One-electron reduction of 2,1,3-benzothiadiazole-4,7-dicarbonitrile in aqueous solution

Abstract

Some of the electrochemical properties of 2,1,3-benzothiadiazole-4,7-dicarbonitrile (BTDN) have been studied at various pH values by pulse radiolysis and cyclic voltammetry. The optical absorption spectrum of the one-electron-reduced species, BTDN^–˙, exhibits absorption maxima at 360, 460, and 500, 630, and 680 nm. The spectrum is independent of pH in the range 2.6–7.0, indicating a low pK_a for BTDN^–˙. This finding is supported by ab initio calculations on the anion radical species. The e.s.r. spectrum of BTDN^–˙ can be interpreted as arising from a radical with a^H 0.24, a^NA 0.39, and a^NB 0.08 mT (NA and NB are the ring and cyano nitrogen atoms, respectively). The one-electron reduction potential for the equilibrium BTDN + e^–⇌ BTDN^–˙ has been measured as –506 ± 10 and –490 ± 10 mV by cyclic voltammetry and pulse radiolysis, respectively. In the presence of O₂, BTDN^–˙, the natural lifetime of which is 10 s at pH 7.0, decays with a rate constant of (1.5 ± 0.2)× 10⁸ dm³ mol^–1 s^–1.