Structural Origins of Forbidden Emission in Two-Dimensional Lead Halide Perovskites

Abstract In bulk semiconductors, luminescence is treated under the electric dipole approximation, where magnetic dipole (MD) and higher-order multipolar transitions are assumed negligible under parity selection rules. However, two-dimensional (2D) metal-halide perovskites frequently exhibit anomalous low-energy emission signatures arising from out-of-plane oriented MD transitions. As these transitions are nominally disallowed due to parity selection rules, we aim to identify the symmetry-breaking mechanisms that activate these forbidden emissive pathways. Here, we demonstrate that anomalous MD emission in 2D HOIPs is correlated with the deviation of interoctahedral bond angles from 180°. Beyond establishing a fundamental structure–property link, this finding equips researchers with a synthetic design rule for harvesting multipolar emission from 2D perovskites for next-generation optoelectronic devices. Our findings also reveal that modular metal cation substitution─specifically replacing Pb with Sn─provides a robust mechanism for tuning forbidden emission intensity through lone-pair-driven dynamic structural distortions.