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.

Automated summary

Intramolecular charge transfer (intra-CT) and intermolecular charge transfer (inter-CT) are two general molecular design strategies for thermally activated delayed fluorescence (TADF) materials. By tuning the doping concentrations, the influence of molecular conjugation on charge transfer can be overcome. The planar molecular skeleton enhances intramolecular electronic coupling, reduces steric hindrance, and shortens intermolecular distance, leading to improved charge transfer efficiency.