Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 139
Fig. 12 Structural formula of two possible MLCT transitions in ruthenium complexes with
large polypyridine ligands
not receive a significant contribution from the chelating nitrogens). In these
systems, the lowest-energy MLCT transition (MLCT0)canhavevanishingoscillator
strength and thus it does not significantly contribute to the absorption
spectrum. On the contrary, a closely lying LUMO+1 (centered on a different
moiety of the large ligand) receives a significant contribution from the
chelating nitrogens, so it is largely coupled with the metal-based HOMO(s);
as a consequence, its corresponding MLCT transition (the MLCT1 transition)
dominates the absorption spectrum. Since reduction takes place in the
LUMO, the linear relationship between absorption spectra and redox potential
cannot be followed. This case will also be discussed in Sects. 5 and 6, for
specific systems.
As far as the relationship between emission spectra and redox potentials is
concerned, whether it is followed or not depends on how fast the interconversion
between the MLCT1 and MLCT0 states is, compared to the intrinsic decay
of the MLCT1 excited state (here it is assumed that MLCT0 is lower in energy
than MLCT1; otherwise, the relationship is always followed, except for very
particular cases). Solvent, temperature, driving force, and medium effects are
very important in this regard. For example, at room temperature in fluid