Formation mechanisms of equilibrium component composition of molecular layers of polymethine dyes

We study the concentration dependences for the absorption spectra and component composition of molecular layers from three homologous series of symmetric polymethine dyes of different electron-donating ability of their terminal heterocyclic groups. We find that a change in the layer thickness leads to a change in the width and position of the spectrum due to a change in the number of absorption bands. The number of bands of monomers increases with increasing chain length and electron-donating ability. The concentration ratio of monomers and associated forms depends on the spatial orientation of molecules in the layer. The electron-donating ability of terminal groups affects the angle between the chromophores of molecules that form a dimer and the intensity ratio between the short- and long-wavelength absorption bands of dimers. We conclude that the effect of the thickness of the layer on its spectral parameters is determined by the degree of intramolecular electron asymmetry that arises as a result of the interaction of chemically symmetric molecules with charges of the substrate surface and upon intermolecular interaction. This asymmetry leads to changes in free energies of ground states of monomeric molecules and, as a consequence, to an increase in equilibrium concentrations of cis-stereoisomeric forms in the layer.