Deep-blue thermally activated delayed fluorescence emitter with a very high fluorescence rate

Abstract

Conventional thermally activated delayed fluorescence (TADF) molecules achieve small energy differences between the lowest singlet and triplet excited states (Δ E ST ) by enhancing the intramolecular charge transfer, which inevitably leads to a wide emission spectrum and low fluorescence rate. Here, we prepared a deep blue TADF molecule via a small Δ E ST pyridine-phenol fluoroboron complex as the acceptor. The small Δ E ST is maintained when carbazole donors are attached to the 4-position of the phenyl rings in the fluoroboron complex. Benefiting from the strong electron coupling between the donor (D) and acceptor (A) moieties, the compound Cz-4-BF exhibits a high fluorescence rate of 4.8 × 10 8 s −1 and a small D-A dihedral angle change in the excited state. Consequently, a photoluminescence (PL) quantum yield of nearly 100% and a PL spectrum with full-width at half-maximum (FWHM) < 60 nm were obtained in solution and low-concentration doped films. A TADF-sensitized fluorescence (TSF) device containing Cz-4-BF achieves an external quantum efficiency of 21%, which is higher than the devices employing classical fluorescent emitters and multiple resonance-type TADF emitters. The Cz-4-BF-based TSF device shows significantly improved color coordinates of (0.14, 0.10) versus a control device without Cz-4-BF. • Deep-blue TADF emitter with very high fluorescence rate of 4.8 × 10 8 s −1 . • Full-width at half-maximum of PL and EL spectra <60 nm. • PL and EL efficiencies are higher than those of multiple resonance type TADF emitters.