Linear Au Research Articles

(Invited) Organic Transformations with Strongly Photoreducing Catalysts

The recent advances of synthetic methodologies benefit from molecular photoredoxcatalysis. Molecular photoredox catalysts offer particular advantages in steering reactions that involve key radical intermediates, as they enable large exoergicity for generating radical species under ambient conditions through heterobimolecular photoinduced electron transfer. To expand the catalytic potential, my group investigated photoinduced electron transfer of doublet molecules and electron-rich, low-valent metal complexes. We found that radical anions of organic molecules exhibited strong photoreducing power capable of generating synthetically important aryl radical intermediates from aryl halides which reacted with aryl boronate esters to produce C-C cross-coupled products. Our studies provided convincing evidence that the active catalyst species was the excited-state radical anion which could be generated through consecutive absorption of a photon, an electron, and another photon. In particular, femto- and nanosecond transient absorption spectroscopic investigations provided direct evidence for the generation and oxidative quenching of the excited-state radical anion. As an alternative strategy to utilize the strong photoreducing power of a molecular catalyst, we seek electron-rich, low-valent transition metal complexes as photoredoxcatalysts. Our exploration enabled us to identify heteroleptic, linear Au(I) complexes as potent photoreducing catalysts with an excited-state oxidation potential as cathodic as -2.23 V vs standard calomel electrode. These Au(I) complexes can catalyse heteroaryl C-C cross-coupling reactions of redox-resistant aryl chlorides. Our investigations demonstrate that the Au(I) complexes offer several benefits, including strong visible-light absorption, a long excited-state lifetime, and the capacity of a 91% yield in the production of free-radical intermediates.

Antibacterial activity of Au(I), Pt(II), and Ir(III) biotin conjugates prepared by the iClick reaction: influence of the metal coordination sphere on the biological activity.

A series of biotin-functionalized transition metal complexes was prepared by iClick reaction from the corresponding azido complexes with a novel alkyne-functionalized biotin derivative ([Au(triazolatoR,R')(PPh3)], [Pt(dpb)(triazolatoR,R')], [Pt(triazolatoR,R')(terpy)]PF6, and [Ir(ppy)(triazolatoR,R')(terpy)]PF6 with dpb = 1,3-di(2-pyridyl)benzene, ppy = 2-phenylpyridine, and terpy = 2,2':6',2''-terpyridine and R = C6H5, R' = biotin). The complexes were compared to reference compounds lacking the biotin moiety. The binding affinity toward avidin and streptavidin was evaluated with the HABA assay as well as isothermal titration calorimetry (ITC). All compounds exhibit the same binding stoichiometry of complex-to-avidin of 4:1, but the ITC results show that the octahedral Ir(III) compound exhibits a higher binding affinity than the square-planar Pt(II) complex. The antibacterial activity of the compounds was evaluated on a series of Gram-negative and Gram-positive bacterial strains. In particular, the neutral Au(I) and Pt(II) complexes showed significant antibacterial activity against Staphylococcus aureus and Enterococcus faecium at very low micromolar concentrations. The cytotoxicity against a range of eukaryotic cell lines was studied and revealed that the octahedral Ir(III) complex was non-toxic, while the square-planar Pt(II) and linear Au(I) complexes displayed non-selective micromolar activity.

Unraveling ligand exchange reactions in linear neutral Au(i) and Cu(i) N-heterocyclic carbene complexes for biological applications

Linear complexes of the form [M(NHC)Cl] (M = Au(i) or Cu(i), NHC = N-heterocyclic carbene) are promising drug candidates due to their potent in vitro antitumor, antibacterial, and antiparasitic activities.

Computational study of conductance through Cu, Ag, Au and Pt atomic chain contacts

Atomic size wires are basic devices of molecular electronics and a common model for simulation of transport properties of other molecular devices, where hypothetical semi-infinite equidistant atomic chains are considered as electrodes. In this work, structure of infinite linear Cu, Ag, Au and Pt atomic chains was studied at nonrelativistic, scalar and spin–orbit zeroth order relativistic approximation levels. Bonds in chains were characterized within Quantum Theory of “Atoms In Molecules” approach. Conductance through finite chains of six atom length between bulk leads for bias voltage range of 0.0–1.0 V was calculated within non-equilibrium Green’s function approach combined with density functional theory. Chain conductance at high bias voltages correlates with product of Jenkins’ metallicity and positively defined kinetic energy density at bond critical points of corresponding infinite atomic chains. Variation of number of atoms in finite Au and Pt chains showed weak “odd-even” oscillations of chain conductance at some bias voltages.

High‐Performance Blue Electroluminescence Devices Based on Linear Gold(I) Complexes as Ultrafast Triplet Exciton Harvesters

AbstractAchieving large external quantum efficiencies, narrow bandwidths, and a long operational lifetime at high brightness remains the largest hurdle to developing organic blue‐emitting devices. Here, a material strategy is demonstrated that can meet these conditions. The strategy is based on linear heteroleptic Au(I) complex exciton harvesters and multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters. The organic electroluminescence devices produce blue emission with Commission Internationale de l'Eclairage chromaticity coordinates of (0.108, 0.160), a narrow full‐width at half‐maximum value of 20 nm, and a maximum external quantum efficiency (EQE) as high as 30.2%. Notably, the EQE value remains 22.2% at 2000 cd m−2, whereas conventional control devices with an organic exciton harvester suffer from huge roll‐offs in quantum efficiency. An additional benefit of the device is a one‐order‐of‐magnitude improvement in its operational lifetime compared with that of the control device. Finally, the investigations reveal that the improvements are attributable to the unique ability of the Au(I) complexes for ultrafast triplet exciton harvest. In addition, the Au(I) complexes can facilitate Förster energy transfer to the MR‐TADF emitter, with effective suppression of hazardous triplet–triplet Dexter energy transfer. It is believed that the research is helpful in commercializing high‐efficiency and stable blue electroluminescence devices.

Ion Pairing in Cationic Au(I)(μ‐H)2WCp2 Bimetallic Dihydrides

AbstractThe interionic structure in solution of a series of [LAu(μ‐H)2WCp2][BF4] ion pairs (L=N‐heterocyclic carbene, phosphine or phosphite) has been elucidated by means of heteronuclear NOE and diffusion NMR. Selective ion pairing based on specific metallocene‐anion interactions has been observed for all species. The ancillary ligand L plays only a minor role in dictating the availability of secondary ion pair structures, mainly owing to steric factors. DFT calculations have indicated that the tungstenocene fragment delocalizes most of the positive charge and acts as the preferential anion anchoring point. Even though the interaction is highly specific, ion association in CD2Cl2 is not enhanced and the typical aggregation behavior of linear Au(I) ion pairs is observed.

Porous Supramolecular Assembly of Pentiptycene-Containing Gold(I) Complexes: Persistent Excited-State Aurophilicity and Inclusion-Induced Emission Enhancement.

We report herein a porous supramolecular framework formed by a linear mononuclear Au(I) complex (1) via the tongue-and-groove-like joinery between the pentiptycene U-cavities (grooves) and the rod-shaped π-conjugated backbone and alkyl chains (tongues) with the assistance of C-H···π and aurophilic interactions. The framework contains distorted tetrahedral Au4 units, which undergo stepwise and persistent photoinduced Au(I)-Au(I) bond shortening (excited-state aurophilicity), leading to multicolored luminescence photochromism. The one-dimensional pore channels could accommodate different solvates and guests, and the guest inclusion-induced luminescence enhancement (up to 300%) and/or vapochromism are characterized. A correlation between the aurophilic bonding and the luminescence activity is uncovered by TDDFT calculations. Isostructural derivatives 2 and 3 corroborate both the robustness of the porous supramolecular assembly and the mechanisms of the stimulation-induced luminescence properties of 1. This work demonstrates the cooperation of aurophilicity and structural porosity and adaptability in achieving novel supramolecular photochemical properties.

Calibration of BAS-TR image plate response to GeV gold ions.

The response of the BAS-TR image plate (IP) was absolutely calibrated using a CR-39 track detector for high linear energy transfer Au ions up to ∼1.6 GeV (8.2 MeV/nucleon), accelerated by high-power lasers. The calibration was carried out by employing a high-resolution Thomson parabola spectrometer, which allowed resolving Au ions with closely spaced ionization states up to 58+. A response function was obtained by fitting the photo-stimulated luminescence per Au ion for different ion energies, which is broadly in agreement with that expected from ion stopping in the active layer of the IP. This calibration would allow quantifying the ion energy spectra for high energy Au ions, which is important for further investigation of the laser-based acceleration of heavy ion beams.

A 59-Electron Non-Magic-Number Gold Nanocluster Au99(C≡CR)40 Showing Unexpectedly High Stability.

An atomically resolved gold nanocluster Au99(C≡CC6H3-2,4-F2)40 (Au99) with an unusual 59 valence electrons has been synthesized. Single-crystal X-ray diffraction reveals that its Au79 kernel is a Au49 Marks decahedron capped by two Au15 units. The surface structure of Au99 consists of 20 linear Au(C≡CR)2 staples. Intercluster interactions are observed between these D5 symmetric clusters. The existence of an unpaired electron is verified by magnetic measurement. Interestingly, this open-shell gold cluster Au99 stays intact in toluene solution at 80 °C for more than a week, and it has good charging-discharging capability under electrochemical conditions. The compact ligand shell protection around the symmetric core accounts for the high stability. This work suggests that geometric factors may play a crucial role in determining the stability of a metal nanocluster, even though the cluster has an open-shell electronic structure.

Tuning the Au–Au interactions by varying the degree of polymerisation in linear polymeric Au(i) N-heterocyclic carbene complexes

Gold(i) N-heterocyclic carbene (Au(i) NHC) polymers were successfully synthesised with alkyl and alkoxy spacers, wherein polymers with different degrees of polymerisation were isolated for the first time by varying the reaction time.

Electron Transport of the Nanojunctions of (BN) n (n = 1-4) Linear Chains: A First-Principles Study.

We applied the density functional theory and nonequilibrium Green’s function method (DFT + NEGF) to investigate the relationship between the conductance and chain length in the stretching process, the asymmetric coupling of contact points, and the influence of positive and negative biases on the electron transport properties of the nanojunctions formed by the coupling of (BN)n (n = 1–4) linear chains and Au(100)-3 × 3 semi-infinite electrodes. We find that the BN junction has the lowest stability and the (BN)2 junction has the highest stability. Under zero bias, the equilibrium conductance decreases as the chain length increases; px and py orbitals play a leading role in electron transport. In the bias range of −1.6 to 1.6 V, the current of the (BN)n (n = 1–4) linear chains increases linearly with increasing voltage. Under the same bias voltage, (BN)1 has the largest current, so its electron transport property is the best. The rectification effect reflects the asymmetry of the structure of BN linear chains themselves and the asymmetry of coupling with the Au electrode surfaces at both ends. With the chain length increasing, the transmission spectrum near Ef is suppressed, the tunneling current decreases, and the rectification ratio increases. (BN)4 molecular junctions have the largest rectification ratio, reaching 13.32 when the bias voltage is 1.6 V. Additionally, the Au–N strong coupling is more conducive to the electron transport of the molecular chain than the Au–B weak coupling. Our calculations provide an important theoretical reference for the design and development of BN linear-chain nanodevices.

QM study of interaction between arginine amino acid and Au clusters and the effects on arginine acidity

Understanding the various structures of gold clusters and the interaction modes between Au clusters and biomolecules is an important issue in material science such as biosensors and catalysts. The binding of small gold clusters (Aun n = 2–5) with neutral and anionic forms of arginine (Arg) amino acid is investigated in this study using density functional theory (DFT) and B3LYP level. The relative stability among different forms of Au clusters including linear, zigzag, planar, and three-dimensional Au clusters was estimated, initially. The calculated findings show that the zigzag structure for Au3 and the planar structure for Au4 and Au5 are the best form. Furthermore, the different modes of interaction were taken into account from thermodynamic view point between the most stable conformers of Arg and Arg− with gold clusters. Finally, the arginine is considered as a weak organic acid to investigate the impact of Au clusters on the gas phase acidity. The acidity of isolated arginine and the acidity of [Aun/Arg] complexes were also compared. Based on the obtained results, upon the complexation with Au clusters at 298 K, for the interaction of Au, Au2, Au3, Au4, and Au5 clusters with arginine, the gas phase acidity (GPA) of arginine alters from 342.12 to 314.17, 303.04, 299.42, 303.41, and 331.66 kcal/mol respectively. These calculated values predict that when a weak organic acid is complexed with Au clusters, it will be altered to super acid. Furthermore, for isolated and complexed species of Arg, pKa values were evaluated in water solvent.

QTT-isogeometric solver in two dimensions

Elliptical PDEs are at the core of many computational problems. Sometimes it is necessary to solve them on fine meshes, which entail huge memory footprints and low computational speeds. We provide a method to solve elliptical PDEs on fine grids with lowered memory consumption and improved convergence.This paper considers one typical elliptical PDE – the Poisson equation on various polygonal domains with Dirichlet boundary conditions. The Finite Element Method (FEM) is used for numerical solution. FEM approximates a two-dimensional PDE as a system of linear equations Au=f. For an n-by-n mesh grid the sparse representation of A has the size of O(n2). We replace the sparse matrix representation with a Quantized Tensor Train (QTT) representation to obtain O((log⁡n)α) time and memory complexity to construct both A and f. This is ensured by constant-bounded QTT approximation ranks. AMEn solver is used on the final linear system, and its iterations are faster for low rank QTT approximation. To avoid rank growth caused by the intrinsic structure of A we introduce a new operation z-kron which constructs a matrix with rows and columns permuted into so called z-order. An algorithm to construct A in z-order directly in QTT with O(1) ranks w.r.t. n is provided.The proposed method is used to solve the Poisson equation on two different polygonal domains. Experiments show that our approach significantly improves memory consumption and speed over classical sparse-matrix based partial differential equation solvers like FEniCS.

Gold(I) Complexes Nuclearity in Constrained Ferrocenyl Diphosphines: Dramatic Effect in Gold-Catalyzed Enyne Cycloisomerization.

Di-tert-butylated-bis(phosphino)ferrocene ligands bearing phosphino substituents R (R=phenyl, cyclohexyl, iso-propyl, mesityl, or furyl) allow tuning the selective formation of Au(I) halide complexes. Thus, dinuclear linear two-coordinate, but also rare mononuclear trigonal three-coordinate and tetrahedral four-coordinate complexes were formed upon tuning of the conditions. Both Au(I) chloride and rarer Au(I) iodide complexes were synthesized, and their X-ray diffraction analysis are reported. The significance of the control of structure and nuclearity in Au(I) complexes is further illustrated herein by its strong effect on the efficiency and selectivity of gold-catalysed cycloisomerization. Cationic linear digold(I) bis(dicyclohexylphosphino) ferrocenes outperform other catalysts in the demanding regioselective cycloisomerization of enyne sulphonamides into cyclohexadienes. Conversely, tetrahedral and trigonal cationic monogold(I) complexes were found incompetent for enyne cycloaddition. We used the two-coordinate linear electron-rich Au(I) complex 2 b (R=Cy) to extend the scope of selective intramolecular cycloaddition of different 1,6-enyne sulfonylamines with high activity and excellent selectivity to the endo cyclohexadiene products.

Synthesis and Reactivity of (NHC)AuI–Mercaptopyridine Complexes

Reaction of (NHC)Au–X (X = halide) complexes with 2- or 4-mercaptopyridine under basic conditions allows for the preparation of a series of (NHC)AuI–mercaptopyridine complexes 1–4 in high yields. All complexes have been fully characterized by means of 1H and 13C NMR spectroscopy, melting point, and elemental analysis. Single crystals of complexes 1–3 were obtained, and the solid-state structures display a linear Au(I) center with coordination to the NHC ligand and the sulfur atom of the mercaptopyridine moiety. Interestingly, weak γ-anagostic C–H···Au interactions are found in complexes 1–3 regardless of the relative N or S position in the mercaptopyridine. Taking advantage of the free pyridine nitrogen, reaction of complexes 1 and 2 with palladium allyl chloride dimer permits the isolation of heteronuclear Au/Pd complexes 5 and 8 featuring a bridging ambidentate mercaptopyridine ligand. Further reactivity of complexes 1–3 toward B(C6F5)3 produces complexes 6, 7, and 9, which display frustrated Lewis pair reactivity with the activation of a H–F bond.

Surface enhanced IR absorption by cortisol molecules

We report on the study of surface enhanced infrared absorption (SEIRA) by steroid hormone cortisol molecules with different concentrations deposited on linear Au nanoantenna arrays. Localized surface plasmon resonances (LSPRs) arise in the nanoantennas under the influence of external electromagnetic radiation. LSPR frequency depends mainly on the nanoantenna length and can vary from the visible to the terahertz range. We establish the ratio between the structural parameters of nanoantenna arrays and LSPR frequencies based on the results of 3D electrodynamic simulations. Using nanolithography we fabricate nanoantenna arrays having LSPR frequencies close to the frequencies of the most intense absorption modes of steroid hormone cortisol. We deposit cortisol molecules onto the surface of nanoantenna arrays by drop– coating. SEIRA spectra of the nanoantenna arrays make it possible the determination of the presence of cortisol and establishing the sensitivity limit of this method. Thus, we show possibility of SEIRA application for cortisol concentration analysis.

Infrared Spectroscopy of Au+(CH4)n Complexes and Vibrationally-Enhanced C\u2013H Activation Reactions

A combined spectroscopic and computational study of gas-phase Au+(CH4)n (n = 3–8) complexes reveals a strongly-bound linear Au+(CH4)2 core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH4a1 and t2 fundamental transitions reveals essentially free internal rotation of the core ligands about the H4C–Au+–CH4 axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t2 vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au+(CH4)n=2,3 complexes corresponding, presumably, to the loss of two H2 molecules.

Well-defined linear Au n (n = 2-4) chains encapsulated in SWCNTs: a DFT study.

One-dimensional (1D) gold nanostructures have been extensively studied due to their potential applications in nanoelectronic devices. Using first-principles calculations, composites consisting of a well-defined linear Au n (n = 2-4) chain encapsulated in a (9,0) single-walled carbon nanotube (SWCNT) were studied. The translational energy barrier of a single Au atom in a (9,0) SWCNT was found to be 0.03eV. This low barrier guaranteed the formation of Au n @ (9,0) SWCNT (n = 1-4) composites. Bond lengths, differential charge densities, and electronic band structures of the composites were studied. The average Au-Au bond lengths in the composites were found to be almost the same as those in the corresponding free-standing linear Au n . The average bond length increased as the number of Au atoms increased. Charge transfer in all of these composites was slight, although a few valence electrons were transferred from the (9,0) SWCNT and the Au chains to intercalations. The conductivities of the encapsulated linear Au n (n = 2-4) chains were enhanced to some extent by encapsulating them in the SWCNT.

Localized surface plasmons in structures with linear Au nanoantennas on a SiO2/Si surface.

The study of infrared absorption by linear gold nanoantennas fabricated on a Si surface with underlying SiO2 layers of various thicknesses allowed the penetration depth of localized surface plasmons into SiO2 to be determined. The value of the penetration depth derived experimentally (20 ± 10 nm) corresponds to that obtained from electromagnetic simulations (12.9–30.0 nm). Coupling between plasmonic excitations of gold nanoantennas and optical phonons in SiO2 leads to the appearance of new plasmon–phonon modes observed in the infrared transmission spectra the frequencies of which are well predicted by the simulations.

A Bis(Diphosphanyl N-Heterocyclic Carbene) Gold Complex: A Synthon for Luminescent Rigid AuAg2 Arrays and Au5 and Cu6 Double Arrays.

A mononuclear bis(NHC)/Au(I) (NHC=N-heterocyclic carbene) cationic complex with a rigid bis(phosphane)-functionalized NHC ligand (PC(NHC)P) was used to construct linear Au3 and Ag2 Au arrays, a Au5 cluster with two intersecting crosslike Au3 arrays, and an unprecedented Cu6 complex with two parallel Cu3 arrays. The impact of metallophilic interactions on photoluminescence was studied experimentally.