SENSITIZED PHOTOOXIDATION OF DIHYDROXYBENZENES AND CHLORINATED DERIVATIVES - A KINETIC-STUDY

The dye-sensitized photo-oxidation of dihydroxbenzenes and chlorinated derivatives was studied as a function of solvent polarity. Rate constants for the disappearance of singlet molecular oxygen (O2(1-delta-g)), determined by time-resolved phosphorescence at lambda > 1050 nm, are in the range 5 x 10(5) to 3 x 10(8) M-1 s-1 depending on the solvent polarity and substitution pattern of the reactant. The ratio between the rate constant for reactive quenching k(r) and the total quenching constant k(t) is also strongly dependent on substitution pattern and solvent. The values of k(r) indicate that the O2(1-delta-g)-mediated photo-oxidation of the dihydroxybenzenes is a viable possibility for their degradation in an aqueous environment. A charge-transfer mechanism is suggested for the photo-oxygenation. Dihydroxybenzenes are fairly good O2(1-delta-g) self-sensitizers of photo-oxidation on irradiation at 308 nm, with O2(1-delta-g) quantum yields of approximately 0.2 in the most favourable case. For hydroquinones, this presumably occurs through the excitation of the corresponding quinone derivatives which are always present in traces in the aerated solutions.