Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters

Heptazine-based materials have recently emerged as a promising motif for thermally activated delayed fluorescence, as their near-zero or negative singlet-triplet energy gaps enable extremely fast reverse intersystem crossing (rISC) rates. Another method for achieving a high rate of rISC is through the use of highly symmetric emitters, which benefit from energy-level degeneracies and a high density of states. Here, we investigate the effect of combining these two design strategies on the excited-state dynamics of C-3-symmetric emitters containing heptazine cores. We find that in two of the four emitters studied, the S-1 state has a high degree of locally excited (LE) character with density on the heptazine moiety, preventing excited-state localization and a loss of symmetry in the energy-minimized S-1 geometry. Surprisingly, these symmetric molecules still suffer from a loss of density of triplet states below the S-1 state. Overall, we find that maintaining C-3 symmetry will not necessarily maintain density of states, but that heptazine-based materials with LE S-1 states still benefit from maximized rISC rates via increased spin-orbit coupling with low-lying charge-transfer triplet states and exhibit advantageous photophysical properties, such as near-unity photoluminescence quantum yields and high colour purity.