A Free-Standing and Self-Healable 2D Supramolecular Material Based on Hydrogen Bonding: A Nanowire Array with Sub-2-nm Resolution

Light-driven self-assembly of spiropyran-functionalized covalent organic framework

Controlling the number of molecular switches and their relative positioning within porous materials is critical to their functionality and properties. The proximity of many molecular switches to one another can hinder or completely suppress their response. Herein, a synthetic strategy involving mixed linkers is used to control the distribution of spiropyran-functionalized linkers in a covalent organic framework (COF). The COF contains a spiropyran in each pore which exhibits excellent reversible photoswitching behavior to its merocyanine form in the solid state in response to UV/Vis light. The spiro-COF possesses an urchin-shaped morphology and exhibits a morphological transition to 2D nanosheets and vesicles in solution upon UV light irradiation. The merocyanine-equipped COFs are extremely stable and possess a more ordered structure with enhanced photoluminescence. This approach to modulating structural isomerization in the solid state is used to develop inkless printing media, while the photomediated polarity change is used for water harvesting applications.

Ultra-Thin 2D Ionic Salt Supported with Strong Hydrogen-Bonding Assisted Ionic Interaction

2D materials have attracted great interest since the report of graphene. However, because of the fragile stability of ultra-thin nanosheets, most studies are restricted to sheets maintained by strong covalent or coordination bonds. The research on which kind of bonds can maintain the free-standing existence of 2D nanosheets is still of great significance. Recently, 2D ionic salts are successfully synthesized on substrates, but whether 2D ionic salts can free-stand is still a problem. Herein this problem is addressed by a free-standing 2D ionic salt (thickness: ≈2 nm) exfoliated from a 4,4'-bipyridinium hydrochloride salt crystal. The stability of this 2D salt is supported by a strong NH···Cl hydrogen (H)-bonding assisted ionic interaction (17.99 kcal mol^-1 ), which is verified by density functional theory calculation and natural bond orbital analysis. The salt crystal has strong air-stable radicals inside and the 2D ionic salt exhibits red fluorescence in solution and in solid-state, especially in solution the stokes shifts are very large (≈ 386 nm). This breakthrough work is not only beneficial for the construction of novel 2D materials but also for the understanding of H-bonding interactions.

Water-Phase Lateral Interconnecting Quantum Dots as Free-Floating 2D Film Assembled by Hydrogen-Bonding Interactions to Acquire Excellent Electrocatalytic Activity

Supramolecular self-assembly of nanoparticles in two orthogonal directions would potentially allow one to fabricate nanomaterials with fascinating properties. In this study of a hydrothermal polycondensation of melamine/cyanuric acid, graphitic carbon nitride-based quantum dots (CNQD, ∼2 nm) are in situ arranged along two orthogonal directions through lateral hydrogen bonding, and free-floating two-dimensional hydrogen-bonded films of CNQD (2D CNQD) are built. On the basis of the universality of this hydrothermal in situ supramolecular self-assembly technique, 2D films linked by other quantum dots such as sulfur-doped graphitic carbon nitride and CdTe are also constructed. With the benefits of stimuli responsiveness and the reversibility of hydrogen bonds, controllable assembly/disassembly of the 2D CNQD film is feasibly achieved by external stimuli such as inletting CO₂/N₂, which endows the assembled 2D CNQD films optimal electrochemical superiorities of both 2D film and zero-dimensional (0D) quantum dots. Accordingly, the 2D CNQD film delivers a high bifunctional activity in both a nitrogen reduction reaction (NRR) and an oxygen evolution reaction (OER). Especially in NRR, it exhibits the high yield rate of NH₃ reaching 75.07 μg h^-1 mg^-1 at -0.85 V versus reversible hydrogen electrode at ambient condition. Strikingly, the power density of the rechargeable Zn-N₂ battery using 2D CNQD film as cathode reaches 31.94 mW cm^-2, outperforming the majority of Zn-N₂ batteries. Density functional theory calculations proved the promoted adsorption of N₂ and stabilized NRR intermediates on 2D CNQD cooperated by multiply hydrogen-bonding interactions are the main reasons for the excellent NRR electrocatalytic performances. This work hints that hydrothermal in situ supramolecular self-assembly is a feasible and direct way to integrate 0D quantum dots into 2D directional arrays, and the hydrogen bond that interlinks enables this free-floating 2D structure to maintain the electrochemical superiority of both 0D and 2D structures.

Crystal Structures of Lignocellulosic Furfuryl Biobased Polydiacetylenes with Hydrogen-Bond Networks: Influencing the Direction of Solid-State Polymerization through Modification of the Spacer Length

We present the topochemical polymerization of two lignocellulosic biobased diacetylenes (DAs) that only differ by an alkyl spacer length of 1 methylene (n = 1) or 3 methylene units (n = 3) between the diyne and carbamate functionalities. Their crystalline molecular organizations have the distinctive feature of being suitable for polymerization in two potential directions, either parallel or skewed to the hydrogen-bonded (HB) network. However, single-crystal structures of the final polydiacetylenes (PDAs) demonstrate that the resulting orientation of the conjugated backbones is different for these two derivatives, which lead to HB supramolecular polymer networks (2D nanosheets) for n = 1 and to independent linear PDA chains with intramolecular HBs for n = 3. Thus, spacer length modification can be considered a new strategy to influence the molecular orientation of conjugated polymer chains, which is crucial for developing the next generation of materials with optimal mechanical and optoelectronic properties. Calculations were performed on model oligodiacetylenes to evaluate the cooperativity effect of HBs in the different crystalline supramolecular packing motifs and the energy profile related to the torsion of the conjugated backbone of a PDA chain (i.e., its ability to adopt planar or helical conformations).

Topochemical polymerizations for the solid-state synthesis of organic polymers

Topochemical polymerizations are solid-state reactions driven by the alignment of monomers in the crystalline state. The molecular confinement in the monomer crystal lattice offers precise control over the tacticity, packing and crystallinity of the polymer formed in the topochemical reaction. As topochemical reactions occur under solvent- and catalyst-free conditions, giving products in high yield and selectivity/specificity that do not require tedious chromatographic purification, topochemical polymerizations are highly attractive over traditional solution-phase polymer synthesis. By this method, polymers having sophisticated structures and desired topologies can be availed. Often, such ordered packing confers attractive properties to the topochemically-synthesized polymers. Diverse categories of topochemical polymerizations are known, such as polymerizations via [2+2], [4+4], [4+2], and [3+2] cycloadditions, and polymerization of diynes, triynes, dienes, trienes, and quinodimethanes, each of which proceed under suitable stimuli like heat, light or pressure. Each class of these reactions requires a unique packing arrangement of the corresponding monomers for the smooth reaction and produces polymers with distinct properties. This review is penned with the intent of bringing all the types of topochemical polymerizations into a single platform and communicating the versatility of these lattice-controlled polymerizations. We present a brief history of the development of each category and comprehensively review the topochemical synthesis of fully-organic polymers reported in the last twenty years, particularly in crystals. We mainly focus on the various molecular designs and crystal engineering strategies adopted to align monomers in a suitable orientation for polymerization. Finally, we analyze the current challenges and future perspectives in this research field.

Highly Sensitive Polydiacetylene Ensembles for Biosensing and Bioimaging

Polydiacetylenes are prepared from amphiphilic diacetylenes first through self-assembly and then polymerization. Different from common supramolecular assemblies, polydiacetylenes have stable structure and very special optical properties such as absorption, fluorescence, and Raman. The hydrophilic head of PDAs is easy to be chemically modified with functional groups for detection and imaging applications. PDAs will undergo a specific color change from blue to red, fluorescence enhancement and Raman spectrum changes in the presence of receptor ligands. These properties allow PDA-based sensors to have high sensitivity and specificity during analysis. Therefore, the PDAs have been widely used for detection of viruses, bacteria, proteins, antibiotics, hormones, sialic acid, metal ions and as probes for bioimaging in recent years. In this review, the preparation, polymerization, and detection mechanisms of PDAs are discussed, and some representative research advances in the field of bio-detection and bioimaging are highlighted.

A C-N Coupling Polymerization on Ice-Surface towards Decimeter-Sized 2D Covalent Materials with High Catalytic Activity for Water-Splitting

Compared to other solid templates (metal, ceramic, carbon, etc.), polymerization on ice-surface has many advantages. However, the popularity of this method has been impeded by the lack of appropriate polymerization reactions. To date, only few oxidation polymerizations have been reported to occur on ice-surface, and unfortunately they can only produce supramolecular films rather than fully covalent films. Herein for the first time, 2D covalent materials have been created on ice-surface even at -16 °C through a C-N coupling polymerization, and two free-standing 2D polyarylamines (2DPA)s were synthesized. Both 2DPAs have decimeter-size and nanometer thickness. This study provides the first polymerization reaction to synthesize 2D covalent materials on ice-surface, which can be used to fabricate materials/tools/robots with specific 2D structure by copying ice morphologies. Furthermore, both 2DPAs exhibit higher hydrogen evolution reaction activity than many metal-free catalysts and even some metal-based catalysts, so this study also provides further insight for the development of metal-free catalysts for water-splitting.

Creation of Centimeter-Sized 2 D Crystalline Film by Crystallization of Homopolymer in Solution

The 2 D assembly of polymers to form free-standing and large crystalline films is quite appealing but very challenging. Although there have been some works using interface templates, reports of in situ assembly in solution are still rare. Herein, a simple strategy is developed for the creation of a free-standing and centimeter-sized 2 D crystalline polymer film through crystallization of an amphiphilic brush polydiacetylene (PDA) in solution. The film exhibits good shape memory, a low dielectric constant, and good carrier mobility. This strategy may be applied extensively to produce a variety of other macroscopic 2 D crystalline polymer films for applications in electronics, catalysis, and so on.

Reversibly Thermochromic Cyclic Dipeptide Nanotubes

Owing to their capability of forming extensive hydrogen bondings and the facile introduction of chirality, cyclic dipeptides (CDPs) have gained great attention as scaffolds for functional supramolecules. Surprisingly, introduction of a photopolymerizable diacetylene (DA) moiety to the CDP afforded nanotubular structures with enhanced stability and reversible thermochromism. A series of CDP-containing DAs (CDP-DAs) are prepared by coupling 10,12-pentacosadiynoic acid with CDPs, cyclo(-Gly-Ser) and cis/trans cyclo(-Ser-Ser). Fabrication of CDP-DA self-assemblies in a polar chloroform and methanol solvent mixture affords nanotubes comprising single-wall and multiwall structures. The self-assembly behavior and morphology characteristic are examined by scanning electron microscopy and transmission electron microscopy. Next, X-ray diffraction analysis confirms well-ordered lamellar structures with a perfect agreement with the bilayer formation leading to the tubular structure via lamellar scrolling behavior. Upon UV irradiation, monomeric CDP-DA tubular assemblies result in the blue-colored CDP/polydiacetylene (PDA) nanotubes. Interestingly, CDP/PDA nanotubes exhibit a reversible blue-to-red color change for over 10 consecutive thermal cycles. The CDP-DA/PDA supramolecular system demonstrates potential applications in developing stimulus-responsive functional materials.

Synthesis of a 2D phosphorus material in a MOF-based 2D nano-reactor

A 2D nano-reactor was constructed within a pillar–layer-MOF, in which a moderately stable 2D phosphorus material was synthesized.

Synthesis of two dimensional (2D) materials mainly relies on interface-templates, exfoliation of layered crystals and surfactant-assistance. Here we have realized another strategy of polymerization in a 2D nano-reactor. As an application of this strategy, a 2D nano-reactor was constructed within a new pillar–layer metal organic framework (MOF) and in this nano-reactor a moderately stable 2D phosphorus material was synthesized from white phosphorus (P₄). Our research may provide a new approach for the synthesis of various 2D materials.

Self-Healing Natural Rubber with Tailorable Mechanical Properties Based on Ionic Supramolecular Hybrid Network

In most cases, the strength of self-healing supramolecular rubber based on noncovalent bonds is in the order of KPa, which is a challenge for their further applications. Incorporation of conventional fillers can effectively enhance the strength of rubbers, but usually accompanied by a sacrifice of self-healing capability due to that the filler system is independent of the reversible supramolecular network. In the present work, in situ reaction of methacrylic acid (MAA) and excess zinc oxide (ZnO) was realized in natural rubber (NR). Ionic cross-links in NR matrix were obtained by limiting the covalent cross-linking of NR molecules and allowing the in situ polymerization of MAA/ZnO. Because of the natural affinity between Zn²⁺ ion-rich domains and ZnO, the residual nano ZnO participated in formation of a reversible ionic supramolecular hybrid network, thus having little obstructions on the reconstruction of ionic cross-links. Meanwhile, the well dispersed residual ZnO could tailor the mechanical properties of NR by changing the MAA/ZnO molar ratios. The present study thus provides a simple method to fabricate a new self-healing NR with tailorable mechanical properties that may have more potential applications.