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

The synthesis of unsymmetrical, disubstituted quinoline-based N^N ligands was achieved using a range of isatin reagents. Subsequent complexation within heteroleptic Ir(III) leads to the formation of luminescent complexes which emit ca. 675 nm.

Abstract

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 Lⁿ 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.

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