Electronic Structures and Photoluminescence of Monolayer-Protected Metal Clusters

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We have developed a synthetic method of thiolate-protected gold clusters with well-defined chemical compositions. The method is based on size separation by using polyacrylamide gel electrophoresis (PAGE) and gel permeation chromatography (GPC) and mass spectrometric characterization. Electronic and geometrical structures of the thiolated gold (Au : SR) clusters thus synthesized were studied by using various spectroscopic methods and theoretical calculations. The findings are summarized as follows.1)The Au : SR clusters formed by reduction of the corresponding Au(I)-SR polymer correspond to trapped intermediates of the growing clusters. We observed several magic-numbered gold clusters such as Au25(SG)18 and Au55(SC18)32, where GSH and C18SH represent glutathione and octadecanethiol, respectively.2)The small Au : SR clusters exhibit photoluminescence with quantum yield of 10^<-3>- 10^<-4>, which is significantly larger than that of bulk gold (10^<-10>). This enhancement is attributable to the quasi-molecular electronic structures. The emission wavelengths are nearly constant irrespective of the core size. This suggests that the PL originates from the Au-SR ring structures attached on the gold cluster surface. This structure model is supported by theoretical studies.