F. Rollet, C. Richard
Jul 1, 2006
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Journal
Photochemical & Photobiological Sciences
Abstract
The reactivity of Br^− and Cl^− with triplet of anionic 6-chloropicolinic acid (pH = 5.4) and with triplets of 6-chloro and 6-bromopicolinic acids in zwitterionic forms (pH = 0.9) was studied by laser flash photolysis and steady-state irradiation. Br^− was found to trap the three triplets. Triplet lifetime measurements gave quenching rate constants of 8 × 10^8 mol^−1 dm^3 s^−1 for the zwitterion of 6-chloropicolinic acid and of 3.4 × 10^5 mol^−1 dm^3 s^−1 for the anionic counterpart. No secondary transient species were observed indicating that the charge transfer intermediates are subject to dissipative processes. Cl^− trapped triplet of zwitterions only, and reactions were found to be associated with a high quantum yield of radicals. The photolysis of 6-bromopicolinic acid photolysis was drastically enhanced by Cl^−, 6-chloropicolinic acid being produced with a chemical yield of about 90%. The 6-bromo-2-carboxypyridinyl radical could be characterized ( λ _max/nm = 318 with shoulder at 370 nm and ε /mol^−1 dm^3 cm^−1 = 8100).