Ultrafast Near-Infrared Luminescence from Cyclometalated Iridium(III) Complexes with Indirubin as Ancillary Ligand

Being a constitutional isomer of the indigo dye, indirubin has been known as a purple colourant for a long time. On the other hand, its structural and photophysical properties as ligand in metal complexes are virtually unexplored. Herein, for the first time, we utilized indirubin as ancillary chelator to develop two new heteroleptic iridium(III) complexes equipped with two 2-phenylpyridine units as archetypical cyclometalating ligands. These new complexes display fully reversible oxidation and reduction processes, and show a panchromatic absorption extending up to 900 nm in acetonitrile solution at 298 K. Moreover, an unexpected broad and unstructured emission band is detected for both complexes in the near-infrared region, peaking at approx. 1100 nm and having a lifetime of approx. 60 ps; such an emission is attributed to a ligand-centred triplet excited state (3LC) located on the indirubin ligand itself, as proved by DFT calculations. These findings may pave the way for further exploration of the indirubin dye as a ligand for different types of metal centres to create efficient near-infrared triplet emitters without the need for difficult synthetic procedures.