Photochemistry and Photophysics of Coordination Compounds

12 V. Balzani et al.
induced [30–34] and light-generating [35, 36] electron-transfer processes.
Such studies were further boosted by the fact that, after the energy crisis of
the early 1970s, several photochemists became involved in the problem of solar
energy conversion. Particular interest arose around photosensitized water
splitting [37–41] and it was soon realized [42] that [Ru(bpy)3] 2+ and related
complexes, because of their excited-state redox properties, might function as
photocatalysts for such a process.
As a matter of fact, in the period 1975–1985 a real revolution occurred in
the field of the photochemistry of coordination compounds. The study of intramolecular
ligand photosubstitution, photoredox decomposition, and photoisomerization
reactions was almost completely set apart, about 300 Ru(II)
bipyridine-type complexes were synthesized and investigated in an attempt
(mostly vain) to improve the already outstanding excited-state properties
of [Ru(bpy)3] 2+ [43], and, thanks to an extensive use of pulsed techniques,
huge amounts of data were collected on the rate constants of bimolecular
processes [44]. The high exergonicity of the excited-state electron-transfer
reactions (and/or of their back reactions) offered the opportunity for the
first time to investigate some fundamental aspects of electron-transfer theories
[45], with particular attention to the so-called Marcus inverted region.
4
Supramolecular Photochemistry
4.1
Operational Definition of Supramolecular Species
In the late 1980s, following the award of the 1987 Nobel prize to Pedersen,
Cram, and Lehn, there was a sudden increase of interest in supramolecular
chemistry, a highly interdisciplinary field based on concepts such as molecular
recognition, preorganization, and self-assembling.
The classical definition of supramolecular chemistry is that given by
J.-M. Lehn, namely “the chemistry beyond the molecule, bearing on organized
entities of higher complexity that result from the association of two
or more chemical species held together by intermolecular forces” [46]. There
is, however, a problem with this definition. With supramolecular chemistry
there has been a change in focus from molecules to molecular assemblies
or multicomponent systems. According to the original definition, however,
when the components of a chemical system are linked by covalent bonds, the
system should not be considered a supramolecular species, but a molecule.
This point is particularly important in dealing with light-powered molecular
devices and machines (vide infra), which are usually multicomponent systems
in which the components can be linked by chemical bonds of various
natures.