The molecules of colour. The real blue indigo: photostability mechanisms, new functional derivatives, and hybrids

Indigo is in the origin of the first blue mankind used for various purposes. Together with the inorganic Prussian Blue, it constituted, for centuries, the favourite source of blue for a plethora of applications. In the past recent years this molecule of colour has underlined more its epithet of timeless molecule, with new findings both in its excited state decay mechanisms and potential applications. This covers research fields such as excited state proton transfer, linked to its photostability, but also, when proton transfer is blocked (e.g., by N,N′-substitution), to the development of new molecular photoswitches, incorporation in low band gap conjugated organic polymers (with potential applications in organic solar cells and as NIR detectors), and to its combination with inorganic guests (as in Maya Blue) in the development of new organic–inorganic hybrids (OIH). New perspectives opened by this molecule will be reviewed in this contribution.