Theoretical Study On Structure-property Relationship Of Equal Size Au Nanoclusters

Gold nanoclusters(Au NCs)protected by ligands(such as organosulfur and phosphine)have attracted extensive attention in recent years due to their precise atomic structure,ultrasmall size and easy functionalization.they have been used in catalytic conversion,fluorescent probes,electrochemistry,biomedicine and chemical sensors.So far,generous researches have been reported on the structure,properties and applications of gold nanoclusters.The size effect is a phenomenon that the physical-chemical properties(and three-dimensional structure)change with the size of nanoclusters,which is important to nanoclusters.In this context,studying the structure-property relationship of clusters with the same size(i.e.a group of clusters with the same number of metal atoms),focusing on the influence of different bonding modes of metal atoms on their three-dimensional configuration and physical-chemical properties,which can provide beneficial information for revealing the formation mechanism of clusters.In this paper,two groups of gold nanoclusters of equal size synthesized experimentally are selected,and the structure-property relationship is studied using density functional theory.The specific research contents are as follows:The first research is mainly aimed at the structure-property relationship of a group of equal size gold nanoclusters protected by thiolate ligands.We selected the two nanoclusters of Au₂₁(S-^tBu)₁₅ and Au₂₁(S-Adm)₁₅(HS-^tBu and HS-Adm are tertiary butylthiol and 1-adamantanethiol,respectively)with the same amount of gold atoms and thiolate ligands reported in our group.The two nanoclusters correspond to an extended definition of isomersm,because the number and type of metal atoms and ligands are consistent.Due to the core structures of Au₂₁(S-^tBu)₁₅ and Au₂₁(S-Adm)₁₅are hexagonal closest packed(hcp)and face centered cubic packing(fcc),respectively,their structures are denoted by Au₂₁^hcp and Au₂₁^fcc.The similarities and differences of the three-dimensional structure and electronic structure of the two clusters are analyzed by adopting density functional theory(DFT)and time-dependent density functional theory(TD-DFT).It was found that in both clusters,the Au-Au interactions within the metallic core are relatively stronger than those between core-shell Au-Au interactions;the presence of more interfacial Au-S bonds in Au₂₁^hcp results in the relatively weaker core Au-Au interactions than those in Au₂₁^fcc,and more positive charge distribution in the core metal atoms.To this end,the higher diffusing ability of the core metal orbitals(the weaker covalent bonding character)further results in the easier electron transition in Au₂₁^hcp and HOMO-LUMO(HOMO:the lower highest occupied molecular orbital;LUMO:the lowest unoccupied molecular orbital)gap.The insights into the correlations between the isomerism will benefit the deep understanding on structure-property correlations of more isomeric nanoclusters.The main goal of the second research aimed at the interconversion mechanism of a group of gold clusters with the equal size protected by phosphine ligands.We selected the redox induced interconversion of[Au₈(dppp)₄]²⁺and[Au₈(dppp)₄Cl₂]²⁺(dppp:1,3?bis(diphenylphosphino)propane)reported by Konishi group as a modle reaction,which are denoted Au₈Cl₂²⁺and Au₈²⁺respectively.DFT calculations were performed to shed light on the mechanism of redox-induced interconversion between Au₈²⁺and Au₈Cl₂²⁺nanoclusters.The conversion of Au₈Cl₂²⁺to Au₈²⁺is initiated by the 2e reduction,following with the removal of chloride and dissociation of Au-Au bond on different corner Au atoms.After the formation of Au-Au bonds,the edge-sharing tri-tetrahedral core structure was constructed,and Au-Au bonding network facilitates the removal of the second chloride.Meanwhile,the conversion of Au₈²⁺to Au₈Cl₂²⁺is initiated by the2e oxidation,increasing the electrophilicity of the metallic core,and weakens the metallic bonding,especially in the outside moieties.To this end,the addition of chloride is favored by both electronic and steric effects.After that,the introduced chlorides further enhanced the electron density of the metallic core,resulting in the electronic repulsion between the electron-rich moieties and re-construction of the metal core.The Au-Cl bond on the electron-rich precursors(e.g.reduction state)and Au atoms with high coordination number is more labile to dissociate(i.e.remove chloride),while the outmost Au-Au bonds is preferentially activated along the oxidation processes,leading to the subsequent coordination of the electron-rich ligands and the reconstruction of the metal core.The proposed structure-activity correlations might appeal to the deep understanding on dynamics and development of more strategies on redox-induced size-control/conversion of metal nanoclusters.