Scanning tunneling microscopy of prochiral anthracene derivatives on graphite: chain length effects on monolayer morphology

The morphology of monolayers formed upon adsorption of prochiral 1,5-substituted anthracene derivatives on highly oriented pyrolytic graphite is investigated using scanning tunneling microscopy at the liquid-solid interface. The adsorption orientation of these prochiral anthracene derivatives positions one of their enantiotopic faces in contact with the graphite. The molecules adsorb in rows with contact between adjacent anthracenes. The anthracene side chains extend perpendicular to the direction of the row repeat. All molecules within a single row adsorb via the same enantiotopic face. Anthracenes with side chains containing an even number of non-hydrogenic atoms (C, S) form monolayers in which molecules in adjacent rows adsorb via opposite enantiotopic faces. Anthracenes with side chains that contain an odd number of non-hydrogenic atoms form two-dimensional chiral domains in which all rows contain molecules adsorbed via the same enantiotopic face. This chain length effect on monolayer morphology represents a generalized example of structural effects previously observed in alkanoic acid monolayers formed on HOPG. The variation of the STM current with position in the vicinity of the anthracenes indicates that the highest occupied molecular orbital is the predominant mediator of tunneling for the aromatic group.     

A hexa-peri-hexabenzocoronene cyclophane: an addition to the toolbox for molecular electronics

Cyclophanes with the largest-to-date polycyclic aromatic hydrocarbon (hexa-peri-hexabenzocoronene, HBC) to be entrained in such a structural motif are reported. The two disks are covalently captured by intermolecular ring-closing olefin metathesis of dienes in good yield. DSC, optical microscopy, and WAXD show the new cyclophanes to self-assemble to thermotropic columnar liquid crystal mesophases similar to monomeric analogues. Solution spectroscopic studies reveal that the two disks within a single unit lie face-to-face, with a small average lateral offset. Self-assembly into two-dimensional crystals at a solid-liquid interface was visualized by STM, and the electrical properties of single molecules were assessed by scanning tunneling spectroscopy revealing a diode-like behavior which is similar to that previously reported for single HBC disks, laying the groundwork for future electrical interrogations of dynamic molecular complexes.     

吡嗪衍生物在石墨表面上的自组装单层膜结构

利用扫描隧道显微镜研究了荧光液晶分子2, 5-二-[2-(3, 4-二-十二烷氧基-苯基)-乙烯基]-3, 6-二甲基吡嗪(BPDP12)在石墨表面上自组装单层膜的结构. 实验结果表明, 该化合物在石墨表面形成两种自组装结构：一种是稳定的, 分子的共轭中心相互平行, 烷基链相互交错的密排结构；另一种是不稳定的, 分子的共轭中心彼此为烷基链所分隔的非密排结构. 分子之间较强的π-π作用和分子烷基链之间的范德华作用力对分子组装的取向形成竞争, 是产生两种不同组装结构的根本原因.     

A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.     

Immobilization of paracetamol and benzocaine pro-drug derivatives as long-range self-organized monolayers on graphite

We show here by means of scanning tunneling microscopy (STM) at the liquid/solid interface that paracetamol and benzocaine molecules bearing a long aliphatic chain can be immobilized on highly oriented pyrolitic graphite (HOPG) as perfectly ordered two-dimensional domains extending over several hundreds of nanometers. In both cases, high-resolution STM images reveal that compounds 1 and 2 self-assemble into parallel lamellae having a head-to-head arrangement. The paracetamol heads of 1 are in a zigzag position with entangled n-dodecyloxy side chains while benzocaine heads of compound 2 are perfectly aligned as a double row and have their palmitic side chains on either sides of the head alignment. We attribute the very long-range ordering of these two pro-drug derivatives on HOPG to the combined effects of intermolecular H-bonding on one side and Van der Waals interactions between aliphatic side chains and graphite on the other side. The 2D immobilization of pro-drug derivatives via a non-destructive physisorption mechanism could prove to be useful for applications such as drug delivery if it can be realized on a biocompatible substrate.     

Scanning tunneling microscopy study of metal-free phthalocyanine monolayer structures on graphite

Low temperature scanning tunneling microscopy (STM) studies of metal-free phthalocyanine (H2Pc) adsorbed on highly oriented pyrolytic graphite (HOPG) have shown ordered arrangement of molecules for low coverages up to 1 ML. Evaporation of H2Pc onto HOPG and annealing of the sample to 670 K result in a densely packed structure of the molecules. Arrangements of submonolayer, monolayer, and monolayer with additional adsorbed molecules have been investigated. The high resolution of our investigations has permitted us to image single molecule orientation. The molecular plane is found to be oriented parallel to the substrate surface and a square adsorption unit cell of the molecules is reported. In addition, depending on the bias voltage, different electronic states of the molecules have been probed. The characterized molecular states are in excellent agreement with density functional theory ground state simulations of a single molecule. Additional molecules adsorbed on the monolayer structures have been observed, and it is found that the second layer molecules adsorb flat and on top of the molecules in the first layer. All STM measurements presented here have been performed at a sample temperature of 70 K.     

Self-assembly of amphiphilic hexapyridinium cations at the air/water interface and on HOPG surfaces.

Mono- and multilayers of a novel amphiphilic hexapyridinium cation with six eicosyl chains (3) are spread at the air/water interface as well as on highly ordered pyrolytic graphite (HOPG). On water, the monolayer of 3 is investigated by recording surface pressure/area and surface potential/area isotherms, and by Brewster angle microscopy (BAM). Self-organized tubular micelles with an internal edge-on orientation of molecules form at the air/water interface at low surface pressure whereas multilayers are present at high surface pressure, after a phase transition. Packing motifs suggesting a tubular arrangement of the constituting molecules were gleaned from atomic force microscopy (AFM) investigations of Langmuir-Blodgett (LB) monolayers being transferred on HOPG at different surface pressures. These LB film structures are compared to the self-assembled monolayer (SAM) of 3 formed via adsorption from a supersaturated solution, which is studied by scanning tunnelling microscopy (STM). On HOPG the SAM of 3 consists of nanorods with a highly ordered edge-on packing of the aromatic rings and an arrangement of alkyl chains which resembles the packing of molecules at the air/water interface at low surface pressure. Additional details of the molecular packing were gleaned from single-crystal X-ray structure analysis of the hexapyridinium model compound 2b, which possesses methyl instead of eicosyl residues.     

Self-organized monolayer of meso-tetradodecylporphyrin coordinated to Au(111)

The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H2Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H2Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au(111), H2Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-C=N-) nitrogen atoms of the porphyrin macrocycle to Au(111).     

Two-dimensional structure control by molecular width variation with metal coordination

The self-assembled monolayer of bipyridine derivative 1, which has two alkyl chains on each end, at the HOPG/1-phenyloctane interface was studied by in situ scanning tunneling microscopy (STM). The detailed mechanism of a spontaneous change in the monolayer packing pattern by Pd coordination was studied. Uncomplexed 1 existed in a bent form in the monolayer, and the alkyl chains were interdigitated, whereas Pd-complexed 1 was in a straight form and the alkyl chains were not interdigitated. An intermediate state of 1 was successfully observed during metal coordination. The structure was the bent form with noninterdigitated alkyl chains. Equilibrium intermolecular distances reported from ab initio calculations indicate that the molecular width of the central aromatic part of uncomplexed 1 (7.5 A) is substantially smaller than that of the peripheral alkyl chain part (9.2 A). The bent form was suitable for covering up the surface to maximize the packing density. However, the molecular width of the aromatic unit of Pd-complexed 1 (9.1 A) was almost identical to that of the alkyl chain unit (9.2 A). Therefore, Pd-complexed 1 took the straight form in the monolayer. The observation of surface coverage by STM suggests that the bent form increases the packing density by as much as 16% compared with that of the straight form. These results indicate that the control of molecular width can be used to design molecular templates for nanostructure formation.     

β-联碳酰基类衍生物有序自组装膜的STM研究

在大气条件下, 利用扫描隧道显微镜研究了四个β-联碳酰基类衍生物在高定向裂解石墨(HOPG)表面的自组装结构. 研究分子的结构中均包含π电子共轭体系和烷基链. 实验研究了分子结构对自组装结构的影响, 并利用分子结构的变化实现了自组装膜结构的调控. 结果表明, 在甲苯溶剂中制备的这些自组装结构均长程有序, 分子间氢键和偶极相互作用是影响自组装膜结构变化的重要因素.     

取代烷基侧链长度对二维纳米结构的影响

合成了一系列烷基取代的间苯三酚衍生物,并在大气条件下用扫描隧道显微镜研究了它们在高定向裂解石墨表面的吸附和组装行为.实验结果表明,这些自组装分子具有条状结构特征.在链长较短的分子图像中,两条平行的烷氧基链肩并肩地排列在苯环的一侧,另一条烷氧基链则排列在苯环的另一侧,链与链之间彼此相互交错排列形成均一的烷基条带.当链长增加时,这种高稳定性和密排结构遭到破坏,出现单个分子和分子对共存的组装结构.这是由于烷基链与烷基链之间以及烷基链与基底之间的作用力共同决定的.通过调控分子烷基链的长度可以得到不同的表面二维纳米结构.     

Self-assembly of small polycyclic aromatic hydrocarbons on graphite: a combined scanning tunneling microscopy and theoretical approach

Self-assembled monolayers of chrysene and indene on graphite have been observed and characterized individually with scanning tunneling microscopy (STM) at 80 K under low-temperature, ultrahigh vacuum conditions. These molecules are small, polycyclic aromatic hydrocarbons (PAHs) containing no alkyl chains or functional groups that are known to promote two-dimensional self-assembly. Energy minimization and molecular dynamics simulations performed for small groups of the molecules physisorbed on graphite provide insight into the monolayer structure and forces that drive the self-assembly. The adsorption energy for a single chrysene molecule on a model graphite substrate is calculated to be 32 kcal/mol, while that for indene is 17 kcal/mol. Two distinct monolayer structures have been observed for chrysene, corresponding to high- and low-density assemblies. High-resolution STM images taken of chrysene with different bias polarities reveal distinct nodal structure that is characteristic of the molecular electronic state(s) mediating the tunneling process. Density functional theory calculations are utilized in the assignment of the observed electronic states and possible tunneling mechanism. These results are discussed within the context of PAH and soot particle formation, because both chrysene and indene are known reaction products from the combustion of small hydrocarbons. They are also of fundamental interest in the fields of nanotechnology and molecular electronics.     

Hydrogen-bonding versus van der Waals interactions in self-assembled monolayers of substituted isophthalic acids

Self-assembled monolayers of a series of isophthalic acids (5-octadecyloxyisophthalic acid, 5-decyloxyisophthalic acid, 5-hexyloxyisophthalic acid, and 5-pentyloxyisophthalic acid) formed on highly ordered pyrolytic graphite (HOPG) at the solid-liquid interface were studied using scanning tunneling microscopy (STM). Although these molecules have the same dicarboxyl headgroup, their hydrocarbon tails are of different lengths. Hydrogen-bonding between headgroups and van der Waals interactions between the hydrocarbon tails control the final morphology of the monolayer. The STM images show that both van der Waals interactions (vdWs) and hydrogen-bonding (H-B) compete to control the structure, but the final structure of the monolayer is determined by balance between the two interactions.     

正十八烷醇在HOPG上形成自组装膜的吸附特性

研究了正十八烷醇在高定向热解石墨(HOPG)上形成自组装膜的吸附特性, 正十八烷醇在室温下从溶液中吸附至HOPG上形成整齐定向排列的单层自组装膜. 通过扫描隧道显微镜(STM)、接触角测量和X射线光电子能谱(XPS)分析了正十八烷醇单层自组装膜在HOPG上的结构. 实验结果表明, 正十八烷醇自组装膜在基底上成平铺或直立形态, 由于分子在基底上覆盖程度的不同, 会导致它在基底上排列的方式有所不同.     

Molecular architecture of oligothiophene on a highly oriented pyrolytic graphite surface by employing hydrogen bondings

To achieve a controllable and predictable molecular architecture on a two-dimensional (2D) surface, a series of oligothiophenes with carboxylic groups and alkane chains were synthesized. The alkane chains and carboxylic groups, which can form hydrogen bonding, were intentionally designed in different positions of the oligothiophenes. The resulted molecular architectures by using the so-prepared oligothiophenes on a highly oriented pyrolytic graphite (HOPG) surface were investigated by scanning tunneling microscopy (STM) and density functional theory (DFT). It is found that the hydrogen bonding plays an essential role in the formation of the ordered assemblies. A controlled 2D molecular assembly could be fabricated by using hydrogen bondings.     

Structural evolution of hexa-peri-hexabenzocoronene adlayers in heteroepitaxy on n-pentacontane template monolayers

The growth and structure of self-assembled adlayers of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) adsorbed on highly ordered pyrolitic graphite (HOPG) decorated by an n-pentacontane (n-C50H102) monolayer have been investigated by scanning tunneling microscopy (STM). Whereas on HOPG the HBC-C12 molecules readily self-assemble into a unique stable 2D structure, on the [n-C50H102 monolayer/graphite] system we observe morphological phase transitions with formation of time dependent alpha, beta, and gamma phases (alpha-->beta-->gamma). The initial alpha-phase is similar to that obtained on bare graphite, while intermediate beta- and final gamma-structures present molecular dimers and rows, respectively. The observed two-dimensional polymorphism is due to weak interaction between HBC-C12 molecules and n-C50H102-modified graphite substrate. Our results constitute an important step toward the control of the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.     

Self‐Assembly of a Mononuclear [FeIII(L)(EtOH)2] Complex Bearing an n‐Dodecyl Chain on Solid Highly Oriented Pyrolytic Graphite Surfaces

The synthesis and structures of the N‐[(2‐hydroxy‐3‐methyl‐5‐dodecylphenyl)methyl]‐N‐(carboxymethyl)glycine disodium salt (H L ) ligand and its neutral mononuclear complex [Fe^III( L )(EtOH)₂] ( 1 ) are reported. Structural and electronic properties of 1 were investigated by using scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy (CITS) techniques. These studies reveal that molecules of 1 form well‐ordered self‐assemblies when deposited on a highly oriented pyrolytic graphite (HOPG) surface. At low concentrations, single or double chains (i.e., nanowires) of the complex were observed, whereas at high concentration the complex forms crystals and densely packed one‐dimensional structures. In STM topographies, the dimensions of assemblies of 1 found on the surface are consistent with dimensions obtained from X‐ray crystallography, which indicates the strong similarities between the crystal form and surface assembled states. Double chains are attributed to hydrogen‐bonding interactions and the molecules align preferentially along graphite defects. In the CITS image of complex 1 a strong tunneling current contrast at the positions of the metal ions was observed. These data were interpreted and reveal that the bonds coordinating the metal ions are weaker than those of the surrounding ligands; therefore the energy levels next to the Fermi energy of the molecule should be dominated by metal‐ion orbitals.     

Adsorption characteristic of self‐assembled corrole dimers on HOPG

Our study first focus on two types of corrole dimers oxidized and reduced forms on highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM), X‐ray photoelectron spectroscopy (XPS) and contact angle measurement (CAM) were used to investigate the self‐assembled monolayers of corrole dimers adsorbed on HOPG surfaces at room temperature in air. XPS and CAM results have confirmed both two molecules adsorbed on an HOPG surface and formed self‐assembled films, and STM experiments found that the corrole dimers adsorbed on HOPG surfaces form similar lobes. The different stable space structure of the oxidized form molecule (OFM) and reduced form molecule (RFM), led to the diversity of the tetramer structural dimensions. The occurrence of molecular aggregations and assembly was controlled by the interactions between molecular–molecular and molecule–substrate. The electrostatic interactions between the molecules control the geometrical sizes and molecule–substrate interactions determine topographical shapes of the self‐assembled corrole dimers on HOPG surface. Copyright © 2009 John Wiley & Sons, Ltd.     

Molecular Dissociation on the SiC(0001) 3×3 Surface

In the framework of density functional theory, the adsorption of the halogenated polycyclic aromatic hydrocarbon 2,11‐diiodohexabenzocoronene (HBC‐I₂) on the SiC(0001) 3×3 surface has been investigated. Nondissociative and dissociative molecular adsorption is considered, and simulated scanning tunneling microscopy (STM) images are compared with the corresponding experimental observations. Calculations show that dissociative adsorption is favorable and reveal the crucial importance of the extended flat carbon core on molecule–surface interactions in dissociative adsorption; the iodine atom–surface interaction is of minor importance. Indeed, removing iodine atoms does not significantly affect the STM images of the central part of the molecule. This study shows that the dissociation of large halogenated polycyclic aromatic hydrocarbon molecules can occur on the SiC surface. This opens up interesting perspectives in the chemical reactivity and functionalization of wide band gap semiconductors.     

Incorporation and manipulation of coronene in an organic template structure

A two-dimensional molecular template structure of 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA) was formed on a highly oriented pyrolytic graphite surface (HOPG) by self-assembly at the liquid-solid interface. Scanning tunneling microscopy (STM) investigations show high-resolution images of the porous structure on the surface. After the host structure was created, coronene molecules were inserted as guest molecules into the pores. STM results indicate that some of the guest molecules rotate inside their molecular bearing. Further investigations show that single coronene molecules can be directly kicked out of their pores by means of STM.