Synthesis, Crystal Growth, and Characterization of an Organic

CRYSTAL GROWTH & DESIGN

Synthesis, Crystal Growth, and Characterization of an Organic Nonlinear Optical Donor-π-Acceptor Single Crystal: 2-Amino-5-nitropyridinium-Toluenesulfonate G. Anandha Babu,† R. Perumal Ramasamy,† P. Ramasamy,*,† and V. Krishna Kumar‡

2009 VOL. 9, NO. 7 3333–3337

Centre for Crystal Growth, SSN College of Engineering, SSN Nagar, Tamilnadu- 603 110, India, and Department of Physics, Periyar UniVersity, Salem- 636 011, Tamilnadu, India ReceiVed February 6, 2009; ReVised Manuscript ReceiVed June 11, 2009

ABSTRACT: A second-harmonic generation (SHG) crystal 2-amino-5-nitropyridinium-toluenesulfonate (2A5NPT) has been synthesized. Good quality single crystal of 2A5NPT was successfully grown by the slow evaporation method with dimensions 16 × 15 × 8 mm3. The unit cell dimensions were determined from single crystal X-ray diffraction studies. GAUSSIAN 98W has been used to calculate the first-order hyperpolarizability of 2A5NPT. The calculated first-order hyperpolarizability of 2A5NPT is 142 × 10-30 esu. The structural perfection of the grown crystals has been analyzed by high-resolution X-ray diffraction (HRXRD) rocking curve measurements. Fourier transform infrared (FTIR) spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the 2A5NPT have been identified by UV-vis-NIR studies. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study its thermal properties. Powder test with Nd:YAG laser radiation shows a high second harmonic generation. Introduction New materials exhibiting nonlinear optical effects have been explored with a view to developing optical devices, for example, optical modulators and frequency-doubling devices.1 It is necessary to use molecules with high molecular hyperpolarizability (β) and which form an acentric crystal structure to obtain crystals with high second-order nonlinear susceptibility (χ(2)). Molecular salts composed of a cation with high β and a counter anion have been reported, in which the anion was introduced to improve the arrangement of the molecules.2,3 The 2-amino5-nitropyridine (2A5NP) has a nitro group as an electron acceptor and an amino group as an electron donor to induce high nonlinear optical (NLO) character. The p-toluene sulfonic acid has a methyl group as an electron donor and a sulfonate group as an electron acceptor to induce high NLO character. The 2A5NP derivatives crystals contain herringbone motifs of chromophores anchored onto inorganic or organic host matrices building noncentrosymmetric frameworks.4-6 Short and multiple hydrogen-bonded networks observed in 2A5NP derivatives structures provide the crystalline materials with improved thermal, chemical, and mechanical stabilities compared with those observed in molecular compounds built up with the same chromophore. A remarkable structural property observed in many of the crystal structures of these materials is the arrangement of 2A5NP+ cations in herringbone motifs connected to anionic layers. Masse et. al have published a series of organic-inorganic crystals of 2A5NP and inorganic acids such as arsenic acid, which were designed for NLO materials. 2A5NP allows for the growth of numerous salts, such as 2-amino-5nitropyridium dihydrogenphosphate, 2-amino-5-nitropyridium dihydrogen arsenate, 2-amino-5-nitropyridium acetophosphate, 2-amino-5-nitropyridium chloride (2A5NPCl), 2-amino-5-nitropyridium bromide (2A5NPBr), and 2-amino-5-nitropyridinium-L-monohydrogentartrate (2A5NPLT).6-9 Herein we report organic-organic crystals of 2A5NP combined with * Corresponding authors: E-mail: [email protected] (P.R.), anandcgc@ gmail.com (G.A.B.). † SSN College of Engineering. ‡ Periyar University.

Scheme 1. Reaction of 2A5NP with p-Toluenesulfonic Acid

p-toluenesulfonic acid and their bulk growth, structural, optical, thermal, and second harmonic generation (SHG) properties. Experimental Procedures Material Synthesis. The commercially available 2A5NP (Merck, Purity >98%) is a weak Bronsted base and can acquire a proton in a strongly acidic aqueous medium (pH < 2). This induces the dissolution of this molecule in an aqueous acidic medium by forming the 2A5NP+ cation and leads to the synthesis of hydrogen-bonded salts with the conjugated bases of strong or medium acids. The organic-organic 2-amino-5-nitropyridinium-toluenesulfonate salt is obtained by dissolving the 2A5NP in p-toluenesulfonic acid at 50 °C in Millipore water of resistivity 18.2 MΩ cm. The reaction scheme and their chemical structures are illustrated in Scheme 1. Solubility and Crystal Growth. To realize the practical applications, good-quality single crystals of reasonable size are essential. The material was purified from aqueous solution by the recrystallization process. The single-crystal growth of 2A5NPT has been performed from aqueous solution. The solubility of 2A5NPT in water was assessed as a function of temperature in the range 25-40 °C. A thermostatically controlled vessel (100 mL) was filled with an aqueous solution of 2A5NPT with some undissolved 2A5NPT and stirred for 6 h using a motorized magnetic stirrer by ensuring homogeneous temperature and concentration throughout the volume of the solution. The concentration of the solute was determined gravimetrically. The experiment was carried out in a constant temperature bath with a cryostat facility. The different experimental solutions were prepared at the desired saturation temperature by adding 2A5NPT. Then the saturated solution was heated 5 °C above the saturation temperature and kept there for 1 h. The solution was cooled at 4 °C/h until nucleation occurred. The difference between saturation and nucleation temperatures was taken as the metastable zone width (MZW). The knowledge of MZW is very important in terms of designing crystallization processes and obtaining desired crystal sizes,

10.1021/cg9001384 CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

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Figure 1. Solubility and nucleation curve of 2A5NPT in water.

Figure 2. Grown single crystals of 2A5NPT. shapes, and purities. Figure 1 shows the solubility and nucleation curve for 2A5NPT. The 2A5NPT exhibits positive solubility in water. Single crystals of 2A5NPT have been grown from saturated solution of the synthesized salt of 2A5NPT by the slow evaporation technique at room temperature using a 250 mL crystallizer. Single crystal of size 16 × 15 × 8 mm3 has been obtained after a typical period of 60 days. Grown single crystals of 2A5NPT are shown in Figure 2. Computational Details. The density functional theory (DFT) has emerged as a promising quantum chemical computational methodology owing to its ability to predict molecular geometry. Ab initio computations are performed using the Gaussian 98 package. The first-order static hyperpolarizability has been calculated using HF/3-21 G (d, p) on the basis of the finite field theory approach and aiming to estimate the ground state dipole moment. The finite field method offers a straightforward approach for the calculation of hyperpolarizabilities.10 The 3-21 (d, p) basis set gives remarkably good geometries for such a small basis set, and in fact it is used for the geometry optimization of some high accuracy energy methods. And it is so fast for optimization of giant molecules such as the title compound. Before calculating the hyperpolarizability for the title compound, the geometry taken from the starting structures based on its crystallographic data was optimized in the unrestricted open-shell Hartree-Fock (UHF) level. Molecular geometries were fully optimized by Berny’s optimization algorithm using redundant internal coordinates. All optimized structures were confirmed to be minimum energy conformations. An optimization is complete when it is converged, that is, when it has reached a minimum on the potential energy surface, thereby predicting the equilibrium structures of the molecules. This criterion is very important in geometry optimization. In the optimized structure, no imaginary frequency modes were obtained proving that a true minimum on the potential energy surface was found. The dispersion free first-order hyperpolarizability was calculated using finite field method. All the calculations were

Babu et al. carried out by the density functional triple parameter hybrid model DFT/ B3LYP and HF/3-21 G (d, p) basis set. Structral, Spectral, Thermal, and Optical Characterizations. The grown crystals were subjected to X-ray diffraction studies using Nonius CAD4/MACH 3 single crystal X-ray difffractometer, using Mo KR (λ ) 0.71073 Å). The crystalline perfection of the 2A5NPT single crystals grown by the slow evaporation solution growth technique (SEST) was characterized by high-resolution X-ray diffraction (HRXRD) by employing a multicrystal X-ray diffractometer developed at NPL. The well-collimated and monochromated Mo KR1 beam obtained from the three monochromator Si crystals set in dispersive (+,-,-) configuration has been used as the exploring X-ray beam. The specimen crystal is aligned in the (+,-,-,+) configuration. Because of dispersive configuration, though the lattice constants of the monochromator crystal(s) and the specimen are different, the unwanted dispersion broadening in the diffraction curve (DC) of the specimen crystal is insignificant. The specimen can be rotated about the vertical axis, which is perpendicular to the plane of diffraction, with minimum angular interval of 0.4 arc sec. The DC was recorded by the so-called ω scan wherein the detector was kept at the same angular position 2θB with wide opening for its slit. The various functional groups of 2A5NPT crystal were identified by the KBr pellet technique using a Perkin-Elmer FTIR spectrometer in the range 4000-450 cm-1. The transmission spectrum of the 2A5NPT crystal was studied in the range 200-1100 nm by PerkinElmer spectrometer. Transparent single crystal of 1 mm thickness was used for this study. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments were carried out on a NETZSCH STA 409 instrument with a heating rate of 10 °C/min from 30 to 600 °C. Samples were weighed in an Al2O3 crucible. Quantitative measurement of relative efficiency of 2A5NPT with respect to KDP was made by the Kurtz and Perry powder technique.11 The finely powdered crystal of 2A5NPT was packed in capillary tube. An Nd:YAG laser (DCR11) was used as a light source. A laser beam of fundamental wavelength 1064 nm, 8 ns pulse width, with 10 Hz pulse rate was made to fall normally on the sample cell. The power of the incident beam was measured using a power meter. The transmitted fundamental wave was passed over a monochromator (Czemy Turner monochromator), which separates 532 nm (second harmonic signal) from 1064 nm, and absorbed by a CuSO4 solution, which removes the 1064 nm light, and passed through BG-34 filter to remove the residual 1064 nm light and an interference filter with a bandwidth of 4 nm and central wavelength of 532 nm.

Results and Discussion From the single crystal X-ray diffraction studies, it is observed that the crystal belongs to the monoclinic system with the space group of Pc and the unit cell parameters are a ) 11.529(5) Å, b ) 7.914(7) Å, c ) 15.213(5) Å, and β ) 92.10(3)°. The crystal structure was already reported by Koshima et al.12 The 2A5NPT molecule is stabilized by hydrogen bonds. The 2amino-5-nitropyridium cation and toluene sulfonate anion are linked by strong PyN+-H · · · Oa-S, N-Ha · · · Ob-S, N-Hb · · · Oc-S, N-Hb · · · Oa-S, and O-H · · · Ob · · · S hydrogen bonding. The 2-amino-5-nitropyridium cation stack alternately with toluene sulfonate anion to give independent zigzag structures (Figure 3). Quantum chemical calculations are precious and inexpensive tools for predicting the molecular NLO properties of molecules before synthesis. In the present work, we calculated the hyperpolarizability of title compound. The static responses hyperpolarizability of a molecular system is calculated as the derivatives of the energy with respect to the electric field components, taken at zero field:

dipole moment ) µi ) components of polarizability tensor:

[ ] ∂2E ∂Fi∂Fj

0

Nonlinear Optical Donor-π-Acceptor Single Crystal

Rij ) -

[ ] ∂E ∂Fi

Table 1. Hyperpolarizability of 2A5NPT (×10-30) esu

0

components of hyperpolarizability tensor:

βijk ) -

[

∂3E ∂Fi∂Fj∂Fk

Crystal Growth & Design, Vol. 9, No. 7, 2009 3335

]

0

In the presence of an applied electric field, the energy of a system is a function of the electric field, and the first hyperpolarizability is a third-rank tensor that can be described by a 3 × 3 × 3 matrix. The first-order static hyperpolarizability is also significantly affected by the basis set in the calculation. The 27 components of 3D matrix can be reduced to 10 components because of the Kleinmann symmetry.13 The matrix can be given in the lower tetrahedral format. It is obvious that the lower part of the 3 × 3 × 3 matrix is a tetrahedral.

βtot ) [(βxxx + βxyy + βxzz)2 + (βyyy + βyzz + βyxx)2 + (βzzz + βzxx + βzyy)2] The calculated first-order hyperpolarizability (βtotal) of 2A5NPT is 142 × 10-30 esu, which is nearly 680 times that of urea (0.20920 × 10-30 esu). The calculated first-order hyperpolarizability values (β) are presented in Table 1. The theoretical calculation seems to be more helpful in the determination of particular components of β tensor than in establishing the real values of β. Domination of particular components indicates on a substantial delocalization of charges in those directions. The biggest values of hyperpolarizability are noted in the βxzz direction, and subsequently delocalization of electron cloud is more in that particular direction. Here the β values are calculated based on the structure derived from single crystal XRD of the title compound. To confirm our analysis, we calculated the hyperpolarizabilities of neutral NLO chromophores and the protonated species of the title crystal. The geometries of the two chromophores were optimized by ab initio DFT methods with HF/3-21 G (d, p) using Gaussian 98 software. The β tensor components were then computed from these optimized structures and the finite field method. All the chromophores are planar. The results for β are shown in Table 2. The 2A5NPT molecule has two electron

Figure 3. Molecular arrangements in 2A5NPT crystals. Yellow, red, blue, dark blue, and white atoms represent sulfur, oxygen, nitrogen, carbon, and hydrogen, respectively.12

βxxx βxxy βxyy βyyy βxxz βxyz βyyz βxzz βyzz βzzz βtotal

-10.3 -0.9625 7.867215 1.21277 -7.120376 -2.4688 -6.61622 -138.83 -3.71708 8.9891325 141.9643

donating amino and methyl groups, and NO2 and sulfonate acting as an electron accepting group. Thus, the molecule possesses a D-π-A type structure where charge transfer is from the donor to acceptor group. In the 2A5NPT crystals, π-π interaction between the pyridine and phenyl moieties as well as the multidimensional hydrogen bonds control the crystal structure to lead to the alternate stacking of 2A5NP+ cations and p-toluenesulfonate anions and the formation of column, zigzag, layer, and herringbone structure. Our calculations clearly indicate that the hyperpolarizabilities are increased by the proton transfer. The hyperpolarizability is multifold for 2A5NP title crystal when toluene sulfonic acid loses its proton to become sulfonate. The dense noncentrosymmetric packing of chromophores, all of which have nonzero values of β, gives rise to a strong second harmonic signal when a powder sample is illuminated by Nd:YAG laser radiation. When protonation occurs, the transfer of proton from toluene sulfonic acid to 2A5NP thus tends to increase molecular hyperpolarizabilities of these components of crystal relative to the β of the corresponding neutral molecules. Figure 4 shows the high-resolution diffraction curve (DC) recorded for a typical 2A5NPT single crystal specimen using (411) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer described in the Structural, Spectral, Thermal, and Optical Characterizations section with Mo KR1 radiation. The solid line (convoluted curve) is well fitted with the experimental points represented by the filled circles. On deconvolution of the diffraction curve, it is clear that the curve contains an additional peak, which is 42 arc sec away from the main peak. This additional peak depicts an internal structural very low angle (tilt angle e1 arc min) boundary whose tilt angle (misorientation angle between the two crystalline regions on both sides of the structural grain boundary) is 42 arc sec from its adjoining region.14 The full width at half-maximum (fwhm) of the main peak, and the very low angle grain boundary are respectively 22 and 84 arc sec. Though the specimen contains a very low angle grain boundary, the relatively low angular spread of around 300 arc sec of the diffraction curve and the low fwhm values show that the crystalline perfection is reasonably good. Thermal fluctuations or mechanical disturbances during the growth process could be responsible for the observed very low angle grain boundary. It may be mentioned here that such very low angle grain boundaries (which do not really deteriorate the properties) 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. Vibrational spectroscopy provides evidence for the charge transfer interaction between the donor and acceptor groups through π-electron movement. The intramolecular hydrogen bonding network formed between amino hydrogen of 2A5NP atoms and sulfonate oxygen atoms of toluene sulfonate. The 2A5NPT compound consists of 2-amino-5-nitropyridium as the cation and p-toluene sulfonate as the anion. The recorded

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Table 2. Calculated Values of Molecular Hyperpolarizabilities for 2-Amino-5-nitropyridine (2A5NP), p-Toluenesulfonic Acid, and 2-Amino-5-nitropyridinium-toluenesulfonate

infrared spectrum is shown in Figure 5. The amino of the 2-amino-5-nitropyridium cation and SO3 group in p-toluene sulfonate anion are held together by the intramolecular hydrogen bonds. The amino N of the 2-amino-5-nitropyridium cation forms an N-H · · · O hydrogen bonds with the O atoms of the toluene sulfonate anion. In the case of 2A5NPT, there are two types of hydrogen bonds: O-H · · · O bonds that are stronger than N-H · · · O bonds. The intramolecular N-H · · · O hydrogen

Table 3. Spectral Data and their Assignments for 2A5NPT FTIR (cm-1)

assignments

3329 3111 1676 1651 1561 1530 1497 1452 1357 1248 1156 1036 637 564

νas(NH2) ν(NH+) ν (CdN) δ (CdN) νas (NO2) ν (CdN) ν (N-O) ν (N-O) ν (NO2) δ (C-H) δ in plane (C-H) δ in plane (C-H) δ (NH2) νas (SO3)-

Figure 4. Diffraction curve recorded for 2A5NPT single crystal for (411) diffracting planes by employing the multicrystal X-ray diffractometer with Mo KR1 radiation.

bonding is formed due to the overlap between n (O) and σ*(N-H) causing stabilization of hydrogen-bonded systems. The increase in population of the N-H antibonding orbital weakens the N-H bond. The observed wave numbers and the assignments made from the recorded spectra for 2A5NPT crystal are given in Table 3. SHG materials should have a good transparency window to use them for frequency doubling extending from the visible down to the UV region. The wider the transparency window, the better will be the practical applicability; however, the crystal should be phase matchable in the entire transparency region for an effective use. The UV-vis-NIR spectrum of 2A5NPT is shown in Figure 6. The 2A5NPT crystal is transparent in the visible to NIR region with an optical cutoff at 420 nm. The protonation of

Figure 5. FTIR spectrum of 2A5NPT.

Figure 6. UV-vis-NIR Spectrum of 2A5NPT.

Nonlinear Optical Donor-π-Acceptor Single Crystal

Crystal Growth & Design, Vol. 9, No. 7, 2009 3337

Table 4. Properties of Some NLO Materials

a

crystals

melting point

λcutoff (nm)

2A5NPLT15 2A5NPBr16 2A5NPCl17 2A5NPFB18 2A5NPTa

181 188 150 145 204

410 410 410 420 420

Present work.

2A5NP in the acid medium shortens the conjugated bond lengths CdC, CdN, C-NH2, and lengthens the C-NO2 bond length. Such changes indicate an alternation in the intramolecular charge transfer, that is, NH being an electron-donor group in competition with the electron-acceptor NO2 group. The presence of the protonated N heteroatom in the aromatic ring is believed to account for the inclination of the molecular transition dipole-moment with respect to the nitro-amino direction in the nonprotonated molecule. This protonation modifies the transparency of the original molecule8 (2A5NP) and consequently shifts the crystal transparency (40 nm) limit toward a visible length. A comparison of the melting point and cutoff wavelength for some selected 2A5NP derivatives crystals is presented in Table 4. The thermal behavior of 2A5NPT has been identified from the TG/DTA. Figure 7 shows the thermal properties of the 2A5NPT crystal carried out by TGA and DTA. In the differential thermogram, sharp endothermic peak was found at 204 °C. The endothermic is assigned to the melting point at which no weight loss in TG has been noticed. The sharp exothermic reaction around 305 °C may be possibly due to some complex formation. There is steep loss of weight starting around 250 °C and after complex formation the weight loss is relatively gradual. The final residue weight left was about 40% after heating to 600 °C. The 2A5NPT material has a fairly high melting point compared to other 2A5NP derivative crystals.

Figure 7. TG-DTA curves of 2A5NPT. Table 5. Comparison of Powder Signal Output input power (µJ/pulse)

KDP (mV)

urea (mV)

2A5NPT (mV)

630

4.3

28

390

Table 5 shows the comparison of SHG signal output for the title compound 2A5NPT with that of standard KDP and urea. The beam energy is 630 µJ/pulse. The SHG signal outputs are 4.3 mV and 390 mV for KDP and 2A5NPT, respectively. Powder test with Nd:YAG laser radiation shows a high SHG. This strongly suggests the title compound as a potential candidate for SHG applications compared to other 2-amino-5nitropyridine derivatives crystals. This larger efficiency is probably due to the density of the chromophores, which is the highest observed among the herringbone structure built with the 2A5NP+ cations. Conclusion Optical quality single crystals of 2A5NPT were grown using the solution growth technique. The unit-cell parameters of 2A5NPT were confirmed by single crystal X-ray diffraction analysis. GAUSSIAN 98W was used to calculate the first-order hyperpolarizability of 2A5NPT. The calculated first-order hyperpolarizability of 2A5NPT is 142 × 10-30 esu. The high resolution X-ray diffraction curve (DC) measurements substantiate the good quality of the crystals. The functional group was confirmed by FTIR. In the transmittance spectra, it is evident that the 2A5NPT crystal has a wide transparency range in the entire visible and NIR range. The thermal behavior of the grown crystals was studied by using TG-DTA. Powder test with Nd: YAG laser radiation shows a high SHG. Thus, 2A5NPT seems to be a promising material for NLO application. Acknowledgment. The authors thank Prof. P. K. Das, Indian Institute of Science, Bangalore, for support with the SHG measurements. We thank Dr. G. Bhagavannarayana, National Physical Laboratory, New Delhi, for the HRXRD studies.

References (1) Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of Organic Molecules and Crystals; Academic Press; New York, 1987; Vols. 1 and 2. (2) Masse, R.; Zyss, J. Mol. Eng. 1991, 1, 141–152. (3) Zyss, J.; Oudar, J. L. Phys. ReV A. 1982, 26 (4), 2028–2048. (4) Masse, R.; Bagieu-Beucher, M.; Pecaut, J.; Levy, J. P.; Zyss, J. Nonlinear Opt. 1993, 5, 413. (5) Pecaut, J.; Masse, R. Acta Crystallogr. 1993, B49, 277. (6) Zyss, J.; Masse, R.; Bagieu-Beucher, M.; Levy, J. P. AdV. Mater. 1993, 5 (2), 120. (7) Zaccaro, J.; Capelle, B.; Ibanez, A. J. Cryst. Growth 1997, 180, 229– 237. (8) Pecaut, J.; Levy, J. P.; Masse, R. J. Mater. Chem. 1993, 3, 999. (9) Pecaut, J.; Masse, R. J. Mater. Chem. 1994, 4, 1851–1854. (10) Cohen, H. D.; Roothan, C. C. J. J. Chem. Phys. 1965, 34, 435. (11) Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798. (12) Koshima, H.; Hamada, M.; Yagi, I.; Uosaki, K. Cryst. Growth Des. 2001, 6, 467. (13) Kleinman, D. A. Phys. ReV. 1977, 126, 1962. (14) Bhagavannarayana, G.; Ananthamurthy, R. V.; Budakoti, G. C.; Kumar, B.; Bartwal, K. S. J. Appl. Crystallogr. 2005, 38, 768–771. (15) Manikandan, S.; Dhanuskodi, S. Spectrochim. Acta Part A 2007, 67, 160. (16) Dhanuskodi, S.; Pricilla Jeyakumari, A.; Manivannan, S. J. Cryst. Growth 2005, 282, 72–78. (17) Dhanuskodi, S.; Manikandan, S. Radiat. Eff. Defects Solid 2004, 159, 173–180. (18) Manivannan, S.; Dhanuskodi, S.; Kirschbaum, K.; Tiwari, S. K. Cryst. Growth Des. 2005, 5 (4), 1463.

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