Rigid Orthogonal Bis-TEMPO Biradicals with Improved Solubility for Dynamic Nuclear Polarization

The synthesis and characterization of oxidized bis-thioketal-trispiro
dinitroxide biradicals that orient the nitroxides in a rigid, approximately
orthogonal geometry are reported. The biradicals show better performance
as polarizing agents in dynamic nuclear polarization (DNP) NMR experiments
as compared to biradicals lacking the constrained geometry. In addition,
the biradicals display improved solubility in aqueous media due to
the presence of polar sulfoxides. The results suggest that the orientation
of the radicals is not dramatically affected by the oxidation state
of the sulfur atoms in the biradical, and we conclude that a biradical
polarizing agent containing a mixture of oxidation states can be used
for improved solubility without a loss in performance.