CHANGING PARADIGM IN DNP POLARIZING AGENTS DESIGN – CHARMING

This project can be decomposed into four parts: (i) design and synthesis of an organic radical precursor enabling the simultaneous generation of two transient radicals centered on two different heteroatoms for which the difference between their Larmor frequency fit with the one of 13C, (ii) preparation of mesoporous silicas with these radical precursors embedded in the walls, (iii) characterization of these polyradical systems by continuous wave and pulsed EPR experiments and (iv) appraisal of these materials as PA for DNP NMR.

As a first step, a preliminary study determined that the optimum distance between two radicals, face-to-face in the walls of an SBA-15 silica, to obtain optimal NMR signal exaltation was of the order of 0.7 nm. To this end, a wide range of silicas functionalized with stable radicals for which the distance between them increases by 0.2 nm were prepared and characterized. This parameter was used to design the models for the rest of the study.
Three families of transient radical precursors, with different fragmentation mechanisms, were synthesized and incorporated into various SBA-15 mesoporous silicas by direct sol-gel synthesis. These materials were characterized by physico-chemical and spectroscopic (Electron Paramagnetic Resonance) methods. The relaxation times measured for the sulfur and oxygen radicals positioned within the silicas are long enough to envisage their use as polarizing agents for DNP NMR. The quantity of radicals formed depends on the nature of the precursor, i.e. the fragmentation mechanism, and on the shaping of the sample during irradiation. These systems were evaluated as polarizing agents for DNP NMR and no significant exaltation factors were recorded under any experimental conditions.

It would be very interesting to evaluate all silicas functionalized with transient radicals in dissolution DNP NMR.

- “Probing the efficiency of thermal and photochemical bond homolysis in functionalized nanostructured SBA-15 silicas”, Nabokoff, P.; Gastaldi, S.; Besson, E. Micropor. Mesopor. Mat. 2021, 311, 110674. DOI: 10.1016/j.micromeso.2020.110674.

- “Monitoring Crystallization Processes in Confined Porous Materials by Dynamic Nuclear Polarization Solid-state Nuclear Magnetic Resonance”, Juramy, M.; Chèvre, R.; Cerreia Vioglio, P.; Ziarelli, F.; Besson, E.; Gastaldi, S. ; Viel, S.; Thureau, P.; Harris, K. D. M.; Mollica, G. J. Am. Chem. Soc. 2021, 143, 6095-6103. DOI: 10.1021/jacs.0c12982.

- “Investigating the efficiency of silica materials with wall-embedded nitroxide radicals for dynamic nuclear polarisation NMR”, Besson, E; Vebr, A.; Ziarelli, F.; Bloch, E.; Gerbaud, G.; Queyroy, S.; Thureau, P.; Viel, S.; Gastaldi, S. Phys. Chem. Chem Phys. 2022, 24, 25279-25286. DOI: 10.1039/D2CP02872G.

- “Synthesis and Characterization of Face-to-Face Arylsulfanyl and Phenoxyl Radicals Embedded in Mesoporous Silicas”, Nabokoff, P.; Brulay, G.; Dol, C.; Gerbaud, G.; Guigliarelli, B.; Etienne, E.; Bloch, E.; Ziarelli, F.; Besson, E.; Gastaldi, S.; J. Phys. Chem. C. 2023, 127, 9699-9706. DOI: 10.1021/acs.jpcc.3c01550.

Solid-state nuclear magnetic resonance is a well-established spectroscopic technique in many areas of science, which non-destructively provides atomic-level information on a large range of materials. Its main limitation, however, lies in its intrinsically low sensitivity. This issue could be greatly overcome using dynamic nuclear polarisation (DNP), which enhances nuclear magnetisation through the microwave-driven transfer of electron spin polarisation to nuclei, typically via exogenous paramagnetic centres referred to as polarising agents (PA). Unfortunately, in most cases, the performance of existing PA remains far from the theoretical maximum. This project aims at introducing a novel paradigm in the design of PA by exploiting the tremendous increase in lifetime of transient radicals embedded in nanostructured silicas. Such new materials are tools of choice to investigate DNP from a theoretical and experimental point of view.