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

Mixing behavior of p-terphenyl-3,5,3',5'-tetracarboxylic acid with trimesic acid at the solid-liquid interface

The molecular self-assembly of carboxylic acid molecules on a solid surface plays an important role in understanding the nanoscale-precision construction of functional patterns. In this study, the mixing behavior of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC) and trimesic acid (TMA) on a highly oriented pyrolytic graphite surface was studied by scanning tunneling microscopy (STM). The STM images show how the presence of a small percentage of TPTC molecules adsorbed onto TMA molecules can drastically change the on-surface self-assembly behavior of aromatic tetracarboxylic acid by initiating the nucleation and growth of a different polymorph. Molecular mechanics and density functional theory simulations of the adsorption energy and the additional stabilizing energy, induced by hydrogen bonds during assembly formations, provide insights into the relative stability of different assembled structures. Moreover, STM-based "nanoshaving" was conducted to confirm that the template layer underneath the second layer is indeed a random network.

Fabrication of 2D Hetero-Complexes With Nucleic-Acid-Base Adenine and Fatty-Acid Stearic Acid at Liquid/Solid Interface

Designing and fabricating hetero-complexes composed of organic and biological compounds had become an exciting area referring to biological recognition, molecular devices etc. Here, hydrogen-bonded complex of nucleic-acid-base (adenine, A) and fatty-acid (stearic acid, SA) was designed, fabricated and investigated at liquid/solid interface. The interesting striped-shaped structure composed of SA-A-SA trimers was formed after introducing adenine molecules. Meanwhile, the primary lamella-shape characteristic of the assembly of SA molecules was kept because of the collaboration of non-covalent interactions of molecule-molecule and molecule-substrate. With a series of experimental characterization and theoretical simulation, the origination of the as-prepared 2D hetero-complexes was gradually exhibited from the assembled structures of two building blocks of stearic acid and adenine. Our study provides a blueprint for designing additional multi-component complexes based on the existing molecular assembled architectures.

Two-dimensional supramolecular crystal engineering: chirality manipulation

Two dimensional (2D) supramolecular crystal engineering, one of the most important strategies towards nanotechnology, is both a science and an industry. In the present review, the recent advances in 2D supramolecular crystal engineering through chirality manipulation on solid surfaces are summarized, with the aid of the scanning tunneling microscopy technique. On-surface chirality manipulation includes surface confined structural chirality formation, chirality transformation, chirality separation as well as chirality elimination, by using component exchange and different external stimuli. Under this principle, host-guest supramolecular interactions, solvent induction, temperature regulation and STM-tip driven orientation control and reorientation effects under equilibrium or out-of-equilibrium conditions, towards the generation of the best-adapted chiral or achiral 2D nanostructures, are mainly described and highlighted. Future challenges and opportunities in this exciting area are also then discussed.

Two solvent-induced variable host-guest two-dimensional binary frameworks mediated by hydrogen bonding

Two-dimensional binary hydrogen-bonded organic frameworks constructed from 1,3,5-benzenetricarboxylic acid (TMA) and 4,4'-biphenyldicarboxylic acid (BDA) on highly oriented pyrolytic graphite (HOPG) were investigated by scanning tunneling microscopy (STM) in heptanoic acid and octanoic acid solvents. High-resolution STM observations demonstrated that various assemblies of hydrogen-bonded networks are strongly dependent on the nature of the solvent. Well-ordered porous rectangular flowerlike networks were revealed at the heptanoic acid/HOPG interface, whereas two different coexisting densely packed guest-host BDA/TMA structures were observed at the octanoic acid/HOPG interface. It is suggested that the stabilization of the binary networks is possibly associated with the solvent chain length, and longer-chain solvents favored the formation of porous polymorphic networks.

An STM/STS study of site-selective adsorption of C70 molecules onto arc-shaped BODIPY molecular-networks

In this article, a donor-acceptor H-T-BO/C70 system was studied by a STM/STS method on the molecular level. STM results revealed that H-T-BO, a BODIPY-based derivative, can form a semi-closed molecular network at the 1-phenyloctane/HOPG interface. After introducing C70 fullerene molecules into the network, two kinds of self-assembled nanoarrays were observed by STM. Density functional theory has been utilized to reveal the formation mechanism of the molecular nanoarrays. Scanning Tunneling Spectroscopy (STS) measurements were performed to investigate the electronic properties of H-T-BO/C70 systems. I-V spectra combined with theoretical analyses showed that the introduction of C70 into the H-T-BO system induced a great drop of the band gap, which should be a result of electrons transferring from the donor H-T-BO to the acceptor C70 molecules.

Investigating the Co-Adsorption Behavior of Nucleic-Acid Base (Thymine and Cytosine) and Melamine at Liquid/Solid Interface

The co-adsorption behavior of nucleic-acid base (thymine; cytosine) and melamine was investigated by scanning tunneling microscopy (STM) technique at liquid/solid (1-octanol/graphite) interface. STM characterization results indicate that phase separation happened after dropping the mixed solution of thymine-melamine onto highly oriented pyrolytic graphite (HOPG) surface, while the hetero-component cluster-like structure was observed when cytosine-melamine binary assembly system is used. From the viewpoints of non-covalent interactions calculated by using density functional theory (DFT) method, the formation mechanisms of these assembled structures were explored in detail. This work will supply a methodology to design the supramolecular assembled structures and the hetero-component materials composed by biological and chemical compound.

A dynamic study of the structural change in the binary network in response to guest inclusion

In the present work flexible binary networks of 1,3,5-benzenetricarboxylic acid (TMA) with 4,4'-bipyridine (Bpy) or 1,3,5-tris(4-pyridyl)-2,4,6-triazine(TPTZ) molecules at the liquid-solid interface were constructed. When coronene (COR) molecules are introduced into these systems, the binary networks collapse and at the same time, new COR/TMA host-guest structures are formed. Both experiments and calculations unambiguously indicate that the COR/TMA host-guest complex structure has stronger adsorption energy, resulting in the deconstruction-reconstruction phenomenon.

Site-selection and adaptive reconstruction in a two-dimensional nanoporous network in response to guest inclusion

We coronene (COR) molecule is added into the flexible binary network formed by tetraacidic azobenzene (NN4A) and trans-1,2-bis(4-pyridyl)ethylene (DPE), the binary network breaks and reconstruction structures of NN4A/COR host–guest systems are subsequently formed.

A template-confined fabrication of controllable gold nanoparticles based on the two-dimensional nanostructure of macrocycles

An amine-substituted macrocycle 6Y, a rigid hexagonal ring, self-assembled into a network architecture on the surface, which could be used as a molecular template to directly fabricate gold nanoparticles (AuNPs) with narrow size distribution (2.2 ± 0.1 nm).

Investigation of the non-covalent interactions of molecular self-assembly by scanning tunneling microscopy using the association of aromatic structures in pyrene-4,5,9,10-tetraone and phenanthrene-9,10-dione molecules

The self-assembled monolayers of aromatic molecules (pyrene-4,5,9,10-tetraone and phenanthrene-9,10-dione) were investigated at the liquid/solid (1-phenyloctane/graphite) interface using scanning tunneling microscopy, respectively.

Adaptive reorganization of 2D molecular nanoporous network induced by coadsorbed guest molecule

The ordered array of nanovoids in nanoporous networks, such as honeycomb, Kagome, and square, provides a molecular template for the accommodation of "guest molecules". Compared with the commonly studied guest molecules featuring high symmetry evenly incorporated into the template, guest molecules featuring lower symmetry are rare to report. Herein, we report the formation of a distinct patterned superlattice of guest molecules by selective trapping of guest molecules into the honeycomb network of trimesic acid (TMA). Two distinct surface patterns have been achieved by the guest inclusion induced adaptive reconstruction of a 2D molecular nanoporous network. The honeycomb networks can synergetically tune the arrangement upon inclusion of the guest molecules with different core size but similar peripherals groups, resulting in a trihexagonal Kagome or triangular patterns.

On-surface single molecule synthesis chemistry: a promising bottom-up approach towards functional surfaces

In this review, we introduce recent progress on surface synthesis and focus on supramolecular self-assembled structures driven by several typical chemical reactions at solid surfaces, with the aid of scanning tunneling microscopy (STM). We also emphasize the relationship between the non-covalent self-assembly and surface reactivity, by which we hope to find an effective way for further controllable nano-manufacture.

Surface tectonics of nanoporous networks of melamine-capped molecular building blocks formed through interface Schiff-base reactions

Control over the assembly of molecules on a surface is of great importance for the fabrication of molecule-based miniature devices. Melamine (MA) and molecules with terminal MA units are promising candidates for supramolecular interfacial packing patterning, owing to their multiple hydrogen-bonding sites. Herein, we report the formation of self-assembled structures of MA-capped molecules through a simple on-surface synthetic route. MA terminal groups were successfully fabricated onto rigid molecular cores with 2-fold and 3-fold symmetry through interfacial Schiff-base reactions between MA and aldehyde groups. Sub-molecular scanning tunneling microscopy (STM) imaging of the resultant adlayer revealed the formation of nanoporous networks. Detailed structural analysis indicated that strong hydrogen-bonding interactions between the MA groups persistently drove the formation of nanoporous networks. Herein, we demonstrate that functional groups with strong hydrogen-bond-formation ability are promising building blocks for the guided assembly of nanoporous networks and other hierarchical 2D assemblies.

Moiré pattern induced by the electronic coupling between 1-octanol self-assembled monolayers and graphite surface

Two-dimensional self-assembly of 1-octanol molecules on a graphite surface is investigated using scanning tunneling microscopy (STM) at the solid/liquid interface. STM images reveal that this molecule self-assembles into a compact hydrogen-bonded herringbone nanoarchitecture. Molecules are preferentially arranged in a head-to-head and tail-to-tail fashion. A Moiré pattern appears in the STM images when the 1-octanol layer is covering the graphite surface. The large Moiré stripes are perpendicular to the 1-octanol lamellae. Interpretation of the STM images suggests that the Moiré periodicity is governed by the electronic properties of the graphite surface and the 1-octanol layer periodicity.