CIS-TRANS-ISOMERIZATION OF CATIONIC-ANIONIC POLYMETHINE DYES

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.