Pyrylium salt-photosensitized degradation of phenolic contaminants derivedfrom cinnamic acid with solar light - Correlation of the observed reactivities with fluorescence quenching

Solar light photodegradation of three cinnamic acid derivatives (ferulic ac id, caffeic acid and p-coumaric acid) present in olive oil industry wastewa ter has been achieved using 2,4,6-triphenylpyrylium hydrogen sulfate as an electron transfer photosensitiser. The parent cinnamic acid undergoes no ph otodegradation under the employed reaction conditions. The relative reactiv ities are in agreement with expectations based on qualitative reasoning: th e substrates with two electron-donating groups, ferulic and caffeic acid, r eact faster than p-coumaric acid, which has only one activating substituent ; while the non-activated cinnamic acid is nearly unreactive. The data obta ined from fluorescence measurements show that the excited singlet state of the pyrylium salt is quenched by the acids, with the following rate constan ts: 3.6 x 10(9) M-1 s(-1) for cinnamic acid, 4.4 x 10(10) M-1 s(-1) in the cast: of p-coumaric acid, 8.7 x 10(10) M-1 s(-1) for caffeic acid, and 1.0 x 10(11) M-1 s(-1) for ferulic acid. These values are in good agreement wit h the order of reactivities observed in the preparative solar reactions, (C ) 2000 Elsevier Science B.V. All rights reserved.