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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
Author(s) -
Cai Xinyi,
Gao Bin,
Li XiangLong,
Cao Yong,
Su ShiJian
Publication year - 2016
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201603520
Subject(s) - intersystem crossing , homo/lumo , hypsochromic shift , materials science , singlet state , density functional theory , photochemistry , molecular orbital , triplet state , acceptor , excited state , molecule , fluorescence , chemistry , atomic physics , computational chemistry , physics , optics , organic chemistry , condensed matter physics
A barely reached balance between weak intramolecular‐charge‐transfer (ICT) and small singlet–triplet splitting energy (ΔE 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–π* transition character, hypsochromic color, a large radiative rate (k F ), and small ΔE 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 ΔE 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.