Aryl-Substituted\nAcridine Donor Derivatives Modulate\nthe Transition Dipole Moment Orientation and Exciton Harvesting Properties\nof Donor–Acceptor TADF Emitters

Thermally activated\ndelayed fluorescence (TADF) compounds\nare highly\nattractive as sensitizing and emitting materials for organic light-emitting\ndiodes (OLEDs). The efficiency of the OLED depends on multiple parameters,\nmost of which rely on the properties of the emitter including those\nthat govern the internal quantum and outcoupling efficiencies. Herein,\nwe investigate a series of aryl substituted acridine donor derivatives\nof the donor–acceptor TADF emitter <b>DMAC-TRZ</b>, with\nthe objective of correlating their properties, such as triplet harvesting\nefficiency and transition dipole moment orientation, with their corresponding\ndevice efficiency. The decoration of the DMAC donor with substituted\naryl groups not only modifies the molecular weight and length of the\nemitter but also affects the emission color and the capacity for the\nemitters to efficiently harvest triplet excitons. The presence of\nelectron-withdrawing 4-cyanophenyl and 4-trifluoromethylphenyl groups\nin, respectively, <b>CNPh-DMAC-TRZ</b> and <b>CF3Ph-DMAC-TRZ</b>, blue-shifts the emission spectrum but slows down the reverse intersystem\ncrossing rate constant (<i>k</i><sub>RISC</sub>), while\nthe opposite occurs in the presence of electron-donating groups in <b><sup><i>t</i></sup>BuPh-DMAC-TRZ</b> and <b>OMePh-DMAC-TRZ</b> (red-shifted emission spectrum and faster <i>k</i><sub>RISC</sub>). In contrast to our expectations, the OLED performance\nof the five DMAC-TRZ derivatives does not scale with their degree\nof horizontal emitter orientation but follows the <i>k</i><sub>RISC</sub> rates. This, in turn, demonstrates that triplet harvesting\n(and not horizontal emitter orientation) is the dominant effect for\ndevice efficiency using this family of emitters. Nonetheless, highly\nefficient OLEDs were fabricated with <b><sup><i>t</i></sup>BuPh-DMAC-TRZ</b> and <b>OMePh-DMAC-TRZ</b> as emitters,\nwith improved EQE<sub>max</sub> (∼28%) compared to the reference <b>DMAC-TRZ</b> devices.