Photoluminescence electron transfer quenching of ruthenium(II)-polypyridyl complexes with biologically important phenolate ions in aqueous acetonitrile solution

Abstract

involve the oxidation of phenols. Phenoxyl-type radicals are involved in biological redox processes and in the biosynthesis of natural products. The conversion of phenol to phenoxyl radical is of interest to chemists because of its involvement in important biological and industrial processes and in the degradation of chemical waste [8][9][10][11]. The one electron oxidation of phenolate to the resulting phenoxyl radical is a key step in the oxidation of phenols. The generation of phenoxyl radical involves either H-atom or electron abstraction from phenol and phenolate ion, respectively. The study of the kinetic and thermodynamic aspects of electron transfer (ET) to generate phenoxyl radicals bearing bulky groups in the ortho-and para-positions may help to understand the different biological roles of phenols [12][13][14][15].The oxidation potential of a polyphenol provides an estimate of the energy required to donate an electron; the lower the oxidation potential, lesser the energy required to donate an electron, hence it undergoes oxidation easily. The mechanism of oxidation of polyphenols and their stability in solution depend on pH [16,17]. The oxidation potential decreases with an increase in the number of phenolic -OH groups [18].Ruthenium polypyridyl complexes, such as [Ru(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine), are among the most investigated in fields that include solar energy conversion [19,20], artificial photosynthesis [21], optical sensing [22,23], and luminescent probes for characterizing microheterogeneous environments, owing to their favorable photophysical properties, excited state reactivity, and chemical stability [24,25]. The excited state properties like emission lifetime, quantum yield, wavelength of emission maximum and redox potential of [Ru(bpy) 3 ] 2+ are largely affected by the introduction of electron-donating and electron-withdrawing groups in the 4,4′-position of 2,2′-bipyridine [26]. AbstractThe photoinduced electron transfer reactions of three Ru(II) complexes with phenolate ions of polyphenols (gallic acid, quercetin, p-coumaric acid, and ferulic acid) and thymol have been measured in 50 % aqueous acetonitrile at pH 11 and the observed quenching constant (k q ) values are sensitive to the nature of the ligand and the structure of the phenolate ion. The change of k q values with ΔG 0 is in accordance with the Marcus semiclassical theory of electron transfer. The static as well as dynamic nature of quenching is confirmed from the ground-state absorption studies. The reductive quenching of the Ru(II) complexes by the phenolate ions has been confirmed from the transient absorption spectra. The formation of phenoxyl radical as a transient is confirmed by its characteristic absorption at 400 nm.