CRYSTAL GROWTH & DESIGN
Growth, Thermal, Spectroscopic, and Optical Studies of L-Alaninium Maleate, a New Organic Nonlinear Optical Material S.
Natarajan,*,†
S. A. Martin
Britto,†
and E.
2006 VOL. 6, NO. 1 137-140
Ramachandran‡
School of Physics, Madurai Kamaraj UniVersity, Madurai-625 021, India, and Department of Physics, ThiruValluVar College, Papanasam-627 425, Tamilnadu, India ReceiVed June 1, 2005; ReVised Manuscript ReceiVed July 18, 2005
ABSTRACT: L-Alaninium maleate (C3H8NO2+C4H3O4-), a new organic nonlinear optical (NLO) material, is synthesized. Optical behavior such as UV-visible-NIR absorption spectra and second harmonic generation (SHG) were investigated to explore the NLO characteristics of the material for the first time. Thermal analysis and Fourier transform infrared (FTIR) spectroscopic studies of the specimen were also conducted. Characterization of the crystals was made using single-crystal X-ray diffraction and density determinations. 1. Introduction Many research efforts are undertaken to synthesize and characterize new molecules for second-order nonlinear optical (NLO) applications such as high-speed information processing, optical communications, and optical data storage.1-3 These applications depend on the various properties of the materials, such as transparency, birefringence, refractive index, dielectric constant, and thermal, photochemical, and chemical stability.4 Among NLO materials, organic NLO materials are generally believed to be more versatile than their inorganic counterparts due to their more favorable nonlinear response. In the organic class, R-amino acids exhibit some specific features5 such as molecular chirality, weak van der Waals and hydrogen bonds and the absence of strongly conjugated bonds, wide transparency ranges in the visible and UV spectral regions, and zwitterionic nature of the molecule which favors crystal hardness.6 Other advantages of organic compounds apart from the above include, amenability for synthesis, multifunctional substitutions, higher resistance to optical damage, and maneuverability for device applications, etc. L-Arginine acetate,7 L-alanine,8 and γ-glycine9 are a few such compounds reported earlier. Chart 1. Molecular Structure of L-Alaninium Maleate
which were unsuitable for optical investigations. Presently, the crystal growth of bulk crystals of LAM for optical studies was undertaken. Characterization of LAM was done using singlecrystal X-ray diffraction (XRD). Synthesis, structural studies using Fourier transform infrared (FTIR) spectroscopy, thermal analysis (TA), and optical studies were conducted and are presented. 2. Experimental Procedures 2.1 Crystal Growth. L-Alanine and maleic acid obtained from E. Merck (India) Ltd. were mixed in a stoichiometric ratio of 1:1 in doubly distilled water and stirred continuously for an hour to get a saturated solution. The solution was filtered and transferred to crystal growth vessels, and crystallization was allowed to take place by slow evaporation at room temperature (27 °C). Well-defined, transparent crystals of good quality were obtained. These crystals were used as seeds for getting bulk single crystals from a saturated solution taken in a crystallizer, a modified growth apparatus, using submerged seed solution growth. Transparent bulk crystals of size 29.0 × 10.0 × 8.0 mm3 crystallized within two weeks. Attempts were made to use methanol-water solution as a solvent for the crystallization; the size of the crystal increased, but the quality of the crystal was inferior. The crystals were carefully removed and were photographed (Figure 1). The density of the crystal was determined using the flotation method. Xylene (density: 0.89 g/cm3) and bromoform (density: 2.89 g/cm3) were used for the experiment. The density of LAM was determined as 1.39(3) g/cm3. The expected density (F) of the complex was calculated from the crystallographic data,12 using the well-known expression (1)
F ) MZ/NV
Many oxalates are reported to be second harmonic generation (SHG)-active,10 and also such behavior of L-alaninium oxalate was recently reported.11 Hence, it may be useful to synthesize the amino acid complexes with other carboxylic acids and study their properties. Attempts to crystallize such complexes are under progress in our laboratory, and L-alaninium maleate (LAM)12 was successfully grown and its crystal structure was elucidated. In this case, only very small crystals were grown, * Corresponding author. Phone: +91-0452-2458471-352 (extn). E-mail:
[email protected]. † Madurai Kamaraj University. ‡ Thiruvalluvar College.
(1)
where M is molecular weight, Z is the number of molecules in the unit cell, N is the Avogadro number, and V is the volume of the unit cell. This value of density was calculated as 1.375 g/cm3, and it agreed well with the density determined from the flotation method, confirming the identity of the substance. 2.2 Single-Crystal X-ray Diffraction Studies. Nonius CAD-4/ MACH 3 diffractometer with Mo KR (0.71073 Å) radiation was used to obtain the accurate cell parameters of the grown crystals at room temperature. Cell parameters were obtained from least-squares refinement of the setting angles of 25 reflections. The lattice parameters are a ) 5.588(2) Å, b ) 7.380(4) Å, c ) 23.699(1) Å, and the space group is P212121. These values agreed well with the reported values.12
3. Results and Discussion Large and optically transparent crystals of LAM made up of faces belonging to the families {002}, {011}, and {101} were obtained by the slow cooling method. The morphology of the LAM crystal is depicted in Figure 2 and is rhombic (Epsomite)
10.1021/cg0502439 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/20/2005
138 Crystal Growth & Design, Vol. 6, No. 1, 2006
Figure 1.
L-Alaninium
maleate crystal.
Figure 2. Morphology of LAM crystal.
with 10 developed faces. Of these faces, the largest faces were {011} and the smallest was {002}. The figures for habit faces and orientation of faces were obtained using the program SHAPE.13 Simultaneous thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out for the asgrown LAM crystals, using a NETZSCH-Gera¨tebau STA 409 PC thermal analyzer to test the thermal stability. The characteristic curves are shown in Figure 3. Fine powder of the crystals was used for the TGA/DTA carried out in the temperature range of 26 to 700 °C with a heating rate of 5 °C/min. The crucible used was of alumina (Al2O3), which served as a reference for
Natarajan et al.
the sample. During the endothermic reaction in the TGA/DTA, at around 100 °C, DTA reaches a maximum and starts decreasing. A dip at 105 °C gives an expectation for phase transition. The melting point of maleic acid is 139 °C14 and that of L-alanine is 314 °C14 An endothermic peak observed at 162.2 °C in DTG corresponds to the melting point of LAM and is different from that of maleic acid and L-alanine. Maleic acid melts at a relatively lower temperature (139 °C), but this is elevated due to the utilization of thermal energy to overcome the valence bonding between the alaninium cation and the maleate anion, which happens during the initial stage of decomposition. During next stage, maleic acid decomposes and becomes anhydride15 and results in the further release of CO2 and CO molecules at 205 and 258 °C at the third stage, which is evident from the DTG. This accounts for the loss of mass of 21 and 13.7% of the substance, respectively. The reactions of simplest amino acids induced by heating include the condensation reactions of carboxyl and amino groups leading to the formation of peptide bonds. In the dehydration at the first stage, H2O molecule is not liberated immediately; instead, 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 the DTG. A loss of mass of 17.6% results due to the release of water molecules from the amino acid. CO and CH4 molecules are liberated at around 450 °C. Further heating results in the liberation of the NH3 molecule at 520 °C. The FTIR spectra (Figure 4) of the grown L-alanine maleate was recorded in the KBr phase in the frequency region 4004000 cm-1 using a Jasco spectrometer FTIR model 410 at a resolution of 4 cm-1 and with a scanning speed of 2 mm/s. The recorded FTIR spectra were compared with the standard spectra of the functional groups.16 Asymmetric NH2 stretching vibrations in saturated amines give rise to a band between 3380 and 3350 cm-1, and a symmetric stretch will appear between 3310 and 3280 cm-1. Because of protonation, this will shift to a lower wavenumber.17 Primary amines have asymmetric NH2 stretch (3400-3300 cm-1) and symmetric stretch (3320-3290 cm-1).18 Broad bands of medium intensity at 3209 and 2943
Figure 3. TGA/DTA of L-alaninium maleate (a) DTA, (b) DTG, and (c) TGA.
Studies of L-Alaninium Maleate
Crystal Growth & Design, Vol. 6, No. 1, 2006 139
Figure 4. FTIR of L-alaninium maleate. Table 1. FTIR Spectral Data of L-Alaninium Maleatea wavenumber (cm-1) 3209 m 2943 m, b 2762 w, sh 2522 w, b 2002 w, sh 1869 w 1722 s 1572 vs 1504 vs 1377 s 1259 s 1215 s 1101 s 978 s 908 m 866 m 814 w 760 m 658 m 586 w
tentative assignments + asym
NH3 str NH3+ sym str/CH3 asym str O-H str O-H str O-H‚‚‚O valence str C-C overtone CdO str NH3+ asym bending/COO asym str NH3+ sym bending CH3 sym bending COO str COO str C-H deformation O-H out-of-plane deformation O-H‚‚‚O out-of-plane deformation C-C str C-H out-of-plane deformation N-H deformation COO in-plane vibration C-C deformation
a s, strong; w, weak; sym, symmetrical; b, broad; str, stretching; asym, asymetrical; m, medium.
cm-1 are due to NH3+ asymmetric and symmetric stretching, respectively, and this overlaps with the C-H stretching vibrations. However, the absorption band at 2943 cm-1 may also be assigned to the asymmetric stretching due to the CH3 group. Furthermore, another band of strong intensity due to a methyl group is observed at 1377 cm-1. A valence stretching combinational weak absorption band at 2002 cm-1 is due to weak Van der Waals interactions of the O-H‚‚‚O bond.19 Also, the bands at 2762 and 2522 cm-1 are assigned to the stretching vibrations due to the hydroxyl group of the maleic acid anion. A strong absorption band at 1722 cm-1 occurred due to the covalent bond CdO, and the others observed at 1259 and 1215 cm-1 are attributed to COO stretching. In-plane vibrations due to COO give rise to a band of medium intensity at 658 cm-1. Very strong absorption peaks due to the symmetric and asymmetric deformation of NH3+ occurred at 1572 and 1504 cm-1, respectively. O-H out-of-plane deformation (978 and 908 cm-1), C-C stretching (866 cm-1), C-H deformation (1101 and 814 cm-1), N-H deformation (760 cm-1), and the C-C deformation (580 cm-1) vibrations are observed.20 The observed vibrational frequencies and their tentative assignments are listed in Table 1. Optical window width is an important characteristic of an NLO material. Hence the study of the transmission of electromagnetic waves of the UV-Vis-NIR range through the NLO
Figure 5. Transmittance spectrum of L-alaninium maleate.
material is necessary. The transmission spectrum was recorded using a VARIAN (Cary 500) UV-Vis-NIR spectrophotometer in the range 200-1100 nm covering the entire near-UV, visible, and NIR regions (Figure 5). Transparent single crystal of 1.0 mm thickness was used for this study. No absorption was found in the visible region of the UV-vis-NIR spectra. The transmittance due to absorbance decreases rapidly around 310 nm leading to electronic excitation in this region. The absence of absorption in the region between 320 and 1100 nm shows that this crystal could be used for optical window applications.2,4,10 The SHG may be used as a tool to evaluate at least qualitatively the bulk homogeneity of the samples under investigation.21 The SHG behavior of the powdered material of the LAM crystal was tested using the Kurtz and Perry method.22 A high-intensity Nd:YAG laser (λ ) 1064 nm) was passed through the palletized sample. The SHG behavior is confirmed from the output of the laser beam having the bright green emission (λ ) 532 nm) from the crystal. Intensity of the bright emission is found to be 15% of that of the standard urea crystal. 4. Conclusions Transparent bulk crystals of LAM, a new NLO material from the amino acid-carboxylic acid family, was successfully grown using a slow evaporation technique at room temperature and characterized by single-crystal X-ray diffraction and density determinations. Structural studies were carried out using TGA/ DTA and FTIR. Optical studies confirmed that LAM crystals may find useful optical window applications in the wavelength region 310-1100 nm. Furthermore, the intensity of the output signal in the SHG test was found to be the maximum for the wavelength 532 nm and was comparable with that of the standard urea crystal.
140 Crystal Growth & Design, Vol. 6, No. 1, 2006
Acknowledgment. The authors thank the UGC-SAP and DST-FIST Programmes. E.R. thanks the Secretary, Thiruvalluvar College, Papanasam, for the encouragement and support provided. References (1) Dmitriev, V. G.; Gurzadyan, G. G.; Nicogosyan, D. N. Handbook of Nonlinear Optical Crystals; Springer-Verlag: New York, 1999. (2) Razzetti, C.; Ardoino, M.; Zanotti, L.; Zha, M.; Paorici, C. Cryst. Res. Technol. 2002, 37, 456-465. (3) Wong, M. S.; Bosshard, C.; Pan, F.; Gunter, P. AdV. Mater. 1996, 8, 677-680. (4) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers; Wiley: New York, 1991. (5) Nicoud, J. F.; Twieg, R. J. Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: London, 1987; pp 227-296. (6) Delfino, M. Mol. Cryst. Liq. Cryst. 1979, 52, 271-284. (7) Pal, T.; Kar, T.; Bocelli, G.; Rigi, L. Cryst. Growth Des. 2003, 3, 13-16. (8) Razzetti, C.; Ardoino, M.; Zanotti, L.; Zha, M.; Paorici, C. Cryst. Res. Technol. 37 2002, 5, 456-465.
Natarajan et al. (9) Bhat, N.; Dharmaprakash, S. M. J. Cryst. Growth 2002, 236, 376380. (10) Bhat, H. L. Bull. Mater. Sci. 1994, 17, 1233-1249. (11) Dhanuskodi, S.; Vasantha, K. Cryst. Res. Technol. 2004, 39, 259265. (12) Alagar, M.; Krishnakumar, R. V.; Subha Nandhini, M.; Natarajan, S. Acta Crystallogr. 2001, E57, o855-o857. (13) Dowty, E. SHAPE, Version 7.1; Kingsport, TN, 2003. (14) Weast, R. C. CRC Handbook of Chemistry and Physics, 54th ed.; CRC Press: Ohio, 1973. (15) Felthouse, T. R.; Burnett, J. C; Horrell, B.; Mummey, M. J.; Kuo, Y.-J., www.huntsman.com/performance_ products/Media/ KOMaleic.pdf. (16) Socrates, G. Infrared Characteristic Group Frequencies; WileyInterscience: Chichester, 1980. (17) Silverstein, R. M.; Webster, F. M. Spectroscopic Identification of Organic Compounds, 6th ed.; John Wiley & Son: New York, 1998. (18) Bernstein, H. J. Spectrachim. Acta 1962, 18, 161-170. (19) Khanna, K.; Moore, M. H. Spectrochim. Acta 1999, A55, 961-967. (20) Brauer, B.; Chaban, G. M.; Benny Gerber, R. Phys. Chem. Chem. Phys. 2004, 6, 2543-2556. (21) Sherwood, J. N. Philos. Trans. R. Soc. London 1990, A330, 127-. (22) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798.
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