Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds 11
energy transfer:
∗ A+B→ A + +B –
∗ A+B→ A – +B +
oxidative electron transfer (6)
reductive electron transfer (7)
∗ ∗
A+B→ A+ B energy transfer . (8)
Bimolecular electron- and energy-transfer processes are important because
they can be used (1) to quench an electronically excited state, i.e., to prevent
its luminescence and/or intramolecular reactivity, and (2) to sensitize other
species, for example, to cause chemical changes of, or luminescence from,
species that do not absorb light.
Simple kinetic arguments show that only the excited states that live longer
than ca. 10 –9 s may have a chance to be involved in encounters with other solute
molecules. Usually, in the case of metal complexes only the lowest excited
state satisfies this requirement. The kinetic aspects of energy- and electrontransfer
processes are discussed in detail elsewhere [17, 20, 23]. A point that
must be stressed is that an electronically excited state is a species with quite
different properties compared with those of the ground-state molecule. In
particular, because of its higher energy content, an excited state is both
a stronger reductant and a stronger oxidant than the corresponding ground
state. To a first approximation, the redox potentials of the excited-state couples
may be calculated from the potentials of the ground-state couples and the
one-electron potential corresponding to the zero–zero excited-state energy,
E0–0 , as shown by Eqs. 9 and 10 [25]:
E � A + / ∗ A � ≈ E � A + /A � – E 0–0
E � ∗ A/A – � ≈ E � A/A –� + E 0–0 . (10)
3.2
Bimolecular Processes Involving Metal Complexes
From an exhaustive monograph that appeared in 1970 [2] and a multiauthored
volume of 1975 [26], it clearly appears that most of the interest was
then focused on ligand photosubstitution reactions, photoredox decomposition,
and photoisomerization reactions, while bimolecular processes were
barely investigated. This picture, however, changed profoundly in a few years
following the extensive work carried out by several research groups on the luminescence
of coordination compounds [27–29] and the discovery that the
lowest excited state of a number of Cr(III), Ru(II), and Os(II) complexes exhibits
a sufficiently long lifetime in fluid solution to be able to participate
as a reactant in bimolecular reactions [25, 30]. A further advantage offered
by Ru(II) and Os(II) bipyridine-type complexes is that they can undergo reversible
redox reactions both in the ground and excited state, so they were
soon used as reactants and, even more interesting, as mediators, in light-
(9)