Interaction of clusters with chiral surfaces

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In the present research proposal, we aim to induce chirality in catalytically active, size-selected metal clusters through interaction with chiral surfaces and focus on the detection and the fundamental understanding of the origin of this asymmetrization. While the induced cluster chirality may promote enantioselectivity, these systems may also reveal high activity, since the catalyst will be fully accessible for reactant molecules from the gas phase. This approach for designing asymmetric catalyst is fundamentally different from the various attempts, taken in the past to heterogenize efficient, asymmetric homogeneous catalysts by immobilization on a variety of support materials. In detail, the objective of this research proposal is to prepare and characterize a variety of chiral surfaces ranging from chiral molecular monolayers to chiral oxides and to detect possible chirality transfer to metal clusters upon deposition in a size range from a couple of atoms up to about 100 atoms. We will particularly focus on metal clusters and clusters sizes with high reactivity and well positioned optical transitions, i.e. with optical transitions within the spectral range available to us and not interfering with the ones of the chiral support. Up to date, there are no reports on the successful detection of chirality transfer into supported metal clusters. To this end we have overcome the main challenge, i.e. the incapability to detect small chiroptical features by developing a new highly sensitive spectroscope based on nonlinear methods. Although second harmonic generation circular dichroism (SHG-CD) allows for detecting small optical activities, it is limited, according to the corresponding selection rules, to optical transitions with a transition dipole moment non-parallel to the surface. Accordingly, another challenge will not only be to discover combinations of size-selected metal clusters and chiral surfaces with an efficient chirality transfer but also to find a system where the optical response of the catalyst is not destructively influenced by the underlying chiral surface. In this respect chiral oxide surfaces may be particularly promising. Once the chirality transfer has been successfully identified, we will also study the thermal stability and the accessibility of reaction sites.