Multiple resonance thermally activated delayed fluorescence enhanced by halogen atoms

We synthesized halogen-substituted multiple-resonance (MR) thermally activated delayed fluorescence emitters, namely 2,12-dichloro-N,N,5,9-tetrakis(4-chlorophenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-7-amine (Cl-MR) and 2,12-dibromo-N,N,5,9-tetrakis(4-bromophenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-7-amine (Br-MR). Cl-MR and Br-MR exhibited a decreased delayed fluorescence lifetime and an enhanced reverse intersystem crossing rate without any changes in Delta E-ST and orbital distribution compared with a non-halogenated MR emitter. Cl-MR exhibited a high photoluminescence quantum yield (PLQY) of 85% and external quantum efficiency (EQE) of 17%; however, Br-MR did not exhibit any enhancement in the PLQY and EQE. The different performances of Cl-MR and Br-MR were rationalized by analysing the rate constants of the excited states and bond dissociation energies of the carbon-halogen bonds.