Dissertation

|3.2 Bisphenanthroline: A Suitable Molecular Bridge?|
Furthermore, phenphen has no electron storage moiety which compares to the pyrazine moiety of
the tpphz ligand. According to Campagna et al., this results in more negative reduction potentials
for the bridge in [(bpy) 2 M(µ-phenphen)M(bpy) 2 ] 4+ (M = Ru, Os, Ir), when compared to tpphz
complexes. [131]
These two facts, missing electron storage capacity and negative reduction potentials, hold the
potential that a reduction of other metal centers such as Pt(II), Rh(III) or Co(III) would be possible
after excitation of the MLCT in a heteronuclear phenphen-bridged complex.
Thus, the use of phenphen shall be applied in heteronuclear P ∼ B ∼ C-type systems. Leaving
the chromophore unchanged but focusing on the bridging ligand and reaction center allows
characterizing and understanding the nature of charge separation and electron transfer in the
supramolecular catalyst. Therefore, one of the two [Ru(bpy) 3 ]-type centers at the binuclear
phenphen bridged ruthenium complex of Campagna et al. may represent the chromophore
which offers high synthetic flexibility, high stability and well understood photophysical and redox
properties. The second metal center has to be replaced by a variety of reaction centers such as
noble metals Rh, Re, Pd, Pt or other metal centers such as Co or Ni which have been found to be
suitable for different kinds of redox catalysis. [88]
With respect to the resulting P ∼ B ∼ C-system, two major drawbacks were identified until today
which reduce the quantum yield of the overall reaction:
On the one hand, decomplexation processes of the catalyst-systems with possible colloid
formation or chromophore decomposition are side reactions that may occur after photo
reduction.
[94, 132]
On
the other hand, it is difficult but essential to tune the kinetic and
thermodynamic properties of the bridging ligand (e.g. electronic communication/insulation or
electron storage to a greater or lesser extent as previously discussed for the tpphz ligand) because
these factors may potentially limit the type of reducible metal centers and the rate of side reactions
or charge recombination. [80]
In the following section, the synthesis of mononuclear and hetero-dinuclear phenphen-bridged
complexes will be presented. The characterization of the photo- and electrochemical properties
will be discussed toward the application potential within P ∼ phenphen ∼ C-type systems in light
driven catalysis. Special emphasis will be placed on the characterization of stereoisomers.
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