Visible light generation of Ru(III)-polypyridyl complexes and their reactivity toward s-alkyl- and s-aryl-L-cysteines: Reorganization energy favors s-aryl-L-cysteines

S-aryl-L-cysteines react faster with photochemically generated ruthenium (III)-polypyridyl ([Ru(NN)3]3 + ) complexes compared to s-alkyl-L-cysteines, however ∆G° value is in favor of s-alkyl-L-cysteines. The detection of a two-center three-electron bond in sulfur-sulfur dimer radical cation using nanosecond laser flash photolysis technique indicates that the reaction proceeds through the electron transfer (ET) mechanism. The transients observed and successful application of the semi-classical theory of ET to the reaction of [Ru(NN)]3+ complexes with s-alkyl-and s-aryl-L-cysteines support the ET mechanism. The photocatalytic oxidation of s-alkyl-and s-aryl-L-cysteines follows MLCT (metal to ligand charge transfer) from Ru(dπ) to ligand (π*) was confirmed by UV-Visible and DFT studies for [Ru(NN)3]3+ complexes and it provides sulfoxide as a major product. The reaction is strongly enthalpy controlled and the negative value of ΔS≠ indicates the compactness of the transition state. The experimental and theoretical data support the low inner-sphere reorganization energy accompanying s-aryl-L-cysteines facilitate faster ET reactions in this study.