Formation and decay of the methylviologen radical cation in the solid phase

Absorption and emission spectra of the methylviologen (MV²⁺)/tris (2,2'-bipyridine) ruthenium(II)[Ru(bpy)²⁺₃ system in a variety of solid systems—ethylene glycol-water glass, polymer foil, cellulose, and wool—have been studied at 4.2, 77, and 298 K. It was found that MV²⁺ always quenches the luminescence of $\text{∗}\text{Ru(bpy)}{}^{\mathrm{2+}}{}_3$. However, the MV^+. radical cation, which is generated at 298 K as a result of a quenching process, is not generated at 77 and 4.2 K. Lowering of the temperature probably prevents the diffusion of electron donor to the oxidized form of photosensitizer and enables instantaneous back reaction. The formation of the MV^+. radical cation under the influence of light λ > 300 nm at low and ambient temperatures has been studied. The crucial role of water in the oxidation of MV^+. was confirmed in a pulse radiolysis experiment.