CHARACTERIZATION OF HEMICYANINE AGGREGATES IN AIR-WATER MONOLAYERS

Absorption spectra of a docosyl derivative of the hemicyanine chromoph ore were measured at the air-water interface as a function of surface concentration. At low compression, a broad band is observed correspond ing to the monomer species. With increasing surface pressure two new b ands grow in concurrently at the expense of the monomer band. Principa l factor analysis of the spectra indicates that the two aggregate band s are produced by a single aggregate species. In terms of exciton theo ry, this corresponds to the aggregate geometry having two molecules pe r unit cell. The sensitivity of the aggregate structure to the local e nvironment is shown by the shift in position of the absorption bands f or a mixed hemicyanine-cationic diluent monolayer. The polyion complex ed hemicyanine monolayer exists only as the monomer at all surface con centrations, in contrast to the monomer-aggregate coexistence observed on a pure water subphase.