Cucurbit[10]uril binding of heteroleptic iridium(iii) complexes: synthesis and photophysical characterization

The supramolecular host-guest interaction between heteroleptic iridium(iii) complexes and cucurbit[10]uril (Q[10]) in an aqueous medium was investigated in this work. Both studied iridium complexes, [Ir(ppy)₂(bpy-(CHO)₂)]⁺ (complex 1) and [Ir(ppy)₂(bpy-(COOH)₂)]⁺ (complex 2), possessed two phenylpyridine ligands and a single R-bipyridine ligand. The formation of the encapsulated species (Q[10]·1 and Q[10]·2) was demonstrated by ¹H NMR and luminescence studies. A significant improvement was observed in the luminescence properties of both iridium complexes (emission intensity, quantum yield and lifetime) upon the addition of Q[10] in an aqueous medium. Results suggested a major effect of the hydrophobic cavity in the destabilization of the ³MLCT state (lowest excited state) of the iridium complexes. The binding study of both complexes with Q[10] revealed the formation of 1 : 1 and 1 : 2 host-guest species, with the binary complex dominating the emission behavior. The equilibrium between the emitting species was significantly influenced by the temperature, wherein the 1 : 1 inclusion complex was less favorable at elevated temperatures. The effect of pH on the emission profiles of the free and encapsulated iridium complexes was also investigated in this study. Density functional theory (DFT) calculations showed that introducing different substituent groups (CHO and COOH) on the bpy ligand had a negligible effect on the orientation of these complexes within the Q[10] cavity.