Cyclometalated Ir(III) complexes containing N-aryl picolinamide ancillary ligands

Abstract

The reaction of the cyclometalated chloro-bridged iridium(III) dimer, [(ppy) 2 Ir(μ-Cl)] 2 (ppy – 2-phenyl pyridine) with N-aryl picolinamides (LH, LH–NO 2, LH–CH 3, LH–l, LH–F) resulted in the formation of neutral heteroleptic complexes [Ir(ppy) 2L] ( 1), [Ir(ppy) 2L–NO 2]( 2), [Ir(ppy) 2L–CH 3]( 3), [Ir(ppy) 2L–Cl]( 4) and [Ir(ppy) 2L–F] ( 5). These complexes contain a six-coordinate iridium with a 2C, 4N coordination environment. The N-aryl picolinamide ligands are deprotonated during complexation and the resulting amidates bind to iridium in a chelating manner ( N, N). Optical spectroscopic studies revealed that the complexes 1– 5 exhibited intense π→π ∗ absorptions in the ultraviolet region. In addition low energy transitions due to 1MLCT, 1LLCT and 3MLCT are also seen. The emission spectra of 1– 5, upon excitation at 450 nm, show a single emission with a λ max around 513 nm. The lifetimes of this emission are in between 7.4 and 9.6 μs while the quantum yields are quite high and range from 0.2 to 0.5. Based on density functional theory (DFT) calculations on 1 and 3, the three highest occupied orbitals are composed of ligand π orbitals mixed with Ir-d orbitals while the three lowest unoccupied orbitals are mostly π orbitals of the ligands. From the time dependent DFT calculations it is revealed that the lowest energy electronic singlet and triplet excitations are a mixture of MLCT and LLCT.