Towards a spectroscopic and structural understanding of indigo: dye widely used during the historical periods

Indigo is one of the most famous and important dyes and is found in different heritage materials such as carpets, painting, manuscripts, etc. However, its spectroscopic behavior in heritage materials hasn’t been fully understood until now. Thus, our objectives consist of multi-technical fingerprinting of indigo and elucidating its spectroscopic behavior. To achieve our objectives, several calculation methods were applied and the obtained results are compared to experimental data. The correlation between the spectroscopic and structural properties revealed that: i) Indigo molecule would undergo trans-cis photoisomerization thereby causing the rupture of the two intramolecular hydrogen bonds, followed by a dimerization process occurring through the establishment of two new intermolecular hydrogen bonds connecting the two monomers; ii) the weakness of indigo fluorescence is due probably to the intermolecular hydrogen bonding interaction; iii) in the Raman spectrum, the shift in C = C vibration theoretical and experimental values is likely due to intermolecular interactions.