Probing the Indigo Molecule in Maya Blue Simulants with Resonance Raman Spectroscopy

Abstract

Maya blue (MB) simulants were prepared from mixtures of indigo and palygorskite (ind–paly) and investigated by resonance Raman and UV–vis absorption spectroscopies together with thermogravimetric analysis, aiming to enlarge the understanding of the dye–clay interaction, the relationship between color and chemical stability, and the alleged formation of dehydroindigo (DHI); for comparative purposes, other simulants were prepared using sepiolite, laponite, and montmorillonite. The results obtained here suggest that the greenish hue that develops when the ind–paly is heated seems to be linked with a decrease in the indigo molecular symmetry, which causes an increase in the oscillator strength of an absorption band at 500 nm (forbidden under the C2h symmetry) and a bathochromic shift and narrowing of the intense electronic transition at 657 nm (indigo). The DHI characteristic features are not observed in the resonance Raman spectrum (457.9 nm) of the ind–paly system, contrary to what happens with sepiolite, L...