Intervalence Charge Transfer (IVCT) in Trinuclear and Tetranuclear Complexes of Iron, Ruthenium, and Osmium

Electron and energy transfer are ubiquitous in biological, chemical, and physical processes, which has led to extensive multidisciplinary research efforts to elucidate the factors influencing mechanistic pathways. Of considerable importance in these studies have been investigations of intramolecular electron and energy transfer within polymetallic assemblies as a result of the diversity (coordination number, ligand environment, stereochemistry, and redox characteristics) provided by the metal centers in such structures. Because of their multicomponent nature, these structures have considerable design potential to exploit the cooperation between the metals and/or other redox-active centers. Novel photochemical molecular devices (PMDs) may be constructed which are capable of performing useful light- and redox-induced functionsincluding artificial photosynthesis and photoinduced energy- and electron-transfer processes in light-harvesting “antenna” systems. Metallosupramolecular assemblies have also been designed to mimic the photoinduced charge separation function in photosynthetic organisms, in an attempt to elucidate the complex electron- and energy-transfer mechanisms which occur in natural systems. The possibility of multiple electron transferby absorption of several photons by linked chromophores, or the design of systems which generate more than one electron upon absorption of one photonhas significant implications in catalytic schemes, as well as understanding long-range electron transfer in biological systems, and the conductivity of “molecular wires”. In molecules involving delocalized unpaired electrons, polarizability may be present so that the species exhibit interesting nonlinear optical or magnetic properties. Polypyridyl complexes of the d⁶ metals FeII, RuII, and OsII have attracted particular attention as the basis of these assemblies due to a combination of favorable photophysical and redox characteristics, the longevity of their excited states, and their chemical inertness in a variety of oxidation states. An important feature of these complexes is the capability of systematic variation of the ground- and excited-state properties by the judicious choice of the coordinating ligands.