Efficient TADF from carbon−carbon bonded donor−acceptor molecules based on boron-carbonyl hybrid acceptor

We present a new type of thermally activated delayed fluorescence (TADF) emitters, namely, PCzBCO and BuPCzBCO, composed of the donor (D) and acceptor (A) units linked by a C–C bond. The two emitters have N-aryl carbazole as a donor and a planarized boron-carbonyl (BCO) unit as an acceptor. The crystal structure of BuPCzBCO reveals a moderate twist between the D and A units with a dihedral angle of 49.2°. Although both emitters exhibit the prompt fluorescence only in solution, they exhibit distinct TADF characteristics in the rigid state, with the high photoluminescence quantum yields of 81% and 93% in an mCBP host, respectively. Importantly, fast reverse intersystem crossing (RISC) with the rate constants of ∼106 s−1 is obtained in their host films. Along with the small singlet−triplet energy gap below 50 meV, theoretical studies demonstrate that RISC is facilitated by the strong spin-orbit coupling between the charge-transfer singlet excited state (1CT, S1) and the BCO-centered, local triplet excited state (3LE, T2) with a 3nπ* character. With the PCzBCO and BuPCzBCO molecules as emitters, efficient greenish blue TADF-OLEDs with similar maximum external quantum efficiency of 16.2% are realized. These findings highlight the potential of C–C bonded D−A molecules as efficient TADF emitters.