Influence of molecular structure of squaraine dyes on their aggregation in aqueous solutions

The influence of the molecular structure of several squaraine dyes on their aggregation in aqueous solutions and its spectral manifestation were studied. The results are analyzed using quantum-chemical modeling. It is shown that a transverse dipole moment in squaraine molecules contributes to aggregation with an antiparallel orientation of the molecules whereas a longitudinal dipole moment leads to formation of J-type aggregation with a longitudinal orientation of the molecules. In some instances both aggregate types are present in the solution over a wide concentration range.