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Interaction of clusters with chiral surfaces

Subject Area Physical Chemistry of Solids and Surfaces, Material Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Term from 2014 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 262670452
 
Final Report Year 2023

Final Report Abstract

In asymmetric heterogeneous catalysis the existence of a chiral active center is key. Despite the acknowledged superiority of heterogeneous catalysis over its homogeneous counterpart large scale industrial applications of asymmetric heterogeneous catalysis are still very scarce because of the difficulty to design asymmetric reactive centers. In principle there are two ways to obtain such centers: (i) immobilization of homogeneous asymmetric catalysts, (ii) asymmetrization of heterogeneous catalysts. In this proposal we focused on the second approach from a fundamental point of view and first developed the spectroscopic tools to investigate chiral centers on surfaces and second, to understand the phenomenon of the chirality transfer from a chiral environment to an achiral entity. We introduced second harmonic generation circular dichroism (SHG-CD) and optical rotation dispersion (SHG-ORD) as a highly surface sensitive spectroscopic approach to measure optical activity on supported chiral species and extended this approach to a relatively broad wavelength range. This technique led to a 106 higher sensitivity than the linear counterparts, as shown in the case of adsorbed binol. This high sensitivity was then used to measure the optical activity of single monolayers of UCAs (upstanding chiral architectures) which is beyond the capability of commercially available methods. Stimulated by these findings we decided to gain a more fundamental understanding of the chirality transfer. To this aim we investigated this phenomenon in a variety of environments to disentangle the various contributions and started from the systems that consisted of Ag nanoparticles stabilized by chiral ligands. We increased the complexity by studying the induced CD in dye molecules embedded in thin films of binol and then in chiral water soluble gels. These studies reveal that the two main mechanisms of chirality transfer (direct and indirect) both contribute to the observed chirality transfer and cannot be easily isolated. In order to exclude one contribution we investigated the induced chirality into a single Pd-atom from chiral ligands in a catalytically relevant complex. We could show that the optically active transitions of the complex possess an appreciable contribution of metal orbitals and thus we could then conclude the direct mechanism of chirality transfer to be responsible for the observation. In the future we will use the available spectroscopic toolbox and the obtained know-how together with the fundamental insights to design and study new chiral materials that are not only relevant for asymmetric catalysis but also for other applications such as chiral sensors, chiroptical materials or chiral theranostics.

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