Molecular Design Approach Managing Molecular Orbital Superposition for High Efficiency without Color Shift in Thermally Activated Delayed Fluorescent Organic Lightâ Emitting Diodes

Abstract

Molecular design principles of thermally activated delayed fluorescent (TADF) emitters having a high quantum efficiency and a color tuning capability was investigated by synthesizing three TADF emitters with donors at different positions of a benzonitrile acceptor. The position rendering a large overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) enhances the quantum efficiency of the TADF emitter. Regarding the orbital overlap, donor attachments at 2â and 6â positions of the benzonitrile were more beneficial than 3â and 5â substitutions. Moreover, an additional attachment of a weak donor at the 4â position further increased the quantum efficiency without decreasing the emission energy. Therefore, the molecular design strategy of substituting strong donors at the positions allowing a large molecular orbital overlap and an extra weak donor is a good approach to achieve both high quantum efficiency and a slightly increased emission energy.Overlap to emit: The substitution of strong donors at the positions rendering a large HOMOâ LUMO overlap and the addition of a weak donor constitute an effective design approach to realize TADF emitters having high efficiency.