Medium effects on the isomerization of an anionic polymethine dye

Strong effects of electrophilic solvents on the processes of photoisomerization and thermal back isomerization of the photoisomer were found for an anionic polymethine dye, 9-fluorosubstituted oxonol. In particular, the photoisomerization quantum yield and the Arrhenius activation energy for the back isomerization decrease with the increase of the Dimroth–Reichardt solvation parameter ET(30), whereas the back isomerization rate constant increases with growing ET(30). The effects can be explained considering the electronic structure of oxonols, which possess lone electron pairs on oxygen atoms susceptible to electrophilic solvation.