Spin-Phonon Engineering in Tetrahedral Mn ²⁺ Single Crystals: Toward X-ray Scintillators with Ultrahigh Environmental Stability

Single-crystalline (HPPh₃)₂MnCl₄ was synthesized via antisolvent vapor diffusion. The compound crystallizes in the monoclinic P21/n space group and features a distorted tetrahedral [MnCl₄P] core with Mn-Cl bond lengths of 2.3 Å and Cl-Mn-Cl bond angles ranging from 104-116°. Narrowband green emission (516 nm, full width at half-maximum (fwhm) = 47 nm, photoluminescence quantum yield (PLQY) = 49%) is achieved due to suppressed Jahn-Teller distortion, which results from the structural rigidity imparted by interligand π-π stacking and C-H···Cl hydrogen bonds. Spectroscopic and theoretical analyses confirm that the luminescence is dominated by the crystal field (10Dq = 8723 cm^-1). Furthermore, spin-phonon engineering drives an anomalous thermal response, characterized by a thermally activated delayed fluorescence (TADF)-mediated intensity enhancement between -50 and 75 °C (⁴T₁-²E gap <1500 cm^{- 1}) and Jahn-Teller quenching with an activation energy of 0.36 eV. For radiation detection, the crystal achieves record light yield (47950 photons/MeV) and ultralow detection limit (0.25 μGy·s^-1). A 150 μm flexible scintillator screen demonstrates real-time X-ray imagingat clinical dose rates (1 μGy·s^-1). This work establishes supramolecularly engineered Mn²⁺ phosphors as the next-generation scintillators.