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

A barely reached balance between weak intramolecular-charge-transfer (ICT) and small singlet-triplet splitting energy (Delta E-ST) 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-pi* transition character, hypsochromic color, a large radiative rate (k(F)), and small Delta E-ST 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 k(F) 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 k(F) but significantly lowers the local triplet excited state, indicating small HOMO and LUMO overlap is not a sufficient, but necessary condition for reduced Delta E-ST. 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.