Pyridinium Quenchers of Ru(bpy)32+*. Charge Effects of the Yield of Electron Transfer

Article abstract of DOI:10.1021/jo00350a068

The quenching of the lumunescent state of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)3²⁺) by a series of pyridinium ions has been studied. 4-Acetyl-, 4-cyano-, 4-carbomethoxy-, or 4-carboxy-1-methylpyridinium, along with the well-known electron-transfer agent methyl viologen (MV²⁺), were employed as electron-acceptor quenchers in order to reveal the effects of charge type on the yield of photoinduced electron transfer involving the Ru(II) luminescent state as electron donor.Rates of quenching by the pyridinium ions were measured by using steady irradiation techniques and compared with expectations based on the calculated energetics of electron transfer.Electron transfer yields were obtained by measurement of the transient absorbances of photogenerated radicals (e.g., pyridinyls) using conventional flash photolysis (broad band visible excitation, pH 5, μ = 0.5).The pyridiniums were less effective electron relays for photocleavage of water in terms of the rate of hydrogen production using the Ru(II) complex as sensitizer, the selected acceptor quenchers, the sacrificial relay, EDTA, and a colloidal platinum catalyst at pH = 0.5.The reduced yield of electron transfer (and photoformation of hydrogen) for quenching by pyridinium ions was consistent with an effect of electrostatic charge in the nascent radical pair produced on electron transfer quenching of Ru(bpy)3^2+* by viologens/pyridiniums.The transient photobleaching of Ru(bpy)3²⁺ by 4-carboxy-1-methylpyridinium was studied by using a Nd:YAG laser with results showing pH control of the charge type and yield of net electron transfer due to in-cage protonation of photogenerated geminate radical pairs.The electrochemical properties of the pyridinium ions were also examined by cyclic voltammetry and a Hammett correlation was made of the reduction potentials.