Theoretical and experimental investigation on the spectroscopic properties of indigo dye

The spectroscopic properties of indigo, one of the most important natural dyes present in nature, have been investigated by means of DFT and TD-DFT calculations and Raman and IR spectroscopies. The absorption spectra of this dye, in vacuo and in different solvents, have been computed. The formation of aggregates in solvent have been investigated by computing the electronic absorption spectra of the dimer and the trimer, thus evaluating the effects of the aggregation on the optical properties of indigo.The IR and Raman spectra have been measured and computed. The comparison between the experimental and theoretical spectra and the potential energy distribution (PED) of the computed normal modes have been used to perform the assignment of the experimental features in terms of functional group displacements. Finally, the effects of the intermolecular hydrogen bond present in the solid state have been evaluated by computing the vibrational spectra of the dimer.The intention of the present work is to give an insight into both the vibrational and optical properties of indigo as well as to evaluate DFT and TD-DFT potentialities in the study of organic dyes’ spectroscopic properties of interest in the cultural heritage field.