Reactivities of Isomerization, Oxidation, and Dimerization of Radical Cations of Stilbene Derivatives

Article abstract of DOI:10.1021/jp9609904

Reactions of radical cations of eight stilbene derivatives (S(.+)) have been studied using pulse radiolysis and γ-ray radiolysis in 1,2-dichloroethane or butyl chloride.Unimolecular isomerization from cis-S(.+) to trans-S(.+) and bimolecular reactions with O2 (oxidation) and a neutral stilbene (dimerization) occur depending on the substituents.The unimolecular c-t isomerization and the oxidation proceed preferably in S(.+) substituted with a p-methoxyl group (as an electron-donating substituent) with rate constants of k_i = 4.5E6 to 1,4E7 s^-1 and k₀₂ = (1.2-4.5)E7 M^-1 s^-1, respectively.On the basis of transient absorption measurements, it is concluded that separation and localization of a positive charge and an unpaired electron play the most important role as the controlling factors in the reactivities of the unimolecular isomerization and the oxidation.The dimerization involves initial formation of a ?-complex with overlapping of two benzene rings and is inhibited by steric hindrance of substituents on the benzene rings and olefinic carbons.