Doublet-Triplet Energy Transfer-Dominated Photon Upconversion

Stable luminescent pi-radicals with doublet emission have aroused a growing interest for functional molecular materials. We have demonstrated a neutral pi-radical dye (4-N-carbazoly1-2,6-dichlorophenyl)bis-(2,4,6-trichlorophenyl)-methyl (TTM-1Cz) with remarkable doublet emission, which could be used as triplet sensitizer to initiate the photophysical process of triplet triplet annihilation photon upconversion (TTA-UC). Dexter-like excited doublet-triplet energy transfer (DTET) was confirmed by theoretical calculation. With the same sensitizer, a mixed solution of TTM-1Cz and aromatic emitters could upconvert red light (lambda = 635 nm) to blue or ryan light. An anti-Stokes energy shift as large as 0.92 eV was observed from red to blue light upconversion. This finding of DTET phenomena offers a new kind of triplet sensitizer for TTA-UC.