Mechanistic details of formation of two indo dyes have been investigated using spectroelectrochemistry. These dyes absorb strongly in the visible region and are formed when a primary amine such as p-phenylene diamine (PPD) is initiated electrochemically to form the radical cation. The PPD radical cation quickly couples with another coexisting aromatic amine, viz., m-phenylene diamine (MPD) or p-amino-o-cresol (PAOC) to yield bright colored indo-amine or indo-cresol dyes. Unlike the conventional chemical oxidation methods, the electrochemical oxidation provides selectivity in oxidizing the desired aromatic amine (viz., PPD) and allows us to probe the pathways with which the bright colored dye is formed. Spectroelectrochemical studies that elucidate the indo dye formation and its dependence on the concentration of the color coupler are described. These in situ studies probe the reactivity of aromatic amines in self-coupling and cross-coupling reactions. The electrochemical approach described in this study has also enabled us to synthesize pure form of these dyes.
Dr. Zeena S. Pillai and Prashant V. Kamat, “Spectroelectrochemistry of aromatic amine oxidation: an insight into the indo dye formation”, Res Chem Intermediat , vol. Volume 31, no. Issue 1, 2005.