Viologen acceptors

Excitation of the complexes leads to photoinduced electron transfer from the excited ruthenium polypyridyl site to the viologen acceptor. The Ru2+ site is restored through electron transfer from the TEOA or back-electron transfer from the bipyridine, while the viologen is oxidized by the electrode, thus generating the photocurrent. As illustrated in Figure 5.51, this mechanism is supported by experiments in which the electron acceptor 4ZV (see Figure 5.50) reduced the... 

Application of Hush theory to the observed IPCT bands yielded information about the relationship between optical and thermal ET in these systems. The redox potentials of both the metal dithiolene donors and the viologen acceptors can be systematically varied, which, in turn, tunes the thermodynamic driving force for electron transfer. The researchers found that the IPCT band energy increases linearly with more positive free energy AG for ET, and that the reorganization energy (x) remains constant with variation in the metal or cation redox potentials (66, 67). 

The rate of photoinduced ET, from the locally excited MLCT state of the Ru(bpy)3 + donor to the viologen acceptor in acetonitrile was found to decay exponentially with increasing chain length, from n = 1-5 and becomes roughly constant for n = 5,1, and 8. These results were interpreted in terms of TB-mediated ET... 

Figure 16.7 PCET reaction in a model fortyrosine oxidation in photosystem II. Inthe first step of the experiment, the ruthenium-tris-bipyridine portion absorbs light, and the excited electron is transferred to an external methyl viologen acceptor. In the second step, which is shown here, the tyrosine portion transfers an electron to the ruthenium and is deprotonated. Reproduced from Ref. [63].

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