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

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)-10<i>H</i>-phen­ox­azine (<b>SPXZPO</b>), 10,10′-(4,4′-(phenyl­phos­phoryl)­bis­(4,1-phenylene))­bis­(10<i>H</i>-phen­ox­azine) (<b>DPXZPO</b>), and 10,10′,10″-(4,4′,4″-phos­phoryl­tris­(benzene-4,1-diyl))­tris­(10<i>H</i>-phen­ox­azine) (<b>TPXZPO</b>). 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 <b>TPXZPO</b> enhance the intra­molecular charge transfer (ICT), facilitating reverse inter­system crossing for higher TADF efficiency and shorter lifetime. As a consequence, <b>TPXZPO</b> realized the desired pure-blue electro­lumin­escence peak at 464 nm, accompanied by a favorable external quantum efficiency (η_ext) up to 15.3%, 100% exciton utilization, and reduced efficiency roll-offs. Its complementary full-TADF white organic light-emitting diodes also achieved η_ext as high as 16.3%, among the best results reported so far for white TADF devices. The success of <b>TPXZPO</b>, 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