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

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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, Δ E 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.