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The synthesis of air-stable, highly water-soluble organic radicals containing a 1,3-bis(diphenylene)-2-phenylallyl (BDPA) core is reported. A sulfonated derivative, SA-BDPA, retains the narrow electron paramagnetic resonance linewidth (<30 MHz at 5 T) of the parent BDPA in highly concentrated glycerol/water solutions (40 mM), which enables its use as polarizing agent for solid effect dynamic nuclear polarization (SE DNP). A sensitivity enhancement of 110 was obtained in high-field magic-angle-spinning (MAS) NMR experiments. The ease of synthesis and high maximum enhancements obtained with the BDPA-based radicals constitute a major advance over the trityl-type narrow-line polarization agents.
Haze et al. (Thu,) studied this question.