Ligand‐Tuneable, Red‐Emitting Iridium(III) Complexes for Efficient Triplet–Triplet Annihilation Upconversion Performance

A series of substituted 2‐phenylquinoxaline ligands have been explored to finely tune the visible emission properties of a corresponding set of cationic, cyclometalated iridium(III) complexes. The electronic and redox properties of the complexes were investigated using experimental (including time‐resolved luminescence and transient absorption spectroscopy) and theoretical methods. The complexes display absorption and phosphorescent emission in the visible region attributed to MLCT transitions. The different substitution patterns of the ligands induce variations in these parameters. TD‐DFT studies support these assignments and show that there is likely to be a strong spin‐forbidden contribution to the visible absorption bands at 500‐600 nm. Calculation also reliably predict the magnitude and trends in triplet emitting wavelengths for the series of complexes. The complexes were assessed as potential sensitizers in triplet‐triplet annihilation upconversion experiments using 9,10‐diphenylanthracene as the acceptor, with the methylated variants performing especially well with impressive upconversion quantum yields up to 39.3 %.