Synthesis and characterization of thienyl-substituted pyridinium salts for second-order nonlinear optics

The synthesis of a series of thienyl-substituted pyridinium salts with different counter-anions and their second-order nonlinear optical (NLO) properties are described. Kurtz powder tests revealed that, out of the twelve new pyridinium salts, four possess noncentrosymmetric structures and the maximum powder second harmonic generation (SHG) efficiency is 0.6 times of DAST. Single crystals of three new salts were successfully obtained from methanol by slow evaporation method. X-Ray studies exhibited that they crystallized in the monoclinic space groupP21/c, P21, and P21/n, respectively. (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium 4-hydroxybenzenesulfonate (3c) forms a hydrated noncentrosymmetric phase with a large macroscopic optical nonlinearity 0.5 times of that of DAST at the non-resonant wavelength of λ = 2109 nm, while both (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium 2-amino-3,5-dimethylbenzenesulfonate (3i) and (E)-4-(2-(5-(dimethylamino)thiophen-2-yl)vinyl)-1-methylpyridinium iodide (3a) crystallize in centrosymmetric structures.