exhibit practically identical photophysical properties, since the
lowest energy subunit(s) is in all the cases the identical periph-
eral (bpy) 2 Ru!m-dpp CT state(s).
For heterometallic species, on increasing nuclearity, an unidi-
rectional gradient (center-to-periphery or vice versa) for energy
transfer is hardly obtained with only two types of metals (com-
monly, Ru(II) and Os(II)) and ligands (bpy and 2,3-dpp). In fact,
by using a divergent synthetic approach starting from a metal-
based core, it becomes unavoidable that metal-based building
blocks with high-energy excited states (high-energy subunits)
are interposed between donor and acceptor subunits of the
energy-transfer processes( 17 ). For example, while in the tetra-
nuclear Os[(m-dpp)Ru(bpy) 2 ] 38 þ (OsRu 3 ) species, in which a
central {Os(m-dpp) 3 }^2 þ subunit is surrounded by three {Ru
(bpy) 2 }^2 þ subunits, only the osmium-based core emission is
obtained (acetonitrile, room temperature: lmax¼860 nm;
t¼18 ns; Fem¼ 1 10 ^3 )( 22 ), indicating quantitative energy
transfer from the peripheral Ru-based chromophore to the
center Os-based site, for the larger systems peripheral Ru-based
emission is not quenched ( 17 ). This highlights that although
downhill or even isoergonic energy transfer between nearby
building blocks in the dendrimers based on 2,3-dpp bridging
ligand is fast and efficient, direct downhill energy transfer
between partners separated by high-energy subunits is much
slower and can be highly inefficient. This problem has been over-
come (i) by using a third type of metal center, namely, a Pt(II)
one, to prepare decanuclear species (second-generation
dendrimers) having different metal centers in each“generation”
layer (schematically, OsRu 3 Pt 6 species) ( 23 ) or, more recently,
(ii) in a heptanuclear dendron where the barrier made of high-
energy subunits is bypassed via the occurrence of consecutive
electron-transfer steps ( 24 ). Quite interestingly, this latter study
also suggests that long-range photo-induced electron-transfer
processes do not appear to be dramatically slowed down by inter-
posed high-energy subunits in this class of dendrimers.
IV. Coordination of Metal Ions Inside DendrimersDendrimers containing coordination sites (e.g., amine, amide
groups) in their structure can act as ligands of metal ions. We
can distinguish dendrimers containing a single coordination site,
for example, in the core (Fig. 2c), or multiple coordination sites in
the branches (Fig. 2d).
PHOTOCHEMISTRY & PHOTOPHYSICS OF METAL COMPLEXES 117