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 E-T(30), whereas the back isomerization rate constant increases with growing E-T(30). The effects can be explained considering the electronic structure of oxonols, which possess lone electron pairs on oxygen atoms susceptible to electrophilic solvation. (c) 2007 Elsevier B.V. All rights reserved.