Site-specific bromination on an organic cation scaffold: a minimal-variable platform reveals structure-luminescence relationships in manganese halide hybrids.

Organic-inorganic hybrid manganese halides (OIMnHs) are promising stimuli-responsive materials due to their structural tunability and rich photophysical properties. However, current research relies largely on empirical screening of numerous organic cations, and a rational design model that decouples the multiple structural variables affecting luminescence remains scarce. Here, a site-specific chemical modification strategy on a 2-amino-3,5-dimethylpyridinium scaffold [A]2[MnCl4] is proposed to systematically decouple the effects of the anionic halide site (X-site) and the organic cation site (A-site). Through two orthogonal bromination pathways: substituting Cl- with Br- at the X-site and -CH3 with -Br at the functional group of the A-site, six Mn-based hybrid crystals are synthesized, which form a tightly interlinked comparative platform with minimal structural perturbation. Although A-site Br substitution reduces the PLQY from 32.4% (35DM-C) to approximately 5% (3BG and 5BG), it simultaneously imparts distinctive humidity-responsive behaviors. At 75% relative humidity, 3BG and 5BG undergo humidity-induced structural transformations into 3BY and 5BR, respectively, boosting PLQY dramatically (∼11-fold) to 55.0% and 52.9%. Taking advantage of the humidity response and broad spectral coverage of these materials, their potential for information encryption and high-color-rendering white-light illumination is further demonstrated, achieving a color-rendering index of 91.6. Systematic experiments and theoretical calculations reveal that local chemical modification of the organic cation regulates excited-state dynamics, enhancing radiative transitions and suppressing nonradiative pathways by tuning weak-interaction networks, which in turn governs the humidity-induced structural transformation and luminescence switching. This stepwise, interlocking research framework enables clear structure-property correlations.