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.

The Brπ halogen bond assisted self-assembly of an asymmetric molecule regulated by concentration

The two-dimensional (2D) self-assembly behavior of an asymmetric thienophenanthrene derivative (M1) has been theoretically predicted and further probed via STM. The barely exploited Brπ halogen bonds play an assisting role in the structural formation, and a strong cooperative effect from the C-HBr bonds is shown. Such π-type halogen bond assisted self-assembly reveals self-adaption properties, which is of great interest for flexible light-emitting devices and self-healing materials.

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.

Surface Adatom Mediated Structural Transformation in Bromoarene Monolayers: Precursor Phases in Surface Ullmann Reaction

Structural transformations of supramolecular systems triggered by external stimuli maintain great potential for application in the fabrication of molecular storage devices. Using combined ultrahigh vacuum scanning tunneling microscopy, X-ray photoemission spectroscopy, and density functional theory calculations, we observed the surface adatom mediated structural transformation from 4,4''-dibromo- m-terphenyl (DMTP)-based halogen-bonded networks to DMTP-Cu(Ag) coordination networks on Cu(111) and Ag(111) at low temperatures. The halogen-bonded networks, which were formed on Cu(111) at 97 K and on Ag(111) at 93 K, consist of intact DMTP molecules stabilized by triple Br···Br bonds. The DMTP-Cu(Ag) coordination networks form on Cu(111) at 113 K and on Ag(111) at 103 K. They contain alternatingly arranged intact DMTP molecules and Cu(Ag) adatoms stabilized by weak C-Br···Cu(Ag) coordination bonds. Annealing the DMTP-Ag structure to 333 K leads to the initiation of C-Br bond scission. This observation suggests that the DMTP-Ag coordination network represents the intermediate phase ready for dehalogenation, which is the first step of the surface Ullmann reaction.

A size, shape and concentration controlled self-assembling structure with host-guest recognition at the liquid-solid interface studied by STM

In the present investigation, we reported the fabrication of host networks formed by two newly prepared phenanthrene-butadiynylene macrocycles (PBMs) at the liquid-solid interface. Size, shape and concentration controlled experiments have been performed to investigate the PBMs/coronene (COR) host-guest system with the structural polymorphism phenomenon. Initially, PBM1 could form a regular linear network structure and PBM2 form a well-ordered nanoporous network structure. When the COR molecules were introduced, the self-assembled structure of PBM1 remained unchanged, while COR could be entrapped into the cavities of the PBM2 nanoporous network, and the co-assembly of the PBM2/COR host-guest systems underwent a structural transformation with the increase of concentration of COR. Scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations are utilized to reveal the formation mechanism of the molecular nanoarrays controlled by the solution concentration.

Solution concentration controlled self-assembling structure with host-guest recognition at the liquid-solid interface

In the present investigation, we reported the fabrication of a chicken-wire porous 2D network formed by triphenylene-2,6,10-tricarboxylic acid (H3TTCA) at the liquid-solid interface. When coronene (COR) molecules were added into the system, the H3TTCA honey-comb network was broken and the reconstructed structures of the H3TTCA/COR host-guest systems were subsequently formed. Scanning tunneling microscopic (STM) measurements and density function theory (DFT) calculations were utilized to reveal the structural variety in the co-assembly of H3TTCA/COR controlled by the solution concentration at 1-heptanoic acid/HOPG interface.

Two-dimensional self-assembly of single-, poly- and co-crystals at the liquid/solid interface

The observation of two polymorphs indicates that C2 cannot form single crystals because of an increase in molecular flexibility.

Crystallization of micrometer-sized particles with molecular contours

The crystallization of micrometer-sized particles with shapes mimicking those of tetrabenzoheptacene (TBH) and 1,2:5,6-dibenzanthracene (DBT), both flat polyacenes, in an electric field results in the formation of ordered 2D packings that mimic the plane group symmetries in their respective molecular crystal equivalents. Whereas the particles packed in low-density disordered arrangements under a gravitational gradient, dielectrophoresis (under an ac electric field) produced ordered high-density packings with readily identifiable plane group symmetry. The ordered colloidal assemblies were stable for hours, with the packing density decreasing slowly but with recognizable symmetry for up to 12 h for the TBH-shaped particles and up to 4 h for the DBT-shaped particles. This unexpected stability is attributed to jamming behavior associated with interlocking of the dogbone-shaped (TBH) and Z-block (DBT) particles, contrasting with the more rapid reduction of packing density and loss of hexagonal symmetry for disk-shaped particles upon removal of the electric field. The TBH-shaped and DBT-shaped particles assemble into the p2 plane group, which corresponds to the densest particle packing among the possible close-packed plane groups for these particle symmetries. The p2 symmetry observed for the TBH-shaped and DBT-shaped colloid crystal emulates the p2 symmetry of the (010) layers in their respective molecular crystals, which crystallize in monoclinic lattices. Notably, DBT-shaped particles also form ordered domains with pgg symmetry, replicating the plane group symmetry of the (100) layer in the orthorhombic polymorph of DBT. These observations illustrate that the 2D ordering of colloid particles can mimic the packing of molecules with similar shapes, demonstrating that packing can transcend length scales from the molecular to the colloidal.

Principles of molecular assemblies leading to molecular nanostructures

Self-assembled physisorbed monolayers consist of regular two-dimensional arrays of molecules. Two-dimensional self-assembly of organic and metal-organic building blocks is a widely used strategy for nanoscale functionalization of surfaces. These supramolecular nanostructures are typically sustained by weak non-covalent forces such as van der Waals, electrostatic, metal-ligand, dipole-dipole and hydrogen bonding interactions. A wide variety of structurally very diverse monolayers have been fabricated under ambient conditions at the liquid-solid and air-solid interface or under ultra-high-vacuum (UHV) conditions at the UHV-solid interface. The outcome of the molecular self-assembly process depends on a variety of factors such as the nature of functional groups present on assembling molecules, the type of solvent, the temperature at which the molecules assemble and the concentration of the building blocks. The objective of this review is to provide a brief account of the progress in understanding various parameters affecting two-dimensional molecular self-assembly through illustration of some key examples from contemporary literature.

Functionalization of two-component molecular networks: recognition of Fe³⁺

Two-component supramolecular networks have been constructed with a symmetric triphenylene derivative with three carboxyl groups (sym-TTT) and melamine. Two kinds of hydrogen bonds with different strength are involved in the multi-component self-assembly, one is H-bond between carboxyl group of sym-TTT and melamine, the other is intermolecular H-bond between melamine molecules. These interactions drive a structural transformation from close-packed network to hexagonal network with active amino groups inside of the cavity. Scanning tunneling microscopy (STM) measurements reveal that the functionalized network of sym-TTT/melamine could recognise Fe(3+). These results could be helpful for designing functionalized molecular networks by multi-component self-assembling strategy.