Ordered arrays of semi-crown ligands on an Au(111) electrode surface: <Emphasis Type="Italic">in situ</Emphasis> STM study

In situ scanning tunneling microscopy (STM) and cyclic voltammetry were employed to investigate the adsorption structures of three semi-crown ligands on an Au(111) surface under the potential control. It is found that all the molecules formed ordered arrays in 0.1 mol/L HClO₄ solution, although their geometric structures are complex and asymmetric. The driving force was supposed to come from the balance between intermolecular and molecule-substrate interactions. High resolution STM images revealed internal molecular structures, orientations and packing arrangements in the ordered adlayers. The results are useful for preparing ordered arrays of transition metal-mediated nanostructures.     

Structural comparison of self-organized adlayers of ligands and their metal-coordinated complexes on a Au(111) surface: an STM study

Scanning tunneling microscopy (STM) was employed to investigate the adsorption of the linear-spacer-bridged ligands bis(pyrrol-2-yl-methyleneamine) (BPMB and BPMmB), and their Zn(II)-coordinated complexes, BPMB/Zn(II) and BPMmB/Zn(II), onto a Au(111) surface in 0.1 M HClO(4) solution. Both the ligands, with different spacer bridges, and their Zn(II) complexes adsorb onto the Au(111) surface and self-organize into highly ordered two-dimensional arrays. The complexes BPMB/Zn(II) and BPMmB/Zn(II) appear in helical and triangular conformations, respectively, consistent with their chemical structures. Although the metal complexes include ligands, the assembled structures and adlayer symmetries of the ligands and complexes are totally different. The structures and intramolecular features obtained by high-resolution STM imaging are discussed. The results should be important in fabricating surface supramolecular structures.     

Formation of heterocaryotic and homonuclear bridged–dimeric complexes on surface

The formation of coordinated dimeric complexes bridged by axial ligands on surface is observed with the help of a 1,3,5-tris(10-carboxydecyloxy)benzene(TCDB) template through scanning tunneling microscopy(STM). STM images of molecular adlayers of zinc tetraphenylporphyrin(Zn TPP), zinc phthalocyanine(Zn Pc), and their mixture are reported. Zn TPP and Zn Pc can spontaneously form highly an ordered structure with a 1:1 molar ratio, which is different from that of individual Zn Pc. The coordinated bimolecular complexes bridged with axial ligands, simply as Zn Pc–DPP–Zn TPP and Zn Pc–DPE–Zn Pc, are presented and the corresponding surface structures are compared. Zn Pc and Zn TPP can be connected by an axial ligand DPP and formed assembled structures out of surface. Two types of arrays with entirely new structure are obtained for the Zn Pc–DPE–Zn Pc complex. These bridged hybrid complexes provide an example of design of self-organized crystals on the basis of coordination through non-covalent interactions.     

Dynamic combinatorial olefin metathesis: templated synthesis of porphyrin boxes

A porphyrin macrocyclic square is efficiently prepared by a dynamic combinatorial approach to olefin metathesis and shown by scanning tunneling microscopy (STM) to self-assemble into highly ordered arrays on a graphite surface.     

异代酯基枝状分子表面组装及结构转化机理的STM研究

利用扫描隧道显微技术(STM)研究了3种异代酯基Janus枝状分子(G₁G₂COOCH₃, G₀G₃COOCH₃, G₁G₃COOCH₃)在高定向裂解石墨(HOPG)表面的组装结构. 室温下, G₁G₂COOCH₃, G₀G₃COOCH₃和G₁G₃COOCH₃分别在表面形成有序、局部有序及无序结构. 将样品加热至60~80 ℃后, 表面无序结构可以转化为有序的二聚体密堆积结构. 实验结果表明, 随着枝状分子代数的增加, 分子之间及分子与基底之间作用力增强, 室温下分子组装行为更依赖于分子吸附动力学过程, 组装结构趋于无序化, 而加热可以增强枝状分子在表面的活动性, 调整分子的位置关系, 使表面无序结构转化为热力学稳定的有序二聚体密堆积组装结构.     

Orientation‐Controlled Single‐Molecule Junctions

The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature by a combination of molecular assembly, scanning tunneling microscopy (STM) imaging, and STM break junction (STM‐BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat‐oriented mesitylene molecules on Au(111) to enable direct tip/π contacts, a result that is not possible by conventional methods. The measured conductance of Au/π/Au junction is about 0.1 G_o , two orders of magnitude higher than the conductance of phenyl rings connected to the electrodes by standard anchoring groups. Our experiments suggest that long‐range ordered structures, which hold the aromatic ring in place and parallel to the surface, are essential to increase probability of the formation of orientation‐controlled molecular junctions.     

STM investigation of the photoisomerization of an azobis-(benzo-15-crown-5) molecule and its self-assembly on Au(111)

Scanning tunneling microscopy (STM) has been employed to investigate the photoisomerization of azobis-(benzo-15-crown-5) on Au(111). A self-assembled monolayer of azobis-(benzo-15-crown-5) with a (3 x 7) symmetry was observed on Au(111). After ex-situ UV-light irradiation, a new adlayer structure with a different molecular arrangement could be seen on Au(111), although the domain size of the ordered packing is small. On the basis of a high-resolution STM image and photochemical reaction result, schematic models have been proposed for the original and ex-situ irradiated adlayers. After in-situ UV-light irradiation, the ordered arrays disappeared and many bright clusters emerged at domain boundaries. The results presented here provide direct evidence at the molecular level for a photochemical reaction.     

Schiff碱N-aete-N在Au(111)上自组装单分子膜的电化学及STM研究

利用电化学技术及扫描隧道显微镜(STM),于0.1mol/LHClO₄溶液中研究了Schiff碱N-aete-N在单晶Au(111)面上所形成的自组装单分子膜(SAMs)的电化学性质及结构.N-aete-N在Au(111)电极表面的吸附抑制了金的阳极氧化,同时使固/液界面双层电容明显降低.观察到N-aete-NSAMs的高分辨STM图像.N-aete-N分子在Au(111)表面上以(6×7)结构单胞呈二维有序排列,其表面浓度为5.5×10^-11mol/cm².     

Solvothermal Transformation of a Calcium Oleate Precursor into Large‐Sized Highly Ordered Arrays of Ultralong Hydroxyapatite Microtubes

Hydroxyapatite (HAP), a well‐known member of the calcium phosphate family, is the major inorganic component of bones and teeth in vertebrates. The highly ordered arrays of HAP structures are of great significance for hard tissue repair and for understanding the formation mechanisms of bones and teeth. However, the synthesis of highly ordered HAP structure arrays remains a great challenge. In this work, inspired by the ordered structure of tooth enamel, we have successfully synthesized three‐dimensional bulk materials with large sizes (millimeter scale) that are made of highly ordered arrays of ultralong HAP microtubes (HOAUHMs) by solvothermal transformation of calcium oleate precursor. The core–shell‐structured oblate sphere consists of a core that is composed of HAP nanorods and a shell that consists of highly ordered HAP microtube arrays. The prepared HOAUHMs are large: 6.0 mm in diameter and up to 1.4 mm in thickness. With increasing solvothermal reaction time, the HOAUHMs grow larger; the microtubes become more uniform and more ordered. This work provides a new synthetic method for synthesizing highly ordered arrays of uniform HAP ultralong microtubes that are promising for biomedical applications.     

Highly ordered adlayers of three calix[4]arene derivatives on Au(1 1 1) surface in HClO4 solution: in situ STM study

Highly ordered adlayers of three calix[4]arene derivatives have been prepared on Au(1 1 1) surface in HClO₄ solution. The adlayer structures are characterized by in situ scanning tunneling microscopy (STM). High-resolution STM images reveal the molecular orientation and packing arrangement in the ordered adlayers. All the molecules are upright on Au(1 1 1) surface in pinched cone conformation. The structural difference of these molecules is proposed to be from the different functional groups at the upper and lower positions. The structural models are tentatively proposed for three ordered adlayers.     

Physical structure of standing-up aromatic SAMs revealed by scanning tunneling microscopy

Long-range-ordered aromatic SAMs are formed on Au(111) using 4-nitrophenyl sulfenyl chloride as a precursor. Although the main structure is a √3 × √3 with a molecular density similar to that usually found for aliphatic SAMs, particular spots presenting specific shapes are also observed by STM. These include hexagons, partial hexagons, parallelograms, and zigzags resulting from specific arrangements of adsorbed molecules. These molecular arrangements are reversible as they form and dissociate or "vanish" in various areas on the surface. STM shows that these particular structures provide some order to their surrounding because areas void of these structures look less ordered. More interestingly, STM shows submolecular details of the molecules involved in forming these structures, hence providing direct experimental evidence for the ability of the STM to provide physical structure information of standing up SAMs. This is indeed a heavily debated question, and this work reports the first experimental example where submolecular physical structure is revealed by STM for standing-up SAMs.     

Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.     

Elucidation of the deposition processes and spatial structures of alkanethiol and arylthiol molecules adsorbed on Pt111 electrodes with in situ scanning tunneling microscopy

In situ scanning tunneling microscopy (STM) was used to examine the spatial structures of n-alkane thiols (1-hexanethiol, 1-nonanethiol, and 1-octahexanethiol) and arylthiols (benzenethiol and 4-hydroxybenzenethiol) adsorbed on well-ordered Pt111 electrodes in 0.1 M HClO4. The electrochemical potential and molecular flux were found to be the dominant factors in determining the growth mechanisms, final coverages, and spatial structures of these organic adlayers. Depending on the concentrations of the thiols, deposition of self-assembled monolayers (SAMs) followed either the nucleation-and-growth mechanism or the random fill-in mechanism. Low and high thiol concentrations respectively produced two ordered structures, (2 x 2) and (square root of 3 x square root of 3)R30 degrees , between 0.05 and 0.3 V. On average, an ordered domain spanned 500 A when the SAMs were made at 0.15 V, but this dimension shrank substantially once the potential was raised above 0.3 V. This potential-induced order-to-disorder phase transition resulted from a continuous deposition of thiols, preferentially at domain boundaries of (square root of 3 x square root of 3 x )R30 degrees arrays. All molecular adlayers were completely disordered by 0.6 V, and this restructuring event was irreversible with potential modulation. Since all thiols were arranged in a manner similar to that adopted by sulfur adatoms (Sung et al. J. Am. Chem. Soc. 1997, 119, 194), it is likely that they were adsorbed mainly through their sulfur headgroups in a tilted configuration, irrespective of the coverage. Both the sulfur and phenyl groups of benzenethiol admolecules gave rise to features with different corrugation heights in the molecular-resolution STM images. All thiols were adsorbed strongly enough that they remained intact at a potential as negative as -1.0 V in 0.1 M KOH.     

Adsorptive behavior of dimethylglyoxime on Au(111)

Dimethylglyoxime (DMG) adsorbed on Au(111) was investigated using electrochemical scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM experiments revealed three different structures of adsorbed DMG at open circuit potential (~0.07 V versus Ag/AgCl): (2√3×2√3)R30°-α, (2√3×4√3)R30°-β, and (2√3×4√3)R30°-γ. The coverage of adsorbed DMG obtained using XPS was 0.33. A combination of structural and quantitative information identified the adsorbed DMG as an anionic tetramer, held together by intermolecular hydrogen bonding and arrayed in three ordered patterns. Domains of adsorbed DMG underwent phase transitions between the observed structures, most likely due to the influence of the STM tip. However, a significant correlation between the observed structures and the imaging conditions was not found. The ordered layers existed only at open circuit potential as evidenced by their disappearance when the potential was shifted to 0.2 or -0.15 V. The ordered layers were also removed by immersion in a solution of Ni(2+), implying that the adsorbed DMG was converted to a soluble dimer complex with the Ni(2+) ion. This particular observation is discussed in terms of the rigidity of the organic network.     

Electrochemical self-assembly of melanin films on gold

An electrochemical method for self-assembling melanin films on the Au(111) surface from melanin aggregates in alkaline media is reported. Electrochemical data combined with scanning tunneling microscopy (STM), atomic force microscopy, and Auger electron spectoscopy show that the amount and structure of the deposited melanin film depend on the potential (E) applied to the electrochemical interface and deposition time. Film formation takes place at a noticeable rate at E = -1.0 V (vs SCE). High-resolution STM images at the early stages of growth show small particles, 5-8 nm in size and 0.3-0.4 nm in height, forming ordered arrays that follow closely the Au(111) topography. The size of the melanin particles increases as the film thickness increases, reaching 150 nm for deposits grown for 16 h. The deposited films are electrochemically active, showing well-defined redox couples preceding the hydrogen evolution reaction.     

The initial growth behavior of perylene on Cu(100)

Using scanning tunneling microscopy (STM) together with density functional theory (DFT) the growth behavior of perylene on the Cu(100) substrate has been investigated. As revealed by STM images, perylene molecules prefer to adopt lying configuration with their molecular plane parallel to the substrate, and two symmetrically equivalent ordered domains were observed. DFT calculations show that perylene molecule prefers to adsorb on the top site of substrate Cu atoms with its long molecular axis aligning along the [011] or [01-1] azimuth of the substrate which is the most stable adsorption geometry according to its highest binding energy. Consequently, two adsorption structures of c(8×4) and c(8×6), each containing two perylene molecules per unit cell, are proposed based on our STM images. The growth mechanism for ordered perylene domains on Cu(100) can be attributed to the balance between weak adsorbate-adsorbate interaction and comparable adsorbate-substrate interaction.     

Synthesis of nanometer-scale porphyrin wheels of variable size

Starting from 1,3-phenylene linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag I-promoted coupling reactions provided linear oligomers from 2nZA up to 128ZA. Of these zigzag shaped porphyrin arrays, the Ag I-promoted intramolecular cyclization reaction of 2 nZA (n=5, 6, 8, 9, 12, and 16) under dilute conditions gave the corresponding cyclic porphyrin wheels C2nZA (n=5, 6, 8, 9, 12, and 16), whereas large arrays 2nZA (n=24, 32, and 48) did not provide cyclic porphyrin products. These large discrete porphyrin arrays and wheels were fully characterized by means of 1H NMR spectroscopy, MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, GPC-HPLC analysis, and the scanning tunneling microscopy (STM) technique. The STM images of C12ZA and C18ZA reveal their large circular structures. In the cyclic structures of C2nZA in solution, however, the gradual decrease in fluorescence quantum yields and fluorescence lifetimes are observed, reflecting some conformational heterogeneities. Collectively, the present work provides an important contribution to the construction of fully covalently linked large cyclic arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.     

Giant porphyrin disks: control of their self-assembly at liquid-solid interfaces through metal-ligand interactions

The synthesis and self-assembly behaviour of porphyrin dodecamers 1H(2) and Zn-1, which consist of twelve porphyrins that are covalently attached to a central aromatic core, is described. According to STM, 1D and 2D NMR studies, and molecular modelling calculations, the porphyrin dodecamers have a yo-yo-shaped structure. Their large pi surface, in combination with their disk-like shape, allows them to form self-assembled structures, which in the case of Zn-1 can be tuned by adding bidentate ligands. The self-assembly of the molecules at the liquid-solid interface of 1-phenyloctane with highly oriented pyrolytic graphite or Au(111) was imaged by using STM. The porphyrin disks in the self-assembled arrays have an edge-on orientation on the surface. The addition of bidentate axial ligands to the Zn-1 molecules in the arrays allows their intermolecular distance to be precisely controlled.     

STM images of individual porphyrin hexamers; meso-meso singly linked orthogonal hexamer and meso-meso, beta-beta, beta-beta triply-linked planar hexamer on Cu(100) surface

Geometrical structures of chain porphyrin arrays adsorbed on Cu(100) are observed by STM: a bridge-like bent structure for meso-meso singly linked orthogonal hexamer, whereas a rigid planar and one-dimensionally stacked structure for meso-meso, beta-beta, beta-beta triply-linked hexamer.     

Fine-tuning of relative metal-metal distances within highly ordered chiral 2D nanopatterns

The two-dimensional nanopatterning of a series of neutral alkoxy/alkyl-functionalised bis(salicylaldehydato)/bis(aldiminato)copper(II) and -palladium(II) complexes at a liquid/solid (highly oriented pyrolytic graphite, HOPG) interface has been studied by scanning tunnelling microscopy (STM). The relative metal-metal distances were tuned stepwise in two dimensions by ligand design. Exchange of the carbonyl O-atom for NH or N-alkyl units effects different intermolecular interactions such as weak hydrogen bonds and steric effects that determine, together with the van der Waals forces between the alkyl chains, the relative arrangements of the complexes. Further variation of the length and position of the alkoxy side chains as well as the exchange of CuII for PdII affords an absolute fine-tuning of the surface structures. Highly resolved STM images of the resultant highly ordered adlayers allow us to establish detailed models of the molecular 2D arrays and to classify them into three basic chiral pattern types. Homochirality within the individual domains is induced by the highly regular deposition of the prochiral complexes from the same enantiotopic face. In the case of the C12 O-substituted bis(salicylaldiminato) (NH) Cu(II) complex Cu5, a secondary structure occurs as a racemic mixture of two chiral surface species deposited in a distinct alternating order.