Photochemistry and Photophysics of Coordination Compounds

162 S. Campagna et al.
(for similar systems, see Sect. 4.4) [194]. A recently investigated, closely related
tetranuclear dendritic-shaped compound, where the bridging ligand
is the asymmetric PHEHAT ligand (PHEHAT = 1,10-phenanthrolino[5,6b]-1,4,5,8,9,12-hexaazatriphenylene),
displays similar photophysical properties
[274].
Mixed-metal Os(II)–Ru(II) dendrimers whose bridging ligands contain
ether linkages have also been reported: compounds 38 and 39 are two examples
[275, 276]. The tetranuclear species in 38 exhibits quantitative energy
transfer from the Ru(II) chromophores to the Os(II) core [275], whereas
for 39 the efficiency of the energy transfer process is highly temperature
dependent; in fact the Ru-to-Os energy transfer is highly efficient at low temperature,
whereas at room temperature it does not compete with the intrinsic
decay of the Ru(II)-based subunits [276].
In most of the mixed-metal dendrimers featuring energy transfer and
containing Ru(II) subunits, the Ru(II) centers play the role of the energy
donor components (and usually Os(II) centers are the acceptors). Examples
of photoactive dendrimers in which Ru(II) subunits behave as acceptor components
are the tetranuclear compound 40 [277] and its higher-generation,
Y-shaped octanuclear species, in which four additional Ir(III) chromophores
have been connected at the two peripheral Ir(III) subunits of the compound
40 (JAG Williams, personal communication). Here, efficient energy transfer
takes place for the Ir(III) cyclometallated subunits toward the single Ru(II)
polypyridine chromophore, which acts as the energy trap of the assemblies.
The Ru(II) subunit then emits with relatively high quantum yield (0.12 in
degassed acetonitrile at room temperature) and long lifetime (1.6 µs) [277].