Light-emitting iridium complexes with tridentate ligands.

Once the Cinderella amongst the Platinum Group Metals at the Photochemistry Ball, iridium has become of intense interest since the beginning of the decade. Complexes of iridium(III) can be prepared that are highly luminescent, with emission wavelengths tuneable over the whole of the visible region. Whilst most studies have focused on tris-bidentate complexes, a rich and varied chemistry is also possible using tridentate ligands. In this review, we discuss the synthesis and excited-state properties of such complexes, exploring in particular how the number of cyclometallating carbon atoms in the coordination sphere of the metal ion influences the luminescence. Moving from [IrN6]3+ to [IrN3X3] coordination via [IrN5X]2+ and cis/trans-[IrN4X2]+ complexes, where N is a heterocyclic nitrogen and X is an anionic ligand or cyclometallated carbon, a whole range of luminescence efficiencies are encountered, ranging from the barely detectable to room temperature quantum yields approaching unity. We consider the extent to which these profound differences, arising as a result of subtle changes in molecular structure, can be rationalised in terms of the nature of the frontier orbitals.