Solvent-induced organization of squaraine dyes in solution capillary layers and adsorbed films

Unusual behavior of indolenine and hydroxyphenyl squaraines has been observed in solution capillary layers and adsorbed films. The confined solutions showed anomalous aggregation of squaraine molecules in contrast to their monomer behavior in the bulk solutions of the same concentration, along with formation of a macroscopic cell-like structure in the confined solution layer, with the diameter of cells being 3-5 microm. The aggregate structure, as observed through electronic absorption spectra, was strongly dependent on the chemical structure of squaraine used and solvent used, and it also was different from squaraine aggregates observed in aqueous solutions and films prepared by vacuum evaporation. It has been found that indolenine squaraine is capable of forming H-aggregates in confined dimethylformamide solutions and hydroxyphenyl squaraine is capable of forming J-aggregates in confined dimethylformamide solutions and adsorbed films. The results were compared with pseudoisocyanine, which forms J-aggregates in aqueous bulk solutions readily; however, no J-aggregates have been found in their capillary layers. The interplay of dye-dye, dye-surface, and dye-solvent interactions resulting in the above effects is discussed.