Res. Chem. Intermed.
, Vol. 31, No. 1–3, pp. 103–112 (2005)
Also available online - www.vsppub.com
Spectroelectrochemistry of aromatic amine oxidation:
an insight into the indo dye formation
ZEENA S. PILLAI and PRASHANT V. KAMAT
Radiation Laboratory and Department of Chemical Engineering and Biomolecular Engineering,
University of Notre Dame, Notre Dame, IN 46556, USA
Received 15 May 2004; accepted 31 May 2004
Abstract—Mechanistic details of formation of two indo dyes have been investigated using spectro-
electrochemistry. 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 selec-
tivity 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
Keywords: Spectroelectrochemistry; indo dye; primary amine; aromatic amine; electrochemical
Popularity of permanent hair dyes has increased considerably in recent years [1, 2].
Three major ingredients, viz., a primary amine, color coupler and an oxidant
) are required for the formation of indo dye, which constitute a major
component of permanent hair dyes. Usually o-orp-diamino or aminohydroxy
benzenes are used as primary amines whose oxidation initiates the coupling
reaction. Color couplers, such as phenols, m-disubstituted phenylene diamines,
aminophenols and resorcinols, react with the oxidized form of primary amine to
produce indodyes having a wide range of colors.
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