Benefits of the Exciplex-like Framework in Reducing the Singlet-Triplet Energy Difference: A Theoretical Perspective on the Role of the Exciton Binding Energy.

By means of post-Hartree-Fock and density functional theory calculations, we compare the exciplex-like U-type and conventional S-type thermally activated delayed fluorescence emitters which are composed of electron-donor (D), linker (L), and electron-acceptor (A) units: 10-phenyl-9,10-dihydroacridine, fluorene, and 2,4,6-triphenyl-1,3,5-triazine analogues, respectively. We found that the singlet-triplet energy difference, Δ E ST , consistently decreases in going from the S-type emitters to their U-type counterparts, and this reduction in Δ E ST is ascribed to the substantially more stable S 1 state of the latter, while their T 1 states remain similar in energy. Natural transition orbital pictures and excitation energy decomposition analyses demonstrate that the S 1 states of the emitters are dominated by the charge transfer (CT) character and stabilized by the exciton binding energy, E B , which substantially enhances when the hole and electron are in close proximity. Without relying on the vague notion of through-space vs through-bond CT characters, we clearly showed that the exciplex-like molecular framework can effectively reduce Δ E ST by taking advantage of the short distance between the D and A units and subsequently reinforcing E B for the D-to-A CT state.