An Mn²⁺-based organic-inorganic hybrid with spatially separated charge transfer for efficient green phosphorescence

A novel manganese(II)-based organic-inorganic hybrid material, bis[(cyanomethyl)triphenylphosphonium] tetrachloridomanganate(II), (C₂₀H₁₇NP)₂[MnCl₄] or [CTP]₂[MnCl₄], has been synthesized and structurally characterized. The compound crystallizes in the monoclinic space group P2₁/n, with its three-dimensional packing stabilized by a network of weak intermolecular forces, primarily C-H...Cl hydrogen bonds and van der Waals interactions. Under UV excitation, the material exhibits an intense green photoluminescence centred at 520 nm, characteristic of the spin-forbidden ⁴T₁(G)→⁶A₁(S) transition of the tetrahedrally coordinated Mn²⁺ ion. Transient decay analysis confirms its phosphorescent nature, revealing a remarkably long lifetime of 2.92 ms. Periodic density functional theory (DFT) calculations reveal a direct band gap of 1.94 eV and, critically, a spatially separated electronic structure where the highest occupied crystalline orbital is localized exclusively on the inorganic [MnCl₄]^2- anion and the lowest unoccupied orbital resides solely on the organic [CTP]⁺ cation. This clear spatial separation defines the primary photoexcitation as an inter-ionic charge transfer from the anion to the cation, which effectively suppresses nonradiative recombination pathways and is identified as the key factor enabling the observed long-lived phosphorescence. This study not only reports a new efficient green phosphor based on an earth-abundant metal, but also demonstrates that engineering a spatially segregated charge-transfer state is a viable strategy for achieving persistent luminescence in hybrid materials.