Thiolate-protected Au(20) 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)(SCH(2)CH(2)Ph)(16). The Au(20)(SCH(2)CH(2)Ph)(16) clusters exhibit a stepwise, multiple-band optical absorption spectrum, reminiscent of quantum confinement behavior. The optical energy gap is determined to be E(g) approximately 2.15 eV; this HOMO-LUMO gap is remarkably larger than that of Au(25)(SR)(18) (1.3 eV). The Au(20)(SCH(2)CH(2)Ph)(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)(SCH(2)CH(2)Ph)(16) and Au(n)(SG)(m) (n < 25). The crystal structure of the Au(20)(SCH(2)CH(2)Ph)(16) cluster remains to be unraveled in future work.