| Faculty of Life Sciences |  |
Institute of Physical and Theoretical Chemistry |
As a chemist with a strong background in the field of bioinorganic and coordination chemistry as
well as catalysis my research is focused on “Photo- and Electrocatalysis Based on Abundant Metal
Complexes Inspired by Nature”. Special attention is given to the catalytic splitting of water, the
reduction of CO2 and light mediated C-C bond formation reactions. In particular,
molecular hydrogen is considered to be an efficient and sustainable energy carrier of the future,
whereas the conversion of carbon dioxide is of high industrial relevance. Therefore, it is necessary to
develop systems enabling the photo- and electrocatalytic reduction of protons or carbon dioxide,
e.g. by mimicking reactions carried out by Photosystem II (PSII) or by hydrogenase enzymes (H2ase)
as well as by carbon monoxide dehydrogenase enzymes (CODH). As a main goal the proposed
synthetic models should mimic structural and functional principles in each case. In this respect, the
usage of more abundant metals for both, the required photosensitizers and catalysts, is an important
step towards a broader application. Mechanistic studies using different spectroscopic methods form the basis for the elucidation of structure-property-relationships and for a stepwise improvement within an iterative process. Moreover, to bring such systems into practice, there is a need for immobilization in order to obtain stable and recyclable heterogeneous materials. Consequently, my research is divided into the following topics, which are closely connected:
Photosynthesis is characterized as the fundamental reaction of plants that convert water and CO2 into carbohydrates by the utilization of sunlight. To enable the conversion of light energy into chemical energy the absorption over a broad range of the incident light is desired. Therefore, the development of suited molecular photosensitizers (PS) is a key issue in the field of artificial photosynthesis. Since the 1970's complexes based on a variety of noble metals, including Ru, Rh or Ir have been successfully applied as PS. In contrast, less expensive noble-metal-free PS are still scarce, which render the more abundant heteroleptic Cu(I) complexes a promising alternative. Unfortunately, the currently used copper complexes of the type [(P^P)Cu(N^N)]+ still suffer from a limited stability and absorption range in the visible as well as low extinction coefficients. Nevertheless, these copper photosensitizers offer a high structural flexibility, which allows the fine tuning of the photophysical and electrochemical properties. The chemical nature, size and position of the ligands and substituents can strongly influence the ground state (absorption), but also the excited state (emission) properties and the redox potentials. A possibility to increase the extinction coefficients and to red-shift the absorption is the extension of the π-conjugated system. In order to improve the stability, Cu(I) complexes with mixed chelate ligands P^N or tridentate ligands P^N^P are in the center of interest (see Fig.). These ligands will replace the traditional bisphosphines P^P, since some of them are only weakly bound to copper and tend to decompose under catalytic conditions.
| A Noble-Metal-Free System for Photocatalytic Hydrogen Production from Water E. Mejía, S.-P. Luo, M. Karnahl, A. Friedrich, S. Tschierlei, A.-E. Surkus, H. Junge, S. Gladiali, S. Lochbrunner & M. Beller Chem. Eur. J., 2013, 19, 15972-15978. |
Hydrogen, which is generated by the direct photocatalytic splitting of water, is considered to be an efficient (high energy density of 119 kJ/g), environmentally friendly and clean energy carrier. Furthermore, there is an increasing interest to use CO2 as raw material for the conversion into carbon monoxide, as industrial relevant building block, or to directly form liquid carbon based fuels (e.g. formic acid).
Consequently, synthetic approaches to solar fuel production require the development of suitable reduction catalysts that are able to either generate hydrogen or to activate carbon dioxide. In addition, these catalysts need to be driven by the reduction equivalents from excited molecular photosensitizers, where the sun light serves as the source of energy. As a main goal within this topic this should be realized by a fully noble-metal-free system, which may be achieved by the application of the above described copper photosensitizers (Topic 1).
With respect to proton reduction especially [FeFe] hydrogenase mimics will be considered (see Fig. left), due to their ability to convert protons to hydrogen. Moreover, the fact that the catalytic center is fully based on cheap and abundant iron is important for a future application. Special attention will be given to a class of penta-coordinated and mononuclear Fe(II)-carbonyl complexes, which contain an essential free coordination site for substrate binding in order to imitate the active distal Fed. Another focus lies on the key role of pendant amines that function as possible proton relays in such H2-evolving catalysts.
Regarding CO2 reduction the active sites of the carbon monoxide dehydrogenase enzymes (CODHs) will serve as prototypes for the development of bio-inspired model complexes. There are two different basic classes, i) the oxygen sensitive Ni-Fe containing (CODHNi) enzymes and ii) Cu and Mo containing enzymes (CODHMo). While the CODHNi enzyme reveals a [Ni-4Fe-5S] cluster, the CODHMo active site provides a {MoIV(=O)(OH)-S-Cu} system (see Fig. right side).
| Pentacoordinate iron complexes as functional models of the distal iron in [FeFe] hydrogenases M. Beyler, S. Ezzaher, M. Karnahl, M.-P. Santoni, R. Lomoth & S. Ott Chem. Commun., 2011, 47, 11662-11664. |
| Coordination and conformational isomers in mononuclear iron complexes with pertinence to the [FeFe] hydrogenase active site A. Orthaber, M. Karnahl, S. Tschierlei, D. Streich, M. Stein & S. Ott Dalton Trans., 2014, 43, 4537-4549. |
The novel photosensitizers and reduction catalysts (Topic 2) are tested in a number of light-driven reactions such as the photocatalytic reduction of protons or CO2. The photocatalytic reaction is typically accomplished within a three-component system, consisting of the photosensitizer (PS), the reduction catalyst (RC) and a sacrificial reductant (SR). As described in the general catalytic cycle (see Fig.) the sacrificial reductant, most often an amine, an alcohol or an acid, is required to regenerate the photosensitizer after electron transfer. On the one hand, these intermolecular systems offer an excellent opportunity for the independent optimization of photosensitizer and reduction catalyst, but on the other hand they are strongly dependent on the collision probability and a good interaction between PS and RC in solution. In addition, essential reaction parameters such as concentration, wavelength, solvent composition etc. need to be adjusted to the different catalytic systems. Once the photocatalytic system has been established, a systematic investigation of the photochemical and catalytic properties upon structural changes of the molecularly defined systems is required. In particular, elucidation of structure-activity-relationships and investigation of the electron-transfer mechanism are important.
Accompanying spectroscopic studies (e.g. UV/vis-, Emission-, IR-, EPR and time-resolved spectroscopy) in combination with electrochemical measurements and DFT-calculations are crucial for a better understanding of the underlying catalytic processes and intermediates. These photophysical and mechanistic investigations are performed in close cooperation with different cooperation partners.
| Death and Rebirth: Photocatalytic Hydrogen Production by a Self-Organizing Copper-Iron System S. Fischer, D. Hollmann, S. Tschierlei, M. Karnahl, N. Rockstroh, E. Barsch, P. Schwarzbach, S.-P. Luo, H. Junge, M. Beller, S. Lochbrunner, R. Ludwig & A. Brückner ACS Catal., 2014, 4, 1845-1849 |
| Substitution-Controlled Excited State Processes in Heteroleptic Copper(I) Photosensitizers Used in Hydrogen Evolving Systems S. Tschierlei, M. Karnahl, N. Rockstroh, H. Junge, M. Beller & S. Lochbrunner ChemPhysChem, 2014, 15 (17), 3709-3713. |
In addition to the photocatalytic reduction of H+ and CO2(Topic 3a) the newly developed copper complexes are also applied as photoredoxcatalysts in visible-light-mediated cross-dehydrogenative coupling reactions (CDC) or atom transfer radical additions (ATRA). In this case the heteroleptic Cu(I)-compounds serves as photo and catalytic center at the same time and offer unique possibilities in the field of organic synthesis by catalyzing the direct formation of carbon-carbon bonds. As a result, valuable products can be produced in high yields under mild conditions without the need for sacrificial donors.
In order to bring the above described molecular systems into practice an immobilization onto a suitable support, which might be a semiconductor (e.g. TiO2 or NiO), an electrode material (e.g. carbon) or a polymer is desired. This is due to some beneficial aspects of heterogeneous materials such as high stability, easy separation and recyclability. Therefore, the aim of this topic is the development of novel composites, where molecular photosensitizers and/or catalysts are sensitized on a heterogeneous support, to improve the photocatalytic activity. This is possible, because the applied photosensitizers show significantly higher light harvesting efficiencies in the visible range than most of the supports. Moreover, the immobilized complexes may also serve as a co-catalyst by providing additional active sites.
For the preparation of such composite materials anchor groups like carboxylates, phosphonates or sulfonates are required (see Fig.). Another promising alternative to immobilize these complexes is the usage of azide groups or polycyclic aromatic hydrocarbons (e.g. pyrene derivatives). Such functionalized semiconducting or electrode materials can be applied as photoelectrodes inside a photoelectrochemical cell (PEC cell). As a main advantage, a PEC cell does not require any sacrificial electron donor, because the essential electrons are provided by an external bias. In addition, the molecular functionalized photoelectrodes may show a reduced overpotential compared to classical carbon or metal based electrodes.
| Photocatalytic Hydrogen Production with Copper Photosensitizer-Titanium Dioxide Composites M. Karnahl, E. Mejía, N. Rockstroh, S. Tschierlei, S.-P. Luo, K. Grabow, A. Kruth, V. Brüser, H. Junge, S. Lochbrunner & M. Beller ChemCatChem, 2014, 6, 82-86. |
| Light-Driven Electron Transfer between a Photosensitizer and a Proton-Reducing Catalyst Co-adsorbed to NiO J. M. Gardner, M. Beyler, M. Karnahl, S. Tschierlei, S. Ott & L. Hammarström J. Am. Chem. Soc., 2012, 134, 19322-19325. |