A simple and broadly applicable synthesis of fluorene-coupled D-σ-A type molecules: towards high-triplet-energy bipolar hosts for efficient blue thermally-activated delayed fluorescence

An effective two-step synthesis of high-triplet-energy (E_T) bipolar hosts that is simple and broadly applicable is introduced. Electron-donating (D = (9-phenyl-9H-carbazol-3-yl/4-(9H-carbazol-9-yl)phenyl)) and -accepting (A = (tetrafluoropyridin-4-yl/4,6-diphenyl-1,3,5-triazin-2-yl)) units have been coupled to the C9 atom of fluorene to afford the four hosts 3CzFTFP, 9PhFTFP, 3CzFDPhTz, and 9PhFDPhTz with sp³ C9-centered bulky ternary structures, excellent thermal/morphological stabilities, tunable abilities for bipolar charge carrier injection/transport, and identical high E_T (2.87 eV). Blue thermally-activated delayed fluorescence (TADF) devices using these new hosts and 2CzPN as the host-guest system exhibited efficient electroluminescence efficiencies of 38 cd A^-1/30 lm W^-1/17%, which are among the best for 2CzPN-based TADF devices. The generality of this synthetic strategy to high-E_T fluorene-coupled D-σ-A type hosts was confirmed by successfully appending other D/A groups ((diphenylamino)phenyl/phenylsulfonyl) to fluorene, affording TPAFBSO.