Aggregation of Surfactant Squaraine Dyes in Aqueous Solution and Microheterogeneous Media:  Correlation of Aggregation Behavior with Molecular Structure

The synthesis of several amphiphilic squaraine dyes and a study of their aggregation behavior and photophysics are reported. The several different squaraines are found to give spectrally blue-shifted aggregates in aqueous and mixed aqueous-organic solution and in microheterogeneous media (bilayer vesicles). While in some cases an intermediate dimer can be detected in the monomer to aggregate conversion process, in others direct conversion of monomer to aggregate is observed. The aggregation number can be determined together with the equilibrium constant and thermodynamic parameters for some of the squaraines in different environments. In several cases the aggregation number is found to be ca. 4. The finding of a strong induced circular dichroism signal when the aggregate (but not dimer or monomer) is generated in the presence of a chiral host (or counterion) suggests that the aggregate is chiral. From these results and molecular simulations indicating that an extended monolayer of some of the squaraines adopts a glide or herringbone lattice we propose a chiral “pinwheel” structure for the unit aggregate and suggest that extended aggregate structures or crystals may be a mosaic of these unit aggregates. In contrast to the monomers, which are strongly fluorescent, the squaraine dimers and aggregates are nonfluorescent and have extremely short exciton lifetimes, as indicated by transient spectroscopy.