Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
reaction (ET) which leads to the formation of the groundstate sensitizer in an oxidized (oxidative
quenching) or reduced form (reductive quenching). These redox reaction involving quenching
mechanisms lead to the formation of charge pairs and, hence, represent a particularly important
step toward long term charge separation.
In accordance to the HOMO-LUMO 1 MLCT-absorption, also ground state redox chemistry
typically involves the metal centered HOMO and the LUMO which is localized on the ligand.
Hence, electrochemical GS-oxidation of [Ru(bpy) 3 ] 2+ -type complexes takes place at the metal
fragment with simultaneous formation of a complex with π M (t 2g ) 5 -configuration (compare
figure 19). Whereas GS-reduction takes place at a ligand centered orbital with retention of the
π M (t 2g ) 6 -configuration of the metal fragment. [40] The oxidation and reduction potential of the
ground state ions (-1.28 and +1.26 V in water vs. NHE) are stronger than that of the excited
biradical species and may also be used to oxidize or reduce water.
It is important that these processes are quasi reversible so that a reductively or oxidatively
quenched chromophore can be regenerated without decomposition (requirement 1). This
regeneration of the Ru(II) chromophore by a second component S may complete the hypothetical
light-driven catalytic electron transfer cycle (excitation, quenching, regeneration), e.g. in the
photoreduction of water.
It follows that *[Ru(bpy) 3 ] 2+ is at the same time a fairly good energy donor, electron acceptor,
and electron donor. [36] Whereas the potentials of charged quenching products are very high
and are definitely sufficient to oxidize or reduce water respectively. Although its excited state
*[Ru(bipy) 3 ] 2+ is thermodynamically capable of both oxidizing and reducing water at neutral pH,
this has not been found to occur.
1.9.2 Redox Systems for Charge Separation
As described before, light excitation increases both the oxidizing and the reducing power of a
chromophore. Therefore, light excitation of P can often be utilized by subsequent electron transfer
(ET) processes. Unfortunately, a direct oxidation or reduction of water by *[Ru(bipy) 3 ] 2+ -type
complexes has not been found to occur. Thus, suitable redox mediators R have to account for the
fast and irreversible charge separation via quenching mechanisms to gather or deliver electrons
from or to the chromophore. Furthermore, such redox ferries have the function to mediate the
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