Theoretical Study On The Structure,optical And Catalytic Properties Of Atomically Precise Noble Metal Nanoclusters

Noble metal nanoclusters have great potential in both fundamental theoretical research and industrial applications,due to their unique stability,biocompatibility,optical properties,and catalytic properties.Atomically precise noble metal nanoclusters have the characteristics of designable structure controllable properties,which create opportunities for the development of new luminescent materials and catalytic materials.Based on density functional theory（DFT）and time-dependent density functional theory（TD-DFT）,the structural evolution,catalytic properties and optical properties of atomic accurate noble metal nanoclusters have been studied.Based on the atomically precise structures of Au₆₀S₆（SR）₃₆ and Au₆₀S₇（SR）₃₆,this paper theoretically predicts the structure and property evolution law of Au₆₀S_n（SR）₃₆（n=0-12）clusters with dual properties of clusters and quantum dots.A distinct feature of Au₆₀S_n（SR）₃₆ nanoclusters is that the doped S atoms tend to occupy the Au tetrahedral units in the outer layer first and then occupies the inner layer.With the increase of the number of doping S atoms show a gradual transition from the monolayer-protected metal nanoclusters to the ligand-protected Au S quantum dots.During this transition process,the HOMO-LUMO energy gap of Au₆₀S_n（SR）₃₆ clusters gradually increased,and the UV absorption peak gradually blueshifted.The reaction mechanism of S atom doping into the core of gold clusters protected by thiol ligands can be divided into three steps.Firstly,Ph CH₂SH forms a Ph CH₂S-SCH₂Ph intermediate by dimerization,and then forms（Ph CH₂）₂S=S by isomerization,finally the thiol ligand-protected gold cluster obtains an S atom from（PhCH₂）₂S=S to forms a SAu₄unit in the metal core.The structure of Au₆₀（SR）₃₆ clusters withoutμ₄-S has linear anisotropy,while the“flat”Au₆₀S_n（SR）₃₆ clusters are more thermodynamically stable whenμ₄-S is doped.The structural isomerism of atomically precise ligand-protected Au₂₈ clusters(Au₂₈（CHT）₂₀ and Au₂₈（TBBT）₂₀)regulating their luminescence properties was investigated.It was found that Au₂₈（TBBT）₂₀ exhibited more charge transfer（CT）characteristics during excitation,while Au₂₈（CHT）₂₀ exhibited more localized excitation（LE）characteristics.Different from the traditional S₁→S₀ emission process in Au₂₈（TBBT）₂₀,ligand shell isomerization leads to the emission of high-energy excited state（S₂）in Au₂₈（CHT）₂₀.This phenomenon can be attributed to the larger Franck-Condon overlap integral of S₂→S₀ and the smaller Franck-Condon overlap integral of S₂→S₁ in Au₂₈（CHT）₂₀,resulting in that S₂→S₀ can be combined with S₂→S₁ internal conversion competition to realize S₂→S₀ direct emission.In addition,the structure of Au₂₈（CHT）₂₀ has higher symmetry,and less energy is dissipated during the structural relaxation process,which leads to its higher quantum yield.The TADF mechanism of atomically precise Ag₆ clusters(Ag₆N₆S₁₂C₃₆H₆₀ and Ag₆N₆S₁₂C₅₄H₄₈ are abbreviated as Ag₆L₆and Ag₆PL₆)protected by different ligands was investigated.A reverse intersystem crossing"h RSIC"pathway of the high-energy triplet state（T₄→S₁）was found in Ag₆L₆and Ag₆PL₆,which promoted the transformation of triplet excitons from the high-energy excited state（T₄）to S₁.This is the reason why both Ag₆L₆and Ag₆PL₆ have high TADF quantum yields（>50%）.In addition,it was found that the ligands indirectly regulated the excited state characteristics of Ag₆L₆and Ag₆PL₆,which in turn affected the relative magnitude of the intersystem crossing and the reverse intersystem crossing rate.In Ag₆L₆,the rate of S₁→T₄is faster than that of T₄→S₁ due to the larger spin-orbit coupling in S₁→T₄.In Ag₆PL₆,due to the the high-frequency C-N=C stretching vibration,the vibration coupling of the intersystem crossing from S₁ to T₄is small,which makes the S₁→T₄rate slower than the T₄→S₁ rate,resulting in a more efficient high-energy excited triplet reverse intersystem crossing.This is the reason that the quantum yield of TADF in Ag₆PL₆ is higher than that of Ag₆L₆.Based on atomically precise ligand-free Au₂₈ cluster,we designed two different types of N-doped graphene surfaces,namely the graphitic-like nitrogen-doped graphene G_{n N} and the pyridinic-like nitrogen-doped graphene G_{n N,n’V}.The interaction between supports and gold clusters and catalytic activity in Au₂₈/G_{n N} and Au₂₈/G_{n N,n’V}systems were systematically studied.We found that in the Au₂₈/G_{n N} system,the Au atoms interact withμ_3N-C atoms.However,in the Au₂₈/G_{n N,n’V} system,the Au atoms interact with the N atoms.Specifically,the different bonding modes result in the charge state of the Au cluster being negative on the G_{n N} surface,whereas on the G_{n N,n’V} surface,the Au cluster is of positive charge.Therefore,it is possible to change the type of C-N bonding structure to control the charge types of gold clusters on nitrogen-doped graphene.Compared with the Au₂₈/G_{n N,n’V} system with a positive charge on the surface of the gold cluster,the Au₂₈/G_{n N} system with a negative charge on the surface of the gold cluster has better catalytic activity.