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

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 (λ_max/nm = 318 with shoulder at 370 nm and ε/mol⁻¹ dm³ cm⁻¹ = 8100).