Photochemical generation and consequent stabilization of electron-transfer products on separate like-charged polyelectrolytes

The mechanism of the reactions taking place in a photoredox system which exhibits extremely long lifetimes of redox products was investigated using photochemical, electrochemical, and radiation chemical methods. The system contains a tris(2,2′-bipyridine)ruthenium(II) derivative anchored to a positive poly-3,3-ionene polyelectrolyte (P3,3-Ru(bpy)₃²⁺) as a photosensitizer, 4-methoxy-N,N-dimethylaniline (4-MeODMA) as a quencher and a N,N,N′,N′-tetramethyl-p-phenylenediamine derivative covalently bound to another poly-3,3-ionene polyelectrolyte (P3,3-TMPD) as a secondary donor. Synthesis and characterization of these species are described in this work. The lowest emitting excited state of P3,3-Ru(bpy)₃²⁺ was found to possess an emission maximum at 618 nm and a lifetime of 502 ns. Stern-Volmer plots indicate that it is quenched by 4-MeODMA with a rate constant of (1.1 ± 0.1) × 10⁹ M^-1 s^-1. The yield and back reaction of the electron-transfer products of this process were followed using laser flash photolysis while the electron transfer scavenging of the 4-MeODMA^•+ product by P3,3-TMPD was studied using the pulse radiolysis method. The lifetimes of the two ultimate photoredox products, P3,3-Ru(bpy)₃⁺ and P3,3-TMPD⁺, were found to be in the time range of minutes under our experimental conditions corresponding to a factor of inhibition of their back reaction of more than 5 orders of magnitude as compared to the corresponding reaction of their monomer analogues in acetonitrile solution. This reaction appears to occur via reoxidation of a small fraction of 4-MeODMA.