Luminescent metal complexes: Diversity of excited states

The photoluminescence of metal complexes has attracted much recent interest since it can be utilized for a variety of applications such as optical sensors and LEDs. Moreover, the emission behavior provides a probe for the investigation of photoreactions including artificial photosynthesis. In this review, emitting compounds are classified according to the nature of their excited states: metal-centered, ligand-to-metal charge transfer, metal-to-ligand charge transfer, ligand-to-ligand charge transfer, metal-to-metal charge transfer, ligand-centered (or intra ligand), and intraligand charge transfer excited states. Complexes of transition metals (d(n) with n = 0-10), main group metals (s(2)),lanthanides and actinides (f(n)) are included in our discussion. However, this review does not cover the photoluminescence of metal complexes comprehensively, but illustrates this subject by selected examples. The viewpoint is that of a coordination chemist and not of a spectroscopist. Accordingly, molecular complexes which emit under ambient conditions are preferably chosen.