Real-Space Investigation of the Multiple Halogen Bonds by Ultrahigh-Resolution Scanning Probe Microscopy.

The chemical bond is of central interest in chemistry, and it is of significance to study the nature of intermolecular bonds in real-space. Herein, non-contact atomic force microscopy (nc-AFM) and low-temperature scanning tunneling microscopy (LT-STM) are employed to acquire real-space atomic information of molecular clusters, i.e., monomer, dimer, trimer, tetramer, formed on Au(111). The formation of the various molecular clusters is due to the diversity of halogen bonds. DFT calculation also suggests the formation of three distinct halogen bonds among the molecular clusters, which originates from the noncovalent interactions of Br-atoms with the positive potential H-atoms, neutral potential Br-atoms, and negative potential N-atoms, respectively. This work demonstrates the real-space investigation of the multiple halogen bonds by nc-AFM/LT-STM, indicating the potential use of this technique to study other intermolecular bonds and to understand complex supramolecular assemblies at the atomic/sub-molecular level.

Low-Dimensional Porous Carbon Networks Using Single-/Triple-Coupling Polycyclic Hydrocarbon Precursors.

Polycyclic hydrocarbons (PHs) share the same hexagonal structure of sp2 carbons as graphene but possess an energy gap due to quantum confinement effect. PHs can be synthesized by a bottom-up strategy starting from small building blocks covalently bonded into large 2D organic sheets. Further investigation of the role of the covalent bonding/coupling ways on their electronic properties is needed. Here, we demonstrate a surface-mediated synthesis of hexa-peri-hexabenzocoronene (HBC) and its extended HBC oligomers (dimers, trimers, and tetramers) via single- and triple-coupling ways and reveal the implication of different covalent bonding on their electronic properties. High-resolution low-temperature scanning tunneling microscopy and noncontact atomic force microscopy are employed to in situ determine the atomic structures of as-synthesized HBC oligomers. Scanning tunneling spectroscopy measurements show that the length extension of HBC oligomers narrows the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, the energy gaps of triple-coupling HBC oligomers are smaller and decrease more significantly than that of the single-coupling ones. We hypothesize that the triple coupling promotes a more effective delocalization of π-electrons than the single coupling, according to density functional theory calculations. We also demonstrate that the HBC oligomers can further extend across the substrate steps to achieve conjugated polymers and large-area porous carbon networks.

Stable Triarylmethyl Radicals and Cobalt(II) Ions Based 1D/2D Coordination Polymers.

The incorporation of organic radicals into coordination polymers was considered as a promising strategy to promote metal-ligand exchange interactions, but there are only a very limited number of stable organic radical-based ligands that can serve well such a purpose. Herein, we report two new tris(2,4,6-trichlorophenyl)methyl (TTM) radical-based ligands L1 and L2 with two and three imidazole substituents, respectively. The imidazole unit serves as a coordination site and it can also stabilize the TTM radical by intramolecular donor-acceptor interaction. Coordination of L1 and L2 with cobalt(II) ions gave the corresponding one- (CoCP-1) and two-dimensional (CoCP-2) coordination polymers, the structures of which were confirmed by X-ray crystallographic analysis. Magnetic measurements and theoretical calculations suggest antiferromagnetic coupling between the paramagnetic cobalt(II) ions and the radical ligands. Our study provides a rational design for stable organic radical-based ligands and further demonstrated the feasibility of a metal-radical approach toward magnetic materials.

Supramolecular Tiling of a Conformationally Flexible Precursor.

Supramolecular self-assembly offers a possible pathway for nanopatterning and functionality. In particular, molecular tiling such as trihexagonal tiling (also known as the Kagome lattice) has promising chemical and physical properties. Distorted Kagome lattices are not well understood due to their complexity, and studies of their controllable fabrication are few. Here, by employing a conformationally flexible precursor, 2,4,6-tris(3-bromophenyl)-1,3,5-triazine (mTBPT), we demonstrate two-dimensional distorted Kagome lattice p3, (333) by supramolecular self-assembly and achieve tuning of the metastable phases, including the homochiral porous network and distorted Kagome lattice p3, (333) by steering deposition rates on a cold Ag(111) substrate. By a combination of scanning tunneling microscopy and density functional theory calculations, the distorted Kagome lattice is energetically unfavorable but can be trapped at a high deposition rate, and the process mainly depends on surface kinetics. This work using conformationally flexible mTBPT molecules provides a pathway for the controllable growth of different phases, including metastable Kagome lattices.

Realizing Two-Dimensional Supramolecular Arrays of a Spin Molecule via Halogen Bonding.

Well-ordered spin arrays are desirable for next-generation molecule-based magnetic devices, yet their synthetic method remains a challenging task. Herein, we demonstrate the realization of two-dimensional supramolecular spin arrays on surfaces via halogen-bonding molecular self-assembly. A bromine-terminated perchlorotriphenylmethyl radical with net carbon spin was synthesized and deposited on Au(111) to achieve two-dimensional supramolecular spin arrays. By taking advantage of the diversity of halogen bonds, five supramolecular spin arrays form and are probed by low-temperature scanning tunneling microscopy at the single-molecule level. First-principles calculations verify that the formation of three distinct types of halogen bonds can be used to tailor supramolecular spin arrays via molecular coverage and annealing temperature. Our work suggests that supramolecular self-assembly can be a promising method to engineer two-dimensional molecular spin arrays.

On-Surface Synthesis of Variable Bandgap Nanoporous Graphene.

Tuning the bandgap of nanoporous graphene is desirable for applications such as the charge transport layer in organic-hybrid devices. The holy grail in the field is the ability to synthesize 2D nanoporous graphene with variable pore sizes, and hence tunable band gaps. Herein, the on-surface synthesis of nanoporous graphene with variable bandgaps is demonstrated. Two types of nanoporous graphene are synthesized via hierarchical CC coupling, and are verified by low-temperature scanning tunneling microscopy and non-contact atomic force microscopy. Nanoporous graphene-1 is non-planar, and nanoporous graphene-2 is a single-atom thick planar sheet. Scanning tunneling spectroscopy measurements reveal that nanoporous graphene-2 has a bandgap of 3.8 eV, while nanoporous graphene-1 has a larger bandgap of 5.0 eV. Corroborated by first-principles calculations, it is proposed that the large bandgap opening is governed by the confinement of π-electrons induced by pore generation and the non-planar structure. The finding shows that by introducing nanopores or a twisted structure, semi metallic graphene is converted into semiconducting nanoporous graphene-2 or insulating wide-bandgap nanoporous graphene-1.

Atomic-Level Electronic Properties of Carbon Nitride Monolayers.

Heteroatom-doped carbon-based materials are of significance for clean energy conversion and storage because of their fascinating electronic properties, low cost, high durability, and environmental friendliness. Atomically precise fabrication of carbon-based materials with well-defined heteroatom-dopant positions and atomic-scale understanding of their atomic-level electronic properties is a challenge. Herein, we demonstrate the bottom-up on-surface synthesis of 1D and 2D monolayer carbon nitride nanostructures with precise control of the nitrogen-atom doping sites and pore sizes. We also observe an electronic band offset at the C-N heterojunction. Using high-resolution scanning tunneling microscopy, the atomic structure of the as-prepared carbon nitride nanoporous monolayers are revealed, indicating successful and precise control of the structures and N atom doping sites. Furthermore, corroborated by theoretical calculations, scanning tunneling spectroscopy measurements reveal a valence band shift of 140 meV that results in an electric field of 2.9 × 108 V m-1 at the C-N heterojunction, indicating efficient separation of the electron-hole pair at the N doping site. Our finding offers direct atomic-level insights into the local electronic structure of the heteroatom-doped carbon-based materials.

Thermally Induced Chiral Aggregation of Dihydrobenzopyrenone on Au(111).

The realization of chiral supramolecular architectures on solid surfaces has triggered much interest due to its potential enantiospecific applications. An in-depth study of chiral aggregation on surfaces is significant for developing functional chiral surfaces. Herein, we report thermally induced chiral aggregation of dihydrobenzopyrenone on Au(111). By high-resolution low-temperature scanning tunneling microscopy, a racemate monolayer consisting of levorotatory and dextrorotatory dihydrobenzopyrenones was found to aggregate into conglomerate domains after moderate annealing treatment. Combined with first-principles calculations, we suggest that the intermolecular dipole-dipole interaction plays an important role in chiral aggregation, which takes place via molecular in-plane diffusion rather than molecular out-of-plane flipping. This work unveils one underlying mechanism of thermally induced chiral aggregation, thus enabling potential applications such as fabricating supramolecular architectures for functional chiral surfaces.

Catalyst-Free Growth of Two-Dimensional BCxN Materials on Dielectrics by Temperature-Dependent Plasma-Enhanced Chemical Vapor Deposition.

Traditional methods to prepare two-dimensional (2D) B-C-N ternary materials (BCxN), such as chemical vapor deposition (CVD), require sophisticated experimental conditions such as high temperature, delicate control of precursors, and postgrowth transfer from catalytic substrates, and the products are generally thick or bulky films without the atomically mixed phase of B-C-N, hampering practical applications of these materials. Here, for the first time, we develop a temperature-dependent plasma-enhanced chemical vapor deposition (PECVD) method to grow 2D BCxN materials directly on noncatalytic dielectrics at low temperature with high controllability. The C, N, and B compositions can be tuned by simply changing the growth temperature. Thus, the properties of the as-made materials including band gap and conductivity are modulated, which is hardly achieved by other methods. A 2D hybridized BC2N film with a mixed BC2N phase is produced, for the first time, with a band gap of about 2.3 eV. The growth temperature is 580-620 °C, much lower than that of traditional catalytic CVD for growing BCxN. The product has a p-type conducting property and can be directly applied in field-effect transistors and sensors without postgrowth transfer, showing great promise for this method in future applications.

Toward Two-Dimensional π-Conjugated Covalent Organic Radical Frameworks.

Reported is the synthesis, characterization, and material properties of the first π-conjugated two-dimensional covalent organic radical framework (CORF), PTM-CORF, based on the stable polychlorotriphenylmethyl (PTM) radical. The covalent organic framework (COF) precursor (PTM-H-COF) was first synthesized by liquid/liquid interfacial acetylenic homocoupling of a triethynylpolychlorotriphenylmethane monomer, and showed crystalline features with a hexagonal diffraction pattern matching that of A-B-C stacking. Subsequent deprotonation and oxidation of the PTM units in PTM-H-COF gave PTM-CORF. Magnetic measurements revealed that the neighboring PTM radicals in the PTM-CORF are anti-ferromagnetically coupled each other, with a moderate exchange interaction (J=-375 cm-1 ). The PTM-CORF has a small energy gap (ca. 0.88 eV) and a low-lying LUMO energy level (-4.72 eV), and exhibits high electrocatalytic activity and durability toward the oxygen reduction reaction.

Light-Triggered CO2 Breathing Foam via Nonsurfactant High Internal Phase Emulsion.

Solid materials for CO2 capture and storage have attracted enormous attention for gaseous separation, environmental protection, and climate governance. However, their preparation and recovery meet the problems of high energy and financial cost. Herein, a controllable CO2 capture and storage process is accomplished in an emulsion-templated polymer foam, in which CO2 is breathed-in under dark and breathed-out under light illumination. Such a process is likely to become a relay of natural CO2 capture by plants that on the contrary breathe out CO2 at night. Recyclable CO2 capture at room temperature and release under light irradiation guarantee its convenient and cost-effective regeneration in industry. Furthermore, CO2 mixed with CH4 is successfully separated through this reversible breathing in and out system, which offers great promise for CO2 enrichment and practical methane purification.

A Reversed Photosynthesis-like Process for Light-Triggered CO2 Capture, Release, and Conversion.

Materials for CO2 capture have been extensively exploited for climate governance and gas separation. However, their regeneration is facing the problems of high energy cost and secondary CO2 contamination. Herein, a reversed photosynthesis-like process is proposed, in which CO2 is absorbed in darkness while being released under light illumination. The process is likely supplementary to natural photosynthesis of plants, in which, on the contrary, CO2 is released during the night. Remarkably, the material used here is able to capture 9.6 wt.% CO2 according to its active component. Repeatable CO2 capture at room temperature and release under light irradiation ensures its convenient and cost-effective regeneration. Furthermore, CO2 released from the system is successfully converted into a stable compound in tandem with specific catalysts.

Rapid Self-Assembly of Block Copolymers for Flower-Like Particles with High Throughput.

The self-assembly of block copolymers has evolved into a foremost bottom-up approach for building polymeric materials. Historical challenges exist within this lively field, including the scalability and elegant simplicity of self-assembled aggregates with predictable structures. Here, we report a generally applicable strategy for the rapid self-assembly of poly(ethylene glycol)-block-poly(l-lactic acid) with the help of a single oil-in-water emulsion. A kind of flower-like polymer particle with filamentous surface branches is rapidly formed after removing the oil phase from the emulsion system. Moreover, the dimension of the branched filaments and the spherical internal core can be controlled through regulating the block ratio and the emulsification conditions. In particular, we propose an explosion theory as a balance between phase separation and interchain interaction for explaining the formation of the branched structures of the flower-like particles. The particles with high throughput are further functionalized with polypyrrole for their use in enhanced photoelectric-sensing applications.

Surfactant-Free Emulsions with Erasable Triggered Phase Inversions.

Complex emulsions including double emulsions and high-internal-phase emulsions (HIPEs) are wonderful templates for producing porous polymeric materials. Yet, surfactants and multiple emulsifications are generally needed. In this work, surfactant-free complex emulsions are successfully prepared using a CO2-responsive block copolymer through one-step emulsification. Phase inversion from HIPEs to double emulsions happens in one system upon the change in polymer amphiphilicity as a result of CO2 triggering. The one-step emulsification method offers great convenience for converting the block copolymer into porous 3D scaffolds and particles. Moreover, CO2 triggering is erasable so that the polymer can be repeatedly used for controllable complex emulsions as well as porous materials.

Emulsion Templating Cyclic Polymers as Microscopic Particles with Tunable Porous Morphology.

Cyclic polymers are a particular class of macromolecules without terminal groups. Most studies has involved their physical properties and polymer composition, while attention has rarely been paid to their emulsification in an oil-water system. Herein we synthesized a cyclic polymer with polystyrene side chains via ring-expansion metathesis polymerization and click-chemistry. This cyclic polymer was compared with linear polystyrene in order to investigate the effect of cyclic topology on preparing porous particles by emulsion templating methods. The contribution of cyclic topology to emulsification originates from the formation of hollow microspheres with the use of cyclic polymer while linear polymer only afforded solid microspheres. With addition of hexadecane as soft template, both cyclic polymer and linear polymer emulsions were successfully converted into porous particles. Superior to linear polymer, cyclic polymer enables the stabilization of emulsion droplets and the tuning of porous morphology. It is revealed that cyclic polymer with nanoring shape tends to assemble at the interfacial area, leading to the Pickering effect that decelerates the macrophase separation. Furthermore, the unique porous feature of polymer particles affords a convenient application for the detection of trace explosive.

Tuning polymeric amphiphilicity via Se-N interactions: towards one-step double emulsion for highly selective enzyme mimics.

A selenium-containing small molecule is exploited to controllably tune the polymer amphiphilicity, leading to fabrication of appropriate polymer surfactants through which one-step double emulsions can be obtained in a facile, scalable, surfactant-free approach. After solvent evaporation, these resulting porous microparticles are shown to be the exceptional artificial GPx enzyme mimics.

Managing the phase separation in double emulsion by tuning amphiphilicity via a supramolecular route.

Double emulsion has attracted intense scientific investigation on account of its use in a wide range of applications. However, the process of its solidification is usually accompanied by the problem of uncontrollable phase separation. In this work, a supramolecular route is proposed to manage the phase separation in double emulsion. Different degrees of phase separations, from complete wetting to partial wetting and complete dewetting, have been achieved in an emulsion system consisting of P4VP-oleic acid. Partial wetting offers a strategy for generating polymer particles with controllable anisotropic structures. It is demonstrated that the amphiphilicity of polymer matrix, relying on the change of polymer-acid ratio or the chain length of aliphatic acid, is of vital importance for determining the degree of phase separation. A spreading and wetting theory is established to predict and explain the formation of partial wetting.