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

Single-crystalline (HPPh 3 ) 2 MnCl 4 was synthesized via antisolvent vapor diffusion. The compound crystallizes in the monoclinic P21/n space group and features a distorted tetrahedral [MnCl 4 P] 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 ( 4 T 1 – 2 E 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 Mn 2+ phosphors as the next-generation scintillators.