Phototautomerism of triazolo-triazole scaffold

It is shown that 4-methyl-7-(4-hydroxyphenyl)-[1,2,4]-triazolo[3,2-c][1,2,4]triazole exhibits a rich photoinduced protolytic behavior: Förster cycle shows that the protonated nitrogen of the triazolo-triazole ring is a weak photoacid, with ?pKa?-3; furthermore, at moderately basic pH its deprotonated monoanion exhibits a long distance water mediated phototautomerism, in which the hydroxyl group releases a proton to solvent and a basic nitrogen of the triazolo-triazole fused ring, different from that protonated in the neutral species, is protonated by the solvent.