Synthesis, Growth, and Characterization of L-Arginine Acetate Crystal: A Potential NLO Material Tanusri Pal,† Tanusree Kar,*,† Gabriele Bocelli,‡ and Lara Rigi‡ Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India, and Centro di Studio per la Strutturistica Diffrattometrica del CNR, Parco Area delleScienze 17a, 43100 Parma, Italy Received September 13, 2002;
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 1 13-16
Revised Manuscript ReceivedNovember 5, 2002
ABSTRACT: Single crystals of nonlinear optical material L-arginine acetate (LAA) (C6H14N4O2‚CH3COOH), space group P21, were successfully grown for the first time by the temperature-lowering method and also by the slow evaporation method at constant temperature (30 °C) from its aqueous solution with pH at 6 and dimension of 21 × 15 × 3 mm3. Initially, solubility tests were carried out for four solvents such as water, water and methanol, water and ethanol, and water and acetone. Among the four solvents, the solubility of LAA was found to be the highest in water, so crystallization of LAA was done from its aqueous solution. Morphological analysis reveals that the crystal is a polyhedron with 16 developed faces with major face forms {100}, {001}, and {102} (pinacoids) parallel to the polar axis. As grown crystals were characterized by chemical analysis, density measurement, and X-ray powder diffraction studies. Infrared spectroscopy, thermogravimetric analysis, and differential thermal analysis measurements were performed to study the molecular vibration and thermal behavior of LAA crystals. Thermal analysis does not show any structural phase transition. 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 Organic materials have been of particular interest because the nonlinear optical response in this broad class of materials is microscopic in origin, offering an opportunity to use theoretical modeling coupled with synthetic flexibility to design and produce novel materials.1,2 Also, organic nonlinear optical materials are attracting a great deal of attention, as they have large optical susceptibilities, inherent ultrafast response times, and high optical thresholds for laser power as compared with inorganic materials. Organic molecules with significant nonlinear optical activity generally consist of a π-electronconjugated moiety substituted by an electron donor group on one end of the conjugated structure and an electron acceptor group on the other end, forming a “push-pull” conjugated structure. The conjugated π-electron moiety provides a pathway for the entire length of conjugation under the perturbation of an external electric field. The donor and acceptor groups provide the ground state charge asymmetry of the molecule, which is required for secondorder nonlinearity. In this context, amino acids are interesting materials for NLO application as they contain a proton donor carboxyl acid (-COO) group and the proton acceptor amino (-NH2) group in them. L-Arginine is an amino acid, which forms a number of complexes on reaction with different acids and has attractive NLO properties as discovered by Monaco et al.3 L-Arginine acetate (LAA) is one of the most attractive salts that belongs to this arginine complex family. Vijayan et al.4 solved the single crystal structure of LAA. It crystallized to the monoclinic system with space group P21 and Z ) 2. The cell parameters were a ) 9.229(2) Å, b ) 5.178(3) Å, c ) 13.271(4) Å, β ) 114.4(1)°, and dm ) 1.346 gm/cc. Monaco et al.3 have studied the linear and nonlinear * To whom correspondence should be addressed. E-mail: mstk@ mahendra.iacs.res.in. † Indian Association for the Cultivation of Science. ‡ Centro di Studio per la Strutturistica Diffrattometrica del CNR.
optical properties of this complex using crystals as small as 50-100 µm. Apart from these studies, no other studies on the physical properties of these LAA crystals have been carried out to date and no attempts have been made to grow larger crystals of LAA. So, we have tried to grow large size single crystals of LAA for the first time from the aqueous solution of its salt by solvent evaporation and the slow-cooling method (0.5 °C/day) followed by characterization by chemical analysis, Fourier transform infrared (FTIR) studies, and X-ray powder diffraction studies. Beside these, we have also studied the morphology and solubility of LAA along with differential thermal analysis (DTA) and thermogravimetric analysis (TGA). For device purposes, it is necessary to assess the quality of the grown crystals by X-ray Lang topography, chemical etching, and other methods. However, work in this direction will be reported in a future paper. LAA was synthesized by dissolving 1 equiv amount of strongly basic amino acid, L-arginine (Lobachemie) (NH2)NHCNH(CH2)3CH(NH2)COOH, in double-distilled water containing 1 equiv amount of acetic acid. The synthesized salt was then purified by repeated crystallization until optically clear crystals were obtained. The purity of the material was also checked by measuring the melting point (about 221 °C) after every crystallization. The chemical composition of the synthesized salt was then established by CHN analysis. To confirm the identity of the synthesized salt, unit cell parameters were calculated from the X-ray diffraction pattern of the salt and also verified with density measurement by flotation method. The X-ray powder diffraction pattern of LAA (Figure 1) was recorded on a Philips microprocessor-controlled X-ray diffractometer (APD1710) using nickel-filtered Cu KR radiation (36 KV, 20mA) from a Philips X-ray generator (PW1310). The powdered sample was scanned in steps of 0.02° for a time interval of 2 s over a 2θ range of 10-50°. All of the observed reflections were indexed, and the unit cell parameters were calculated using the computer program POWD.5 The density of as grown crystal was measured by the flotation method in a mixture of carbon tetrachloride (CCl4) and chloroform (CHCl3) and found to be 1.345 gm/cc. This agreed well with the theoretical value
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Figure 1. X-ray powder diffraction pattern of LAA. Table 1. Unit Cell Parameters of LAA
a (Å) b (Å) c (Å) β (°) v
unit cell parameters from powder data (present case)
unit cell parameters from single crystal data4
9.214(3) 5.182(2) 13.222(3) 111.4(1) 587
9.229(2) 5.178(3) 13.271(4) 111.4(1) 586
of 1.346 gm/cc calculated from the unit cell dimensions and the molecular weight of LACH3COOH. The solubility of LAA in different solvents such as water, methanol and water, ethanol and water, and acetone and water was determined as a function of temperature in the temperature range of 28-50 °C. Thermostatically controlled vessels (100 mL) were filled with different solutions of LAA with some undissolved LAA and stirred for 24 h. On the next day, a small amount of solution from each vessel was pipetted out and its composition was determined gravimetrically. Single crystals of LAA were grown from saturated aqueous solution of the salt of LACH3COOH by the slow evaporation technique at constant temperature (30 °C) and also by the slow-cooling method (0.5 °C/day) from 45 to 28 °C. During the slow evaporation method, we varied the pH of the solution from 4 to 8. Optically clear crystals obtained by the slow evaporation method were then used as a seed crystal for further growth by the slow-cooling method. Crystal dimensions and the angle between faces of the crystal were measured using the “Zweikreis Reflections Goniometer” (P. Stoe Company, Heidelberg, Germany). These data were then correlated with the X-ray intensity data with the help of the program MORPHO,6 and finally, the figures for habit faces and orientation of faces were obtained with the program SHAPE.7 A nicolet MAGNA-IR 750 (series II) FTIR spectrometer was used to record the infrared spectra of LAA in the range of 400-4000 cm-1 with reference to potassium bromide pellets. From this spectra, we tried to identify different functional groups present in LAA. DTA and TGA of LAA crystals were carried out simultaneously employing an automatic recording apparatus Shimadzu DT30 differential thermal analyzer. The sample was heated at a rate of 20 °C/min in inert nitrogen atmosphere. The weight of the sample was 20 mg. Chemical analysis confirms that the LAA crystal does not contain any water molecules, and the composition of the synthesized material is established as C6H14N4O2‚CH3COOH. Table 1 shows the unit cell parameters obtained by X-ray powder diffraction analysis along with the re-
Figure 2. Solubility graph of LAA in various solvents.
ported values.4 Figure 2 shows the solubility graph of LAA in different solvents. From this curve, we find that the solubility of LAA in water shows a regular behavior with a solubility coefficient (dS/dT)/S0 ) 0.013 °C-1 for T0 ) 30 °C (where S is the solubility and T is the temperature, S0 is the solubility at T0). The solubility coefficients lie between 0.011 and 0.013 °C-1 in the temperature range of 30-45 °C and hence are suitable for the growth of LAA by the slow-cooling and slow evaporation methods. On the other hand, the solubility of LAA in other mixed solvents (methanol, ethanol and acetone in water) is almost constant with temperature, and hence, growth will not occur. Thus, water is selected as a potentially suitable solvent for the growth of LAA. Rodlike tiny crystals with well-developed prism faces only were obtained from saturated aqueous solution of LAA near the pH value of 4. From solutions with pH values above 4 and below 8, well-developed crystals with all faces present were harvested but the best and optically clear crystals were obtained from the solution of pH value of 6. Figure 3 shows the as grown LAA crystal obtained by the slow-cooling method. The morphological analysis as shown in Figure 4 reveals that the crystal is a polyhedron with 16 developed faces. There is a pair of parallel faces, which is a pinacoid and indexed as {100}. This is the most prominent face and dominates the crystal morphology. The other pinacoids are {001} and {102}. They are inclined with the {100} faces and meet with each other along the longest dimension of the crystal and belong to the zone [010] parallel to polar axis. Different faces of the crystal in order of importance are as follows: {100} > {001} > {102} > {012} > {112}. To analyze the IR spectrum (Figure 5) of LAA, we have to consider the single crystal structure of LAA,4 which shows that in crystalline state, the arginine molecule is deprotonated at the carboxyl group (COO-) and protonated at the guanidyl (+(H2N)2CNH) and amino (NH3+) groups. Thus, the structure of the LAA crystal consists of an L-arginine molecule in the ionized form and an acetate ion. For simplicity of analysis, we have considered three different regions. The high wavenumber region (2000-3500 cm-1) consists of bands due to NH, NH3+, NH2, CH, CH2, and CH3 stretching vibration. The lower wavenumber region (below 1000 cm-1) contains bands due to deformation vibrations of different groups. The presence of three
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Figure 5. Infrared spectra of LAA.
Figure 3. Optical photograph of LAA crystal (21 × 15 × 3 mm3) grown from aqueous solution by slow cooling method, with polar axis vertical.
Figure 6. TG and DTA curves of LAA.
Figure 4. Observed morphology of LAA with polar axis along the long dimension of the crystal.
characterization bands at 610, 545, and 467 cm-1 below 1000 cm-1 corresponding to COO- in plane deformation, COO- wagging mode, and NH3+ torsion mode indicates the protonation of the NH3+ group and deprotonation of COO-.
Besides this presence of absorption band in the region of 2000-2048 cm-1, because of the combination of NH3+ deformation and NH3+ torsion, this is an indicator band for the identification of the charged NH3+ group and this confirms the protonation of the amino group. The presence of a strong absorption band at 1403 cm-1 and a medium band at 1600 cm-1 corresponding to the symmetric and asymmetric stretching of the COO- group also confirms the deprotanation of the carboxyl group. An intense absorption band appears at 3374 cm-1, which is attributed to the NH stretching vibration. So, with the help of available data on the vibrational frequencies of amino acids,8 we have identified the characteristic IR bands for different molecular groups present in the LAA. Figure 6 shows the thermogram for DTA and TGA of LACH3COOH. The DTA curve shows a major endothermic peak, which corresponds to the melting point of the compound at 221 °C, after which the compound decomposes into a sticky viscous substance. At higher temperature, this decomposition process continues up to 800 °C with the removal of almost all of the compound as gaseous products. LAA (C6H14N4O2‚CH3COOH) has been synthesized by reacting equivalent amounts of acetic acid and L-arginine
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in water. Solubility tests that were performed with different solvents reveal that water is the best solvent for growing single crystals. Thus, crystals of different morphologies were grown by the slow evaporation method at constant temperature from the aqueous solution of the LAA salt within a pH range of 4-8 and the best crystal was obtained at pH 6. Morphological analysis shows that the LAA crystal is a polyhedron with 16 developed faces, and the (100) face is the most prominent one. Chemical analysis, X-ray diffraction studies, and density measurement confirmed the identity of the synthesized material. DTA and TGA coupled with FTIR studies have established that LAA does not contain any water molecule. Thermal analysis has revealed that LAA is stable up to 221 °C; after that, it undergoes a physical transformation associated with mass changes. Thus, during thermal treatment, the possibility of any change in crystal structure of the original compound is ruled out.
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References (1) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers; Wiley: New York, 1991. (2) Chemla, D. S., Zyss, J., Eds. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press: New York, 1987. (3) Monaco, S. B.; Davis, L. E.; Velsko, S. P.; Wang, F. T.; Eimerl, D.; Zalkin, A. J. Cryst. Growth 1987, 85, 252-255. (4) Suresh, C. G.; Vijayan, M. Int. J. Pept. Protein 1983, 21, 223-226. (5) Wu, E. J. Appl. Crystallogr. 1989, 22, 506-510. (6) Nardelli, M. MORPHO, an Utility Program for Correlating X-ray Data with the Morphology of Crystals. Presented at the sixth European Crystallographic Meeting, Barcelona, Spain, 1980. (7) Dowty, E. SHAPE, Version 6.0.1; Kingsport, TN, 2000. (8) Krishnan, R. S.; Sankaranarayanan, K.; Krishnan, K. J. Indian Inst. Sci. 1973, 55, 66-116.
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