The molecules of colour

Indigo, alizarin, mauveine, brazilin and brazilein (the latter two being major colour constituents of Brazilwood) are molecules that have had a strong impact in our civilization. The colour and longevity of these molecules is linked to their photostability, which have different origins. In this contribution the photochemistry and photophysics of these and other molecules will be reviewed, and linked with relevant issues such as the photostability, photoisomerization, mechanisms of photodegradation, the origin of the colour, and the incorporation in different matrices. In some cases, stability is associated with excited state proton transfer, while in the case of indigo it also forms Maya Blue, the pigment of the ancient Central American civilizations, with the clay palygorskite.