Polar Thiazole\nDerivative-Induced Second Harmonic\nResponse in Hybrid Bismuth Chlorate with Reversible Photochromism

Inorganic–organic hybrid bismuth halides demonstrate\ngreat\nprospect in the field of second-order nonlinear optical (NLO) crystals\nbecause the ns<sup>2</sup> electron on Bi(III) could lead to large\nmolecular polarization and high second harmonic generation (SHG) coefficient\non a noncentrosymmetric structure. However, researchers cannot yet\ncontrol the effective arrangements of the bismuth halide functional\nmotif, which results in SHG-active hybrid bismuth halides being rare.\nHerein, thiazole derivatives with a polar donor−π–acceptor\nsystem are designed to explore hybrid bismuth halide NLO crystals.\nThe protonated 2-(4-hydroxyphenyl) thiazole (denoted as hpt) interacts\nwith the (BiCl<sub>6</sub>) motif via H···Cl hydrogen\nbonds to prevent antiparallel arrangements, which therefore enhance\nthe NLO property. <b>(Hhpt)</b><sub><b>3</b></sub><b>[BiCl</b><sub><b>6</b></sub><b>]</b> crystallizes\nin the monoclinic polar <i>P</i>2<sub>1</sub> space group,\nwhich exhibits a strong SHG response and reversible photochromic behavior.\nStructural analysis and theory calculations reveal that the synergistic\neffect between the polar thiazole derivative and the distorted inorganic\n[BiCl<sub>6</sub>]<sup>3–</sup> motif is responsible for the\nSHG response. This is the first report of polar thiazole derivative-induced\nSHG-active hybrid bismuth halide coupled with reversible photochromism.