The life and times of excited states of organometallic and coordination compounds

The photochemistry of transition metal organometallic and coordination compounds is discussed from the perspective of time domains, in which excited-state reactions occur. Ultrafast photochemical processes, which are competitive with nuclear motion, are distinguished from reactions of long-lived, thermally equilibrated, excited states. Many special photochemical features of transition metal complexes and organometallic compounds stem from the simultaneous presence of excited states of different localisations and orbital origins in the same chromophoric molecule, together with high state densities. A brief survey of recently studied systems and reactions shows the present level of understanding and highlights the scientific challenges and possible applications of photoprocesses occurring on different time scales. Factors that influence excited-state lifetimes are discussed and differences in the chemical properties of electronically excited and ground states demonstrated using the radical-like behavior of [Pt-2(II)(P2O5H2)(4)](4-) and electron-transfer reactivity of [Ru(bpy)(3)](2+) as typical examples. It is shown that reactions of long-lived excited states can become ultrafast when a chromophore is inserted into a redox-active supramolecular assembly or attached to an electrode surface. This has important implications for light-energy conversion and manipulation of information at a molecular level. Optically prepared Franck-Condon excited states of many transition metal complexes have an ultrafast chemistry of their own. This includes relaxation to lower excited states, electron transfer, energy transfer or bond splitting. Typical examples are discussed with an emphasis on phenomena occurring only on very short time scales. A final outlook points to the most challenging questions in mechanistic inorganic photochemistry and to possible applications. (C) 2000 Elsevier Science S.A. All rights reserved.