Direct determination of rate constants for coupling between aromatic radical anions and alkyl and benzyl radicals by laser-flash photolysis

Abstract

Coupling rates between the radicals methyl, n-, sec-, tert-butyl and benzyl (R˙) and the aromatic radical anions of 1,4-dicyanonaphthalene, 9,10-dicyanoanthracene and fluorenone (A⁻˙) have been obtained using a new laser-flash photolysis method. The radicals R˙ and the radical anions A⁻˙ were generated by a photoinduced electron transfer reaction between the aromatic compound A and the alkyl or benzyl triphenylborate anion RB(Ph)₃⁻. For the first time the rate constants of the coupling reaction between methyl and benzyl radicals with aromatic radical anions have been obtained. For all the measured coupling rate constants an average value of k₁ = 1.9 × 10⁹ M⁻¹ s⁻¹ was found with a relatively small variation in the coupling rates (0.8–2.9 × 10⁹ M⁻¹ s⁻¹). The results demonstrate that the coupling rate k₁ is insensitive to changes in the steric and electronic properties of the radicals and the structure and standard potentials of the aromatic radical anions.