ns ² Electron Engineering in Zero-Dimensional Metal Halides for Modulating Emission Behavior

The photophysical behavior of ns2 metal-based zero-dimensional (0D) halides, particularly their broad emission driven by self-trapped excitons (STEs), makes them unique and promising for light-emitting technologies. The stereochemical activity of the ns2 lone pair plays a decisive role in dictating the structural and photophysical properties of such metal halides. However, a systematic and generalized framework correlating the factors associated with ns2 electron engineering, e.g., metal identity, local coordination geometry, electronic energy level of the organic cation, and dynamical off-centering in tuning the emission characteristics, remains limited. In this work, using state-of-the-art density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations on ns2 metal (Pb2+, Sn2+, and Sb3+) bromides incorporating Cs+, aliphatic, and aromatic organic cations having octahedral, disphenoidal, and square-pyramidal coordination environments, we have studied the ground and excited-state behavior and framed a generalized structure–emission characteristic correlation. Our results demonstrate that, in higher-coordination environments, ns2 lone pair exposure primarily determines the emission behavior, with Sn2+ exhibiting more stable STE characteristics and Pb2+ remaining largely inactive. However, in lower-coordination environments, the photophysical response appears as an interplay between ns2 lone pair exposure and its coordination geometry; Sn2+, having disphenoidal coordination, displays a prominent emission characteristic compared to that of Pb2+ disphenoidal and Sb3+ square-pyramidal geometries. We find that the STE responses are largely hole-driven with a lesser role for electrons. Furthermore, our study reveals that, while A-site cation substitution has a minimal effect on ground-state hybridization, it profoundly alters excited-state behavior, where aromatic cations promote charge separation and non-STE-like excitons, whereas aliphatic cations favor STE formation. AIMD calculations further reveal that higher-coordination systems show lone pair activity through dynamic off-centering, whereas the lone pair of lower-coordination systems is stereochemically inactive to dynamic off-centering due to deviation from the optimal spatial availability of the lone pairs. Therefore, our results demonstrate that, despite the ns2 lone pair’s population at the valence band edge, ground-state treatment is insufficient to fully capture the stereochemical nature; instead, it is dictated by the excited state and dynamical treatment. These insights establish a robust atomistic framework linking the stereochemical activity, coordination geometry, and exciton localization of the lone pair, thus providing atomistic interpretation of experimentally observed trends and offering a fundamental and generalized perspective for analyzing the emission behavior and thereby providing design guidelines for engineering efficient 0D metal halide emitters.