Three donor-acceptor-type thermally activated delayed fluorescence (TADF) emitters (PXZBAO (1), PXZBTO (2), and PXZBPO (3)) comprising a phenoxazine (PXZ) donor and differently π-expanded boron-carbonyl (BCO) hybrid acceptor units are proposed. The emitters exhibit red (1) to orange (3) emissions with an increase in the π-expansion in the BCO acceptors. The control of the strength of local aromaticity for the BCO unit and the corresponding LUMO level is attributed to inducing the unusual emission color shifts. The photoluminescence quantum yield and delayed fluorescence lifetime of the emitters are also adjusted by the π-expansion. Notably, although 1 possesses a 3nπ* state in the acceptor unit as a local triplet excited state (3LE, T2), the T2 states of 2 and 3 mainly comprise a 3ππ* state in the acceptor. Consequently, all of the emitters exhibit strong spin-orbit coupling between their T2 and excited singlet (S1) states, leading to a fast reverse intersystem crossing with rate constants of ∼106 s-1. By employing the emitters as dopants, we realize efficient red-to-orange TADF-OLEDs. Maximum external quantum efficiencies of 17.7% for the yellowish-orange (3), 15.5% for the orange (2), and 13.9% for the red (1) devices are achieved, and the values are very close to the theoretical limit predicted from the optical simulation.
Keywords: OLEDs; TADF; boron-carbonyl acceptor; fast spin-flip; π-expansion.