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Structural investigations and postsynthetic modifications in covalent organic frameworks
Structural investigations and postsynthetic modifications in covalent organic frameworks
The research on porous functional materials has seen a great increase in activities with the introduction of the first metal-organic frameworks (MOFs) in 1999. The construction of porous frameworks from secondary building units and organic linkers allows for the modular construction of porous crystalline frameworks. With the development of the purely organic covalent organic frameworks (COFs) in 2005, it is now possible to transfer this concept to the world of organic polymers and to synthesize organic porous and crystalline materials with a defined control over their composition and geometry. In covalent organic frameworks, organic linkers are connected via reversible organic reactions. While the reversibility of their formation reaction allows for the arrangement of the building blocks in an ordered fashion, the symmetry and connectivity of the linkers predefines the symmetry of the lattice and the size and shape of the pores in this networks. Furthermore, by decorating the building blocks with certain functionalities, the chemical character of the pore walls in the COF can be defined. A comprehensive overview over the structures realized in COFs to date alongside with a detailed discussion of synthesis strategies and applications of COFs is given in Chapter 1. Matrices are used to depict the linker combinations that have been reported for COF synthesis. The practical part of this work focuses on two main tasks in COF research: the control of the crystallinity of COFs and their postsynthetic modification to tune the properties of the frameworks. In addition, an application of postsynthetically modified COFs in liquid adsorption is presented. In the first project of this thesis, the structural impact of functional side groups in COFs is investigated. For that, the benzodithiophene linker in BDT COF is functionalized with ethoxy side chains. With this linker, the COF BDT-OEt is synthesized as crystalline, highly porous framework. To investigate the structural changes upon incorporation of OEt functionalized linker, a series of COFs with increasing OEt content is prepared. A gradual, almost linear decrease of both the pore diameter and the crystallite domain size is found for an increased content of BDT-OEt. Furthermore, the composition of the resulting framework can be pre-determined by the ratio of the linkers used in the synthesis allowing for a fine-tuning of the structural properties of the mixed-linker COF. Theoretical calculations applying Monte Carlo and DFT simulations reveal an increasing lateral offset of the COF layers upon incorporation of higher amounts of the OEt-functionalized linker. In the second project, the impact of a self-assembled monolayer on the growth of COF-5 as thin film on substrates is investigated. The study reveals a formation of thin films on gold substrates that are non-functionalized as well as on mercaptohexadecane SAMs that are functionalized with CH3, OH, and COOH, respectively. The films feature preferential orientation of the crystallites with the c-direction being aligned perpendicular to the sub- strate surface plane. For substrates functionalized with -OH or -COOH terminated SAMs, a remarkably high degree of orientation of the crystallites was found, indicating that the -OH and -COOH groups can act as modulating agent to promote the formation of highly ordered COF crystallites. Postsynthetic modification is a versatile strategy to increase the available pool of functionalizations in solid materials. In COFs, this approach is to date limited to only a few reaction pathways. In order to broaden the spectrum of postsynthetic modification reactions in covalent organic frameworks, a reaction pathway for the incorporation of primary amines as functional groups in COFs is developed in the third project. First the nitro-functionalized chemically highly stable beta-ketoenamine-linked COF TpBD(NO2)2 is synthesized. The nitro groups are then reduced to amines to yield the COF TpBD(NH2)2 while preserving the crystallinity and porosity of the framework. The accessibility of the established amino groups is demonstrated by a sequential modification, the aminolysis of acetic anhydride to create the amide COF TpBD(NHCOCH3)2. Furthermore, the performance of the COFs is tested in the adsorption of lactic acid, a major precursor for biodegradable plastics. A strong impact of the COF functionalization on the adsorption performance was found, with the amino-functionalization leading to a distinct increase in lactic acid adsorption compared to the pristine nitro-functionalized COF. In the last project, postsynthetically introduced primary amines are utilized to amend the optical absorption of a COF by postsynthetic modification. The amino COF TpBD(NH2)2 is applied in azo coupling with N,N-dimethylaniline and diphenylamine, respectively. The degree of modification can be increased up to a quantitative conversion of the amino groups to azo groups. Diffuse reflectance UV-vis measurements reveal the impact of the azo dye functionalization on the visible absorption of the COF. With diphenylamine as coupling agent and a conversion of 50% of the amino groups, a strong bathochromic shift of the UV-vis absorption can be observed, demonstrating the tunability of the optical absorption in COFs via postsynthetic modification.
Covalent Organic Frameworks, Porous Materials, Postsynthetic Modification
Lohse, Maria
2017
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Lohse, Maria (2017): Structural investigations and postsynthetic modifications in covalent organic frameworks. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

The research on porous functional materials has seen a great increase in activities with the introduction of the first metal-organic frameworks (MOFs) in 1999. The construction of porous frameworks from secondary building units and organic linkers allows for the modular construction of porous crystalline frameworks. With the development of the purely organic covalent organic frameworks (COFs) in 2005, it is now possible to transfer this concept to the world of organic polymers and to synthesize organic porous and crystalline materials with a defined control over their composition and geometry. In covalent organic frameworks, organic linkers are connected via reversible organic reactions. While the reversibility of their formation reaction allows for the arrangement of the building blocks in an ordered fashion, the symmetry and connectivity of the linkers predefines the symmetry of the lattice and the size and shape of the pores in this networks. Furthermore, by decorating the building blocks with certain functionalities, the chemical character of the pore walls in the COF can be defined. A comprehensive overview over the structures realized in COFs to date alongside with a detailed discussion of synthesis strategies and applications of COFs is given in Chapter 1. Matrices are used to depict the linker combinations that have been reported for COF synthesis. The practical part of this work focuses on two main tasks in COF research: the control of the crystallinity of COFs and their postsynthetic modification to tune the properties of the frameworks. In addition, an application of postsynthetically modified COFs in liquid adsorption is presented. In the first project of this thesis, the structural impact of functional side groups in COFs is investigated. For that, the benzodithiophene linker in BDT COF is functionalized with ethoxy side chains. With this linker, the COF BDT-OEt is synthesized as crystalline, highly porous framework. To investigate the structural changes upon incorporation of OEt functionalized linker, a series of COFs with increasing OEt content is prepared. A gradual, almost linear decrease of both the pore diameter and the crystallite domain size is found for an increased content of BDT-OEt. Furthermore, the composition of the resulting framework can be pre-determined by the ratio of the linkers used in the synthesis allowing for a fine-tuning of the structural properties of the mixed-linker COF. Theoretical calculations applying Monte Carlo and DFT simulations reveal an increasing lateral offset of the COF layers upon incorporation of higher amounts of the OEt-functionalized linker. In the second project, the impact of a self-assembled monolayer on the growth of COF-5 as thin film on substrates is investigated. The study reveals a formation of thin films on gold substrates that are non-functionalized as well as on mercaptohexadecane SAMs that are functionalized with CH3, OH, and COOH, respectively. The films feature preferential orientation of the crystallites with the c-direction being aligned perpendicular to the sub- strate surface plane. For substrates functionalized with -OH or -COOH terminated SAMs, a remarkably high degree of orientation of the crystallites was found, indicating that the -OH and -COOH groups can act as modulating agent to promote the formation of highly ordered COF crystallites. Postsynthetic modification is a versatile strategy to increase the available pool of functionalizations in solid materials. In COFs, this approach is to date limited to only a few reaction pathways. In order to broaden the spectrum of postsynthetic modification reactions in covalent organic frameworks, a reaction pathway for the incorporation of primary amines as functional groups in COFs is developed in the third project. First the nitro-functionalized chemically highly stable beta-ketoenamine-linked COF TpBD(NO2)2 is synthesized. The nitro groups are then reduced to amines to yield the COF TpBD(NH2)2 while preserving the crystallinity and porosity of the framework. The accessibility of the established amino groups is demonstrated by a sequential modification, the aminolysis of acetic anhydride to create the amide COF TpBD(NHCOCH3)2. Furthermore, the performance of the COFs is tested in the adsorption of lactic acid, a major precursor for biodegradable plastics. A strong impact of the COF functionalization on the adsorption performance was found, with the amino-functionalization leading to a distinct increase in lactic acid adsorption compared to the pristine nitro-functionalized COF. In the last project, postsynthetically introduced primary amines are utilized to amend the optical absorption of a COF by postsynthetic modification. The amino COF TpBD(NH2)2 is applied in azo coupling with N,N-dimethylaniline and diphenylamine, respectively. The degree of modification can be increased up to a quantitative conversion of the amino groups to azo groups. Diffuse reflectance UV-vis measurements reveal the impact of the azo dye functionalization on the visible absorption of the COF. With diphenylamine as coupling agent and a conversion of 50% of the amino groups, a strong bathochromic shift of the UV-vis absorption can be observed, demonstrating the tunability of the optical absorption in COFs via postsynthetic modification.