Aggregation of amphiphilic squaraines at the air-water interface and in Langmuir-Blodgett films

A series of amphiphiles incorporating the squaraine chromophore (1-3) has been synthesized and these amphiphiles have been studied as films at the air-water interface and supported Langmuir-Blodgett (LB) films on glass. For the spread films at the air-water interface, aggregate formation is observable even at very low surface pressures and in relatively diluted mixtures; in certain cases the type of aggregate formed is sensitive to the surface pressure. The most frequently encountered spectrally blue-shifted or H-aggregate (lambda(max) = 530 nm, compared to the monomer, lambda(max) = 630 nm), is attributed to a ''unit aggregate'' which we have shown previously to be a cyclic, chiral tetramer. The extended aggregate in compressed films is thus a mosaic of these unit aggregates which exist even before compression. For certain squaraine amphiphiles and mixtures we obtain evidence for a second species which has spectral characteristics consistent with the red-shifted or J-aggregate, and they form only under compression. This species is metastable in several cases and can be converted to the H-aggregate under a variety of conditions. The relationships between amphiphile structure, microenvironment, aggregates formed, and aggregate stability are discussed.