Authors: Torben Sick
Source: Electronic Theses of LMU Munich (Ludwig-Maximilians-Universität München) (article, 2018-01-01)
Field: Materials Science — Materials Chemistry

Synthesis and characterization of nanoporous covalent organic frameworks for optoelectronic applications Nanostructured materials represent an intriguing and growing field of research aimed at developing new, application-targeted materials. Different techniques have been established for the synthesis of porous materials, particularly for those with defined pore size, volume and surface area. Covalent organic frameworks (COFs) are a class of lightweight and highly crystalline open porous networks, where different organic building blocks are connected via covalent bonds to form polymers. By the choice of the building blocks’ geometry, dimensionality and incorporated functionality, the pore shape and size can be tailored. COFs exhibit high thermal and reasonable chemical stability in combination with high specific surface area and a long-range order. With these features in hand, COFs allow for a precise spatial arrangement of variable building blocks. This allows for the examination of potential correlations between the degree of crystallinity and porosity, aiming at other desired properties and behavior like host-guest interactions, light absorption, diffusion, and conductivity. While 3D COFs are interconnected in all three dimensions, in 2D COFs polymeric sheets stack in the third dimension via comparably weak dispersive forces (π-stacking). Thereby, the distance of building blocks within adjacent layers can be much smaller compared to the 3D structures, which allows for enhanced interactions in the stacking direction. The respective stacking distance mainly depends on the spatial requirements, steric hindrances and stacking affinities of the incorporated building blocks. As COF growth is a highly dynamic process simultaneously occurring at different reactive sites, and at a high reaction rate by reversible bond formation, crystallites with small domain sizes and orientations are most likely to form. As the reversible bond formation enables the self-healing mechanism of COF crystallites and defines the degree of overall order, influencing the reversibility and the reaction rate is an important goal in COF syntheses. In chapter three, we introduce a strategy to slow down the overall reaction rate by the addition of competitive molecular agents named modulators. The modulator agents act on the covalent bond formation and on the formation of the 2D sheets. The modulator molecule usually lacks symmetric reactive linking groups and thereby, under reversible reaction conditions, can attach and leave the growing fragment. It temporarily truncates the active linking sites of the respective counterpart. Furthermore, this approach allows for the incorporation of accessible functional groups or stabilizing polymer chains, as the modulators finally most probably partially decorate the COF crystallite outer surface. By employing the modulator approach, enlarged domain sizes are obtained. Furthermore, the overall porosity and accessible surface area are positively affected. As the first COFs were constructed by connecting building blocks through the formation of sigma bonds, charge migration within the polymeric sheets was effectively inhibited. To enable charge carrier mobility within the sheets, conjugated COFs like imines or imides were developed. Importantly, in COFs consisting of completely planar and rigid building blocks, the incorporation of heteroatoms can result in lateral offsets between adjacent layers due to the electrostatic repulsion and bond polarization. Thereby, adjacent COF layers start to stack – energetically driven – in a staggered mode without any preferential displacement direction and slight lateral offsets are the result. This inconsistent displacement can result in comparably small domain sizes in all three dimensions of the individual crystallites. In chapter four we addressed this issue by the incorporation of propeller-like building blocks (4,4′,4′′,4′′′-(ethylene-1,1,2,2-tetrayl)tetraaniline (ETTA), 1,3,5-tris(4-aminophenyl)benzene (TAPB) and tris(4-aminophenyl)amine (TAPA)). These sterically demanding building blocks serve as anchors by predefining a thermodynamically favored docking site of attachment. In chapter five we describe the incorporation of an electroactive counterpart, namely 2,6-imine connected benzo[1,2-b:4,5-b']dithiophene (BDT) to the ETTA building block, which provides a highly crystalline and open porous, kagome-structured COF. Oriented thin films of the BDT-ETTA-COF were fabricated using the well-established in-situ film growth. By variation of the precursor concentration, thin COF films with a high degree of orientation and different thicknesses could be synthesized on different substrates like quartz or conducting oxides (indium tin oxide (ITO) and fluorine-doped tin oxide (FTO)). Finally, the very first COF-photocathodes were fabricated and utilized in noble metal-free photoelectrochemical water splitting to hydrogen and oxygen. The deposition of platinum nanoparticle co-catalysts on top of the photoabsorbing COF allowed for a significant increase in photocurrent. In chapter six, the realization of an isoreticular series of the propeller-like TAPB building block featuring hexagonal COF pores is shown. The TAPB-COFs with comparably short linear linkers such as 1,4-imine connected phenylene (TA), 2,5-imine connected thieno[3,2-b]thiophene (TT) or BDT revealed structural instability towards post-synthetic treatments. Neither the desired property of crystallinity nor of porosity could be preserved upon usual powder isolation treatments like filtering, washing, or solvent exchange. The development of a very successful and fast work up procedure with supercritical carbon dioxide (scCO2) resulted in the desired highly crystalline and open porous frameworks, which were stable under ambient conditions for months. However, treatments with solvents or even solvent vapors drove these COFs back to poorly crystalline non-porous materials. To overcome this stability issue, an extension of the π-system such as 2,7-imine connected pyrene (Pyrene-2,7) or alkoxy functionalization of the BDT cores (4,8-dialkoxy-functionalized BDT: BDT-OMe, BDT-OEt, and BDT-OPr) dramatically increased the COFs’ stabilities. Even a continuous Soxhlet-extraction became a possible work-up procedure for these robust COFs. The incorporation of the pyrene building block results in a comparably closer stacking distance between the adjacent Pyrene-2,7 TAPB layers. This emphasizes the role of the linear counterpart in the overall TAPB-COF structural stability. Furthermore, it illustrates that the TAPB core features a somewhat flexible propeller configuration rather than providing a rigid docking site dictating the COF stacking distance. Depending on the employed linear counterpart that was previously viewed to serve only as a bridge between the two rigid propeller nodes, this approach has the potential to modulate and dictate the stacking profile of the final TAPB-COF. Surprisingly, all TAPB-COFs, whether fragile or robust, exhibit a high degree of sensitivity towards the main component of the reaction mixtures, namely 1,4-dioxane. Upon exposure to 1,4-dioxane vapor, time-dependent PXRD measurements revealed a significant impact on the π-stacking distance of adjacent layers, which was enlarged in all COFs. We propose a molecular layer intercalation mechanism, which in the case of 1,4-dioxane results in an amorphization of the COF structures. In 1,4-dioxane atmosphere, the π–attractive forces between adjacent COF layers are weakened, resulting in a complete layer displacement in all three dimensions. Strikingly, we were able to reestablish all 1,4-dioxane treated, amorphous COF powders using the scCO2 treatment, thus converting the amorphous powder back into crystalline and highly porous COF structures. This process can be cycled as often as desired and shows that scCO2 treatment is an excellent, fast, and efficient work-up procedure for all investigated TAPB-COFs, whether robust or fragile. In chapter seven we introduce chrysene as the core of a four-arm ETTA-like building block, i.e. dibenzo[g,p]chrysene-2,7,10,15-tetraamine (DBCA). In condensation reactions with linear bifunctional aldehyde counterparts dual pore kagome DBC-COF structures with imine linkages are obtained. With propeller-like molecular building blocks such as ETTA featuring docking sites for COF construction, the stacking distance of adjacent layers still is comparably large with about 4.7 Å. Close stacking distances of the adjacent COF layers with preservation of the structural guidance provided by a rigid docking site are beneficial for an improved π-orbital overlap of the building blocks and thereby electronic interactions of the adjacent layers. This feature should allow for enhanced charge migration within the COF. In this context, the central naphthyl unit of DBC instead of the central ethylene unit of ETTA offers two advantages. Firstly, a fully conjugated π-system in the DBC core. In DBC all phenyl groups contribute to one π-system. Secondly, while the rotation of the phenyl groups in ETTA provides for the desired molecular docking sites, it simultaneously results in a weaker π-overlap. In DBC the tilting is minimized upon incorporation of two new chemical bonds, resulting in a large π-overlap as well as a closer stacking distance of adjacent layers. Nanoporous ; Characterization (materials science) ; Materials science ; Nanotechnology ; Covalent bond ; Optoelectronics ; Chemistry ; Organic chemistry Covalent Organic Framework Applications ; Metal-Organic Frameworks: Synthesis and Applications ; Advanced Photocatalysis Techniques