Cyclometalated Ir(iii) complexes of deprotonated N-methylbipyridinium ligands: effects of quaternised N centre position on luminescence

Abstract

The optical emission behaviour of tricationic Ir ^III complexes depends markedly on the position of the N-methyl unit in cyclometalating ligands.

Abstract

Six new tricationic Ir ^III complexes of cyclometalating ligands derived from 1-methyl-2-(2′-pyridyl)pyridinium or 1-methyl-4-(2′-pyridyl)pyridinium are described. These complexes of the form [Ir ^III(C^N) ₂(N^N)] ³⁺ (C^N = cyclometalating ligand; N^N = α-diimine) have been isolated and characterised as their PF ₆ ⁻ and Cl ⁻ salts. Four of the PF ₆ ⁻ salts have been studied by X-ray crystallography, and structures have been obtained also for two complex salts containing MeCN and Cl ⁻ or two Cl ⁻ ligands instead of N^N. The influence of the position of the quaternised N atom in C^N and the substituents on N^N on the electronic/optical properties are compared with those of the analogous complexes where C^N derives from 1-methyl-3-(2′-pyridyl)pyridinium (B. J. Coe, et al., Dalton Trans., 2015, 44, 15420). Voltammetric studies reveal one irreversible oxidation and multiple reduction processes which are mostly reversible. The new complexes show intramolecular charge-transfer absorptions between 350 and 450 nm, and exhibit bright green luminescence, with λ _max values in the range 508–530 nm in both aqueous and acetonitrile solutions. In order to gain insights into the factors that govern the emission properties, density functional theory (DFT) and time-dependent DFT calculations have been carried out. The results confirm that the emission arises largely from triplet excited states of the C^N ligand ( ³LC), with some triplet metal-to-ligand charge-transfer ( ³MLCT) contributions.