Towards a spectroscopic and structural understanding of indigo: Dye widely used during the ancient and Islamic periods

Indigo is one of the most famous and important dyes and is found in different heritage materials such as carpets, textiles, artworks, manuscripts, etc. However, until now, scientists never understood its spectroscopic behavior in heritage materials. Indeed, some bands or peaks undergo important shifts and others sometimes disappear. Thus, our objectives consist of multi-technical fingerprinting of this very important dye and elucidating its spectroscopic behavior. To achieve our objectives, several calculation methods have been applied and the obtained results are compared to77 experimental data. The correlation between the spectroscopic and structural properties revealed that: i) During the dyeing process, the 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) In heritage materials, the absence of indigo fluorescence is due most probably to the quenching phenomenon. iii) In the Raman spectrum, the shift in C = C vibration values (theoretical and experimental) is likely due to intermolecular interactions.