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

Abstract

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, ΔEST, consistently decreases in going from the S-type emitters to their U-type counterparts, and this reduction in ΔEST is ascribed to the substantially more stable S1 state of the latter, while their T1 states remain similar in energy. Natural transition orbital pictures and excitation energy decomposition analyses demonstrate that the S1 states of the emitters are dominated by the charge transfer (CT) character and stabilized by the exciton binding energy, EB, 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 ΔEST by taking advantage of the short distance between the D and A units and subsequently reinforcing EB for the D-to-A CT state.