AGGREGATION OF A SURFACTANT SQUARAINE IN LANGMUIR-BLODGETT-FILMS, SOLIDS, AND SOLUTION

The aggregation of a surfactant squaraine, 4-(N-methyl-N-(carboxypropyl)amino)phenyl-4'-(N,N-dibutylamino)phenylsquaraine (SBA), has been studied in a variety of media, including organic solvents, aqueous cyclodextrin (CD) solutions, vesicles, monolayers, Langmuir-Blodgett films, and pure solid dye films. As a monomer in organic solvent, SBA exhibits sharp and intense absorption with lambda(max) ranging from 622 to 656 nm and fluorescence emission at lambda(F) 637-678 nm. In dilute aqueous solution the monomer absorbs at 650 nm and gives a fluorescence at 678 nm. The dimer absorbs at 594 nm and is nonfluorescent. This structural assignment is supported by studying the spectroscopic properties of SBA in CD solutions. SBA forms 1:1 inclusion complexes with alpha- and beta-CD which absorb at lambda(max) similar to 650 nm and emit at lambda(F) similar to 672 nm with a fluorescence quantum yield a factor of 2-3 higher than that in pure water. The inclusion of the squaraine chromophore in alpha- and beta-CD is also indicated by H-1 NMR spectral data which suggest that the squaraine chromophore is in a relatively nonpolar environment. In gamma-CD solution, due to the larger cavity size (9.5 Angstrom vs 5.7 and 7.8 Angstrom in alpha- and beta-CD, respectively), a 2:1 complex between SBA and gamma-CD is formed. This 2:1 complex which essentially is the dimer of SBA absorbs at 594 nm and does not fluoresce. A further blue-shift in absorption, lambda(max) at 500-540 nm, is observed when SBA is incorporated in vesicles (at high concentrations), monolayers, supported LB films, and pure solid dye film. From available crystal structure data and the compression isotherm data in this study, it is calculated that SBA adopts either a translation layer or a glide layer structure in these aggregates. Interestingly, the intermolecular interaction between the squaraine chromophores in these aggregates is shown to be similar to that in microcrystals of bis(4-methoxyphenyl)squaraine, which also shows blue-shifted absorption in the aggregated state relative to the monomer.