Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation-Adjustable CN-Substituted Pyridine and Pyrimidine Acceptor Units

A series of twisted D-pi-A type emitters based on the acridine donor unit and CN-substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge-transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN-substitution pattern but also by the subtle control of in-acceptor conformation (twist angles). Small triplet-to-singlet energy gaps (Delta E-ST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D-pi-A compounds with sufficient acceptor strengths and tunable in-acceptor conformation, yielding a series of efficient blue-green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%-100%. Highly efficient blue-green to yellow TADF organic light-emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%-31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.