Enhanced blue TADF in a D-A-D type naphthyridine derivative with an asymmetric carbazole-donor motif

Efficient triplet-to-singlet conversion in conventional donor-acceptor TADF compounds relies on charge-transfer (CT) and locally-excited (LE) triplet state mixing as this provides the required spin-orbit coupling. In this work, an asymmetric carbazole-donor motif is shown to facilitate the coupling and spin-flip process in a naphthyridine-acceptor based TADF emitter due to the enhanced LE character of the lowest triplet state. This was revealed from the delayed fluorescence measurements, which clearly signified a doubled RISC rate (up to 1.33 x 10(6) s(-1)) and a shortened delayed fluorescence lifetime (down to 4.4 mu s) in an asymmetric compound compared to those of symmetric or singly-carbazole-substituted compounds. The promising features of the asymmetric blue TADF emitter were demonstrated to result in high efficiency OLEDs (external quantum efficiency of 21%) with a reduced efficiency roll-off.