Energy Transfer from a Surface-Bound Arene to the Gold Core in ω-Fluorenyl-Alkane-1-Thiolate Monolayer-Protected Gold Clusters

Abstract

9-(9-Fluorenyl)-nonane-1-thiolate and 12-(9-fluorenyl)-dodecane-1-thiolate (fluorenyl-alkane-1-thiolates) monolayer-protected gold clusters (Au-MPCs) have been prepared by place exchange of fluorenyl-alkane-1-thiol with alkane-1-thiolate Au-MPCs. The structures of these composites were established by TEM, elemental analysis, and 1H NMR spectroscopy. Changes in optical and physical properties caused by attachment of the fluorenyl groups to the gold nanoparticles were studied by absorption, Fourier transform infrared, and fluorescence spectroscopies. Emission from the end-attached fluorenyl groups is substantially quenched upon attachment to a gold cluster so that less than 5% of emission intensity from an optically matched solution of the analogous freely dissolved fluorene and alkane-1-thiolate Au-MPCs is retained in the fluorenyl-alkane-1-thiolate Au-MPCs. Nanosecond flash photolysis shows that intersystem crossing (from the fluorenyl singlet to the corresponding triplet) was suppressed because energy transfer was much faster. No direct evidence of electron transfer from the excited fluorenyl group to the gold cluster could be observed, implying that electronic coupling through energy transfer accounts for most of the observed emission quenching and suppressed intersystem crossing.