Involvement of triplet excited states in the electron transfer photodegradation of cinnamic acids using pyrylium and thiapyrylium salts as photocatalysts

Abstract

The mechanistic pathway for degradation of cinnamic acids using 2,4,6-triphenylpyrylium as well as 2,4,6-triphenyl(thia)pyrylium salts (1a,b) as solar photocatalysts has been unambiguously established. Results obtained in steady-state experiments have been correlated with time-resolved photophysical studies. High percentages of photodegradation (60–70%) were achieved when aqueous solutions of caffeic and ferulic acids (2a,b) as model pollutants were submitted to irradiation in the presence of 1a,b. Electron-transfer quenching of both the singlet and triplet excited states of 1a,b by 2a,b has been proved, and the quenching rate constants (close to diffusion control) have been determined. However, the percentages of singlet quenching by 2a,b, even at relatively high concentrations of the model pollutants, is lower than 5%. In addition to this, growth of the signal corresponding to the pyranyl radical occurs in the microsecond timescale, incompatible with the singlet state as precursor. Thus, photodegradation of 2a,b mainly involves the triplet state of the photocatalysts.