MODULATION AND SECONDARY BOND ASSISTED FORMATION OF COVALENT ORGANIC FRAMEWORK

ABSTRACTAN ABSTRACT OF THE DISSERTATION OF Lihong Gao, for the Doctor of Philosophy degree in Chemistry, presented on April 1, 2022, at Southern Illinois University Carbondale. TITLE: MODULATION AND SECONDARY BOND ASSISTED FORMATION OF COVALENT ORGANIC FRAMEWORK MAJOR PROFESSOR: Dr.Yuqing Hou Crystalline imidazole-linked covalent organic frameworks (COFs) bearing bromide with rigid subunits were formed via multicomponent reactions (MCRs) under solvothermal conditions. This allows the introduction of functional groups to the COF skeleton via post-synthetic modification (PSD) such as Suzuki coupling reactions, whereas such functional groups may not be able to tolerate long reaction time.Porosity and crystallinity are important to COFs for many properties including adsorption, diffusion and electron transfer. Only reactions with sufficient functional ‘error-correcting’ mechanisms, i.e., covalent bonds can be formed, broken, and reformed at adequately rate, will lead to crystalline frameworks. Amorphous or partially crystalline imine-linked COFs have been reported frequently. We have studied the effects of, modulation, catalysis and supercritical CO2 activation on the porosity and crystallinity on imine-linked COFs. Supercritical CO2 activation increased the pore size and surface area (BET value), but not to a very significant extent. Crystalline COFs were successfully obtained through benzaldehyde modulation during the COF formation. Enhanced crystallinity may be beneficial for applications, for instance, catalysis, electron transfer and gas separation. Chapter 3 covers the conversion of COFs via oxidation of hydrazone to oxadiazole-linked COFs. Crystalline oxadiazole-linked COF was obtained by conversion of hydrazone-linked COF at mild condition (I2, 60 ℃) without metal catalysts, which may be trapped in the pore of COFs. Iodine can be easily removed after the conversion. We believe this strategy paves a way for various functional oxadiazole-linked COFs. In Chapter 4, hydrogen bond assisted rapid COF synthesis is discussed. Free building blocks used for the COF linkers which are rigid or rotationally unrestricted either hard to react or easily result with amorphous materials due to the free intramolecular bond rotation. Thus, secondary bonding, such as intramolecular hydrogen bonding, could be used to restrict bond rotation, orienting the incoming building blocks to form highly crystalline COF. The inter-layer hydrogen bond between COF layers further limits the random sliding, leading to a periodically stacked structure. Dipole-induced rapid construction of COFs with various percentage of terminal alkynes were obtained in one hour under open air. These COFs bearing terminal alkynes can be post-modified with click reaction, reduction, addition, etc. The reaction can be easily scaled up to gram scale, thus, the rapid formation of crystalline COF at large scale lays the ground for further applications of these COFs, as, terminal alkyne present in these COFs makes it possible to tether functional groups onto the framework