Molecular design on substituted DAST derivatives for second-order nonlinear optics

Abstract Molecular design of the derivatives of 1-methyl-4-(2-(4-(dimethylamino)phenyl)ethynyl)pyridinium (DAS) was investigated from the following two points, i.e., simple substitution of one aromatic hydrogen atom to enhance hyperpolarizability β and fluorine substitution to decrease optical loss due to overtones of C–H bond vibration. By the screening using semiempirical calculation, 2-cyano-1-methyl-4-(2-(4-(dimethylamino)phenyl)ethynyl)pyridinium 7, 2,3,5,6-tetrafluoro-1-methyl-4-(2-(4-(dimethylamino)-2,3,5,6-tetrafluorophenyl)ethynyl)pyridinium 10, etc. were expected to have larger β than that of DAS. The salts of 7 and 1-methyl-4-(2-(4-(dimethylamino)-2,3,5,6-tetrafluorophenyl)ethynyl)pyridinium as a related cation of 10 were synthesized and four crystals showing second-harmonic generation were found.