Dissertation

|1.10 Intramolecular Photoredoxcatalysts|
(1) intrinsic and intercomponent stability toward thermal, photo- and electrochemical
decomposition and reaction products or intermediates
(2) reversible redox behavior
(3) distance control of the interlinked metal centers
(4) suitable electronic coupling between the components
(5) suitable intracomponent HOMO and LUMO energies (redox-, ground state- and
excited state potentials)
(6) unidirectional charge separation or long lifetimes of the charge-separated state
(7) sufficiently small energy gap between relevant excited/redox states to guarantee for
(6), while conserving maximal redox activity
(8) high energy/potential of the reactive excited/redox state
(9) good kinetic factors for inner sphere electron transfer reactions for:
(10) electron storage capacity
(11) high efficiency of population of the reactive (excited) redox state at the catalyst site
Nevertheless, it is difficult to predict the properties of a single component, the bridging ligand in
particular, without considering the other components on the supramolecular structure.
The series of [Ru(bpy) 2 (tpphz)]-type complexes (tpphz = tetrapyrido[3,2-a:2’,3’-c:3”,2”-h:2”’,3”’-
j]phenazine) in figure 31 nicely exemplifies the difficult interdependences and interactions in a
supramolecule.
Interestingly, the bridging ligand tpphz exhibits two types of empty π*-orbitals in close energetic
proximity, the LUMO bpy , which is mainly localized on the bipyridine like moieties and the
LUMO pz which is localized on the pyrazine moiety of the molecule. The orbital energies of these
LUMOs will be shifted in dependence of the oxidation state and the type of metal fragments,
coordinated to it. As a result of that, differently localized LUMO orbitals will be involved in the
charge transfer processes and charge separated states between the attached metals and the bridging
ligand which will be indicated by MLCT bpy and MLCT pz in the following passages.
The mononuclear complex [Ru(phen) 2
(tpphz)] 2+ (Ru(tpphz), phen = 1,10-phenanthroline) shows
the typical 1 MLCT bpy -absorption between 400 and 500 nm and MLCT bpy -emission from its
LUMO bpy (λ max = 625 nm, τ = 1.25 ms, Φ = 0.07, depicted in orange). The electron-transfer process
after excitation of the molecule, leading to deactivation of MLCT bpy and population of MLCT pz
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