Persistent Radicals of Self-assembled Benzophenone bis-Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid-state DNP MAS Spectroscopy

5 years ago

Persistent Radicals of Self-assembled Benzophenone bis-Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid-state DNP MAS Spectroscopy

Linda S. Shimizu, Johan van Tol, Frederic Mentink-Vigier, Alexander M. Brugh, Clifford R. Bowers, Thomas M. Makris, Malcolm D. E. Forbes, Baillie A. DeHaven, John T. Tokarski, Arthur A. Korous

UV-irradiation of a self-assembled benzophenone bis-urea macrocycle generates μm amounts of radicals that persist for weeks under ambient conditions. High-field EPR and variable-temperature X-band EPR studies suggest a resonance stabilized radical pair through H-abstraction. These endogenous radicals were applied as a polarizing agent for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR enhancement. The field-stepped DNP enhancement profile exhibits a sharp peak with a maximum enhancement of ϵon/off=4 superimposed on a nearly constant DNP enhancement of ϵon/off=2 over a broad field range. This maximum coincides with the high field EPR absorption spectrum, consistent with an Overhauser effect mechanism. DNP enhancement was observed for both the host and guests, suggesting that even low levels of endogenous radicals can facilitate the study of host–guest relationships in the solid-state. Macrocycle magic! Persistent organic radicals are generated upon UV-irradiation as a consequence of their self-assembled structure. The endogenous host framework radicals, present in μm quantities, are characterized and applied as a polarizing agent for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR spectroscopy where they displayed a maximum enhancement of four for both host and guest.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701705