Halide‐Dependent Photoluminescence and Heavy‐Atom Effects in Low‐Melting Organic–Inorganic Manganese Halides

ABSTRACT Low‐melting ionic solids with stirring luminescent properties hold significant promise for optoelectronic applications. Here, we compare and contrast the structural and spectroscopic correlations of two highly luminescent organic‐inorganic manganese halides (C 4 Py) 2 [MnCl 4 ] and (C 4 Py) 2 [MnBr 4 ], synthesized from their respective manganese halides and N‐butyl pyridinium halide ionic liquids. Although both compounds exhibit very similar bulk structures (determined by single‐crystal and powder X‐ray diffraction) and overall similar electronic structures (as indicated by the density of states), they differ notably in their optical properties. The chloride salt, (C 4 Py) 2 [MnCl 4 ], has a photoluminescence decay lifetime ten times longer than its bromide analogue, (C 4 Py) 2 [MnBr 4 ]. Furthermore, PL‐quantum yield of (C 4 Py) 2 [MnBr 4 ] is 1.6 times higher than that of (C 4 Py) 2 [MnCl 4 ], which was attributed to the heavy atom effect of bromine atoms, based on periodic density functional calculations (with and without spin‐orbit coupling). Although photoluminescence is only exhibited in the solid state, EXAFS analysis confirms that the coordination environment of manganese is remarkably similar in crystalline and molten states, potentially suggesting that photoluminescence is associated with the long‐range crystalline order, which is lost upon melting. Building on these fundamental studies, the potential of (C 4 Py) 2 [MnCl 4 ] as a luminescent security ink for anticounterfeiting applications has been demonstrated.