Tailoring the features of covalent organic frameworks

Covalent organic frameworks (COFs) represent a new and emerging class of functional materials built from organic subunits. These networks are formed via reversible co-condensation reactions resulting in covalent bonds based on diverse binding motifs. Due to the great variety and large number of accessible building blocks, structural and functional diversity can be achieved easily. By combining desired subunits, crystalline and porous two- or three-dimensional frameworks can be synthesized exhibiting a defined pore size and a high surface area. Crystallinity and porosity are of central importance for many characteristics of COFs such as adsorption, diffusion, and electronic transport. The possible role of COF materials applied for sensing, catalysis, and optoelectronic applications is based on their tunable characteristics which can be tailored by implementing suitable subunits. Furthermore, postsynthetic modifications can be used to adjust properties of the framework while maintaining the main aspects. In general, COFs can be synthesized during solvothermal reaction conditions followed by diverse work-up steps which have to be adapted and optimized for each system. Therefore, finding the right parameters is key for the successful synthesis of crystalline materials and mastering them enables the formation of thin films on suitable substrates. This is particularly important for device applications in the optoelectronic area. This thesis focuses on the synthesis of novel two-dimensional COFs, introducing new characteristic features into such frameworks. The different COF materials were fully characterized by various methods to ensure high crystallinity and porosity as well as elucidating their morphology and structure. Next to neat bulk and film materials, also full electronic devices were fabricated for further analysis. In addition, postsynthetic treatments concerning structural improvement and pore wall modifications were investigated. The possible applications arising from introducing additional functionality after the synthesis were a substantial part of this work. The experimental results of this thesis can be separated into three main parts: structural control as well as implementation of dyes and electroactive subunits, electrical characterization, and postsynthetic treatment methods. In general, while Chapter 1 of this thesis is introducing the main aspects of COFs and the theoretical background, Chapter 2 contains descriptions of the methods used for materials characterization. The first part of the experimental results (Chapter 3 and 4) deals with structural control including the implementation of dyes and the comparison of imine and boronate ester binding motifs. Chapter 3 is focused on the precise construction of template-free nano- and microstructures of dye-containing porous materials. The implemented organic dye diketopyrrolopyrrole was functionalized with accessible aldehyde groups. After reaction with a tetra amine-functionalized porphyrin building block the resulting DPP-TAPP-COF showed enhanced absorption capabilities. Regarding structural control, the obtained COF exhibited spontaneous aggregation into hollow microtubular assemblies with well-defined and controllable outer and inner tube diameters. A detailed mechanistic morphology investigation revealed that the whole process is time-dependent and undergoes a traceable transformation starting with sheet-like agglomerates into stable tubular microstructures via a rolling-up mechanism. As already mentioned in the third chapter (the implementation of sterically demanding dyes as building blocks for COF systems), the integration of diketopyrrolopyrrole molecules is still hampered by limited control of the binding motif in combination with the size of the backbone. In Chapter 4, the results of using a flat, rigid, and non-conjugated boronate ester bond combined with the dye and a suitable counterpart are reported. Here, structural control was achieved and led to enhanced properties and enforced specific stacking behavior. The COF was successfully synthesized and crystallinity as well as porosity could be improved as compared to the imine-connected counter-part, with even shorter reaction times. The boronate ester coupling motif guides the formation of a planar and rigid backbone and long-range molecular stacks. Furthermore, the COF exhibits application-relevant optical properties including strong absorption over the visible spectral range, broad emission into the near infrared region (NIR), and a long singlet lifetime. All the findings can be attributed to the controlled formation of molecular stacks with J-type interactions between the subcomponents in the COF. In addition, these molecular stacks showed an influence on the electronic behavior revealing electrical conductivity values of crystalline COF pellets up to 10 6 Scm-1. Further investigations on electrical conductivity and charge carrier mobility studies on different COF systems are described in Chapter 5. Here, a series of acene-based building blocks was implemented into a scaffold, i.e. benzene and anthracene dialdehyde-functionalized subunits, which show high charge carrier mobilities comparable to organic materials in single crystals. The building blocks were designed in a way to tailor the length of the resulting aromatic backbone pointing into the pore of the framework without changing the overall unit cell dimensions, i.e. the molecules were inserted perpendicular to the binding direction. This allows for a better comparison of the structures and the resulting properties. The measurements revealed that the length of the backbone has a strong influence on the achieved electrical conductivity and mobility values. Moreover, different measurement methods for conductivity in combination with mobility are compared due to the diverse theoretical backgrounds each method is based on, yielding technique-specific values. In addition, all COF samples revealed surprisingly high Hall mobility values for pressed powder pellets as well as for thin films and devices. The measured hole-only devices exhibited up to 10 3 cm2V-1s-1 for the anthracene COF, which is one of the largest values of intrinsic mobility for COFs so far. The results point towards the importance of π-overlap and hence the length of the acene unit. Further extension of the series towards the even longer pentacene unit was initiated but will be part of future studies. The third part of the experimental results deals with the different possibilities of postsynthetic treatments. In Chapter 6, a new postsynthetic treatment for covalent organic frameworks is introduced. Here, a newly synthesized anthracene-based COF (from Chapter 5) is reported followed by a postsynthetic treatment based on light-induced defect reduction. The applied laser light apparently leads to opening and reformation of imine bonds resulting in a significant decrease of defect sites and therefore a striking increase in photoluminescence. This effect was further supported by IR investigations showing a reduced intensity of aldehyde and amine vibrations and a gain of imine vibration intensity upon laser irradiation of a stoichiometric precursor mix. Another approach regarding postsynthetic treatment is presented in Chapter 7, based on chemical modification after successful COF synthesis. A novel terphenyldiboronic acid-based COF is reported which features accessible hydroxyl groups at the inner and outer pore wall environment. These functional groups serve as anchor sites for a fluorescence label which can be installed by a postsynthetic modification approach. By forming o thiocarbamate bonds, fluorescein molecules were immobilized on the inner as well as at the outer surface of the pore system. This reaction was further extended to another COF system and to other grafting moieties. In conclusion, this thesis mainly focused on the fundamental synthetic, structural, and functional characteristics of new optoelectronic COF materials. Next to synthesis and morphology control, electrical and optical characterization was performed, giving insights on the stacking behavior and electronic landscape within the framework. The COF was used as a new tool for directed structural control and stacking of molecular chromophore units. Furthermore, exceptionally high intrinsic charge carrier mobilities were found even on the macroscopic scale of devices, possibly enabling new applications in sensing and optoelectronic devices. Additionally, postsynthetic processes were developed to extend the portfolio of applications for synthesized COFs through modification or to improve the performance of materials through defect reduction, which is of particular interest for optoelectronic thin film-based devices