Near-Full-Spectrum Emission Control in Copper(I) Iodides via Inorganic Structural Engineering Within a Single-Cation Host

Abstract

Hybrid copper(I) halides have emerged as a new class of optoelectronic materials due to their tunable structure and photophysical properties. However, systematically correlating inorganic polyhedra configurations with emission characteristics remains challenging. Herein, we address this by synthesizing a homologous series of copper(I) iodides templated solely by the [C₁₃H₂₄N]⁺ cation. Precise control reaction conditions yielded distinct inorganic polyhedral configurations, monomeric [CuI₃]²⁻ (1), dimeric [Cu₂I₄]²⁻ (2), trimeric [Cu₃I₆]³⁻ (3), and tetrameric [Cu₄I₆]²⁻ (4). We establish a direct correlation where increasing inorganic aggregation systematically reduces the bandgap and dictates the luminescence color across a near-full visible spectrum, from blue (1) to cyan (2), red (3), and yellow (4). Detailed spectroscopic and theoretical analyses reveal the self-trapped excitons emission mechanism dependent on the Cu-I configuration, in which the closed [Cu₄I₆]²⁻ configuration is more resistant to excited lattice deformation, thereby resulting in a lowest Stokes shift energy. Furthermore, stimuli-responsive sequential phase transitions between these well-defined structures were demonstrated, offering insights into their structural dynamics. This work provides critical fundamental understanding of how inorganic framework engineering within a fixed organic host precisely controls both electronic structure and excited-state relaxation pathways in hybrid copper(I) halides, paving the way for rational design of materials with tailored optical properties.