Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
unidirectional electron transfer between electron donor D and acceptor A via ground state redox
processes (see figure 20).
D +
R -
A
D A -
R
Figure 20: Reversible redox cycle of an electron ferry which can be used in a multicomponent
system to allow for long distant charge separation and electron transfer between distant reactants,
such as D and A in fluid media.
According to Lehn et al., a suitable shuttle species in a multicomponent setup for water reduction
should have the following characteristics: [34]
(1) reversible redox behavior;
(2) suitable redox potentials;
(3) ability to accumulate multiple electrons;
(4) ability to accumulate multiple protons;
(5) good kinetic factors for outer sphere electron (and proton) transfer reactions;
(6) stability toward thermal and photochemical decomposition;
Depending on the particular quenching mechanism, it should be reducible by the excited
chromophore P* or by the photoreduced species P − in the catalytic system (e.g. *[Ru(bpy) 3 ] 2+ or
[Ru(bpy) 3 ] + ). And secondly, the shuttle species itself must be able to reduce water or the reduction
catalyst (-0.86(-1.28) V < E 0 < -0.41 V, at pH 7 vs. NHE) to takeover transfer functionalities
in the reduction half reaction. The converse argumentation applies for the water oxidation
half reaction. To be a proper redox shuttle in the reduction chain, it can be helpful if the
mediator is able to accumulate two or more electrons at about the same reduction potential to
potentially allow for dielectronic reduction. According to natural systems it can also be helpful
if reversible combination with protons can be coupled to the reduction reaction (e.g. quinones
in photosynthesis or homogeneous hydrogenation catalysts). Proton coupled electron transfer
offers a route to compensate charge effects in intercomponent ET reactions. In addition this may
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