Design Principles for Enhancing Photoluminescence Quantum Yield in Hybrid Manganese Bromides

Hybrid manganese halides have attracted widespread attention because of their highly emissive optical properties. To understand the underlying structural factors that dictate the photoluminescence quantum yield (PLQY) of these materials, we report five new hybrid manganese bromides with the general formula A_mMnBr₄ [m = 1 or 2, A = dimethyl­ammonium (DMA), 3-methyl­piperidinium (3MP), 3-aminomethyl­piperidinium (3AMP), heptamethyl­enimine (HEP), and trimethylphenyl­ammonium (TMPEA)]. By studying the crystal structures and optical properties of these materials and combining our results with the findings from previously reported analogs, we have found a direct correlation between Mn···Mn distance and the PLQY, where high PLQYs are associated with long Mn···Mn distances. This effect can be viewed as a manifestation of the concentration-quenching effect, except these are in stoichiometric compounds with precise interatomic distances rather than random alloys. To gain better separation of the Mn centers and prevent energy transfer, a bulky singly protonated cation that avoids H-bonding is ideal. We have demonstrated this principle in one of our newly reported material, (TMPEA)₂MnBr₄, where a PLQY of 70.8% for a powder sample and 98% for a large single crystal sample is achieved. Our study reveals a generalized method for improving PLQYs in hybrid manganese bromides and is readily extended to designing all varieties of highly emissive hybrid materials.