Theoretical Study On Structure Prediction And Growth Regularity Of Thiolate-Protected Gold Nanoclusters

As a bridge connecting molecules and macroscopic materials,nanoclusters（NCs）have unique physicochemical properties.The study of their structure and performance will help explore their broad application prospects.As a typical representative of nanoclusters,gold clusters protected by ligands have achieved rapid development in the field of nanoscience in the past two decades.Among them,due to the high thermodynamic and chemical stability and good biocompatibility of the thiolate-protected gold clusters（RS-AuNCs）,the exploration of their structure and properties is the current research focus in the field of nanoclusters.Since the properties depend on the structure,a lot of experimental and theoretical works are devoted to the determination or prediction of the cluster structures.However,although many important advances have been made in the study of cluster structure,there are still some key intermediate cluster species that have not been determined,which hinders people’s in-depth understanding of the evolution mechanism and properties of clusters.In view of this,this paper takes the structure of clusters as the main line,predicts some new cluster structures by means of density functional theory（DFT）,and on this basis,discusses their possible growth mechanism and optical properties.Au₂₉（SR）₁₉ has shown its considerable abundance in early mass spectrometry.However,its structure has not been reported experimentally or theoretically.Based on the crystal structure characteristics of Au₂₈(S-c-C₆H₁₁)₂₀ and Au₃₀（S-t Bu）₁₈,we theoretically predicted the structure of 10e Au₂₉（SR）₁₉,thus filling the vacancy of the10e cluster structure in the valence electron number sequence.Furthermore,combining the similarity of the structures of Au₂₈（SR）₂₀,Au₂₉（SR）₁₉ and Au₃₀（SR）₁₈ as well as the experimental structures of Au_24±1（SAdm）₁₆ and Au₃₀S（SR）₁₈,we have proposed a structure growth mechanism from Au₂₈（SR）₂₀ to Au₂₉（SR）₁₉ and then to Au₃₀（SR）₁₈:‘gold atom insertion,thiolate-group elimination’rule.The key step in this growth mechanism is the formation of triangular Au₃ units in the gold core:first,a foreign Au atom is inserted between two adjacent S in the ligand layer,and then a three-coordinatedμ₃-SR group is eliminated.Based on the analysis of the experimental crystal structure of the Au₂₁（S-t Bu）₁₅cluster,we believe that it is a key intermediate in the structure evolution from Au₂₀（SR）₁₆ to Au₂₃（SR）^-₁₆.The seemingly complex clusters may contain some evolutionary laws.Inspired by this,combined with the structural analyses of other small and medium-sized fcc gold clusters,we proposed a structural evolution map.From the as proposed structural evolution blueprint,we can find that there are some obvious structural regularities among these small and medium-sized clusters.Based on this,we predicted two new cluster structures:Au₁₇（SR）₁₃ and Au₂₈（SR）₂₀ isomers as well as the TTG structure growth rules to reveal the structure evolution mode of these small-sized fcc configuration gold clusters.In addition,we put forward a new topological structure-electronic structure model.We believe that the molecular orbital of the cluster core is a linear combination of molecular orbitals that make up its basic modular units(such as tetrahedral Au₄²⁺and triangular Au₃⁺),which can reasonably explain the stability of the non-superatom clusters(e.g.Au₂₃（SR）^-₁₆)with magic number of valence electrons（8e）.Under the similarity of the structures of Au₄₀（o-MBT）₂₄and Au₄₉（2,4-DMBT）₂₇,we constructed the intermediate cluster structures that may exist during the evolution of the two clusters,including Au₄₃（SR）₂₅,Au₄₆（SR）₂₆ and Au₄₉（SR）₂₇-iso.At the same time,we investigated their relative stability based on the formation energy.In addition,inspired by the 2e gradual growth mechanism observed by Xie et al.,we proposed the evolution mechanism from Au₄₀（SR）₂₄ to Au₄₉（SR）₂₇:seed mediated growth mechanism of carboxyzation and decarboxylation with CO as model reductant.DFT calculations show that the growth path based on this mechanism is an energetically favorable process.Taking into account the D₃ symmetry of Au₄₀（SR）₂₄,we further constructed some intermediate cluster structures that evolved from Au₄₉（SR）₂₇ to Au₅₈（SR）₃₀,such as Au₅₂（SR）₂₈,Au₅₅（SR）₂₉,Au₅₈（SR）₃₀-iso1,and investigated the feasibility of this growth path.The calculation results show that this growth process is energetically unfavorable.The main reason is the low relative stability of these intermediate clusters.The source of this instability lies in the excessive tension in their structures.The successful determination of Au₂₄₆（p-MBT）₈₀ in the exciton state and Au₂₇₉（TBBT）₈₄ in the metal state provides new opportunities for exploring the origin of SPR.However,the large-sized cluster structures near the critical point of property transition has not been reported.Inspired by the structure of Au₆₀S₆（SCH₂Ph）₃₆ clusters,we constructed an Au-S network structure in fcc configuration.By cutting out n×m×l cubes of different sizes,we constructed a series of large-sized RS-AuNCs with fcc configurations and examined their relative stability.Then,based on the relatively stable clusters,their properties are analyzed by calculating their ultraviolet-visible absorption spectra（UV-Vis）,and further the critical point of transition from the exciton state to the metal state is judged.The calculation results show that the absorption spectrum curve of Au₂₉₆（SR）₁₀₄ is very similar to that of Au₂₇₉（SR）₈₄,and both show a single strong absorption peak.On the contrary,the absorption spectrum of Au₂₃₂（SR）₈₈,which has a relatively small size,shows multi-peak characteristics,indicating the nature of its excitons.In particular,the spectrum of Au₂₆₄（SR）₉₆ contains both a strong absorption peak and two weak absorption peaks in the shortwave direction.Based on this,we believe that Au₂₆₄（SR）₉₆ has both excitonic and metal properties.Therefore,from the perspective of the absorption spectrum,we hold that the transition from the exciton state to the metal state does not happen suddenly,but a smooth transition.