Green and Red Photoluminescent Manganese Bromides with Aminomethylpyridine Isomers

Two positional isomers, 4-amino-3-methylpyridine and 3-amino-5-methylpyridine, produce 4-amino-3-methylpyridinium and 5-methylpyridin-3-aminium, respectively, under acidic conditions. The two protonated isomers create different hydrogen bonding networks, resulting in different coordination environments of the [MnX₄]^2- unit embedded in molecular compounds such as 4-amino-3-methylpyridinium manganese bromide, [(C₆H₉N₂)₂MnBr₄] and 5-methylpyridin-3-aminium manganese bromide, [(C₆H₉N₂)₄MnBr₄(H₂O)·(MnBr₄)]. Both compounds can be prepared using the slow evaporation method or mechanochemical synthetic procedures. Single-crystal structure analysis of [(C₆H₉N₂)₂MnBr₄] and [(C₆H₉N₂)₄MnBr₄(H₂O)·(MnBr₄)] revealed different manganese halide units, including tetrahedral and tetrahedral with distorted trigonal bipyramidal structures, which emit photoluminescence in the green (527 nm) and red (607 nm) regions, respectively. Electronic structure calculations were conducted to support the validity and interpretation of the UV-vis and photoluminescence (PL) spectral data. Thin films deposited using the [(C₆H₉N₂)₂MnBr₄] precursor also exhibit PL properties. The diverse pseudo-three-dimensional networks can be constructed by using positional isomers with different hydrogen bonding pathways and π-π stacking of organic units, in which the design strategy successfully enables the tuning of various optical properties.