Dye-sensitized chemiluminescence of luminol and related cyclic hydrazides

Triplet state methylene blue and eosin Y are potent oxidants which initiate the chemiluminescence (CL) of luminol and related phthalhydrazides. Reductive quenching of the triplet dyes results in the formation of phthalhydrazine radicals and semireduced dye radicals. The efficiency of the radical production is affected by the following factors: the formation of a complex between the ground-state dye and phthalhydrazide and the deprotonation of the geminate radical pairs by OH − . The dye-sensitizer catalytic cycle includes photogeneration of the triplet dye molecules, their reductive quenching by the phthalhydrazide, and the sensitizer recovery in the reaction of the semireduced dye with oxygen. The latter reaction is also a source of superoxide radicals. Recombination of the phthalhydrazide radicals with superoxide leads to CL. The side reaction of singlet oxygen with phthalhydrazide produces an inhibitor, which acts as a non-stoichiometric scavenger of superoxide and/or phthalhydrazide radicals.