Simultaneously optimizing radiative decay and up-conversion of triphenylamine-based thermally activated delayed fluorescence emitters to achieve efficient deep-red organic light-emitting diodes

Abstract

Triphenylamine (TPA) has been commonly used as the electron donor to construct red and near-infrared thermally activated delayed fluorescence (TADF) emitters as the strong intramolecular charge transfer with electron acceptors to induce rapid radiative decay to compete with the small energy gap. For TPA-derivates, even stronger conjugation, accelerated up-conversion and enhanced rigidity are highly desired to further overcome the serious non-radiative decay and promote exciton utilization. Targeting at optimizing TPA-based emitters, herein, three new molecules were developed. Among them, 4,4′,4′'-(pyrazino[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline-3,6,11-triyl)tris(N,N-diphenylaniline) (TTPA-DCPPm) were designed by incorporating intramolecular hydrogen bonding and quasi-degenerate molecular orbitals. The rigidity is enhanced and radiative decay as well as reverse intersystem crossing (RISC) are promoted. Accordingly, TTPA-DCPPm demonstrates an order of magnitude smaller non-radiative decay rate, a 2.6 times larger fluorescence decay rate and a 1.8 times faster RISC rate compared with the control group, which supports its OLED with a maximum external efficiency of 26.5% with an emission peak at 640 nm. This work paves the way for developing efficient TPA-based deep-red emitters.