Planar D-?-A type thermally activated delayed fluorescence material with Intra- and intermolecular charge transfer characteristics

Intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT) are two general molecular design strategies of thermally activated delayed fluorescence (TADF) materials for organic light-emitting diodes (OLEDs). Herein, a diphenylamine (DPA) donor and a 2,4-diphenyl-1,3,5-triazine (TRZ) acceptor are connected by two planar units, non-conjugated 9,9-dimethyl-9H-xanthene (XA) and conjugated dibenzo[b,d] furan (FR), to give D-sigma-A type emitter DPA-XA-TRZ and D-pi-A type emitter DPA-FR-TRZ. Coexistence of intraand inter-CT is found for DPA-XA-TRZ and is readily tuned by various doping concentrations owing to the nonconjugated linkage. In contrast, DPA-FR-TRZ shows a dominant intra-CT characteristic due to the strong molecular conjugation. The planar molecular skeleton of DPA-XA-TRZ could not only enhance intramolecular electronic coupling, but also reduce steric hindrance and shorten intermolecular distance to enhance inter-CT process. A maximum external quantum efficiency (EQEmax) of 21.9 % is achieved for the OLED device based on DPA-XA-TRZ, which is 2.5 times higher than that of DPA-FR-TRZ (8.4 %). DPA-XA-TRZ can also be utilized as an assistant host to sensitize multiple resonance TADF emitter DMAc-BN, and an EQEmax of 31.7 % is realized owing to its dual CT processes and high reverse intersystem crossing rate (kRISC), which is significantly superior to the conventional mCBP host and mCBP:PO-T2T exciplex cohost.