Received 6 February 2016; accepted 27 March 2016; published 30 March 2016
Organic nonlinear optical materials are of great interest due to its demand in optical communication technologies. These materials have good mechanical and chemical stability, and sufficiently much larger number of design possibilities, which are used in optical information processing, optical disk storage, optical computing and telecommunications  . A large number of organic nonlinear optical materials have been accounted because of its nonlinear optical and photonics applications. Amino acid mixed organic crystals have great attention in the optical applications compared to other materials  . The conjugated pi molecules containing an electron acceptor and donor groups provide a large degree of second order optical nonlinearity   . The acceptor and donor groups with large transition dipole moments and large difference in energy between ground and excited states exhibit large second order optical nonlinearity of the molecules. The basic amino acid L-Arginine gains more interest in the development of nonlinear optical materials and the maleic acid―a basic dicarboxylic acid has great attention due to its large pi conjugation  . The growth of the crystal, L-Arginine maleate dihydrate and its characterization is already reported  -  . In all these works, the growth is carried out using low temperature solution growth technique. In this paper, L-Arginine maleate dihydrate is grown from liquid diffusion method and the grown crystals were characterized by Single and Powder crystal X-Ray Diffraction analyses (XRD), Fourier Transform Infra Red Spectroscopy (FTIR), Fourier Transform Raman Spectroscopy (FT Ramam), Nuclear Magnetic Resonance Spectroscopy (NMR), Thermal studies, optical characterization like UV-VIS-NIR Spectroscopy, and Second Harmonic Generation (SHG) property.
Basically, some salts of amino acid with different organic or inorganic acids can be grown from the standard method of synthesizing the aqueous solution of the required materials mixed in a molar proportion, and purifying the solution and then evaporating the solvent by slow evaporation technique. But in some cases, this method does not lead to the expected product, in which no reaction takes place between the starting materials, and the resulting crystal formation is simply any one of the starting materials. The liquid diffusion technique is employed to overcome such difficulties. In this technique, two solvents are chosen. One solvent is allowed to diffuse into the other solvent. An aqueous solution of the compound is prepared and chosen as the first solvent. A less dense solvent, in which the compound is insoluble, is chosen as the second solvent. Two solvents are added so that they form a distinct layer, where the lower dense solvent adds diffuses slowly as the precipitant in to an aqueous solution of the compound with the formation of crystals at the liquid boundary.
2.1. Materials and Methods
Crystals of L-Arginine maleate dihydrate were obtained from liquid diffusion method. Aqueous solution of the amino acid L-Arginine and the maleic acid was prepared in an equimolar proportion. The solvent acetonitrile is added to the prepared compound, since the compound is insoluble in the solvent. Crystals of L-Arginine maleate dihydrate were grown when the solvent acetonitrile, diffuses slowly as the precipitant into an aqueous solution of L-Arginine and maleic acid. Transparent crystals of L-Arginine maleate dihydrate were grown in a period of one week. The formation of crystal follows the reaction.
L-Arginine Maleic acid L-Arginine maleate dihydrate
The grown L-Arginine maleate dihydrate crystal was subjected to various characterization techniques like single and powder crystal X-ray diffraction, Fourier transform infrared and Fourier transform Raman spectral studies, UV-VIS spectral analysis, Nuclear magnetic resonance spectral analyses, thermal analysis and nonlinear optical studies. Nonius CAD4/MACH 3 single crystal X-Ray diffractometer with MoKα (λ = 0.71069 Å) radiation and D8 phaser Bruker powder diffractometer were used to find the lattice parameter values. Fourier Transform Infra Red spectrum was recorded by the KBr pellet technique using a SPECTROMRX1 FTIR spectrometer and BRUKER RFS 27 spectrometer records the FT Raman spectrum to confirm the functional groups. The optical absorption and transmission spectra were recorded in the region 190 nm to 1100 nm using λ35 model PerkinElmer double beam UV-VIS-NIR spectrometer. The instrument Bruker 300 MHz (ultrasheild)TM at room temperature (1H-NMR at 300MHz, 13C-NMR at 75 MHz) records the 1H-NMR and 13C-NMR spectra by dissolving the crystal in (D2O) heavy water for the confirmation of molecular structure. The thermal characterization was determined from SDT Q600 V20.9 Build 20 instrument in a nitrogen atmosphere in temperature range 30˚C - 1100˚C. Kurtz Perry powder technique was used to measure the second harmonic generation efficiency. Nd:YAG Q-switched mode locked laser with a harmonic output at 1064 nm, and an input energy of 1.6 mJ/ pulse with a pulse width of 10 ns at a repetition rate of 10 Hz was used for the measurement of an efficiency of second harmonic generation.
3. Results and Discussion
3.1. Single Crystal and Powder XRD Studies
Single crystal X-ray diffraction analysis was carried out to find the lattice parameters. This study reveals that the grown crystal of L-Arginine maleate dihydrate belongs to the Triclinic system with P1 space group. The determined lattice parameters are listed in Table 1.
The crystallinity and structure of the L-Arginine maleate dihydrate crystal have been confirmed by powder diffraction analysis. The crushed powder sample was subjected to intense X-rays of wavelength 1.5418 Å (CuKα) at a scan speed of 1˚/minute. The observed powder XRD pattern in Figure 1 has been indexed by Rietveld Index software package. The lattice parameters have been calculated by Rietveld Unit Cell software package and they are shown in Table 1. It is observed that lattice parameters of L-Arginine maleate dihydrate from single and powder crystal XRD data were in good agreement with the already reported values  .
Table 1. Lattice parameters of L-Arginine maleate dihydrate crystal.
Figure 1. Powder XRD pattern of L-Arginine maleate dihydrate crystal.
3.2. Vibrational Spectral Analyses
FTIR and FT Raman studies are the useful techniques for the identification of compounds. The recorded FTIR and FT Raman spectra of L-Arginine maleate dihydrate crystal are depicted in Figure 2 and Figure 3.
The IR band at 2951 cm−1 is assigned to CH3 asymmetric stretching vibration, whereas its Raman peak is located at 2925 cm−1. The peaks at 1677 cm−1 in FTIR and its Raman equivalent observed at 1680 cm−1 is due to asymmetric deformation of NH3+ group. The IR bands at 1627 cm−1 and 1389 cm−1 are assigned to COO− asymmetric and symmetric stretching vibrations. Their Raman counterparts are resolved at 1610 cm−1 and 1392 cm−1 respectively. The rocking and stretching of and CN groups both in IR and Raman occurs at 1166 cm−1, 1111 cm−1, 1172 cm−1 and 1109 cm−1 respectively. The bands in FTIR and Raman at 868 cm−1 and 851 cm−1 are assigned to COO− rocking vibration. The IR band at 661 cm−1 is due to the rocking of CH2 group, and its corresponding band in Raman occurs at 662 cm−1. The peak at 573 cm−1 and 545 cm−1 in IR and Raman are ascribed to COO− wagging mode of vibration. The detailed assignments based on the recorded FTIR and FT Raman spectra are presented in Table 2 and Table 3. From Table 3, most of the vibrations present in both Infrared and Raman spectra are same, thus confirming the non-centrosymmetric nature of the crystal.
3.3. Optical Studies
The UV-VIS-NIR absorption and transmission spectra were recorded in the range between 190 nm to1100 nm. The observed spectra are shown in Figure 4 and Figure 5. From the observed spectrum, the lower cut-off wavelength is found to be 238 nm and there is no absorption in the visible region and near infrared region. All amino acids possess the property, that the absorption of radiation is absent in the entire visible region   . The absence of absorption in this region provides as the source for second harmonic generation. Thus, the absorption studies reveal that the crystal L-Arginine maleate dihydrate acts as a suitable material for second harmonic generation in the visible and near infrared regions.
3.4. NMR Studies
The carbon-hydrogen bonded network of the crystal was analyzed by 1H-NMR and 13C-NMR spectra. The 1H-NMR and 13C-NMR spectra of L-Arginine maleate dihydrate are shown in Figure 6 and Figure 7 respectively and their chemical shifts with the assignments are tabulated in Table 4 and Table 5.
Figure 2. FTIR Spectrum of L-Arginine maleate dihydrate crystal.
Figure 3. FT-Raman Spectrum of L-Arginine maleate dihydrate crystal.
Table 2. FTIR vibrational assignments of L-Arginine maleate dihydrate crystal.
The resonance peaks at δ = 1.629 ppm and at δ = 1.889 ppm in the 1H-NMR spectrum is due to the CH2 group of L-Arginine. The spectrum shows two peaks at δ = 3.201 ppm and δ = 3.717 ppm corresponds to the CH2 and CH groups of L-Arginine. The resonance peak observed at δ = 6.285 ppm exhibits the presence of CH2 methy-
Figure 4. UV-VIS-NIR absorption Spectrum of L-Arginine maleate dihydrate crystal.
Figure 5. UV-VIS-NIR transmission Spectrum of L-Argi- nine maleate dihydrate crystal.
lene proton of maleic acid. The absence of peaks for NH2 and COOH group indicates that they are ionic in nature and are involved in secondary forces. It is interesting to note that the CH peak observed at 3.27 ppm for L-Arginine is shifted to 3.717 ppm for L-Arginine maleate dihydrate. This is due to the protonation of L-Argi- nine () by maleic acid (COO−) as shown below.
L-Arginine Maleic acid L-Arginine maleate dihydrate
The 13C-NMR spectrum of L-Arginine maleate dihydrate contains eight signals. The resonance peaks at δ = 174.27 ppm and δ = 170.81 ppm is due to the carboxylic groups of L-Arginine and maleic acid. The resonance signal observed at δ = 156.71 ppm is due to the carbon attached to HN=C−NH2 group. The carbon attached to amino group shows its resonance peak at δ = 54.24 ppm. The peaks at δ = 40.45 ppm, δ = 27.49 ppm and δ = 23.84 ppm are due to the carbon environments of CH2 groups of L-Arginine. The CH group of maleic acid gives
Figure 6. 1H-NMR Spectrum of L-Arginine maleate dihydrate crystal.
Figure 7. 13C-NMR Spectrum of L-Arginine maleate dihydrate crystal.
rise to a signal at δ = 134.35 ppm.
3.5. Thermal Analysis
The thermo gravimetric analysis and differential thermal analysis spectra are obtained for L-Arginine maleate dihydrate crystal. The TGA/DTA curve is given in Figure 8. The initial mass of the materials used for the analy-
Table 3. FTIR and FT Raman spectral assignments of L-Arginine maleate dihydrate crystal.
Table 4. The chemical shifts in 1H-NMR spectrum of L-Arginine maleate dihydrate crystal.
Figure 8. TGA/DTA curve of L-Arginine maleate dihydrate crystal.
sis was 5.735 mg and the final mass left out after the experiment was only −0.1420% (−0.008142 mg) of initial mass. The material starts decomposing at 107˚C with the loss of water of crystallization. The TGA curve also shows that the decomposition between the temperature ranges 178˚C - 301˚C results in the liberation of carbondioxide in the compound. The endothermic peak at 178˚C corresponds to the decomposition of maleic acid. The next stage of decomposition occurring between 301˚C and 538˚C is due to the removal of ammonia present in the compound. After this, the decomposition of L-Arginine maleate dihydrate continues upto 700˚C.
The DSC curve shown in Figure 9 reveals that the endothermic peak at 107˚C is assigned to the decomposition of the material which is also evident in the DTA curve. Further, endothermic peaks were due to the volatilization of the compound observed during the decomposition stages which matches with the TG/DTA curves.
3.6. Second Harmonic Generation Studies
The powder second harmonic generation (SHG) test was carried out for L-Arginine maleate dihydrate crystal using Kurtz and Perry technique  . The powdered sample of the crystal was tightly packed in a capillary tube of 1.5 mm diameter and was illuminated by a high intense beam of laser radiation of wavelength 1064 nm, with a pulse width of 10 ns and an input beam energy of 1.6 mJ/pulse. The material exhibits nonlinear optical property thus by the emission of 532 nm wavelength of green radiation. The second harmonic generation efficiency was found to be 1.4 times that of KDP.
Figure 9. DSC curve of L-Arginine maleate dihydrate crystal.
Table 5. The chemical shifts in 13C-NMR spectrum of L-Arginine maleate dihydrate crystal.
L-Arginine maleate dihydrate crystal was grown by liquid diffusion technique at room temperature. Single crystal XRD studies reveal that the crystal L-Arginine maleate dihydrate belongs to triclinic structure and noncentrosymmetric space group. The functional groups of the crystal were identified by FT-IR and FT Raman spectroscopy. The optical absorption spectrum reveals that the crystal is transparent in the entire UV-VIS-NIR region with the lower cut-off wavelength of 238 nm. 1H-NMR and 13C-NMR spectral analyses determine the structure of the grown crystal. From the thermal studies, the thermal stability was analyzed. Kurtz and Perry powder technique confirms that L-Arginine maleate dihydrate exhibits the nonlinear optical property.
The authors wish to thank SASTRA University for the Powder XRD, NMR and TG/DTA and DSC characterization facilities. The authors thank St. Joseph College, Tiruchirappalli for FTIR and UV spectra and IIT, Chennai for single crystal XRD and FT-Raman studies. The authors are also grateful to Dr. P. K. Das, Indian Institute of Science, Bangalore for the measurement of SHG efficiency.