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 the ir aggregation behavior and photophysics are reported. The several dif ferent squaraines are found to give spectrally blue-shifted aggregates in aqueous and mixed aqueous-organic solution and in microheterogeneo us media (bilayer vesicles). While in some cases an intermediate dimer can be detected in the monomer to aggregate conversion process, in ot hers direct conversion of monomer to aggregate is observed. The aggreg ation number can be determined together with the equilibrium constant and thermodynamic parameters for some of the squaraines in different e nvironments. In several cases the aggregation number is found to be ca . 4. The finding of a strong induced circular dichroism signal when th e 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 extend ed monolayer of some of the squaraines adopts a glide or herringbone l attice we propose a chiral ''pinwheel'' structure for the unit aggrega te and suggest that extended aggregate structures or crystals may be a mosaic of these unit aggregates. In contrast to the monomers, which a re strongly fluorescent, the squaraine dimers and aggregates are nonfl uorescent and have extremely short exciton lifetimes, as indicated by transient spectroscopy.