ON THE LOCALIZATION OF THE ELECTRONIC EXCITATION IN SUPRAMOLECULES BUILT-UP BY EQUIVALENT UNITS LINKED BY HYDROGEN-BONDS

The electronic distribution of molecular systems formed by two identic al 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 hav ing identical geometries) the minimum energy structure for the first p i-->pi excited singlet electronic state turns out to be asymmetric. T his asymmetry is responsible for the localization of the electronic ex citation in only one of the monomers. The factors that drive the elect ronic excited state to prefer an asymmetric geometry are studied. This analysis can have important consequences in the fields of photochemis try and supramolecular chemistry. (C) 1998 Elsevier Science B.V.