Photochemistry and Photophysics of Coordination Compounds

128 S. Campagna et al.
of stabilization of the cationic [Ru(bpy)3] 2+ species, whereas formation of
neutral [Ru(bpy)2X2] complexes is favored in low-polarity solvents. Photoracemization
of [Ru(bpy)3] 2+ [86] also occurs with low quantum yield
(2.9 × 10 –4 in water at 25 ◦ C). This process can be accounted for by a rearrangement
of the square pyramidal primary photoproduct into a trigonal
bipyramidal intermediate which can lead back to either the ∆ or the Λ isomer
[64].
Ligand photodissociation is, of course, a drawback for the use of [Ru
(bpy)3] 2+ in practical applications. To avoid ligand photodissociation one
should prevent population of 3 MC and/or ligand dissociation from 3 MC.
Population of 3 MC can be prevented or at least reduced by: (a) addition of
sufficient quencher to capture 3 MLCT before surface crossing to 3 MC can
occur; (b) working at low temperature; (c) increasing the energy gap between
3 MLCT and 3 MC; and (d) increasing pressure [87, 88]. Ligand dissociation
from 3 MC can also be reduced by (e) avoiding coordinating anions in solvent
of low dielectric constant and (f) linking together the three bpy ligands so
as to form a single caging ligand which encapsulates the metal ion. Point (a)
is experimentally difficult, since thermal equilibration is quite a fast process.
Points (c) and (f) are particularly interesting and much effort has been
made along such directions [1, 89, 90]. It should be considered that in most
of the [Ru(bpy)3] 2+ derivatives, the 3 MLCT state is shifted to lower energies
[1], whereas the energy of the 3 MC state usually does not change. This
leads to an increased energy gap between MLCT and MC states and decreased
photolability. As a consequence, photosubstitution is a minor problem in
most ruthenium polypyridine complexes. It should be considered, however,
that decreasing the energy of the 3 MLCT level increases the Franck–Condon
factors for radiationless decay to the ground state, leading to decreased luminescence
lifetimes and quantum yields. The rate of radiationless decay can
be decreased by extending the delocalization of the promoted electron on
suitable aromatic ligands [78, 91, 92].
Finally, it should also be noted that the photolabilization of ligands can
be a profitable photochemical process: for example, a synthetic route to trisheteroleptic
Ru complexes involves photosubstitution of ligands [93] and
photochemical, reversible ligand exchange has been proposed to be used to
photoswitch the complexation activity in a ruthenium complex containing
a scorpionate terpyridine ligand [94].
3.5
Quenching of the 3 MLCT Excited State:
Energy and Electron Transfer Processes
The lowest 3 MLCT excited state of [Ru(bpy)3] 2+ lives long enough to encounter
other solute molecules (even when these are present at relatively low
concentration) and possesses suitable properties to play the role of energy