One-Pot Synthesis Of Phenylmethanethiolate-Protected Au₂₀（SR）₁₆ And Au₂₄（SR）₂₀ Nanoclusters And Insight Into The Kinetic Control

Noble metals are world wide welcome as jewelry and currency due to their expensive price and stable chemical properties. Since Brust created a Bottom-up preparation of gold nanoparticles in1994, size-focus synthetic methods of gold nanoclusters have been fast developing, prompting atomically precise gold nanoclusters have been synthesized and through a series of characterization. Because of the mature preparation methods and well known crystal structure, Au25, Au38and Au102nanoclusters has been the representative of organic monolayer protected gold nanoclusters, whose properties’research have been a hot spot for nearly a decade. Besides these, Au20and AU24also attracted the wide attention from scientists due to their special structures and properties.In the nanocluster size regime, the specific propertities of nanoclusters are fundamentally altered. Quantum sized nanoclusters preform multiple optical absorbtion rather than the single surface plasmon resonance(SPR) peak at520nm for bulk gold nanocrystal with spherical sturcture. Furthmore, the nanoparticles typically adopted face-centered cubic(fcc) structure while the nanoclusters often have non-fcc structure. Thus gold nanoclusters,as well as doping of Au nanoclusters with foreign atoms, have particular fluorescent, magnetic and chiral properties.In early research the total structure of Au nanoclusters was difficult to determine at large limiting to its potential energy surface. However, so far scientific research in the synthesis of gold nanoclusters have led to a number of atomically monodisperse gold nanoclusters with size ranging from Au5to AU333. More importantly, the breakthroughs in total structure determinations of Au13, Au25and Au102have shed exciting light on the exploring the mechanism of atomically precise nanocluters catalysis and intriguing atomic packing structure of these nanoclusters. These ligands protected nanoclusters are well-defined to atomic level rather than the size characterization in the conventional nanoparticle synthsis.We report two synthetic routes for concurrent formation of phenylmethanethiolate (-SCH2Ph)-protected Au20(SR)16and Au24(SR)24nanoclusters in one-pot by kinetic control. Unlike the previously reported methods for thiolate-protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH4and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold-thiolate cluster synthesis. 1. We demonstrate that the synthesis of-SCH2Ph-protected Au20and AU24nanoclusters can be obtained through two different, kinetically con-trolled methods. Specifically, route1employs slow addition of a relatively large amount of NaBH4under slow stirring of the reaction mixture, while route2employs rapid addition of a small amount of NaBH4under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinet-ics, but interestingly they give rise to exactly the same product (i.e., the cop-roductionofAu2o(CH2Ph)i6andAu24(CH2Ph)20clusters. Our results explicitly demonstrate the complex inter-play between the kinetic factors that include the addition speed and amount of NaBH4solution as well as the stir-ring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters.2. We also compared the photoluminescence and electrochemical properties of PhCH2S-protected Au20and Au24nanoclusters with the PhC2H4S-protected counterparts. A surprising2.5times photoluminescence enhancement was observed for the PhCH2S-capped nanoclusters when compared to the PhC2H4S-capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.According to the obtained structure of gold clusters, the different feature of the configuration of gold clusters can attribute to three factors. First and foremost, the properties of ligands including the steric hindrance and electronic conjugation play a vital role in determining the atomic packing form of gold clusters. Secondly, the synthetic conditions of preparation of gold clusters (i.e. thermodynamic selection and kinetic control in synthesis of glod nanoclusters) also have a profound influence on the size of gold nanoclusters. Last but not least, the structure can be changed to some extent under special environment. The case in point is the valence state of Au25(SCH2CH2Ph)18transfer from-1to0under the oxidizing atmosphere with the distortion of Au core. A similar situation maybe happen in the future studies.