Ultrabright scintillators via triplet exciton recycling in organolanthanide systems
Current high-performance inorganic scintillators, including ceramics and perovskites, suffer from complex manufacturing, environmental toxicity, self-absorption, and stability issues, while organic alternatives exhibit weak X-ray absorption and inefficient triplet exciton utilization. Lanthanide ions offer high energy conversion efficiency and tunable luminescence, yet organolanthanide molecules remain underexplored due to limited understanding of excitonic dynamics. This work introduces a molecular design strategy achieving ultrabright scintillation through lanthanide-assisted near-unity triplet exciton recycling. By engineering organic ligands to reclaim energy lost during secondary X-ray relaxation, radioluminescence (RL) enhancements exceeding three orders of magnitude are demonstrated relative to existing organic and commercial inorganic scintillators. Precise alignment of ligand triplet energy levels with emissive states of Eu3+ and Tb3+ enables near-unity energy transfer efficiency (>99%) via resonance energy transfer. Europium complexes with aromatic diketones, particularly 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (NTA), exhibit superior RL intensity due to a tailored triplet energy of 19,600 cm−1, slightly exceeding the 5D0 level of Eu3+ (17,200 cm−1), suppressing back-transfer and yielding 100-fold RL enhancement over EuCl3. Coordination saturation with DPEPO and PHEN further optimizes exciton confinement, achieving a 1000-fold RL enhancement for Eu(NTA)3DPEPO. Mechanistic studies reveal that f-f transition systems achieve >99% triplet exciton recycling via Dexter-mediated energy transfer, whereas d-f systems (Ce3+, Eu2+) rely on direct 5d-4f excitation with high PLQY (~100%) but limited X-ray sensitivity due to narrow ligand absorption (<250 nm) and ultrafast decay (50 ns). These findings establish triplet exciton harvesting, not PLQY, as the key determinant of scintillation performance.