Kinetic studies of the photoreduction of 4,4′-di-n-alkylbipyridylium salts by alkanols in aqueous solution

Abstract

The photoreduction of dilute (≤10^–3M) aqueous solutions of bipyridylium salts RNC₅H₄–C₅H₄NR (R = Me, Et, Prⁿ, Buⁿ, n-C₅H₁₁, n-C₆H₁₃, n-C₇H₁₅) by alcohols R¹R²CHOH (R¹= H, R²= Me, Et, Prⁿ, Buⁿ, and R¹= R²= Me) on irradiation with light of 254 nm wavelength is investigated. Triplet excited bipyridylium dication reacts initially with the alcohol to give an exciplex, which subsequently leads to cation radical formation via hydrogen atom transfer. Halide ions exert a strong quenching effect. The lengths of the substituent alkyl chains R (in the salt), and R²(in the alcohol) greatly influence the net quantum yield of photoreduction.