REACTIVITIES OF ISOMERIZATION, OXIDATION, AND DIMERIZATION OF RADICALCATIONS OF STILBENE DERIVATIVES

Reactions of radical cations of eight stilbene derivatives (S-.+) have been studied using pulse radiolysis and gamma-ray radiolysis in 1,2-d ichloroethane or butyl chloride. Unimolecular isomerization from cis-S -.+ to trans-S-.+ and bimolecular reactions with O-2 (oxidation) and a neutral stilbene (dimerization) occur depending on the substituents. The unimolecular c-t isomerization and the oxidation proceed preferabl y in S-.+ substituted with a p-methoxyl group (as an electron-donating substituent) with rate constants of k(i) = 4.5 x 10(6) to 1.4 x 10(7) s(-1) and k(02) = (1.2-4.5) x 10(7) M(-1) s(-1), respectively. On the basis of transient absorption measurements, it is concluded that sepa ration and localization of a positive charge and an unpaired electron play the most important role as the controlling factors in the reactiv ities of the unimolecular isomerization and the oxidation. The dimeriz ation involves initial formation of a pi-complex with overlapping of t wo benzene rings and is inhibited by steric hindrance of substituents on the benzene rings and olefinic carbons.