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 fluorescent (TADF) emitters and their use in organic light-emitting diodes. However, most of the studies have focused disparately on donor/acceptor combinations to yield efficient emitters, while it is rare that there is a methodological study to investigate the influence of the nitrogen (N) doping ratios on the ground and excited states of PAH acceptors. 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 (ΔE_ST) decreases and the rate constant for reverse intersystem crossing (k_RISC) increases. In particular, introducing nitrogen at the 10-position of the 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 (λEL = 640 nm) with an EQE_max of 5.4%.