Contracted, Not Converted: Photoluminescence Thermochromism in Ultrabright Pure Blue Emissive Hybrid Copper(I) Halide

Abstract Thermochromic luminescent materials are promising for applications such as sensors or display technologies. Understanding the mechanism of thermochromic luminescence in these materials plays an important role for their targeted applications. Here, a 0D copper(I) halide (TMP) 2 CuBr 3 and 1D (TMP)Ag 2 Br 3 (TMP = tetramethylphosphonium) are reported. While (TMP)Ag 2 Br 3 is a weak light emitter, (TMP) 2 CuBr 3 demonstrates ultrabright pure blue emission with photoluminescence (PL) quantum yield exceeding 90% within the temperature range 300–80 K. The emission in (TMP) 2 CuBr 3 is attributed to the radiative recombination of self‐trapped excitons (STEs) that arise localized on [CuBr 3 ] 2− anions. (TMP) 2 CuBr 3 demonstrates excitation‐selective PL thermochromism at low temperatures. In the literature, the origin of multiple STEs is often unclear in luminescent metal halides. Here, a detailed temperature‐dependent spectroscopic and structural analysis suggests that the PL thermochromism in (TMP) 2 CuBr 3 is associated with the appearance of a second STE state. The appearance of second STE is expected to have a structural origin, and it may be associated with the observed anomaly in thermal contraction near 200 K. The distinct PL features of melt‐processible (TMP) 2 CuBr 3 coupled with its remarkable photo‐ and thermal stability allow its consideration for practical optical applications, including temperature sensing.