Crystal Structure and Properties of a New Organic Nonlinear Optical Material
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The single crystals of a binary organic 1:1 molecular complex were grown from different solvents. The single crystal x-ray diffraction study revealed that the binary material has crystallized with a noncentrosymmetric space group (P21) entirely different from that of the parent components. Second harmonic generation measurement on polycrystalline material shows green light emission with the molecular complex. The differential scanning calorimeter and the thermogravimetric analysis show eminent higher melting point and decomposition temperature, respectively for the binary molecular complex compared to that of urea and m-nitrobenzoic acid. The optical spectrum of newly grown crystal shows the cutoff wavelength (398 nm) near the UV region, and the micro hardness measurement confirmed the greater hardness of the binary crystal.
I. INTRODUCTION
Modern science demands both newer materials with specific properties and novel techniques to prepare efficient materials with required properties. Recently, organic materials have been reported for their various electronic applications1–3 as well as for nonlinear optical (NLO) and electro-optic applications.4,5 The credibility of organic materials is due to the ease in changing polarizability of the electrons in the -bonding orbital in contrast to the conventional inorganic materials where the lattice vibrations play a dominant role and require higher energy. However, more that 90% of the achiral organic molecules crystallize in a centrosymmetric space group,6,7 and there are no conclusive ways to control crystallization behavior of organic materials as well as to control mechanical, chemical, and thermal stabilities. m-Nitrobenzoic acid (NBA) belongs to the monoclinic system with centrosymmetric space group P21/c whereas urea belongs to the tetragonal system with space group P4¯21m and point group symmetry 4¯2 m. The wide range of transparency (200 to 1400 nm), together with birefringence, enables urea phase matching over an extensive wavelength region. Although urea has been used for frequency conversion8 and others applications9–10 by immersing in an index matching fluid, it could not be exposed to the normal atmosphere due to its hygroscopic nature.
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 7, Jul 2002
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During the phase diagram and physicochemical investigation by one of the authors,11 it was found that the binary material of urea and m-nitrobenzoic acid (UNBA) is not hygroscopic and molecular association is occurring between urea and m-nitrobenzoic acid via hydrogen bonding. Recently, organic chemists have shown that molecular complexation can be used as a designing tool in the crystal engineering of noncentrosymmetric structures,12–14 whereas the efforts for the synthesis of organic intermolecular species based on the basic understandings of phase diagram and physicochemical investigations by solid-state chemists have not received due consideration. Theref
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