Peripheral halogen atoms in multi-resonant thermally activated delayed fluorescence emitters: the role of heavy atom on intermolecular interactions and spin orbit coupling

Multi-resonant thermally activated delayed fluorescence materials (MR-TADF) can show narrow-band emission with high photoluminescence quantum efficiency, desirable for applications in organic light emitting diodes (OLEDS). However, they frequently suffer from slow reverse intersystem crossing (RISC) compared to established donor-acceptor TADF emitters, leading to severe device efficiency roll-off at high exciton densities. Introducing heavy atom effects (HAE) by core-substitution has been previously shown to enhance spin orbit coupling and thus RISC in MR-TADF emitters, frequently with oxygen atoms replaced by isoelectronic sulfur or selenium. Here, we explore an alternate HAE strategy using peripheral halogenation of the MR-TADF DiKTa core, comparing tBr-DiKTa and dBr-tBu-DiKTa with non-halogenated _Mes3-DiKTa. The two brominated emitters demonstrate improved kRISC because of the HAE, while the rate appears to improve by an additional order of magnitude in mCP host, because of intermolecular (guest-host) interaction. Despite the beneficial hetero-intermolecular interactions, strong homo-intermolecular interactions result in enhanced non-radiative pathways and lower photoluminescence quantum yields. OLEDs of dBr-tBu-DiKTa hence showed comparable EQEmax with Mes₃-DiKTa (21%) and improved efficiency roll-off until 500 cd m^-2, although with accelerated roll-off beyond a critical current density. Together with comparisons in less strongly doped devices, these results show that the HAE provided by peripheral halogens improves the device performance up to 500 cd m-2, but also support detrimental intermolecular interactions that dominate at higher device currents.