High-Strength, Microporous, Two-Dimensional Polymer Thin Films with Rigid Benzoxazole Linkage

Facial synthesis of fluorine-engineered magnetic covalent organic framework for selective and ultrasensitive determination of fipronil, its metabolites and analogs in food samples

To improve the adsorption affinity and selectivity of fipronils (FPNs), including fipronil, its metabolites and analogs, a magnetic covalent organic framework (Fe3O4@COF-F) with copious fluorine affinity sites was innovatively designed as an adsorbent of magnetic solid-phase extraction (MSPE). The enhanced surface area, pore size, crystallinity of Fe3O4@COF-F and its exponential adsorption capacities (187.3-231.5 mg g-1) towards fipronils were investigated. Combining MSPE with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), an analytical method was established for the selective determination of fipronils in milk and milk powder samples. This method achieved high sensitivity (LODs: 0.004-0.075 ng g-1), satisfactory repeatability and accuracy with spiked recoveries ranging from 89.9% to 100.3% (RSDs≤5.1%). Overall, the constructed Fe3O4@COF-F displayed great potential for the selective enrichment of fipronils, which could be ascribed to fluorine‑fluorine interaction. This method proposed a feasible and promising strategy for the development of functionalized COF and broadened its application in fluorine containing hazards detection.

Rigid covalent organic frameworks with thiazole linkage to boost oxygen activation for photocatalytic water purification

Owing to their capability to produce reactive oxygen species (ROS) under solar irradiation, covalent organic frameworks (COFs) with pre-designable structure and unique architectures show great potentials for water purification. However, the sluggish charge separation, inefficient oxygen activation and poor structure stability in COFs restrict their practical applications to decontaminate water. Herein, via a facile one-pot synthetic strategy, we show the direct conversion of reversible imine linkage into rigid thiazole linkage can adjust the π-conjugation and local charge polarization of skeleton to boost the exciton dissociation on COFs. The rigid linkage can also improve the robustness of skeleton and the stability of COFs during the consecutive utilization process. More importantly, the thiazole linkage in COFs with optimal C 2p states (COF-S) effectively increases the activities of neighboring benzene unit to directly modulate the O2-adsorption energy barrier and improve the ROS production efficiency, resulting in the excellent photocatalytic degradation efficiency of seven toxic emerging contaminants (e.g. degrading ~99% of 5 mg L-1 paracetamol in only 7 min) and effective bacterial/algal inactivation performance. Besides, COF-S can be immobilized in continuous-flow reactor and in enlarged reactor to efficiently eliminate pollutants under natural sunlight irradiation, demonstrating the feasibility for practical application.

Tunable 2D Conjugated Porous Organic Polymer Films for Precise Molecular Nanofiltration and Optoelectronics

Structural design of 2D conjugated porous organic polymer films (2D CPOPs), by tuning linkage chemistries and pore sizes, provides great adaptability for various applications, including membrane separation. Here, four free-standing 2D CPOP films of imine- or hydrazone-linked polymers (ILP/HLP) in combination with benzene (B-ILP/HLP) and triphenylbenzene (TPB-ILP/HLP) aromatic cores are synthesized. The anisotropic disordered films, composed of polymeric layered structures, can be exfoliated into ultrathin 2D-nanosheets with layer-dependent electrical properties. The bulk CPOP films exhibit structure-dependent optical properties, triboelectric nanogenerator output, and robust mechanical properties, rivaling previously reported 2D polymers and porous materials. The exfoliation energies of the 2D CPOPs and their mechanical behavior at the molecular level are investigated using density function theory (DFT) and molecular dynamics (MD) simulations, respectively. Exploiting the structural tunability, the comparative organic solvent nanofiltration (OSN) performance of six membranes having different pore sizes and linkages to yield valuable trends in molecular weight selectivity is investigated. Interestingly, the OSN performances follow the predicted transport modeling values based on theoretical pore size calculations, signifying the existence of permanent porosity in these materials. The membranes exhibit excellent stability in organic solvents at high pressures devoid of any structural deformations, revealing their potential in practical OSN applications.

Changing Benzoxazole Ring into Nonring Imine Linkages on Covalent Organic Frameworks with Tuning H2O2 Photosynthesis Performance

Two π-conjugated covalent organic frameworks (COFs) with nonring imine or benzoxazole ring linkages were prepared by reacting 3,3'-dihydrooxybenzidine (BDOH) with 3,5-triformylbenzene (Tb) in the presence or absence of benzimidazole (BDOH-Tb-IM and BDOH-Tb-BO). Although two COFs indicated similar composition, crystalline structures, and morphologies, imine-based BDOH-Tb-IM exhibited a photocatalytic H2O2 production rate of 2490 μmol·g-1·h-1 in sacrificial reagent-free pure water, higher than that of benzoxazole-based BDOH-Tb-BO-a (1168 μmol·g-1·h-1). The higher photocatalytic activity of BDOH-Tb-IM was attributed to its more efficient photoinduced charge separation and utilization efficiency and different 2e- ORR active sites over the two COFs. This study demonstrated an available ring effect to adjust photocatalytic performance between π-conjugated COFs.

Imine-Linked 2D Conjugated Porous Organic Polymer Films for Tunable Acid Vapor Sensing

Two-dimensional (2D) films of conjugated porous organic polymers (C-POPs) can translate the rich in-plane functionalities of conjugated frameworks into diverse optical and electronic applications while addressing the processability issues of their crystalline analogs for adaptable device architectures. However, the lack of facile single-step synthetic routes to obtain large-area high-quality films of 2D-C-POPs has limited their application possibilities so far. Here, we report the synthesis of four mechanically robust imine-linked 2D-C-POP free-standing films using a single-step fast condensation route that is scalable and tunable. The rigid covalently bonded 2D structures of the C-POP films offer high stability for volatile gas sensing in harsh environments while simultaneously enhancing site accessibility for gas molecules due to mesoporosity by structural design. Structurally, all films were composed of exfoliable layers of 2D polymeric nanosheets (NSs) that displayed anisotropy from disordered stacking, evinced by out-of-plane birefringent properties. The tunable in-plane conjugation, different nitrogen centers, and porous structures allow the films to act as ultraresponsive colorimetric sensors for acid sensing via reversible imine bond protonation. All the films could detect hydrogen chloride (HCl) gas down to 0.05 ppm, far exceeding the Occupational Safety and Health Administration's permissible exposure limit of 5 ppm with fast response time and good recyclability. Computational insights elucidated the effect of conjugation and tertiary nitrogen in the structures on the sensitivity and response time of the films. Furthermore, we exploited the exfoliated large 2D NSs and anisotropic optoelectronic properties of the films to adapt them into micro-optical and triboelectric devices to demonstrate their real-time sensing capabilities.

Catalytic Linkage Engineering of Covalent Organic Frameworks for the Oxygen Reduction Reaction

Metal-free covalent organic frameworks (COFs) have been employed to catalyze the oxygen reduction reaction (ORR). To achieve high activity and selectivity, various building blocks containing heteroatoms and groups linked by imine bonds were used to create catalytic COFs. However, the roles of linkages of COFs in ORR have not been investigated. In this work, the catalytic linkage engineering has been employed to modulate the catalytic behaviors. To create single catalytic sites while avoiding other possible catalytic sites, we synthesized COFs from benzene units linked by various bonds, such as imine, amide, azine, and oxazole bonds. Among these COFs, the oxazole-linkage in COFs enables to catalyze the ORR with the highest activity, which achieved a half-wave potential of 0.75 V and a limited current density of 5.5 mA cm-2 . Moreover, the oxazole-linked COF achieved a conversion frequency (TOF) value of 0.0133 S-1 , which were 1.9, 1.3, and 7.4-times that of azine-, amide- and imine-COFs, respectively. The theoretical calculation showed that the carbon atoms in oxazole linkages facilitated the formation of OOH* and promoted protonation of O* to form the OH*, thus advancing the catalytic activity. This work guides us on which linkages in COFs are suitable for ORR.

2D Covalent Organic Frameworks Based on Heteroacene Units

Covalent organic frameworks (COFs) are a unique new class of porous materials that arrange building units into periodic ordered frameworks through strong covalent bonds. Accompanied with structural rigidity and well-defined geometry, heteroacene-based COFs have natural advantages in constructing COFs with high stability and crystallinity. Heteroacene-based COFs usually have high physical and chemical properties, and their extended π-conjugation also leads to relatively low energy gap, effectively promoting π-electron delocalization between network units. Owing to excellent electron-withdrawing or -donating ability, heteroacene units have incomparable advantages in the preparation of donor-acceptor type COFs. Therefore, the physicochemical robust and fully conjugated heteroacene-based COFs solve the problem of traditional COFs lacking π-π interaction and chemical stability. In recent years, significant breakthroughs are made in this field, the choice of various linking modes and building blocks has fundamentally ensured the final applications of COFs. It is of great significance to summarize the heteroacene-based COFs for improving its complexity and controllability. This review first introduces the linkages in heteroacene-based COFs, including reversible and irreversible linkages. Subsequently, some representative building blocks are summarized, and their related applications are especially emphasized. Finally, conclusion and perspectives for future research on heteroacene-based COFs are presented.

Direct Construction of Isomeric Benzobisoxazole-Vinylene-Linked Covalent Organic Frameworks with Distinct Photocatalytic Properties

Vinylene/olefin-linked two-dimensional covalent organic frameworks (v-2D-COFs) have emerged as advanced semiconducting materials with excellent in-plane conjugation, high chemical stabilities, and precisely tunable electronic structures. Exploring new linkage chemistry for the reticular construction of v-2D-COFs remains in infancy and challenging. Herein, we present a solid-state benzobisoxazole-mediated aldol polycondensation reaction for the construction of two novel isomeric benzobisoxazole-bridged v-2D-COFs (v-2D-COF-NO1 and v-2D-COF-NO2) with trans and cis configurations of benzobisoxazole. Interestingly, the isomeric benzobisoxazole linkers endow the two v-2D-COFs with distinct optoelectronic and electrochemical properties, ranging from light absorption and emission to charge-transfer properties. When employed as the photocathode, v-2D-COF-NO1 exhibits a photocurrent of up to ∼18 μA/cm² under AM 1.5G irradiation at -0.3 V vs reversible hydrogen electrode (RHE), which is twice the value of v-2D-COF-NO2 (∼9.1 μA/cm²). With Pt as a cocatalyst, v-2D-COF-NO1 demonstrates a photocatalytic hydrogen evolution rate of ∼1.97 mmol h^-1 g^-1, also in clear contrast to that of v-2D-COF-NO2 (∼0.86 mmol h^-1 g^-1) under identical conditions. This work demonstrates the synthesis of v-2D-COFs via benzobisoxazole-mediated aldol polycondensation with isomeric structures and distinct photocatalytic properties.