Scheme 1. Donor–acceptor Stenhouse adducts based on barbituric acid. These molecules can be isomerised between a coloured linear form (a) and a colourless cyclic form (b) with visible light. Where unsymmetrical amines are used, two major linear isomers (a, a′) are present in solution with the most abundant isomer (a) having the smaller substituent in the R position. The cyclic isomer exists as either the zwitterionic enolate form (b), or the keto form (b′), depending on the amine.