Photochemistry and Photophysics of Coordination Compounds

176 S. Campagna et al.
signature. This characterization was made by extensive work (including spectroelectrochemistry
in various solvents, as well as at different pH values in
aqueous solution) to identify the intermediates and to clarify the effect of
pH on the processes [315–317]. Indeed, it has been demonstrated that in
some solvents, processes 2 and 4 take place simultaneously, so it would be
more correct to talk of proton-coupled electron transfer instead of successive
electron/proton transfer events. At pH 4, only a fully reversible, coupled twoelectron/two-proton
transfer process is observed for 51. This is a rare example
of a proton-coupled multielectron transfer reaction [317] (FM MacDonnell,
personal communication).
Another interesting example of photoinduced multielectron collection,
this time at a metal center rather than at a ligand site, has been reported for
a series of trimetallic mixed-metal species [318–320]. The most recent example
of the series is 53 [320]. In this species, two Ru(II) chromophores (the
light absorber units, LA) are linked to one Rh(III) center, which represents the
electron collection (EC) core, through polyazine bridging ligands (BLs). The
absorption spectrum of this species is dominated by the absorption of the Ru
LA subunits, while the reduction properties identify the Rh(dσ ∗ ) orbital as
the LUMO [319]. Upon excitation of the peripheral Ru(II) chromophore, an
oxidative electron transfer to the Rh(III) center takes place (k = 1.2 × 10 8 s –1 ),
populating a triplet Ru(II)-to-Rh(III) charge transfer state. In the presence of
a sacrificial donor, dimethylaniline, which restores the Ru(II) center(s), the
starting compound undergoes a net photoreduction process with formation
of [{(bpy)2Ru(dpp)}2Rh I ] 5+ , as also demonstrated by spectroelectrochemistry,
in which two chlorides have been lost (probably by irreversible chlo-