Photochemistry of 2,3,4,4-tetrachloro-1-oxo-1,4-dihydronaphthalene and 2,2,3,4-tetrachloro-1-oxo-1,2-dihydronaphthalene in liquid solutions

Abstract

The stable products, which result from the u.v. photolysis of 2,3,4,4-tetrachloro-1-oxo-1,4-dihydronaphthalene (β-TKN) and 2,2,3,4-tetrachloro-1-oxo-1,2-dihydronaphthalene (α-TKN) in cyclohexane and in CCl₄ have been isolated and identified. From the hydrogen abstraction reactions of the two compounds in their lowest triplet state it is concluded that this is a ³(n–π*) state in both compounds. Heterolytic photodissociation proceeds from the first excited singlet state of α-TKN ¹(π–π*) and of β-TKN ¹(n–π*). Also homolytic photodissociation of β-TKN is observed, after excitation into the first absorption band. The branching ratio of triplet formation and homolytic and heterolytic photodissociation is found to differ for the two compounds. The carbenium ion formed in the heterolytic photodissociation has a lifetime which varies with the solvent used. The decay of the carbenium ion is determined by both ion recombination and reactions with other photoproducts in the solution. The carbenium ion is responsible for the photochromic effect of α- and β-TKN in CCl₄.