Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
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180 S. Campagna et al.<br />
charges to the complex thus making feasible oxidation to the Mn2 III,IV state,<br />
which would have otherwise been impossible on thermodynamic grounds.<br />
The lig<strong>and</strong> exchange <strong>and</strong> reorganization <strong>of</strong> the manganese ion coordination<br />
sphere, which has a nonneutral effect from the viewpoint <strong>of</strong> the charge, would<br />
be a charge compensating process, analogous to proton release occurring<br />
in most <strong>of</strong> the oxidation states <strong>of</strong> the “manganese cluster” in natural systems<br />
[321, 325]. It can be inferred that charge compensating processes are<br />
needed requirements to maintain the oxidation potential <strong>of</strong> the redox-active<br />
catalytic site roughly constant when moving along the various steps <strong>of</strong> the<br />
overall hole accumulating process, an aspect that should be well taken into<br />
account in designing new systems.<br />
Recently a mixed Ru–Mn2 species featuring a photoinduced chargeseparation<br />
state with an impressive lifetime (0.6 ms at room temperature<br />
<strong>and</strong> 0.1–1 sat140 K, comparable to many <strong>of</strong> the naturally occurring chargeseparated<br />
states in photosynthetic systems) has been reported [334]. The<br />
slow charge-recombination rate obtained for such a species has been mainly<br />
attributed to the large reorganization energy connected with the inner reorganization<br />
<strong>of</strong> the manganese subunit already mentioned (about 2 eV for the<br />
compound in [334]). This would suggest that there is no needed to look for<br />
charge-recombination processes occurring in the Marcus inverted region to<br />
obtain long-lived separated states, since the large inner reorganization energy<br />
typical <strong>of</strong> the manganese systems could lead to the same (or better) result.<br />
5.8<br />
Photocatalytic Processes Operated by Supramolecular Species<br />
5.8.1<br />
Photogeneration <strong>of</strong> Hydrogen<br />
Since the early papers appeared in the 1970s [335–339], Ru(II) polypyridine<br />
complexes have been extensively used to produce hydrogen in heterogeneous<br />
cycles under light irradiation, by using sacrificial donor species (most commonly<br />
amines), electron acceptor relays (usually methyl viologens), <strong>and</strong> colloidal<br />
metal catalysts (Pt, Rh, etc.). This aspect <strong>of</strong> Ru(II) photochemistry has<br />
been extensively reviewed [1, 340] <strong>and</strong> will not be discussed in detail here. We<br />
will discuss some recent papers in which a (supramolecular) multicomponent<br />
approach is used.<br />
One <strong>of</strong> the important steps in designing a multicomponent hydrogen<br />
evolving system would be to assemble in the same (supramolecular) system<br />
as many key components as possible. Key components would be (based on the<br />
systems operating in heterogeneous schemes): (a) light-harvesting units (antennae);<br />
(b) a charge-separation unit made <strong>of</strong> a photosensitizer (the energy<br />
trap <strong>of</strong> the antenna, if an antenna is present), an electron acceptor, <strong>and</strong> an<br />
electron donor; <strong>and</strong> (c) a catalyst [280, 297, 341–345]. The compound 60 [346]