Molecular design of a proton-induced molecular switch based on rod-shaped Ru dinuclear complexes with bis-tridentate 2,6-bis(benzimidazol-2-yl)pyridine derivatives

Abstract

New dinuclear Ru complexes of bis-tridentate 2,6-bis(benzimidazol-2-yl)pyridine derivatives, [Ru₂(terpy)₂(H4Ln)]⁴⁺ (terpy = 2,2′:6′,2″-terpyridine, n = 0∼2), have been synthesized. The Ru complexes act as tetrabasic acids, in which N–H protons on benzimidazole moieties are responsible for a deprotonation site. Both the absorption spectra and oxidation potentials are strongly dependent on the solution pH, which leads to the basis of a proton-induced molecular switch. The dinuclear Ru complexes bridged by bis-tridentate bis{2,6-bis(benzimidazol-2-yl)pyridine} show a lower Ru(II/III) oxidation potential but almost similar MLCT absorption maxima, compared to the corresponding dinuclear Ru complexes with “back-to-back” bis-2,2′:6′,2″-terpyridine bridging ligands. These results indicate that the bis-tridentate bis{2,6-bis(benzimidazol-2-yl)pyridine} ligand has a stronger σ/π donor property and a weaker π-acceptor property than the bis-2,2′:6′,2″-terpyridine bridging ligand. The solubility of Ru complexes in solution is progressively decreased with increasing number of phenyl group in the bridging ligand, and therefore it becomes difficult to study the change of chemical properties for external stimuli such as pH change. Immobilization of complexes on a solid surface is one of the approaches to overcome their low solubility. The [Ru₂(bpbbip)₂(H4L0)]⁴⁺ complex with phosphonate groups (bpbbip = 2,6-bis(1-(4-diphosphonyl)butylbenzimidazol-2-yl)pyridine) was successfully immobilized on an ITO electrode and characterized by means of XPS, and cyclic voltammetry. The Ru complex monolayers exhibit a reversible Ru(II/III) oxidation at +0.80 V vs. Ag/AgCl in 0.1 M aqueous HClO₄. The immobilized Ru complex monolayer is stable over the pH range 1 < pH < 10. The oxidation potential, E_½, vs. pH plot reveals several lines, indicating that the proton-coupled oxidative reactions occur on the ITO surface. The phosphonate-immobilized Ru dinuclear complex monolayers exhibited a stable electrochromic response on an ITO electrode.