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Dogru, Mirjam (2012): Functionalization of covalent organic frameworks. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



Covalent Organic Frameworks (COFs) are a novel class of highly stable, purely organic crystalline frameworks made of molecular building blocks. For example, the condensation of boronic acids with appropriate polyols in principle allows the design of precisely controllable structures since their chemical and physical properties can be easily tuned through the selection of the building blocks. The young research field of COFs has attracted scientists due to their extraordinary and versatile properties, however, strategies to control the topology and the properties of the backbone as well as the inner surface are still not well established. With support of Prof. Knochel and his group, who contributed numerous new organic COF linkers, this thesis aims to extend the functionalization strategies for the design of Covalent Organic Frameworks. Investigation of the structural modification and the associated change in physical and chemical properties should lead to progress regarding the applicability of these materials. Employing the concept of reticular chemistry in combination with High Throughput Synthesis Techniques, the formation of a very large Covalent Organic Framework BTP-COF with 4 nm open pores was successfully carried out. The solvothermal co-condensation of 1,3,5-benzenetris(4-phenylboronic acid) (BTPA) and 2,3,6,7-tetrahydroxy-9,10-dimethyl-anthracene (THDMA) was carried out using microwave irradiation instead of conventional synthesis in an oven, thus synthesis time of BTP-COF was reduced from initially 72 h to 5 min. Extending the open pore diameter of a crystalline material to 4 nm, in combination with the resulting high accessible surface area of 2000 m2/g offers great potential to exploit organic reactions in the pores and enables the incorporation of large functional guests, such as polymers or dyes. Bearing these results in mind the scope of functionalization possibilities was expanded from the geometric extension to the chemical modification of the inner surface of COFs. Decorating the organic building blocks with small functional active groups, such as methyl-, -methoxy- and hydroxy- allowed for the successful synthesis of several organic frameworks. Chemical and physical properties of the backbone and the inner surface can be precisely tailored by chemical modification of the building blocks. In order to investigate post-synthetic modification strategies, the methyl- and hydroxy-groups were used as reaction anchor points to covalently attach molecules after framework formation. The co-condensation of benzene-1,3,5-triyltriboronic acid (BTBA) and the 9,10-dimethyl-anthracene-2,3,6,7-tetraol (DMAT) succeeded in the formation of AT-COF-Me. In a radical bromination reaction the methyl groups of an anthracene linker were successfully brominated giving AT-COF-Br without degrading the crystalline framework of AT-COF-Me. The formation of the resulting benzylic bromine was monitored with IR spectroscopy and solid state NMR, respectively. Elemental analysis results correspond to the bromination of half the -CH3 groups. Reaction of (2',5'-dihydroxy-[1,1':4',1''-terphenyl]-4,4''-diyl)diboronic acid (HTDBA) and 2,3,6,7,10,11-hexahydroxytri-phenylene (HHTP) The terphenyl-based hydroxyl substituted T-COF-OH, formed by (2',5'-dihydroxy-[1,1':4',1''-terphenyl]-4,4''-diyl)diboronic acid (HTDBA) and 2,3,6,7,10,11-hexahydroxytri-phenylene (HHTP), was tested in several nucleophilic substitution reactions. Esterification of the –OH group was achieved with either acetylchloride or in a Steglich type reaction with 4-pentynoic acid. X-ray diffraction analysis after the post-synthetic modification shows that the crystallinity of the framework was preserved. This indicates that T-COF-OH is compatible with the reaction conditions. The detection of the newly formed ester moieties in IR and in solid state NMR spectra proves the successful post-synthetic esterification of the –OH groups. Another approach to tailor functionality in COFs is to assemble monomers with distinct properties in COF synthesis. Modification of the backbone of the framework was realized with two heterocyclic building blocks. Benzothiadiazole (BTD) and thienothiophene (TT) monomers are known as building blocks of semiconducting polymers. These molecules were equipped with boronic acid or boronate ester moieties in para position. The linkers were then used in co-condensation reactions with HHTP. The synthesis of BTD-COF was carried out in a two step microwave synthesis procedure: first the pinacolboronate 4,7-Bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)benzo[c][1,2,5]thiadiazole (BTDA) was cleaved with HCl, in a second step addition of HHTP resulted in the crystalline product in only 60 min. TT-COF was synthesized in a conventional co-condensation reaction of thieno[3,2-b]thiophene-2,5-diyldiboronic acid (TTBA) with HHTP; the black TT-COF showed aborbance over the whole spectrum of the visible light. Upon irradiation with light the system showed significant photoconductivity. The 3 nm pores of the hole-transporting TT-COF offer enough space to incorporate large fullerene-based electron-transporting materials such as PCBM. This inclusion leads to a significant quenching of the luminescence of TT-COF, indicating light-induced charge transfer at the interface of these two materials. The oriented growth of thin films of porous COF-10, a product of the condensation of 4,4’-biphenyldiboronic acid(BPBA) and HHTP, and TT-COF on self-assembled monomer (SAM)-functionalized gold surfaces is shown. Films grown on boronic acid terminated SAMs result in a parallel orientation of the pores along the substrate. Scanning electron microscopy was used to investigate the morphology of the films. Homogenous films with thicknesses of around 150 nm and a total coverage of the substrates were obtained. In summary, several functionalization strategies are shown to control or tune the topology and properties of Covalent Organic Frameworks. Tuning the topology and functionality to large open pore systems or intrinsic semiconductivity allows incorporation of large functional molecules and study the host-guest interactions. The post-synthetic modification of COFs offers a synthetic pathway to integrate organic functionalities, which cannot be synthesized directly by co-condensation. These strategies provide the means necessary for a precise control of the pore environment and design a porous material for specific applications. A facile and rapid method to produce thin oriented COF films will pave the way for this material to fabricate technological devices, such as photovoltaic devices, sensors of OFETs.