Supramolecular Photocatalyst for the Reduction of Au(III) to Au(l) and High-Turnover Generation of Gold Nanocrystals

We report a photocatalyst composed of a diphenylanthracene core appended with two lipophilic thioether side-chains that binds gold(III) chloride. Upon excitation using visible light, the Au-III ions are smoothly reduced to Au-1 which, in the presence of water, lead to the formation of crystalline gold nanoparticles of 20-50 nm diameter that are devoid of sulfur-containing capping agents. Ultrafast transient absorption spectroscopy shows that the anthracene excited state is quenched with a rate k = 3.5 x 10(10) s(-1), assigned to intramolecular energy transfer to the bound gold ions, which then oxidizes the solvent to produce an intermediate low-valency gold(I) species. In the absence of water, the latter is stable and can be used as a homogeneous Au-1 catalyst. When employed in a biphasic reactor, the photocatalyst shows average turnover numbers of 150 atoms of Au-III reduced to Au-0 per molecule of photocatalyst.