(1.59 eV) are significantly stabilized (Figure and Tables ). This trend in S1 energies mirrors the trend in the HOMO-LUMO gaps. The T1 state of TPPQ, mainly located on the pyrazine rings, is 2.07 eV and, like the S1 state, also has locally-excited (LE) character. The T1 state of 4DMAC-TPPQ at 2.01 eV is slightly lower in energy, while the T1 state of 4PXZ-TPPQ at 1.57 eV is significantly stabilized, reflecting their CT character (Figure and Table ). The corresponding calculated singlet-triplet energy gap (DEST) of 4DMAC-TPPQ (0.01 eV) and 4PXZ-TPPQ (0.02 eV) are much smaller than that of TPPQ (1.03 eV), Figure . There are multiple high-lying triplet states in 4DMAC-TPPQ and 4PXZ-TPPQ, some of which have different orbital type to S1, and may participate in the TADF process in these two compounds (Figure and Table ). Additionally, at an excited T1 geometry, there is decrease in the S1-T1 spin-orbit coupling matrix element (SOCME) from 3.99 cm -1 in TPPQ to 0.01 cm -1 in 4DMAC-TPPQ and 0.02 cm -1 in 4PXZ-TPPQ (Table ). It should be noted that the SOCME of S1-Tn (n>1) in 4DMAC-TPPQ (<S1|HSO|T3> 0.03 cm -1 ) and in 4PXZ-TPPQ (<S1|HSO|T3> is 0.06 cm -1 ) is larger because of the large changes in orbital character between these two states (Table ). All these studies support a RISC process could proceed via the T3 to S1 in 4DMAC-TPPQ and the T3 to S1 in 4PXZ-TPPQ as T3 lies just 0.06 eV above S1 for 4DMAC-TPPQ and 0.05 eV above S1 for 4PXZ-TPPQ, so is readily accessible because of their relatively large SOCME between these states.