Switching Colors: Multiple Emission in a Manganese(II) Bromide Compound

A novel organic-inorganic hybrid compound, [H3L][MnBr4]∙[H2O] 1, with the protonated 1-hydroxy-4-methyl-2-(pyridin-2-yl)-5-phenylimidazole has been synthesized. Single crystal X-ray diffraction reveals that 1 crystallizes in the noncentrosymmetric space group Pca21, with crystals exhibiting opposite polar orientations. The crystal structure is stabilized by non-covalent interactions, including intermolecular OH⋯O, CH⋯Br, and Br⋯Br contacts and an intramolecular NH⋯O hydrogen bond. Based on QTAIM analyses, the strong OH⋯O contact (40.2 kcal/mol) between the {[H3L]}2+ and {[H2O]}0 moieties suggests a potential pathway for proton transfers, leading to the formation of metastable {[H2L][HBrMnBr3][H2O]} state. Electron paramagnetic resonance spectroscopy shows that the Mn(II) ions have a 6A1(S) ground state with an axial zero-field splitting (|D| = 2250 MHz). Compound 1 exhibits multi-band, excitation wavelength-dependent photoluminescence in the visible region. This emission is attributed to both d–d transitions within the [MnBr4]2– anion and transitions within the organic cation, which are coupled via the non-covalent interactions. The unique structure and tunable dual-emission properties, mediated by proton-transfer processes, suggest the potential application of 1 in smart optical devices.