Role of Substitution on the Photophysical Properties of 5,5′-Diary1-2,2′-bipyridine (bpy*) in [Ir(ppy)2(bpy*)PF6 Complexes: A Combined Experimental and Theoretical Study

The synthesis of a family of 4'-functionalized 5,5'-diaryl-2,2'-bipyridines (bpy*; 6a-6g) is reported. These ligands were reacted with the dimer [(ppy)(2)IrCl](2) (ppyH = 2-phenylpyridine) and afforded, after subsequent counterion exchange, a new series of luminescent cationic heteroleptic iridium(III) complexes, [(ppy)(2)Ir(bpy*)]PF6 (8a-8g). These complexes were characterized by electrochemical and spectroscopic methods. The crystal structures of two of these complexes (8a and 8g) are reported. All of the complexes except for 8c and 8f exhibit intense and long-lived emission in both 2-MeTHF and ACN at 77 K and room temperature. The origin of this emission has been assigned by computational modeling to be an admixture of ligand-to-ligand charge-transfer [(LLCT)-L-3; pi(ppy) -> pi*(bpy*)] and metal-to-ligand charge-transfer [(MLCT)-M-3; d pi(Ir) -> pi*(bpy*)] excited states that are primarily composed of the former. The luminescent properties for 8a-8c are dependent upon the functionalization at the 4' position of the aryl substituents affixed to the diimine ligand, while those for 8d-8g are essentially independent because of an electronic decoupling of the aryls and bpy due to the substitution of o,o-dimethyl groups on the aryls, causing a near 900 angle between the aryl and bipyridyl moieties. A combined density functional theory (DFT)/time-dependent DFT study was conducted in order to understand the origin of the transitions in the absorption and emission spectra and to predict accurately emission energies for these complexes.