Thermal quenching (TQ) in luminescent materials tend to be unavoidable, which is caused by the rapid increase of the nonradiative relaxations of excited state electrons with temperature, has greatly limited the practical applications of luminescence. Therefore, the development of effective strategies to mitigate TQ or even enhance luminescence at elevated temperatures has remained an urgent research topic. In recent years, some negative thermal expansion (NTE) materials provide a way to overcome TQ, where the chemical bond contracts along certain crystallographic orientation upon heating. If the NTE behavior could be incorporated into the emission center, TQ may be alleviated or even luminescence would be enhanced at elevated temperatures. Herein, we report three metal−organic frameworks Cu(I) halide MOFs [CuCl(bpy)]n (CuCl-bpy, bpy = 4,4′-bipyridine), [CuBr(bpy)]n (CuBr-bpy), and [CuI(bpy)]n (CuI-bpy) , with obvious anisotropic NTE. We found the strongest NTE leads to a contraction of the Cu...Cu distance in CuCl-bpy, which further intensifies the luminescence emission. This phenomenon indicates the existence of thermally enhanced charge transfer. Moreover, the origin of the distinction in charge transfer attributed to the different valence states of the copper is investigated through the combined studies of X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and density functional theory calculations. Our research provides a new approach to modulating the luminescence thermal enhancement by NTE.

Overcoming thermal quenching with negative thermal expansion molecular materials

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