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

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 C(2)h 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, Laponite and montmorillonite that, however, do not present the chemical stability observed for MB. Resonance Raman, UV-vis absorption spectroscopy, and thermal analysis provided firm evidences that in ind-paly, the dye is inside the micropores, interacting through hydrogen bonding with water molecules coordinated with the metal ions of the clay framework.