Aggregation of Amphiphilic Squaraines at the Air−Water Interface and in Langmuir−Blodgett Films

Abstract

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 (λmax = 530 nm, compared to the monomer, λmax = 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.