Preparation and crystal structures of new colorless 4-amino-1-methylpyridinium benzenesulfonate salts for second-order nonlinear optics

A series of ionic 4-amino-1-methylpyridinium benzenesulfonate salts have been prepared by quaternization reaction of 4-aminopyridine with methyl benzenesulfonates having a substituent of hydroxy, methoxy, methyl, chloro, or bromo group at para position. The X-ray structure analysis has been performed for their single crystals which were obtained by slow evaporation of their methanol solutions. The salts were found to crystallize into noncentrosymmetric structures; triclinic space group P1 for the hydroxy derivative, monoclinic space group P2₁ for the methoxy and methyl derivatives, and monoclinic space group Cc for the chloro and bromo derivatives. By using the molecular structures and the molecular first hyperpolarizability (β) of the corresponding cation and anions, the components of second-harmonic tensor coefficient d of the crystals were evaluated by the oriented-gas model. The diagonal d components of the crystals were estimated to be as large as one-fourth of that of 2-methyl-4-nitroaniline (MNA) at 1064-nm laser wavelength irrespective of their very short absorption cutoff wavelength in the UV region. These colorless crystals could be a new series of prospective crystalline materials for second-order nonlinear optical applications using conventional diode lasers.