Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 159
a second-generation dendrimer of this family (in the sketch of the compound,
the peripheral ligands, schematized as NˆN, stand for 2,2 ′ -bipyridine) [254].
For the dendrimers of this series, the modular synthetic strategy [252] 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, of
its coordination sphere (which in its turn depends on the metal position, inner
or outer, within the dendritic array) and on 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 metalbased
subunit belonging to the same dendritic layer. Therefore, the synthetic
control translates into control of specific properties, such as the direction
of electronic energy flow within the dendritic array (antenna effect) [245].
For example, in 35 the lowest excited-state level involves the peripheral subunit(s),
and the emission of the species (acetonitrile, room temperature:
λmax = 780 nm; τ = 60 ns; Φ = 3 × 10 –3 ) is assigned to a (bpy)2Ru→ µ-dpp
MLCT triplet state [254]. Excitation spectroscopy indicates that quantitative
energy transfer takes place from inner subunits to the peripheral ones [254].
Because of the energy gradient between the dendritic layers, the energy transfer
is ultrafast (see later), occurring in the femtosecond timescale. On the
basis of the above discussion, it is not surprising that all the homometallic
dendrimers of the same family, independent of the number of Ru subunits
(i.e, tetranuclear [255, 256], decanuclear [253, 254], and docosanuclear [251–
253], as well as hexanuclear [257, 258], heptanuclear [259], and tridecanuclear
species [260], which have particular connections/geometries), exhibit
practically identical photophysical properties, since the lowest-energy subunit(s)
is in all cases the identical peripheral (bpy)2Ru(µ-dpp) MLCT state(s).
A nonanuclear species has also been prepared [261], but its photophysical
properties have not yet been reported.
On increasing nuclearity, a unidirectional gradient (center-to-periphery
or vice versa) for energy transfer is hardly obtained with only two types of
metals (commonly, 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 [245]. For example, while in
the tetranuclear [Os{(µ-dpp)Ru(bpy)2}3] 8+ (OsRu3) species, in which a central
{Os(µ-dpp)3} 2+ subunit is surrounded by three {Ru(bpy)2} 2+ subunits,
only the osmium-based core emission is obtained (acetonitrile, room temperature:
λmax = 860 nm; τ = 18 ns; Φ = 1 × 10 –3 ) [262], indicating quantitative
energy transfer from the peripheral Ru-based chromophore to the central
Os-based site; for the larger systems the peripheral Ru-based emission is not
quenched [245, 253]. This result highlights that although downhill or even
isoergonic energy transfer between nearby building blocks in the dendrimers