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

Perylene-Based Covalent Organic Frameworks for Acid Vapor Sensing.

Traditionally, the properties and functions of covalent organic frameworks (COFs) are defined by their constituting building blocks, while the chemical bonds that connect the individual subunits have not attracted much attention as functional components of the final material. We have developed a new series of dual-pore perylene-based COFs and demonstrated that their imine bonds can be protonated reversibly, causing significant protonation-induced color shifts toward the near-infrared, while the structure and crystallinity of the frameworks are fully retained. Thin films of these COFs are highly sensitive colorimetric acid vapor sensors with a detection limit as low as 35 μg L-1 and a response range of at least 4 orders of magnitude. Since the acidochromism in our COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously the concentration and protonation strength of nonaqueous acid solutions, in which pH electrodes are not applicable, and to distinguish between different acids. Including the imine bonds as function-determining constituents of the framework provides an additional handle for constructing multifunctional COFs and extending the range of their 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

Spectrally Switchable Photodetection with Near-Infrared-Absorbing Covalent Organic Frameworks

Most covalent organic frameworks (COFs) to date are made from relatively small aromatic subunits, which can only absorb the high-energy part of the visible spectrum. We have developed near-infrared-absorbing low bandgap COFs by incorporating donor–acceptor-type isoindigo- and thieno­isoindigo-based building blocks. The new materials are intensely colored solids with a high degree of long-range order and a pseudo-quadratic pore geometry. Growing the COF as a vertically oriented thin film allows for the construction of an ordered interdigitated heterojunction through infiltration with a complementary semiconductor. Applying a thieno­isoindigo-COF:fullerene heterojunction as the photoactive component, we realized the first COF-based UV- to NIR-responsive photodetector. We found that the spectral response of the device is reversibly switchable between blue- and red-sensitive, and green- and NIR-responsive. To the best of our knowledge, this is the first time that such nearly complete inversion of spectral sensitivity of a photodetector has been achieved. This effect could lead to potential applications in information technology or spectral imaging

The Hermeneutic Bond between Translation and Literature

The theory of translation and its critical appraisal can be traced as far back as the 60s, albeit translation per se is a time-honoured practice. This awareness has produced a corpus of theoretical works examining the great complexity of the issue, and another huge paratextual one (prefaces, forewords, introductions, editor’s words) usually added to the texts by the translators. The act of translating is viewed as a transcultural process which establishes the inextricable vehicular function and, ultimately, the raison d’être of translation itself. The essay discusses the practice of literary translation starting with some examples of Italian translations of "A Room with a View" by E. M. Forster. It emerges that translation is only prima facie an exercise based on language, and that both translation and literature necessarily inhabit the same world because they preside over the mental capacity to form ideologies. Translation is therefore defined as a creative act, the art of transformation as opposed to mere repetition

Research data supporting "Excited State Dynamics in Fully-Conjugated 2D Covalent Organic Frameworks"

Data behind the figures in the main text and the Supporting Information. Results of Photoluminescence (PL), Absorption (UV-VIS), Photothermal Deflection (PDS), and Transient Absorption (TA) Spectroscopy, as well as Time-Correlated Single Photon Counting (TCSPC). A detailed description can be found in the ReadMe.tx

Excited-State Dynamics in Fully Conjugated 2D Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are a highly versatile group of porous materials constructed from molecular building blocks, enabling deliberate tuning of their final bulk properties for a broad range of applications. Understanding their excited-state dynamics is essential for identifying suitable COF materials for applications in electronic devices such as transistors, photovoltaic cells, and water-splitting electrodes. Here, we report on the ultrafast excited-state dynamics of a series of fully conjugated two-dimensional (2D) COFs in which different molecular subunits are connected through imine bonds, using transient absorption spectroscopy. Although these COFs feature different topologies and chromophores, we find that excited states behave similarly across the series. We therefore present a unified model in which charges are generated through rapid singlet-singlet annihilation and show lifetimes of several tens of microseconds. These long-lived charges are of particular interest for optoelectronic devices, and our results point toward the importance of controlling the singlet-singlet annihilation step in order to increase the yield of separated charges

Oligothiophene-Bridged Conjugated Covalent Organic Frameworks

Two-dimensional covalent organic frameworks (2D-COFs) are crystalline, porous materials comprising aligned columns of π-stacked building blocks. With a view toward the application of these materials in organic electronics and optoelectronics, the construction of oligothiophene-based COFs would be highly desirable. The realization of such materials, however, has remained a challenge, in particular with respect to laterally conjugated imine-linked COFs. We have developed a new building block design employing an asymmetric modification on an otherwise symmetric backbone that allows us to construct a series of highly crystalline quaterthiophene-derived COFs with tunable electronic properties. Studying the optical response of these materials, we have observed for the first time the formation of a charge transfer state between the COF subunits across the imine bond. We believe that our new building block design provides a general strategy for the construction of well-ordered COFs from various extended building blocks, thus greatly expanding the range of applicable molecules

Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks

Covalent organic frameworks (COFs), formed by reversible condensation of rigid organic building blocks, are crystalline and porous materials of great potential for catalysis and organic electronics. Particularly with a view of organic electronics, achieving a maximum degree of crystallinity and large domain sizes while allowing for a tightly π-stacked topology would be highly desirable. We present a design concept that uses the 3D geometry of the building blocks to generate a lattice of uniquely defined docking sites for the attachment of consecutive layers, thus allowing us to achieve a greatly improved degree of order within a given average number of attachment and detachment cycles during COF growth. Synchronization of the molecular geometry across several hundred nanometers promotes the growth of highly crystalline frameworks with unprecedented domain sizes. Spectroscopic data indicate considerable delocalization of excitations along the π-stacked columns and the feasibility of donor–acceptor excitations across the imine bonds. The frameworks developed in this study can serve as a blueprint for the design of a broad range of tailor-made 2D COFs with extended π-conjugated building blocks for applications in photocatalysis and optoelectronics