Zero-Dimensional (Piperidinium)2MnBr4: 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)₂MnBr₄ (PipMBr). It crystallizes in a monoclinic P2₁/n structure, with isolated MnBr₄ 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.