ELECTRONIC EXCITATIONS IN ORGANIZED MOLECULAR-SYSTEMS - A MODEL FOR COLUMNAR AGGREGATES OF IONIC COMPOUNDS

A model for the description of electronic excitations in organized molecular systems of finite size in which intermolecular distances are comparable to molecular dimensions is presented. A methodology based on the excitonic theory coupled to quantum chemistry is developed and applied to columnar aggregates of triarylpyrylium tetrafluoroborates. Excitation energy and interactions among transition moments (diagonal and off-diagonal terms of the matrix Hamiltonian) are calculated taking into account the precise geometry of the aggregate. Its energetic topography shows that diagonal energy is sensitive to edge and orientational effects. Diagonalization of the electronic Hamiltonian in the strong exciton interaction limit, provides the eigenstates from which localization indexes, radiative lifetimes, absorption and fluorescence spectra are calculated. It is found that aggregate growth induces a blue-shift in absorption and an increase in radiative lifetime. Orientational disorder causes a red-shift in fluorescence. Previously published experimental data are discussed in the frame of the present model.