Engineering of Organic Chromophores with Large Second-Order Optical Nonlinearity and Superior Crystal Growth Ability

We present two D-π-A organic cationic core structures leading to highly efficient nonlinear optical (NLO) salts (E)-2-(4-(dimethylamino)­styryl)-1,1,3-trimethyl-1H-benzo­[e]­indol-3-ium iodide (P–BI) and (E)-2-(2-(5-(dimethylamino)­thiophen-2-yl) vinyl)-1,1,3 -trimethyl-1H-benzo­[e]­indol-3-ium iodide (S–BI). Single crystals of the above two materials were successfully obtained by the slow evaporation method. Two different polymorphic crystals of both P–BI and S–BI were obtained from different polar solvents. Kurtz powder tests revealed that the maximum second harmonic generation (SHG) efficiency of P–BI with monoclinic space group P21 is 1.14 times the benchmark DAST­(4-N,N-dimethylamino-4,-N,-methylstilbazolium tosylate). Bulk single crystals of P–BI were obtained with size of up to 17.0 × 6.0 × 2.0 mm3 without using seed crystals. This demonstrates that this material exhibits great crystal growth ability along with a high second-order optical nonlinearity, making it a very attractive candidate for NLO applications such as electro-optics and THz-wave generation.