The work performed towards the objectives of the project was divided into the following areas: (1) development of improved polarizing agents for DNP-enhanced NMR, (2) development of sample formulations that maintain favorable conditions for DNP-enhanced NMR at high temperatures, and (3) development of room temperature DNP in liquids.
(1) Development of polarizing agents: Significant work was carried out to better understand the function of DNP polarizing agents and develop strategies to improve them. DNP is typically performed with organic radical dopants, but the relationship between their atomic-structure and the overall DNP efficiency is not clearly understood. A major milestone towards this goal was accomplished (Angew. Chem. Int. Ed. 2023, e202304844). In this study, the mechanism of NMR signal enhancement via the so-called ‘cross effect’ in the polarizing agent ‘TEKPol’ was studied, which led to the invention of the best-performing polarizing agent in organic solvents to date, dubbed ‘NaphPol’. In addition, work was carried out to evaluate the factors that determine the overall sensitivity provided by DNP using a series of polarizing agents, and strategies to design new DNP polarizing agents. Our results provide new polarizing agents that are effective in both organic and water-based solvents, demonstrating their high versatility.
On the other hand, Gadolinium (Gd)-based polarizing agents are promising for DNP due to their stability in highly reactive chemical environments. However, previously reported Gadolinium polarizing agents perform poorly in comparison to organic radicals. We developed a model that clearly establishes the relationship between the symmetry of the environment around the metal center in Gd complexes and the DNP enhancement (J. Phys. Chem. C 2022, 126, 11310-11317). These results will guide the design of improved Gd-based polarizing agents.
(2) The efficacy of DNP at high temperatures (–70 to 30 °C) depends largely on the solvent used to disperse the polarizing agent. We evaluated the factors that limit the overall DNP efficiency at room temperature in polymers which include the concentration of hydrogen atoms, impurities, and dissolved polarizing agent, which are critical for high DNP enhancements. Our results will provide a recipe for enabling DNP at high temperatures.
(3) Finally, room temperature DNP was successfully achieved in liquids (J. Phys. Chem. Lett. 2022, 13, 7749-7755). While previous research in this area was primarily limited to carbon nuclei in organic molecules, a strategy to transfer polarization to the highly abundant hydrogen atoms was developed. This result opens new possibilities to study challenging systems that contain hydrogen atoms, at room temperature.
In addition to the above, DNP was applied to address challenges in materials science such as the atomic-level characterization of cementitious materials and growth mechanism of shells on nanoparticles. These results impact the development of sustainable materials for society.
