Covalent organic frameworks

The first members of covalent organic frameworks (COF) have been designed and successfully synthesized by condensation reactions of phenyl diboronic acid C6H4[B(OH)2]2 and hexahydroxytriphenylene C18H6(OH)6. The high crystallinity of the products (C3H2BO)6 (C9H12)1 (COF-1) and C9H4BO2 (COF-5) has allowed definitive resolution of their structure by powder X-ray diffraction methods which reveal expanded porous graphitic layers that are either staggered (COF-1, P63/mmc) or eclipsed (COF-5, P6/mmm). They exhibit high thermal stability (to temperatures up to 500- to 600-C), permanent porosity, and high surface areas (711 and 1590 m2/g, respectively) surpassing those of related inorganic frameworks. A similar approach has been used for the design of other extended structures

Covalent organic frameworks

Covalent organic frameworks (COFs) are a new and emerging class of porous and crystalline materials that are formed via the connection of organic subunits through covalent bonds. Their great structural flexibility allows for the realisation of COFs based on a modular principle, where the respective building blocks can be hand-picked and designed regarding features like pore size, pore geometry or specific functionalities of the resulting material. Potential for application has been demonstrated amongst others in gas storage, gas separation, sensing, drug delivery or (opto)electronics. As COFs are polymers linked in two or three dimensions, the realisation of crystalline materials is challenging and only possible when the covalent bond formation mechanism is reversible, allowing the network to self-heal during synthesis. This healing mechanism, however, is only applicable to a limited number of attachment and detachment cycles until the building blocks get ultimately trapped in the growing network. This way, defects are inevitably incorporated in the resulting COF. Building blocks that are used in conventional 2D COF syntheses exhibit a combination of two properties potentially fraught with problems: (1) They prefer to stack with a lateral offset and (2) exhibit symmetry elements like rotational axes. Due to symmetry reasons, there is hence no preferred direction for the offset of adjacent COF layers. When growing islands on top of a perfect layer feature different offsets along symmetry-equivalent directions, they cannot merge into each other, resulting in lattice strain, defects and an overall compromised crystallinity. Potential applications like optoelectronic devices would benefit to a great extent from highly crystalline, error-free domains for successful charge-transport, so the first part of this thesis is focused on the realisation of COFs with a very high degree of order. By applying tetraphenylethylene building blocks with a unique propeller-shaped three dimensional geometry, the individual COF sheets are locked in place as the molecules can stack perfectly eclipsed upon each other like puzzle pieces. Each building block can act as a docking site for newly attaching molecules during crystal growth, preventing stacking faults and dislocations. Studying a series of COFs comprising different linear linkers enabled us to observe that the molecular conformation of the bridge itself plays a crucial role in the realisation of error-free crystallites. To ensure that only the correct propeller enantiomer is incorporated within one COF domain, bridges with C2 rotational axis synchronize adjacent core molecules by transmitting configurational information from one propeller to the other. In the next part of this thesis, we extended our lock-and-key concept further and made it accessible to a broader range of bridging units. Switching from our initial building block that enforces strictly eclipsed packing to a tightly π-stacked central core unit that enables offset-stacking, we were able to realise conjugated COF single crystallites on the order of 0.5 μm. The armchair conformation of the tetraphenylpyrene core is synchronised via flat and rigid π-stacked bridges, which additionally allow for electronic communication between all subunits of the framework. Tuning the electron density of the bridging entitiy we were further able to modulate the optoelectronic properties of the respective COFs. In the third part of this thesis we used our docking concept to realise highly crystalline and stable COF films that can change their electronic structure reversibly depending on the surrounding atmosphere. By combining electron-rich and -deficient building blocks, we synthesised the first solvatochromic COFs that show a strong charge-transfer induced colour change when exposed to humidity or solvent vapours. The extent of the colour change is dependent on the vapour concentration and the solvent polarity, allowing for contactless sensing of probe molecules. The growth of the COFs as oriented films guarantees highly accessible pores and thus ultrafast response times below 200 ms, outperforming even commercially available sensing devices. As a proof of concept, we constructed a humidity sensor with full reversibility and stability over at least 4000 cycles by applying a solvatochromic COF film as a light filter between a LED and a photoresistor. Although many intriguing functionalities have been demonstrated with COFs, reversible structural flexibility has not been reported for 2D COFs yet. We surmised that a high degree of lateral displacement between individual COF layers combined with tightly interlocked π-stacks would enable the linear bridging units to move almost freely upon applying an external stimulus. Indeed, the design of multidentate COF linkers based on perylene-3,4,9,10-tetracarboxylic acid diimide allowed us to realise the first breathing 2D COFs that reversibly change their crystal and electronic structure when in contact with solvent molecules. During these “wine-rack” breathing transitions, the distance between the perylene-3,4,9,10-tetracarboxylic acid diimides can be tuned, allowing for switching on and off in-plane electronic coupling. Taking this concept further, we showed that slight modifications of the linear bridging unit can again inhibit the dynamic response due to steric effects. The last part of this thesis was focused on structural requirements of building blocks for constructing large-pore COFs. We elaborated boundary conditions for linear bridging units as well as multidentate building blocks, taking into account multiple aspects like building block offset, alkyl chain packing and tilt angles. To achieve crystalline packing in such large-pore COF systems, we established that both building blocks have to be matched appropriately, allowing the COF to adapt one single, well-defined structure. In conclusion, this thesis has been focused on exploring the fundamental relationships between linker design and resulting structural and functional characteristics of the respective covalent organic framework. The ability to realise highly crystalline networks with reversibly tuneable electronic, optical and geometric properties will help this young class of materials to evolve from a purely academic field of research and broaden the scope of possible applications

Solvatochromic covalent organic frameworks.

Covalent organic frameworks (COFs) are an emerging class of highly tuneable crystalline, porous materials. Here we report the first COFs that change their electronic structure reversibly depending on the surrounding atmosphere. These COFs can act as solid-state supramolecular solvatochromic sensors that show a strong colour change when exposed to humidity or solvent vapours, dependent on vapour concentration and solvent polarity. The excellent accessibility of the pores in vertically oriented films results in ultrafast response times below 200 ms, outperforming commercially available humidity sensors by more than an order of magnitude. Employing a solvatochromic COF film as a vapour-sensitive light filter, we demonstrate a fast humidity sensor with full reversibility and stability over at least 4000 cycles. Considering their immense chemical diversity and modular design, COFs with fine-tuned solvatochromic properties could broaden the range of possible applications for these materials in sensing and optoelectronics

Solvatochromic covalent organic frameworks

Covalent organic frameworks (COFs) are an emerging class of highly tuneable crystalline, porous materials. Here we report the first COFs that change their electronic structure reversibly depending on the surrounding atmosphere. These COFs can act as solid-state supramolecular solvatochromic sensors that show a strong colour change when exposed to humidity or solvent vapours, dependent on vapour concentration and solvent polarity. The excellent accessibility of the pores in vertically oriented films results in ultrafast response times below 200 ms, outperforming commercially available humidity sensors by more than an order of magnitude. Employing a solvatochromic COF film as a vapour-sensitive light filter, we demonstrate a fast humidity sensor with full reversibility and stability over at least 4000 cycles. Considering their immense chemical diversity and modular design, COFs with fine-tuned solvatochromic properties could broaden the range of possible applications for these materials in sensing and optoelectronics

Design of Covalent Organic Frameworks for Methane Storage

We designed 14 new covalent organic frameworks (COFs), which are expected to adsorb large amounts of methane (CH_4) at 298 K and up to 300 bar. We have calculated their delivery uptake using grand canonical Monte Carlo (GCMC) simulations. We also report their thermodynamic stability based on 7.5 ns molecular dynamics simulations. Two new frameworks, COF-103-Eth-trans and COF-102-Ant, are found to exceed the DOE target of 180 v(STP)/v at 35 bar for methane storage. Their performance is comparable to the best previously reported materials: PCN-14 and Ni-MOF-74. Our results indicate that using thin vinyl bridging groups aid performance by minimizing the interaction methane-COF at low pressure. This is a new feature that can be used to enhance loading in addition to the common practice of adding extra fused benzene rings. Most importantly, this report shows that pure nonbonding interactions, van der Waals (vdW) and electrostatic forces in light elements (C, O, B, H, and Si), can rival the enhancement in uptake obtained for microporous materials derived from early transition metals

Covalent Organic Frameworks (COFS): A Review

The search for supramolecular promising porous crystalline materials with diverse applications such as gas storage, catalysis, chemo-sensing, energy storage, and optoelectronic have led to the design and construction of Covalent Organic Frameworks (COFs). COFs are a class of porous crystalline polymers that allow the precise integration of organic building blocks and linkage motifs to create predesigned skeletons and nano-porous materials. In this review article, a historic overview of the chemistry of COFs, survey of the advances in topology design and synthetic reactions, basic design principles that govern the formation of COFs as porous crystalline polymers as well as common synthetic procedures and characterization techniques are discussed. Furthermore some challenges associate with the synthesis of COFs are highlighted. We hope that this review will help researchers, industrialists and academics in no mean feat

Vinylene-Linked Covalent Organic Frameworks by Base-Catalyzed Aldol Condensation

Two 2D covalent organic frameworks (COFs) linked by vinylene (−CH=CH−) groups (V‐COF‐1 and V‐COF‐2) are synthesized by exploiting the electron deficient nature of the aromatic s‐triazine unit of C3‐symmetric 2,4,6‐trimethyl‐s‐triazine (TMT). The acidic terminal methyl hydrogens of TMT can easily be abstracted by a base, resulting in a stabilized carbanion, which further undergoes aldol condensation with multitopic aryl aldehydes to be reticulated into extended crystalline frameworks (V‐COFs). Both V‐COF‐1 (with terepthalaldehyde (TA)) and V‐COF‐2 (with 1,3,5‐tris(p‐formylphenyl)benzene (TFPB)) are polycrystalline and exhibit permanent porosity and BET surface areas of 1341 m2 g−1 and 627 m2 g−1, respectively. Owing to the close proximity (3.52 Å) of the pre‐organized vinylene linkages within adjacent 2D layers stacked in eclipsed fashion, [2+2] photo‐cycloadditon in V‐COF‐1 formed covalent crosslinks between the COF layers.TU Berlin, Open-Access-Mittel - 2019DFG, 390540038, EXC 2008: UniSysCa

Hydrogen Adsorption Into Covalent Organic Frameworks

The practicality of hydrogen power vehicles relies on the existence of an effective onboard storage method. Using expanded Wang-Landau simulations, we study the adsorption of hydrogen in a series of covalent organic frameworks (COF-102, COF-105 and COF-108). From which adsorption isotherms of H2 are generated at temperatures of 77 K and 298 K. At 77 K the COFs are on par with the Department of Energy\u27s 2015 targets, but fall short at 298 K. Molecular dynamic simulations of hydrogen in the COFs were also performed. From which the mean square displacement of the H2 molecules was measured to obtain the diffusion coefficients of H2 inside the COFs. As to be expected, the diffusion coefficients were found to be lower than bulk H2 and possessed a relative order that corresponds with the pore sizes of the COFs

Catalytically Active Imine-based Covalent Organic Frameworks for Detoxification of Nerve Agent Simulants in Aqueous Media

A series of imine-based covalent organic frameworks decorated in their cavities with di erent alkynyl, pyrrolidine, and N-methylpyrrolidine functional groups have been synthetized. These materials exhibit catalytic activity in aqueous media for the hydrolytic detoxification of nerve agents, as exemplified with nerve gas simulant diisopropylfluorophosphate (DIFP). These preliminary results suggest imine-based covalent organic frameworks (COFs) as promising materials for detoxification of highly toxic molecules.MINECO (MAT2016-77608-C3-1-P and 2-P, CTQ2017-84692-R) and EU FEDER fundin