Singlet–Triplet Splitting Energy Management via Acceptor Substitution: Complanation Molecular Design for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters and Organic Light‐Emitting Diodes Application

Abstract

A barely reached balance between weak intramolecular‐charge‐transfer (ICT) and small singlet–triplet splitting energy (ΔEST) for reverse intersystem crossing from non‐emissive triplet state to radiative singlet state impedes the realization of deep‐blue thermally activated delayed fluorescence (TADF) materials. By discarding the twisted‐ICT framework for a flattened molecular backbone and introducing a strong acceptor possessing n–π* transition character, hypsochromic color, a large radiative rate (kF), and small ΔEST are achieved simultaneously. Six molecules with a 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine (i‐DMAc) donor are synthesized and investigated. Coinciding with time‐dependent density functional theory, the reduced dihedral angles between donor (D) and acceptor (A) weaken ICT from dispersed charge density and enable a large kF from increased frontier molecular orbitals overlap. Despite the separated highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) population, the intercalation of phenyl bridges between D–A increases kF but significantly lowers the local triplet excited state, indicating small HOMO and LUMO overlap is not a sufficient, but necessary condition for reduced ΔEST. Integrating short conjugation length and carbonyl or triazine acceptors into the complanation molecules, deep‐blue TADF organic light‐emitting diodes demonstrate maximum external quantum efficiencies of 11.5% and 10.9% with Commission Internationale de l'Eclairage coordinates of (0.16, 0.09) and (0.15, 0.11), respectively, which is quite close to the stringent National Television System Committee blue standard.