  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-carbazolyl-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. A mixed solution of TTM-1Cz
and aromatic emitters could ambidextrous upconvert red light to cyan light or
green light = 532 nm) to blue light. This finding of DTET phenomena provides a
new perspective in designing new triplet sensitizer for TTA-UC.
