Photochemistry and Photophysics of Coordination Compounds

250 M.T. Indelli et al.
chemical lesion in DNA. Investigations of photoinitiated repair of duplexes
containing a single thymine dimer lesion were carried out with visible light
(400 nm) using both nontethered and tethered complexes (Fig. 17).
The quantum yield for photorepair with a Rh(III)-tethered complex is substantially
(about ca. 30 fold) reduced compared to the noncovalently bound
complex. Since the repair efficiency does not appear to be very sensitive to the
distance between intercalated rhodium complex and the thymine dimer, the
authors suggest that the observed disparity likely results from differences in
π-stacking. In addition, evidences that the repair efficiency diminished with
disruption of the intervening π-stack confirm that the DNA helix mediates
this long-range oxidative repair reaction.
5
Conclusion
A large number of rhodium(III) polypyridine complexes and their cyclometalated
analogues have been investigated from the viewpoint of photochemistry,
photophysics and of their possible applications.
As mononuclear species, Rh(III) polypyridine complexes display interesting
photophysical properties, with lowest excited states of LC type for tris
bis-chelated species, and increasing role of MC states for mixed-ligand halopolypyridine
species. In Rh(III) cyclometalated complexes, the covalent character
of the C – Rh bonds makes the excited state classification less clearcut,
with strong mixing of LC, MLCT, and LLCT character.
Many polynuclear and supramolecular systems containing Rh(III) polypyridine
and related units have been synthesized and studied, taking advantage
of the favorable properties of these units as good electron acceptors and
strong photo-oxidants. In particular, Ru(II)-Rh(IIII) dyads have been actively
investigated for the study of photoinduced electron transfer, with specific
interest in driving force, distance, and bridging ligand effects. A limited number
of supramolecular systems of higher nuclearity have also been produced.
Among these, of particular interest are trinuclear species containing rhodium
dihalo polypyridine units, which can act as two-electron storage components
thanks to their Rh(III)/Rh(I) redox behavior.
Finally, a large amount of work has been devoted to the use of Rh(III)
polypyridine complexes as intercalators for DNA. In this role, they have
shown a very versatile behavior, being used for direct strand photocleavage
marking the site of intercalation, to induce long-distance photochemical
damage or dimer repair, or to act as electron acceptors in long-range electron
transfer processes.
Acknowledgements Financial support from MUR (PRIN 2006) is gratefully acknowledged.