The collective electronic oscillator (CEO) approach recently proposed by Mukamel, which is based on an INDO/S Hamiltonian in the framework of the time-dependent Hartree–Fock approximation, is used to investigate the effect of the supramolecular architecture on the photogenerated excitations in different aggregates of a large-sized oligodiacetylene. The method gives the transition energies and the corresponding one-electron transition density matrices directly, which allows one to analyze, in terms of electron–hole motions in real space, the electronic redistribution upon photon absorption. Electronic excitations depend strongly on the arrangement of the oligomer molecules in the cluster, appreciable interchain effects being obtained even when the arrangement of the oligomer molecules corresponds to a minimum interchain interaction. Interchain interactions give rise to new transitions whose oscillator strengths depend on the nature of the aggregate (neutral or charged), and which can significantly modify the optical properties of the isolated molecular system.