CRYSTAL GROWTH & DESIGN
Synthesis, Growth, and Characterization of L-Proline Cadmium Chloride Monohydrate (L-PCCM) Crystals: A New Nonlinear Optical Material A. Kandasamy,†,‡ R. Siddeswaran,§,⊥ P. Murugakoothan,§ P. Suresh Kumar,⊥ and R. Mohan*,† Department of Physics, Presidency College, Chennai 600 005, India, Department of Physics, Sriram Engineering College, Perumalpattu 602 024, India, PG & Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India, and Materials Research Centre, Department of Physics, Velammal Engineering College, Chennai 600 066, India
2007 VOL. 7, NO. 2 183-186
ReceiVed July 11, 2006; ReVised Manuscript ReceiVed December 10, 2006
ABSTRACT: Single crystals of a semi-organic nonlinear optical (NLO) material, L-proline cadmium chloride monohydrate (L-PCCM), were grown by the slow evaporation and slow cooling method. The synthesized material was purified by repeated recrystallization. The grown crystals were transparent and had dimensions 16 × 8 × 5 mm3; they were characterized by single-crystal XRD, FT-IR, CHN analysis, density, melting point measurements, and UV-vis-NIR techniques. The SHG efficiency of L-PCCM is twice that of potassium dihydrogen phosphate (KDP). 1. Introduction Nonlinear optics (NLO) is at the forefront of current research because of its importance in providing the key functions of frequency shifting, optical modulation, optical switching, optical logic, and optical memory for the emerging technologies in areas such as telecommunications, signal processing, and optical interconnections.1,2 Inorganic materials are widely used in these applications because of their high melting point, high mechanical strength, and high degree of chemical inertness. The optical nonlinearity of these materials is poor. Organic compounds are often formed by weak van der Waal’s and hydrogen bonds and possess a high degree of delocalization. Hence, they are optically more nonlinear than inorganic materials. Some of the advantages of organic materials include flexibility in the methods of synthesis, scope for altering the properties by functional substitution, inherently high nonlinearity, high damage resistance, etc. A major drawback of crystalline organic NLO materials is the difficulty in growing large, opticalquality single crystals; also, the often-fragile nature of these crystals makes them difficult to process. The inherent limitations on the maximum attainable nonlinearity in inorganic materials and the moderate success in growing device-grade organic single crystals have led scientists to adopt alternate strategies. The obvious one was to develop hybrid organic-inorganic materials with some tradeoff in their respective advantages. This new class of materials has come to be known as semiorganics. One approach to highefficiency optical quality organic-based NLO materials in this class is to form compounds in which a polarizable organic molecule is stochiometrically bonded to an inorganic host. Amino acids and their complexes belong to a family of organic materials that have applications in NLO.3-6 Amino acids are interesting materials for NLO applications as they contain a proton donor carboxyl acid (COO) group and the proton acceptor amino (NH2) group in them. L-Arginine and L-arginine phosphate, for example, have shown promising results as efficient second harmonic generators and are being applied in devices such as optical parametric amplifiers.7 Presently, crystals of L-proline cadmium chloride mono hydrate (LPCCM), a salt of the amino acid L-proline, have been studied. Synthesis, bulk crystal growth, and characterization of L-PCCM have also been discussed. * To whom correspondence should be addressed. Tel: 91-44-2854 4894. Fax: 91-44-2851 0732. E-mail:
[email protected]. † Presidency College. ‡ Sriram Engineering College. § Pachaiyappa’s College. ⊥ Velammal Engineering College.
Figure 1. As-grown single crystal of L-PCCM from slow cooling technique.
2. Experimental Section 2.1. Materials Synthesis. The molecule L-proline (C5H9NO2) has two groups (a guanadyl and amino) that can be protonated. The starting material was synthesized by taking cadmium chloride and L-proline in a 1:1 stoichiometric ratio. The required amount of starting materials for the synthesis of L-PCCM salt was calculated according to the reaction
CdCl2‚H2O + NHCH2CH2CH2CHCOOH f Cd(NHCH2CH2CH2CHCOOH) Cl2‚H2O The calculated amount of cadmium chloride was first dissolved in deionized water. L-Proline was then added to the solution slowly by stirring. The prepared solution was allowed to dry at room temperature and the salts were obtained by slow evaporation technique. The purity of the synthesized salt was further improved by successive recrystallization process. Bulk growth of L-PCCM single crystal was carried out from aqueous solution by the slow cooling technique, in a constant temperature bath controlled to an accuracy of ( 0.1 °C. Three hundred milliliters of the solution was saturated at 45 °C and then filtered to remove any insoluble impurities. The seed obtained from the slow evaporation method was used for the bulk growth. The solution was maintained at 45 °C for 2 days before seeding. The temperature was reduced at a rate of 0.1-0.2 °C per day as the growth progressed. The period of growth ranged from 30 to 35 days. Figure 1 shows the as-grown crystal of L-PCCM with an optimized solution pH value of 6. It is observed that unlike L-arginine phosphate (LAP) solutions, no microbes were formed during the growth of L-PCCM, probably
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184 Crystal Growth & Design, Vol. 7, No. 2, 2007
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Figure 2. (a) L-PCCM atom numbering scheme of non-hydrogen atoms. (b) Molecular projection of L-PCCM along the c-axis.
Figure 3. FTIR spectrum of L-PCCM crystals.
due to the chlorine compounds, which act destructively over the growth of microbes. 3. Characterization Studies To study the chemical composition of the synthesized compound, we carried out CHN analysis on the recrystallized sample using the instrument Elementar Vario EL III CHNS analyzer. Singlecrystal X-ray diffraction analysis was carried out using an Enraf CAD-4 diffractometer with MoKR (λ ) 0.1770 Å) radiation to identify the structure and estimate the lattice parameter values. The FTIR spectra of L-PCCM crystals were recorded in the range 4004000 cm-1 employing a Perkin-Elmer spectrometer by the KBr pellet method to study the metal complex coordination. Linear optical properties of the crystals were studied using a Shimadzu UV-visible spectrophotometer. To confirm the nonlinear optical property, we performed the Kurtz-Perry powder SHG test on the grown crystals. An actively Q-switched diode array side pumped Nd:YAG laser was used to measure the laser-induced damage threshold studies of the grown crystal.
Table 1. CHN Analysis of L-PCCM element present carbon hydrogen nitrogen chlorine cadmium
experimental
calcd
18.93 3.03 4.56 22.43 35.42
18.98 3.50 4.43 22.41 35.52
3.1. Results and Discussion. To confirm the chemical composition of the synthesized compound, we carried out CHN analysis on the recrystallized sample using the instrument Elementar Vario EL III CHNS analyzer. The results of the analysis are presented in Table 1. Theoretical values of CHN were found by the molecular formula CdCl2 (C5H9NO2)·H2O. The experimental and calculated values of C, H, and N agree with each other, confirming the formation of L-PCCM. The atom numbering schemes of the atoms are shown in Figure 2a. L-PCCM consists of a one-dimensional polymer bridged by chlorine atoms and carboxyl oxygen atoms. The four chlorine atoms, which coordinate to a cadmium atom,
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Crystal Growth & Design, Vol. 7, No. 2, 2007 185
Table 2. Single-Crystal X-ray Data of L-PCCM Crystal CdCl2(C5H9NO2)‚H2O orthorhombic a ) 9.952(1) Å b ) 13.484(3) Å c ) 7.255(1) Å µ(M0KR) ) 26.7 cm-1
FW ) 316.45 space group P212121 Z)4 V ) 973.60(9) Å3 Dm ) 2.13 g cm-3
Table 3. Band Assignment of FTIR Spectra for L-PCCM Single Crystals wave number (cm-1) experimental
observed
assignments
3480 1543 1431 1368 1332, 1310 1170 1038 943 917 853 782 630 464
3430 1557 1410 1383 1328, 1302 1177 1052 919 905 851 687 643 453
stretching vibration of H2O molecule NH2+ in-plane deformation COO- symmetric stretching wagging NH2+ wagging CH2+ twisting NH2+ C-N stretch rocking CH2 rocking NH2+ rocking CH2 in-plane deformation of COOwagging COOrocking COO-
are almost on a plane. The plane extended zigzag in the direction of the c-axis like an infinite folding screen, sharing opposite edges. The two carboxyl oxygen atoms of L-proline coordinate with the two cadmium atoms. L-PCCM does not contain any intermolecular hydrogen bonds. Thus, the pyrrolidine ring is not fixed and its position is far from the coordination atoms in L-proline. A water molecule is connected to the nitrogen atom in the pyrrolidine ring by a hydrogen bond. Figure 2b shows the molecular projection of L-PCCM along the c-axis. L-PCCM belongs to the orthorhombic system with space group P212121. The lattice parameters of L-PCCM are a ) 9.952 Å, b ) 13.484 Å, c ) 7.255 Å, V ) 973.60 Å3, Z ) 4, and density Dc ) 2.13 g/cm3, in close agreement with the reported values,8 and are presented in Table 2. The FT-IR spectral analysis of L-PCCM crystals was shown in Figure 3. In L-PCCM, the peak at 3480 cm-1 is assigned to the OH stretching vibration of H2O (O-H). The peak at 3138 cm-1 corresponds to the NH stretching vibration. Ratajczak et.al. determined that the lack of any strong IR band at 1700 cm-1 clearly indicates the existence of the COO- ion in zwitterionic form.9 The peaks at 1543 cm-1 are assigned to NH2+ in-plane deformation of L-proline cadmium chloride monohydrate. The rocking and wagging vibrations of COO- are observed at 464 and 630 cm-1, respectively. The peaks at 1368-1588 cm-1 and the peaks at 2736-3138 cm-1 are the characteristics of C5H10NO2+, the 1431 cm-1 feature is characteristic of the N-H vibration, and the feature at 1588 cm-1 is associated with CdO symmetric stretching vibration. The other peaks at 1332 and 1310 cm-1 are assigned to wagging of CH2 group of the L-PCCM. The observed wavenumbers are presented in Table 3. The measurement of density is one of the important techniques required for the study of crystal purity. The most sensitive method for determining the density of the grown crystal available in small size is the floatation method. The experimental value measured by the floatation method was 2.25 gm/cm3. This agrees quite well with the literature value of 2.13 gm/cm3.10 The melting point determined using the capillary tube method was 210 °C. The UV-vis-NIR spectrum is shown in Figure 4. It is evident that the L-PCCM crystal has a wide frequency in the entiree visible spectra; its transparent power is 80% in this range. Also, it is evident that L-PCCM crystal has a UV cut off at 230 nm, which is sufficient for SHG laser radiation of 1064 nm or other application in the blue region. The Kurtz-Perry powder technique remains an extremely valuable tool for initial screening of materials for second harmonic generation. The fundamental beam 1064 nm from Q-switched Nd:YAG laser
Figure 4. Transmittance spectrum of L-PCCM crystals.
(Pro Lab 170 Quanta ray) was used to test the second harmonic generation (SHG) property of the L-PCCM crystal by using the Kurtz-Perry powder technique.11 A pulse energy of 4 mJ/pulse, pulse width of 10 ns, and repetition rate of 10 Hz are used. Ninety degree geometry was employed. Using IR filter filtered the fundamental beam. A photo multiplier tube (Philips Photonics) was used as detector. Powdered samples of standard KDP and L-PCCM with the same particle size were considered for powder SHG measurements; it was found that the output power was twice that of standard KDP. Other semi-organic material such as L-APCl, L-AHCl, L-AHBr, and L-AHClBr were only 0.3-0.4 times that of KDP.12-13 The UV-vis-NIR spectrum shows that the crystal possesses wide transparency in the entire visible spectrum, which is quite suitable for SHG in the green or blue region. 4. Conclusion Optical good-quality single crystals of L-PCCM have been grown using a slow solvent evaporation technique and slow cooling method. Compound confirmation was done by a CHN analyzer. The unit-cell parameters of L-PCCM were confirmed by singlecrystal X-ray diffraction analysis. The functional group was confirmed by FTIR. In the transmittance spectra, it is evident that the L-PCCM crystal has a wide transparency range in the entire visible range. The SHG efficiency of L-PCCM is twice that of KDP. Thus, L-PCCM seems to be a promising material for NLO application in view of its superior optical properties. Acknowledgment. The authors thank Dr. R. Jayavel, Director, Centre for Nanoscience and Technology, Anna University, Chennai 600 025, India, for his constant encouragement and help.
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