Reversible Phase Transformation of Thermal Quenching and Anti‐Thermal Quenching in Cu(I) Halides for Anti‐Counterfeiting

Abstract Zero‐dimensional (0D) organic‐inorganic hybrid metal halides have attracted intensive attention due to the excellent structure‐dependent photophysical property. Nevertheless, the controllable structural transformations remains enormous challenge. Additionally, the common phenomenon of thermal quenching (TQ) effect exists in luminescent materials further hindering their applications for optoelectronic field. Herein, two new 0D Cu(I)‐based halides of [BTPP] ₂ Cu ₂ Br ₄ and [BTPP] ₂ Cu ₄ Br ₆ (BTPP = Benzyltriphenylphosphonium cations) exhibit stimuli‐responsive reversible phase transformations under ethanol or [BTPP]Br treatment is reported. [BTPP] ₂ Cu ₂ Br ₄ shows weak green light emission with a low photoluminescence quantum yield (PLQY) of 3.47%, while [BTPP] ₂ Cu ₄ Br ₆ exhibits strong orange light emission with near unity PLQY, which all stemming from the self‐trapped exciton (STE). Intriguingly, [BTPP] ₂ Cu ₂ Br ₄ displays normal TQ phenomenon while [BTPP] ₂ Cu ₄ Br ₆ features anti‐TQ effect in a large temperature range from 80 K to 400 K. The detailed experiments demonstrate the anti‐TQ effect is attribute to the detrapping of excitons from shallow surface defect states to STE emission energy. The controllable reversible phase transformation without requiring additional complicated treatment allowing two halides applied as high level of anti‐counterfeiting under room temperature. This work provides a new view of phase transformation with TQ and anti‐TQ effect in 0D hybrid halides.