Ligand substitution in organic–inorganic hybrid zinc iodides for second-harmonic generation

Noncentrosymmetric metal halides are promising nonlinear optical (NLO) materials that have prospective applications in optical switching technologies. However, obtaining zinc halides with a large second harmonic generation (SHG) effect is of great challenge due to their ideal tetrahedral configurations. In this study, four organic-inorganic hybrid zinc iodides, namely, (HIm)2[ZnI4]·H2O (1), (HIm)[ZnI3(Im)] (2), ZnI2(Im)2 (3), and [ZnI(Im)3]I (4) where Im = imidazole, have been synthesized by a ligand substitution approach. Structural analysis shows that compound 2 is noncentrosymmetric, while the remaining three compounds are centrosymmetric. Notably, compound 2 exhibits a strong SHG response 4.0 times that of KH2PO4 under 1064 nm laser irradiation. Theoretical studies indicate that the SHG activity of compound 2 is attributed to the synergistic effect of the distorted zinc-centered tetrahedra and π-conjugated organic components. This research sheds new light on the development of zinc halide NLO materials through ligand engineering.