Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
formula for the activation energy ∆G ‡ , based on the reorganization energy λ and free reaction
enthalpy ∆G ET .
∆G ‡ = (∆G ET + λ) 2
4λ
Interestingly, the quadratic relationship between ∆G ‡ and ∆G ET causes a dependency of the rate
constant for the ET to pass through a maximum k ET, max in the case of -∆G ET = λ (∆G ‡ = 0).
Otherwise, in the case of -∆G ET > λ or -∆G ET < λ the ET reaction will be slower, even though
the reaction will be very exergonic. This consequence of the Marcus theory is termed Marcus
inverted region.
A combination of the above reaction gives the basic equation of Marcus theory:
k ET = 2π̷h ∣ H AB ∣ 2 1
√ 4πλ kB T exp {−(∆G ET + λ) 2
}
4λ k B T
After having covered photophysical, photochemical and electron transfer aspects the essential
influence of the catalytic centers on the over all efficiency shall be discussed.
1.9.3 Oxidation Catalysts - Solar Fuels
From the above discussion it is clear that the cleavage of water into oxygen and hydrogen
represents a four-electron redox reaction which can be subdivided into oxidation and reduction
half-reactions and which requires an energy of 1.23 eV per transferred electron. The main problem
is the large overpotential (activation energy) for the multi-electron processes which has to be
applied to the electrodes in an electrochemical setup in addition to the required energy of the
thermodynamically uphill process. [45, 28] A key step for light-induced water splitting will be the
discovery of an efficient catalyst which achieves a stepwise one-electron transfer oxidation reaction
and, therefore, bears a lower overpotential toward water oxidation (see figure 23).
Such molecules have to be multiply reducible or have to exhibit multiple electron “holes” to gather
up to four electrons. Between the binding of H 2 O and the release of O 2 , three ideal intermediates
have to be considered to be involved in the water oxidation reaction at the catalyst: C Ox –OH,
C Ox =O and C Ox –OOH. Accordingly, fitting binding sites and fine-tuned redox potentials of the
involved processes have to be present in the catalyst.
Plants, algae, and cyanobacteria oxidize water with the help of solar energy and a µ-oxo bridged
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