Prolate-shaped Gold Cluster Series with Unique Optical Properties: Au₆ as a New Building Unit

Subnano-sized gold clusters bearing ligands have been an attracting research topic, because they are expected to exhibit nuclearity- and structure-dependent physicochemical properties. During our systematic study on the gold clusters ligated with bidentate diphosphines, we found that a series of prolate-shaped Au_n clusters (n = 6, 7, 8, 11) have a common Au₆ unit, and exhibit the lowest-energy intense absorption bands in the visible region [1-3]. These bands, which have never been found in the absorption spectra of the conventional spherical gold clusters ligated with monophosphines, were all assigned to HOMO-LUMO intraband transitions. Furthermore, the correlation between the Au₆-based structures and the HOMO-LUMO gaps was revealed from density functional theory calculation. In addition, optical tunabilities of these clusters were also demonstrated through ligand modifications around the gold cores [4,5]. [1] J. Am. Chem. Soc. 136, 12892 (2014). [2] Chem. Commun. 48, 7559 (2012). [3] Angew. Chem. Int. Ed. 50, 7442 (2011). [4] Nat. Commun. 8, 576 (2017). [5] J. Phys. Chem. Lett. 7, 4267 (2016).