Cobaltocenium Amides - Photoinduced Electron Transfer Processes in Donor-Acceptor Amides
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Abstract
Natural processes like photosynthesis, energy storage, energy transformation and energy transport are very efficient. Scientists are still working on the understanding of the basic principles of these processes, whereby the technology transfer on biomimetic model complexes and efficient applications is forced. Thus, the dependency on fossil fuels should be reduced and efficient energy storage and transport should be facilitated.
Organometallic „Sandwich“ complexes, like ferrocene and cobaltocenium, are competent to reversibly accept and donate electrons. Linked by peptide bridges, oligometallocenes can be used as nanowires for information transfer. By oxidation or reduction, respectively, the oligomers can change their conformation and operate as molecular switches. The development of a synthesis for N cobaltocenium peptides [Cc-NHCO-R]+ makes the manufacturing of oligopeptides, consisting of a cationic cobaltocenium backbone, possible. The new substitution pattern will expand the bandwidth of possible redox properties for substituted cobaltocenium complexes, which opens new fields for research. The here presented N cobaltocenium amides (R = Ph, Fc, Cc+) were prepared and analyzed by multiple techniques, like NMR, IR and UV/vis spectroscopy, cyclic voltammetry, spectroelectrochemistry and single crystal X-ray diffraction. Here, electron transfer bands could be observed, which were assigned by Density Functional Theory calculations. By Hush analysis of these charge transfer bands, the properties of the investigated bimetallocenes could be compared with similar bimetallocenes and understood.
Porphyrins are chromophores, which are similar to the natural pigments of the photosynthesis cascade and possess distinctive properties for light absorption. By irradiation, the porphyrin is excited and energy is emitted via fluorescence or phosphorescence. With potential electron acceptors close by, photoinduced electron transfer to this acceptor is possible. Cobaltocenium is a suitable electron acceptor, which is a powerful reductant after reduction to cobaltocene. The reduction force is strong enough to reduce protons to molecular hydrogen. The here presented amide linked cobaltocenium porphyrin conjugate was investigated in terms of the catalytic hydrogen evolution. NMR, IR und UV/vis spectroscopy, cyclic voltammetry, spectroelectrochemistry, transient-absorption spectroscopy and single crystal X-ray diffraction were used to understand the principle electron transfer processes of this donor acceptor peptide. The redox potentials of the dyad show, that the charge-shifted state is able to reduce protons. Photoinduced electron transfers from porphyrin to cobaltocenium, starting from the excited singlet S1 and triplet T1 states of the porphyrin, were demonstrated.