Effect of acidity on the reactivity of the triplet state of 2-nitrothiophen

Abstract

Laser flash photolysis of aqueous acidic solutions of 2-nitrothiophen (NT) over a wide range of acidity has enabled the pK_a of the lowest excited triplet state of 2-nitrothiophen (³NT) to be determined as –0.95 ± 0.20. The effect of acidity on the reactivity of ³NT towards a number of one-electron donors has been examined. In particular, it was found that protonated ³NT (³NTH⁺) oxidizes chloride ion with a second-order rate constant, k₂, which is more than an order of magnitude higher than that measured for ³NT. For the quenching of ³NT by inorganic anions (e.g. NO^–₂, SO^2–₃) which protonate at lower acidities than ³NT the values of k₂ depend on the acid–base properties of the quencher. Finally, the second-order rate constants for the quenching of ³NT by metal ions in acidic media are generally lower than would be predicted on the basis of their thermodynamic potentials.