Thiolate-Protected Au20 Clusters with a Large Energy Gap of 2.1 eV

We report a kinetically controlled approach to synthesizing thiolate-capped 20-atom gold clusters. ESI mass spectrometry analysis in combination with other methods, including elemental analysis, XPS, NMR, and thermogravimetric analysis, determines the cluster composition to be Au-20(SCH2CH2Ph)(16). The Au-20(SCH2CH2Ph)(16) Clusters exhibit a stepwise, multiple-band optical absorption spectrum, reminiscent of quantum confinement behavior. The optical energy gap is determined to be E. 2.15 eV; this HOMO-LUMO gap is remarkably larger than that of Au-25(SR)(18) (1.3 eV). The Au-20(SCH2CH2Ph)(16) clusters were also found to be particularly robust again excess thiol etching, in contrast to the previous report on the thiol etching stability of a series of glutathione-capped Au-n(SG)(m) (n < 25) clusters. This stability difference might indicate some structural differences between Au-20(SCH2CH2Ph)(16) and Au-n(SG)(m) (n < 25). The crystal structure of the Au-20(SCH2CH2Ph)(16) cluster remains to be unraveled in future work.