Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 119
photochemistry, photophysics, photocatalysis, electrochemistry, photoelectrochemistry,
chemi- and electrochemiluminescence, and electron and energy
transfer. Mostly in the last 15 years, Ru(II) polypyridine complexes have
also contributed highly to the development of supramolecular photochemistry,
and in particular to its aspects related to photoinduced electron and energy
transfer processes within multicomponent (supramolecular) assemblies,
including luminescent polynuclear metal complexes, light-active dendrimers,
artificial light-harvesting antennae, photoinduced charge-separation devices,
luminescent sensors, and light-powered molecular machines.
Because of the enormous number of Ru(II) complexes investigated from
a photochemical viewpoint and the variety of multicomponent structures
prepared and light-based functions explored, it is impossible to make an exhaustive
review. In this chapter, we recall some basic concepts on ruthenium
photochemistry and discuss in some detail a few selected topics, particularly
those that have developed or emerged during the last 15 years. In this way we
also hope to give an overview of some research directions which ruthenium
photochemistry allows to be explored. An exhaustive review [1] published
about 20 years ago collects photochemical, photophysical, and redox data of
several hundreds of Ru(II) polypyridine complexes. Another extensive review
was published about 10 years ago [2], dealing with the luminescence
properties of polynuclear transition metal complexes, most of them containing
Ru(II) polypyridine subunits (interestingly, in the former review [1] less
than ten polynuclear Ru complexes were reported). A review focused on the
photophysical properties of Ru(II) complexes with tridentate polypyridine
ligands [3] has also been published. All these review articles contain more or
less comprehensive tables of data. Enlightening articles on some basic properties
of Ru(II) polypyridine complexes are also available [4–8].
The very large majority of photochemical investigations on ruthenium
complexes deal with Ru(II) polypyridine species. For such a reason, as also
implicitly suggested above, we will limit our discussion to these species. Other
photoactive compounds containing ruthenium metals, including ruthenium
porphyrins, are not included in this article.
2
Structure, Bonding, and Excited States of Ru(II) Polypyridine Complexes
Ru2+ is a d6 system and the polypyridine ligands are usually colorless molecules
possessing σ donor orbitals localized on the nitrogen atoms and
π donor and π∗ acceptor orbitals more or less delocalized on aromatic rings.
Following a single-configuration one-electron description of the excited state
in octahedral symmetry (Fig. 1a), promotion of an electron from a πM metal
ligand orbitals gives rise to metal-to-ligand charge transfer
orbital to the π∗ L
(MLCT) excited states, whereas promotion of an electron from πM to σ∗ M or-