From the difference in the transient PL decay behavior under aerated and degassed conditions in toluene it is clear that the delayed emission is significantly quenched by the presence of oxygen (Figure ); interestingly, the quenching of the PL in DMACBP and DMACBPN is more significant than in DMACPyBP and DMACPyBPN, the latter two of which contain an additional pyridine ring fused to the pyrazine in the acceptor. The increased quenching of the PL of the former two compounds may be due to their longer delayed lifetimes. The emission intensity is likewise strongly affected by the presence of oxygen, especially for DMACBP and DMACBPN, while for DMACPyBP and DMACPyBPN, the enhancement in intensity upon oxygen removal was much smaller (Figure ). The PL decays with biexponential kinetics, with prompt fluorescence lifetimes, tp, of 18 ns, 21 ns, 27 ns and 10 ns (Figure We next investigated the photophysical properties of the four emitters in an OLED-relevant host, 4,4′-N,N′-dicarbazolebiphenyl (CBP), as this host matrix has a sufficiently high triplet energy (T1 = 2.56 eV) to confine the excitons onto the emitter. The PL of the doped films of varying dopant concentration from 2-10 wt% of emitters were firstly investigated (Figure ) and the corresponding FPL values are collated in Table . As the doping concentration is increased, the emission of all of the compounds is red-shifted from yellow to red (Figure ), and the highest FPL values are at a doping concentration of 2 wt%. The SSPL spectra of DMACBP, DMACPyBP, DMACBPN and DMACPyBPN at 2 wt% doping concentration exhibit broad and unstructured emission with lPL at 568, 601, 586, and 606 nm, respectively (Figures 5a). The trend in lPL mirror those in toluene (Figure ). The FPL values of these films of DMACBP, DMACPyBP, DMACBPN and DMACPyBPN are 75, 47, 71 and 37%, respectively, under a N2 atmosphere (Figure , Table ). The decrease of FPL for DMACPyBP/DMACPyBPN can be ascribed to their smaller energy gaps, thus leading to increased non-radiative decay. However, the difference of the distribution of the N atoms in the BP and BPN (two additional nitrogen atoms at the 4-and 5-positions) acceptors has only a relatively minor impact on the acceptor strength, with a small red-shift in the emission from 568 nm for DMACBP to 586 nm for DMACBPN; as well, there is only a negligible decrease in the FPL between DMACBP and DMACBPN (Table , Figure ). The FPL values of DMACBP and DMACBPN exhibited a larger decrease in the presence of air at 56 and 52%, respectively, compared to 41% for DMACPyBP and average delayed emission lifetimes, average td, of 130.3, 5.9, 47.2, and 2.9 µs at room temperature for DMACBP, DMACPyBP, DMACBPN and DMACPyBPN, respectively. The corresponding kISC for DMACBP, DMACPyBP, DMACBPN and DMACPyBPN in the 2 wt% doped films in CBP are 1.7´10 7 s -1 , 0.6´10 7 s -1 , 1.2´10 7 s -1 and 0.8´10 7 s -1 , respectively, while the kRISC for DMACPyBP and DMACPyBPN reached 1.94´10 5 s -1 and 4.26´10 5 s -1 , more than ten times higher than those of DMACBP at 0.10´10 5 s -1 and DMACBPN at 0.29´10 5 s -1 (Figure ), following a similar trend to that observed in toluene. The introduction of a nitrogen atom at the 10-position in DMACPyBP and DMACPyBPN leads to much stronger CT and faster kRISC. Temperature-dependent time-resolved PL (TRPL) analysis shows that the intensity of the delayed emission increases with increasing temperatures for all 4 compounds, confirming that these compounds emit via TADF (Figure ).