Enhancing Reverse Intersystem Crossing via Secondary Acceptors: toward Sky-Blue Fluorescent Diodes with 10-Fold Improved External Quantum Efficiency.

How to simply but effectively facilitate reverse intersystem crossing (RISC) transition is always the key issue for developing high-performance thermally activated delayed fluorescence dyes. In this work, as a proof of concept, a feasible strategy named "acceptor enhancement" is demonstrated with a series of ternary blue emitters ( xCz mPO nTPTZ) using diphenylphosphine oxide (PO) as secondary acceptors. Compared with its PO-free binary analogue, such a simple introduction of PO groups in pCzPO2TPTZ dramatically enhances its RISC rate constant ( kRISC) by 10 times the level of ∼105 s-1, accompanied by RISC efficiency (ηRISC) of 92%, which further improves the triplet-to-singlet upconversion for effective triplet harvesting in its devices. As a result, on the basis of a trilayer device structure, pCzPO2TPTZ realized a state-of-the-art external quantum efficiency beyond 20% with a 10-fold improvement.