Substitution Effects on a New Pyridylbenzimidazole Acceptor for Thermally Activated Delayed Fluorescence and Their Use in Organic Light-Emitting Diodes

In this work a new acceptor is used for use in thermally activated delayed fluorescence (TADF) emitters, pyridylbenzimidazole, which when coupled with phenoxazine allows efficient TADF to occur. N-functionalization of the benzimidazole using methyl, phenyl, and tert-butyl groups permits color tuning and suppression of aggregation-caused quenching (ACQ) with minimal impact on the TADF efficiency. The functionalized derivatives support a higher doping of 7 wt% before a fall-off in photoluminescence quantum yields is observed, in contrast with the parent compound, which undergoes ACQ at doping concentrations greater than 1 wt%. Complex conformational dynamics, reflected in the time-resolved decay profile, is found. The singlet−triplet energy gap, ΔE_ST, is modulated by N-substituents of the benzimidazole and ranges of between 0.22 and 0.32 eV in doped films. Vacuum-deposited organic light-emitting diodes, prepared using three of the four analogs, show maximum external quantum efficiencies, EQE_max, of 23.9%, 22.2%, and 18.6% for BIm(Me)PyPXZ, BIm(Ph)PyPXZ, and BImPyPXZ, respectively, with a correlated and modest efficiency roll-off at 100 cd m^–2 of 19% 13%, and 24% of the EQE_max, respectively.