Halogen‐Driven Trade‐Off Between Humidity and Thermal Stability in Zero‐Dimensional Hybrid Manganese Halides for Dual‐Stimulus Encryption

ABSTRACT Zero‐dimensional (0D) hybrid manganese halides are attractive lead‐free luminescent materials, yet the simultaneous demands for moisture and thermal stability pose a fundamental challenge. Here we demonstrate a halogen‐driven trade‐off between humidity and thermal stability in (BrPhTMN) 2 MnX 4 (X = Cl, Br, I) with a non‐hydrogen‐bonding cation. Replacing Cl with I dramatically increases the photoluminescence quantum yield (from 20.2% to 84.3%) but drastically reduces humidity resistance at 90% relative humidity (RH) (from ∼51 h for the bromide to <14 min for the iodide). Remarkably, the thermal stability trend is reversed: the iodide shows enhanced emission at 430 K, whereas the chloride is heavily quenched. This paradoxical thermal robustness of the iodide, despite its large electron–phonon coupling (Huang–Rhys factor S = 4.43), can be rationalized by the energy‐gap law for multiphonon relaxation. Raman spectroscopy reveals an exceptionally low Mn–I phonon frequency (111 cm −1 ), which exponentially suppresses multiphonon non‐radiative decay, effectively closing the non‐radiative channel even under strong coupling. Exploiting these orthogonal stabilities, we construct a dual‐stimulus encryption device responsive to specific humidity‐heat sequences. This work establishes a design principle for anti‐thermal‐quenching luminescent materials and opens avenues for advanced stimulus‐responsive systems.