Steric Rooted Multi-Resonant Thermally Activated Delayed Fluorescent Emitters for Pure Blue Organic Light Emitting Diodes with Ultralow Efficiency Roll-Off

Abstract

Multi-resonant thermally activated delayed fluorescent (MR-TADF) materials are blooming for high-resolution organic light-emitting diodes (OLEDs). However, boron/nitrogen (B/N)-integrated MR-TADF emitters suffer severe efficiency roll-off from their strong inter-molecular π–π interactions. Herein, versatile narrowband pure blue emitters (mono-mx-CzDABNA and tri-mx-CzDABNA) are demonstrated featuring a ring-fused extended π-skeleton: a classic steric hindrance and rigidity accessed by integrating with meta-xylene (mx) rotors. tri-mx-CzDABNA shows a narrowband (FWHM, 26 nm) pure blue emission (λmax, 462 nm) with substantial hypsochromic shift (12 nm) while maintaining MR-TADF characteristics. The key solid-state analyses conclude that they conceivably suppress the non-radiative energy loss, thus improving the photoluminescence quantum yield (PLQY > 90%) and rate of reverse intersystem crossing (RISC) (kRISC ≈2.85 × 105 s−1). The integration of tri meta-xylene significantly leads to an enhanced horizontal dipole ratio (HDR) from 65% to 85%. Hyperfluorescent-OLEDs are fabricated using designed MR-TADF as terminal emitter, achieving a narrowband (FWHM, 34 nm) pure blue electroluminescence (λmax, 472 nm) and maximum external quantum efficiency (EQEmax) of 26.97% with magnificently suppressed efficiency roll-off (7.8%) at 1000 cd m−2. So, it is believed that regulation of internal efficiencies and high color purity can amplify the route to achieving a narrowband pure blue emission through new synthetic MR-TADF approaches. © 2023 Wiley-VCH GmbH.