The molecules of colour. New structural derivatives from indigo: tryptanthrin and indirubin

Abstract

Two compounds, which are structurally related to indigo, tryptanthrin and indirubin, are here described in the context of recent findings in this new chapter of the Molecules of Colour. The two are naturally occurring compounds, with diversified applications. Indirubin, the red isomer of the blue indigo, shows, in comparison with indigo, a more efficient radiationless decay process, whose mechanism has been recently detailed and constitutes a step forward in the complexity of indigo’s deactivation and associated photostability. In contrast with indigo, tryptanthrin, which can be synthetically obtained from the former, has decay processes with efficient triplet state formation and singlet oxygen sensitization. Tryptanthrin, with its synthetically modified water-soluble compound, sulfonated-tryptanthrin, has also been shown to be electrochemically stable and a viable active organic material for eco-friendly all-organic redox flow batteries (AORFB). This new application of tryptanthrin, in an emerging source of energy power storage (RFB), is also discussed here.