Halogen Anion-Mediated Substitution of Nitrate: Toward Polar Materials with Intense Second-Harmonic Generation Effects

Organic–inorganic hybrid halides have attracted extensive research efforts due to their promising nonlinear optical (NLO) properties, and enhancing the polarizability of these materials can effectively improve their second-harmonic generation (SHG) efficiency. In this work, three isostructural NLO crystals, (C 5 H 5 NS) 2 MCl 2 (M = Zn, Cd, Hg), were successfully synthesized by an equivalent anion substitution strategy. The crystal structure is constructed by tetrahedral [MCl 2 S 2 ] 4– and π-conjugated (C 5 H 5 NS) units. (C 5 H 5 NS) 2 MCl 2 shows strong SHG intensities of 2.78, 3.96, and 4.60 times those of KH 2 PO 4 (KDP) at 1064 nm and 0.31, 0.34, and 0.50 times those of AgGaS 2 (AGS) at 2100 nm for M = Zn, Cd, and Hg, respectively. Theoretical calculations indicate that the SHG responses in these compounds arise from the synergistic contribution of the organic π-conjugated rings and the distorted metal-centered moieties. This work has enriched the material repertoire of NLO crystals constructed via the integration of pyridine-based groups and metal halides.