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.