Covalent organic frameworks

Ascherl, Laura Patricia (2018): Covalent organic frameworks: form follows function. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

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.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Covalent Organic Frameworks, organic synthesis, porous materials, physical chemistry
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 29. März 2018
• 1. Bericht­erstatter:in: Bein, Thomas