Molecular engineering of stilbazolium derivatives for second-order nonlinear optics

Three series of ionic stilbazolium salts with different counteranions have been synthesized to investigate the effect of counteranion size to crystal packing. Single crystals of five new salts were successfully grown from solution by the slow cooling and slow evaporation techniques. X-ray studies revealed that three of them show the triclinic space group P1 and two crystallize in the monoclinic space group Cc and P2(1)/c, respectively. Kurtz powder tests revealed that out of the eight new stilbazolium salts, six possess noncentrosymmetric structures and three show large powder second-harmonic generation efficiencies higher or similar to that of 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST). The optimal crystalline packing for electro-optics with perfectly aligned chromophores was achieved in three of the investigated compounds. The reasons for considerably different macroscopic nonlinearities of these compounds have been explored in terms of intermolecular interactions.