Bis-cyclometalated iridium(iii) complexes with terpyridine analogues: syntheses, structures, spectroscopy and computational studies

Two ligands based upon a 2,6-disubstituted pyridine bridge introduce bis-quinoxalinyl units in a fashion that yields analogues to the archetypal terdentate ligand, 2,2 ':6 ',2 ''-terpyridine. The ligands were synthesised from the key intermediate 2,6-bis(bromoacetyl)pyridine: a new, high-yielding route is described for this reagent. Two ligand variants (differentiated by H/Me substituents on the quinoxaline ring) were explored as coordinating moieties for iridium(iii) in the development of luminescent complexes. Computational studies (DFT approaches employing B3LYP, B3LYP/LANL2DZ, and M062X/LANL2DZ levels) were used to investigate the geometric and coordination mode preferences of the new ligands and two possibilities arose from theoretical investigations: [Ir(N<^>N<^>N)(2)](3+) and [Ir(N<^>N<^>C)(2)](+), with the former predicted to be more energetically favourable. Upon synthesis and isolation of the Ir(iii) complexes, X-ray crystallographic studies revealed coordination spheres that were cyclometalated, the structures both showing a [Ir(N<^>N<^>C)(2)]PF6 arrangement. Further spectroscopic characterization via NMR confirmed the ligand arrangements in the complexes, and photophysical studies, supported by DFT, showed that a mixture of metal-to-ligand charge transfer (MLCT) and intra-ligand charge transfer (ILCT) character is likely to contribute to the emission features of the complexes, which phosphoresce orange-red (lambda(em) = 580-618 nm). The emission wavelength was influenced by the substituents on the quinoxaline ring (H vs. Me), thereby implying further tuneability is possible with future ligand iterations.

Automated summary

Two ligands based on a 2,6-disubstituted pyridine bridge were synthesized to introduce bis-quinoxalinyl units in iridium(iii) luminescent complexes. The emission features of the complexes were found to be influenced by both metal-to-ligand charge transfer and intra-ligand charge transfer, with emission wavelength tunable by substituents on the quinoxaline ring.