completely quenched with concomitant sensitization of the
orange [Ru(bpy) 3 ]^2 þ phosphorescence (lmax¼610 nm). These
results show that energy-transfer processes with very high effi-
ciency (ca. 90%) take place from the very short lived (nanosecond
time scale) potentially fluorescent excited states of the aromatic
units of the wedges to the relatively long lived (microsecond time
scale)^3 MLCT level of metal-based dendritic core. Dendrimer 12 þ
is therefore an example of a light-harvesting antenna system, as
well as of a species capable of changing the color of the incident
light. It should also be noted that in aerated solution, the phos-
phorescence intensity of the [Ru(bpy) 3 ]^2 þdendritic core is more
than twice intense as that of the“free”[Ru(bpy) 3 ]^2 þparent com-
pound because the dendrimer branches protect the core from
dioxygen quenching( 13 ).
Alight-harvestingantennabasedonthesamechromophoreshas
been obtained by a proton-driven self-assembly of the [Ru(bpy)
(CN) 4 ]^2 complex and a dendrimer containing a 1,4,8,11-
tetraazacyclotetradecane (cyclam) core appended with four den-
drons identical to that of compound 12 þ( 14 ). In the self-assembled
structure, the two protons are shared between the cyclam core and
the cyanide ligand of the Ru(II) complex. The advantage of the
present system is the possibility of tuning the emission wave-
length by careful choice of a different metal complex without a
time-consuming synthesis of a new dendrimer.
B. DENDRIMERS WITHMETALCOMPLEXES ASBRANCHINGCENTERS
In dendrimers with metal complexes as branching centers
(Fig. 2b), a key role is played by polytopic-chelating ligands (the
bridging ligands), which can coordinate more than a single metal
center and control the shape of the polynuclear array and the
electronic interaction between metal chromophores, thereby
allowing for intercomponent energy or electron-transfer
processes.
Among such type of dendrimers, an important family is based
on the 2,3-bis(2^0 -pyridyl) pyrazine (dpp) bridging ligand. Within
such a family, the largest species contains 22 metal centers
( 15 ). The decanuclear compound 2 shown in Fig. 6 is a second-
generation dendrimer of this family ( 16 ). For the dendrimers of
this series, the modular synthetic strategy allows a high degree
of synthetic control in terms of the nature and position of metal
centers, bridging ligands, and terminal ligands. Since the
excited-state level of each metal center in the dendrimer depends
on the nature of the metal, its coordination sphere (which in its
PHOTOCHEMISTRY & PHOTOPHYSICS OF METAL COMPLEXES 115