Pressure Elucidates Self-Trapped Exciton Emissions and Piezochromism in Hybrid Copper Halide

Abstract

The remarkable piezochromism and quest to elucidate the emission origin of structurally diverse low-dimensional hybrid copper halides represent a central challenge and urgently need to be addressed. To resolve this, we introduce high-pressure engineering to study the emission mechanism in 0D hybrid (TBA)₂Cu₄Br₆. Under pressure, the material exhibits a pronounced piezochromic response: the emission evolves from a single peak into a doublet, and both peaks undergo blue shifts. Concurrently, a dynamic competition between the two emissive states occurs, accompanied by a switch of the dominant channel. The concomitant reduction in Cu–Cu distances during the blue shift rules out the triplet cluster-centered (³CC) as the origin of ambient-pressure emission. Instead, it confirms the predominance of self-trapped exciton (STE) emission. Furthermore, the pristine sample exhibits a high linear sensitivity of 56.7 nm/GPa below 1 GPa and its photoluminescence quantum yield increases by 2.5-fold during compression. Following an initial irreversible phase transition caused by the first compression-decompression cycle, the sample reveals a broader and reversible piezochromic range of up to 192 nm in subsequent cycles. This study establishes high pressure as an efficient tool to elucidate emission mechanisms, providing new insights for designing promising piezochromic materials for pressure sensing through STE manipulation.