Aggregation of perylene dyes in lipid vesicles: The effect of optically active substituents

Different optically active and inactive derivatives of the perylene dye, perylene-3,4:9,10-tetracarboxylic bisimide, solubilized in unilamellar lipid vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) have been studied by means of polarized light spectroscopic methods. Absorption and fluorescence spectra become distorted at molar ratios greater than one dye molecule per 1000 molecules of DOPC. These observations are compatible with the formation of ground-state aggregates. Circular dichroism (CD) was not detectable for any perylene dye dissolved in liquid solutions, or diluted in lipid vesicles. However, dyes with optically active substituents reveal CD spectra at dye:lipid ratios of greater than 1:1000. For dyes with R and S enantiomers, CD spectra are mirror images, while no CD of racemic mixtures, or optically inactive perylene dyes could be detected. The observed absorption and CD spectra are qualitatively explained by exciton coupling within dimers and their molecular symmetry.