Coulomb effect on the energy levels in molecular crystals

The effect of aggregation on the ionization energy of cationic dye molecules, adsorbed in a monolayer on a substrate, is investigated using a SCF-LCAO approximation. The calculations are performed on a model system which is based on molecules of the dye pinacyanol adsorbed on a cadmium sulphide substrate. Starting from the calculation for the isolated molecule, we use an iterative procedure to calculate the coulombic effect. The charge densities at the atom sites of nearby molecules are introduced into the calculation and new charge densities are computed. The calculation is repeated until substantially unvarying charge densities are found, and the ionization energy is computed. This ionization energy is lower than that of an isolated molecule by 0·6 eV. The aggregation effect is a result of redistribution of charge in the aggregate ground state, as compared to the isolated molecule, due to the first order coulomb interaction between nearby molecules in the aggregate and the one which is studied. The first and last molecules in a linear array are examined separately; their ionization energy is higher or lower than that of the intermediate molecules in the array. The results are compared with the experimental photoionization energy; taking into account the substrate effect, a reasonable vapour ionization energy is inferred.