Squaraine Chemistry. Effect of Orientation on the Aggregation of Surfactant Squaraines in Langmuir-Blodgett Films

Article abstract of DOI:10.1021/j100343a058

Surfactant squaraines DSSQ (4-(distearylamino)phenyl-4'-(dimethylamino)phenylsquaraine), MSSQ (bis(4-methylstearylamino)phenyl)squaraine), and TSSQ (bis(4-(distearylamino)phenyl)squaraine), which were designed to orient the "bricklike" squaraine chromophore in three different orientations when organized in monolayers or Langmuir-Blodgett (LB) films, have been synthesized.Surface pressure-area isotherm studies show that the squaraine chromophores of DSSQ, MSSQ, and TSSQ orient as designed, namely, being laid down vertically along the long axis, vertically along the short axis, and flat, respectively, on water.Monolayers of TSSQ were found to collapse upon compression.Although stable monolayers can be obtained by mixing TSSQ with cadmium stearate, TSSQ was not studied in any great extent in this work due to the chemical decomposition of the squaraine chromophore by water at the air-water interface.Stable monolayers and LB films were obtained for DSSQ and MSSQ.The aggregation of the squaraine chromophore in these LB films was studied by absorption spectroscopy.Spectral data show that the absorption spectra of DSSQ and MSSQ in LB films not only are different from their solution absorption spectra but are also different from each other.By comparison of the absorption of model squaraine aggregates in the solid state, we conclude that the squaraine chromophores of DSSQ and MSSQ form aggregates in the LB films and that there is an orientational effect, which is induced by the LB film technique, on the aggregation.The stabilities of the LB films of DSSQ and MSSQ were also examined.While the LB film of DSSQ is very stable, a change in absorption maximum and a decrease in absorbance were observed in the LB film of MSSQ during storage.A model for the change in optical absorption is proposed and methodologies for preparing stable LB films of MSSQ are discussed.