•The introduction of POM leads to red shift and broadened absorption peak.•The energy gaps for systems 4–6 are smaller than those of systems 1–3.•Thiophene as π linker in system 5 has the best performance.A series of hexamolybdate-based organic-inorganic hybrids containing double donor-π-linker-acceptor chains were studied by using density functional theory and time-dependent density functional theory methods. The energy levels of frontier molecular orbitals, absorption spectra, electronic transition characters and photovoltaic parameters of dyes 1–6 were systematically evaluated. The results show that the absorption spectra are broadened and red-shifted by introducing hexamolybdate and different π-linkers. The absorption spectrum of dye 5 (thiophene as π-linker) has three strong and broad absorption bands in 300–750 nm. The hole injection energy, dye regeneration energy and charge recombination energy of dye 5 are smaller than others, which are advantageous to the hole injection and dye regeneration. The introduction of thiophene (dye 5) in this study is the most effective strategy to improve the absorption and increase the efficiency of dye in dye-sensitized solar cell (DSSC).A theoretical design and study on hexamolybdate-based organic-inorganic hybrids with double D-π-A chains for high performance p-type dye-sensitized solar cells.

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

•Charge transfer directions are all from donor side to POM, and injecting into TiO2 via COOH.•The introduction of POM results in stronger, longer, and tighter CT process.•The introduction of POM leads to a larger responsive range of spectraA series of donor-acceptor-π-bridge-acceptor (D-A-π-A) type hybrids were designed by introducing Lindqvist-type polyoxometalate (POM) into synthesized dyes aiming at revealing the impact of POMs on the performance of dye. The spectroscopic properties and charge transfer (CT) characters were studied using density functional theory (DFT) and time-dependent DFT methods. Compared with the synthesized dyes, designed dyes show larger responsive range of spectra. CT of the designed dyes are all from donor side to POM and COOH, suggesting strong electronic coupling with TiO2. Further analysis on CT parameters indicate that the introduction of POM results in stronger, longer, and tighter CT process. These designed dyes can be promising candidates for DSSCs.Four D-A-π-A hybrids were designed by introducing polyoxometalate for improving photovoltaic performance.Download high-res image (278KB)Download full-size image

Using density functional theory (DFT) in combination with non-equilibrium Green's functions, we have investigated the electronic structures, magnetization, and quantum transport properties of zigzag graphene nanoribbons (ZGNRs) functionalized with conventional conductive metal adatoms (Al, Cu, Ag and Au). On the basis of the adsorption energies, our simulation demonstrates that Al and Cu adatoms are chemically bonded with ZGNRs, while the adsorptions for Ag and Au are between weak chemisorption and strong physisorption. The properties of charge transfer and magnetic moment are in reasonable agreement with the previous calculations. The adsorption of metal adatoms induce a net magnetic moment of −1 μB in 6ZGNR–metal systems. On the other hand, the transport studies of metal adatoms adsorbed ZGNRs suggest that the metal adatoms play an important role in the transport properties of devices and exhibit different effects on the transport properties of 6ZGNR-based and 7ZGNR-based devices. The 7ZGNR-based devices show the opposite conductive order in 6ZGNR-based devices. For 6ZGNR-based devices, the transport current in 6ZGNRs can be enhanced effectively by the adsorption of metal adatoms. However, the currents in 7ZGNR functionalized with conductive metal atoms are obviously smaller than that in pristine 7ZGNR, implying that metal adsorptions reduce the electrical conductivity of 7ZGNR-based devices. In contrast to the properties of the bulk materials, the conductivity of 6ZGNR–Al is highest among 6ZGNR–metal systems, which is in agreement with that of single atomic wires of Ag, Al, Au, and Cu.

A beneficial “microenvironment effect” on the efficiency of confined electrocatalysts is predicted by theory. However, examples of its experimental confirmation are scarce for catalysts based on polyoxometalates for the hydrogen evolution reaction (HER). For this purpose, the cyclic 48-tungsto-8-phosphate [H7P8W48O184]33− (P8W48) was fixed in a 3D configuration on reduced graphene oxide sheets (rGO) to boost its HER activity. The HRTEM imaging and the solid state 31P NMR spectrum of P8W48/rGO reveal a strong interaction between individual P8W48 and transparent rGO sheets. The calculation of the interaction between P8W48 and graphene (G) sheets is difficult to perform within a reasonable period of time because of the large size and very high overall negative charge of P8W48. However, as P8W48 is symmetrical, a quarter of its structure [H2P2W12O48]12− (P2W12) was extracted as a DFT calculation model. As P8W48 in P8W48/rGO is neutral, due to surrounding counter cations, the calculation model P2W12 is neutral with protons considering the affordable computational time. The adsorption energy for P2W12 on G (−1.55 eV) and the charge transfer between P2W12 and G (0.66 |e|) indicate that a strong interaction between P2W12 and G sheets exists. Kinetic studies show that the P8W48/rGO hybrids display excellent HER activity in acid, further confirmed by reproducible generation of hydrogen with quantitative faradaic yield and a high turnover frequency (11 s−1 at 295 mV overpotential) for a noble metal-free electrocatalyst. Importantly, the overpotentials required for the HER compare well with those of the commercial Pt/C (20 wt% Pt), which indicates that P8W48/rGO is a promising cheap HER electrocatalyst. We demonstrate here the most convincing experimental evidence of the “microenvironment effect” on HER electrocatalysis by a polyoxometalate.

•The effects of different functionals on the bond length of [PW12O40]3− were studied.•The effects of different functionals on the FMO energies of [PW12O40]3− were systematically investigated.•B3P86 with 13% HF exchange offers the reasonable results of M3PW12O40, M4SiW12O40 and M6P2W18O62 (M = Na, K).To obtain more reasonable Frontier Molecular Orbital (FMO) energies of α-Keggin anion [PW12O40]3−, the influence of different density functional theory (DFT) functionals were systematically investigated. Except LC-ωPBE, M06HF, LC-BLYP, HF and LC-BP86, the functionals we tested overestimate the bond length with mean unsigned error (MUE) values ranging from 0.02 Å to 0.03 Å. All the functionals used show the typical trend that the FMO energy levels are overestimated. The hybrid generalized gradient approximation (hGGA), such as B3P86 (MUE 0.465 eV), performs better than others. The results on Na3PW12O40 are more reasonable than those of [PW12O40]3− as the errors caused by the negative charge were compensated. B3P86 is superior to B3PW91 and B3LYP in the description on Na3PW12O40 with MUE 0.461 eV and mean signed error (MSE) 0.043 eV. B3P86 with 13% HF exchange offers the reasonable MUEs (≈0.12 eV) for M3PW12O40, M4SiW12O40 and M6P2W18O62 (M = Na, K).

The electronic and transport properties of a series of 11-ASiNRs (armchair silicene nanoribbons) at different torsion angles were studied by using density functional theory combined with nonequilibrium Green's function method. Several key factors determining the transport properties, such as the electron transmission coefficient and band structure, have been discussed. The interesting results suggest that the transport properties of ASiNRs are insensitive to the torsional silicene nanoribbon configuration in the scattering region. With the increase of the torsion angle, the transmission coefficient is still well maintained within the limits of the torsion angle. Although the torsion angle is increased to 120°, the current dropped by just 22% compared to the initial 11-ASiNRs at a torsion angle of 0°. Furthermore, all the configurations of 11-ASiNRs in this study behave as conventional conductors with nearly linear current–voltage dependence. On the basis of these distinctive transport properties with metabolic structure, ASiNRs present potential promising applications in silicon-based electronic nanodevices.

A thorough theoretical analysis was carried out on the sulfoxidation with H2O2 catalyzed by a tetranuclear peroxotungstate [SiO4{WO(O2)2}4]4−. The active species is the [SiO4{WO(O2)2}4(H2O2)]4− (SiW4(H2O2)) complex rather than [SiO4{WO(O2)2}4]4− (SiW4). The catalytic cycle consists of three elementary processes: oxygen transfer, sulfoxide dissociation, and catalyst regeneration. The oxygen transfer occurs from the peroxo oxygen atom O1 of SiW4(H2O2) to the sulfur center of dimethyl sulfide with a moderate Gibbs activation energy (ΔG°‡) of 17.1 kcal mol−1. By comparing potential energy surfaces and condensed Fukui functions (ƒ+), the electrophilicity of the outer peroxo atoms in SiW4(H2O2) determines which oxygen transfers to the dimethyl sulfide. Then, the sulfoxide dissociation proceeds with a small ΔG°‡ value of 2.3 kcal mol−1 by elongation of the peroxo O1–O4 distance and elimination of the product dimethylsulfoxide. Finally, the catalyst regeneration is found to occur via two successive proton transfers from H2O2 to the oxygen atoms of peroxotungstates with the ΔG°‡ values of 15.9 and 15.3 kcal mol−1, which has been firstly examined in the present study. All of these steps occur easily with moderate ΔG°‡ values, but the oxygen transfer is the rate-determining step of this catalytic reaction. In addition, the catalytic activity of peroxotungstates can be effectively tuned by changing the heteroatom X of [XO4{WO(O2)2}4(H2O2)]n− in the order: SeVI ≈ SVI > AsV ≈ PV > SiIV.

Quantum chemical calculations were performed to explore the structural and electronic properties of the two polyoxoaurates, [AuIII4AsV4O20]8− (Au4As4) and [AuIII4SeIV4O16]4− (Au4Se4), known to date, and a number of hypothetical polyoxoaurate derivatives comprising heteroatoms different from arsenic and selenium (namely, Si, Ge and P). In addition, the interactions of [AuIII4X4Om]n− (X = As, Se) with alkali-metal cations (Li+, Na+, K+ and Rb+) are also analysed. The studies suggest that the geometry structure, electronic properties and nucleophilicity of oxygen atoms of these polyoxoaurates are tuned by the size or electronegativity of the heteroatoms (Si, Ge, P, As and Se). Then, the geometry of [AuIII4X4Om]n− (X = As and Se) coordinating with alkali cations from Li+ to Rb+ and the complexation energy between [AuIII4X4Om]n− and alkali cations were compared. The results show that the stability and electronic structure of heteropolyoxoaurates depend on the entrapped cations. On the basis of the complexation energy, it can be concluded that the ion-pairing effect in arsenate-capped oxoaurate is stronger than that in selenite-capped oxoaurate. These heteropolyoxoaurates are expected to play a role in aqueous behaviour, self-assembly characteristics of polyoxoaurates, ion recognition, selectivity studies and may exhibit potential guest-switchable redox properties.

Polyoxometalate (POM)-based organic–inorganic hybrid systems II1–II7 are designed as p-type dyes containing double D–A1–π–A2 chains. The A1 spacers are thiophene, 1,2,3-triazole, 1,3,4-oxadlazole, thienothiadiazole units or their combinations and the A2 spacer is hexamolybdate. The electronic structures, absorption spectra, and electronic transition characteristics of systems were systematically studied on the basis of density functional theory (DFT) and time-dependent DFT (TDDFT). The highest occupied molecular orbital (HOMO) levels of systems II1–II7 were below the valence bond (VB) of NiO and the lowest unoccupied molecular orbital (LUMO) levels of studied systems were higher than the I2/I3− redox level, which benefit hole injection and dye regeneration. The HOMOs of systems II1–II4 were predominantly delocalized over the organic groups and MoN, which are more helpful to hole injection than systems II5–II7. Introduction of thienothiadiazole units is an effective way to improve the light absorption ability of dyes, and inserting thiophene and 1,2,3-triazole as A1 spacers can increase the efficiency of dye in dye-sensitized solar cells (DSSC).

Based on a porphyrin derivative (system 1), Lindqvist-, Keggin-, and Anderson-type polyoxometalate (POM) organic–inorganic hybrids (systems 2–4) were designed with the aim of investigating their charge transfer character and screening them as high performance p-type sensitizers. The electronic structures and absorption spectra of systems 1–4 were systematically investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The results indicate that Lindqvist- and Keggin-type POMs affect the lowest unoccupied molecular orbital (LUMO) energy levels, while the Anderson-type POM does not contribute to the frontier molecular orbitals (FMOs). Furthermore, the absorption spectrum of the Lindqvist-type POM porphyrin derivative (system 2) exhibits strong and broad absorption in the visible region and is red shifted about 100 nm in comparison with system 1. Further studies point out that system 2 can balance the photovoltaic parameters, LHE, HJE, CRE and DRE, indicating that it will be a promising high performance dye sensitizer in p-type dye-sensitized solar cells (DSSCs).

A series of polyoxometalates (POMs)-based dyes with electron donating/withdrawing groups were investigated by using density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The light harvesting efficiency (LHE), dye regeneration efficiency (DRE), charge recombination efficiency (CRE), holes injecting efficiency (HJE) and reorganization energy (Ereorg) were systematically evaluated. The maximum absorptions of the designed dyes with electron donating groups (systems 1–3) are red shifted comparing with those containing the electron withdrawing groups (systems 4 and 5). Different electron donating groups have significant differences on the binding energy of dye–(NiO)4 system. In the dye regeneration, the I2 interacts with the terminal oxygen of POM cluster. System 1 (–NH2) balances all parameters, and it may perform better than others. Therefore, the introduction of –NH2 into POM-based organic–inorganic hybrids may improve the performance of dye in dye-sensitized solar cells (DSSCs).

Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis were carried out to investigate the electronic structures and bonding features between the ruthenium(II) atom and the SO2 molecule in two ruthenium–sulfur dioxide (SO2) adducts, trans-Ru(NH3)4(SO2)Cl+ and [{SiW11O39}RuII(SO2)]6−. In addition, the bonding interactions between SO2 and the metal-ruthenium fragment were determined by binding energy (ΔEabs) calculation and electronic structures. The results indicate that the η1-S-planar model in both trans-Ru(NH3)4(SO2)Cl+ and [{SiW11O39}RuII(SO2)]6− are more favorable. NBO analysis of the bonding interaction between ruthenium and sulfur centers in the [{SiW11O39}RuII(SO2)]6− complex shows that it possesses a σ and a π bond. It predicts that the polyoxometalate [SiW11O39Ru]6− can serve as a potential adsorbent for the SO2 molecule because of the strong Ru–S bond relative to Ru(NH3)4Cl+.

•The second-order NLO behaviors can be switched by reversible redox.•The absorption bands possess remarkably shift with the manipulable redox states.•The redox processes have influence on intramolecular donor or acceptor character.•The complex with reversible redox states might be excellent switchable NLO materials.•The β0 value of system, SP2−, is largest and is 525.94*10−30 esu.In this paper, the relationship between reversible redox properties and second-order nonlinear optical (NLO) responses for photochromic unsymmetrical Mn-Anderson type polyoxometalate covalently linked to one spiropyran has been systematically investigated by using density functional theory (DFT) method. It is noticeable that the second-order NLO behaviors can be switched by reversible redox for this complex. The gaining and losing of electrons cause significant change on NLO response. Among these studied systems, the β0 value of two-electron-oxidized system, SP2−, is largest and ∼48 times as large as that of system SP. The charge-transfer transition corresponding to the dominant contributions to β0 values indicates that redox processes affect on intramolecular donor and acceptor character. Therefore, this kind of complex with the facile and reversible redox states might potentially be used for switching purposes in NLO molecular devices.The unsymmetrical spiropyran–polyoxometalate complex with the facile and reversible redox states might become excellent switchable NLO material.

Using a pH-dependent synthetic approach, the combination of different simple metal salts or metal coordination complexes with SeO32– heteroanion templates was employed to synthesize five distinct assemblies of Keggin-/Dawson-type tungstoselenites: (C2H8N)10KNa[(α-SeW9O34){Zr(H2O)}{WO(H2O)}(WO2)(SeO3){α-SeW8O31Zr(H2O)}]2·14H2O (1) at pH = 1.3; (C2H8N)10KNa5[(Se2W18O60)2(μ2-O)4]·12H2O (2) at pH = 2.5; (C2H8N)4Na4[Se2W18O62(H2O)2]·13H2O (3) at pH = 3.6; (C2H8N)4K3Na10[(α-SeW9O33)2{Ce2(CH3COO)(H2O)3W3O6}(α-Se2W14O52)]·26H2O (4) at pH = 4.5; K10Na5[(α-SeW9O33)2{Ce2(H2O)4W3O6}{α-Se2W14O51(OH)}]·24H2O (5) at pH = 4.5. All five compounds were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric, UV/vis spectroscopy, and ESI-MS. Moreover, their electrochemical properties were investigated. Keggin-type polyoxoanion of 1 remains the first reported Zr-containing tungstoselenites based on {α-SeW9} building blocks. X-ray analysis revealed that the 4d metal Zr centers have seven- and eight-coordinated modes, and SeO32– acts as the templates as well as the linkers. With the increasing of the pH, Dawson-type polyoxoanions of 2 and 3 based on the first reported basic lacunary {α-Se2W14} building blocks are obtained by using 3d-4f metal coordination complexes. Polyoxoanions of 4 and 5 remain similar structures stabilized by the 4f metal Ce centers at pH = 4.5 and that contain the basic Keggin-type {α-SeW9} and Dawson-type {α-Se2W14} building blocks in 1–3 at the same time, presenting the mixed multiple lacunary building blocks being combined into the single polyoxoanion architecture. Furthermore, the density functional theory calculations have been performed on polyoxoanions of 1 and 5 as the representatives to investigate their electronic properties.

•Novel hexamolybdates dyes were designed with different electron donors.•The absorption spectra of substituted hexamolybdates dyes remarkably red shift.•The theoretical examination was performed on the key parameters.•Dye 6 containing a biTT unit has the largest λmax at 733 nm and shows a higher Jsc.•These dyes are promising candidates to obtain high performance solar cell materials.Novel organoimido-substituted hexamolybdates dyes were designed by introducing 3,4-ethylenedioxythiophene (EDOT) or thienothiophene (TT) unit as electron donor based on [Mo6O18(MBTH)]2−. The electronic structures, absorption spectra and transition natures of designed systems have been theoretically investigated according to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. Compared with dye 1, the absorption spectra of these designed organoimido-substituted hexamolybdates dyes exhibit both strong and broad absorptions from 400 to 800 nm, as well as remarkably red shift owing to the long π-conjugated bridge and high delocalization. Especially for dye 6, which contains a biTT unit, it has the largest maximum absorption wavelength (λmax) at 733 nm and may show a higher short-circuit current density (Jsc) as it possesses higher light harvesting efficiency (LHE) and reasonable driving force (ΔERP). Our work reveals that the designed molecule 6 is promising candidate for high performance solar cell materials.

We use time-dependent density functional theory methods to discuss the absorption spectra, electronic transition properties, and photovoltaic performance of metalloporphyrin–polyoxometalates (POM) complexes for p-type dye-sensitized solar cells (DSSCs). The results show that the energy levels of the frontier molecular orbitals for dyes 2–6 match the requirements of p-type DSSCs. The absorption spectra of dyes 2–6 exhibit larger and broader absorptions compared to that of dye 1 by the introduction of POM. In addition, the photovoltaic performances of dyes 2–6 are suitable for high-efficiency DSSCs. This paper is expected to advance the design of metalloporphyrin–POM hybrid dyes with excellent performance in DSSCs.

•The novel dyes of POM-based organic–inorganic hybrids were designed for DSSCs.•The absorption spectra of systems were systematically studied.•The electron injection efficiency (Φinj) from dyes to TiO2 was evaluated.•The light harvesting efficiency (LHE) of dyes was assessed.•The incident photon-to-electron conversion efficiency (IPCE) of dyes was assessed.A series of POM-based organic–inorganic hybrids with different π-conjugated bridges are investigated as sensitizers for application in dye-sensitized solar cells (DSSCs). A combination of density functional theory (DFT) and time-dependent DFT (TDDFT) approaches is employed. The effects of π-conjugated spacer size and π-conjugation length on the spectra of designed systems are demonstrated. The results show that the absorption spectra are systematically broadened and red-shifted with increasing sizes of the π-conjugated spacer and the length of π-conjugation. The theoretical examination was performed on the key parameters of incident photon-to-electron conversion efficiency (IPCE), light harvesting efficiency (LHE) and electron injection efficiency (Φinj). The result suggests that system 2 with thieno[3,2-b]thiophene demonstrates a balance of the two crucial factors and may result in the highest IPCE of DSSCs. This study is expected to deepen our understanding of POM-based organic–inorganic hybrid dyes and assist the molecular design of new dyes for the further optimization of DSSCs.

The static second-order nonlinear optical (NLO) responses of a series of lacunary γ-Keggin polyanion derivatives, [XM2W10O38(μ-OH)2]n− (X = SiIV, GeIV, PV, AsV, M = AlIII, TiIV, VV; X = ZnII, VV, M = AlIII), were investigated by using density functional theory (DFT) methods. The results show that the molecular NLO activity of lacunary Keggin POM derivatives can be modified by replacing the central heteroatom (X) and the substituted addenda metal atom (M), which reveals the general rules to design the system with large optical nonlinearity.Graphical abstractHighlights► These inorganic complexes have evidently large static second-order polarizability. ► The molecular NLO activity of lacunary Keggin POM derivatives can be modified. ► These POMs seem to be promising inorganic candidates for application in nonlinear optics.

•The UV/ECD spectra of chiral POM enantiomer were studied using TDDFT method.•UV/ECD spectra of enantiomer R in gas phase and different solvents were calculated.•UV/ECD spectra of chiral POM in gas phase, polar, non-polar solvent are different.•Our work provides the first-hand structure–property information about chiral POMs.The ultraviolet–visible and electronic circular dichroism (UV–vis/ECD) spectra of diphosphonate-functionalized asymmetric cantilever-type chiral polyoxomolybdate (POM) enantiomer R-{Mo2O5[(Mo2O6)NH3CH2CH2CH2C(O)(PO3)2]2}6− (R) were systematically investigated using time-dependent density functional theory (TDDFT) method. From the view of molecular structure and relative energy, we inferred that there is likely a structural conversion from enantiomers R to S-{Mo2O5[(Mo2O6)NH3CH2CH2CH2C(O)(PO3)2]2}6− (S) via the intermediate configuration (IN). The ECD spectra of the enantiomer R were produced over the range of 3.0–6.3 eV. The UV–vis and ECD spectra of enantiomer R in the gas phase and different solvents were calculated. The results reveal that the UV–vis and ECD spectra of the chiral POM in gas phase, polar solvent, or non-polar solvent are different. The calculated electron density difference maps (EDDMs) display that the POM cluster is a chiroptical chromophore in studied compound.The electronic structures, chiroptical properties and the solvent effect on the CD spectra of diphosphonate-functionalized asymmetric polyoxomolybdates were systematically investigated using time-dependent density functional theory (TDDFT) method.

•The NLO properties of Keggin-type POMs organic–inorganic hybrids are investigated.•The metalloporphyrin is helpful in enhancing the optical nonlinearity.•The solvent effect affects the NLO response of the complex.The second-order nonlinear optical (NLO) properties of hybrid complexes via coordination interaction between porphyrin and Keggin-type polyoxometalates (POMs) α-[MSiW11O39]3− (M = NbV or VV) are investigated by time-dependent density functional theory (TDDFT). The calculated results show that this kind of organic–inorganic hybrid complexes possesses remarkably large molecular second-order NLO polarizability, especially for the ZnP3P-CC-4-Py-[VSiW11O39]3− (complex 4), which has a computed β0 value of 261,410 a.u. and might be an excellent second-order NLO material. The effects of substituted metal atom (M), metalloporphyrin, and π-conjugation on NLO response are analyzed, the substituted metal atom (M) with a large electronegativity, the metalloporphyrin, and the lengthening of π-conjugation are helpful in enhancing the optical nonlinearity of these systems, which reveal the general rules to design the complexes with large optical nonlinearities. Furthermore, the solvent effect largely affects the first-order hyperpolarizability of the complex, it implies that the second-order polarizabilities increased with the increase of the solvent in polarity.The second-order nonlinear optical (NLO) properties of hybrid complexes via coordination interaction between porphyrin and Keggin-type polyoxometalates (POMs) α-[MSiW11O39]3− (M = NbV or VV) are investigated by time-dependent density functional theory (TDDFT).

The novel dyes of organoimido-substituted hexamolybdates for positive type dye-sensitized solar cells (p-type DSSCs) have been studied on the basis of time-dependent density functional theory (TDDFT) calculations. The electronic absorption spectra, light harvesting efficiency (LHE), charge separation efficiency (CSE), and holes injecting efficiency (HJE) of designed systems have been systematically investigated. The results reveal that the long π-conjugated bridge and auxochrome play crucial roles in red-shifting the absorption bands and reinforcing the intensity of the bands. Based on [(n-C4H9)4N]2[Mo6O18(N-1-C10H6-2-CH3)], the designed systems 6 and 4 are good candidates for p-type DSSC dyes due to the strong absorption in the visible region as well as high LHE, CSE, and HJE. The maximum absorption of the one-electron-reduced system obviously red-shifts to the visible region. Therefore, the highly efficient dyes of p-type DSSC can be prepared by reducing POM-based organic–inorganic hybrids which have both long π-conjugated bridge and auxochrome.

The electronic circular dichroism (ECD) and UV–visible absorption (UV–vis) spectra of Strandberg-type polyoxometalates (POMs) (R, R)-[(R*PO3)2M5O15]2- (R* = CH3CH(NH3), (M = Mo, W)) have been explored using the time-dependent density functional theory (TDDFT) method. It demonstrates that the absolute configurations of chiral systems can be determined by chiroptical spectroscopic methods combined with DFT calculations. The calculated ECD spectra of the Strandberg-type molybdate were produced over the range of 3.3–6.5 eV, which are generally in agreement with the experimental spectra. In addition, the ECD spectra of (R, R)-[(R*PO3)2W5O15]2- (R* = CH3CH(NH3)) were produced over the range of 4.5–8.5 eV. The Becke’s half-and-half hybrid exchange-correlation functional (BHandHLYP) with the HF exchange fraction to 55% hybrid functional was found to well predict the excitation energies of studied systems. The origins of the ECD bands of two systems are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to metal atoms in polyanion. The results suggest that the polyanion are chiroptical chromophores. The polyanion plays a role as an optically active chromophore and contribute to the absorptions of ECD spectra. The difference of the UV–vis/ECD spectra between two systems shows that the transition metal atom significantly influences on the chiroptical properties of the studied Strandberg-type POMs.

We theoretically investigate a novel switching phenomenon based on the divacant Keggin-type polyoxotungstate bearing chiral organophosphonate [{NH2CH(CH3)PO}2(γ-SiW10O36)]4−, that is the synchronous chiroptical and nonlinear optical (NLO) switch triggered by redox. The ECD calculations on the Boltzmann weighted conformations of the three oxidation states of this chiral polyoxometalate (POM) clearly present a chiroptical switching process. The electronic transition and the bond-length alternation studies show that the chirality transfer from chiral carbon atom to POM cage increases as the polyanion is reduced. Simultaneously, the static first hyperpolarizability of studied chiral POM quadrupled from the oxidized state to the 1e-reduced state, and is further doubled to the 2e-reduced state, which is mainly due to the increasing electronic-dipole-allowed d–d charge transfer transitions in the POM cage. This work firstly reproduces the ECD spectrum of chiral POM with high accuracy and proves the possibility for confirming the molecular conformations of flexible chiral POMs in solution by the aid of ECD calculations. Most importantly, a sensitive diplex switch based on a chiral POM is predicted in theory, which may aid the design of novel POM-based switches.

The electronic properties and stabilities of five [Nb2W4O18OCH3]3− isomers have been investigated using a density functional theory method. The results show that the isomer with the methoxy group occupying a bridging position between two tungsten atoms (two tungsten atoms in the plane that contains two niobium atoms) in the [Nb2W4O18OCH3]3− framework is the most stable isomer in acetonitrile. The stability of the one-electron-reduced isomers changes little. The most stable one-electron-reduced isomer has the methoxy group occupying a bridging position between niobium atoms in the [Nb2W4O18OCH3]4− framework. The M-Ob (M = Nb, W; b denotes bridging) bond lengths in anions in which the metal atoms are connected by a methoxy group are longer than those in [Nb2W4O19]4−. The highest occupied molecular orbitals (HOMO) in [Nb2W4O19]4− mainly delocalize over the bridging oxygen atoms of two niobium atoms and two tungsten atoms located in the equatorial plane, and the bridging oxygen atoms on the axial surface. The lowest unoccupied molecular orbitals (LUMO) of [Nb2W4O19]4− are mainly concentrated on the tungsten atoms and antibonding oxygen atoms. Methoxy substitution modifies the electronic properties of the [Nb2W4O18OCH3]3− isomers. The HOMOs in the five isomers formally delocalize over the bridging oxygen atoms, which are distant from the surface containing the methoxy group and four metal atoms. The LUMOs delocalize over the d-shells of the four metal atoms that are close to the methoxy group, and the p-orbitals of oxygen. One-electron reduction occurred at the tungsten atoms, not the niobium atoms.

The UV/CD spectra of tin-bearing acetonyl-substituted Wells–Dawson polyoxotungstates α1- and α2-[P2W17O61{SnCH2CH2C(═O)}]6– were systematically investigated using the time-dependent density functional theory (TDDFT) method. The electronic circular dichroism (ECD) spectra were produced over the range of 3.3–5.8 eV. The calculated ECD spectra of the α1-R isomer were generally in agreement with the experimental spectra. The CAM-B3LYP hybrid functional was found to predict the excitation energies of tin-containing polyoxotungstates well. The fact that the UV/ECD spectra of α1-isomers are different from those of α2-isomers demonstrates the effect of the tin substitution site on the chiroptical properties of the studied isomers. The origins of the ECD bands are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to W atoms, from organic fragments to W atoms, or from the combination of two CT transitions. The results suggest that the organic fragment and polyoxometalate (POM) cage are chiroptical chromophores.

The chiroptical properties of bisarylimidos bearing o-alkoxy chain-substituted polyoxomolybdates [Mo6O17(2,2′-NC6H4OCnH2nOC6H4N)]2− [n = 4(2), 6(3±), 8(4)] were investigated using the time-dependent density functional method. The results showed that the studied chiral polyoxometalates (POMs) manifested similar absorption sites but displayed different shapes and magnitudes in their electronic circular dichroism (ECD) spectra. The ECD spectra of the studied chiral POMs originated from charge-transfer (CT) transitions from arylimido fragments to the POM cages and from oxygen atoms to the molybdenum atoms in the POM cages. The o-alkoxy chain served as a scaffold for generating chirality rather than contributing to the ECD spectrum of the studied POMs. The induced chiralities of the POM cages were defined by the CT transitions, which were completely localized on the POM cages. Furthermore, the long-range corrected CAM-B3LYP hybrid functional and a basis set that is larger than Lanl2DZ should be used for ECD calculations of chiral POMs. Our work establishes the use of computational studies to investigate the chiroptical properties of chiral POMs and provides theoretical interpretations.Graphical abstractHighlights► The geometry, electronic, and chiroptical properties of polyoxometalatocyclophanes are studied. ► The rigidity of POM in solution leads to the intramolecular chirality. ► The ECD spectra of POMs vary in signs and magnitudes, while show similar absorption sites. ► The o-alkoxy chain is scaffold for generating chirality, rather than contributes to ECD absorption. ► The induced chirality of POM cage is confirmed by electron transition assignment.

Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis were carried out to investigate the electronic structures and bonding features between the ruthenium(II) atom and the SO2 molecule in two ruthenium–sulfur dioxide (SO2) adducts, trans-Ru(NH3)4(SO2)Cl+ and [{SiW11O39}RuII(SO2)]6−. In addition, the bonding interactions between SO2 and the metal-ruthenium fragment were determined by binding energy (ΔEabs) calculation and electronic structures. The results indicate that the η1-S-planar model in both trans-Ru(NH3)4(SO2)Cl+ and [{SiW11O39}RuII(SO2)]6− are more favorable. NBO analysis of the bonding interaction between ruthenium and sulfur centers in the [{SiW11O39}RuII(SO2)]6− complex shows that it possesses a σ and a π bond. It predicts that the polyoxometalate [SiW11O39Ru]6− can serve as a potential adsorbent for the SO2 molecule because of the strong Ru–S bond relative to Ru(NH3)4Cl+.

The electronic and transport properties of a series of 11-ASiNRs (armchair silicene nanoribbons) at different torsion angles were studied by using density functional theory combined with nonequilibrium Green's function method. Several key factors determining the transport properties, such as the electron transmission coefficient and band structure, have been discussed. The interesting results suggest that the transport properties of ASiNRs are insensitive to the torsional silicene nanoribbon configuration in the scattering region. With the increase of the torsion angle, the transmission coefficient is still well maintained within the limits of the torsion angle. Although the torsion angle is increased to 120°, the current dropped by just 22% compared to the initial 11-ASiNRs at a torsion angle of 0°. Furthermore, all the configurations of 11-ASiNRs in this study behave as conventional conductors with nearly linear current–voltage dependence. On the basis of these distinctive transport properties with metabolic structure, ASiNRs present potential promising applications in silicon-based electronic nanodevices.

We theoretically investigate a novel switching phenomenon based on the divacant Keggin-type polyoxotungstate bearing chiral organophosphonate [{NH2CH(CH3)PO}2(γ-SiW10O36)]4−, that is the synchronous chiroptical and nonlinear optical (NLO) switch triggered by redox. The ECD calculations on the Boltzmann weighted conformations of the three oxidation states of this chiral polyoxometalate (POM) clearly present a chiroptical switching process. The electronic transition and the bond-length alternation studies show that the chirality transfer from chiral carbon atom to POM cage increases as the polyanion is reduced. Simultaneously, the static first hyperpolarizability of studied chiral POM quadrupled from the oxidized state to the 1e-reduced state, and is further doubled to the 2e-reduced state, which is mainly due to the increasing electronic-dipole-allowed d–d charge transfer transitions in the POM cage. This work firstly reproduces the ECD spectrum of chiral POM with high accuracy and proves the possibility for confirming the molecular conformations of flexible chiral POMs in solution by the aid of ECD calculations. Most importantly, a sensitive diplex switch based on a chiral POM is predicted in theory, which may aid the design of novel POM-based switches.

A thorough theoretical analysis was carried out on the sulfoxidation with H2O2 catalyzed by a tetranuclear peroxotungstate [SiO4{WO(O2)2}4]4−. The active species is the [SiO4{WO(O2)2}4(H2O2)]4− (SiW4(H2O2)) complex rather than [SiO4{WO(O2)2}4]4− (SiW4). The catalytic cycle consists of three elementary processes: oxygen transfer, sulfoxide dissociation, and catalyst regeneration. The oxygen transfer occurs from the peroxo oxygen atom O1 of SiW4(H2O2) to the sulfur center of dimethyl sulfide with a moderate Gibbs activation energy (ΔG°‡) of 17.1 kcal mol−1. By comparing potential energy surfaces and condensed Fukui functions (ƒ+), the electrophilicity of the outer peroxo atoms in SiW4(H2O2) determines which oxygen transfers to the dimethyl sulfide. Then, the sulfoxide dissociation proceeds with a small ΔG°‡ value of 2.3 kcal mol−1 by elongation of the peroxo O1–O4 distance and elimination of the product dimethylsulfoxide. Finally, the catalyst regeneration is found to occur via two successive proton transfers from H2O2 to the oxygen atoms of peroxotungstates with the ΔG°‡ values of 15.9 and 15.3 kcal mol−1, which has been firstly examined in the present study. All of these steps occur easily with moderate ΔG°‡ values, but the oxygen transfer is the rate-determining step of this catalytic reaction. In addition, the catalytic activity of peroxotungstates can be effectively tuned by changing the heteroatom X of [XO4{WO(O2)2}4(H2O2)]n− in the order: SeVI ≈ SVI > AsV ≈ PV > SiIV.

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

Development of a highly efficient and stable hydrogen evolution reaction (HER) electrocatalyst based on non-precious-metal remains a great challenge. Herein, we report P-modified tungsten carbide encapsulated in N-doped carbon (P-W2C@NC) as an efficient and stable HER electrocatalyst over the whole pH range (0–14), prepared by annealing polyoxometalate (H3PW12O40·xH2O) and dicyandiamide at a high temperature. To achieve a current density of 10 mA cm−2, P-W2C@NC required an overpotential of 89 mV in acid, 63 mV in alkaline solution and 185 mV in neutral solution. This is the first case of a tungsten-carbide-based HER electrocatalyst operating at all pH values with high activity and stability, and might provide some new routes for the exploration of tungsten-based Pt-like electrocatalysts for the HER.

Using density functional theory (DFT) in combination with non-equilibrium Green's functions, we have investigated the electronic structures, magnetization, and quantum transport properties of zigzag graphene nanoribbons (ZGNRs) functionalized with conventional conductive metal adatoms (Al, Cu, Ag and Au). On the basis of the adsorption energies, our simulation demonstrates that Al and Cu adatoms are chemically bonded with ZGNRs, while the adsorptions for Ag and Au are between weak chemisorption and strong physisorption. The properties of charge transfer and magnetic moment are in reasonable agreement with the previous calculations. The adsorption of metal adatoms induce a net magnetic moment of −1 μB in 6ZGNR–metal systems. On the other hand, the transport studies of metal adatoms adsorbed ZGNRs suggest that the metal adatoms play an important role in the transport properties of devices and exhibit different effects on the transport properties of 6ZGNR-based and 7ZGNR-based devices. The 7ZGNR-based devices show the opposite conductive order in 6ZGNR-based devices. For 6ZGNR-based devices, the transport current in 6ZGNRs can be enhanced effectively by the adsorption of metal adatoms. However, the currents in 7ZGNR functionalized with conductive metal atoms are obviously smaller than that in pristine 7ZGNR, implying that metal adsorptions reduce the electrical conductivity of 7ZGNR-based devices. In contrast to the properties of the bulk materials, the conductivity of 6ZGNR–Al is highest among 6ZGNR–metal systems, which is in agreement with that of single atomic wires of Ag, Al, Au, and Cu.

Polyoxometalate (POM)-based organic–inorganic hybrid systems II1–II7 are designed as p-type dyes containing double D–A1–π–A2 chains. The A1 spacers are thiophene, 1,2,3-triazole, 1,3,4-oxadlazole, thienothiadiazole units or their combinations and the A2 spacer is hexamolybdate. The electronic structures, absorption spectra, and electronic transition characteristics of systems were systematically studied on the basis of density functional theory (DFT) and time-dependent DFT (TDDFT). The highest occupied molecular orbital (HOMO) levels of systems II1–II7 were below the valence bond (VB) of NiO and the lowest unoccupied molecular orbital (LUMO) levels of studied systems were higher than the I2/I3− redox level, which benefit hole injection and dye regeneration. The HOMOs of systems II1–II4 were predominantly delocalized over the organic groups and MoN, which are more helpful to hole injection than systems II5–II7. Introduction of thienothiadiazole units is an effective way to improve the light absorption ability of dyes, and inserting thiophene and 1,2,3-triazole as A1 spacers can increase the efficiency of dye in dye-sensitized solar cells (DSSC).

Quantum chemical calculations were performed to explore the structural and electronic properties of the two polyoxoaurates, [AuIII4AsV4O20]8− (Au4As4) and [AuIII4SeIV4O16]4− (Au4Se4), known to date, and a number of hypothetical polyoxoaurate derivatives comprising heteroatoms different from arsenic and selenium (namely, Si, Ge and P). In addition, the interactions of [AuIII4X4Om]n− (X = As, Se) with alkali-metal cations (Li+, Na+, K+ and Rb+) are also analysed. The studies suggest that the geometry structure, electronic properties and nucleophilicity of oxygen atoms of these polyoxoaurates are tuned by the size or electronegativity of the heteroatoms (Si, Ge, P, As and Se). Then, the geometry of [AuIII4X4Om]n− (X = As and Se) coordinating with alkali cations from Li+ to Rb+ and the complexation energy between [AuIII4X4Om]n− and alkali cations were compared. The results show that the stability and electronic structure of heteropolyoxoaurates depend on the entrapped cations. On the basis of the complexation energy, it can be concluded that the ion-pairing effect in arsenate-capped oxoaurate is stronger than that in selenite-capped oxoaurate. These heteropolyoxoaurates are expected to play a role in aqueous behaviour, self-assembly characteristics of polyoxoaurates, ion recognition, selectivity studies and may exhibit potential guest-switchable redox properties.