Photolysis of pyrene and chloropyrene in the presence of triethylamine in acetonitrile: dehalogenation assisted by potassium cyanide

Abstract

Kinetics and transient absorption spectra of pyrene– and 1-chloropyrene–triethylamine (TEA) systems, in the presence and absence of cyanide ions, have been studied by a nanosecond time-resolved laser flash photolysis method in dry acetonitrile at room temperature. Schemes for chloropyrene–triethylamine– and chloropyrene–triethylamine–CN^– systems have been developed and individual rate constants determined either by direct measurements or by use of computer analysis of the experimental decay curves. The reaction results in the dehalogenation of chloropyrene to yield pyrene as the reaction product, but high quantum yields (0.2–0.4) are obtained only in the presence of cyanide ions. The hydrogen atoms which replace the chlorines originate from the solvent. This was determined by deuterium labelling. The reaction proceeds via an exciplex to radical ions, which recombine to form ion pairs. Cyanide ions attract chlorohydropyrene radicals, which are mainly formed in the reaction between chloropyrene anion radicals and the solvent. The cyanohydropyrenyl radicals formed are in equilibrium with the relatively stable cyanopyrene anion radicals and, by cleavage of the cyano radicals, yield the reaction product.