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 d imeric systems are usually cast in terms of perturbed localized spin-a llowed excitations on each chromophore, enhanced by the addition of el ectron-transfer excitations between them. We show that locally spin-fo rbidden processes on individual chromophores, when coupled weakly to e ither unpaired spins or other spin-forbidden processes on other chromo phores, become spin-allowed and may perturb spectra and excited state photochemistry. In neutral dimers, some doubly excited states are quit e low lying, and it is possibly for one of these to constitute the low est lying spin-allowed excited state. In dimer radical ions, a (possib ly quite intense) band is generally expected to lie very close to the triplet absorption energy of each isolated monomer. Particular example s are considered, as well as a model system consisting of an ethylene dimer in a stacked configuration typical of that found in large pi-sys tems such as the primary electron donor in the photosynthetic reaction centre, stacked phthalocyanines, acene dimers, stretched norbornadien e-type systems, etc. The results apply more generally than this, howev er, being relevant to all weakly interacting molecular or inorganic sy stems.