The red shift of the absorption band of the PPC films with increase of n is due to the approach of the energies of the frontier orbitals of the dye molecules to each other with increase in the length of the polymethine chain (Table ). At a dye concentration of ≥ 5-7 wt.% the absorption bands of the polymethine dyes 1-3 become substantially broader (Fig. ). Their shape deviates significantly from the universal contour. This is a result of strong interaction between the dye molecules as a result of their significant approach to each other during increase of the concentration.