Synthesis,Characterization Of Radical Covalent Organic Frameworks And Their Application In Dynamic Nuclear Polarization

Covalent organic frameworks(COFs)have potential applications in the areas of gas adsorption/separation,catalysis,optoelectricity,sensing,and so on due to their high crystallinity and uniformed porosity.It is therefore expected that crystallinity,porosity,and functionality be facilely integrated,precisely adjusted,and further utilized for unique applications.Dynamic Nuclear Polarization(DNP)is a technique to enhance NMR signal intensities and has significant application in characterization of materials.One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents,the basic criteria for which are homogenous distribution and fixed orientation of unpaired electrons.We therefore envisioned that the crystalline and porous structures of COFs,if evenly embedded with radicals,may work as efficient polarizing agents for DNP experiments.So,this thesis mainly focuses on the designed synthesis of PR(x)-COFs via multivariate and bottom-up synthetic strategy,and exploring their applications for DNP.In Chapter 1,we first briefly introduced the background knowledge of covalent organic framework,and then focused on the knowledge of dynamic nuclear polarization.In Chapter 3,we had constructed a series of PR(x)-COFs with different radical concentrations through a multivariate and bottom-up synthesis strategy.FT-IR and ¹³C CP/MAS SSNMR spectra indicated that PR(x)-COFs were formed via imine bonds.PXRD,Nitrogen adsorption and desorption,and ¹²⁹Xe NMR spectroscopy showed that PR(x)-COFs were crystalline materials and had large BET surface areas/pore volumes.Then,we determined the radical concentration of PR(x)-COFs by electron paramagnetic resonance experiment.The results showed that by adjusting the ratio of radical precursor and its reduced form,it is possible to controllably synthesize PR(x)-COFs with different radical concentrations.Finally,we used PR(100)-COF as an example to characterize its magnetic properties and the results showed that it is paramagnetic.In Chapter 4,we used PR(x)-COFs as polarizing agents and performed NMR experiments of the material itself and the absorbed small analytes by DNP method.Firstly,the central-line width of the electron paramagnetic resonance spectra at low temperature showed that the radicals in PR(x)-COFs were uniformly distributed.After illuminating this point,we investigated the effect of radical concentration of PR(x)-COFs on the DNP enhancement and found that as the radical concentration increased,the DNP enhancement first increased and then decreased,among which PR(20)-COF gave the best DNP efficiency,and the ¹H NMR signal was enhanced by 64 times.Then we used DNP method to perform ¹³C,¹⁵N CP/MAS SSNMR experiments of PR(10)-COF.Finally,PR(15)-COF was applied as a polarizing agent to polarize the adsorbed alanine,and an enhancement factor of 44 for ¹³C signal of alanine was obtained.The above results indicated that PR(x)-COFs were really effective polarizing agents for DNP.In Chapter 5,we explored the structure of PROXYL radical COFs We utilized a short-chain radical precursor to react with 1,3,5-Benzenetricarboxaldehyde to synthesize two-dimensional PR-COF with smaller pore-size.In addition,we synthesized three-dimensional radical COF(3D-PR-COF)by using Tetrakis(4-formylphenyl)silane and short-chain radical precursor as monomers.Unfortunately,even though numerous conditions were conducted,crystallinities of these COFs are not satisfactory.We will further attempt to optimize the crystalline structure of these radical COFs.