Pushing the Understanding of ns2 Lone-Pair Exposure in Zero-Dimensional Metal Halides: Molecular Dynamics and Machine...

Combining density functional theory, ab initio molecular dynamics, machine learning, and excited-state analysis, we uncover structural factors governing ns2 lone-pair expression and emission across diverse inorganic and hybrid 0D metal halides, addressing an unresolved unified understanding of STE emission in high-symmetry systems, the stronger stereochemical activity of Sn than Pb, and halide-dependent energy-level alignment. We show that emission is controlled by valence-edge s-p mixing driven by lone-pair activation, with its extent governed by two distinct coordination-dependent structural modes. In highly coordinated octahedral systems, lone-pair exposure emerges through angular fluctuation-mediated transient off-centering, explaining finite luminescence in apparently centrosymmetric compounds. In lower-coordinate systems, intrinsic cage asymmetry produces persistent lone-pair exposure, often stabilized by organic cations, leading to stronger STE stabilization and enhanced emission. The higher stereochemical activity of Sn relative to Pb is rooted in greater dynamical structural degrees of freedom, opening more channels to activate the 5s2 lone pair. This coordination- and valence-shell-dependent effect is amplified in softer bromides with favorable energy-level alignment with ns2 cations. Overall, our results redefine the understanding of lone-pair stereochemistry, concluding that lone-pair stereochemistry in 0D metal halides is largely dynamical, activated by selective structural descriptors that govern excited-state localization and emission, providing a general design framework for efficient emitters.