Tetranuclear Ru Complex Linked via Redox-Active Azobis(bipyridine) Ligands as a Redox-Responsive Photoswitch

A tetranuclear Ru complex linked by 4,4"-azobis(2,2ʹ-bipyridine) (azobpy), [Ru{(azobpy)Ru(bpy)2}3][PF6]8 (bpy = 2,2ʹ-bipyridine) has been prepared and fully characterized. Cyclic and differential pulse voltammetries show that two kinds of oxidation occur, the one at the less positive potential being assigned to the oxidation of the three peripheral Ru ions, while the other to that of the central Ru ion. On a cathodic scan, a pair of three-electron (one electron on each azobpy) reductions of azobpy ligands appear at less negative potentials than terminal bpy ligands. A low-energy metal-to-ligand charge transfer (MLCT) absorption band involving the (pi)*orbitals of azobpy ligands is observed in addition to the usual MLCT band involving terminal bpy ligands. These results indicate the presence of a low-lying (pi)* level of azobpy. A selective reduction of azobpy ligands renders the non-luminescent complex into a moderately luminescent species. This light switch is reversible, implying the utility of this branched structure in energy delivering molecular systems.