Raman spectrum of monobromoindigo

Monobromoindigo is a component of Tyrian purple, a purple–red natural colorant extracted from various species of sea snails, which was possibly first produced by the ancient Phoenicians and has been employed as a symbol of royalty and power by several civilizations over the centuries. Raman spectroscopy has proved to be an effective analytical technique to detect historical dyes, as it allows rapid and accurate identification of unknowns in a nondestructive way. Although other constituents of Tyrian purple have been comprehensively investigated by Raman spectroscopy in the past years, monobromoindigo, a molecule that has been correlated with a specific snail species, Hexaplex trunculus (also known as Murex trunculus), has never been investigated by this technique. In the present work, we provide a complete Raman spectral characterization of monobromoindigo, including experimental spectra recorded at 488 and 785 nm, which were compared with those collected from indigo in the same conditions. Moreover, theoretical Raman spectra for both molecules were obtained using density functional theory calculations, and a detailed assignment of the Raman lines is presented.