Using fullerene fragments as acceptors to construct thermally activated delayed fluorescence emitters for high-efficiency organic light-emitting diodes

Fragments of fullerene are used as acceptor segments for the first time to construct two thermally activated delayed fluorescence (TADF) compounds DBCP and FAP. Both compounds exhibit similar highly twisted geometries and separated frontier molecular orbital distributions. While on the other hand, their locally exited triplet energy levels from acceptor moieties ((LEA)-L-3) are fine-tunable owning to the different structural stress in the fragments, resulting in different lowest triplet excited state (T-1) features. In DBCP, the (LEA)-L-3 is the high lying state and thus DBCP exhibits a small singlet-triplet energy difference of 0.10 eV and a high photoluminescence quantum yield (phi(PL)) of 89% with excellent TADF characteristics; while T1 of FAP is (LEA)-L-3, and thus, FAP displays poor triplet exciton utilization with phi(PL) of only 54%. In organic light-emitting diodes (OLEDs) based on DBCP and FAP as dopants, maximum external quantum efficiencies of 20.2% and 12.8% are respectively achieved. This work not only demonstrates for the first time that fullerene fragments can be used as key building blocks for TADF emitters, it also proves in principle that pure hydrocarbon mobilities without any electronegative heteroatom can also be employed as acceptor segment in high performance TADF emitters.