Synthesis, Crystal Growth and Characterization of L-Proline Lithium Chloride Monohydrate: A New Semiorganic Nonlinear Optical Material T. Uma Devi,*,† N. Lawrence,‡ R. Ramesh Babu,§ S. Selvanayagam,| Helen Stoeckli-Evans,⊥ and K. Ramamurthi§
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 3 1370–1374
Department of Physics, CauVery College for Women, Tiruchirappalli - 620018, India, Department of Physics, St. Joseph’s College (Autonomous), Tiruchirappalli - 620002, India, Crystal Growth and Thin Film Laboratory, School of Physics, Bharathidasan UniVersity, Tiruchirappalli - 620024, India, Department of Physics, Kalasalingam UniVersity, Krishnan Koil - 626 190, India, and Institute of Physics, UniVersity of Neuchaˆtel, Rue Emile-Argand 11, CP 158, CH-2009 Neuchaˆtel 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, a crystal of size 16 × 6 × 5 mm3 was grown from aqueous solution. The cell dimensions obtained by single crystal X-ray diffraction studies reveal that the crystal belongs to the monoclinic system. Functional groups of the grown crystal were identified from FTIR spectral analysis. UV-vis-NIR studies show that the crystal is transparent in the wavelength range of 300-1100 nm. Second harmonic generation conversion efficiency found using the Kurtz and Perry method is about 0.2 times that of KDP. The thermal stability of the compound was determined by TG-DTA analyses of the specimen. The microhardness test was carried out, and the load dependent hardness was measured. Introduction Second-order nonlinear optical (SNLO) materials have attracted much attention because of their potential applications in emerging optoelectronic technologies.1,2 Organic materials have 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 coupled with synthetic flexibility to design and produce novel materials. Inorganic materials have excellent mechanical and chemical properties but are often of limited use because they possess low nonlinear coefficients when compared with organic counterparts or it is difficult and expensive to grow these crystals. Because of the properties of organic and inorganic materials, semiorganic materials have the potential for combining the high optical nonlinearity and chemical flexibility of organics with the physical ruggedness of inorganic materials.3 Thus, extensive investigation in this direction has resulted in the discovery of a series of new semiorganic NLO crystals.4 A close survey of the literature shows that the various amino acids offer a wide range of choice to synthesize new semiorganic materials exhibiting enhanced NLO properties.5-7 Among the amino acids, all except glycine, are characterized by chiral carbons, a proton donating carboxyl (-COOH) group, and a proton-accepting amino (-NH2) group. Proline is an abundant amino acid in collagen and is exceptional among the amino acids because it is the only one in which the amine group is part of a pyrrolidine ring, thus making it rigid and directional in biological systems.8 L-Proline has been exploited for the formation of salts with different organic and inorganic acids.9 L-Prolinium picrate exhibits relative second harmonic generation * Corresponding author. E-mail:
[email protected]. Phone: 91-4312751232. Fax: +91-431-2407045. † Cauvery College for Women. ‡ St. Joseph’s College. § Bharathidasan University. | Kalasalingam University. ⊥ University of Neuchaˆtel.
Scheme 1. The Reaction Mechanism Involved in the Synthesis of LPLCM
(SHG) efficiency 52 times higher than that of KDP.10 Growth and characterization of L-proline cadmium chloride monohydrate11 single crystal was reported recently. As metal atoms or ions occur widely in association with proteins and show a variety of functions, one can expect that synthesizing the amino acid complexes with metal salts and characterizing them would yield useful and informative results.12-14 Hence, in this work we report on the synthesis and growth of a new semiorganic NLO crystal L-proline lithium chloride monohydrate (LPLCM) from aqueous solution by a temperature reduction method for the first time. Experimental Procedures Synthesis. LPLCM was synthesized by the reaction between lithium chloride (Loba Chemie) and L-proline (Loba Chemie) taken in an equimolar ratio. The calculated amount of lithium chloride was first dissolved in double distilled water. L-Proline was then slowly added to the solution and stirred well using a temperature controlled magnetic stirrer to yield a homogeneous mixture of solution. Then the solution was allowed to evaporate at room temperature, which yielded the crystalline salt of LPLCM. The reaction mechanism involved in the synthesis of LPLCM is given in Scheme 1. Solubility. The amount of LPLCM required to saturate the aqua solution at 30 °C was estimated from a gravimetric method, and this process was repeated for different temperatures. The solubility data obtained in this work was used to estimate the metastable zonewidth. One hundred milliliters of solution was preheated to 5 °C above the
10.1021/cg800589m CCC: $40.75 2009 American Chemical Society Published on Web 01/15/2009
Characterization of L-Proline Lithium Chloride Monohydrate
Crystal Growth & Design, Vol. 9, No. 3, 2009 1371
Figure 1. The solubility curve and metastable zonewidth of LPLCM.
saturation temperature. Metastable zonewidth was estimated by the conventional polythermal method,15 where the equilibrium saturated solution is cooled from the superheated temperature to a temperature at which the first speck is observed. This corresponds to metastable zonewidth at that particular temperature. The solubility curve along with the metastable zonewidth is represented in Figure 1. One can observe that the metastable zonewidth decreases with increasing temperatures. Crystal Growth. Saturated aqua solution of LPLCM was prepared at 35 °C from recrystallized salt, and this solution was filtered with microfilters. About 200 mL of this solution was taken in a beaker and placed in a constant temperature bath having an accuracy of (0.01 °C. One of the better quality crystals obtained from slow evaporation of the solvent at room temperature was used as a seed crystal (Figure 2a). Single crystal of LPLCM was grown by reducing the temperature from 35 to 33.5 °C at the rate of 0.1 °C per day. Optically clear and well-shaped crystal of size 16 × 6 × 5 mm3, harvested in a growth period of 15 days, is shown in Figure 2b.
Results and Discussion The three-dimensional crystal structure of LPLCM was determined by single crystal X-ray diffraction analysis. Suitable crystals of LPLCM were obtained as colorless blocks by the slow evaporation technique. Data set was obtained using a Stoe Image Plate Diffraction System16 using Mo KR graphite monochromated radiation at 193 K. The structure was solved by Direct methods using the program SHELXS-9717 and refined by full-matrix least-squares method using SHELXL-97.17 The R-value of the full-matrix least-squares refinement is given in Table 1. The H-atoms could all be located in Fourier difference maps. The water H-atoms were freely refined, while the remainder of the H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The X-ray study confirmed the molecular structure and atomic connectivity for the title compound as illustrated in the PLATON18 drawing (Figure 3). All the C-C and C-N bond lengths in the five-membered ring are comparable to the related literature values. The molecular structure is influenced by strong intramolecular N-H · · · O hydrogen bond involving atoms
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 crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) R1 WR2
175.54 monoclinic P21 7.6799 (10) 5.0744 (5) 10.3360 (15) 90 105.861 (16) 90 387.47 (9) 0.0411 0.1050
N1-H1N · · · O1. Chlorine atom plays a major role for molecular packing in the form of three intermolecular hydrogen bonds. The water molecule hydrogen atoms is involved in an O-H · · · Cl intermolecular hydrogen bond with two neighboring molecules in the equivalent positions 1 + x, y, z and 1 + x, 1 + y, z. In addition to this, atom N1 is involved in two intermolecular hydrogen bond with the chlorine atom at (1 + x, y, z and 1 x, -1/2 + y, 1 - z) through the hydrogen atom H2N. One of these hydrogen bonds creates a C(9) chain motif along the “bc” plane in the unit cell (Figure 4). The molecules are further stabilized by weak C-H · · · O intermolecular interaction involving atoms C2-H2 · · · O1. The FTIR spectrum of the grown LPLCM crystal was recorded using KBr pellet technique in the frequency region 400-4000 cm-1 (Figure 5) employing Perkin-Elmer spectrometer. In LPLCM, the peak at 3401 cm-1 is assigned to the OH stretching vibration of H2O (O-H). The peak at 3191 cm-1 corresponds to the NH stretching vibration. The absence of any strong IR band at 1700 cm-1 indicates the existence of the COO- ion in zwitterionic form. The peak at 1527 cm-1 is
1372 Crystal Growth & Design, Vol. 9, No. 3, 2009
Devi et al. Table 2. Band Assignment of FTIR Spectrum for LPLCM LPLCM
LPCC11
frequency assignments11
1527 1423 1370 1329, 1299
1543 1431 1368 1332 1310 1170 1038 943 917 853 782 630
NH2+ in-plane deformation COO- symmetric stretching NH2+ wagging CH2+ wagging
1168 1031 972 922 845 778 661
Figure 3. The molecular structure and crystallographic numbering scheme (50% probability).
Figure 4. Crystal packing viewed along the b axis O-H · · · Cl and N-H · · · Cl hydrogen bonds are shown as dotted blue lines.
NH2+ twisting C-N stretching CH2 rocking NH2+ rocking CH2 rocking COO- in-plane deformation COO- wagging
the N-H vibration. The peak at 1329 is assigned to wagging of the CH2 group of the LPLCM. The vibrational frequencies of LPLCM are compared in Table 2 with the corresponding frequencies of L-proline cadmium chloride monohydrate. The optical transmittance spectrum of LPLCM was recorded using Shimadzu model 1601 spectrometer in the wavelength range of 300-1100 nm (Figure 6). Optically clear single crystal of thickness about 2 mm was used for this study. There is no appreciable absorption in the wavelength range 300-1100 nm as is the case of the amino acids,19 and the transmittance is approximately 60% in this wavelength range. The lower cutoff of wavelength occurs at 350 nm. Thus the optical transmittance study shows that LPLCM is a good candidate for SHG. The study of NLO efficiency of powder LPLCM was carried out using Kurtz and Perry set up.20 A Q-switched Nd:YAG laser beam 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 a uniform particle size and then packed in a microcapillary tube of uniform pore size and exposed to laser radiation. The output from the sample was monochromated to collect the intensity of 532 nm component and to eliminate the fundamental frequency. Second harmonic radiation generated by the randomly oriented microcrystals was focused by a lens and detected by a photomultiplier tube. The generation of the second harmonics was confirmed by the emission of green light. A sample of potassium dihydrogen phosphate (KDP), also powdered to the same particle size as the experimental sample of LPLCM, was used as a reference material and the relative SHG efficiency of LPLCM is found to be about 0.2 times that of KDP. The relative efficiency of LPLCM is found almost equal to the semiorganic single crystals of L-arginine hydrochloride, L-arginine hydrobromide, and L-arginine hydrochloride bromide.21,22
Figure 5. FTIR spectrum of LPLCM.
assigned to NH2+ in-plane deformation of LPLCM. The rocking and wagging vibrations of COO- are observed at 414 and 603 cm-1, respectively. The peak at 1423 cm-1 is characteristic of
Figure 6. UV-vis-NIR spectrum of LPLCM.
Characterization of L-Proline Lithium Chloride Monohydrate
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Figure 7. TG-DTA of LPLCM.
Differential thermal and thermogravimetric spectra for LPLCM were recorded using a simultaneous thermal analyzer SDT Q600 V8.2 Build 100. A ceramic crucible was used for heating the sample, and the analyses were carried out in an atmosphere of nitrogen at a heating rate of 20 °C/min for the temperature range 30-800 °C. The initial mass of the material subjected to the analyses was 1.2950 mg. The thermogravimetric-differential thermal (TG-DTA) curves of LPLCM are illustrated in Figure 7. The TG analysis shows that between 112 and 136 °C the weight loss is about 13%. This indicates the loss of water of hydration (H2O). The strong endothermic peak in DTA around 126 °C with the associated shoulders indicates the stepwise removal of water during this temperature range. There is another strong endothermic peak in DTA around 253 °C, indicating melting of the substance. The sharpness of this endothermic peak shows the good degree of crystallinity and purity of the sample.23 This is associated with a loss of weight of about 29% in the TG curve (between 208 and 298 °C) and another weight loss of about 20% up to 387 °C. These weight losses are due to dissociation of the substance and evaporation of volatile substances. There is a gradual and significant weight loss as the temperature is increased above the melting point. There is no endothermic or exothermic peak up to 800 °C in the DTA curve, whereas TGA shows almost complete weight loss and the residual weight obtained at 800 °C is only 3.9%. Vicker’s microhardness measurements were carried out on LPLCM crystal using an Ultra Microhardness Tester fitted with a diamond indenter. The indentations were made using a Vickers pyramidal indentor for various loads from 25 to 200 g. The diagonals of the impressions were measured using a Reichert Polyvar 2 MET microscope with a Microduromat 4000E hardness controller. The measurements were made on a welldeveloped face. Vickers microhardness number (Hv) was evaluated from the relation Hv ) 1.8544P/d2 (kg/mm2), where P is the indenter load in kilogram and d is the diagonal length of the impression in millimeters. The variation of microhardness values with the applied load is shown in Figure 8. The hardness value increases with increasing load. The plane considered for study exhibits the reverse indentation size effect. The specimen
Figure 8. Variation of microhardness values with applied load.
does not offer resistance or undergo elastic recovery, but undergoes relaxation involving a release of the indentation stress away from the indentation site. This leads to a larger indentation size which gives rise to a lower hardness at low loads.24 For loads above 200 g, cracks started developing around the indentation mark. It is concluded that the hardness of the LPLCM crystal is moderately good. Conclusion A new semiorganic crystal LPLCM of dimension 16 × 6 × 5 mm3 was grown by the temperature reduction method. The crystal is transparent and colorless with a well-defined external appearance. Single crystal XRD shows that LPLCM crystallizes in the monoclinic system. The observed unit cell parameters are a ) 7.6799(10) Å, b ) 5.0744(5) Å, c ) 10.3360(15) Å, R ) γ ) 90°, β ) 105.861(16)°, and the crystal belongs to the space group P21. FTIR spectral analysis confirms the presence of functional groups constituting LPLCM such as NH2+, CH2, C-N, and COO-. The optical study shows that the crystal is transparent in the wavelength region of 300-1100 nm. Its SHG
1374 Crystal Growth & Design, Vol. 9, No. 3, 2009
efficiency is about 0.2 times that of KDP. The microhardness studies reveal that the hardness of crystal is moderately good. Thus, LPLCM is a new candidate for NLO application in view of its superior optical, mechanical properties, and moderate thermal stability. Further effort to modify this system with other possible derivatives may be expected to lead to new materials with improved SHG efficiency. Acknowledgment. One of the authors (T.U.) acknowledges Cauvery College for Women (Reddy Educational Trust) for providing laboratory facilities to carry out the research work. The authors acknowledge Prof. P. K. Das, Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, for extending the laser facilities for the SHG measurement.
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CG800589M
