Formation of Ordered Coronene Clusters in Template Utilizing the Structural Transformation of Hexaphenylbenzene Derivative Networks on Graphite Surface

Study on 2D Molecular Networks of Flexible Pentacarboxylic Acid Ligands Induced by Ether Bonds in Response to Selective Guest Inclusion

The flexibility of ligands allows for their bending, twisting, or rotation to adopt various conformations, leading to distinct symmetries during the self-assembled process. Flexible aromatic acid ligands modified by ether bonds are a promising type of self-assembled module when it comes to surfaces. Here, two pentacarboxylic acid ligands (H5L1 and H5L2) with minor skeleton differences have successfully self-assembled into disparate porous networks on the graphite surface and demonstrated excellent potential for the inclusion of guest molecules. The H5L1 molecule's network structure only accommodates coronene (COR) molecules. With fewer COR molecules, H5L1 molecules act as a host template to accommodate the COR molecules. When there are too many COR molecules, COR molecules will induce H5L1 molecules to transform into a new host-guest nanostructure. Additionally, H5L2 molecules showed the ability to capture C70 molecules and exhibited cavity selectivity. However, the assembled network of H5L2 was slightly deformed in attempts to trap the COR molecules. To understand these phenomena more deeply, various assembled mechanisms were analyzed in combination with building theoretical models and energy analysis. These results reveal the great potential of flexible aromatic acid ligands in two-dimensional self-assembly and host-guest systems for their application in related fields.

Celebrating 20 Years of NCNST: Innovation in Nanoscience and Nanotechnology

With the development of nanoscience and technology, it has become an essential part of various research directions, changing our way of life, such as advanced accurate manufacturing in nanotechnology that facilitates reducing chip sizes, progress made in health care via nanoscience that provides hope to patients, and so on. As the nation's flagship institution of nanoscience and technology in China, the National Center for Nanoscience and Technology, China (NCNST), established in December 2003, has played a crucial role in promoting cutting-edge technologies in the field of nanoscience and expediting interdisciplinary fusion. With a strong research team and state-of-the-art research equipment, NCNST currently carries out frontier research and world-class technology innovation, including nanosystems and hierarchical fabrication, biological effects of nanomaterials and nanosafety, standardization and measurements for nanotechnology, as well as theoretical simulations. Serving as one of the most prestigious institutions in nanoscience and nanotechnology in China, NCNST will continue to foster impactful international cooperation, cultivate young talents, and boost inspiring innovation.

Surface supramolecular assemblies tailored by chemical/physical and synergistic stimuli: a scanning tunneling microscopy study

Supramolecular self-assemblies formed by various non-covalent interactions can produce diverse functional networks on solid surfaces. These networks have recently attracted much interest from both fundamental and application points of view. Unlike covalent organic frameworks (COFs), the properties of the assemblies differ from each other depending on the constituent motifs. These various motifs may find diverse applications such as in crystal engineering, surface modification, and molecular electronics. Significantly, these interactions between/among the molecular tectonics are relatively weak and reversible, which makes them responsive to external stimuli. Moreover, for a liquid-solid-interface environment, the dynamic processes are amenable to in situ observation using scanning tunneling microscopy (STM). In the literature, most review articles focus on supramolecular self-assembly interactions. This review summarizes the recent literature in which stimulation sources, including chemical, physical, and their combined stimuli, cooperatively tailor supramolecular assemblies on surfaces. The appropriate design and synthesis of functional molecules that can be integrated on different surfaces permits the use of nanostructured materials and devices for bottom-up nanotechnology. Finally, we discuss synergic effect on materials science.

Coronene guest molecule selectivity in host templates formed by hydrogen bonding and van der Waals forces at liquid/solid interfaces

COR guest molecules are selectively adsorbed in the hexagonal cavities in both H6PDB and H6PAB self-assembled systems and preferentially adsorbed in the H6PAB host template in the H6PAB/HPB system.

Bidirectional Phase Transformation of Supramolecular Networks Using Two Molecular Signals

Reversible control of molecular self-assembly is omnipresent in adaptive biological systems, yet its realization in artificial systems remains a major challenge. Using scanning tunneling microscopy and density functional theory calculations, we show that a 2D supramolecular network formed by terthienobenzenetricarboxylic acid (TTBTA) can undergo a reversible structural transition between a porous and dense phase in response to different molecular signals (trimethyltripyrazolotriazine (TMTPT) and C₆₀). TMTPT molecules can induce a phase transition from the TTBTA honeycomb to the dense phase, whereas a reverse transition can be triggered by introducing C₆₀ molecules. This response stems from the selective association between signal molecules and TTBTA polymorphs. The successful realization of reversible molecular transformation represents important progress in controlling supramolecular surface nanostructures and could be potentially applicable in various areas of nanotechnology, including phase control, molecular sensing, and "smart" switchable surfaces.

Two-Dimensional Molecular Network Built from Hierarchy Self-Assembly of Perylene Bisimide Derivatives

The structures of two-dimensional (2D) self-assembled molecular networks formed by a series of perylene bisimide (PBI) derivatives were investigated by scanning tunneling microscopy (STM). By introducing different functional groups to the PBI rings, we successfully built a different self-assembled molecular network on the liquid-solid interface. When the substituent is propanol, PBI is aligned in lines. When we introduced either an ester group or an amide group to the PBI compounds, they tended to form dimers and trimers. Especially, the PBI with the amide groups can form a 2D porous molecular network by hierarchy self-assembly. The 2D porous molecular network has a great potential to be the host molecule for the accommodation of a guest molecule, coronene (COR), and the structure of the 2D porous molecular network can be tuned by varying the concentration. The density functional theory calculations were also performed to disclose the mechanisms involved.

Hexaarylbenzene based high-performance p-channel molecules for electronic applications

Hexaarylbenzene-based molecules find potential applications in organic electronics due to wider energy gap, high HOMO level, higher photoconductivity, electron-rich nature, and high hole-transporting property. Due to the unique propeller structure, these molecules show low susceptibility towards self-aggregation. This property can be tailored by proper molecular engineering by the incorporation of appropriate groups. Therefore, hexaarylbenzene chromophores are widely used as the materials for high-efficiency light-emitting materials, charge transport materials, host materials, redox materials, photochemical switches, and molecular receptors. This review highlights the diverse structural modification techniques used for the synthesis of symmetrical and unsymmetrical structures. Also, the potential applications of these molecules in organic light-emitting diodes, organic field-effect transistors, organic photovoltaics, organic memory devices, and logic circuits are discussed.

Controlled Construction of an Exquisite Three-Component Co-assembly Supramolecular Structure at the Liquid-Solid Interface

A three-component supramolecular co-assembly structure formed at the liquid-solid interface by employing a shape-persistent π-conjugated macrocycle (1_6mer) and two guest molecules (COR and C₆₀) is demonstrated. Scanning tunneling microscopy (STM) observations revealed that 1_6mer can serve as a versatile host molecule that can co-assemble with both COR and C₆₀ guest molecules to form stable two-component structures, where the COR guest molecule filled in the gap between the side chains of adjacent 1_6mer molecules, and the C₆₀ guest molecule entered the inner cavity of 1_6mer. It was found that the adding sequence of COR and C₆₀ guest molecules is crucial to the resulting co-adsorption structure in the three-component system. To obtain the intriguing 1_6mer-COR-C₆₀ three-component co-assembly structure, the 1_6mer and COR two-component co-assembly structure should first be constructed on a HOPG surface, followed by addition of C₆₀. Based on the analysis of the STM results and the density functional theory (DFT) calculations, the formation mechanism of the assembled structures was revealed.

Tilting in coronene layers on Au(111)

Control of molecule adsorption and ordering on metal surfaces is of critical importance for the design and fabrication of molecule-based functional materials. In the present work, the molecule layer structures of coronene on Au(111) and HOPG are studied by combining scanning tunneling microscopy with image analysis techniques to unravel small changes in molecule adsorption geometry. Coronene forms a densely packed layer on Au(111) and HOPG at room temperature, but does not preferentially decorate the herringbone reconstruction. The molecule layer structure is confirmed by histograms of molecule radius and apparent height obtained from STM images using a python based open source code. Annealing at 116 °C initiates a tilting of coronene molecules on Au(111) by about 11 ± 4° which is deduced from statistical image analysis. The structural analysis is combined with an assessment of apparent height modulation with bias voltage to ascertain the reliability of the statistical analysis. Our work illustrates that the combination of advanced image analysis processing and STM images allows one to extract even small changes in a molecule layer structure.

Dynamic host-guest behavior in halogen-bonded two-dimensional molecular networks investigated by scanning tunneling microscopy at the solid/liquid interface

The fabrication of supramolecularly engineered two-dimensional (2D) networks using simple molecular building blocks is an effective means for studying host-guest chemistry at surfaces toward the potential application of such systems in nanoelectronics and molecular devices. In this study, halogen-bonded molecular networks were constructed by the combination of linear halogen-bond donor and acceptor ligands, and their 2D structures at the highly oriented pyrolytic graphite/1-phenyloctane interface were studied by scanning tunneling microscopy. The bi-component blend of the molecular building blocks possessing tetradecyloxy chains formed a lozenge structure via halogen bonding. Upon the introduction of an appropriate guest molecule (e.g., coronene) into the system, the 2D structure transformed into a hexagonal array, and the central pore of this array was occupied by the guest molecules. Remarkably, the halogen bonding of the original structure was maintained after the introduction of the guest molecule. Thus, the halogen-bonded molecular networks are applicable for assembling guest species on the substrate without the requirement of the conventional rigid molecular building blocks with C 3 symmetry.

Dynamic surface-assisted assembly behaviours mediated by external stimuli

Supramolecular self-assembly behaviors on solid substrates have been widely investigated in the last few decades. Owing to the complexity of interfacial assembly systems, the precise regulation of supramolecular nanostructures is still challenging and waits to be solved. The supramolecular nanostructures are governed by non-covalent bonds, so they can be disrupted and influenced by an external environment. In this review, the dynamic supramolecular nanostructures that are mediated by external stimuli containing guest species, light irradiation, temperature and electric field are discussed in detail. The research studies mentioned in this article are all accomplished by STM, and the effects of these external stimuli on the assembled nanostructures have been elucidated exhaustively here.

On-Surface Crystallization Behaviors of H-Bond Donor-Acceptor Complexes at Liquid/Solid Interfaces

Two-dimensional (2D) crystallization behaviors of A-TPC _n ( n = 4, 6, 10), T3C₄, and hydrogen-bonded complexes T3C₄@TPC _n ( n = 4, 6, 10) are investigated by means of scanning tunneling microscope (STM) observations and density functional theory (DFT) calculations. The STM observations reveal that A-TPC₄, A-TPC₁₀, and T3C₄ self-organize into dumbbell-shaped structures, well-ordered bright arrays, and zigzag structures, respectively. Interestingly, T3C₄@TPC₁₀ fails to form the cage-ball structure, whereas T3C₄@TPC₄ and T3C₄@TPC₆ co-assemble into cage-ball structures with the same lattice parameters. The filling rates of the balls of these two kinds of cage-ball structures depend heavily on the deposition sequence. As a result, the filling rates of the cages in T3C₄/A-TPC _n ( n = 4, 6) with deposition of T3C₄ anterior to A-TPC _n are higher than those in A-TPC _n/T3C₄ ( n = 4, 6) with the opposite deposition sequence. Furthermore, lattice defects formed by T3C₄ coexist with the cage-ball structures. Moreover, the similar energy per unit area of lattice defects (-0.101 kcal mol^-1 Å^-2) and the two cage-ball networks (-0.194 and -0.208 kcal mol^-1 Å^-2, respectively), illustrating the similar stabilities of lattice defects and cage-ball networks, demonstrates the rationality of lattice defects. Combining STM investigations and DFT calculations, this work could provide a useful approach to investigate the 2D crystallization mechanisms of supramolecular liquid crystals on surfaces.

Assembly structures and electronic properties of truxene-porphyrin compounds studied by STM/STS

The self-assembly of functional molecules into uniform nanostructures with innovational properties has attracted extensive research interest. In the present work, the assembly structures and electronic properties of a novel type of truxene derivative, e.g. truxene-porphyrin derivatives, were studied, for the first time, on a highly oriented pyrolytic graphite (HOPG) surface. Scanning tunneling microscopy (STM) images revealed that the truxene-porphyrin compounds could be parallelly arranged into long-ranged lamellar patterns. Density functional theory (DFT) calculations helped explain the assembly mechanisms further. Besides, order distribution of the smaller compound 1T1P in the 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) host network was achieved, which is a reflection of the dimensional effect in the host-guest assembly. Furthermore, together with theoretical analyses, scanning tunneling spectroscopy (STS) measurements were conducted to investigate the electronic properties of truxene-porphyrin compounds. Results showed that the metalation of the porphyrin units could have a significant effect on the band gap and the position of the gap center. The study enhances our understanding of the assembly mechanism of truxene derivatives at the molecular level and paves the way towards fabricating truxene-based functional nanodevices.

Hierarchical Hybrid Nanostructures Constructed by Fullerene and Molecular Tweezer

For the construction of well-defined hierarchical superstructures of pristine [60]fullerene (C₆₀) arrays, pyrene-based molecular tweezers (PT) were used as host molecules for catching and arranging C₆₀ guest molecules. The formation of host-guest complexes was systematically studied in solution as well as in the solid state. Two-dimensional proton nuclear magnetic resonance spectroscopic studies revealed that PT-host and C₆₀-guest complexes were closely related to the molecular self-assembly of PT. Ultraviolet and fluorescence spectroscopic titrations indicated the formation of stable 1:1 and 2:1 (PT/C₆₀) complexes. From the nonlinear curve-fitting analysis, equilibrium constants for the 1:1 (log K₁) and 2:1 (log K₂) complexes were estimated to be 4.96 and 5.01, respectively. X-ray diffraction results combined with transmission electron microscopy observations clearly exhibited the construction of well-defined layered superstructures of the PT-host and C₆₀-guest complexes. From electron mobility measurements, it was demonstrated that the well-defined hierarchical hybrid nanostructure incorporating a C₆₀ array exhibited a high electron mobility of 1.7 × 10^-2 cm² V^-1 s^-1. This study can provide a guideline for the hierarchical hybrid nanostructures of host-guest complex and its applications.

Dependence of the photo-response behavior of self-assembled 2D Azo-derivatives on the functional groups on a solid surface

Here, by means of scanning tunneling microscopy, we found that 2D self-assembled monolayers of four azobenzene derivatives exhibited different isomerization behaviors when taken from dark to irradiation conditions.

Peptide Self-Assembly and Its Modulation: Imaging on the Nanoscale

This chapter intends to review the progress in obtaining site-specific structural information for peptide assemblies using scanning tunneling microscopy. The effects on assembly propensity due to mutations and modifications in peptide sequences, small organic molecules and conformational transitions of peptides are identified. The obtained structural insights into the sequence-dependent assembly propensity could inspire rational design of peptide architectures at the molecular level.

Programmed Hierarchical Hybrid Nanostructures from Fullerene-Dendrons and Pyrene-Dendrons

The construction of fullerene (C₆₀ ) hierarchical nanostructures with the help of amphiphilic molecules remains a challenging task in nanoscience and nanotechnology. Utilizing the host-guest complex concept, sub-10 nm layered superstructures are constructed from a monofunctionalized C₆₀ dendron (C₆₀ D, guest) and tweezer-like pyrene dendron (PD, host). Since C₆₀ D and PD are asymmetric shape amphiphiles having liquid crystal (LC) dendrons, both C₆₀ D and PD construct head-to-head bilayer superstructures by themselves. From fluorescence titration experiments, it is realized that the host-guest complex shows 1:1 stoichiometric binding with a binding constant (K_sv = 2.45 × 10⁵ m^-1 ). Based on the morphological observations and scattering analyses, it is found that buckle-like asymmetric building blocks (C₆₀ D·PD) are self-assembled by the host-guest complex and construct multilayer hybrid nanostructures. The hierarchical hybrid nanostructures consist of the self-assembled C₆₀ D·PD bilayer with a 2D C₆₀ ·P nanoarray sandwiched between LC dendrons. This advanced strategy is expected to be a practicable and rational guideline for the fabrication of programmed hierarchical hybrid nanostructures.

Nanotribological Study of Supramolecular Template Networks Induced by Hydrogen Bonds and van der Waals Forces

Nanotribology has been given increasing attention by researchers in pursuing the nature of friction. In the present work, an approach that combines the supramolecular assembly and nanotribology is introduced. Herein, the nanotribological study was carried out on seven supramolecular template networks [namely, hydrogen bond induced tricarboxylic acids and van der Waals force induced hexaphenylbenzene (HPB) derivatives]. The template networks, as well as the host-guest assemblies of template molecules induced by different forces, were constructed on the highly oriented pyrolytic graphite (HOPG) surface and explicitly characterized using scanning tunneling microscopy (STM). Meanwhile, the nanotribological properties of the template networks were measured using atomic force microscopy (AFM). Together with the theoretical calculation using the density functional theory (DFT) method, it was revealed that the friction coefficients were positively correlated with the interaction strength. The frictional energy dissipation mainly derives from both the intermolecular interaction energy and the interaction energy between molecules and the substrate. The efforts not only help us gain insight into the competitive mechanisms of hydrogen bond and van der Waals force in supramolecular assembly but also shed light on the origin of friction and the relationship between the assembly structures and the nanotribological properties at the molecular level.

Formation of Coronene Clusters in Concentration and Temperature Controlled Two-Dimensional Porous Network

In this research, with the assistance of scanning tunneling microscopy (STM), we observed the two-dimensional (2D) self-assembly transition of a star-shaped oligofluorene derivative (StOF) from disordered structure to honeycomb network by adjusting StOF concentration in solution. By introducing guest molecules coronene (COR), we for the first time achieved novel triangle-shaped COR trimers on liquid/highly oriented pyrolytic graphite (HOPG) interface in both honeycomb network and the disordered structure. In thermal tests, the COR/StOF-disorder system underwent a structural conformation to form well-ordered ladder structures, while the COR/StOF-honeycomb system remained the perfect hexagonal network with COR trimers included in the cavities. Density functional theory (DFT) calculations have been employed to investigate the forming mechanism of the molecular nanoarrays. These studies are expected to enhance controlling on 2D self-assembly and provide a facial approach toward constructing on-surface molecular clusters.