Liquid Graphite Interface Research Articles

Heptagonal Molecular Tiling via Self-Assembly of Heptagonal Phenylene-Ethynylene Macrocycle at the Liquid-Solid Interface.

The molecular-level scrutinization of on-surface tiling garners considerable interest among scientists. Herein, we demonstrate molecular-level heptagonal tiling using the self-assembly of a heptagonal meta-phenylene-ethynylene macrocycle featuring 14 long alkoxy substituents at the liquid-graphite interface using scanning tunneling microscopy. This heptagonal macrocycle produces an antiparallel pattern at the 1-phenyloctane-graphite interface through van der Waals interactions between the alkoxy chains. This pattern resembles the densely packed pattern of heptagonal tiles, albeit with variations in the orientations and spacing of heptagonal cores owing to intermolecular interactions between the alkoxy chains. Conversely, at the 1,2,4-trichlorobenzene-graphite interface, the heptagonal molecule forms an oblique pattern composed of four independent molecular orientations. This phenomenon arises from core distortion induced by the coadsorption of the solvent molecules within the intrinsic macrocyclic pores. This study elucidates the design strategy - specifically, the choice of heptagonal molecular building block - for heptagonal tiling and fills a crucial gap in the field of two-dimensional crystal engineering.

The polymorphism of porphyrin 2D assemblies at the liquid-graphite interface: the effect of a polar solvent additive and a flexible spacer on the face-on and edge-on type molecular arrangements.

Self-assembly structures of 5,10,15,20-tetrakis(4-substituted phenyl)porphyrins at the liquid-graphite interface were investigated by scanning tunneling microscopy. We found that the presence of a small amount of a polar solvent, i.e., only 0.5 vol% of octanoic acid in phenyloctane, significantly affected the selective formation of the face-on polymorph over the edge-on one likely due to the solvent-molecule interactions at the 2D interface.

Halogenated building blocks for 2D crystal engineering on solid surfaces: lessons from hydrogen bonding.

Halogen bonding has emerged as a promising tool in two-dimensional (2D) crystal engineering. Since halogen bonds are similar to hydrogen bonds in a number of aspects, the existing knowledge of hydrogen bonded systems can be applied to halogenated systems. Here we evaluate the applicability of a retrosynthetic approach based on topological similarity between hydrogen and halogen bonds to obtain predictable halogen bonded networks. The self-assembly of 1,3-dibromo-5-alkoxybenzene derivatives was studied in analogy with well-explored alkoxy isophthalic acids using a combination of experimental and theoretical tools. Scanning tunneling microscopy (STM) characterization of the networks formed at the liquid-graphite interface revealed that while the retrosynthetic approach works at the level of small clusters of molecules within the 2D network, the overall structure of the network deviates from the anticipated structure. The monolayers consist of fractured rows of halogen-bonded modules instead of the expected continuous lamellar structure. Each module consists of a discrete number of halogen-bonded molecules. The interactions responsible for the stabilization of halogen bonded dimers are delineated through detailed density functional theory (DFT) calculations coupled with natural bonding orbitals (NBO) and perturbation analysis. A modified force field that includes an extra charged site to imitate the σ hole on the halogen atom was developed and applied to extract total potential energies of the anticipated and observed networks. Plausible reasons for the deviation from the anticipated structure are discussed. Finally, a modified molecular design that allows successful application of the hydrogen bond-halogen bond analogy was tested experimentally.

Discrimination between Conglomerates and Pseudoracemates Using Surface Coverage Plots in 2D Self-Assemblies at the Liquid-Graphite Interface.

The two-dimensional (2D) molecular ordering of two photochromic diarylethenes was investigated using scanning tunneling microscopy at the liquid-graphite interface. The racemic mixture of the closed-ring isomer of one of the diarylethenes showed spontaneous separation of its enantiomers upon 2D crystallization, whereas that of the other diarylethene formed a pseudoracemic crystal in which two enantiomers coexist in a 2D ordering domain. The mixing of enantiomers in 2D assemblies can be analyzed by the dependence of the surface coverage on the concentration of enantiomers at different enantiomeric ratios.

Discrimination between Conglomerates and Pseudoracemates Using Surface Coverage Plots in 2D Self‐Assemblies at the Liquid–Graphite Interface

AbstractThe two‐dimensional (2D) molecular ordering of two photochromic diarylethenes was investigated using scanning tunneling microscopy at the liquid–graphite interface. The racemic mixture of the closed‐ring isomer of one of the diarylethenes showed spontaneous separation of its enantiomers upon 2D crystallization, whereas that of the other diarylethene formed a pseudoracemic crystal in which two enantiomers coexist in a 2D ordering domain. The mixing of enantiomers in 2D assemblies can be analyzed by the dependence of the surface coverage on the concentration of enantiomers at different enantiomeric ratios.

Two-dimensional crystals from reduced symmetry analogues of trimesic acid.

The two-dimensional assembly of multicarboxylated arenes is explored at the liquid-graphite interface using scanning tunneling microscopy. Symmetry variations were introduced via phenylene spacer addition and the influence of these perturbations on the formation of hydrogen-bonded motifs from an alkanoic acid solvent is observed. This work demonstrates the importance of symmetry in 2D crystal formation and draws possible links of this behavior to prediction of coordination modes in three-dimensional coordination polymers.

Bias voltage dependence of molecular orientation of dialkyl ketone and fatty acid alkyl ester at the liquid–graphite interface

Molecular orientations of self-assembled 18-pentatriacontanone (as ketone) and stearyl stearate (as ester) monolayers adsorbed on a graphite surface were studied by scanning tunneling microscopy (STM) at the liquid–solid interface. At a positive sample bias, the central areas of the dialkyl ketone and fatty acid alkyl ester molecules in the STM images appeared as two bright regions on both sides of a dim spot and a bright region on one side of a dim spot, whereas at a negative sample bias, the areas appeared dim. This contrast variation indicates that a perpendicular carbon skeleton-plane orientation with the CO pointing down on the surface is favorable for a substrate with positive charge and vice versa because of the greater electronegativity of the oxygen atom. Upon the bias voltage reversal, the delay time for the STM image contrast change in the region was observed on a time scale of minutes. The difference between the delay time lengths for the direction of bias polarity change indicates that the perpendicular configuration with CO pointing up is more stable than that with CO pointing down. These results indicate that the use of an electric field along a direction vertical to the monolayer on the substrate provides control over the orientations of the molecules between two stable states at the liquid–solid interface.

Cooperative rotation of alkylated sulfide molecules at the liquid-graphite interface.

Self-assembled monolayers of hexadecyl sulfide adsorbed on a graphite surface were studied using scanning tunneling microscopy (STM). STM images of the molecule indicate a bright spot and two relatively dim thin bands that correspond to the sulfur atom positioned at the center of the molecule and two alkyl chains that extend linearly from the sulfur atom, respectively. The contrast of the bands changes reversibly between the zigzag and aligned bright spots, which correspond to the zigzag methylene units of the alkyl chains; accordingly, the methylene units alternate between parallel and perpendicular orientations to the surface, respectively, on a time scale of minutes. The variation in contrast indicates cooperative rotation along the long axis of the molecules in the monolayer at the liquid-graphite interface. The reversibility of the contrast suggests that the solvent influences the difference in the free energies of the parallel and perpendicular configurations of the molecules with respect to the graphite surface.

3-Dimensional atomic scale structure of the ionic liquid-graphite interface elucidated by AM-AFM and quantum chemical simulations.

In situ amplitude modulated atomic force microscopy (AM-AFM) and quantum chemical simulations are used to resolve the structure of the highly ordered pyrolytic graphite (HOPG)-bulk propylammonium nitrate (PAN) interface with resolution comparable with that achieved for frozen ionic liquid (IL) monolayers using STM. This is the first time that (a) molecular resolution images of bulk IL-solid interfaces have been achieved, (b) the lateral structure of the IL graphite interface has been imaged for any IL, (c) AM-AFM has elucidated molecular level structure immersed in a viscous liquid and (d) it has been demonstrated that the IL structure at solid surfaces is a consequence of both thermodynamic and kinetic effects. The lateral structure of the PAN-graphite interface is highly ordered and consists of remarkably well-defined domains of a rhomboidal superstructure composed of propylammonium cations preferentially aligned along two of the three directions in the underlying graphite lattice. The nanostructure is primarily determined by the cation. Van der Waals interactions between the propylammonium chains and the surface mean that the cation is enriched in the surface layer, and is much less mobile than the anion. The presence of a heterogeneous lateral structure at an ionic liquid-solid interface has wide ranging ramifications for ionic liquid applications, including lubrication, capacitive charge storage and electrodeposition.

Alkyl Chain Length Defines 2D Architecture of Salophen Complexes on Liquid–Graphite Interface

AbstractA series of CuII, NiII and CoII salophen [salophen = N,N′‐(o‐phenylene)bis(salicylideneimine)] complexes were prepared, and their two‐dimensional (2D) assemblies on a liquid–graphite interface were studied by scanning tunnelling microscopy (STM). Depending on their varying alkyl chain lengths (C8, C10 or C12), two different new adlayers, parallelogram and honeycomb, were recorded. According to STM data, the driving force for these assemblies is a subtle interplay between a maximum amount of attractive van der Waals interaction of the alkyl chains and weak hydrogen bonding of the phenoxy moieties. Thus, the alkyl chain length defines the favoured 2D structure for each metal complex. As shown here, the self‐assembled monolayers of the salophen complexes studied provide an attractive approach to varying the surface architecture.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Bias-dependent visualization of electron donor (D) and electron acceptor (A) moieties in a chiral DAD triad molecule.

The 2D crystal lattice structure and bias-dependent contrast of a chiral electron donor-acceptor-donor triad system, composed of two oligo(p-phenylene vinylene) electron donors and a perylenediimide electron acceptor (OPV4-PDI-OPV4), have been studied by means of scanning tunneling microscopy (STM) at the liquid-graphite interface. OPV4-PDI-OPV4 is ordered in rows and forms a well-ordered 2D crystal lattice structure. The electrical properties of the donor and acceptor parts are distinguished by the contrast in bias-dependent STM imaging.

Mixing behaviour in 2D layers of linear alkanes adsorbed on graphite

Solid monolayers of binary mixtures of linear alkanes adsorbed on graphite are investigated at 10 K by X-ray diffraction. Monolayers of n-heptane and n-nonane, which belong to the same 2D space group, mix continuously over the whole range of composition. In contrast, n-nonane and n-decane belonging to different space groups have only limited miscibility. Interestingly, the isomorphous monolayers of n-octane and n-decane form a molecular compound. Such variety of the mixing behaviour is substantially different from that observed for the solid monolayers formed at the liquid–graphite interface.

Scanning tunneling microscopy: a unique tool in the study of chirality, dynamics, and reactivity in physisorbed organic monolayers.

Scanning tunneling microscopy (STM) is applied to study organic monolayers, physisorbed at the liquid-graphite interface. Due to the very local nature of the probing, the structure of these adlayers has been imaged with very high detail. The high resolution allowed us to investigate the effect of molecular chirality on the monolayer formation and provided a unique way to study chemical reactions at the liquid-graphite interface. Making use of a fast scanning mode, dynamic processes in these adlayers have been visualized.

STM imaging of a heptanuclear ruthenium(II) dendrimer, mono-add layer on graphite

Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.

Observation of highly ordered, two-dimensional n-alkane and n-alkanol structures on graphite

We have directly imaged n-alkane and n-alkanol layers at the liquid–graphite interface using a scanning tunneling microscope (STM). The layers possessed a high degree of two-dimensional ordering, and the adsorbate was observed to enhance the tunneling current. The results of this study agree with calorimetric and surface mass measurements, and show that these macroscopic measurements can aid in the selection of systems suitable for imaging with the STM.