Enhanced spin-orbit coupling driven by state mixing in organic molecules for OLED applications

We investigate the energy gap variation as well as spin-orbit coupling (SOC) integrals between various low-lying singlet and triplet excited states for a series of fluorescein derivatives. We find that when the electron-donating property of the substituent group on the benzene moiety of fluorescein is gradually increased, the charge transfer states are lowered in energy and a mixing with nearby ππ* or nπ* states occurs, which causes a twisting in the p orbital on the carbonyl group and a non-zero SOC integral between the originally non-coupled ¹ππ* and ³ππ* states. We also find an enhancement of about 3–4 times in the SOC integrals upon sulfur substitution for the oxygen in the carbonyl groups, and that with substantial energy lowering in ππ* and especially in nπ* states, the SOC between the S₁ state with energetically close triplet states is also increased significantly, signifying the possibility of enhanced phosphorescence or thermally-delayed fluorescence emission.