Photocatalytic Water Reduction WRC and Photosensitizers

Lead by Prof. Roger Alberto

Water Reducing Catalysts (WRCs)

The reductive half reaction in the overall “light to chemical energy” conversion process requires Water Reducing Catalysts (WRCs) with particular electronic and structural properties:

1. Complexes, necessary to go beyond fundamental studies should be long term stable, i.e. not prone to intra- or inter-molecular side reactions in oxidations states encountered during the whole proton reduction process.

2. They must be fast, meaning not to remain in activated states for a long time.

3. In order to switch from homogeneous to heterogeneous catalysis with the same WRCs, surface grafting properties are requests.

4. In addition to these basic properties, WRCs with metal centers of abundant elements are needed for economic reasons.

We developed cobalt-based WRCs stable over several runs, which, thanks to a -OH group in the tertiary bridgehead carbon, can be covalently linked to organic polymeric beads, nanomaterials or surfaces. We carried out experiments with polystyrene based beads allowing for consecutive H2 formation by exchanging the medium repeatedly as shown in Figure 1.

Figure 1 Six cycle of photocatalytic H2 formation with WRC [CoIIBr(TPY-OH)]+, each after refreshing the medium to replace shortcutting DHA (Dalton Trans, 2013, 42, 334)

Two complexes, one with a tetra- and one with a penta-pyridine framework, are depicted in Figures 2 and 3. The connections of the different subunits, pyridines and 2,2’-bipyridine ligands, are decisive for cobalt to act as a water reducing catalyst. Other combinations between pyridines and bipyridines proved to be much less active or even inactive. Several ligand frameworks are currently exploited with similar ligand compositions and bound to heterogeneous materials. Mechanistic and kinetic investigations are an essential part as well as surface grafting and the study of electronic structures. Light harvesting and electron transfer to WRCs are further, integral parts of a water splitting architecture.

Light harvesting

Useful photosensitizers (PSs) absorb over a broad part of the optical spectrum and depict electronic states, which are accessible for coupled reduction/oxidation processes of excited states. Metal complexes are in the focus of research for new PSs, because they are often more stable than organic molecules. In our research we focus on complexes based on poly-pyridine or other poly-hetereoaromatic ligands, thus ligands which are potentially useful in the context of WRCs but can also be applied to PSs. Since hetereoaromatic ligands, assembled in particular frameworks, tend to exhibit very good photophysical properties when coordinated to an appropriate metal centre, we currently develop systems with different metals from the 3d series to find new chromophores with suitable characteristics for acting as PSs. Additionally, we are studying the mechanisms of H2 formation with {Re(CO)3}+ based photosensitizers. Examples are rhenium complexes with alkynyl ligands, systematically synthesized and studied over the last year (Figure 4). However, their stabilities and longevities are not yet satisfying and limit the performance of the water reduction process. Furthermore, rhenium is one of the least abundant elements and certainly not the element of choice. Finally, in addition to rhenium and selected 3d element-based photosensitizers, copper complexes in particular and with extended poly-pyridine frameworks are under investigation.

Electron transfer

Electron/hole carriers connect reversibly water oxidation and water reduction. The search for such molecules, mediating water reduction and – oxidation in combination with homo- and heterogeneous molecular architectures are crucial and part of this subproject. So far, we showed that ascorbic acid, a frequently applied sacrificial electron donor in water reduction, shortcuts the system. Its oxidized form, di-dehydroascorbate, efficiently competes with the WRC for the electron of the reductively quenched photosensitizer. Although reversible electron/hole carriers are crucial for a full water splitting system, they shut down the process if present in only one half of the reaction (Figure 5). Coupling with the oxidative half reaction and physical separation of the two processes in e.g. membranes is therefore a focus of this subproject. All different research topics, outlined in this short description, are performed in close collaboration with the other groups of the LightChEC project. Concepts for new compounds for any purpose remain trial and error if unambiguous insights into physico-chemical data and mechanisms are not available. Support from theoretical chemistry is indispensible since syntheses towards tailor-made WRCs or PSs without backup from theory are time consuming and difficult. Knowledge from material sciences will allow for the development of devices, once the single components are optimized and understood.

Figure 5 Ascorbate/DHA as a reversible electron/hole carrier. DHA shortcuts the electron transfer from reduced PS to WRC (Eur. J. Inorg. Chem. 2012, 1, 59).