Thermally Induced Reversible Fluorochromism by Self-Trapped Excitonic Emission in a Two-Dimensional Hybrid Copper(I)-Halide Single Crystal

Abstract

Organic–inorganic metal halides (OIMHs) have gained significant attention as promising materials for various applications, including lighting, imaging, and energy conversion. The development of Pb-free alternatives to traditional Pb-based materials has become increasingly important for environmental and health reasons. In this study, we report on the thermally induced fluorochromism of a two-dimensional OIMH based on Cu(I), namely, (Bz)₂Cu₂I₄·H₂O (abbreviated as BzCuI). Density functional theory calculations revealed that BzCuI has a direct bandgap of 2.11 eV. Sequential fluorescence spectral shifts were observed in the temperature range of 80 to 300 K, indicating a reduction in the bandgap due to increased electron–phonon interactions at higher temperatures. The Huang–Rhys factor further confirmed the strong coupling between electrons and phonons in BzCuI. Additionally, BzCuI exhibited a unique fluorescence-switching behavior, transitioning from blue to red, which was triggered by a structural phase change involving the trapping and release of water molecules. This finding was supported by the temperature-dependent X-ray diffraction (XRD) pattern, which showed evidence of crystal lattice contraction upon heating. Furthermore, when mixed with silicon oil, BzCuI demonstrated the potential for applications such as anticounterfeiting ink and moisture-sensitivity assays. Compared to other OIMHs, BzCuI exhibited the most significant fluorescence shift within the visual spectrum, making it highly promising for various optical sensing applications.