Exciton effects in the dimer and higher aggregates of a simple merocyanine dye. A CSINDO CI based theoretical study

A simple streptomerocyanine with five methine carbons (Mc) has been used as a suitable model system to investigate theoretically the optical properties in organized assemblies of merocyanine dyes. The subject was treated by a methodology making use of a semiempirical quantum chemistry description of the isolated chromophore within the Frenkel exciton theory. Both an extended CS INDO CIPSI and easier CS INDO SCI (with different parametrizations of electron repulsion integrals) schemes were applied in order to establish the role played by the quality of the wave functions. The exciton states were built on the (1)(pi pi*) (HOMO-LUMO) excited-state responsible for the color band of Me. Exciton effects were first analyzed for two stable dimer structures, having H- and J-type character, located by use of a semiempirical intermolecular potential and the simulated annealing method. Then, attention was shifted from dimers to higher-order model aggregates built in such a way as to obtain typical H (columnar) and J (staircase) arrangements. Exciton-state properties (energies and transition dipole moments) were studied as a function of the aggregation number N (up to N = 50). The results are discussed with reference to the approximations used to calculate the excitonic matrix as well as to the effects of changing from CIPSI to simple SCI schemes.