Reactivity of halides with triplets of 6-chloro and 6-bromopicolinic acids: contrasting effects of bromide and chloride

Photochemical & Photobiological Sciences Pub Date: 2006-04-24 DOI: 10.1039/B601319H

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 × 108 mol?1 dm3 s?1 for the zwitterion of 6-chloropicolinic acid and of 3.4 × 105 mol?1 dm3 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 (λmax/nm = 318 with shoulder at 370 nm and ε/mol?1 dm3 cm?1 = 8100).

Graphical abstract: Reactivity of halides with triplets of 6-chloro and 6-bromopicolinic acids: contrasting effects of bromide and chloride
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