Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes

A material possessing a very small energy gap between its singlet and triplet excited states, Δ E 1 − 3 , which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported. The compound, 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine, breaks the restriction of a large energy gap, with a Δ E 1 − 3 of just 0.11 eV, while maintaining a high fluorescent radiative decay rate ( k r ∼ 10 7 ) . The intense TADF provides a pathway for highly efficient electroluminescence.