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, (C₅H₅NS)₂MCl₂ (M = Zn, Cd, Hg), were successfully synthesized by an equivalent anion substitution strategy. The crystal structure is constructed by tetrahedral [MCl₂S₂]^4- and π-conjugated (C₅H₅NS) units. (C₅H₅NS)₂MCl₂ shows strong SHG intensities of 2.78, 3.96, and 4.60 times those of KH₂PO₄ (KDP) at 1064 nm and 0.31, 0.34, and 0.50 times those of AgGaS₂ (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.