F. Rollet, C. Richard, J. Pilichowski
Jan 1, 2006
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0
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Journal
Photochemical & Photobiological Sciences
Abstract
The photochemistry of 6-chloro and 6-bromopicolinate ions ( I and II , respectively) was investigated by product studies and ns laser flash photolysis (LFP). In deoxygenated pH 5.4 water, I yields 6-hydroxypicolinic acid (70%) and a substituted pyrrole. In 2-propanol-water (1: 1) mixture, the reaction yields, very unselectively, 6-hydroxypicolinic acid, 2-carboxypyridine, pyridine and bipyridines. Photolysis of aqueous II leads to 6-hydroxypicolinic acid (78%) and hydroxybipyridines. Oxygen suppresses the photolysis of I but does not affect that of II . By LFP, we detected a short-lived transient at the pulse end from I ( λ _max = 305 nm, k = (2.8 ± 0.2) × 10^5 s^−1, εϕ = 2200 ± 200 dm^3 mol^−1 cm^−1). This is quenched either by oxygen or methyl acrylate and thus assigned to the triplet excited state. The triplet excited state of II is detected at pH 1 only ( λ _max = 320 nm, k > 3 × 10^7 s^−1). The radical ion Cl_2^−· could be successfully detected by photolysing I in 2-propanol-water (1: 1) in the presence of Cl^−. Similarly, Br_2^−? could be detected by irradiating aqueous II in the presence of Br^−. These results show that the photodehalogenation of I is heterolytic in water and mainly homolytic in 2-propanol-water mixtures while that of II is both heterolytic and homolytic in water. A mechanism in which the triplet excited state undergoes homolysis of the C-X bond and subsequent electron transfer from the carboxypyridyl radical to the halogen atom to form an ion pair may account for these observations.