Ultrafast Proton Conduction in an Aqueous Electrolyte Confined in Adamantane-like Micropores of a Sulfonated, Aromatic Framework

Efficient construction of high-quality sulfonated porous aromatic frameworks by optimizing the swelling state of porous structures

Conventional post-modification methods usually face the fundamental challenge of balancing the high content of functional groups and large surface area for porous organic polymers (POPs). The reason, presumably, stems from ineffective and insufficient swelling of the porous structure of POP materials, which is detrimental to mass transfer and modification of functional groups, especially with large-sized ones. It is important to note that significant differences exist in the porous structures of POP materials in a solvent-free state after thermal activation and solvent swelling state. Herein, we propose that the improvement of the swelling state of the porous structure of POP materials is more conducive to obtaining high-quality sulfonated POP materials, and employ a one-pot modification strategy for preparing sulfonated porous aromatic frameworks (PAFs) to prove the proposal. These results show that the specific surface area of the resulting polymer is 580 m2 g-1 with a sulfur content of up to 13.2 wt%, which is superior to most sulfonated porous materials and the control sample. More importantly, we have also shown that the same conclusion is reached by performing similar treatments on hyper-crosslinked polymers (HCPs) and conjugated microporous polymers (CMPs), proving that our hypothesis is effective and feasible when compared to the conventional post-sulfonation method. The excellent hydrophilicity, rich content of sulfonic acid groups, high specific surface area and hierarchical pore structure make the resulting polymer an ideal adsorbent for micro-pollutants in water. The maximum adsorption capacities for Rhodamine B (RhB), Methylene Blue (MB), Tetracycline (TC) and Ciprofloxacin (CIP) are 1075 mg g-1, 1020 mg g-1, 826 mg g-1 and 1134 mg g-1, respectively. This study not only demonstrates the preparation of efficient sulfonated porous adsorbents for the efficient removal of cationic dyes and antibiotics but also illustrates an effective method for constructing high-quality functional POP materials by optimizing the swelling state of the porous structure.

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.

Structures and Dynamics of Complex Guest Molecules in Confinement, Revealed by Solid-State NMR, Molecular Dynamics, and Calorimetry

This review gives an overview of current trends in the investigation of confined molecules such as water, small and higher alcohols, carbonic acids, ethylene glycol, and non-ionic surfactants, such as polyethylene glycol or Triton-X, as guest molecules in neat and functionalized mesoporous silica materials employing solid-state NMR spectroscopy, supported by calorimetry and molecular dynamics simulations. The combination of steric interactions, hydrogen bonds, and hydrophobic and hydrophilic interactions results in a fascinating phase behavior in the confinement. Combining solid-state NMR and relaxometry, DNP hyperpolarization, molecular dynamics simulations, and general physicochemical techniques, it is possible to monitor these confined molecules and gain deep insights into this phase behavior and the underlying molecular arrangements. In many cases, the competition between hydrogen bonding and electrostatic interactions between polar and non-polar moieties of the guests and the host leads to the formation of ordered structures, despite the cramped surroundings inside the pores.

Thermodynamically Stable Functionalization of Microporous Aromatic Frameworks with Sulfonic Acid Groups by Inserting Methylene Spacers

Porous aromatic frameworks (PAFs) are an auspicious class of materials that allow for the introduction of sulfonic acid groups at the aromatic core units by post-synthetic modification. This makes PAFs promising for proton-exchange materials. However, the limited thermal stability of sulfonic acid groups attached to aromatic cores prevents high-temperature applications. Here, we present a framework based on PAF-303 where the acid groups were added as methylene sulfonic acid side chains in a two-step post-synthetic route (SMPAF-303) via the intermediate chloromethylene PAF (ClMPAF-303). Elemental analysis, NMR spectroscopy, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy were used to characterize both frameworks and corroborate the successful attachment of the side chains. The resulting framework SMPAF-303 features high thermal stability and an ion-exchange capacity of about 1.7 mequiv g-1. The proton conductivity depends strongly on the adsorbed water level. It reaches from about 10-7 S cm-1 for 33% RH to about 10-1 S cm-1 for 100% RH. We attribute the strong change to a locally alternating polarity of the inner surfaces. The latter introduces bottleneck effects for the water molecule and oxonium ion diffusion at lower relative humidities, due to electrolyte clustering. When the pores are completely filled with water, these bottlenecks vanish, leading to an unhindered electrolyte diffusion through the framework, explaining the conductivity rise.