Record Second-Harmonic Generation and Exceptional Optical Anisotropy in High-Symmetry 1D Polar Perovskites

The compositional tunability of hybrid organic-inorganic perovskites (HOIPs) enables access to diverse semiconducting materials with distinct structural features. However, tuning the perovskite structures in a rational way to target physical properties for specific applications remains a critical challenge, especially in systems with high crystallographic symmetry. We report herein a new class of polar HOIPs, (R-/S-HEP)BI3 (R-/S-HEP = R-/S-2-(1-hydroxyethyl)pyridinium, B = Ge, Sn, Pb), which feature one-dimensional (1D) octahedral metal-halide chains and crystallize in a hexagonal lattice with high uniaxial symmetry. The stereochemical activity of the group IV cation lone pairs significantly influences the structural polarity, resulting in a dramatic ca. 30-fold increase in powder second-harmonic generation (SHG) response for (R-/S-HEP)GeI3 (15 × KH2PO4) compared to their Sn and Pb analogs (0.45-0.7 × KH2PO4), and a new record in the HOIP family. This family also exhibits exceptional optical anisotropy with birefringence of 0.518-0.595 at 546 nm, the largest values among perovskites with high uniaxial symmetry. First-principles calculations and crystal structure analyses attribute the SHG enhancement in (R-/S-HEP)GeI3 to the highly polarizable [GeI6] octahedra and their favorable alignment with π-conjugated HEP cations, while the large birefringence is primarily cation-driven. The profound understanding developed herein of the influence of π-conjugated cations and lone-pair cations will point the way to the rational design of 1D perovskites with optimized physical properties for optoelectronic applications.