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

Metal-organic frameworks for organic transformations by photocatalysis and photothermal catalysis

Organic transformation by light-driven catalysis, especially, photocatalysis and photothermal catalysis, denoted as photo(thermal) catalysis, is an efficient, green, and economical route to produce value-added compounds. In recent years, owing to their diverse structure types, tunable pore sizes, and abundant active sites, metal-organic framework (MOF)-based photo(thermal) catalysis has attracted broad interest in organic transformations. In this review, we provide a comprehensive and systematic overview of MOF-based photo(thermal) catalysis for organic transformations. First, the general mechanisms, unique advantages, and strategies to improve the performance of MOFs in photo(thermal) catalysis are discussed. Then, outstanding examples of organic transformations over MOF-based photo(thermal) catalysis are introduced according to the reaction type. In addition, several representative advanced characterization techniques used for revealing the charge reaction kinetics and reaction intermediates of MOF-based organic transformations by photo(thermal) catalysis are presented. Finally, the prospects and challenges in this field are proposed. This review aims to inspire the rational design and development of MOF-based materials with improved performance in organic transformations by photocatalysis and photothermal catalysis.

Fascinating isomeric covalent organic frameworks

Isomeric covalent organic frameworks possessing the same chemical constitutions but different atomic arrangement structures and physicochemical properties are fascinating branches of covalent organic frameworks (COFs). However, the rational design and targeted synthesis of isomeric COFs remain conundrums, so the investigation of isomeric COFs is still in a fledging period. According to the diversity of frameworks, positional isomers with similar structures and framework isomers having distinct constructions are the main existing subspecies of isomeric COFs. In this review, we focus on the research progress and substantial achievements in this fascinating embranchment and systematically summarize and highlight the design principles of both positional isomeric and framework isomeric COFs, which will potentially facilitate further exploitation and investigation of novel isomeric COFs. The application and structure-property relationship of these isomeric COFs have been briefly introduced. Moreover, key constraints of current isomeric COFs and further advancement of this promising field are proposed and anticipated.

Targeted Synthesis of Isomeric Naphthalene-Based 2D Kagome Covalent Organic Frameworks

Targeted synthesis of kagome (kgm) topologic 2D covalent organic frameworks remains challenging, presumably due to the severe dependence on building units and synthetic conditions. Herein, two isomeric "two-in-one" monomers with different lengths of substituted arms based on naphthalene core (p-Naph and m-Naph) are elaborately designed and utilized for the defined synthesis of isomeric kgm Naph-COFs. The two isomeric frameworks exhibit splendid crystallinity and showcase the same chemical composition and topologic structure with, however, different pore channels. Interestingly, C₆₀ is able to uniformly be encapsulated into the triangle channels of m-Naph-COF via in situ incorporation method, while not the isomeric p-Naph-COF, likely due to the different pore structures of the two isomeric COFs. The resulting stable C₆₀ @m-Naph-COF composite exhibits much higher photoconductivity than the m-Naph-COF owing to charge transfer between the conjugated skeletons and C₆₀ guests.

Isomeric Oligo(Phenylenevinylene)-Based Covalent Organic Frameworks with Different Orientation of Imine Bonds and Distinct Photocatalytic Activities

Imine-linked covalent organic frameworks (COFs) have been extensively studied in photocatalysis because of their easy synthesis and excellent crystallinity. The effect of imine-bond orientation on the photocatalytic properties of COFs, however, is still rarely studied. Herein, we report two novel COFs with different orientations of imine bonds using oligo(phenylenevinylene) moieties. The COFs showed similar structures but great differences in their photoelectric properties. COF-932 demonstrated a superior hydrogen evolution performance compared to COF-923 when triethanolamine was used as the sacrificial agent. Interestingly, the use of ascorbic acid led to the protonation of the COFs, further altering the direction of electron transfer. The photocatalytic performances were increased to 23.4 and 0.73 mmol g^-1  h^-1 for protonated COF-923 and COF-932, respectively. This study provides a clear strategy for the design of imine-linked COF-based photocatalysts and advances the development of COFs.

Triangular Topological 2D Covalent Organic Frameworks Constructed via Symmetric or Asymmetric "Two-in-One" Type Monomers

Most of the reported covalent organic frameworks (COFs) so far are prepared from highly symmetric building blocks, which to some extent limits the expansion of COF diversity and complexity. Low-symmetric building blocks can be designed through a desymmetrized vertex strategy, which might be used to construct new topological COFs. But reports of COFs constructed by asymmetric building blocks are thus far very rare. Here, a feasible strategy to design asymmetric building blocks for COF synthesis is introduced, by simply varying the positions of functional groups in the monomer. As a proof of concept, two isomeric hexaphenylbenzene-based "two-in-one" type monomers (1,2,4-HPB-NH₂ and 1,3,5-HPB-NH₂ ) are designed and synthesized. To the authors' surprise, self-polycondensation of the asymmetric 1,2,4-HPB-NH₂ (i.e., the isomer of common C₃ -symmetric 1,3,5-HPB-NH₂ ) also affords highly crystalline COF (1,2,4-HPB-COF) similar to the symmetric 1,3,5-HPB-NH₂ counterpart with identical topological structure. The triangular porous structures of both HPB-based COFs are well resolved by powder X-ray diffraction (PXRD), theoretical simulations, nitrogen sorption, and morphologies analysis. This work demonstrates the "two-in-one" type asymmetric building blocks can also produce highly crystalline frameworks and thus provides a new structural design strategy for reticular chemistry.

Highly conjugated two-dimensional covalent organic frameworks for efficient iodine uptake

Radioactive iodine in nuclear waste would be harmful to nature and human health. The design of adsorbents for iodine capture with high efficiency still remains a challenge. Herein, two highly conjugated two-dimensional covalent organic frameworks (TFPB-BPTA-COF and TFPB-PyTTA-COF) have been successfully constructed. Both COFs possess high porosity, stability, and high π-conjugated framework. Impressively, TFPB-PyTTA-COF exhibits an excellent iodine uptake value up to 5.6 g g -1 , which is superior to most of reported COF-based adsorbents for iodine capture.

Light-Driven Molecular Motors Embedded in Covalent Organic Frameworks

The incorporation of molecular machines into the backbone of porous framework structures will facilitate nano actuation, enhanced molecular transport, and other out-of-equilibrium host-guest phenomena in well-defined 3D solid materials. In...