A simple model for formation of two-dimensional orientation structures and their exciton states

Abstract Mechanisms are discussed which govern the formation of complex orientation structures of molecules adsorbed on solid surfaces. For the molecules of polymethine dyes, a simple model of the competition between the van der Waals and Coulomb interactions is proposed which accounts for the existence of different dye aggregates, with certain molecular orientations relative to the aggregate axis; the observed frequency shifts of exciton lines in the electronic spectra of dye aggregates are also rationalized. For two-dimensional crystals with their unit cell containing two orientationally inequivalent molecules, the Davydov splitting is estimated; in the context of point-dipole approximation, the calculated values agree well with the experimental data.