Chiral Lead-Oxyiodide Nonlinear Optical Crystals Constructed on <scp>l</scp> -Malate Groups

The pursuit of nonlinear optical (NLO) crystals exhibiting an exceptional second-harmonic generation (SHG) response has been a persistent objective in scientific research. Herein, three chiral lead-oxyiodide NLO crystals, [Rb₂I][PbI(LM)] (LM = L-OOCCH₂CH(OH)COO) and A₂[Pb₂I₂(LM)₂] (A = Rb, K), have been rationally synthesized by introducing the l-malate group into halide perovskites to substitute partial halide anions. All of them contain highly distorted lead-oxyiodide polyhedra with large hyperpolarizability and anisotropic polarization. In particular, the highly distorted [PbI₂O₄] polyhedrons in [Rb₂I][PbI(LM)] are shielded by more Rb⁺ cations and aligned into a uniform arrangement. As a result, their microscopic second-order susceptibilities are optimally superposed. Remarkably, [Rb₂I][PbI(LM)] exhibits a very strong SHG response of 7.3 × KDP that is nearly three times those of A₂[Pb₂I₂(LM)₂] (A = Rb, K), as well as the largest among malates to date. Theoretical calculations demonstrate that their SHG responses mainly arise from highly distorted lead-oxyiodide polyhedrons. This study offers a viable strategy for the exploration of high-performance NLO crystals.