Electronic and optical properties of the Au₂₂[1,8-bis(diphenylphosphino) octane]₆ nanoclusters disclosed by DFT and TD-DFT calculations

Time-dependent density functional theory calculations have been used to investigate the electronic and optical properties of a nanocluster composed of two directly bonded Au₁₁ subunits, held together by six bidentate diphosphine ligands: 1,8-bis(diphenylphosphino) octane. Three exchange–correlation functionals have been adopted, a general hybrid (PBE0) and two range-separated hybrids (ωB97X and CAM-B3LYP). The results obtained show that the aforementioned properties are significantly different from those of a previously studied Au₁₁-based nanocluster formed by just one single subunit. In particular, charge transfer excitations from the inner metal core to the outer ligands affect most of the UV–visible spectrum and occur for both alkyl and aromatic ligands. This is particularly evident when thiazole molecules are bonded to the gold core: In this case Au → ligand transitions affect also the first HOMO → LUMO excitation. Moreover, the gold core of this Au₂₂ nanocluster has eight under-coordinated Au surface atoms not engaged in bonds with the ligands. No other known organic-protected gold nanocluster has a similar feature. These gold atoms can be considered as potential in situ active sites for catalysis, their catalytic efficiency and selectivity being modulated by charge distribution.