The influence of nitrogen doping of the acceptor in orange-red thermally activated delayed fluorescence emitters and OLEDs

Nitrogen-containing polycyclic aromatic hydrocarbons (N-PAH) have been widely used as deep lowest unoccupied molecular orbital (LUMO) acceptors in donor-acceptor (D-A) red thermally activated delayed fluorescence (TADF) emitters and in organic light-emitting diodes (OLEDs). However, most of the studies have focused disparately on donor/acceptor combinations to yield efficient emitters, while a methodological study investigating the influence of nitrogen (N) doping ratios on the ground and excited states of PAH acceptors is rare. Here, we report a family of four different N-PAH acceptors containing different numbers of nitrogen atoms within the N-PAH and their use in D-A TADF emitters, DMACBP, DMACPyBP, DMACBPN and DMACPyBPN, when coupled to the same donor, 9,9-dimethyl-9,10-dihydroacridine (DMAC). As the nitrogen content in the acceptor increases the LUMO becomes progressively more stabilized while the singlet-triplet energy gap (& UDelta;EST) decreases and the rate constant for reverse intersystem crossing (kRISC) increases. In particular, introducing nitrogen at the 10-position of dibenzo[a,c]phenazine (BP) leads to a more than ten-fold enhancement in kRISC in DMACPyBP and DMACPyBPN compared to DMACBP and DMACBPN. Among the OLEDs with all four emitters that with DMACBPN demonstrates the highest EQEmax of 19.4% at an emission peak of 588 nm, while the deepest red emitting device employed DMACPyBPN (& lambda;EL = 640 nm) with an EQEmax of 5.4%. We have developed a family of four orange-to-red TADF compounds whose structures differ by the number of nitrogen atoms contained within the acceptor. It was found that increasing the nitrogen atom content results in a more stabilized LUMO, a smaller & UDelta;EST and faster kRISC.