Interaction of clusters with chiral surfaces

在目前的研究计划中，我们的目标是通过与手性表面的相互作用来诱导具有催化活性的、尺寸选择的金属团簇的手性，并重点关注这种不对称的检测和起源的基本理解。虽然诱导的簇手性可能会促进对映体选择性，但这些体系也可能显示出高活性，因为催化剂将从气相完全接近反应物分子。这种设计不对称催化剂的方法与过去通过固定在各种载体材料上来使高效的非对称均相催化剂多相化的各种尝试有着根本的不同。详细地说，本研究计划的目的是制备和表征从手性分子单层到手性氧化物的各种手性表面，并检测在沉积时可能的手性转移到金属团簇，其大小范围从几个原子到大约100个原子。我们将特别关注具有高反应性和定位良好的光学跃迁的金属团簇和团簇尺寸，即在我们可用的光谱范围内的光学跃迁，并且不干扰手性支撑体的光学跃迁。到目前为止，还没有成功检测手性转移到支撑金属团簇的报道。为此，我们开发了一种基于非线性方法的新型高灵敏度分光镜，克服了无法检测微小热力特征的主要挑战。虽然二次谐波产生的圆二色性（SHG-CD）允许检测小的光学活动，但根据相应的选择规则，它仅限于具有非平行于表面的跃迁偶极矩的光学跃迁。因此，另一个挑战将不仅是发现具有有效手性转移的尺寸选择金属团簇和手性表面的组合，而且还将找到一种系统，其中催化剂的光学响应不受潜在手性表面的破坏性影响。在这方面，手性氧化物表面可能特别有前途。一旦手性转移被成功识别，我们还将研究热稳定性和反应位点的可及性。

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.