Selective triplet exciton formation in a single molecule.

The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs)1-7. According to a simple model based on spin statistics, the injected charges form spin-singlet (S1) excitons and spin-triplet (T1) excitons in a 1:3 ratio2-4. After the first report of a highly efficient OLED2 based on phosphorescence, which is produced by the decay of T1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage2-6. Because T1 excitons have lower energy than S1 excitons (owing to the exchange interaction), use of the energy difference could-in principle-enable exclusive production of T1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope8-20 and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule21-25 adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T1 excitons without creating their S1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.

On-Surface Evolution of meso-Isomerism in Two-Dimensional Supramolecular Assemblies.

Chiral structures created through the adsorption of molecules onto achiral surfaces play pivotal roles in many fields of science and engineering. Here, we present a systematic study of a novel chiral phenomenon on a surface in terms of organizational chirality, that is, meso-isomerism, through coverage-driven hierarchical polymorphic transitions of supramolecular assemblies of highly symmetric π-conjugated molecules. Four coverage-dependent phases of dehydrobenzo[12]annulene were uniformly fabricated on Ag(111), exhibiting unique chiral characteristics from the single-molecule level to two-dimensional supramolecular assemblies. All coverage-driven phase transitions stem from adsorption-induced pseudo-diastereomerism, and our observation of a lemniscate-type (∞) supramolecular configuration clearly reveals a drastic chiral phase transition from an enantiomeric chiral domain to a meso-isomeric achiral domain. These findings provide new insights into controlling two-dimensional chiral architectures on surfaces.

In Situ STM and Vibrational Study of Nanometer-Scale Reorganization of a Phospholipid Monolayer Accompanied by Potential-Driven Headgroup Digestion.

In situ dynamic observation of model biological cell membranes, formed on a water/gold substrate interface, has been performed by the combination of electrochemical scanning tunneling microscopy and reflection infrared absorption vibrational spectroscopy. Monolayers of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) were formed on alkanethiol-modified gold surfaces in a buffer solution, and the microscopic phase transitions driven by electrochemical potential control were observed more in detail than our previous study on the same system [Electrochem. Commun. 2007, 9, 645-650]. This time the transitions were associated with the chemistry of DHPC by the aid of vibrational spectroscopy and the utilization of deuterium-labeled DHPC molecules. A negative potential shift solidifies the fluidic lipid layers into static striped or grainy features without notable chemical reactions. The first positive potential shift over the virginal DHPC monolayer breaks DHPC into choline and the corresponding phosphatidic acid (DHPA). This is the first case of a phospholipid electrochemical reaction microscopically detected at the solid surface.

Lateral Hopping of CO on Ag(110) by Multiple Overtone Excitation.

A novel type of action spectrum representing multiple overtone excitations of the v(M-C) mode was observed for lateral hopping of a CO molecule on Ag(110) induced by inelastically tunneled electrons from the tip of a scanning tunneling microscope. The yield of CO hopping shows sharp increases at 261±4 mV, corresponding to the C-O internal stretching mode, and at 61±2, 90±2, and 148±7 mV, even in the absence of corresponding fundamental vibrational modes. The mechanism of lateral CO hopping on Ag(110) was explained by the multistep excitation of overtone modes of v(M-C) based on the numerical fitting of the action spectra, the nonlinear dependence of the hopping rate on the tunneling current, and the hopping barrier obtained from thermal diffusion experiments.

Characterization of nitrogen species incorporated into graphite using low energy nitrogen ion sputtering.

The electronic structures of nitrogen species incorporated into highly oriented pyrolytic graphite (HOPG), prepared by low energy (200 eV) nitrogen ion sputtering and subsequent annealing at 1000 K, were investigated by X-ray photoelectron spectroscopy (XPS), angle-dependent X-ray absorption spectroscopy (XAS), and Raman spectroscopy. An additional peak was observed at higher binding energy of 401.9 eV than 400.9 eV for graphitic1 N (graphitic N in the basal plane) in N 1s XPS, where graphitic2 N (graphitic N in the zigzag edge and/or vacancy sites) has been theoretically expected to appear. N 1s XPS showed that graphitic1 N and graphitic2 N were preferably incorporated under low nitrogen content doping conditions (8 × 10(13) ions cm(-2)), while pyridinic N and graphitic1 N were dominantly observed under high nitrogen content doping conditions. In addition, angle-dependent N 1s XAS showed that the graphitic N and pyridinic N atoms were incorporated into the basal plane of HOPG and thus were highly oriented. Furthermore, Raman spectroscopy revealed that low energy sputtering resulted in almost no fraction of the disturbed graphite surface layers under the lowest nitrogen doping condition. The suitable nitrogen doping condition was discovered for realizing the well-controlled nitrogen doped HOPG. The electrochemical properties for the oxygen reduction reaction of these samples in acidic solution were examined and discussed.

Impact of reduced symmetry on magnetic anisotropy of a single iron phthalocyanine molecule on a Cu substrate.

We study the magnetic anisotropy of a single iron phthalocyanine (FePc) molecule on a Cu(110) (2 × 1)-O by using inelastic electron tunneling spectroscopy (IETS) with low-temperature scanning tunneling microscopy. Two inelastic excitations derived from the splitting of the molecular triplet spin state appear as two pairs of steps symmetrically with respect to zero sample voltage. We measured IETS spectra with external magnetic fields perpendicular and parallel to the molecular plane, and we analyzed the spectral evolution with the effective spin Hamiltonian approach. We determined all parameters related with magnetic anisotropy at a single-molecule level, both the easy- and hard-magnetization directions, zero-field splitting constant, D = - 4.0 meV and E = 1.1 meV, the Lande g-tensor gxx, gyy, gzz=(1.82, 2.02, 2.34), and the constant of spin-orbit coupling λ = - 19.1 meV. We stress that the symmetry breaking caused by the adsorption of FePc on the oxidized Cu(110) significantly impacts the magnetic anisotropy.

Scanning tunneling microscope observation of the phosphatidylserine domains in the phosphatidylcholine monolayer.

A mixed monolayer of 1,2-dihexanoyl-sn-glycero-3-phospho-l-serine (DHPS) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) on an 1-octanethiol-modified gold substrate was visualized on the nanometer scale using in situ scanning tunneling microscopy (STM) in aqueous solution. DHPS clusters were evident as spotty domains. STM enabled us to distinguish DHPS molecules from DHPC molecules depending on their electronic structures. The signal of the DHPS domains was abolished by neutralization with Ca(2+). The addition of the PS + Ca(2+)-binding protein of annexin V to the Ca(2+)-treated monolayer gave a number of spots corresponding to a single annexin V molecule.

Surface hydrogenation reactions at the single-molecule level.

Hydrogenation and dehydrogenation reactions on metal surfaces are among the most important in heterogeneous catalysis. Such reactions can be observed and characterized at the single-molecule level with low temperature scanning tunnelling microscopy (LT-STM). A brief review of such studies is presented. A specific example, the hydrogenation of methyl isocyanide to methyl aminocarbyne on the Pt(111) surface, is described in detail. This reaction was first identified in a study with reflection absorption infrared spectroscopy, a technique that averages over monolayer quantities of molecules. The example illustrates the importance of characterization of surface reactions with complementary techniques in order to properly interpret the single-molecule LT-STM images. A second example of the complementary nature of LT-STM and other surface characterization methods is the tip-induced dehydrogenation on Pt(111) of acetonitrile, the more stable isomer of methyl isocyanide.

Controlling orbital-selective Kondo effects in a single molecule through coordination chemistry.

Iron(II) phthalocyanine (FePc) molecule causes novel Kondo effects derived from the unique electronic structure of multi-spins and multi-orbitals when attached to Au(111). Two unpaired electrons in the d(z)(2) and the degenerate dπ orbitals are screened stepwise, resulting in spin and spin+orbital Kondo effects, respectively. We investigated the impact on the Kondo effects of the coordination of CO and NO molecules to the Fe(2+) ion as chemical stimuli by using scanning tunneling microscopy (STM) and density functional theory calculations. The impacts of the two diatomic molecules are different from each other as a result of the different electronic configurations. The coordination of CO converts the spin state from triplet to singlet, and then the Kondo effects completely disappear. In contrast, an unpaired electron survives in the molecular orbital composed of Fe d(z)(2) and NO 5σ and 2π* orbitals for the coordination of NO, causing a sharp Kondo resonance. The isotropic magnetic response of the peak indicates the origin is the spin Kondo effect. The diatomic molecules attached to the Fe(2+) ion were easily detached by applying a pulsed voltage at the STM junction. These results demonstrate that the single molecule chemistry enables us to switch and control the spin and the many-body quantum states reversibly.

Dissociation pathways of a single dimethyl disulfide on Cu(111): reaction induced by simultaneous excitation of two vibrational modes.

We present a novel reaction mechanism for a single adsorbed molecule that proceeds via simultaneous excitation of two different vibrational modes excited by inelastic tunneling electrons from a scanning tunneling microscope. Specifically, we analyze the dissociation of a single dimethyl disulfide (DMDS, (CH3S)2) molecule on Cu(111) by using a versatile theoretical method, which permits us to simulate reaction rates as a function of sample bias voltage. The reaction is induced by the excitation of C-H stretch and S-S stretch modes by a two-electron process at low positive bias voltages. However, at increased voltages, the dissociation becomes a single-electron process that excites a combination mode of these stretches, where excitation of the C-H stretch is the energy source and excitation of the S-S stretch mode enhances the anharmonic coupling rate. A much smaller dissociation yield (few orders of magnitude) at negative bias voltages is understood in terms of the projected density of states of a single DMDS on Cu(111), which reflects resonant excitation through the molecular orbitals.

Direct observation of adsorption geometry for the van der Waals adsorption of a single π-conjugated hydrocarbon molecule on Au(111).

Weak van der Waals adsorption of π-conjugated hydrocarbon molecules onto the gold surface, Au(111), is one of the essential processes in constructing organic-metal interfaces in organic electronics. Here we provide a first direct observation of adsorption geometry of a single π-conjugated hydrocarbon molecule on Au(111) using an atomically resolved scanning tunneling microscopy study combined with van der Waals density functional methodology. For the purpose, we utilized a highly symmetric π-conjugated hydrocarbon molecule, dehydrobenzo[12]annulene (DBA), which has a definite three-fold symmetry, the same as the Au(111) surface. Interestingly, our observations on an atomically resolved scale clearly indicate that the DBA molecule has only one adsorption configuration on Au(111) in spite of the weak van der Waals adsorption system. Based on the precisely determined adsorption geometry of DBA/Au(111), our calculation results imply that even a very small contribution of the interfacial orbital interaction at the organic-metal interface can play a decisive role in constraining the adsorption geometry even in the van der Waals adsorption system of a π-conjugated hydrocarbon molecule on the noblest Au(111) surface. Our observations provide not only deeper insight into the weak adsorption process, but also new perspectives to organic electronics using π-conjugated hydrocarbon molecules on the Au surface.

Substrate-induced symmetry breaking in silicene.

We demonstrate that silicene, a 2D honeycomb lattice consisting of Si atoms, loses its Dirac fermion characteristics due to substrate-induced symmetry breaking when synthesized on the Ag(111) surface. No Landau level sequences appear in the tunneling spectra under a magnetic field, and density functional theory calculations show that the band structure is drastically modified by the hybridization between the Si and Ag atoms. This is the first direct example demonstrating the lack of Dirac fermions in a single layer honeycomb lattice due to significant symmetry breaking.

Tailoring electronic states of a single molecule using adamantane-based molecular tripods.

Adsorption structures and electronic states of molecular tripods, having a Br atom (BATT) and a ferrocene derivative (Ferrocene-ATT) at the head part of the adamantane-based trithiolate, adsorbed on Au(111) have been investigated using low temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that BATT and Ferrocene-ATT form self-assembled monolayers (SAMs), and their orderings are identical to one another, which suggests that the adsorption structure of adamantane-based molecular tripods is independent of the type of functional substituent attached to the head part. The electronic states originated from the ferrocene group were confirmed in the STS spectrum of Ferrocene-ATT whereas those are absent in the BATT spectrum. We note that the ferrocene part has few interactions with the Au substrate owing not only to the upright geometry of Ferrocene-ATT but also to the insulative properties of the adamantane base. The STS mapping revealed the spatial distribution of the electronic state of Ferrocene-ATT.

Genome editing and molecular analyses of an Arabidopsis transcription factor, LATE FLOWERING.

Correct flower organ formation at the right timing is one of the most important strategies for plants to achieve reproductive success. Ectopic overexpression of LATE FLOWERING (LATE) is known to induce late flowering, partly through suppressing expression of the florigen-encoding gene FLOWERING LOCUS T (FT) in Arabidopsis. LATE is one of the C2H2 zinc finger transcription factors, and it has a canonical transcriptional repression domain called the ethylene-responsive element-binding factor-associated amphiphilic repression (EAR) motif at the end of its C terminus. Therefore, LATE is considered a transcriptional repressor, but its molecular function remains unclear. Our genome-edited late mutants exhibited no distinct phenotype, even in flowering, indicating the presence of redundancy from other factors. To reveal the molecular function of LATE and factors working with it, we investigated its transcriptional activity and interactions with other proteins. Transactivation activity assay showed that LATE possesses transcriptional repression ability, which appears to be attributable to both the EAR motif and other sequences. Yeast two-hybrid assay showed the EAR motif-mediated interaction of LATE with TOPLESS, a transcriptional corepressor. Moreover, LATE could also interact with CRABS CLAW (CRC), one of the most important regulators of floral meristem determinacy, through sequences in LATE other than the EAR motif. Our findings demonstrated the possibility that LATE can form a transcriptional repression complex with CRC for floral meristem determinacy.

On the mechanisms of lysis triggered by perturbations of bacterial cell wall biosynthesis.

Inhibition of bacterial cell wall synthesis by antibiotics such as ß-lactams is thought to cause explosive lysis through loss of cell wall integrity. However, recent studies on a wide range of bacteria have suggested that these antibiotics also perturb central carbon metabolism, contributing to death via oxidative damage. Here, we genetically dissect this connection in Bacillus subtilis perturbed for cell wall synthesis, and identify key enzymatic steps in upstream and downstream pathways that stimulate the generation of reactive oxygen species through cellular respiration. Our results also reveal the critical role of iron homeostasis for the oxidative damage-mediated lethal effects. We show that protection of cells from oxygen radicals via a recently discovered siderophore-like compound uncouples changes in cell morphology normally associated with cell death, from lysis as usually judged by a phase pale microscopic appearance. Phase paling appears to be closely associated with lipid peroxidation.

Ligand field effect at oxide-metal interface on the chemical reactivity of ultrathin oxide film surface.

Ultrathin oxide film is currently one of the paramount candidates for a heterogeneous catalyst because it provides an additional dimension, i.e., film thickness, to control chemical reactivity. Here, we demonstrate that the chemical reactivity of ultrathin MgO film grown on Ag(100) substrate for the dissociation of individual water molecules can be systematically controlled by interface dopants over the film thickness. Density functional theory calculations revealed that adhesion at the oxide-metal interface can be addressed by the ligand field effect and is linearly correlated with the chemical reactivity of the oxide film. In addition, our results indicate that the concentration of dopant at the interface can be controlled by tuning the drawing effect of oxide film. Our study provides not only profound insight into chemical reactivity control of ultrathin oxide film supported by a metal substrate but also an impetus for investigating ultrathin oxide films for a wider range of applications.

Symmetry-driven novel Kondo effect in a molecule.

The Kondo effect caused by the adsorption of iron phthalocyanine (FePc) on Au(111) was investigated by the combination of density functional theory and a numerical renormalization group calculation with scanning tunneling microscopy. We found that a novel Kondo effect is realized for a single FePc molecule on Au(111) by tuning the symmetry of the ligand field through the local coordination to the substrate. For FePc in the on top configuration where fourfold symmetry around the Fe(2+) ion is held, the orbital degrees of freedom survive, resulting in the spin+orbital SU(4) Kondo effect. In contrast, the reduced symmetry in the bridge configuration freezes the orbital degrees of freedom, leading to the spin SU(2) Kondo effect. These results provide a novel example to manipulate the many-body phenomena by tuning the local symmetry.

Combined scanning tunneling microscopy and high-resolution electron energy loss spectroscopy study on the adsorption state of CO on Ag(001).

The adsorption site and vibrational energies of CO on a clean Ag(001) surface were determined using scanning tunneling microscopy, inelastic electron tunneling spectroscopy with a scanning tunneling microscope, and high-resolution electron energy loss spectroscopy. The CO molecules were found to adsorb on the atop site of the Ag(001) surface, which was similar to their adsorption on the Cu(001) surface. The vibrational energy of the CO internal stretching mode was found to be 263 meV, which is only 3 meV less than that of CO in the gas phase. This result indicates that the CO molecules chemisorb very weakly on the Ag(001) surface.

Activation of ultrathin oxide films for chemical reaction by interface defects.

Periodic density functional theory calculations revealed strong enhancement of chemical reactivity by defects located at the oxide-metal interface for water dissociation on ultrathin MgO films deposited on Ag(100) substrate. Accumulation of charge density at the oxide-metal interface due to irregular interface defects influences the chemical reactivity of MgO films by changing the charge distribution at the oxide surface. Our results reveal the importance of buried interface defects in controlling chemical reactions on an ultrathin oxide film supported by a metal substrate.

Nature of electron transport by pyridine-based tripodal anchors: potential for robust and conductive single-molecule junctions with gold electrodes.

We have designed and synthesized a pyridine-based tripodal anchor unit to construct a single-molecule junction with a gold electrode. The advantage of tripodal anchoring to a gold surface was unambiguously demonstrated by cyclic voltammetry measurements. X-ray photoelectron spectroscopy measurements indicated that the π orbital of pyridine contributes to the physical adsorption of the tripodal anchor unit to the gold surface. The conductance of a single-molecule junction that consists of the tripodal anchor and diphenyl acetylene was measured by modified scanning tunneling microscope techniques and successfully determined to be 5 ± 1 × 10(-4)G(0). Finally, by analyzing the transport mechanism based on ab initio calculations, the participation of the π orbital of the anchor moieties was predicted. The tripodal structure is expected to form a robust junction, and pyridine is predicted to achieve π-channel electric transport.