Synthesis, Photophysical and Electrochemical Studies on Tryptanthrin and Indigo Derivatives. Historic Molecules with Modern Applications, by Daniela Pinheiro, within the ALBA-FCT joint collaboration.

In the past few years, tryptanthrin and indigo derivatives have gained renewed interest due to applications in several fields such as photoswitches, electronic devices, sensors and rechargeable batteries.

Indigo, an iconic molecule of colour, is included in the most ancient and important natural dyes used by mankind. The longevity as a colourant and unique properties of this compound are related with its high (photo)stability which is linked to a rapid proton transfer in the excited state (ESPT). Excited state proton transfer (ESPT) is one of the more important reactions in chemistry and, in the case of indigo derivatives, it has motivated several studies on the origin of the decay mechanisms.

Tryptanthrin is a natural compound found in several natural sources and with several applications that can also be synthetically obtained from indigo. Among the interesting properties of tryptanthrin and derivatives, their redox properties make them suitable candidates for application in redox flow batteries (RFBs).

The study and development of an improved and fast approach for the synthesis of this molecules combined with a comprehensive characterization of their electronic spectral and photophysical properties in different solvents and temperatures was undertaken and led to renewal insights into the photo(stability), photoisomerization and deactivation pathways of these amazing historical molecules. A pratical and modern application of storage of chemical energy with, an aqueous redox flow battery (RFB) with sulfonated tryptanthrin was also undertaken and paves the way to the promising development of new tryptanthrin based structures for environmentally friendly aqueous all-organic RFBs.

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