Thermally‐Induced Reversible Phase Transition and Luminescence Switching in 0D Hybrid Copper Halide Isomers for Anti‐Counterfeiting and Thermo‐Responsive Scintillators

Abstract Thermochromic luminescent materials with reversible phase transition and photoluminescence (PL) switching hold promise for stimulus‐response systems and optical encryption, but usually suffer from low photoluminescence quantum yield (PLQY), limited switching reversibility, and severe thermal quenching. Here, a novel pair of 0D hybrid copper halide isomers, α‐ and β‐(ETPA) 2 Cu 2 I 4 (ETPA = ethyltripropylammonium) is reported, which exhibit a reversible structural phase transition and PL color switching upon heating or cooling treatment. At 298 K, α‐(ETPA) 2 Cu 2 I 4 displays a single band blue emission with a near‐unity PLQY. Upon heating treatment, an additional thermally‐activated low‐energy emission band appears ≈650 nm, resulting in a dual‐band yellow emission. Conversely, this yellow emission can gradually revert to the original blue emission during a cooling process. The dynamic interconversion process between these two isomers is monitored using in situ optical and structural measurements, revealing that there are two self‐trapped emitting states in [Cu 2 I 4 ] 2− dimers, whose populations are strongly dependent on the surrounding molecular environment and the thermal activation energy. Moreover, the robust structural and optical reversibility over 45 cycles is demonstrated. Notably, the reversible PL switching in 0D hybrid copper halide isomers makes them promising candidates for applications in anti‐counterfeiting and thermo‐responsive scintillators.