cm-1 indicates the existence of theCOO- ion in zwitterionic form. The peak at 1527 cm-1 is

Figure 1. The solubility curve and metastable zonewidth of LPLCM.

Figure 2. (a) As-grown crystal of LPLCM by slow evaporation method,(b) as-grown crystal of LPLCM by slow cooling method.

Table 1. Single-Crystal X-ray Data of LPLCM Crystal

formula C5H11LiNO3+, Cl-

formula weight 175.54crystal system monoclinicspace group P21

a (Å) 7.6799 (10)b (Å) 5.0744 (5)c (Å) 10.3360 (15)R (°) 90� (°) 105.861 (16)γ (°) 90V (Å3) 387.47 (9)R1 0.0411WR2 0.1050

Characterization of L-Proline Lithium Chloride Monohydrate Crystal Growth & Design, Vol. 9, No. 3, 2009 1371

assigned to NH2+ in-plane deformation of LPLCM. The rocking

and wagging vibrations of COO- are observed at 414 and 603cm-1, respectively. The peak at 1423 cm-1 is characteristic of

the N-H vibration. The peak at 1329 is assigned to waggingof the CH2 group of the LPLCM. The vibrational frequenciesof LPLCM are compared in Table 2 with the correspondingfrequencies of L-proline cadmium chloride monohydrate.

The optical transmittance spectrum of LPLCM was recordedusing Shimadzu model 1601 spectrometer in the wavelengthrange of 300-1100 nm (Figure 6). Optically clear single crystalof thickness about 2 mm was used for this study. There is noappreciable absorption in the wavelength range 300-1100 nmas is the case of the amino acids,19 and the transmittance isapproximately 60% in this wavelength range. The lower cutoffof wavelength occurs at 350 nm. Thus the optical transmittancestudy shows that LPLCM is a good candidate for SHG.

The study of NLO efficiency of powder LPLCM was carriedout using Kurtz and Perry set up.20 A Q-switched Nd:YAG laserbeam of wavelength 1064 nm, with an input power of 5.5 mJ,and pulse width 8 ns with a repetition rate of 10 Hz was used.The grown single crystal of LPLCM was powdered with auniform particle size and then packed in a microcapillary tubeof uniform pore size and exposed to laser radiation. The outputfrom the sample was monochromated to collect the intensity of532 nm component and to eliminate the fundamental frequency.Second harmonic radiation generated by the randomly orientedmicrocrystals was focused by a lens and detected by aphotomultiplier tube. The generation of the second harmonicswas confirmed by the emission of green light. A sample ofpotassium dihydrogen phosphate (KDP), also powdered to thesame particle size as the experimental sample of LPLCM, wasused as a reference material and the relative SHG efficiency ofLPLCM is found to be about 0.2 times that of KDP. The relativeefficiency of LPLCM is found almost equal to the semiorganicsingle crystals of L-arginine hydrochloride, L-arginine hydro-bromide, and L-arginine hydrochloride bromide.21,22

Figure 3. The molecular structure and crystallographic numberingscheme (50% probability).

Figure 4. Crystal packing viewed along the b axis O-H · · ·Cl andN-H · · ·Cl hydrogen bonds are shown as dotted blue lines.

Figure 5. FTIR spectrum of LPLCM.

Table 2. Band Assignment of FTIR Spectrum for LPLCM

LPLCM LPCC11 frequency assignments11

1527 1543 NH2+ in-plane deformation

1423 1431 COO- symmetric stretching1370 1368 NH2

+ wagging1329, 1299 1332 CH2+ wagging

13101168 1170 NH2

+ twisting1031 1038 C-N stretching972 943 CH2 rocking922 917 NH2

+ rocking845 853 CH2 rocking778 782 COO- in-plane deformation661 630 COO- wagging

Figure 6. UV-vis-NIR spectrum of LPLCM.

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Differential thermal and thermogravimetric spectra for LPLCMwere recorded using a simultaneous thermal analyzer SDT Q600V8.2 Build 100. A ceramic crucible was used for heating thesample, and the analyses were carried out in an atmosphere ofnitrogen at a heating rate of 20 °C/min for the temperature range30-800 °C. The initial mass of the material subjected to theanalyses was 1.2950 mg. The thermogravimetric-differentialthermal (TG-DTA) curves of LPLCM are illustrated in Figure7. The TG analysis shows that between 112 and 136 °C theweight loss is about 13%. This indicates the loss of water ofhydration (H2O). The strong endothermic peak in DTA around126 °C with the associated shoulders indicates the stepwiseremoval of water during this temperature range. There is anotherstrong endothermic peak in DTA around 253 °C, indicatingmelting of the substance. The sharpness of this endothermicpeak shows the good degree of crystallinity and purity of thesample.23 This is associated with a loss of weight of about 29%in the TG curve (between 208 and 298 °C) and another weightloss of about 20% up to 387 °C. These weight losses are due todissociation of the substance and evaporation of volatilesubstances. There is a gradual and significant weight loss asthe temperature is increased above the melting point. There isno endothermic or exothermic peak up to 800 °C in the DTAcurve, whereas TGA shows almost complete weight loss andthe residual weight obtained at 800 °C is only 3.9%.

Vicker’s microhardness measurements were carried out onLPLCM crystal using an Ultra Microhardness Tester fitted witha diamond indenter. The indentations were made using a Vickerspyramidal indentor for various loads from 25 to 200 g. Thediagonals of the impressions were measured using a ReichertPolyvar 2 MET microscope with a Microduromat 4000Ehardness controller. The measurements were made on a well-developed face. Vickers microhardness number (Hv) wasevaluated from the relation Hv ) 1.8544P/d2 (kg/mm2), whereP is the indenter load in kilogram and d is the diagonal lengthof the impression in millimeters. The variation of microhardnessvalues with the applied load is shown in Figure 8. The hardnessvalue increases with increasing load. The plane considered forstudy exhibits the reverse indentation size effect. The specimen

does not offer resistance or undergo elastic recovery, butundergoes relaxation involving a release of the indentation stressaway from the indentation site. This leads to a larger indentationsize which gives rise to a lower hardness at low loads.24 Forloads above 200 g, cracks started developing around theindentation mark. It is concluded that the hardness of theLPLCM crystal is moderately good.

Conclusion

A new semiorganic crystal LPLCM of dimension 16 × 6 ×5 mm3 was grown by the temperature reduction method. Thecrystal is transparent and colorless with a well-defined externalappearance. Single crystal XRD shows that LPLCM crystallizesin the monoclinic system. The observed unit cell parametersare a ) 7.6799(10) Å, b ) 5.0744(5) Å, c ) 10.3360(15) Å, R) γ ) 90°, � ) 105.861(16)°, and the crystal belongs to thespace group P21. FTIR spectral analysis confirms the presenceof functional groups constituting LPLCM such as NH2

+, CH2,C-N, and COO-. The optical study shows that the crystal istransparent in the wavelength region of 300-1100 nm. Its SHG

Figure 7. TG-DTA of LPLCM.

Figure 8. Variation of microhardness values with applied load.

Characterization of L-Proline Lithium Chloride Monohydrate Crystal Growth & Design, Vol. 9, No. 3, 2009 1373

efficiency is about 0.2 times that of KDP. The microhardnessstudies reveal that the hardness of crystal is moderately good.Thus, LPLCM is a new candidate for NLO application in viewof its superior optical, mechanical properties, and moderatethermal stability. Further effort to modify this system with otherpossible derivatives may be expected to lead to new materialswith improved SHG efficiency.

Acknowledgment. One of the authors (T.U.) acknowledgesCauvery College for Women (Reddy Educational Trust) forproviding laboratory facilities to carry out the research work.The authors acknowledge Prof. P. K. Das, Department ofInorganic and Physical Chemistry, Indian Institute of Science,Bangalore, for extending the laser facilities for the SHGmeasurement.

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