Nn

1. The trivalent cations of the lanthanides have photoluminescent properties that
are favorable for several kinds of applications. However, it is difficult to generate
this luminescence by direct excitation of the lanthanide ion, because of the ions'
poor ability to absorb light. Organic chromophores do a better job at absorbing
light. The energy absorbed by such chromophores can be transferred to a nearby
lanthanide ion, which is then able to emit its characteristic luminescence. The
organic chromophore acts like some sort of 'antenna'
Various lanthanide complexes containing organic antennae are known to show
efficient photoluminescence. The basic architecture of these systems is depicted
in the picture below.
Typical emission spectra of luminescent lanthanide complexes containing
antenna chromophores are shown below. The emissions are coming from
terbium(III), dysprosium(III), europium(III) and samarium(III), respectively. The
antenna was excited at 337 nm.

3. Making lanthanide ions shine brightly
Sensitization of Lanthanide Luminescence

5. • The lanthanides usually exist as trivalent cations, in which case their
electronic configuration is (Xe) 4fn, with n varying from 1 (Ce3+) to
14 (Lu3+). The transitions of the f-electrons are responsible for the
interesting photophysical properties of the lanthanide ions, such as
long-lived luminescence and sharp absorption and emission lines.
The f-electrons are shielded from external perturbations by filled 5s
and 5p orbitals, thus giving rise to line-like spectra. The f-f
electronic transitions are forbidden, leading to long excited state
lifetimes, in the micro- to millisecond range. The forbidden nature
of the f-f transitions is also reflected in low extinction coefficients,
making direct photoexcitation of lantahide ions difficult. This can be
overcome by using energy transfer from organic chromophores to
lanthanide ions. See also: "Luminescent Lanthanide Ions: Making
Them Shine Brightly" (Sensitization of lanthanide luminescence).
Photophysical Properties of Trivalent
Lanthanide Ions