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

In this work we introduce a new acceptor for use in thermally activated delayed fluorescence (TADF) emitters, pyridylbenzimidazole, which when coupled with phenoxazine, allows efficient TADF occurs. N-functionalization of the benzimidazole using methyl, phenyl and tert-butyl groups permits colour tuning and adjustment of the optoelectronic properties and suppression of aggregation-caused quenching (ACQ) with minimal impact on the TADF mechanism. Good photoluminescence quantum yields, F_PL, (49% - 68%) in mCP-doped films are reported for the emitters. The functionalized derivatives support a higher doping of 7 wt% before a fall-off in F_PL is observed, in contrast with the parent compound, which undergoes ACQ at doping concentrations greater than 1 wt%. We find complex conformational dynamics, reflected in the time-resolved decay profile. DE_ST is modulated by N-substituents of the benzimidazole and ranges of between 0.22 eV and 0.32 eV in doped films. Density Functional Theory (DFT) calculations suggest the importance of intermediate triplet excited states to enhance the RISC rate. Vacuum-deposited organic light-emitting diodes (OLEDs), 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; higher maximum luminance values of 21,000 cd m^-2 and 18,000 cd m^-2 are also obtained for BIm(Ph)PyPXZ and BIm(Me)PyPXZ, respectively, compared to 8,000 cd m^-2 for BImPyPXZ.