Density Functional Theory Study On The Ag(2,2'-bipy)-POMs Hybrids System

Polyoxometalates (POMs) constitute a diverse class of inorganic oxo-metal clusters with defined structures based on octahera of tungstic and/or molybdic oxides. POMs are commonly formed by Mo, W or V and mixtures of these elements. Most POMs possess an extensive and reversible redox chemistry and some contain multiple unpaired d-electrons that make them useful in several fields.POMs are one of the most widely used inorganic components in a wide range of fields, such as catalysis, medicine, molecular magnetism or material science, owing to their extreme variability of composition, structure, electronic properties and applications. In recent years, hybrid organic-inorganic materials have attracted an increasing interest owing to such materials not only combine the advantages of organic materials so as to realize the so-called value-adding properties, but also contribute to exploring the possible synergistic effects.The possibility of combining the different characteristics of the elements to get unusual structures, properties, or applications. Over the past few years, research on POMs has greatly expanded to investigate functionalized POMs via incorporation or coordination of organic or organometallic moieties to produce novel structures. The modification and functionalization of POMs have provided the means to exploit more desirable attributes. Wang and his co-workers have synthesized a novelα-Keggin type polyoxometalate derivative {PW₉V₃O₄₀[Ag(2,2′-bipy)]n (n=2,3), which [Ag(2,2'-bipy)] groups are coordinated to theα-Keggin-type polyoxoanion [PW₉V₃O₄₀]^6- via the surface bridging oxygen atoms. Depending on this molecular, a serious of [Ag(2,2′-bipy)]_n- POMs moleculars have been designed, and investigated by DFT. The investigation includes the electronic, bonding, stability,nonlinear optical (NLO) properties, and redox properties. Our work has been focus on the following four aspects:1. [Ag(2,2′-bipy)]_n-PW₁₂O₄₀ (n=1, 2) are designed to be further investigated by using density functional theory (DFT). The interaction between PW₁₂O₄₀ and Ag(2,2′-bipy) of system 2 is stronger than that between PW₁₂O₄₀[Ag(2,2′-bipy)] and Ag(2,2′-bipy) of system 3. The electronic properties of the anions have been studied with molecular orbital and Mülliken analysis. In system 1, the highest occupied molecular orbital (HOMO) delocalized over the oxo ligands, and the lowest unoccupied molecular orbital (LUMO) delocalized over tungsten atoms. However, when Ag(2,2′-bipy)+ is bonding to [PW₁₂O₄₀]^3-, the frontier molecular orbitals (FMO) distribution of system 2 and 3 was changed. Mülliken analysis shows that the charges of W atoms are different from the formal assignments in system 1～3. Analysis of the main contributions to the second-order polarizability suggests that charge transfer from the z-axis directions plays a key role in the nonlinear optical response. The values ofβvec show that system 2 and 3 possess larger molecular second-order polarizabilities. The first hyperpolarizabilities enhances in the order ofβvec(system 3) >βvec(system 2).