Dynamic nuclear polarization of TEMPO radical cross conjugated with a thioanisole scaffold

Abstract

Nuclear magnetic resonance (NMR) spectroscopy is an important technique for molecular structure determination but is inherently limited by its low sensitivity. Recently, the Dynamic nuclear polarization (DNP) technique has emerged as a solution to overcome the intrinsic low sensitivity of NMR spectroscopy by transferring polarization from the unpaired electron spins to nuclear spins under microwave irradiation, achieving a theoretical sensitivity enhancement of up to 658-fold for the detection of ¹H. In this study, we report the synthesis and characterization of a series of TEMPO(2,2,6,6-tetramethylpiperidine-1-oxyl)-based radicals functionalized with a thioanisole scaffold, designed to facilitate self-assembled monolayers on gold surfaces. The radicals were investigated using electron paramagnetic resonance (EPR) and DNP NMR spectroscopy. These radicals demonstrated properties consistent with the standard TEMPO while maintaining stability and functionality. At 10 mM concentration in TCE (1,1,2,2-tetrachloroethane), Radical-Imine-1 yielded a DNP enhancement factor of 3.2 for ¹H nuclei and that of standard TEMPO is around 2.8 at 14.1 T. Relaxation measurements revealed that longitudinal relaxation times (T₁) decreased with radical concentration, while transverse relaxation times (T₂) remain largely unaffected, indicating minimal perturbation from paramagnetic quenching. The structural stability and surface-binding potential of the methyl thiol group make these derivatives suitable for surface-based DNP applications.