Growth and Characterization of the NLO Crystal 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate (DAST) K. Jagannathan,† S. Kalainathan,*,† T. Gnanasekaran,‡ N. Vijayan,§ and G. Bhagavannarayana§
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 859-863
School of Science and Humanities, Vellore Institute of Technology, Vellore-632 014, India, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India, and Materials Characterization DiVision, National Physical Laboratory, New Delhi-110 012, India ReceiVed April 23, 2006; ReVised Manuscript ReceiVed December 14, 2006
ABSTRACT: In order to satisfy present day technological requirements, one has to search the new organic nonlinear optical (NLO) materials with good efficiency. In this article, the title compound (4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST)) belongs to the class of organic NLO materials, and it is one of the highest of the nonlinear materials among all the known NLO single crystals. In the present study, we have reported the synthesis, solubility, metastable zone width, and induction period of the DAST single crystal. The crystal has been grown by the slope nucleation technique by optimizing the growth conditions, and it has been characterized by different instrumentation methods. The crystal system and lattice constants were found from powder X-ray diffractometry (XRD). Nuclear magnetic resonance (13C NMR) studies confirm the presence of functional groups. The crystalline perfection is moderately good as observed from the high-resolution X-ray diffractometry (HRXRD). A low-angle internal structural grain boundary was observed from this analysis, which seems to be formed by the segregation of solvent molecules at the boundary due to entrapment of solvent molecules during the growth process. Entrapment of solvent molecules in the crystalline matrix has been confirmed by differential scanning calorimetry (DSC). The Kurtz powder second-harmonic generation (SHG) reveals that the SHG efficiency of grown DAST is about 141 times that of urea. From the microhardness measurements, the Meyer’s index and Young’s modulus have been calculated. 1. Introduction In recent years organic nonlinear optical (NLO) materials have been attracting attention because of their second- or third-order hyperpolorizablities compared to those of inorganic NLO materials.1 Many investigations are being done to synthesize new organic materials with large second-order optical nonlinearities in order to satisfy day-to-day technological requirements.2 They have innumerable potential applications including telecommunications, optical computing, optical data storage, etc. In this series, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) is one of the recently discovered potential organic NLO materials. It belongs to the monoclinic system with high electrooptic [r111 ) 77 ( 8 pm/V]1,3,4 and nonlinear optical coefficients. Its low dielectric constant suggests that the materials can be used for optical data storage applications. Some studies have been carried out for the growth of DAST single crystals, and recently Shunichi et al.4 and Mohan Kumar et al.5 have grown the DAST crystal by the slow evaporation method. Hiroaki et al.1 have grown this crystal by the spontaneous nucleation method. By the above said methods it is very difficult to control the nucleation site, which is the main parameter for the growth of a single crystal. But getting a high-quality DAST crystal is still a challenge for crystal growers and materials scientists. The present article deals with the growth of DAST single crystals by the slope nucleation technique,3,6 which is different from the above said conventional methods. In view of the above said facts, a detailed study on the nucleation parameters such as solubility, metastable zone width, and induction period has been made, and the growth conditions have * Corresponding author. † Vellore Institute of Technology. ‡ Indira Gandhi Centre for Atomic Research. § National Physical Laboratory.
been optimized using the above said parameters. The grown crystals have been subjected to different instrumentation methods such as powder XRD, NMR, high-resolution XRD, DSC, SHG, and Vickers and Knoop microhardness. To the best of our knowledge no report is available on the determination of the Meyer’s index number and Young’s modulus for the DAST crystal, and hence we report these for the first time. 2. Synthesis and Crystal Growth of DAST 2.1. Material Synthesis. DAST was synthesized by the condensation of 4-methyl-N-methyl pyridinum tosylate, which was prepared from 4-picoline, methyl toluene sulfonate, and 4-N,N-dimethylamino-benzaldehyde in the presence of piperidine as catalyst. The steps involved during the chemical reactions are as follows: The calculated amounts of picoline (10.31 mL, 0.105 mol %) and methyl toluene sulfonate (15.88 mL, 0.105 mol %) were added in toluene solution (200 mL). The total mixture was taken in a round-bottom flask (500 mL) of a Dean-Stark apparatus. Then the mixture was heated using a heating mantle until it crystallized as a white salt, which is insoluble in toluene. When the mixture was boiling, dimethylformamide (DMF) solution was added until the mixture became a clear solution. After getting the clear solution of the above said mixture, the 4-N,N-dimethyl-benzaldehyde (15.65 g, 0.105 mol %) was added slowly. After the reaction process, the piperidine, which plays the role of catalyst, was added as droplets; then immediately the color of the solution became red. Then the mixture was refluxed with a Dean-Stark trap in order to remove the water. After more than an equivalent amount of water was collected, the reactants were cooled to room temperature and the synthesized salt was collected. The collected material was kept in an oven, and the temperature was maintained around 100 °C for 1 h. Then the DAST was purified by a successive recrystallization process.5
10.1021/cg0602414 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007
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Figure 1. Photograph of a typical harvested crystal.
2.2. Growth of DAST Single Crystals. In the present study, the title compound, DAST, was grown by adopting the slope nucleation method (SNM). Normally, glass beakers encourage the growth of microsegregation at the walls.6 In order to avoid this microsegregation, Teflon beakers were used to grow the DAST crystal. The dried salts were collected, and the supersaturated solution was prepared at a desired temperature. Then the saturated solution was filtered and kept in the Teflon beaker. Then the beaker was housed in a constant temperature bath (CTB) in order to maintain a constant temperature (42 °C). The key procedure of the SNM is that a Teflon plate with grooves is inserted into the growth vessel (Teflon beaker). The remaining procedures are the same as are followed in the conventional slow cooling. The initial temperature setting was slowly reduced by 0.1 °C per day. After 20 days of cooling, the DAST single crystals were spontaneously nucleated in the grooves, which we kept inside the mother solution. In this type of growth we can control the position of spontaneously nucleated DAST crystals, which is not possible in other conventional methods. The maximum size of the crystals harvested after 20 days (Figure 1) is 1.3 × 1.2 × 0.1 cm3. The growth rate of the crystals mainly depends on the inclination angle, width, and depth of the grooves of the Teflon plate. Efforts have been made to increase the growth rate and size of the crystals by changing the above said parameters. 3. Characterization Analyses The grown single crystals of DAST have been subjected to different instrumentation methods such as powder X-ray diffraction, high-resolution X-ray diffraction, thermal analysis, nuclear magnetic resonance (NMR), and powder SHG measurements. Vickers and Knoop microhardness methods have analyzed the mechanical behavior of the grown specimen. The detailed discussions are given in the following sections. 3.1. Powder X-ray Diffractometry. Powder X-ray diffraction analysis has been carried out using (λ ) 1.5418 Å) a powder X-ray diffractometer with a scan speed of 0.05° min-1. The lattice parameters were calculated using the TREOR program (then refined by APPLEMAN) from the observed 2θ values. From these measurements we observed that the DAST crystal belongs to the monoclinic system, and the calculated lattice constants are a ) 10.5473, b ) 11.5747, and c ) 18.3589 Å and β ) 93.652°. The observed values are in good agreement with the reported literature values.7 The recorded spectrum is shown in Figure 2. 3.2. High-Resolution X-ray Diffractometry (HRXRD). To reveal the crystalline perfection of the specimen crystals, highresolution rocking or diffraction curves (DCs) were recorded with the multicrystal X-ray diffractometer developed at the National Physical Laboratory8 in symmetrical Bragg geometry.
Figure 2. Powder X-ray diffraction pattern of DAST.
Figure 3. High-resolution X-ray diffraction or rocking curve for DAST single crystal recorded for (320) diffracting planes using Mo KR1 radiation.
A well-collimated and monochromated Mo KR1 beam obtained from a set of three-plane (111) Si monochromator crystals set in dispersive (+, -, -) configuration has been used as the exploring X-ray beam.9 This arrangement improves the spectral purity (∆λ/λ , 10-5) of the Mo KR1 beam. The divergence of the exploring beam in the horizontal plane (plane of diffraction) was estimated to be ,3 arc s. The specimen crystal is aligned in the (+, -, -, +) configuration. Due to the dispersive configuration, though the lattice constant of the monochromator crystal(s) and the specimen are different, the unwanted dispersion broadening in the diffraction curve of the specimen crystal is insignificant. The specimen can be rotated about a vertical axis, which is perpendicular to the plane of diffraction, with minimum angular interval of 0.5 arc s. The diffracted intensity is measured by using a scintillation counter. Before recording the diffraction curve, the specimen surface was prepared by lapping and polishing, and then it was chemically etched by a nonpreferential chemical etchant mixed with water and acetone in 1:2 ratio. Figure 3 shows the highresolution diffraction curve recorded for (320) diffracting planes of the specimen crystal. The solid line (convoluted curve) is well fitted with the experimental points represented by the filled squares. On deconvolution of the diffraction curve, it is clear that the curve contains an additional peak due to internal structural grain boundary, which is 143 arc s away from the main peak. The additional peak corresponds to an internal structural low-angle boundary (tilt angle g 1 arc min but less
4-Dimethylamino-N-methyl-4-stilbazolium Tosylate
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Figure 4. DSC curve of DAST.
than a degree) whose tilt angle (misorientation angle between the two crystalline regions on both sides of the structural grain boundary) is 143 arc s. Generally, such boundaries are observed either when some impurities are entrapped inside the crystal10 or are due to structural phase transitions associated with volume changes during the cooling cycle in the case of melt-grown crystals.11 In the present solution-grown DAST specimen, the former reason could be appropriate due to probable entrapment of solvent molecules during the growth process. To confirm this, thermal analysis was carried out by the DSC technique. In the DSC curve, a small peak was observed at around 52 °C, which is due to the solvent (methanol) evaporation and confirms that the observed low-angle boundary is due to entrapment of solvent in the crystalline matrix during the growth process. The details of the DSC analysis are described in the forthcoming section. The FWHM (full width at half-maximum) of the main peak and the low-angle boundary are, respectively, 146 and 186 arc s. Though the specimen contains a low-angle boundary, the relatively low angular spread of around 500 arc s of the diffraction curve shows that the crystalline perfection is reasonably good. It may be mentioned here that such low-angle boundaries could be detected with well-resolved peaks in the diffraction curve only because of the high-resolution of the multicrystal X-ray diffractometer used in the present studies. 3.3. Differential Scanning Calorimetry (DSC). In order to know the thermal behavior of the present specimen, DSC analysis was carried out using a METTLER TOLEDO STAR system (DSC821). The recorded curve is shown in Figure 4. The decomposition was recorded from 273 to 770 K for 50 min. In the DSC curve, one small peak was observed at around 52 °C, which is due to the solvent (methanol) evaporation. The endothermic peak at 167 °C is due to the breakage of the CH3 group from the stilbazolium ion. Another sharp endothermic peak was observed at 261 °C, which is attributed to the melting point of the title compound. It is good in agreement with the earlier observed value of 258 °C.12,13 After melting, another exothermic peak is observed around 317 °C because of the complete breakage of the stilbazolium ion from DAST. 3.4. NMR Studies. The NMR technique is used to detect the presence of particular nuclei in a compound for a given nuclear species. It is also an important tool for the identification of molecules and for the examination for their electronic structure.14 The 13C NMR spectral analysis was made on the
Figure 5. NMR spectrum of DAST.
DAST crystal sample by dissolving it in deuterated methanol as solvent. The recorded spectrum is shown in Figure 5. In this spectrum, three sets of CH3 groups were observed at 21.2, 40.1, and 47.1 ppm, respectively. The p-toluene sulfonate aromatic ring is identified from the peaks at 156.1, 145.1, and 124.2 ppm. There are nine peaks, at 131.5, 129.7, 117.8, 141.5, 113.1, 143.8, 129.9, 123.5, and 154 ppm, which are due to the presence of the stilbazolium moiety. The peaks between 113.65 and 141.532 ppm are due to heteroaromatic rings. The δ values from 110 to 140 ppm confirm the aromatics and heteroaromatics (sp2 hybridization). 3.5. Kurtz Powder Test. The SHG efficiency of DAST is measured by using the Kurtz powder technique. The fine powder specimen sample was inserted in a microcapillary tube and then subjected to a Q-switched Nd:YAG laser emitting 1064 nm radiation with 10 ns pulse width. The generated SHG signal (confirmed by the emission of green radiation) at 532 nm was split from the fundamental frequency using an IR separator. A detector connected to a power meter was used to detect the second-harmonic intensity and read the energy input and output. From the SHG test, the relative SHG efficiency of the grown DAST crystal was found to be nearly 141 times greater than that of urea.
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Figure 6. Variation of Vicker’s microhardness with load.
3.6. Vickers Microhardness Test. Vickers microhardness measurement studies were also carried out using a MITUTOYO model HM 112 hardness tester. After the preliminary adjustments, the smooth surface of the specimen has been subjected for the present analysis. Hardness values have been calculated using the relation Hv ) 1.8544P/d2 kg/mm2 (where P is the applied load and d is the diagonal length of the indentation). The graph has been plotted against the Vickers hardness (Hv) and load (P). The plot is shown in Figure 6. From this measurement we found that as the load increases up to 50 g, the Vickers microhardness number decreases. After increasing the load above 50 g, cracks were developed. This may be due to the internal stresses released during the indentation. The Meyer’s index has been calculated using the relation P ) kdn (where k is the materials constant). In the present study, we observed that the value of n is equal to 1.85. According to Onitch,15 a value of “n” lying between 1 and 1.6 indicates hard materials; values above that belong to soft materials. The present specimen sample of DSAT may belong to the soft material category. 3.7. Knoop Microhardness Test. Knoop microhardness measurement studies were also carried out using a MITUTOYO model HM 112 hardness tester. Hardness values have been calculated using the relation Hk ) 14.299P/d2 kg/mm2 (where P is the applied load and d is the diagonal length of the indentation). The graph has been plotted against the Knoop hardness (Hk) and load (P). The plot is shown in Figure 7. From this measurement, it is found that as the load increases up to 50 g, the Knoop microhardness number decreases. From the Knoop microhardness measurements the Young’s modulus (E) of the crystal was calculated using the relation E ) 0.45Hk/ (0.1406 - b/a),16 where Hk is the Knoop microhardness value at a particular load and b and a are the shorter Knoop indentation diagonal and the longer indentation diagonal, respectively. The calculated Young’s modulus is 1.3827 × 1010 N m-2. 4. Conclusions The title compound of DAST was successfully synthesized, and the single crystals have been grown by the slope nucleation technique by optimizing the growth conditions. The present technique allows excellent control of the position of the spontaneously nucleated DAST crystal. The solubility has been studied with different solvents. From these measurements, it is
Jagannathan et al.
Figure 7. Variation of Knoop microhardness with load.
observed that methanol was the suitable solvent. From the powder X-ray diffraction pattern, the lattice parameters were calculated, and the monoclinic structure is confirmed. The lowangle grain boundary observed from the high-resolution diffraction curve was attributed to the entrapment of solvent inside the crystal, which is consistent with the corresponding result obtained from the DSC analysis. The melting point was found to be 261 °C from the DSC measurements, which is in good agreement with the reported literature value. The functional groups were identified from the NMR analysis. The SHG efficiency of DAST was found to be 141 times greater than that of urea. From the mechanical measurements, it was observed that the hardness decreases with the increase of load. From the Meyer’s index measurements, it is found that the crystal belongs to the moderately softer substance group. The Young’s modulus calculated from the Knoop microhardness test for the DAST single crystal is 1.35 × 1010 N m-2. Acknowledgment. The authors are grateful to the visionary management of the Vellore Institute of Technology, Vellore for their constant support and encouragement and the Defense Research and Development Organization (DRDO), Government of India for financial assistance. The authors are also grateful to the Director, National Physical Laboratory for his constant encouragement in carrying out the present investigations. References (1) Adachi, H.; Takahashi, Y.; Yabuzaki, J.; Mori, Y.; Sasaki, T. J. Cryst. Growth 1999, 198/199, 568. (2) Meir, U.; Bosch, M.; Boshard, C.; Gunter, P. Synth. Met. 2000, 109, 19. (3) Tsunesada, F.; Iwai, T.; Watanable, T.; Adachi, H.; Yoshimura, M.; Mori, Y.; Sasaki, T. J. Cryst. Growth 2002, 237-239, 2104. (4) Sohma, S.; Takahashi, H.; Taniuchi, T.; Ito, H. Chem. Phys. 1999, 245, 359. (5) Mohan Kumar, R.; Rajan Babu, D.; Ravi, G.; Jayavel, R. J. Cryst. Growth 2003, 250, 113. (6) Mori, Y.; Takahashi, Y.; Iwai, T.; Yoshimura, M. Jpn. J. Appl. Phys. 2000, 39, 1006. (7) Marder, S. R.; Perry, J. W.; Schaefer, W. P. Science 1989, 245, 626. (8) Lal, K.; Bhagavannarayana, G. J. Appl. Crystallogr. 1989, 22, 209215. (9) Bonse, U.; Hart, M. Appl. Phys. Lett. 1965, 7, 238-240. (10) Choubey, A.; Bhagavannarayana, G.; Shubin, Yu.; Chakraborty, B. R.; Lal, K. Z. Kristallogr. 2002, 217, 515-521. (11) Bhagavannarayana, G.; Ananthamurthy, R. V.; Budakoti, G. C.;
4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Kumar, B.; Bartwal, K. S. J. Appl. Crystallogr. 2005, 38, 768-771. (12) Chen-Yang, Y. W.; Sheu, T. J.; Lin, S. S.; Tu, Y. K. Curr. Appl. Phys. 2002, 2, 349. (13) Rai, R. N.; Jeng, J. M.; Tai, C. Y.; Lan, C. W. J. Chin. Inst. Chem. Eng. 2002, 33.
Crystal Growth & Design, Vol. 7, No. 5, 2007 863 (14) Sharma, B. K. Spectroscopy (Goel, Meerut) 1999. (15) Onitch, E. M. Microskopie 1950, 95, 12. (16) Mukerji, S.; Kar, T. Mater. Res. Bull. 2000, 35, 711-717.
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