ANODIC-OXIDATION MECHANISM OF A SPIROPYRAN

The modulation of chemical events using light is attractive for applic ations in many technologies including displays, information storage an d sensors. Spiropyrans are the most well studied of the technologicall y relevant photochromic compounds. Their oxidation is important from a t least two perspectives: oxidative failure of spiropyrans Limits thei r lifetime, and electrochemical reactions can be expected to alter the ir photochemical properties leading to new uses. Thus, we have underta ken the first study of the oxidation of a spiropyran. Spiropyrans cont ain an aniline-like moiety so thay are expected to be oxidizable. We h ave chosen a particular compound for detailed investigation, ',3'-trim ethylspiro[2H-1-benzopyran-2,2'-indoline] 2. A 6-hydroxy functionaliza tion creates a hydroquinone analogue, which will lead to the possibili ty 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 du e to the oxidation of the indoline moiety. This radical cation convert s to a semiquinone radical which disproportionates, leading to a quino ne and a hydroquinone. The hydroquinone is oxidized to the quinone at higher potentials leading to the second oxidation wave. Reduction of t he quinone to the hydroquinone gives the reduction wave. Bulk electrol ysis, H-1 NMR, C-13 NMR, chronoamperometry, UV-VIS spectroscopy and ch emical experiments support the proposed mechanism. The electrochemistr y of these compounds is compared to the simpler electrochemistry of an other class of photochromics: the diphenylchromenes.