Multi-dipolar Chromophores Featuring Phosphine Oxide as Joint Acceptor: A New Strategy toward High-Efficiency Blue Thermally Activated Delayed Fluorescence Dyes

Abstract

Blue thermally activated delayed fluorescence (TADF) dyes are basically combinations of strong acceptors and weak donors. In this work, a weak acceptor P═O group was employed to construct a series of weak acceptor–strong donor (WASD)-type emitters with a phenoxazine donor, namely 10-(4-(diphenylphosphoryl)phenyl)-10H-phenoxazine (SPXZPO), 10,10′-(4,4′-(phenylphosphoryl)bis(4,1-phenylene))bis(10H-phenoxazine) (DPXZPO), and 10,10′,10″-(4,4′,4″-phosphoryltris(benzene-4,1-diyl))tris(10H-phenoxazine) (TPXZPO). Owing to the insulating effect of P═O on conjugation extension and intramolecular electronic communications, the photoluminescence spectra of these molecules are almost identical, manifesting the superiority of WASD structure in emission color preservation. Simultaneously, the multi-dipolar characteristics of TPXZPO enhance the intramolecular charge transfer (ICT), facilitating reverse intersystem crossing for higher TADF efficiency and shorter lifetime. As a consequence, TPXZPO realized the desired pure-blue electroluminescence peak at 464 nm, accompanied by a favorable external quantum efficiency (ηext) up to 15.3%, 100% exciton utilization, and reduced efficiency roll-offs. Its complementary full-TADF white organic light-emitting diodes also achieved ηext as high as 16.3%, among the best results reported so far for white TADF devices. The success of TPXZPO, the first example of a P═O-based WASD-type blue TADF dye, is attributed to the comprehensive and harmonized effects of the P═O joint on controlling conjugation and intramolecular electronic communication and the multi-dipolar structure on enhancing ICT.