Photochemistry and photophysics of coordination compounds: Overview and general concepts

Investigations in the field of the photochemistry and photophysics of coordination compounds have proceeded along several steps of increasing complexity in the last 50 years. Early studies on ligand photosubstitution and photoredox decomposition reactions of metal complexes of simple inorganic ligands (e.g., NH3, CN-) were followed by accurate investigations on the photophysical behavior (luminescence quantum yields and lifetimes) and use of metal complexes in bimolecular processes (energy and electron transfer). The most significant differences between Jablonski diagrams for organic molecules and coordination compounds are illustrated. A large number of complexes stable toward photo decomposition, but capable of undergoing excited-state redox processes, have been used for interconverting light and chemical energy. The rate constants of a great number of photoinduced energy- and electron-transfer processes involving coordination compounds have been measured in order to prove the validity and/or extend the scope of modern kinetic theories. More recently, the combination of supramolecular chemistry and photochemistry has led to the design and construction of supramolecular systems capable of performing light- induced functions. In this field, luminescent and/or photoredox reactive metal complexes are presently used as essential components for a bottom-up approach to the construction of molecular devices and machines. A few examples of molecular devices for processing light signals and of molecular machines powered by light energy, based on coordination compounds, are briefly illustrated.