A photochemical study on the blue dye indigo: from solution to ancient Andean textiles

The degradation of indigo and its water soluble derivative indigo carmine was investigated under light excitation in the presence and absence of molecular oxygen in solution (homogeneous) and gels (heterogeneous) media. Collagen and carboxymethylcellulose (CMC) aqueous gels were chosen to simulate a natural textile environment, wool and cotton, respectively. Isatin was found to be the major degradation product of indigo. In solution, the photodegradation quantum yields (Phi(R)) were in the order of 10(-4), with the exception of aqueous media (Phi(R) = 9 x 10(-6)), and dependent on the irradiation wavelength. In the case of indigo carmine the Phi(R) values were found to suffer a 2-fold increase upon going from water to gels. The results indicate the absence of degradation products involving singlet oxygen and suggest peroxides, or other oxygen based radicals, to have a key role in the degradation of indigo. Finally, the relevance of the simulation is discussed by comparing the main degradation products to those found in the blues of millenary Andean textiles.