Thermally Activated Delayed Fluorescence Mediated through the Upper Triplet State Manifold in Non-Charge-Transfer Star-Shaped Triphenylamine-Carbazole Molecules

Thermally activated delayed fluorescence has been found in a group of tricarbazolylamines that are purely electron-donating, non-charge-transfer (CT) molecules. We show that the reverse intersystem crossing step in these materials is mediated through upper triplet states. Reverse internal conversion is shown to be the thermally activated mechanism behind the triplet harvesting mechanism. The strongly mixed n-π∗/ π- π∗ character of the lowest energy optical transitions retains high oscillator strength and gives rise to high φ_PL. Organic light-emitting diode devices using these materials were fabricated to show very narrow (full width at half-maximum = 38-41 nm) electroluminescence spectra, clearly demonstrating the excitonic nature of the excited states. This new combination of physicochemical properties of a non-CT molecule yields thermally activated delayed fluorescence, but via a different, physical mechanism, reverse internal conversion delayed fluorescence.