Chromophore-Specific Quenching of Ruthenium Trisbipyridine−Arene Bichromophores by Methyl Viologen

Abstract

The quenching (ηq) and subsequent cage-escape efficiency (ηce) have been measured for three arene-linked ruthenium trisbipyridine complexes quenched by methyl viologen (MV2+). The bichromophoric complexes are of the type [Ru(bpy)2(4-methyl-4‘-(2-arylethyl)-2,2‘-bipyridine)]2+(ClO4-)2 where aryl = 2-naphthyl ([Ru]-naphthalene), 1-pyrenyl ([Ru]-pyrene), and 9-anthryl ([Ru]-anthracene). The overall yield of MV•+ is given by the product ηqηce, which depends both on specific solvent effects and on the nature of the quenched excited state of the bichromophore, i.e., whether it is a metal-to-ligand charge transfer (3MLCT) or aromatic triplet state. In aqueous buffer the production of MV•+ is low for the three bichromophores (<10%). In methanol and acetonitrile MV•+ yields for [Ru]-anthracene and [Ru]-pyrene are >70%, reflecting the arene triplet character of the lowest excited state. This increase is due to an increase in the cage-escape efficiency (ηce) in these solvents. In contrast, for [Ru]-naphthalene the lowest excited state is 3MLCT in character and the yield of MV•+ in acetonitrile and methanol remains <10%. NMR spectra of the linking ethane group suggest that the bichromophores adopt different conformations in the different solvents, which could lead to the observed differences in ηce.