Structural manipulation and triboluminescence of tetrahalomanganese(II) complexes

Triboluminescence is a fascinating luminescence phenomenon induced by mechanical stimuli. Triboluminescent materials have potential applications in lighting, displays, and sensing, owing to their distinctive modes of light generation. However, organic triboluminescent materials are severely limited, and their luminescence mechanism remains unclear. Herein, we found that the luminescent manganese(II) complex [BPP]2[MnBr4] displayed interesting triboluminescence performance. A series of green emissive tetrahalomanganese(II) complexes was rationally designed and synthesized. The associated single crystal structures revealed that all complexes consisted of one [MnX₄]₂− (X = Br or Cl) ion and two organic cationic ligands per unit cell, with a tetrahedral geometrical symmetry around the Mn(II) ion. In addition, the photophysical properties of tetrahalomanganese(II) complexes were easily tuned by varying the organic ligands or halogen ions, which is beneficial for these organic-inorganic hybrid structures. Under UV light irradiation, all tetrahalomanganese(II) complexes in the solid state exhibited bright green luminescence and a broad featureless emission band at 450–650 nm. The time-resolved photoluminescent decay curves demonstrated that the emission lifetimes of the prepared tetrahalomanganese(II) complexes ranged from 260.5 μs to 1.95 ms, which was attributed to phosphorescence. The long-lived emission was mainly due to the spin-forbidden nature of the metal center d–d (⁴T₁(G) →⁶A₁) radiative transition. Thermogravimetric analysis was performed to examine the thermodynamic stabilities of the tetrahalomanganese(II) complexes. The thermal stabilities of manganese(II) complexes with P-based ligands were higher than those of the complexes containing N-based ligands. Upon applying a force to the crystals, the tetrahalomanganese(II) complexes all exhibited prominent triboluminescence that could be observed by the naked eye in the dark. Systematic analysis of the crystals showed that the TL activities of the manganese(II) complexes were related to the intra- and inter-molecular C-H·X (X = Br or Cl) interactions. The intra- and inter-molecular C-H·X interactions significantly reduced the possible energy loss caused by molecular vibrations and rotations in the [MnX₄]²⁻ unit under mechanical stress, improving TL emission. Moreover, a comparison of photoluminescence and triboluminescence indicated that different excitation sources yielded two distinct luminescence processes: Transition of excitons excited by illumination and recombination of electrons and holes on the surface driven by polarization charges. Overall, the results presented herein new opportunities for fundamental research based on the developed class of triboluminescent materials.