Trapping Nanostructures on Surfaces through Weak Interactions

The assembly of imidazole‐functionalized phenanthroline‐strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl₃, THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three‐dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface–molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.     

Controlled Growth of Porphyrin Wires at a Solid‐Liquid Interface

Bis(zinc porphyrin) scaffolds bearing C₈ or C₁₈ alkyl chains and imidazole end groups self‐assembled in a head‐to‐tail fashion into multi‐porphyrin assemblies on both HOPG and mica. Due to weaker molecule surface‐interactions, longer arrays formed on mica than on HOPG. In both cases, it was essential first to generate monomers that were drop casted on the surface, then to allow time for the bis(zinc porphyrins) to assemble. Although thicker fibrous assemblies were observed with the C₈ alkyl substituents than with the longer chains, noncovalent assemblies up to 1 μm long were observed for each molecule. These investigations provide a reproducible, noncovalent method to grow porphyrin arrays that may be of interest in molecular electronics for charge transport.     

Phosphonated calix[4]arene-based amphiphiles as scaffolds for fluorescent nano-fibres

p-Phosphonic acid calix[4]arene bearing lower rim O-C(18)H(37) alkyl chains assemble into 6 nm diameter fibres, which deposit from toluene onto mica and graphite, as characterised using Atomic Force Microscopy (AFM). Molecular simulations support a micelle-like arrangement of calixarenes with the alkyl chains directed inwards, and they form a composite material with a fluorescent molecule.     

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 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.     

Highly linear self-assembled porphyrin wires

An efficient noncovalent assembly process involving high geometrical control was applied to a linear bis(imidazolyl zinc porphyrin) 7Zn, bearing C(18) substitutents, to generate linear multiporphyrin wires. The association process is based on imidazole recognition within the cavity of the phenanthroline-strapped zinc porphyrin. In chlorinated solvents, discrete soluble oligomers were obtained after (7Zn)(n) was end-capped with a terminal single imidazolyl zinc porphyrin derivative 4Zn. These soluble species, as well as their destabilization in the presence of protic solvents, were studied by UV-visible and time-resolved luminescence. In the solid state, assemblies as long as 480 nm, which corresponds to 190 iterative units or a total of 380 porphyrins, were observed by atomic force microscopy measurements on mica. The length and linearity of the porphyrin wires obtained illustrate the potential of phenanthroline-strapped porphyrins for the directional control of self-assembly processes.     

A Supramolecular Polymer Network of Graphene Quantum Dots

Graphene quantum dot (GQD)–organic hybrid compounds (GQD‐ 2 b – e ) were prepared by introducing 3,4,5‐tri(hexadecyloxy)benzyl groups (C16) and linear chains terminated with a 2‐ureido‐4‐[1H]‐pyrimidinone (UPy) moiety onto the periphery of GQD‐ 1 . GQD‐ 2 b – e formed supramolecular assemblies through hydrogen bonding between the UPy units. GPC analysis showed that GQDs with high loadings of the UPy group formed larger assemblies, and this trend was confirmed by DOSY and viscosity measurements. AFM images showed the polymeric network structures of GQD‐ 2 e on mica with flat structures (ca. 1.1 nm in height), but no such structures were observed in GQD‐ 2 a , which only carries the C16 group. GQD‐ 2 c and GQD‐ 2 d formed organogels in n‐decanol, and the gelation properties can be altered by replacing the alkyl chains in the UPy group with ethylene glycol chains (GQD‐ 3 ). GQD can thus be used as a platform for supramolecular polymers and organogelators by suitable chemical functionalization.     

Supramolecular assemblies of DNA with neutral nucleoside amphiphiles

A neutral uridine-based amphiphile is described which condenses plasmid DNA. AFM studies show that the three distinct structural components of the amphiphile (i.e, nucleobase, alkyl chains, and poly(ethylene glycol)) are required for the formation of DNA-amphiphile supramolecular assemblies on a mica surface.     

Nanoscale morphology of melt crystallized polyethylene/graphite (HOPG) interphase

High density polyethylene (PE) was crystallised from the melt on freshly cleaved surface of highly oriented pyrolitic graphite (HOPG) or mica. Atomic force microscopy (AFM) studies of structure of the polymer surface adjacent to the graphite or mica were performed after peeling of from the substrate. Significant differences of crystalline structure on the interface were found between PE crystallised on graphite and mica. The surface of polyethylene crystallised on graphite shows large areas with regularly arranged rectangular structures. These objects (ca 20‐80 nm big) probably represent the nucleation centres of the lamellar growth. The surface of polyethylene crystallised at mica surface shows some dot‐like structures showing no particular arrangement.     

Self-assembled monolayers based on phenanthroline-gold(111) bonding

The self-assembly of long-alkyl-chain substituted phenanthroline derivatives on highly oriented pyrolitic graphite (HOPG) and gold(111) is compared. Whereas the adsorption on HOPG is controlled by the affinity of alkyl chains for the substrate, which leads to flat-lying adsorbed molecules, alignments of upright-oriented molecules are formed on gold(111). This situation is explained by the bonding of chelating species with gold(111) surfaces and by the pi-stacking interaction between conjugated moieties. This intermediate situation between strong thiol-like chemical bonding and the weak n-alkane-like physical adsorption opens the route toward laterally organized functional molecular assemblies.     

Ordered arrays of organometallic iridium complexes with long alkyl chains on graphite

Iridium(III) fac-tris(2-phenylpyridine) fac-[Ir(ppy)3] complexes equipped with long alkyl chains were prepared to examine their capability to form organized arrays on the surface of highly oriented pyrolytic graphite (HOPG). The molecules form lamellar arrays at the 1-phenyloctane/HOPG interface. From the analysis of the STM images, it was concluded that the molecules align with alkyl chains being interdigitated. Similar lamellar arrays were also obtained at the air/HOPG interface upon drop-casting of toluene solutions. The lamellar structure at the molecular level leads to rectangular two-dimensional crystalline domains a few hundred nanometers long (nanoslips). Infrared external reflection spectroscopy suggested that the adsorbed alkyl chains adopt the trans-zigzag conformation in the nanoslip, although the orientations of the zigzag plane of the alkyl groups are mixed. Cyclic voltammetry indicates fast electron transfer between the adsorbed molecules and the substrate and significant intermolecular electronic interactions. It was found that annealing at high temperatures is an effective method to prepare ordered assemblies more than a few micrometer scale (microslips). The orientations of the nanoslips prepared from the racemic mixture exhibited an apparent 12-fold symmetry, while its optically active enantiomer resulted in more irregular domains with a six-fold symmetry, implying an important role of chirality on packing at the molecular level and on the orientation of the domains at larger scales. When drop-cast from more concentrated solutions than a few hundreds of micromolar, multilayers were obtained, in which the alkyl chains in the molecules are more or less perpendicular to the surface. This structure can be transformed into the nanoslips upon standing.     

Perfectly straight nanowires of fullerenes bearing long alkyl chains on graphite

Fullerene derivatives bearing long alkyl chains epitaxially adsorb on the basal plane of graphite forming well-ordered one-dimensional lamellae. Within the lamellae, the C60 moieties are organized in a zigzag-type fashion. The ordering is mainly governed by the substitution pattern and chain length of the alkyl chains. The electrochemical activity of the C60 groups is fully maintained in the surface-confined assemblies.     

AFM imaging of adsorbed Nafion polymer on mica and graphite at molecular level

Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.     

Surface supramolecular organization of a terbium(III) double-decker complex on graphite and its single molecule magnet behavior

The two-dimensional self-assembly of a terbium(III) double-decker phthalocyanine on highly oriented pyrolitic graphite (HOPG) was studied by atomic force microscopy (AFM), and it was shown that it forms highly regular rectangular two-dimensional nanocrystals on the surface, that are aligned with the graphite symmetry axes, in which the molecules are organized in a rectangular lattice as shown by scanning tunneling microscopy. Molecular dynamics simulations were run in order to model the behavior of a collection of the double-decker complexes on HOPG. The results were in excellent agreement with the experiment, showing that-after diffusion on the graphite surface-the molecules self-assemble into nanoscopic islands which align preferentially along the three main graphite axes. These low dimension assemblies of independent magnetic centers are only one molecule thick (as shown by AFM) and are therefore very interesting nanoscopic magnetic objects, in which all of the molecules are in interaction with the graphite substrate and might therefore be affected by it. The magnetic properties of these self-assembled bar-shaped islands on HOPG were studied by X-ray magnetic circular dichroism, confirming that the compounds maintain their properties as single-molecule magnets when they are in close interaction with the graphite surface.     

Electrochemical SPM investigation of the solid electrolyte interphase film formed on HOPG electrodes

Solid electrolyte interphase (SEI) film formation on graphite electrodes was studied on highly oriented pyrolytic graphite (HOPG) in nonaqueous electrolyte by in situ electrochemical atomic force microscopy (AFM). For potentials negative to 0.7 V versus Li|Li⁺ a SEI film is formed on the HOPG electrode surface. After the first cycle the film is rough and covers the surface of the HOPG electrode only partially. After the second cycle the HOPG surface is fully covered by a compact film. The thickness of the SEI film was measured by increasing the pressure of the AFM tip and thus scraping a part of the electrode surface. In this way a thickness of about 25 nm was found for the SEI film formed after two scan cycles between 3 and 0.01 V versus Li|Li⁺.     

Chemical and constitutional influences in the self-assembly of functional supramolecular hydrogen-bonded nanoscopic fibres

A new series of secondary amides bearing long alkyl chains with pi-electron-donor cores has been synthesized and characterised, and their self-assembly upon casting at surfaces has been studied. The different supramolecular assemblies of the materials have been visualized by using atomic force microscopy (AFM) and transmission electron microscopy (TEM). It is possible to obtain well-defined fibres of these aromatic core molecules as a result of the hydrogen bonds between the amide groups. Indeed, by altering the alkyl-chain lengths, constitutions, concentrations and solvent, it is possible to form different rodlike aggregates on graphite. Aggregate sizes with a lower limit of 6-8 nm width have been reached for different amide derivatives, while others show larger aggregates with rodlike morphologies which are several micrometers in length. For one compound that forms nanofibres, doping was performed by using a chemical oxidant, and the resulting layer on graphite was shown to exhibit metallic-like spectroscopy curves when probed with current-sensing AFM. This technique also revealed current maps of the surface of the molecular material. Fibre formation not only takes place on the graphite surface: nanometre scale rods have been imaged by using TEM on a grid after evaporation of solutions of the compounds in chloroform. Molecular modelling proves the importance of the hydrogen bonds in the generation of the fibres, and indicates that the constitution of the molecules is vital for the formation of the desired columnar stacks, results that are consistent with the images obtained by microscopic techniques. The results show the power of noncovalent bonds in self-assembly processes that can lead to electrically conducting nanoscale supramolecular wires.     

Self-assembly of [Et,Et]-bacteriochlorophyll cF on highly oriented pyrolytic graphite revealed by scanning tunneling microscopy

The chlorosomal light-harvesting antennae of green phototrophic bacteria consist of large supramolecular aggregates of bacteriochlorophyll c (BChl c). The supramolecular structure of (3(1)-R/S)-BChl c on highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) has been investigated by scanning tunneling microscopy (STM). On MoS2, we observed single BChl c molecules, dimers or tetramers, depending on the polarity of the solvent. On HOPG, we observed extensive self-assembly of the dimers and tetramers. We propose C=O...H-O...Mg bonding networks for the observed dimer chains, in agreement with former ultraviolet-visible and infrared spectroscopic work. The BChl c moieties in the tetramers are probably linked by four C=O...H-O hydrogen bonds to form a circle and further stabilized by Mg...O-H bondings to underlying BChl c layers. The tetramers form highly ordered, distinct chains and extended two-dimensional networks. We investigated semisynthetic chlorins for comparison by STM but observed that only BChl c self-assembles to well-structured large aggregates on HOPG. The results on the synthetic chlorins support our structure proposition.     

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.     

Graphene功能分子在HOPG上自组装结构的密度泛函研究

Graphene自组装超分子结构,是指FTBC-Cn(n=4,6,8,12)分子有序自组装在高定向裂解石墨(HOPG)上形成的自组装超分子结构,是一种新型二维固体表面材料.其中FTBC-Cn分子是由三角形扶手椅型graphene每边添加2条含有n(n=4,6,8,12)个C原子的烷基链所形成具有曲面结构的一种分子.采用...     

Meso‐Substituted Liquid Porphyrins

Meso‐substituted A₄‐porphyrins bearing 3,4,5‐trialkoxyphenyl substituents are efficiently synthesized and characterized. Porphyrins bearing twelve C10 and C11 alkyl chains turned out to be liquid at room temperature. The remaining porphyrins, bearing C8, C9, C12, and C18 alkyl chains, have low melting points and high solubility in nonpolar solvents. Their differential scanning calorimetry distinctly shows, in most cases, only one phase transition.