Multichromophore Molecular Design for Efficient Thermally Activated Delayed Fluorescence Emitters with Near-Unity Photoluminescence Quantum Yields
Three multichromophore thermally activated delayed fluorescence (TADF) molecules, <b><i>p</i>-di2CzPN</b>, <b><i>m</i>-di2CzPN</b>, and<b> 1,3,5-tri2CzPN</b>, were synthesized and characterized. These molecules were designed by connecting the TADF moiety 4,5-di(9<i>H</i>-carbazol-9-yl)phthalonitrile (<b>2CzPN</b>) to different positions of a central benzene ring scaffold. Three highly soluble emitters all exhibited near quantitative photoluminescence quantum yields (<i></i><sub>PL</sub>) in toluene. High <i></i><sub>PL</sub>s were also achieved in doped films, 59% and 70% for <b><i>p</i>-di2CzPN</b> and <b><i>m</i>-di2CzPN</b> in 10 wt% DPEPO doped films, respectively, and 54% for <b>1,3,5-tri2CzPN</b> in 20 wt% doped CBP film. The rate constant of reverse intersystem crossing (<i>k</i><sub>RISC</sub>) for <b><i>p</i>-di2CzPN</b> and <b><i>m</i>-di2CzPN</b> in DPEPO films reached 1.1 × 10<sup>5</sup> s<sup>−1</sup> and 0.7 × 10<sup>5</sup> s<sup>−1</sup>, respectively, and <i>k</i><sub>RISC</sub> for <b>1,3,5-tri2CzPN</b> in CBP film reached 1.7 × 10<sup>5</sup> s<sup>−1</sup>. A solution-processed organic light-emitting diode based on <b>1,3,5-tri2CzPN </b>exhibited a sky-blue emission with CIE coordinate of (0.22, 0.44), and achieved a maximum external quantum efficiency of 7.1%.
