Characterization of hemicyanine aggregates in air-water monolayers

Absorption spectra of a docosyl derivative of the hemicyanine chromophore were measured at the air-water interface as a function of surface concentration. At low compression, a broad band is observed corresponding to the monomer species. With increasing surface pressure two new bands grow in concurrently at the expense of the monomer band. Principal factor analysis of the spectra indicates that the two aggregate bands are produced by a single aggregate species. In terms of exciton theory, this corresponds to the aggregate geometry having two molecules per unit cell. The sensitivity of the aggregate structure to the local environment is shown by the shift in position of the absorption bands for a mixed hemicyanine-cationic diluent monolayer. The polyion complexed hemicyanine monolayer exists only as the monomer at all surface concentrations, in contrast to the monomer-aggregate coexistence observed on a pure water subphase.