Photochemistry and Photophysics of Coordination Compounds

24 V. Balzani et al.
transfer should be approximately equal to the sum of the attenuation factors
for two separated electron-transfer processes, i.e., βel for electron transfer between
the LUMOs of the donor and acceptor, and βht for the electron transfer
between the HOMOs (superscript ht is for hole transfer from the donor to the
acceptor). This prediction has been confirmed by experiments [74].
The spin selection rules for this type of mechanism arise from the need
to obey spin conservation in the reacting pair as a whole. This allows the
exchange mechanism to be operative in many cases in which the excited
states involved are spin-forbidden in the usual spectroscopic sense. Thus, the
typical example of an efficient exchange mechanism is that of triplet–triplet
energy transfer:
∗
A(T1)–L – B(S0) → A(S0)–L – ∗ B(T1) . (38)
Exchange energy transfer from the lowest spin-forbidden excited state is expected
to be the rule for metal complexes [61, 75].
Although the exchange mechanism was originally formulated in terms of
direct overlap between donor and acceptor orbitals, it is clear that it can be
extended to cover the case in which coupling is mediated by the intervening
medium (i.e., the connecting bridge), as discussed above for electron-transfer
processes (superexchange mechanism) [61].
5
Coordination Compounds as Components
of Photochemical Molecular Devices and Machines
In the last few years, a combination of supramolecular chemistry and photochemistry
has led to the design and construction of supramolecular systems
capable of performing interesting light-induced functions. Photoinduced energy
and electron transfer are indeed basic processes for connecting light
energy inputs with a variety of optical, electrical, and mechanical functions,
i.e., to obtain molecular-level devices and machines [48, 55]. We will now describe
a few classical examples of molecular devices and machines in which
coordination compounds are used to process light signals or to exploit light
energy. Other examples are, of course, described in the chapters dealing with
the complexes of the various metals.
5.1
A Molecular Wire
An important function at the molecular level is photoinduced energy and
electron transfer over long distances and/or along predetermined directions.
This function can be obtained by linking donor and acceptor components by
a rigid spacer, as illustrated in Fig. 14.