Thermodynamic, kinetic, and structural factors in the synthesis of imine-linked dynamic covalent frameworks

Dynamic and solid-state behaviour of bromoisotrianglimine

Solid-state materials formed from discrete imine macrocycles have potential in industrial separations, but dynamic behaviour during both synthesis and crystallisation makes them challenging to exploit. Here, we explore opportunities for structural control by investigating the dynamic nature of a C-5 brominated isotrianglimine in solution and under crystallisation conditions. In solution, the equilibrium between the [3 + 3] and the less reported [2 + 2] macrocycle was investigated, and both macrocycles were fully characterised. Solvent templating during crystallisation was used to form new packing motifs for the [3 + 3] macrocycle and a previously unreported [4 + 4] macrocycle. Finally, chiral self-sorting was used to demonstrate how crystallisation conditions can not only influence packing arrangements but also shift the macrocycle equilibrium to yield new structures. This work thus exemplifies three strategies for exploiting dynamic behaviour to form isotrianglimine materials, and highlights the importance of understanding the dynamic behaviour of a system when designing and crystallising functional materials formed using dynamic covalent chemistry.

Unraveling Early-Stage Dynamics of Cage-to-Covalent Organic Framework Transformation at Liquid-Liquid Interface

Postsynthetic linker exchange (PLE) has emerged as an emerging synthetic strategy for constructing high-quality covalent organic frameworks (COFs) from preassembled entities such as linear polymers, amorphous networks, COFs, and porous organic cages by using the principles of dynamic covalent chemistry. The PLE strategy has recently been extended at the liquid-liquid interface to fabricate highly crystalline two-dimensional (2D)-COF membranes at a faster time scale (24 h). Examining the early stages of the interfacial PLE dynamics becomes essential to understanding the expedited COF growth process. In this regard, pendant drop tensiometry has been employed to probe the initial reaction dynamics of the imine cage-to-COF transformation through dynamic interfacial tension (IFT) measurements. The contrasting trends in IFT profiles between PLE-mediated (from cage) and direct COF synthesis (from parent monomers) are in qualitative agreement with the kinetics of bulk-scale interfacial polymerizations. Further, the distinct early-stage interfacial behaviors between the diverse synthetic routes have been experimentally demonstrated using tensiometry, optical microscopy, electron microscopy, and powder X-ray diffraction (PXRD) analysis. The pivotal role of in situ generated imine intermediates (ImIs) and the phenomenon of spontaneous emulsification toward accelerated interfacial COF growth process was delineated. The current study on deploying the pendant drop tensiometric technique to examine early-stage interfacial polymerization dynamics opens up a gripping avenue for mechanistic exploration in PLE-based COF synthesis. The generality of the developed methodology to study the initial COF growth kinetics was extended to a new interfacial PLE-mediated cage-to-COF transformation.

Covalent Organic Frameworks Meet Titanium Oxide

In order to expand the applicability of materials and improve their performance, the combined use of different materials has increasingly been explored. Among these materials, inorganic-organic hybrid materials often exhibit properties superior to those of single materials. Covalent organic frameworks (COFs) are famous crystalline porous materials constructed by organic building blocks linked by covalent bonds. In recent years, the combination of COFs with other materials has shown interesting properties in diverse fields, and the composite materials of COFs and TiO2 have been investigated more and more. These two outstanding materials are combined through covalent bonding, physical mixing, and other methods and exhibit excellent performance in various fields, including photocatalysis, electrocatalysis, sensors, separation, and energy storage and conversion. In this Review, the current preparation methods and applications of COF-TiO2 hybrid materials are introduced in detail, and their future development and possible problems are discussed and prospected, which is of great significance for related research. It is believed that these interesting hybrid materials will show greater application value as research progresses.

Adamantane-type clusters: compounds with a ubiquitous architecture but a wide variety of compositions and unexpected materials properties

The research into adamantane-type compounds has gained momentum in recent years, yielding remarkable new applications for this class of materials. In particular, organic adamantane derivatives (AdR4) or inorganic adamantane-type compounds of the general formula [(RT)4E6] (R: organic substituent; T: group 14 atom C, Si, Ge, Sn; E: chalcogenide atom S, Se, Te, or CH2) were shown to exhibit strong nonlinear optical (NLO) properties, either second-harmonic generation (SHG) or an unprecedented type of highly-directed white-light generation (WLG) - depending on their respective crystalline or amorphous nature. The (missing) crystallinity, as well as the maximum wavelengths of the optical transitions, are controlled by the clusters' elemental composition and by the nature of the organic groups R. Very recently, it has been additionally shown that cluster cores with increased inhomogeneity, like the one in compounds [RSi{CH2Sn(E)R'}3], not only affect the chemical properties, such as increased robustness and reversible melting behaviour, but that such 'cluster glasses' form a conceptually new basis for their use in light conversion devices. These findings are likely only the tip of the iceberg, as beside elemental combinations including group 14 and group 16 elements, many more adamantane-type clusters (on the one hand) and related architectures representing extensions of adamantane-type clusters (on the other hand) are known, but have not yet been addressed in terms of their opto-electronic properties. In this review, we therefore present a survey of all known classes of adanmantane-type compounds and their respective synthetic access as well as their optical properties, if reported.

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine-based Covalent Organic Frameworks

Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized.

Conducting Silicone-Based Polymers and Their Application

Over the past two decades, both fundamental and applied research in conducting polymers have grown rapidly. Conducting polymers (CPs) are unique due to their ease of synthesis, environmental stability, and simple doping/dedoping chemistry. Electrically conductive silicone polymers are the current state-of-the-art for, e.g., optoelectronic materials. The combination of inorganic elements and organic polymers leads to a highly electrically conductive composite with improved thermal stability. Silicone-based materials have a set of extremely interesting properties, i.e., very low surface energy, excellent gas and moisture permeability, good heat stability, low-temperature flexibility, and biocompatibility. The most effective parameters constructing the physical properties of CPs are conjugation length, degree of crystallinity, and intra- and inter-chain interactions. Conducting polymers, owing to their ease of synthesis, remarkable environmental stability, and high conductivity in the doped form, have remained thoroughly studied due to their varied applications in fields like biological activity, drug release systems, rechargeable batteries, and sensors. For this reason, this review provides an overview of organosilicon polymers that have been reported over the past two decades.

Tetrahedral Silicon-Centered Dibenzoylmethanatoboron Difluorides: Synthesis, Crystal Structure, and Photophysical Behavior in Solution and the Solid State

Four tetrahedral silicon-centered derivatives of dibenzoylmethanatoboron difluoride (DBMBF₂ ) were synthesized and characterized. Their structural and optical features both in solution and the solid state were investigated by using X-ray crystallography, steady-state and time-dependent spectroscopy, and DFT-based calculations. In dilute solutions, the molar absorption coefficient increases from 40500 to 175200 M^-1  cm^-1 as the number of DBMBF₂ fragments in a molecule increases from one to four, while, in contrast, the nonradiative rate constant of fluorescence decay decreases from 0.49 to 0.34. In the solid state, absorption and emission spectra depend on the degree of crystallinity and microcrystal size. The tris-DBMBF₂ derivative forms fully overlapping dimeric structures that exhibit excimer-like fluorescence, which is accurately predicted by the quantum-chemical calculations. The mono-DBMBF₂ derivative exhibits fully reverse mechanofluorochromic behavior.

Design and applications of three dimensional covalent organic frameworks

We summarize in this review the current state-of-the-art development of three dimensional covalent organic frameworks.

Dynamically crosslinked polymer nanocomposites to treat multidrug-resistant bacterial biofilms

Multidrug-resistant biofilms are highly resistant to current antimicrobial therapies. We have developed an antimicrobial platform that integrates the bacteria-killing phytochemical carvacrol into dynamically crosslinked polymer nanocomposites (DCPNs). Taking advantage of a reversibly crosslinked Schiff-base scaffold throughout the engineered emulsions, DCPNs exhibited long-term shelf-life and good stability in serum, while readily disassembling in acidic microenvironments. Furthermore, we demonstrated that DCPNs efficiently penetrate the biofilm matrix, eradicating both Gram-negative/positive bacteria enclosed within. Moreover, DCPNs showed no observable toxicity to fibroblast mammalian cells with the same antimicrobial concentrations necessary to eradicate MDR biofilms. Given their potent antimicrobial and stimuli-responsive dissociation characteristics in a biofilm setting, DCPNs are a suitable therapeutic platform for combating MDR bacterial infections.

Single-crystal x-ray diffraction structures of covalent organic frameworks

The crystallization problem is an outstanding challenge in the chemistry of porous covalent organic frameworks (COFs). Their structural characterization has been limited to modeling and solutions based on powder x-ray or electron diffraction data. Single crystals of COFs amenable to x-ray diffraction characterization have not been reported. Here, we developed a general procedure to grow large single crystals of three-dimensional imine-based COFs (COF-300, hydrated form of COF-300, COF-303, LZU-79, and LZU-111). The high quality of the crystals allowed collection of single-crystal x-ray diffraction data of up to 0.83-angstrom resolution, leading to unambiguous solution and precise anisotropic refinement. Characteristics such as degree of interpenetration, arrangement of water guests, the reversed imine connectivity, linker disorder, and uncommon topology were deciphered with atomic precision—aspects impossible to determine without single crystals.

Imine-linked micron-network polymers with high polyethylene glycol uptake for shaped-stabilized phase change materials

Micron-network polymers with high free interstitial space show high adsorption of PEG (up to 85 wt%) for shape-stabilized phase change materials with high energy storage density.

Dynamic combinatorial/covalent chemistry: a tool to read, generate and modulate the bioactivity of compounds and compound mixtures

Reversible covalent bond formation under thermodynamic control adds reactivity to self-assembled supramolecular systems, and is therefore an ideal tool to assess complexity of chemical and biological systems. Dynamic combinatorial/covalent chemistry (DCC) has been used to read structural information by selectively assembling receptors with the optimum molecular fit around a given template from a mixture of reversibly reacting building blocks. This technique allows access to efficient sensing devices and the generation of new biomolecules, such as small molecule receptor binders for drug discovery, but also larger biomimetic polymers and macromolecules with particular three-dimensional structural architectures. Adding a kinetic factor to a thermodynamically controlled equilibrium results in dynamic resolution and in self-sorting and self-replicating systems, all of which are of major importance in biological systems. Furthermore, the temporary modification of bioactive compounds by reversible combinatorial/covalent derivatisation allows control of their release and facilitates their transport across amphiphilic self-assembled systems such as artificial membranes or cell walls. The goal of this review is to give a conceptual overview of how the impact of DCC on supramolecular assemblies at different levels can allow us to understand, predict and modulate the complexity of biological systems.