New 2,3-disubstituted-5-hydroxyquinoxaline ligands and their coordination chemistry with cyclometallated iridium(iii): syntheses, structures and tunable electronic properties

Abstract

A new series of para-substituted 2,3-diphenyl-5-hydroxyquinoxaline ligands (LHⁿ) were synthesised and characterised. These ligands were prepared in high yield via a two-step synthetic method. Four novel heteroleptic iridium(III) complexes were correspondingly prepared in high yield giving [Ir(ppy)₂(Lⁿ)]. Two X-ray crystallographic studies were undertaken on LH³ and [Ir(ppy)₂(L²)] with each confirming the proposed formulations, with the complex showing the O,N-coordination mode of the quinoxalinato ligand. Density functional theoretical calculations were performed, firstly to compare the coordinated quinoxalinato system with the related quinolinato analogue, and secondly to probe the influence of the variation in para-substitution on the ancillary ligand. The calculations suggest that for either the quinoline or quinoxaline systems ligand-centred character appears to dominate the HOMO and LUMOs. Experimental electrochemical and spectroscopic characterisation showed that the subtle variations in absorption and emission wavelengths are probably due to ligand-dominated transitions that are influenced by the electronic nature of the para-substituted phenyl units in coordinated Lⁿ.