Complexes of the rare earth element (lanthanide) europium have the ability
to emit red light over a longer time span after excitation by, for example,
light of a suitable wavelength or the energy of a prior redox reaction. This
luminescence makes them interesting candidates for applications in
biological test systems or as sensors. In addition, they can also be used in
functional units that emit light. Because europium complexes do not
demonstrate any noteworthy absorption in the visible portion of the light
spectrum, between about 400 and 700 nm, other strongly absorbing molecules
must be used as light-gathering antennas to achieve sufficient
photoluminescence under these conditions. These antennas must absorb the
excitation light and transfer the energy it contains to the europium
complex, which can then emit.
A team of researchers from the Netherlands and Russia has now more closely
examined the use of a transition metal complex as the light gathering
component for the photoluminescence of europium. They combined an
iridium(III) phenylpyridine complex with a europium(III) terpyridine
chelate. When the iridium compound alone is excited by light with a
wavelength of 400 nm, it emits blue-green light (wavelengths between 460 and
491 nm). The europium complex emits red light with a 615 nm wavelength when
stimulated by 350 nm light. When both components are mixed in the right
ratio, they form a stable adduct made of one europium and two iridium units.
This assembly begins to glow white after irradiation with light that has a
wavelength of 400 nm, which is only absorbed by the iridium component.
The iridium component is thus clearly capable of transferring a portion of
the energy it absorbs to the europium complex so that it too is stimulated
to emit its characteristic light. The blue-green iridium radiation and the
red europium radiation then mix, resulting in light that appears white.