Ultraviolet–visible and electron paramagnetic resonance spectroelectrochemical studies of the reduction products of some anthraquinone sulphonates in aqueous solutions

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Abstract
The reductions of a range of anthraquinone sulphonates have been studied by cyclic voltammetry using gold and platinum electrodes. Reductions of the 2-sulphonates in tetraethylammonium hydroxide solutions exhibit two distinct one-electron peaks in the voltammograms in contrast to the single peaks observed from alkali-metal hydroxide solutions. Discrete UV–VIS absorption spectra of the radical anion and quinolate dianion of the 2-sulphonates have been recorded from tetraethylammonium hydroxide solutions using an optically transparent thin-layer electrode (gold minigrid) cell. Studies at different pH values have shown that the most prominent absorption of the 2-sulphonate dianions shows characteristic shifts to shorter wavelengths with each stage of protonation. Observation of the EPR spectra of the radical anion species, electrogenerated in a thin-layer cell, confirms their UV–VIS spectral assignments. The resulting hyperfine splitting constants were assigned to specific proton positions using the additivity principle and the simple Hückel method.