Triplet-mediated photoreduction of viologens by aromatic hydrocarbons. A laser flash-photolysis study

Abstract

Using laser flash photolysis the electron-transfer (e.t.) reactions involving a number of aromatic hydrocarbon triplets (donors) and several bipyridinium and pyridinium salts (acceptors) have been examined in terms of redox quenching rate constants (k_e.t.), e.t.-derived photoproduct yields (η) and rate constants (k_b) for back e.t. reactions among the photoproducts. The observed values for k_e.t., η and k_b with paraquat as the quencher in a 1:9 water + acetonitrile mixture are typically in the ranges (5–10)× 10⁹ dm³ mol^–1 s^–1, 0.7–1.0 and (2–4)× 10⁹ dm³ mol^–1 s^–1, respectively. In both methanol and 1:9 water + acetonitrile the transient absorption spectra of primary photoproducts show the formation of radical cations of aromatic hydrocarbons in addition to the one-electron-reduction products of viologens. For free-energy changes (ΔG) of triplet-mediated electron transfer ranging from 17 to –96 kJ mol^–1, plots of log (k_e.t.) against redox potentials of viologen acceptors or triplet hydrocarbon donors are found to bend at small |ΔG| values with slopes estimated at 8–10 V^–1 in the bending regions; there is no indication of the bending of the plots at very large, negative ΔG values (up to –96 kJ mol^–1).