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

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 D-3 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 (MC)-M-3 resulting in a low emission quantum yield and a large chloride ion photosubstitution quantum yield.