Kinetic study of the singlet molecular oxygen-mediated photodegradation of monohydroxylated N-heteroaromatic compounds

Abstract

The kinetics of the dye-sensitized photooxygenation of the monohydroxylated N-heteroaromatic compounds, 2-hydroxypyridine, 3- hydroxypyridine, 4-hydroxypyridine, 4-hydroxyquinoline, 8-hydroxyquinoline, 2-hydroxypyrimidine and 4-hydroxypyrimidine, with the basic structures of some pesticides, has been studied. Rate constants in the range 10 6–10 8 M −1 s −1 for both the overall and the reactive singlet molecular oxygen [ O 2( 1 Δ g ) ] quenching were determined by time-resolved O 2( 1 Δ g ) phosphorescence detection and polarographic methods. Photooxidation quantum yields were in the range of 0.01–0.50 in the cases where reaction was observed, with values significantly higher in aqueous media. The hydroxy substituent plays a key role in the photooxidative process. In 2- and 4-hydroxypyridines, 4-hydroxyquinoline and 4-hydroxypyrimidines the predominant, and almost exclusive, presence of the tautomeric oxo form drastically suppresses the interaction with O 2( 1 Δ g ) . In the case of 3-hydroxypyridine and 8-hydroxyquinoline, compounds with hydroxy form, the OH-ionization greatly enhances the photooxidative process. Experimental evidence suggests the participation of a charge-transfer mediated mechanism, involving an initial excited encounter complex.