Three-Dimensional Chemically Stable Covalent Organic Frameworks through Hydrophobic Engineering

The development of three-dimensional (3D) covalent organic frameworks (COFs) with high chemical stability is of critical importance for their practical use. In this work, it is demonstrated that the stability of 3D COFs can be improved by periodic decoration of isopropyl groups on their backbones. Owing to the strong hydrophobicity of the alkyl groups, the resultant COFs show high crystallinity, permanent pores, and exceptional stability in harsh environments, such as strong acids (3 m HCl or 3 m H₂ SO₄ for one week), a strong base (20 m NaOH for one week), and boiling water (100 °C for one month). Furthermore, these highly stable and hydrophobic COFs display excellent oil/water separation performance with >99 % separation efficiency over a wide pH range. This work demonstrates the use of alkyl decoration in 3D COFs to tune their chemical stability and expand their potential applications.

Four-dimensional metal-organic frameworks

Recognising timescale as an adjustable dimension in porous solids provides a new perspective to develop novel four-dimensional framework materials. The deliberate design of three-dimensional porous framework architectures is a developed field; however, the understanding of dynamics in open frameworks leaves a number of key questions unanswered: What factors determine the spatiotemporal evolution of deformable networks? Can we deliberately engineer the response of dynamic materials along a time-axis? How can we engineer energy barriers for the selective recognition of molecules? Answering these questions will require significant methodological development to understand structural dynamics across a range of time and length scales.

Metal-Covalent Organic Frameworks (MCOFs): A Bridge Between Metal-Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal-organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal-covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.

Covalent organic framework photocatalysts: structures and applications

In the light of increasing energy demand and environmental pollution, it is urgently required to find a clean and renewable energy source. In these years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or environmental pollutant degradation, has shown great potential to achieve this goal. Among the various photocatalysts, covalent organic frameworks (COFs) are very attractive due to their excellent structural regularity, robust framework, inherent porosity and good activity. Thus, many studies have been carried out to investigate the photocatalytic performance of COFs and COF-based photocatalysts. In this critical review, the recent progress and advances of COF photocatalysts are thoroughly presented. Furthermore, diverse linkers between COF building blocks such as boron-containing connections and nitrogen-containing connections are summarised and compared. The morphologies of COFs and several commonly used strategies pertaining to photocatalytic activity are also discussed. Following this, the applications of COF-based photocatalysts are detailed including photocatalytic hydrogen evolution, CO₂ conversion and degradation of environmental contaminants. Finally, a summary and perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts are given.

Single crystal of a one-dimensional metallo-covalent organic framework

Although polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesised directly from solution. One-dimensional (1D) covalent polymers that are packed into a framework structure can be viewed as a 1D covalent organic framework (COF), but making a single crystal of this has been elusive. Herein, by combining labile metal coordination and dynamic covalent chemistry, we discover a strategy to synthesise single-crystal metallo-COFs under solvothermal conditions. The single-crystal structure is rigorously solved using single-crystal electron diffraction technique. The non-centrosymmetric metallo-COF allows second harmonic generation. Due to the presence of syntactic pendant amine groups along the polymer chains, the metallopolymer crystal can be further cross-linked into a crystalline woven network.

Although polymers have been studied for well over a century, there are few examples of covalently linked polymer crystals synthesized directly from solution. Here, the authors demonstrate a strategy to synthesize single crystalline 1D metallo-covalent organic frameworks by combining dynamic covalent chemistry and metal-ligand coordination.

Rapid, Scalable Construction of Highly Crystalline Acylhydrazone Two-Dimensional Covalent Organic Frameworks via Dipole-Induced Antiparallel Stacking

Covalent organic frameworks are an emerging class of porous crystalline organic materials that can be designed and synthesized from the bottom up. Despite progress made in synthesizing COFs of diverse topologies, the synthesis methods are often tedious and unscalable, hampering practical applications. Herein, we demonstrate a scalable, robust method of producing highly crystalline acylhydrazone two-dimensional (2D) COFs with diversified structures (six examples) under open and stirred conditions, with growth typically completed in only 30 min. Our strategy involves selecting molecular building blocks that have bond dipole moments with spatial orientations that favor antiparallel stacking and whose structure allows the restriction of intramolecular bond rotation (RIR) via intra- and interlayer hydrogen bonding. This method is widely applicable for hydrazide linkers containing various side-chain functionalities and topicities. By this strategy, the gram-scale synthesis of two highly crystalline COFs (up to 1.4 g yield) was obtained in a one-pot reaction within 30 min.

Crystallographic descriptions of regular 2-periodic weavings of threads, loops and nets

Piecewise linear descriptions are presented of weavings of threads, loops and 2-periodic nets. Crystallographic data are provided for regular structures, defined as those with one kind (symmetry-related) of vertex (corner) and edge (stick). These include infinite families of biaxial thread weaves, interwoven square lattices (sql), honeycomb (hcb) nets, and tetragonal and hexagonal polycatenanes.

Modular Total Synthesis in Reticular Chemistry

The idea of modularity in organic total synthesis has promoted the construction of diverse targeted natural products by varying the building blocks and assembly sequences. Yet its utilization has been mainly limited to the synthesis of molecular compounds based on covalent bonds. In this work, we expand the conceptual scope of modular synthesis into framework materials, which bridges metal- and covalent organic frameworks (MOFs and COFs) hierarchically in reticular chemistry. While the assembly sequences are determined by the coordination or the covalent bond strengths, a modular synthesis strategy which progressively links simple building blocks into increasingly sophisticated superstructures was reported. As a result, a series of hierarchical COF-on-MOF structures with architectural intricacy were obtained through sequence-defined reactions of diverse building blocks. The tunability of spatial apportionment, compositions, and functionality was successfully managed in these framework materials. To the best of our knowledge, this is the first report on the synthesis of COF@MOF composites and also the first discovery of controlled COF alignment. This generalizable modularity strategy will not only accelerate the discovery of multicomponent framework materials by the hierarchical assembly of MOFs and COFs but also offer a predictable retrosynthetic route to smart materials with unusual tunability owing to the diverse inorganic or organic building units.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Polymer Networks: From Plastics and Gels to Porous Frameworks

Polymer networks, which are materials composed of many smaller components-referred to as "junctions" and "strands"-connected together via covalent or non-covalent/supramolecular interactions, are arguably the most versatile, widely studied, broadly used, and important materials known. From the first commercial polymers through the plastics revolution of the 20^th century to today, there are almost no aspects of modern life that are not impacted by polymer networks. Nevertheless, there are still many challenges that must be addressed to enable a complete understanding of these materials and facilitate their development for emerging applications ranging from sustainability and energy harvesting/storage to tissue engineering and additive manufacturing. Here, we provide a unifying overview of the fundamentals of polymer network synthesis, structure, and properties, tying together recent trends in the field that are not always associated with classical polymer networks, such as the advent of crystalline "framework" materials. We also highlight recent advances in using molecular design and control of topology to showcase how a deep understanding of structure-property relationships can lead to advanced networks with exceptional properties.

Aurophilicity-Mediated Construction of Emissive Porous Molecular Crystals as Versatile Hosts for Liquid and Solid Guests

The first examples of porous molecular crystals that are assembled through Au⋅⋅⋅Au interactions of gold complex 1 are here reported along with their exchange properties with respect to their guest components. Single-crystal X-ray diffraction (XRD) analyses indicate that the crystal structure of 1/CH₂ Cl₂ ⋅pentane is based on cyclic hexamers of 1, which are formed through six Au⋅⋅⋅Au interactions. The packing of these cyclic hexamers affords a porous architecture, in which the one-dimensional channel segment contains CH₂ Cl₂ and pentane as guests. These guests can be exchanged through operationally simple methods under retention of the host framework of 1, which furnished 1/guest complexes with 26 different guests. A single-crystal XRD analysis of 1/eicosane, which contains the long linear alkane eicosane (n-C₂₀ H₄₂ ), successfully provided its accurately modeled structure within the porous material. These host-guest complexes show chromic luminescence with both blue- and redshifted emissions. Moreover, this porous organometallic material can exhibit luminescent mechanochromism through release of guests.

Untying the Photophysics of Quinolinium-Based Molecular Knots and Links

Complex molecular knots and links are still difficult to synthesize and the properties arising from their topology are mostly unknown. Here, we report on a comparative photophysical study carried out on a family of closely related quinolinium-based knots and links to determine the impact exerted by topology on the molecular backbone. Our results indicate that topology has a negligible influence on the behavior of loosely braided molecules, which mostly behave like their unbraided equivalents. On the other hand, tightly braided molecules display distinct features. Their higher packing density results in a pronounced ability to resist deformation, a significant reduction in the solvent-accessible surface area and favors close-range π-π interactions between the quinolinium units and neighboring aromatics. Finally, the sharp alteration in behavior between loosely and tightly braided molecules sheds light on the factors contributing to braiding tightness.

A homochiral 3D framework of mechanically interlocked 1D loops with solvent-dependent spin-state switching behaviors

An atypical homochiral spin-crossover (SCO) framework (1) constructed from mechanically interlocked 1D molecular loops was prepared. Due to the flexibility of the interlocked structure, the guest solvent molecules can be reversibly exchanged. Consequently, its SCO behavior was capable of modulating between one- and two-stepped transitions in response to acetonitrile and methanol.

Solving the COF trilemma: towards crystalline, stable and functional covalent organic frameworks

Strategies in covalent organic frameworks and adjacent fields are highlighted for designing stable, ordered and functional materials.

Selective extraction and determination of aromatic amine metabolites in urine samples by using magnetic covalent framework nanocomposites and HPLC-MS/MS

Purification and selective enrichment of ultra-low level metabolites in bio-samples is very important for HPLC-MS/MS analysis. A magnetic covalent organic framework (i.e. COFs) (TpPa-1) (i.e. 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde-p-phenylenediamine) material was synthesized and used for a magnetic solid phase extraction (MSPE) method in aromatic amine (AA) metabolites (i.e. 1-naphthylamine, 2-naphthylamine, 3-aminobiphenyl and 4-aminobiphenyl) in urine, and then HPLC-MS/MS was employed for analysis. The morphology, pore structure, surface area, chemical composition, magnetic properties, and thermal stability of the synthesized magnetic COFs were characterized with several analysis techniques, such as nitrogen adsorption–desorption isotherms, HRTEM, FTIR, XRD, and so on. MSPE conditions were optimized and analytical performance of the developed method was characterized. The limit of detection (LOD) and limit of quantification (LOQ) of four AAs ranged from 0.01 to 0.07 ng mL⁻¹ and 0.04 to 0.22 ng mL⁻¹, respectively. The recoveries of 1-NA, 2-NA, 3-ABP, 4-ABP were in the range of 81.9–105%, 87.8–102%, 101–120%, 88.3–117%, respectively. Good intra-day and inter-day precision were obtained with RSD of less than 5.8% and 9.9%, respectively. Furthermore, the synthesized magnetic COFs absorbent could be recycled in MSPE at least 5 times. Analytical results of AA metabolites in real urine samples with the developed method showed significant difference (p < 0.01) between smokers and nonsmokers. Thus, urinary AA metabolites could be exposure biomarkers for cigarette smoke.

Purification and selective enrichment of ultra-low level metabolites in bio-samples is very important for HPLC-MS/MS analysis.

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.

Rational synthesis of interpenetrated 3D covalent organic frameworks for asymmetric photocatalysis

Covalent organic frameworks (COFs) show great promise as heterogeneous photocatalysts, but they have not yet been explored for asymmetric photocatalysis, which is important for the sustainable production of pharmaceuticals and fine chemicals. We report here a pair of twofold interpenetrated 3D COFs adopting a rare (3,4)-connected ffc topology for photocatalytic asymmetric reactions by imine condensation of rectangular and trigonal building blocks. Both COFs containing a photoredox triphenylamine moiety are efficient photocatalysts for the cross-dehydrogenative coupling reactions and asymmetric α-alkylation of aldehydes integrated with a chiral imidazolidinone catalyst. Under visible-light irradiation, the targeted chiral products are produced in satisfactory yields with up to 94% enantiomeric excess, which are comparable to those of reported reactions using molecular metal complexes or organic dyes as photosensitizers. Whereas the COFs became amorphous after catalysis, they can be recrystallized through solvent-assisted linker exchange and reused without performance loss. This is the first report utilizing COFs as photocatalysts to promote enantioselective photochemical reactions.

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-In Functions

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high-order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built-in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

Pore surface engineering of covalent organic frameworks: structural diversity and applications

Connecting molecular building blocks by covalent bonds to form extended crystalline structures has caused a sharp upsurge in the field of porous materials, especially covalent organic frameworks (COFs), thereby translating the accuracy, precision, and versatility of covalent chemistry from discrete molecules to two-dimensional and three-dimensional crystalline structures. COFs are crystalline porous frameworks prepared by a bottom-up approach from predesigned symmetric units with well-defined structural properties such as a high surface area, distinct pores, cavities, channels, thermal and chemical stability, structural flexibility and functional design. Due to the tedious and sometimes impossible introduction of certain functionalities into COFs via de novo synthesis, pore surface engineering through judicious functionalization with a range of substituents under ambient or harsh conditions using the principle of coordination chemistry, chemical conversion, and building block exchange is of profound importance. In this review, we aim to summarize dynamic covalent chemistry and framework linkage in the context of design features, different methods and perspectives of pore surface engineering along with their versatile roles in a plethora of applications such as biomedical, gas storage and separation, catalysis, sensing, energy storage and environmental remediation.

Ligand Mediated Metal Cations Exchanges within Metallo-Dynameric Solid Films

Dynameric solid films may be generated via the adequate imine-bond connection between bis(pyridine-2,6-diimine) core centres, coordinated with different metal cations and diaminoPEG connectors. The adequate selection of metal cations leads to cross-linked metallo-dynameric films, allowing the fine modulation of their colour and mechanical property. The coordination of the metal cations and bis(pyridine-2,6-diimine), results in the formation of interlocked structures, leading to the most probably formation of interweaved structures with better mechanical properties than those formed in the absence of the metallic cations. Removal and addition of metal cations from solid films can be achieved via tris(2-aminoethyl)amine (TREN) complexing agent, which strongly binds the metal cations, followed by subsequent insertion of other metallic cations. It allows a ligand-modulated dynamic release of the metal cations from the solid films, together with colour transfer and change of mechanical strength at the interfaces between various solid films.

Stable 2D Heteroporous Covalent Organic Frameworks for Efficient Ionic Conduction

Two-dimensional (2D) covalent organic frameworks (COFs) feature open and ordered one-dimensional column nanochannels which offer immense possibilities for incorporation of various guests for specific functions. However, the relatively low chemical stability of most COFs originating from the dynamic covalent linkages hinders their practical application. In this work, a highly crystalline and heteroporous dibenzo[g,p]chrysene-based COF (DBC-2P) was synthesized and served as a host material for ionic conduction. DBC-2P exhibits excellent stability both in strong acid and base due to the large conjugated DBC-based knot that reinforces the interlayer interactions. Subsequent encapsulation of linear polyethylene glycol (PEG) and PEG-LiBF₄ salt into the nanochannels of DBC-2P affords a hybrid material with a high ionic conductivity of 2.31×10^-3  S cm^-1 . This work demonstrates an efficient post-synthetic strategy for the development of new COF-polymer composites with intriguing properties.

Electron delocalization in single-layer phthalocyanine-based covalent organic frameworks: a first principle study

In this work, we first investigate the localized electronic states in the band structures of three single-layer COFs based on typical building units of COFs chemistry. Our results confirm that the polar nature of strong bonds in these building units is a hindrance to a fully delocalized structure and disfavors the band-like mechanism of transport. We then show that a rational design of the building units can lead to dispersive band states in the electronic structure and results in conducting single-layer COFs. We demonstrate this strategy by investigating the charge carrier transport in a series of single-layer Ni-phthalocyanine (NiPc) covalent organic frameworks (COFs), namely, NiPc-P, NiPc-2P, and NiPc-3P. Three proposed COFs exhibit semiconducting band gaps ranging from 0.55 to 0.91 eV. Their room-temperature intrinsic mobility is predicted to be in range of 200-600 cm2 V-1 s-1 and 20 000-60 000 cm2 V-1 s-1 for electrons and holes, respectively, which are comparable to those of phosphorene and higher than those of the trigonal prismatic molybdenum disulfide. NiPc are dynamically and mechanically stable and can be synthesized via the co-evaporation between Ni and corresponding tetracyano linkers. Importantly, we demonstrate that the properties of the single-layer COFs can be tuned by engineering the organic building blocks. Our theoretical study not only provides insight into the design principles for semiconducting single-layer COFs but also highlights the significance of reticular chemistry in the development of a new generation of two-dimensional materials for optoelectronic applications.

Controlling the Packing of Metal-Organic Layers by Inclusion of Polymer Guests

The preparation of metal-organic structures with a controlled degree of disorder is currently one of the most promising fields of materials science. Here, we describe the effect of guest polymer chains on the transformation of a metal-organic framework (MOF). Heating a pillared MOF at a controlled temperature resulted in the exclusive removal of the pillar ligands, while the connectivity of the metal-organic square-grid layers was maintained. In the absence of a polymer, 2D-layers rearranged to form a new crystalline phase. In contrast, the presence of a polymer in the MOF inhibited totally the recrystallization, leading to a turbostratic phase with layers threaded and maintained apart by the polymer chains. This work demonstrates a new synthetic approach toward the preparation of anisotropic metal-organic materials with controlled disorder. It also reveals how guests can dramatically modify the conversion of host MOFs, even though no chemical reaction occurs between them.

Three-Dimensional Tetrathiafulvalene-Based Covalent Organic Frameworks for Tunable Electrical Conductivity

The functionalization of three-dimensional (3D) covalent organic frameworks (COFs) is essential to broaden their applications. However, the introduction of organic groups with electroactive abilities into 3D COFs is still very limited. Herein we report the first case of 3D tetrathiafulvalene-based COFs (3D-TTF-COFs) with non- or 2-fold interpenetrated pts topology and tunable electrochemical activity. The obtained COFs show high crystallinity, permanent porosity, and large specific surface area (up to 3000 m²/g). Furthermore, these TTF-based COFs are redox active to form organic salts that exhibit tunable electric conductivity (as high as 1.4 × 10^-2 S cm^-1 at 120 °C) by iodine doping_. These results open a way toward designing 3D electroactive COF materials and promote their applications in molecular electronics and energy storage.

Hexane-1,2,5,6-tetrol as a Versatile and Biobased Building Block for the Synthesis of Sustainable (Chiral) Crystalline Mesoporous Polyboronates

We report on the synthesis and characterization of novel mesoporous chiral polyboronates obtained by condensation of (R,S)/(S,S)-hexane-1,2,5,6-tetrol (HT) with simple aromatic diboronic acids (e.g., 1,3-benzenediboronic acid) (BDB). HT is a cellulose-derived building block comprising two 1,2-diol structures linked by a flexible ethane bridge. It typically consists of two diastereomers one of which [(S,R)-HT] can be made chirally pure. Boronic acids are abundantly available due to their importance in Suzuki-Miyaura coupling reactions. They are generally considered nontoxic and easy to synthesize. Reactive dissolution of generally sparingly soluble HT with BDB, in only a small amount of solvent, yields the mesoporous HT/polyboronate materials by spontaneous precipitation from the reaction mixture. The 3D nature of HT/polyboronate materials results from the entanglement of individual 1D polymeric chains. The obtained BET surface areas (SAs) and pore volumes (PVs) depend strongly on HT's diastereomeric excess and the meta/para orientation of the boronic acids on the phenyl ring. This suggests a strong influence of the curvature(s) of the 1D polymeric chains on the final materials' properties. Maximum SA and PV values are respectively 90 m² g^-1 and 0.44 mL g^-1. Variably sized mesopores, spanning mainly the 5-50 nm range, are evidenced. The obtained pore volumes rival the ones of some covalent organic frameworks (COFs), yet they are obtained in a less expensive and more benign fashion. Moreover, currently no COFs have been reported with pore diameters in excess of 5 nm. In addition, chiral boron-based COFs have presently not been reported. Scanning electron microscopy reveals the presence of micrometer-sized particles, consisting of aggregates of plates, forming channels and cell-like structures. X-ray diffraction shows the crystalline nature of the material, which depends on the nature of the aromatic diboronic acids and, in the specific case of 1,4-benzenediboronic acid, also on the applied diastereomeric excess in HT.

Atomic-Level Characterization of Dynamics of a 3D Covalent Organic Framework by Cryo-Electron Diffraction Tomography

Understanding the dynamics of covalent organic frameworks (COFs) is desirable for developing smart materials with coherent responses to external stimulus. Here we illustrate the structural determination of dynamics at atomic level by cryo-electron diffraction tomography (EDT) with single crystals of COF-300 having only submicrometer sizes. We observe and elucidate the crystal contraction upon H₂O adsorption by ab initio structural solution of all non-hydrogen atoms of framework and unambiguous location of guest molecules in the pores. We also observe the crystal expansion of COF-300 upon inclusion of ionic liquid or polymer synthesized in the channels, whose conformational aspects of frameworks can be confirmed.

Isostructural Three-Dimensional Covalent Organic Frameworks

Herein, we reported the designed synthesis of three isostructural three-dimensional covalent organic frameworks (3D COFs) with -H, -Me, or -F substituents, which have similar crystallinity and topology. Their crystal structures were determined by continuous rotation electron diffraction (cRED), and all three 3D COFs were found to adopt a fivefold interpenetrated pts topology. More importantly, the resolution of these cRED datasets reached up to 0.9-1.0 Å, enabling the localization of all non-hydrogen atomic positions in a COF framework directly by 3D ED techniques for the first time. In addition, the precise control of the pore environments through the use of different functional groups led to different selectivities for CO₂ over N₂ . We have thus confirmed that polycrystalline COFs can be definitely studied to the atomic level as other materials, and this study should also inspire the design and synthesis of 3D COFs with tailored pore environments for interesting applications.

Sub-stoichiometric 2D covalent organic frameworks from tri- and tetratopic linkers

Covalent organic frameworks (COFs) are typically designed by breaking down the desired network into feasible building blocks - either simple and highly symmetric, or more convoluted and thus less symmetric. The linkers are chosen complementary to each other such that an extended, fully condensed network structure can form. We show not only an exception, but a design principle that allows breaking free of such design rules. We show that tri- and tetratopic linkers can be combined to form imine-linked [4 + 3] sub-stoichiometric 2D COFs featuring an unexpected bex net topology, and with periodic uncondensed amine functionalities which enhance CO2 adsorption, can be derivatized in a subsequent reaction, and can also act as organocatalysts. We further extend this class of nets by including a ditopic linker to form [4 + 3 + 2] COFs. The results open up possibilities towards a new class of sub-valent COFs with unique structural, topological and compositional complexities for diverse applications.