Kinetics of the dye-sensitized photooxidation of trihydroxybenzenes

The kinetics of the singlet molecular oxygen [O-2((1)Delta(g))]-mediated photooxidation of the three isomeric trihydroxybenzenes (THBs), compounds of potential environmental significance as aquatic contaminants, has been studied in water solution as a function of pH and ionic strength, as well as in benzene and acetonitrile. Rate constants for chemical and overall interactions with O-2((1)Delta(g)), determined by time-resolved IR phosphorescence detection and polarographic methods, are in the range 0.05 x 10(7)-24.0x10(7) M-1 s(-1), depending on the medium and the particular compound. In water at pH 2, the photooxidation quantum efficiencies of 1,2,3-THB, 1,2,4-THB and 1,3,5-THB are 0.07, 0.07 and 0.19, respectively The photooxidative processes are highly favored by the ionization of the OH groups, the increase of solvent polarity, and the presence of salts in the medium, strongly suggesting the participation of a polar encounter complex of the type [THB-O-2((1)Delta(g))]. This kinetic behavior is similar to that observed in simple phenols and dihydroxybenzenes, although THBs are much easier photooxidizable, even in non-ionized form. These results possess environmental relevance, because they demonstrate that any THE in aqueous media undergoes spontaneous and fast solar-promoted photooxidation under practically any field condition. (C) 2000 Elsevier Science S.A. All rights reserved.