Tautomerism of anthraquinones: IX. Protonated 1,5- and 1,8- dihydroxyanthraquinones

Fine structure of πl,π absorption of mono- and dications derived from 1,5- and 1,8-dihydroxyanthraquinones originates from their existence as dynamic equilibrium mixtures of isomers differing by position of the positive charge, double bond distribution, and number of intramolecular hydrogen bonds. Protonation is accompanied by displacement of isomeric equilibria. Isomeric transformations of protonated dihydroxyanthraquinones involve mainly excited states of their molecules. © 2009 Pleiades Publishing, Ltd.