Luminescence Switching Red‐to‐Green in Zero‐Dimensional Manganese Halides via Dehydration‐Driven Structural Phase Transition for Anti‐Counterfeiting

Stimuli‐responsive luminescent materials hold significant promise for anti‐counterfeiting applications; however, most existing systems are limited by poor color discriminability. Herein, we report an irreversible dehydration‐driven phase transition in C 4 H 12 N 2 MnBr 4 (H 2 O) 2 upon thermal treatment, transforming into C 4 H 12 N 2 MnBr 4 with remarkably enhanced photoluminescence (PL) characteristics. This structural transition converts the Mn 2+ coordination environment from octahedral to tetrahedral, accompanied by the transformation of the inorganic units from [MnBr 4 (H 2 O) 2 ] 2– to [MnBr 4 ] 2– . The phase transition triggers a striking luminescence switching from red‐light (650 nm) to green‐light (520 nm), originating from the d–d transition ( 4 T 1 → 6 A 1 ) of Mn 2+ ions. Notably, the phase transition affords a remarkable enhancement of photoluminescence quantum yield (PLQY) from 19.7% to 80%, along with a narrowing of emission bandwidth from 109 to 40 nm. These changes are attributed to the modulated crystal field strength, suppressed non‐radiative relaxation, and reduced electron‐phonon coupling. Leveraging this high‐contrast stimuli‐responsive behavior, we demonstrate its application in information encryption and anti‐counterfeiting. This work expands the family of thermal‐responsive manganese halides and offers a coordination engineering strategy for designing tunable emitters in low‐dimensional metal halides.