Tuning the electronics of phosphorescent, amide-functionalized, cyclometalated IrIII complexes: Syntheses, structures, spectroscopy and theoretical Studies

Iridium(III) complexes were synthesized with the general form [Ir(L1–6)2(bpy)]PF6 (bpy = 2,2′-bipyridine), where ligands (LH1–6) are based on the N-functionalization of 2-phenyl-N-aryl/alkyl-quinoline-4-carboxamides. Single crystal X-ray diffraction studies were undertaken on two complexes, which show that each adopts a distorted octahedral coordination geometry with retention of the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed the subtle perturbing of theIrIII/IV redox couple as a function of ligand structure. Scalar relativistic DFT studies provided qualitative descriptions of the HOMO and LUMO energy levels of the six complexes. The calculated HOMO is generally located over the Ir(5d) centre (about 45 %) and the amide-substituted 2-phenylquinoline ligand, whilst the LUMO is localized over the ancillary 2,2′-bipyridine ligand. Similar calculations for [Ir(L6)2(bpy)]PF6 revealed a different HOMO depiction with locale on the pendant chromophores. A companion calculation, using an alternative relativistic approach (i.e. incorporating spin–orbit coupling effects) conducted on a simplified model compound, provided HOMO/LUMO depictions that are essentially identical to the non-relativistic calculation, which predicts long-lived phosphorescent emission from the HOMO–LUMO transition. Luminescence studies showed the predictable and tunable phosphorescent emission wavelengths between 585–627 nm. The experimental and theoretical studies suggest that the electronic nature of the pendant amide substituent influences the energy of the emitting state – the strongly electron-withdrawing groups bathochromically shift the luminescence wavelength