Physical, photophysical and structural properties of ruthenium(ii) complexes containing a tetradentate bipyridine ligand

Abstract

The focus of this report is the synthesis and properties of two new analogues of ruthenium(II) tris-bipyridine, a monomer and dimer. The complexes contain the ligand 6,6′-(ethan-1,2-diyl)bis-2,2′-bipyridine (O-bpy) which contains two bipyridine units bridged in the 6,6′ positions by an ethylene bridge. Crystal structures of the two complexes formulated as [Ru(bpy)(O-bpy)](PF₆)₂ and [(Ru(bpy)₂)₂(O-bpy)](PF₆)₄ reveal structures of lower symmetry than D₃ which affects the electronic properties of the complexes as substantiated by density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations. The HOMO lies largely on the ruthenium center; the LUMO spreads its electron density over the bipyridine units, but not equally in the mixed O-bpy–bpy complexes. Calculated Vis/UV spectra using TDDFT methods agree with experimental spectra. The lowest lying triplet excited state for [Ru(bpy)(O-bpy)](PF₆)₂ is ³MC resulting in a low emission quantum yield and a large chloride ion photosubstitution quantum yield.