b-Cyclodextrines, appended to a ruthenium complex, have
been employed as hosts for iridium and osmium complexes bear-
ing adamantyl or biphenyl moieties, which form strong
host–guest complexes withb-cyclodextrines (seeFig. 3). In such
systems, photoinduced energy transfer can occur from the
periphery, upon complexation of the iridium units, toward the
central ruthenium acceptor, or switched in the other direction,
from the ruthenium to the periphery when the osmium moieties
are assembled (see Fig. 3) ( 42 ). The lowest excited state is in fact
localized on the osmium center, while the highest luminescent
excited state belongs to the iridium complex (see Fig. 3 right).
The system can be made even more complex to achieve vecto-
rial energy transfer in distinct steps, making the central unit
asymmetric by substituting the coordinating ligands of the
ruthenium with two different CDs. To introduce directionality
in the photoinduced process, botha- andb-CDs have been linked
onto a ruthenium core, which can act as a junction by the selec-
tive binding of tailor-made photoactive guests. In these cases,
anthracene and osmium derivatives were employed as final
energy donor and acceptor, respectively. After self-assembling
of the three different photoactive components (seeFig. 4), and
upon excitation of one of the peripheral anthracene moiety, a vec-
torial unidirectional photoinduced energy transfer process takes
place (Fig. 4)(43).
O
N N
NRuII N
N N
O
Emission intensityIrIIIguest RuIICD OsIIguestONNNN
NNNNNN
NNNNNNNNDEDEDEhn400 500 600
λ (nm)700 800 900FIG. 3. Schematic representation of the assembled RuCD Ir
system in which it is possible, upon light excitation, to funnel the elec-
tronic excitation to the ruthenium core. To switch the direction of the
energy transfer process, Ir complexes must be replaced with the
osmium analogues. Right: emission spectra for the three complexes.
56 CRISTIAN A. STRASSERTet al.