Most metal ion complexes absorb visible and/or UV radiation; however, very
few reemit even a small fraction of the absorbed energy in the form of UV or
visible photons. This situation is a consequence of facile nonradiative deexcitation
pathways that compete efficiently with radiative modes. The d-electron
excited states of transition metal complexes are strongly coupled to the environment
via the ligand field which provides an efficient deexcitation mechanism,
accounting for the rarity of luminescent complexes of this type. On the other
hand the trivalent lanthanide ions, symbolically Ln(III), and their complexes
have their lowest lying excited states comprised of 4f” configurations. The 4j
orbitals are largely shielded from the environment and are minimally involved
in bonding. The total spread of ligand-field splitting of an f-electron term is
rarely more than a few hundred cm-I, while for transition metal complexes
d-electron term energy splittings of 25,000 cm-’ and more are known. As a
consequence, radiationless deexcitation processes in Ln(II1) complexes are relatively
inefficient, and the emission of radiation as luminescence is able to
compete in many instances. Transitions between states of 4f” configuration
are electric dipole forbiddeh and consequently weak both in absorption and
emission. The probability of such transitions is so low that molar extinction
coefficients rarely exceed 10M-l cm-’ and radiative lifetimes as long as several
msec are common. This behavior contrasts sharply with organic fluorescent
molecules where molar extinction coefficients of tens of thousands and radiant
lifetimes in the nsec range are common. In this article the term luminescence is
used to refer to radiative emission in general. Fluorescence and phosphorescence
are reserved to designate the singlet-singlet and triplet-singlet emission of organic
molecules, respectively.