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The collective electronic oscillator (CEO) method allows to analyze the electronic excitations in molecular systems. Using this technique the excitations are directly connected to the corresponding one-electron transition density matrices, obtained as eigenmodes of the time dependent Hartree–Fock (TDHF) Liouville operator, which represent motions of electron–hole (e–h) pairs. Two-dimensional plots of the density matrices establish the connection between the electronic spectra and the electronic rearrangement due to the photon absorption. In this paper, we use the CEO procedure to study the nature of the electronic excitations on three series of one-dimensional porphyrin systems, showing through the analysis of the CEO eigenmodes how they can be described in terms of Frenkel or Wannier–Mott excitons.

We perform quantum chemical calculations on the singly and doubly charged species of the lowest unsubstituted oligodiacetylenes, in order to achieve information on the charged states originated in the photoexcitation of polydiacetylenes. On the basis of the optimized AM1 geometries of the radical ions and dications we obtain their low-energy vertical transitions in an INDO-SCI framework to ensure a proper treatment of electron correlation effects. We get the simulated absorption spectra of the charged oligomers by a convolution of the transition energies, and compare the linearly extrapolated polymer values with the photoinduced absorption results.