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, cyclometallated iridium(III) complexes. The electronic and redox properties of the complexes were investigated through experimental (including time-resolved luminescence and transient absorption spectroscopy) and theoretical methods. The complexes display absorption and phosphorescent emissions in the visible region that are attributed to metal to ligand charge-transfer transitions. The different substitution patterns of the ligands induce variations in these parameters. Time-dependent DFT studies support these assignments and show that there is likely to be a strong spin-forbidden contribution to the visible absorption bands at λ=500-600 nm. Calculations also reliably predict the magnitude and trends in triplet emitting wavelengths for the series of complexes. The complexes were assessed as potential sensitisers in triplet-triplet annihilation upconversion experiments by using 9,10-diphenylanthracene as the acceptor; the methylated variants performed especially well with impressive upconversion quantum yields of up to 39.3 %.