Chirality-driven dimensionality and broadband emission in lead bromide perovskites

Abstract

Incorporating chiral organic cations into hybrid organic–inorganic perovskites (HOIPs) offers a unique strategy for tailoring structural dimensionality and modulating photophysical properties. Here, we report that lead bromide perovskites synthesized with chiral ligands R- and S-α-methylbenzylammonium (MBA) lead to the formation of two-dimensional (2D) Ruddlesden–Popper structures, while the racemic mixture induces the formation of a hydrated 1D phase, (Rac-MBA)₃PbBr₅·H₂O. DFT calculations show that this transition is driven by electrostatic strain from opposing chiral distortions, which favor octahedral connections with compensated dipoles. Water molecules occupy sites between inorganic chains and mediate hydrogen bonds, stabilizing the 1D motif and relieving strain from racemic reordering. These waters form symmetry-related interstitial networks that mirror the chiral organization, acting as electrostatic compensators in the absence of lateral Pb–Br bonding. Theoretical and experimental characterization studies show that the hydrated structure is energetically stable only when four symmetry-related 1D chains and water are present; otherwise, it becomes metallic or unstable. Electronic calculations reveal Rashba-type spin splitting in chiral phases, and the formation of Br-derived mid-gap states and a Fermi level near the conduction band in the racemic compound. This hydrated phase exhibits pronounced broadband photoluminescence attributed to self-trapped excitons (STEs), and temperature-dependent optical and structural studies reveal that exciton localization and emission dynamics are closely related to octahedral distortions and hydration effects. Our findings demonstrate that stereochemical engineering can offer a robust strategy to modulate dimensionality and optical functionality, with implications for broadband light-emitting applications.