Hydrogen-Bond-Driven Isomers of Hybrid Manganese Bromides: Rapid Assembly, Structural Analysis, and Luminescence Properties

In organic–inorganic hybrid metal halides (OIHMHs), the organic cations exert a considerable influence on the optoelectronic properties by modulating the spatial geometry of the [MnBr 4 ] 2– tetrahedra. In this work, we designed a novel phosphonium–ammonium (P–N) hybrid organic cation and assembled two solvent-free crystalline supermolecular isomers ( 1 and 2 ). Through the optimization of the experimental setup, the two crystallize phases were rapidly assembled, while simultaneously achieving a solvent-induced conversion of 1 to 2 . Structural analysis indicates that multiple intermolecular hydrogen bonds stabilizing the crystal architecture formed between the hydrogen atoms at the α-C positions from the organic cation’s N and P atom centers and the bromide atoms of the [MnBr 4 ] 2– unit. Subtle differences in the hydrogen-bonding density within the system result in greater distortion of the [MnBr 4 ] 2– unit in 2 compared to 1, as evidenced by escalation of the bond angle variance (σ 2 ) from 8.26 in 1 to 51.52 at the Mn2 site in 2 . Such amplified local deformation leads to a stronger crystal field surrounding the Mn 2+ center, leading to a red-shifted emission spectrum. Temperature-dependent PL studies showed that the Huang–Rhys factor of 2 ( S = 24.35) is substantially higher than that of 1 ( S = 10.00), indicating stronger electron–phonon coupling and a softer lattice, which also accounts for its broader full width at FWHM. Density functional theory (DFT) calculations elucidated the disparities in the electronic structure of the photoluminescence mechanism of two crystalline phases. Furthermore, these results establish a foundation for the design of hybrid organic cations, rapid green hydrogen-bond-driven assembly, and the study of structure–property relationships in polymorphic 0D metal halides.