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Within the general subject of photoinduced electron and energy transfer, the study of unimolecular processes in covalently linked supramolecular systems constitutes a recent but very active research field. In the present report, the status of this field is briefly outlined. Particular attention is given to the evolution taking place from studies of fundamental interest on simple two-component systems (dyads) to studies of more oriented character involving increasingly complex systems (triads, tetrads, pentads, etc.). The development of this tendency towards the design of photochemical molecular devices is finally pointed out.

Donor-acceptor interactions at the periphery of a coordination compound can have important consequences on its photochemical and photophysical behavior. Effects of this type have been explored using Ru(I1) bipyridine complexes containing cyanide ligands. The cyanide ligands of these compkxes behave a6 eLectroR donors in interactions with acceptor solvents, protonation, methylation, and bridging to other metals with formation of polynuclear complexes. A variety of effects have been observed, ranging from (i) weak perturbations of the properties of the original complex, to (ii) major changes in the photophysical behavior of the complex via changes in redox potentials and excited state energy ordering, to (iii) induction of completely new properties characteristic of the complex-acceptor "supermolecule". Examples of such effects are discussed in some detail. The use of second-sphere donor-acceptor interactions to tune excited-state properties is stressed.