A mechanistic study on photocatalysis by thiapyrylium salts. Photodegradation of dimethoate, alachlor and pyrimethanil under simulated sunlight

Photodegradation of dimethoate, alachlor, and pyrimethanil using a thiapyrilium salt (TPTP+) as photocatalyst was performed under simulated sunlight. The photocatalyst removes preferentially pyrimethanil (45% after 60 min of irradiation), followed by dimethoate (20%) and alachlor (13%). In the three cases, an important to moderate decrease of the fluorescence intensity upon addition of the pollutants was observed. The active participation of the singlet excited state in the photodegradation of the pollutants was demonstrated; the corresponding dynamic quenching rate constants were found to be 1.9 × 1010 M−1 s−1 (pyrimethanil), 6.7 × 109 M−1 s−1 (dimethoate) and 9.1 × 109 M−1 s−1 (alachlor). Moreover, a decrease in the signal corresponding to the triplet excited state was observed immediately after the laser pulse; again a good correlation was observed between the missing fraction of triplets and the photodegradation rates under solar light. Detection of TPTP+, the reduced species derived from the photocatalyst, as well as Rehm–Weller calculations on the free energy changes, strongly support an electron transfer mechanism occurring from the singlet excited state.

Figure optionsDownload as PowerPoint slideHighlights▸ Photodegradation of the title pesticides occurs using a thiapyrilium salt (TPTP+). ▸ The photoreactivity order is: pyrimethanil > dimethoate > alachlor. ▸ Formation of TPTP+/pesticide complexes is involved in photodegradation. ▸ Quenching of TPTP+ singlet excited state by pesticides leads to photodegradation. ▸ A correlation exists between the missing fraction of triplets and photoreactivity.