Zero-Dimensional (Piperidinium) 2 MnBr 4 : Ring Puckering-Induced Isostructural Transition and Strong Electron-Phonon Coupling-Mediated Self-Trapped Exciton Emission.

We report on the synthesis, structure, and photophysical properties of a lead-free organic-inorganic hybrid halide, (Piperidinium) 2 MnBr 4 (PipMBr). It crystallizes in a monoclinic P 2 1 / n structure, with isolated MnBr 4 tetrahedra representing a zero-dimensional compound. It undergoes a reversible isostructural transition at 422/417 K in the heating/cooling cycle owing to the hydrogen-bonding rearrangement mediated by ring puckering of piperidinium cations. This compound exhibits green emission with a photoluminescence quantum yield of 51%. Interestingly, strong electron-longitudinal optical phonon coupling with γ LO of 237 meV is evidenced from the broadening of the temperature-dependent emission linewidth and the Raman spectrum. Such strong electron-phonon coupling and a relatively low Debye temperature (137 K) suggest the self-trapped exciton emission in this compound.