Point-dipole approximation of the exciton coupling model versus type of bonding and of excitons in porphyrin supramolecular structures

The application of the exciton coupling model to interacting porphyrin chro mophores is discussed. Covalently bonded systems and ionic or electrostatic ally bonded homoassociates require different orientations of the transition dipole moments in order to explain the experimental results: according to the symmetry of the assembly for covalently bonded porphyrins, and assuming isolated chromophores for ionic bonded porphyrins. Further, for covalently bonded systems, an extended exciton coupling has been demonstrated, but th e ionic systems are in agreement with non-extended couplings. The relation of these facts to a molecular description of solid-state Wannier-Mott or Fr enkel excitons is briefly discussed.