Deep Red Emitting Heteroleptic Ir(III) Complexes that Incorporate Unsymmetrical 4-quinoline Carboxylic Acid Derived Ligands

Six disubstituted ligands based upon 2-(2 '-pyridinyl/pyrazinyl)quinoline-4-carboxylic acids have been synthesised, solvent-free, in one step from a range of commercially available isatin derivatives. These species behave as ancillary chelating ligands for Ir(III) complexes of the form [Ir(C<^>N)(2)(N<^>N)]PF6 (where C<^>N=cyclometalating ligand; N<^>N=2-(2 '-pyridinyl/pyrazinyl)quinoline-4-carboxylic acids). An X-ray crystallographic study on one complex shows a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed for the cyclometalating ligands. DFT calculations predicted that variations in N<^>N ligand from 2,2 '-bipyridine to L-1(-6) should localise the LUMO on to the L-n ligand and that the complexes are predicted to display MLCT/LLCT character. All complexes displayed luminescence in the deep red part of the visible region (674-679 nm) and emit from triplet states, but with little apparent tuning as a function of L-1(-6). Further time-resolved transient absorption spectroscopy supports the participation of these triplet states to the excited state character.