The effect of solvent polarity and ion pair formation on cis-trans-iso
merization of cationic-anionic polymethine dyes and of related simple
cationic and anionic dyes is studied by flash photolysis. The change i
n the rate constant for reverse cis-trans- isomerization of photoisome
rs due to the interaction of a cation with an anion in an ion pair is
observed in nonpolar solvents. A drop in the yield of the photoisomers
to zero is observed for a number of cationic-anionic dyes in weakly p
olar and nonpolar solvents which is possibly due to steric hindrances
in the photoisomerization process in ion pairs.