The Electron-Rich and Nucleophilic N-Heterocyclic Imines on Metal Surfaces: Binding Modes and Interfacial Charge Transfer.

The strongly electron-donating N-heterocyclic imines (NHIs) have been employed as excellent surface anchors for the thermodynamic stabilization of electron-deficient species due to their enhanced nucleophilicity. However, the binding mode and interfacial property of these new ligands are still unclear, representing a bottleneck for advanced applications in surface functionalization and catalysis. Here, NHIs with different side groups have been rationally designed, synthesized, and analyzed on various metal surfaces (Cu, Ag). Our results reveal different binding modes depending on the molecular structure and metal surface. The molecular design enables us to achieve a flat-lying or upright configuration and even a transition between these two binding modes depending on the coverage and time. Importantly, the two binding modes exhibit different degrees of interfacial charge transfer between the molecule and the surface. This study provides essential microscopic insight into the NHI adsorption geometry and interfacial charge transfer for the optimization of heterogeneous catalysts in coordination chemistry.

Reversible Self-Assembly of an N-Heterocyclic Carbene on Metal Surfaces.

Self-assembly of cyclohexyl cyclic (alkyl)(amino)carbenes (cyCAAC) can be realized and reversibly switched from a close-packed trimer phase to a chainlike dimer phase, enabled by the ring-flip of the cyclohexyl wingtip. Multiple methods including scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations identified a distinct isomer (axial or equatorial chair conformer) in each phase, and consequently support the conclusion regarding the determination of molecular surface geometry on the self-assembly of cyCAAC. Moreover, various substrates such as Ag (111) and Cu (111) are tested to elucidate the importance of cyCAAC-surface interactions on cyCAAC based nanopatterns. These investigations of patterned surfaces prompted a deep understanding of cyCAAC binding mode, surface geometry and reversible self-assembly, which are of paramount significance in the areas of catalysis, biosensor design and surface functionalization.

A Unidirectional Surface-Anchored N-Heterocyclic Carbene Rotor.

A molecular rotor based on N-heterocyclic carbenes (NHCs) has been rationally designed following theoretical predictions, experimentally realized, and characterized. Utilizing the structural tunability of NHCs, a computational screening protocol was first applied to identify NHCs with asymmetric rotational potentials on a surface as a prerequisite for unidirectional molecular rotors. Suitable candidates were then synthesized and studied using scanning tunneling microscopy/spectroscopy (STM/STS), analytical theoretical models, and molecular dynamics simulations. For our best NHC rotor featuring a mesityl N substituent on one side and a chiral naphthylethyl substituent on the other, unidirectional rotation is driven by inelastic tunneling of electrons from the NHC to the STM tip. While electrons preferentially tunnel through the mesityl N substituent, the chiral naphthylethyl substituent controls the directionality. Such NHC-based surface rotors open up new possibilities for the design and construction of functionalized molecular systems with high catalytic applicability and superior stability compared with other classes of molecular rotors.

Aryl Triflates in On-Surface Chemistry.

The reactivity of aryl triflates in on-surface C-C coupling is reported. It is shown that the triflate group in aryl triflates enables regioselective homo coupling with preceding or concomitant hydrodetriflation on Cu(111). Three different symmetrical π-systems with two and three triflate functionalities were used as monomers leading to oligomeric conjugated π-systems. The cascade, comprising different intermediates at different reaction temperatures as observed for one of the molecules, proceeds via initial removal of the trifluoromethyl sulfonyl group to give an aryloxy radical which in turn is deoxygenated to the corresponding aryl radical. Thermodynamically driven regioselective 1,2-hydrogen atom transfer leads to a translocated aryl radical which in turn undergoes coupling. For a sterically more hindered bistriflate, where one ortho position was blocked, dehydrogenative coupling occurred at remote position with good regioselectivity. Starting materials, intermediates as well as products were analyzed by scanning tunneling microscopy. Structures and suggested mechanism were further supported by DFT calculations.

An Electron-Rich Cyclic (Alkyl)(Amino)Carbene on Au(111), Ag(111), and Cu(111) Surfaces.

The structural properties and binding motif of a strongly σ-electron-donating N-heterocyclic carbene have been investigated on different transition-metal surfaces. The examined cyclic (alkyl)(amino)carbene (CAAC) was found to be mobile on surfaces, and molecular islands with short-range order could be found at high coverage. A combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations highlights how CAACs bind to the surface, which is of tremendous importance to gain an understanding of heterogeneous catalysts bearing CAACs as ligands.

K Atom Promotion of O2 Chemisorption on Au(111) Surface.

Alkali atoms are known to promote or poison surface catalytic chemistry. To explore alkali promotion of catalysis and to characterize discharge species in alkali-oxygen batteries, we examine coadsorption of K and O2 on Au(111) surface at the atomic scale by scanning tunneling microscopy (STM) and density functional theory (DFT). On a clean Au(111) surface, O2 molecules may weakly physisorb, but when Au(111) is decorated with K+ ions, they chemisorb into structures that depend on the adsorbate concentrations and substrate templating. At low K coverages, an ordered quantum lattice of K2O2 complexes forms through intramolecular attractive and intermolecule repulsive interactions. For higher K and O2 coverages, the K2O2 complexes condense first into triangular islands, which further coalesce into rhombohedral islands, and ultimately into incommensurate films. No structures display internal contrast possibly because of high structural mutability. DFT calculations explain the alkali-promoted coadsorption in terms of three center, cation-π interactions where pairs of K+ coordinate the π-orbitals on each side of O2 molecules, and in addition O2 forms a covalent bond to Au(111) surface. The K promoted adsorption of O2 is catalyzed by charge transfer from K atoms to Au(111) substrate and ultimately to O2 molecules, forming O2-δ in a redox state between the peroxo and superoxo. Tunneling d I/d V spectra of K2O2 complexes exhibit inordinately intense inelastic progression involving excitation of the O-O stretching vibration, but absence of a Kondo effect suggests that the magnetic moment of O2 is quenched.

On-Surface Reactive Planarization of Pt(II) Complexes.

A series of Pt(II) complexes with tetradentate luminophores has been designed, synthesized, and deposited on coinage metal surfaces with the aim to produce highly planar self-assembled monolayers. Low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations reveal a significant initial nonplanarity for all complexes. A subsequent metal-catalyzed separation of the nonplanar moiety at the bridging unit via the scission of a C-N bond is observed, leaving behind a largely planar core complex. The activation barrier of this bond scission process is found to depend strongly on the chemical nature of both bridging group and coordination plane, and to increase from Cu(111) through Ag(111) to Au(111).

Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions.

Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump-probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.

Interatomic Spin Coupling in Manganese Clusters Registered on Graphene.

Different interatomic spin interactions in graphene-regulated Mn atomic clusters are investigated by low-temperature scanning tunneling microscopy and magnetic-field-dependent inelastic spin excitation spectroscopy. All dimers observed exhibit an antiferromagnetic (AFM) singlet ground state and spin transition from the singlet to triplet states, but their AFM coupling strength shows a unique dependence on their site registration on the graphene. Intriguing spin coupling can be found in the graphene-mediated Mn trimers, which manifest multilevel spin excitations. In combination with Heisenberg spin modeling and first-principles numerical simulation, an exclusive noncollinear spin configuration of the Mn trimer regulated by the graphene template can be determined, and our observed experimental exchange energies cannot be understood by a direct spin exchange mechanism, but suggest a nonlocal Ruderman-Kittel-Kasuya-Yosida indirect spin exchange mechanism through substrate modulation, which has not yet been achieved in graphene so far.

Kondo effect of cobalt adatoms on a graphene monolayer controlled by substrate-induced ripples.

The Kondo effect, a widely studied phenomenon in which the scattering of conduction electrons by magnetic impurities increases as the temperature T is lowered, depends strongly on the density of states at the Fermi energy. It has been predicted by theory that magnetic impurities on free-standing monolayer graphene exhibit the Kondo effect and that control of the density of states at the Fermi level by external means can be used to switch the effect on and off. However, though transport data for Co adatoms on graphene monolayers on several substrates have been reported, there exists no evidence for a Kondo effect. Here we probe the role of the substrate on the Kondo effect of Co on graphene by combining low-temperature scanning tunneling microscopy and spectroscopy measurements with density functional theory calculations. We use a Ru(0001) substrate that is known to cause graphene to ripple, yielding a moiré superlattice. The experimental data show a sharp Kondo resonance peak near the Fermi energy from only Co adatoms at the edge of atop regions of the moiré pattern. The theoretical results show that the variation of the distance from the graphene to the Ru substrate, which controls the spin polarization and local density of states at the Fermi energy, is the key factor for the appearance of the Kondo resonance. The results suggest that rippling of graphene by suitable substrates is an additional lever for tuning and selectively switching the appearance of the Kondo effect.

Visualizing localized nematic states in twisted double bilayer graphene.

Direct visualization of the states originating from electron-electron interactions is of great importance for engineering the surface and interfacial properties of graphene-based quantum materials. For instance, the rotational symmetry breaking or nematic phase inferred from spectroscopic imaging has confirmed the existence of correlated states in a wide range of moiré materials. Here, we study the atomic-scale spatial distributions and symmetry of wave functions in gate-tunable twisted double bilayer graphene by employing scanning tunneling microscopy/spectroscopy and continuum model calculations. A series of spectroscopic imaging analyses are used to identify dominant symmetry breaking of the emergent states. Interestingly, in non-integer hole fillings, a completely new localized electronic state with rotational symmetry breaking is observed on the left side of the valence flat band. The degree of anisotropy is found to increase from the conduction flat band through the valence flat band to the new state. Our results provide an essential microscopic insight into the flat band and its adjacent state for a full understanding of their electric field response in twisted graphene systems.

Mixture toxicity of pyraclostrobine and metiram to the zebrafish (Danio rerio) and its potential mechanism.

The interplay between pesticides plays a critical role in ecotoxicology since these chemicals rarely emerge as single substances but rather in mixtures with other chemicals. In the present work, we purposed to clarify the combined toxic impacts of pyraclostrobine (PYR) and metiram (MET) on the zebrafish by using numerous indicators. Results exhibited that the 4-day LC50 value of MET to fish embryos was 0.0025 mg a.i. L-1, which was lower compared with PYR (0.019 mg a.i. L-1). Combinations of PYR and MET presented a synergetic impact on fish embryos. Contents of POD, CYP450, and VTG were drastically increased in the plurality of the single and joint treatments relative to the baseline value. Three genes, including vtg1, crh, and il-8, related to the endocrine and immune systems, were also surprisingly up-regulated when fish were challenged by the individual and mixture pesticides compared with the baseline value. These results afforded valuable information on the latent toxicity mechanisms of co-exposure for PYR and MET in the early growth stage of fish. Moreover, our data also revealed that frequent application of these two pesticides might exert a potentially ecotoxicological hazard on aquatic ecosystems. Collectively, the present study provided valuable guidance for the risk evaluation of chemical combinations.

The chromosome-level genome of Cherax quadricarinatus.

Red claw crayfish (Cherax quadricarinatus) is an aquatic crustacean with considerable potential for the commercial culture and an ideal model for studying the mechanism of sex determination. To provide better genomic resources, we assembled a chromosome-level genome with a size of 5.26 Gb and contig N50 of 144.33 kb. Nearly 90% of sequences were anchored to 100 chromosomes, which represents the high-quality crustacean genome with the largest number of chromosomes ever reported. The genome contained 78.69% repeat sequences and 20,460 protein-coding genes, of which 82.40% were functionally annotated. This chromosome-scale genome would be a valuable reference for assemblies of other complex genomes and studies of evolution in crustaceans.

On-surface synthesis of ballbot-type N-heterocyclic carbene polymers.

N-Heterocyclic carbenes (NHCs) are established ligands for metal complexes and surfaces. Here we go beyond monomeric NHCs and report on the synthesis of NHC polymers on gold surfaces, consisting of ballbot-type repeating units bound to single Au adatoms. We designed, synthesized and deposited precursors containing different halogens on gold surfaces under ultrahigh vacuum. Conformational, electronic and charge transport properties were assessed by combining low-temperature scanning tunneling microscopy, non-contact atomic force microscopy, X-ray photoelectron spectroscopy, first-principles calculations and reactive force field simulations. The confirmed ballbot-type nature of the NHCs explains the high surface mobility of the incommensurate NHC polymers, which is prerequisite for their desired spatial alignment. The delicate balance between mobility and polymerization rate allows essential parameters for controlling polymer directionality to be derived. These polymers open up new opportunities in the fields of nanoelectronics, surface functionalization and catalysis.

Conformational evolution following the sequential molecular dehydrogenation of PMDI on a Cu(111) surface.

Molecular spatial conformational evolution following the corresponding chemical reaction pathway at surfaces is important to understand and optimize chemical processes. Combining experimental and theoretical methods, the sequential N-H and C-H dehydrogenation of pyromellitic diimide (PMDI) on a Cu(111) surface are reported. STM experiments and atomistic modeling allow structural analysis at each well-defined reaction step. First, exclusively the aromatic N-H dehydrogenation of the imide group is observed. Subsequently, the C-H group at the benzene core of PMDI gets activated leading to a dehydrogenation reaction forming metalorganic species where Cu adatoms pronouncedly protruding from the surface are coordinated by one or two PMDI ligands at the surface. All reactions of PMDI induce conformational changes at the surface as confirmed by STM imaging and DFT simulations. Such conformational evolution in sequential N-H and C-H activation provides a detailed insight to understand molecular dehydrogenation processes at surfaces.

Paternity assessment: application on estimation of breeding value in body-weight at first egg trait of egg-laying duck (Anas platyrhynchos).

Paternity index was analyzed using five microsatellite loci among Chinese egg-laying ducks (Anas platyrhynchos). Based on the paternity relationship that was identified by paternity index analysis, the estimated breeding value (EBV) was calculated using BLUP (best linear unbiased predictor) method. Body weight at first egg (BWF) is the only considered trait in this study. In total, 12 sires, 31 dams and 77 daughters were involved in the EBV calculation. The results demonstrated that five microsatellite loci's polymorphism information content (PIC) ranged from 0.795 in locus AY493338 to 0.957 in locus AY493264 with average 0.899; the parent-offspring relationships were built by these microsatellites' genotype, 12 families of half sibling and 2 families of full sibling were involved, and the relationship error is smaller than 10(-7). The EBV results suggest that the average EBV was significantly higher in females (average EBV is 10.234 and 0.1045 for mother and daughter, respectively) than males (average EBV is just -26.44). The EBV results on BWF were in good agreement with the principle of GH (growth hormone) expression in poultry. These results show that paternity analyses of Chinese egg-laying ducks were basically resolved using the five microsatellite loci selected. The paternity relationships can apply in Chinese egg-laying duck breeding to quicken the improvement of genetic progress.

Exploring differentially expressed key genes related to development of follicle by RNA-seq in Peking ducks (Anas Platyrhynchos).

Duck follicles enter different reproductive phases throughout life, and follicle gene expression patterns differ according to these phases. In particular, differentially expressed genes and related to development of follicle (mRNAs) play an important role to explore the key genes in this process; however, the expression profiles of these genes remain unclear. In this study, transcriptome sequencing was used to investigate the expression levels of duck ovarian genes, and comparative transcriptional analysis was carried out to identify differential genes, and cluster them into groups and function identification. The results showed differential expression of 593 coding genes between young and laying ducks, and of 518 coding genes between laying and old ducks. In further GO analysis, 35 genes from the comparison between old ducks and laying ducks have significant been changed involved in hormones related to follicle development. They include up-regulated genes StAR, CYP17, EPOX, 3ß-HSD, CYP1B1 CYP19A1 and down-regulated genes SR-B1 in laying ducks hormone synthesis than old ducks. Among which EPOX is a key gene for time special highly expression during egg laying stage, and other key regulatory genes' highly expression showed in young and laying stage, and lower expression showing with follicular development stopping. Therefore, EPOX is a key regulator for duck follicle development in laying period, its expression level increase 100 times higher than in youth and decrease 98% than stop laying period in duck life cycle.

Chemistry of 4-[(4-bromophenyl)ethynyl]pyridine at metal surfaces studied by STM.

Molecular architectures (Kagome networks, coordinated/covalent dimers and branched coordination chains) via self-assembly, Ullmann reaction and pyridine coordination of 4-[(4-bromophenyl)ethynyl]pyridine are found to be sensitive to the underlying metallic surfaces. The molecular species were characterised on the surface by low-temperature scanning tunnelling microscopy (LT-STM) at sub-molecular level.

Seated reach capabilities for ergonomic design and evaluation with consideration of reach difficulties.

This paper aimed to identify the reach capabilities of 26 seated subjects considering the reach difficulty, orientation and other potential factors, and to find a method to model the minimum reach capability surfaces for fixed and adjustable seats. The reach capability radius was used as a measure of the reach capability and theoretically modeled. Based on the test data of seated reach, the distribution of the reach capability radius was analyzed. The strategy to select the minimum reach envelopes was constructed to accommodate a sufficient percentage of the target population for both fixed and adjustable seats. For adjustable seats, a method was developed to derive the reach capability data from the tested individual reach capability data by introducing seating position models to re-position the individual reach capability data. An application case was realized based on the cab packaging data of a mini-van, and the minimum reach envelopes of different difficulties were created and validated to accommodate 90% of the target population.