turn depends on the metal position, inner or outer, within the
dendritic array), and the ligands, each metal-based subunit is
characterized by specific excited-state properties, which because
of the symmetry of the dendritic structure are usually identical
for each metal-based subunit belonging to the same dendritic
layer. Therefore, a judicious design allows that the synthetic con-
trol translates into control on specific properties, such as the
direction of electronic energy flow within the dendritic array
(antenna effect) ( 17 ). For example, in the complex shown in Fig. 6,
the lowest excited-state level involves the peripheral subunit(s),
and the emission of the species (acetonitrile, room temperature:
lmax¼780 nm; t¼60 ns; Fem¼ 3 10 ^3 ) is assigned to a
(bpy) 2 Ru!m-dpp CT triplet state ( 16 ). Excitation spectroscopy
indicates that quantitative energy transfer takes place from
inner subunits to the peripheral ones ( 16 ). Because of the energy
gradient between the dendritic layers, the energy transfer is
ultrafast (femtosecond timescale). On the basis of the above dis-
cussion, it is not surprising that all the homometallic dendrimers
of the same family, independent of the number of Ru subunits
(i.e., tetranuclear ( 18 ), decanuclear ( 15 ), docosanuclear ( 16 ), as
well as hexanuclear ( 19 ), heptanuclear ( 20 ), and tridecanuclear
species ( 21 ), which have particular connections/geometries)
FIG. 6. Formula of a decanuclear Ru^2 þmetal complex with 2,3-bis
(2^0 -pyridyl) pyrazine (dpp) bridging ligands and 2,2^0 - bipyridine (bpy)
peripheral ligands, and the corresponding scheme (Fig. 2b).
116 VINCENZO BALZANIet al.