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

摘要

New dinuclear Ru complexes of bis-tridentate 2,6-bis(benzimidazol-2-yl)pyridine drivatives,[Ru_2(terpy)_2(H4Ln)]~4+ (terpy=2,2':6',2"-terpyridine,n=0 approx 2),have been synthesiaed.The Ru complexes act as tetrabsic acids,in which N-H protons on bezimidazole moieties are responsible for a deprotonation site.Both the absorption spectra and oxidation potentials are strongly dependent on teh solution pH,whihc 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 absorptionmaxima,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 sigma/pi donor property and a weaker pi-acceptor property than the bis-2,2':6',2"-terpyridine bridging ligand.The solubility of Ru complexes in solution is progressively decreased withincreasing 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 overcoem their low solublity.The [Ru_2(bpbbip)_2~- {H4L0)]~4+ complex with phosphonate goups (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 +0.80V vs.Ag/AgCl in 0.1 M aqueous HClO_4.The imobilized Ru complex monolayer is stable ove the pH range 1H<10.The oxidation potential,E_1/2,vs.pH plot reveals several lines,indicating that the proton-coupled oxidative reactions ocur on the ITO surface.The phosphonate-immobilized Ru dinuclear complex monolayers exhibitd a stable electrochromic response on an ITO electrode.