Anodic oxidation mechanism of a spiropyran

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Abstract
The modulation of chemical events using light is attractive for applications in many technologies including displays, information storage and sensors. Spiropyrans are the most well studied of the technologically relevant photochromic compounds. Their oxidation is important from at least two perspectives: oxidative failure of spiropyrans limits their lifetime, and electrochemical reactions can be expected to alter their photochemical properties leading to new uses. Thus, we have undertaken the first study of the oxidation of a spiropyran. Spiropyrans contain an aniline-like moiety so they are expected to be oxidizable. We have chosen a particular compound for detailed investigation, 6-hydroxy-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline]2. A 6-hydroxy functionalization creates a hydroquinone analogue, which will lead to the possibility of reversible electrochemistry. Cyclic voltammetry of 2 at a glassy carbon electrode in acetonitrile shows two anodic waves with a single cathodic wave on the reverse sweep. The first one-electron wave is due to the oxidation of the indoline moiety. This radical cation converts to a semiquinone radical which disproportionates, leading to a quinone and a hydroquinone. The hydroquinone is oxidized to the quinone at higher potentials leading to the second oxidation wave. Reduction of the quinone to the hydroquinone gives the reduction wave. Bulk electrolysis, 1H NMR, 13C NMR, chronoamperometry, UV–VIS spectroscopy and chemical experiments support the proposed mechanism. The electrochemistry of these compounds is compared to the simpler electrochemistry of another class of photochromics: the diphenylchromenes.

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