Interactions of formate ion with triplets of anthraquinone-2-sulfonate, 1,4-naphthoquinone, benzophenone-4-carboxylate, and benzophenone-4-sulfonate

The interaction of formate with the triplet states of naphthoquinone (NQ), anthraquinone-2-sulfonate (AQS), benzophenone-4-carboxylate (BC), and benzophenone-4-sulfonate (BS) was studied by laser flash photolysis. Rate constants were determined either by direct measurement of triplet lifetimes or by inhibition of product yields by competitive reactants. Radical products are formed in two stages, direct reduction by formate and efficient secondary reduction by initially formed CO₂^•- radicals. The quinones react by electron transfer with quenching rate constants k_q(NQ) = 3 × 10⁹ and k_q(AQS) = 4 × 10⁸ M^-1 s^-1, giving anion radicals with primary yields of Φ_R(NQ) ∼ 0.7 and Φ_R(AQS) ∼ 0.3. Formic acid quenches ³AQS much more slowly. The less strongly oxidizing ketone triplets react by H-atom abstraction, k_q(BC) = 1.3 × 10⁷ and k_q(BS) = 5.3 × 10⁷ M^-1 s^-1, giving protonated ketyl radicals with primary yields Φ_R ∼ 0.7. Photoreduction of BC exhibits a deuterium isotope effect, k_H/k_D = 1.6, whereas AQS shows none. A new, short-lived transient, E, is observed in the ³AQS-formate reaction, which may be an exciplex or adduct. The redox potential, E°′(NQ/NQ^•-) = -0.12 V, and rate constants for radical reactions of NQ and O₂ were measured by pulse radiolysis. The results are discussed in terms of pertinent redox potentials, bond strengths, and the nature of the exciplex intermediates.