Polar Thiazole Derivative-Induced Second Harmonic Response in Hybrid Bismuth Chlorate with Reversible Photochromism

Inorganic-organic hybrid bismuth halides demonstrate great prospect in the field of second-order nonlinear optical (NLO) crystals because the ns² electron on Bi(III) could lead to large molecular polarization and high second harmonic generation (SHG) coefficient on a noncentrosymmetric structure. However, researchers cannot yet control the effective arrangements of the bismuth halide functional motif, which results in SHG-active hybrid bismuth halides being rare. Herein, thiazole derivatives with a polar donor-π-acceptor system are designed to explore hybrid bismuth halide NLO crystals. The protonated 2-(4-hydroxyphenyl) thiazole (denoted as hpt) interacts with the (BiCl₆) motif via H···Cl hydrogen bonds to prevent antiparallel arrangements, which therefore enhance the NLO property. <b>(Hhpt)</b>_<b>3</b><b>[BiCl</b>_<b>6</b><b>]</b> crystallizes in the monoclinic polar <i>P</i>2₁ space group, which exhibits a strong SHG response and reversible photochromic behavior. Structural analysis and theory calculations reveal that the synergistic effect between the polar thiazole derivative and the distorted inorganic [BiCl₆]^3- motif is responsible for the SHG response. This is the first report of polar thiazole derivative-induced SHG-active hybrid bismuth halide coupled with reversible photochromism.