Dissertation

|1.10 Intramolecular Photoredoxcatalysts|
In addition, antenna like systems and other supramolecular structures for intramolecular electron
and particularly energy transfer were investigated: [77]
(AP) n P R D/A or AP B N P
Working intramolecular photocatalysts, consisting of photocenter and catalyst to perform a
complete half-reaction were developed very late in the case of water reduction or are still unknown
as in the case of water oxidation: [35]
P B N C Ox/Red H 2 O or P R Q/D/A C Ox/Red H 2 O
It turned out that in addition to organic systems, especially oligonuclear coordination compounds
(dyads, triads, tetrads, ...) possess ideal properties to investigate the desired processes in
supramolecular systems and to reproduce concepts of nature.
1.10.1 Oligonuclear Coordination Compounds
The necessary charge separation for redox-catalysis can be generated with organometallic
chromophores P which represents one component of supramolecular catalyst. In addition, a
second metal center will serve as redox-catalyst C. The use of fine-tuned bridging ligands B will
increase the range and thus the lifetime of the charge separation in the first place. Furthermore, it
will allow for an unidirectional inner sphere electron transfer toward the second metal to allocate
the electrons for consecutive reactions. In general, three different processes exist which transform
the absorbed excitation energy into redox-energy that can be tapped by the catalyst metal (see
figure 30): (a) energy transfer, (b) photoinduced electron transfer and (c) photoinduced charge
separation.
Electronic energy transfer (EnT) is equivalent to electron transfer between different intracomponent-LUMOs
and parallel “hole transfer“ between different intracomponent-HOMOs via
Förster and Dexter mechanisms and shifts the localization of excited state energy to an
energetically lower lying excited state on a different component. Photoinduced electron transfer
(PET) shifts an excited electron from a high intracomponent-LUMO (LUMO+1) toward a lower
lying LUMO (Kasha’s rule) which is localized on a different component. This can be seen as a
intraligand electron transfer process from a high MLCT+1-state which populates a lower MLCT-
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