Hj. Chen et al., AGGREGATION OF AMPHIPHILIC SQUARAINES AT THE AIR-WATER-INTERFACE AND IN LANGMUIR-BLODGETT-FILMS, Journal of physical chemistry, 100(14), 1996, pp. 5949-5955
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 f
ormation is observable even at very low surface pressures and in relat
ively 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 aggr
egate'' 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 squa
raine 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 me
tastable in several cases and can be converted to the H-aggregate unde
r a variety of conditions. The relationships between amphiphile struct
ure, microenvironment, aggregates formed, and aggregate stability are
discussed.