Pulse radiolysis of methyl viologen in aqueous solutions

Abstract

Pulse radiolysis of air-free aqueous methyl viologen (MV²⁺) solutions was carried out at various pH. The attack of e^–_aq on MV²⁺, with k(e^–_aq+ MV²⁺)= 7.5 × 10¹⁰ dm³ mol^–1 s^–1, leads to the formation of the long-lived radical cation (MV^˙+), which possesses two absorption maxima at 392.5 nm (ε_392.5= 4200 m² mol^–1) and 600 nm (ε₆₀₀= 1450 m² mol^–1). The H-atoms react with MV²⁺ at pH 1 forming two species, which have superimposed absorption bands. By means of a computer simulation they are resolved in the absorptions belonging to: (1) a protonated form of the radical cation (MV^˙+H⁺), which is produced with k(H + MV²⁺)=(3.5 ± 0.2)× 10⁸ dm³ mol^–1 s^–1, has 2 absorption maxima at 390 nm (ε₃₉₀= 1700 m² mol^–1) and 595 nm (ε₅₉₅= 760 m² mol^–1) and decays by second-order kinetics with k= 3.5 × 10⁹ dm³ mol^–1 s^–1; (2) an H-adduct (MV^˙2+H) on the ring carbon, which is formed with k(H + MV²⁺)= 2.5 × 10⁸ dm³ mol^–1 s^–1, absorbs at 310 nm (ε₃₁₀= 900 m² mol^–1) and 470 nm (ε₄₇₀= 630 m² mol^–1) and decays by conversion into MV^˙+H⁺ in a first-order process with k= 6 × 10³ s^–1. For the equilibrium MV^˙+H⁺⇌ MV^˙++ H⁺ pK= 2.9 ± 0.1 was determined. The presented data explain, at least partly, the instability of MV²⁺ when used as an electron acceptor in various devices for the utilization of solar energy.