Photochemistry of 5-halogenosalicylic acids: experimental and theoretical evidences of the triplet involvement in the carbene formation.
Résumé
The heterolytic photodehalogenation of 4-halogenophenols in water is known to yield the long-lived triplet carbene 4-oxo-2,5-dienylidene. However, the nature of the excited state from which dehalogenation takes place remains an open issue. To get further insight into the mechanism of carbene formation, we investigated the photolysis of 5-chlorosalicylic acid and 5-bromosalicylic acid from experimental and theoretical points of view. Our results showed that irradiation of monoanionic species in water leaded to the formation of the carbene species and its characteristic photoproducts. Moreover, triplet excited states could be detected by laser flash photolysis of aqueous solutions and their involvement in the photodehalogenation was firmly demonstrated. Identification by quantum calculations of two distinct triplet species supports the experimental results. The short- and long-lived triplets are assigned respectively to the planar and non planar structures. Both were successfully detected experimentally. The non planar triplet either reacted with water to yield the dihydroxyderivative or gave the triplet carbene after heterolytic halogen elimination. These findings shed a new light on our understanding of the halogenophenols photochemistry and could be exploited in further studies.