Efficient donor-σ-acceptor emitters with strengthened intramolecular charge-transfer and their use for high-efficiency organic light-emitting diodes

Abstract

Thermally-activated delayed fluorescence (TADF) materials have emerged as next-generation emitters for organic light-emitting diodes (OLEDs). The donor-σ-acceptor molecule is a promising paradigm for developing TADF, but its radiative decay rate (k_r,s) and photoluminescent efficiency (Ф_PL) require large improvements, due to weak intramolecular charge-transfer (CT). Here, efficient donor-σ-acceptor emitters (1–3) with strengthened intramolecular CT are developed by directly linking the donor and acceptor with a short alkyl chain. 9,9-dimethyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine are employed as the donor and acceptor, respectively, and –CH₂– (for 1), –CH₂CH₂– (for 2) and –CH₂CH₂CH₂– (for 3) are employed as the σ-linkers. The chemical structures of 1–3 have been verified by X-ray crystallography. In dilute solution and lightly doped films, emitters 1–3 show considerably strong intramolecular CT, due to the σ–π hyperconjugation between the donor/acceptor and the alkyl σ-linker. In the 20 wt.% doped films, emitters 1–3 show green–blue TADF with combined intra- and inter-molecular CT, with high Ф_PL k_r,s and reverse intersystem crossing rates up to 0.91, 8.5 × 10⁶ s⁻¹ and 2.6 × 10⁶ s⁻¹, respectively. OLEDs based on emitters 1–3 show green–blue emission with high external quantum efficiencies (EQEs) over 20 %. A hyperfluorescent OLED with emitter 3 as the sensitizer and a typical multiple resonance emitter (DtBuCzB) as the terminal emitter shows narrowband blue-green emission with a high EQE of 28.1 %.