Indium-to-Tin Metal Modulation as a Strategy for Tuning Emission from Yellow to Orange in Zero-Dimensional Metal Halides

Abstract

Metal-ion substitution offers an effective strategy for tailoring the optical properties of low-dimensional organic–inorganic hybrid metal halides (OIMHs). Herein, we demonstrate an indium-to-tin metal modulation approach to systematically modulate the broadband emission from yellow to orange in two zero-dimensional (0D) lead-free hybrid metal halides, (TMSO)3InCl6 and (TMSO)2SnCl6, incorporating the sulfur-based trimethylsulfoxonium (TMSO+) as an organic cation. Single-crystal X-ray diffraction (SCXRD) reveals that (TMSO)3InCl6 crystallizes in the triclinic P1̅ space group, whereas (TMSO)2SnCl6 adopts an orthorhombic Pnnm structure, both comprising isolated distorted [MCl6]n– octahedra surrounded by TMSO+ cations. Under ultraviolet excitation, the In-based compound exhibits intense broadband yellow emission centered in the visible region with a full width at half-maximum (FWHM) of ∼142 nm and a high photoluminescence quantum yield (PLQY) of ∼63.9%, originating from a self-trapped exciton (STE) recombination. In contrast, replacing indium with tin shifts the emission to broadband orange with a wider FWHM (∼165 nm) and a significantly lower PLQY (∼2%). The reduced luminescence efficiency of the Sn analogue is attributed to multiple STE states that facilitate nonradiative decay. Further, the density functional theory calculations reveal that the excited-state polyhedral distortion is substantially greater in (TMSO)3InCl6 than in (TMSO)2SnCl6, suggesting greater polyhedral localization, facilitating STE emission in the former compound. Hydrogen-bonding analysis reveals a greater extent of H-bonding in the In compound as compared to the Sn analogue, leading to more polyhedral distortion and facilitating efficient STE emission, corroborating the theoretical study. This work establishes metal-ion variation as an effective strategy for tuning broadband emission and provides valuable insights into the structure–property relationship governing STE luminescence in lead-free hybrid metal halides.