Aryl-substituted acridine donor derivatives modulate the transition dipole moment orientation and exciton harvesting properties of donor-acceptor thermally activated delayed fluorescence emitters, leading to highly efficient OLEDs

Thermally activated delayed fluorescence (TADF) compounds are highly attractive as sensitising and emitting materials for organic light-emitting diodes (OLEDs) as they can harvest both singlet and triplet excitons and convert them into light. In this study we designed a series of derivatives of the widely studied donor-acceptor TADF emitter DMAC-TRZ with the objective of correlating their structure to their propensity to orient horizontally in vacuum-deposited doped thin films. This is important as the preferential horizontal orientation of the transition dipole moment (TDM) of the emitter, which tends to co-align with the long axis of D-A compounds, leads to enhanced light-outcoupling, and therefore higher maximum external quantum efficiencies (EQEmax) of the OLED. The decoration of the DMAC donor with substituted aryl groups 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. 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.