Functional Theory (DFT) Studies On Structural And Properties Of Ligand Protected Gold Nanoclusters: Thiolate Versus Selenolate

As a new functional nanomaterial, the thiolate protected gold nanoclusters[Aum(SR)n NCs] have attracted wide research interest. At present the researches on this field are conducted constantly in the experimental and theoretical method. The main research is divided into two categories: with and without protected ligands on gold clusters, and the study about thiolate protected Au nanoclusters is more popular.To date, a large amount of single crystal structures of thiolate protected gold clusters have been identified. These results provide us important information to understand the structure evolution of Au clusters. Although researchers have prepared lots of the thiolate protected gold nanoclusters with different composition, the study about nanoclusters need further more reasearches in the experimental and theoretical method, especially how the types of ligands and the properties of organic functional groups in ligands influence the clusters.There are many similar chemical properties bettween thiolate and selenolate.Sulfur(in thiolate) and selenium(in selenolate) belong to the same group in the periodic table, and they have the same number of valence electrons. the electronegativity of selenium is less than the Sulfur. However the binding energy between Au and senenolate is more than this between Au and thiolate. The selenolate can form a stable structure with the Au cluster. To date, the researchers synthesize the selenolate protected gold clusters(Aum(SeR)n) via the ligand exchange of thiolate with selenol in experimental method, and then they study widely their structures and properties. However, so far, the synthesized and characterized selenolate protected gold clusters are few, such as [Au25(SeC8H17)18]-、Au38(SeC12H25)24、Au24(SePh)20、Au18(SePh)14. But it is interesting that the selenolate protected gold clusters have the same number ratio of gold atoms and ligands with the corresponding thiolate protected gold clusters. These results show that the selenolate ligands and thiolate ligands maybe have the same combinative and stable way with the gold atoms.In this study, we predicted and studied eleven selenolate ligand protected gold clusters with different chemical composition: Au102(SeR)44, Au38(SeR)24, Au36(SeR)24,Au28(SeR)20, Au25(SeR)18-, Au24(SeR)20, Au23(SeR)16-, Au20(SeR)16, Au18(SeR)14,Au12(SeR)9+ and Au10(SeR)10. By meanings of density functional theory(DFT), we predict the structures and properties of the selenolate protected gold clusters, andresearch the similarities and differences by comparing with the thiolate protected Au nanoclusters which have prepared and characterized by experimental method. In this thesis, the content mainly includes two aspects: the first part introduces the structures,the ligand exchange reactions, the charge analysis and the differential electron density of the selenolate protected gold clusters; the second part studies the optical absorption properties, the orbital properties and the charge transfer between the Au25- cluster and O2.In the first part of this thesis, based on the selenolate protected gold clusters Au18,Au24, Au25-, Au38 which have synthesized successfully by ligand exchange experiment,we predict the other selenolate protected gold clusters by changing the thiolate to selenolate. With density functional theory calculations, we optimize the structures and study the properties by comparing with the thiolate protected gold clusters. In this chapter, we compare the structure, the ligand exchange reaction, charge analysis and the differential electron density. The results show that the predicted selenolate protected gold clusters have the same basic structures as the thiolate protected gold clusters. By calculating the reaction energy of the ligand exchange reaction, we find that the ligand unit-R group have large influence on the gold clusters Aum(ER)n.However, the atom E has little influence. Charge property analysis shows that in the thiolate protected gold clusters there are more charge transferring from the Au kernel to the ligand than in the selenolate protected gold clusters. And with the differential electron density analysis, we find the charge transfer is similar both in thiolate and selenolate protected gold clusters. The charge transfer mainly occurs between the kernel and the inner ligand, while the charge transfer between gold kernel and the outer ligands are negligible.In the second part of this paper, we calculate the optical properties and orbital properties with the ADF and turbomole package. For the PBE functional, we find that it can well reproduce the optical curve of the thiolate protected gold clusters, However,it is not suitable for selenolate protected gold clusters. To find the suitable functional to simulate the selenolate protected gold clusters. In this chapter, we study the absorption optical curve by four functionals(PBE, PBE0, B-LYP and B-VWN) and compared them with the experimental curves. We find that the hybrid functional PBE0 is the most suitable one for simulating the absorption optical curve of selenolate protected gold clusters among the four functionals. Then, by studying the bond length between gold atoms and ligand terminal S or Se atom(Aucore-SR bond), theHOMO-LUMO gap and coordination number of the clusters, the results show that the tendency is the same both the Aucore-SR bond and the HOMO-LUMO gap. Moreover,with the increase of the coordination number, the HOMO-LUMO gap has a tendency to reduce. Next, we also study the interaction and the charge transfer in the excitation state bettween Au25-cluster and O2 by theoretical method. By simulating the adsorption behaviour of O2, the optical absorption properties and charge transfer in both [Au25(SMe)18]--O2 and [Au25(SeMe)18]--O2 system, the results show that the charge transfer is similar both in the two different systems and the most obvious transfer can be seen between the ground state and the first excited state(T0-T1),however in the higher excited states, the charge transfer will reduce.By predicting and calculating the structures and the physical and chemical properties of selenolate protected gold clusters and comparing with the thiolate protected gold clusters, we can understand the selenolate protected gold clusters to a certain extent. We hope our researches will be a reference to explore the selenolate protected gold clusters in the experimental and theoretical field in future and they can promote the further development of gold clusters.