A Crystalline 1D Dynamic Covalent Polymer

Photoresponsive Supramolecular Polymers Capable of Intrachain Folding and Interchain Aggregation

The competition between polymer chain folding and aggregation is a critical structuring process that determines the physical properties of synthetic and biopolymers. However, supramolecular polymer systems that exhibit both processes have not yet been reported. We herein introduce a system in which folded supramolecular polymers spontaneously undergo interchain aggregation due to a rearrangement in internal molecular order, converting them into crystalline aggregates. These folded supramolecular polymers slowly crystallize over the course of half a day, due to their characteristic higher-order structures. However, the photoisomerization of the trans-azobenzene incorporated into the monomer to the cis isomer leads to unfolding of the polymer, accelerating the intrachain and interchain molecular ordering to a few hours. The intermediate structures visualized by AFM demonstrate that the unfolding is coupled with interchain aggregation.

Enhancing structural control in covalent organic frameworks through steric interaction-driven linker design

Covalent Organic Frameworks (COFs) exhibiting kagome (kgm) structures are promising crystalline porous materials with two distinct pores. However, there are no reliable synthetic methods to exclusively target the kgm over the polymorphic square-lattice (sql) structure. To address this, we introduce a linker design strategy featuring bulky functional groups, which through steric interactions can hinder the sql net formation, thereby leading to a kgm structure. By rigid attachment of the methyl benzoate groups to a tetradentate COF linker, steric interactions with neighbouring linkers depending on the pore size become possible. The steric interaction was tuned by varying the complementary bidentate linear linker lengths, where the shorter phenylenediamine linker leads to steric hindrance and the formation of the kgm lattice, while with the longer benzidine linker, steric interaction is reduced leading to the sql lattice. Thus, control over the net can be exerted through steric interaction strengths. Additionally, structural analysis revealed the formation of the kgm COF with an unusual ABC stacking, leading to pearl string type pores instead of two distinct pore sizes. This COF system shows that steric interaction-driven design enhances control over COF structures, expanding the design toolbox, but also provides valuable insights into network formation and polymorphism.

Construction of Benzoxazine-linked One-Dimensional Covalent Organic Frameworks Using the Mannich Reaction

Covalent polymerization of organic molecules into crystalline one-dimensional (1D) polymers is effective for achieving desired thermal, optical, and electrical properties, yet it remains a persistent synthetic challenge for their inherent tendency to adopt amorphous or semicrystalline phases. Here we report a strategy to synthesize crystalline 1D covalent organic frameworks (COFs) composing quasi-conjugated chains with benzoxazine linkages via the one-pot Mannich reaction. Through [4+2] and [2+2] type Mannich condensation reactions, we fabricated stoichiometric and sub-stoichiometric 1D covalent polymeric chains, respectively, using doubly and singly linked benzoxazine rings. The validity of their crystal structures has been directly visualized through state-of-the-art cryogenic low-dose electron microscopy techniques. Post-synthetic functionalizations of them with a chiral MacMillan catalyst produce crystalline organic photocatalysts that demonstrated excellent catalytic and recyclable performance in light-driven asymmetric alkylation of aldehydes, affording up to 94 % enantiomeric excess.

A redox-active organic cage as a cathode material with improved electrochemical performance

Organic cages offer numerous opportunities for creating novel materials suitable for a wide range of applications. Among these, energy-related applications are beginning to attract attention. We report here the synthesis of a [3 + 6] trigonal prismatic cage constituted by three redox-active dibenzotetraazahexacene subunits. Cathodes formulated with the organic cage show enhanced performance compared to those formulated with the individual subunits, showing improvements in terms of electrochemical stability, lithium-ion diffusivity, and cathode capacity.

Controlling the Degree of Interpenetration in Chiral Three-Dimensional Covalent Organic Frameworks via Steric Tuning

Network interpenetration plays a crucial role in functionalizing porous framework materials. However, controlling the degree of interpenetration in covalent organic frameworks (COFs) to influence their pore sizes, shapes, and functionalities still remains a significant challenge. Here, we demonstrate a steric tuning strategy to control the degree of COF interpenetration and modulate their physicochemical properties. By imine condensations of 1,1'-bi-2-naphthol-derived tetraaldehydes bearing different alkyl substituents with the monomer tetra(p-aminophenyl)-methane, we synthesized and characterized a family of two-component and three-component chiral COFs with different interpenetrated dia networks. The alkyl groups are periodically appended on the pore walls, and their types/contents that can be synthetically tuned control the interpenetration degree of COFs by minimizing repulsive interactions between the alkyl groups. Specifically, the COF with -OH groups adopts an interpenetrated dia-c5 topology, those with -OMe/-OEt groups take an interpenetrated dia-c4 topology, whereas those with the bulky -OnPr/-OnBu groups exhibit a noninterpenetrated dia-c1 topology. The multivariate COFs with both -OH and -OnBu groups display either a noninterpenetrated or dia-c5 topology, depending on the proportion of -OnBu groups. The extent of interpenetration in COFs significantly affects their porosity, thermal stability, and chemical stability, resulting in varying selective performances in the adsorption and separation of dyes and asymmetric catalysis. This work highlights the potential of using steric hindrance to tune and control interpenetration, porosity, stability, and functionalities of COFs materials, broadening the range of their applications.

Dynamic Entwined Topology in Helical Covalent Polymers Dictated by Competing Supramolecular Interactions

Naturally occurring polymeric structures often consist of 1D polymer chains intricately folded and entwined through non-covalent bonds, adopting precise topologies crucial for their functionality. The exploration of crystalline 1D polymers through dynamic covalent chemistry (DCvC) and supramolecular interactions represents a novel approach for developing crystalline polymers. This study shows that sub-angstrom differences in the counter-ion size can lead to various helical covalent polymer (HCP) topologies, including a novel metal-coordination HCP (m-HCP) motif. Single-crystal X-ray diffraction (SCXRD) analysis of HCP-Na revealed that double helical pairs are formed by sodium ions coordinating to spiroborate linkages to form rectangular pores. The double helices are interpenetrated by the unreacted diols coordinating sodium ions. The reticulation of the m-HCP structure was demonstrated by the successful synthesis of HCP-K. Finally, ion-exchange studies were conducted to show the interconversion between HCP structures. This research illustrates how seemingly simple modifications, such as changes in counter-ion size, can significantly influence the polymer topology and determine which supramolecular interactions dominate the crystal lattice.

Advances and applications of microcrystal electron diffraction (MicroED)

Microcrystal electron diffraction, commonly referred to as MicroED, has become a powerful tool for high-resolution structure determination. The method makes use of cryogenic transmission electron microscopes to collect electron diffraction data from crystals that are several orders of magnitude smaller than those used by other conventional diffraction techniques. MicroED has been used on a variety of samples including soluble proteins, membrane proteins, small organic molecules, and materials. Here we will review the MicroED method and highlight recent advancements to the methodology, as well as describe applications of MicroED within the fields of structural biology and chemical crystallography.

Single-Crystal One-Dimensional Porous Ladder Covalent Polymers

The synthesis of single-crystal, one-dimensional (1D) polymers is of great importance but a formidable challenge. Herein, we report the synthesis of single-crystal 1D ladder polymers in solution by dynamic covalent chemistry. The three-dimensional electron diffraction technique was used to rigorously solve the structure of the crystalline polymers, unveiling that each polymer chain is connected by double covalent bridges and all polymer chains are packed in a staggered and interlaced manner by π-π stacking and hydrogen bonding interactions, making the crystalline polymers highly robust in both thermal and chemical stability. The synthesized single-crystal polymers possess permanent micropores and can efficiently remove CO2 from the C2H2/CO2 mixture to obtain high-purity C2H2, validated by dynamic breakthrough experiments. This work demonstrates the first example of constructing single-crystal 1D porous ladder polymers with double covalent bridges in solution for efficient C2H2/CO2 separation.

MXene-Based Nanocomposites for Piezoelectric and Triboelectric Energy Harvesting Applications

Due to its superior advantages in terms of electronegativity, metallic conductivity, mechanical flexibility, customizable surface chemistry, etc., 2D MXenes for nanogenerators have demonstrated significant progress. In order to push scientific design strategies for the practical application of nanogenerators from the viewpoints of the basic aspect and recent advancements, this systematic review covers the most recent developments of MXenes for nanogenerators in its first section. In the second section, the importance of renewable energy and an introduction to nanogenerators, major classifications, and their working principles are discussed. At the end of this section, various materials used for energy harvesting and frequent combos of MXene with other active materials are described in detail together with the essential framework of nanogenerators. In the third, fourth, and fifth sections, the materials used for nanogenerators, MXene synthesis along with its properties, and MXene nanocomposites with polymeric materials are discussed in detail with the recent progress and challenges for their use in nanogenerator applications. In the sixth section, a thorough discussion of the design strategies and internal improvement mechanisms of MXenes and the composite materials for nanogenerators with 3D printing technologies are presented. Finally, we summarize the key points discussed throughout this review and discuss some thoughts on potential approaches for nanocomposite materials based on MXenes that could be used in nanogenerators for better performance.

Controlling block copolymer one-dimensional self-assembly in polymeric matrices

The synthesis of one-dimensional (1D) nanostructures in polymeric matrices has become the focus of much research, as the presence of these highly anisotropic domains determines the transport behaviour and mechanical properties of the resulting nanostructured polymers. In this work, 1D PEO nanocrystals were synthesized in situ from polystyrene-b-polyethylene oxide (PS-b-PEO) self-assembly in a polystyrene matrix. For this, three different block copolymers (BCP) were employed: L-BCP (PS = 32 000 Da and PEO = 11 000 Da), M-BCP, (PS = 59 000 Da and PEO = 31 000 Da), and H-BCP, (PS = 102 000 Da and PEO = 34 000 Da). The formation of 1D nanocrystals starts with the reaction-induced microphase separation of BCP during styrene photopolymerization at room temperature. Then, as matrix polymerizes, the primary crystalline micelles aggregate via epitaxial crystallization by end-to-end coupling. The morphology of the resulting nanocrystals was highly dependent on the BCP employed. While L-BCP self-assembles into 1D ribbon-like nanocrystals, M-BCP macro-phase separates and, H-BCP self-assembles into short disk-like nanocrystals. This dissimilar behavior was mainly associated to the length of the stabilizing corona block. In the case of H-BCP, it was found that 1D self-assembly occurred when the conditions for core thickening were given, that is, when a non-reactive period was introduced in the cure cycle. During such a period, core thickening clears the lateral surface of the nanocrystals, allowing end-to-end coupling. The driving force for crystallization was also modified. An increase in undercooling resulted in an elevated nucleation rate and accelerated crystal growth. This led to a narrower size distribution of shorter 1D nanocrystals. This knowledge will enable the synthesis of customized 1D nanocrystals in a thermoplastic matrix, through the precise selection of the BCP formulation and curing conditions.

Construction of Covalent Organic Frameworks via Multicomponent Reactions

Multicomponent reactions (MCRs) combine at least three reactants to afford the desired product in a highly atom-economic way and are therefore viewed as efficient one-pot combinatorial synthesis tools allowing one to significantly boost molecular complexity and diversity. Nowadays, MCRs are no longer confined to organic synthesis and have found applications in materials chemistry. In particular, MCRs can be used to prepare covalent organic frameworks (COFs), which are crystalline porous materials assembled from organic monomers and exhibit a broad range of properties and applications. This synthetic approach retains the advantages of small-molecule MCRs, not only strengthening the skeletal robustness of COFs, but also providing additional driving forces for their crystallization, and has been used to prepare a series of robust COFs with diverse applications. The present perspective article provides the general background for MCRs, discusses the types of MCRs employed for COF synthesis to date, and addresses the related critical challenges and future perspectives to inspire the MCR-based design of new robust COFs and promote further progress in this emerging field.