Photochemistry of phenylfulgides XX: absence of activation barriers for photoisomerization reactions in the excited singlet state of extremely sterically hindered α,δ-di-(4-alkoxyphenyl)-fulgide isomers

Abstract

The partial quantum yields of the singlet photoisomerization reactions ▪ are nearly independent of the solvent polarity and of the temperature in the investigated range down to 110 K. At low temperatures, a barrier to the ▪ reactions and fluorescence of the ZZ isomers which competes with the isomerization step are observed. Increasing the viscosity of the solutions has no influence on the ▪ reaction. Only in rigid polymer matrices is the rate of the reaction ▪ slightly decreased. We conclude that the rotation of the 4-alkoxyphenyl groups about the exocyclic double bonds of the investigated s-cis-fixed 1,4-di(4-alkoxyphenyl)butadiene systems proceeds without any activation barriers for the isomerization sequence ▪, this effect being caused by the extreme steric hindrance in the EE and EZ isomers. A potential curve model for the isomerization reactions is given.