Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 161
chromophore and interconversion between the initially prepared S3 state
and the low-lying S1 level within the pyrenyl subunits have also been evidenced.
The rate constant of the energy transfer from the pyrenyl groups
to the Ru/Os excited state manifold is in good agreement with the Förster
mechanism when the relaxed S1 pyrene state is taken into account. Energy
transfer from the nonrelaxed state most likely involves folded conformations
in which the pyrenyl subunits are strongly interacting with inner subunits of
the tetranuclear core. Such interactions were also suggested by the groundstate
absorption spectrum of the compound [272].
Because octahedral metal complexes can exist in two chiral forms, Λ and
∆, it could be expected that the photophysical properties of dendrimers
containing metal complexes as branching centers could be different for the
various isomers (the situation can be even more complicated in the case of
geometrical isomers). However, the investigation of optically pure isomers of
dinuclear and dendritic-shaped tetranuclear species (37 is the general structural
formula of the tetranuclear systems: optical geometry is not evidenced)
has shown that stereochemical isomerism does not cause any sizeable difference,
at least for the studied compounds [194].
In 37 the emissive state, which involves the peripheral Ru(II) centers, does
not have a sizeable absorption counterpart, since it is a special type of chargeseparated
state, with the formal “hole” localized on a peripheral Ru(II) center
and the “electron” localized on an orbital mainly centered on the pyrazine
moiety of the bridging ligand: the absorption related to such a state has negligible
oscillator strength, due to the poor overlap of the orbitals involved