The electronic distribution of molecular systems formed by two identical moieties is analyzed. For the dimers of dipropenoic acid and the 7-azaindol, a well-studied model of the DNA base pairs, it is seen that whereas the ground electronic state is symmetric (the two monomers having identical geometries) the minimum energy structure for the first π→π* excited electronic state turns out to be asymmetric. This asymmetry is responsible for the localization of the electronic excitation in only one of the monomers. The factors that drive the electronic excited state to prefer an asymmetric geometry are studied. This analysis can have important consequences in the fields of photochemistry and supramolecular chemistry.