Synthesis and Crystal Structures of Phenylethynylpyridinium Derivatives for Second-Order Nonlinear Optics

Abstract Several salts of 1-methyl-4-[4-(dimethylamino)phenylethynyl]pyridinium 1, whose ethenyl analogue 2 is the cation of DAST (1-methyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}pyridinium p-toluenesulfonate), were synthesized, and these properties were investigated. From a molecular-orbital calculation, the hyperpolarizability (β) of 1 was calculated to be about 80% of that of 2. The absorption maximum wavelength of 1 is shorter than that of 2. Six SHG active salts based on cation 1, including iodide and 4-substituted benzenesulfonates, were obtained in total. Among them, the crystal structures of p-chlorobenzenesulfonate salt (1c) and p-toluenesulfonate salt (1e) could be analyzed. As a result, these two have almost the same structures, and the off-diagonal second-order nonlinear optical coefficient (d) values were estimated to be about twice as large as those of DAST.