Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 185
nected rings is obtained (66). Heating regenerates the starting catenane. The
process can be repeated at will, since both the reactions (coordination and
decoordination) are quantitative. It can be noted that in this system the photosubstitution
process, usually considered as an undesired property for Ru(II)
complexes, is instead used to perform the desired function. More recently,
light-induced motion on a rotaxane system has also been obtained by using
the same strategy [361].
6
Ruthenium Complexes and Biological Systems
The effects of the interaction of photoactive ruthenium complexes with biological
structures have been extensively studied. Because of the outstanding
excited-state properties of Ru(II) polypyridine complexes, these systems
have been employed as probes of biological sites, as well as photocleavage
agents and, in recent times, as inhibitors of biological functions [362–369].
Among the species used as luminescent probes, one of the most studied compounds
in the last 15 years is 67 [200–204, 362–379]. This complex is very
weakly emissive in aqueous solution, but becomes strongly emitting in the
presence of DNA, giving rise to the so-called light-switch effect [200, 201].
The reason for such a behavior lies in the electronic properties of this specific
chromophore and in the intercalation ability of the dipyrido[3,2-a:2 ′ ,3 ′ -
c]phenazine (dppz) ligand. In this species, there are several triplet excited
states quite close in energy: (1) a MLCT state directly populated by light excitation,
in which the excited electron resides in the LUMO+1 centered on
the “bpy-like” portion of dppz ligand; (2) a MLCT state in which the excited
electron is located in the LUMO centered on the “phenazine-like” portion of
the dppz ligand; and (3) a ligand-centered (dppz-based) excited state. This
compound represents another example of interplay between multiple MLCT
states, discussed in more detail in Sect. 4.4. The energy gap and order of the
three low-lying excited states mentioned above, as well as their dynamics,
can be modulated by various parameters, including solvent dielectrics, protic
ability of the solvent, and hydrophobic interactions. In a simplified schema-