Product studies and laser flash photolysis of direct and 2,4,6-triphenylpyrylium–zeolite Y photocatalyzed degradation of fenvalerate

Abstract

Direct irradiation of fenvalerate (FV) in aqueous solution (4 × 10⁻³ M) gives rise to 2,3–diarylisohexanitrile through the homolytic CO—CH(i-Pr) bond cleavage, followed by a rapid decarboxylation of the primary carbonyloxy radical, and radical recombination. The unusual exclusive formation of the cross combination product suggests that the persistent free radical effect is operative in this system and controls product formation. Spectroscopic evidence of the intermediacy of benzylic radicals decaying in microsecond time scale has been obtained by laser flash photolysis. These studies also reveal a remarkable attenuation of radical reactivity toward oxygen resulting from cyano substitution at the radical center. TPY photocatalyzed degradation of FV also leads to decarboxylation products accompanied by oxidative mineralization (38%). The TPY-photocatalyzed degradation can occur through two operating mechanisms involving the generation of OH˙ radicals and/or FV˙⁺ radical cations. Evidence for the latter intermediate has been obtained by laser flash photolysis in acetonitrile solution (detection of TP˙ radical) and by the quenching by FV of the emission from the TPY solid.