Ion pairing of bisdimethylamino pentamethinecyanine perchlorate and its consequences on the cis-trans photoisomerization dynamics

Ion pairing of the title compound is shown to occur in several solvents and solvent mixtures by electrical conductivity measurements, Both solvent separated and contact ion pairs have been found, depending on the solvent nature, Whereas for ion pairs of the former kind the absorption and fluorescence properties are homogeneous with those of the solvated cation and the isomerization dynamics is only slightly modified, significant changes in these properties result from contact ion pair formation, Based on structural and dynamic information obtained from proton-NMR spectra, an interpretation of these observations is attempted, that invokes three different counterion effects, two electrostatic and one steric in nature, to be operative with different consequences and efficiencies.