[21] Luminescence spectroscopy

Luminescence involves the emission of photons from excited state molecules or ions. Luminescence from metal ions stands in sharp contrast to that of organic groups. First of all, metal luminophores are quite rare. Although many metal ions absorb light, very few reemit any. The excited states of most transition metal ions are de-excited by efficient radiationless pathways. The only transition metal ion native to a metalloprotein known to luminesce is Cu(I) which, along with other d10 metals, is discussed in this chapter. Transition metal ions are known to luminesce, particularly in the solid state or, in the case of ruthenium, when complexed by nitrogen heterocycles. Complexes of the tris-(bipyridyl)ruthenium(II) type are well-known and useful extrinsic luminescent probes. Of all the metal ions in the periodic table, only certain of the tripositive lanthanide ions, Ln3+, are capable of luminescing in fluid solution at room temperature when complexed to water or other common biological ligands. Ln3+ ions are, however, not native to enzymes or other metalloproteins, although they are known in the biosphere in trace amounts. This chapter treats only systems where metal ions are sources of luminescence emission and not where they act as acceptors of energy from intrinsic or extrinsic donors.