THE INFLUENCE OF SPIN-FORBIDDEN MONOMER EXCITATIONS ON SPIN-ALLOWED ELECTRON-TRANSFER AND ELECTRON-LOCALIZED STATES OF MIXED-VALENCE AND SINGLE-VALENCE DIMERIC SYSTEMS

Models for the spin-allowed absorption spectra of weakly interacting dimeric systems are usually cast in terms of perturbed localized spin-allowed excitations on each chromophore, enhanced by the addition of electron-transfer excitations between them. We show that locally spin-forbidden processes on individual chromophores, when coupled weakly to either unpaired spins or other spin-forbidden processes on other chromophores, become spin-allowed and may perturb spectra and excited state photochemistry. In neutral dimers, some doubly excited states are quite low lying, and it is possibly for one of these to constitute the lowest lying spin-allowed excited state. In dimer radical ions, a (possibly quite intense) band is generally expected to lie very close to the triplet absorption energy of each isolated monomer. Particular examples are considered, as well as a model system consisting of an ethylene dimer in a stacked configuration typical of that found in large pi-systems such as the primary electron donor in the photosynthetic reaction centre, stacked phthalocyanines, acene dimers, stretched norbornadiene-type systems, etc. The results apply more generally than this, however, being relevant to all weakly interacting molecular or inorganic systems.