Baghdasaryan Ani, Martin Kévin, Lawson Daku Latévi Max, Mastropasqua Talamo Maurizio, Avarvari Narcis, Bürgi Thomas
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland.
MOLTECH-Anjou, UMR 6200, CNRS, UNIV Angers, 2 bd Lavoisier, 49045 ANGERS Cedex, France.
Nanoscale. 2020 Sep 17;12(35):18160-18170. doi: 10.1039/d0nr03824e.
Ligand exchange reactions have become a highly versatile post-synthetic strategy to accurately engineer the ligand shell of atomically precise noble metal nanoclusters. Modifying the chemical structure of the exchanging ligand with chromophore substituents or adding chiral centers allow direct functionalization of the cluster with desired properties. As such, post-functionalized gold nanoclusters with unique physicochemical properties find applications in optoelectronics, catalysis and biomedicine. Herein, we successfully carried out ligand exchange reactions between the chiral Au38(2-PET)24 cluster (both racemic and enantiopure forms) and the helically chiral but configurationally labile 2-thio[4]helicene ligand (TH4). The reaction products with a composition of Au38(2-PET)24-x(TH4)x were analyzed using UV-vis spectroscopy and MALDI mass spectrometry. It was found that up to ten 2-PET ligands can be replaced with the helicene ligand on the cluster surface according to MALDI analysis. Consequently, the UV-vis and CD spectra of the cluster have been strongly affected by the ligand exchange reaction. The intensities of the CD signals of Au38(2-PET)24-x(TH4)x were drastically reduced and red shifted with respect to the reference Au38(2-PET)24 cluster. Moreover, the appearance of the other enantiomer in the HPLC chromatogram revealed the partial racemization of the cluster. DFT calculations were performed and they support the experimental observations and show that the observed chiroptical changes in UV-vis and CD spectra are exchange-site dependent. The calculations also demonstrate that charge transfer (CT) transitions occur between the Au38 cluster and the helicene ligand. Thus the ligand is directly involved in these transitions and contributes to the electronic states comprising those transitions.
配体交换反应已成为一种高度通用的合成后策略，用于精确设计原子精确的贵金属纳米团簇的配体壳层。用发色团取代基修饰交换配体的化学结构或添加手性中心，可以使具有所需性质的团簇直接功能化。因此，具有独特物理化学性质的后功能化金纳米团簇在光电子学、催化和生物医学领域有应用。在此，我们成功地在手性Au38(2-PET)24团簇（外消旋体和对映体纯形式）与螺旋手性但构型不稳定的2-硫代[4]螺旋烯配体（TH4）之间进行了配体交换反应。使用紫外-可见光谱和基质辅助激光解吸电离质谱对组成Au38(2-PET)24-x(TH4)x的反应产物进行了分析。根据基质辅助激光解吸电离分析发现，团簇表面上多达十个2-PET配体可以被螺旋烯配体取代。因此，配体交换反应强烈影响了团簇的紫外-可见光谱和圆二色光谱。相对于参考Au38(2-PET)24团簇，Au38(2-PET)24-x(TH4)x的圆二色信号强度急剧降低并发生红移。此外，高效液相色谱图中另一种对映体的出现表明团簇发生了部分消旋。进行了密度泛函理论计算，这些计算支持了实验观察结果，并表明在紫外-可见光谱和圆二色光谱中观察到的手性光学变化取决于交换位点。计算还表明，在Au38团簇和螺旋烯配体之间发生了电荷转移（CT）跃迁。因此，配体直接参与这些跃迁，并对构成这些跃迁的电子态有贡献。
