Photochemistry and Photophysics of Coordination Compounds

16 V. Balzani et al.
Fig. 9 Schematic representation of excimer and exciplex formation in a supramolecular
system
Compared with the “monomer” emission, the emission of an excimer or exciplex
is always displaced to lower energy (longer wavelengths) and usually
corresponds to a broad and rather weak band.
Excimers are usually obtained when an excited state of an aromatic
molecule interacts with the ground state of a molecule of the same type. For
example, between the excited and ground states of anthracene units. Exciplexes
are obtained when an electron donor (acceptor) excited state interacts
with an electron acceptor (donor) ground-state molecule; for example, between
excited states of aromatic molecules (electron acceptors) and amines
(electron donors). Excited states of coordination compounds are seldom involved
in excimers or exciplexes, since their components (metal and ligands)
have already used their electron donor or acceptor properties in forming
the complex. Furthermore, the three-dimensional structure of coordination
compounds usually prevents strong electronic interaction with other species.
However, for some square planar complexes excimer emission has long been
reported [54] and can indeed be found for some families of Au and Pt complexes,
as discussed in other chapters of this volume.
The working mechanisms of a number of biological and artificial molecular
devices and machines are based on photoinduced electron- and energytransfer
processes [20, 48, 55]. Since these processes have to compete with
the intrinsic decays of the relevant excited states, a key problem is that of
maximizing their rates. It is therefore appropriate to summarize some basic
principles of electron- and energy-transfer kinetics. [56].
4.3
Electron Transfer
4.3.1
Marcus Theory
Electron-transfer processes involving excited-state and/or ground-state molecules
can be dealt with in the frame of the Marcus theory [57] and of the