Aryl-Substituted Acridine Donor Derivatives Modulate the Transition Dipole Moment Orientation and Exciton Harvesting Properties of Donor–Acceptor TADF Emitters

Thermally activated delayed fluorescence (TADF) compounds are highly attractive as sensitizing and emitting materials for organic light-emitting diodes (OLEDs). The efficiency of the OLED depends on multiple parameters, most of which rely on the properties of the emitter including those that govern the internal quantum and outcoupling efficiencies. Herein, we investigate a series of aryl substituted acridine donor derivatives of the donor–acceptor TADF emitter DMAC-TRZ, with the objective of correlating their properties, such as triplet harvesting efficiency and transition dipole moment orientation, with their corresponding device efficiency. The decoration of the DMAC donor with substituted aryl groups not only modifies the molecular weight and length of the emitter but also affects the emission color and the capacity for the emitters to efficiently harvest triplet excitons. The presence of electron-withdrawing 4-cyanophenyl and 4-trifluoromethylphenyl groups in, respectively, CNPh-DMAC-TRZ and CF3Ph-DMAC-TRZ, blue-shifts the emission spectrum but slows down the reverse intersystem crossing rate constant (kRISC), while the opposite occurs in the presence of electron-donating groups in tBuPh-DMAC-TRZ and OMePh-DMAC-TRZ (red-shifted emission spectrum and faster kRISC). In contrast to our expectations, the OLED performance of the five DMAC-TRZ derivatives does not scale with their degree of horizontal emitter orientation but follows the kRISC rates. This, in turn, demonstrates that triplet harvesting (and not horizontal emitter orientation) is the dominant effect for device efficiency using this family of emitters. Nonetheless, highly efficient OLEDs were fabricated with tBuPh-DMAC-TRZ and OMePh-DMAC-TRZ as emitters, with improved EQEmax (∼28%) compared to the reference DMAC-TRZ devices.