Efficient thermally activated delayed fluorophores featuring multi-donor arms and a π-extended acceptor core

Abstract

Thermally activated delayed fluorophores often face challenges related to low solid-state luminescence efficiency and inefficient spin–flip processes, compromising their performances in organic light-emitting diodes. Herein, we introduce two new thermally activated delayed fluorophores, namely 4CzAQ and 4CzDP, based on a π-extended acceptor core (acenaphtho[1,2-b]quinoxaline/dibenzo[a,c]phenazine) and four electron-donating carbazolyl arms. These new emitters feature a propeller-like sterically demanding configuration, which mitigates emission quenching in the solid state. Crucially, this design strategy results in a dense charge-transfer excited-state alignment and strategically introduces locally excited triplet states from the acceptor within the alignment to facilitate spin flipping by a spin–vibronic coupling mechanism. The luminescence properties and spin-flipping efficiency can be finely tuned by varying the acceptor unit, depending on its π-conjugation extension and energy level. A device doped with 10 wt% of the optimal emitter, 4CzAQ, achieves a remarkable electroluminescence performance with an external quantum efficiency of 26.8% and a peak at 555 nm. The combined steric and electronic modulation arising from this molecular design offers a compelling strategy to address critical challenges in thermally activated delayed fluorophores.