$^{\mathrm{13}}$C dynamic nuclear polarization using a mixture of BDPA and trityl OX063

Recently, there have been growing efforts to optimize dynamic nuclear polarization (DNP) in pursuit of achieving the highest enhancements of NMR spectroscopy and imaging (MRI) signals. A substantial portion of such optimization efforts has focused upon the role of free radicals in DNP. In this work, we have examined the effects of changing the concentration of a 1:1 mixture of two narrow line-width free radicals: trityl OX063 (trityl) and 1,3-bisdiphenylene-2-phenylallyl (BDPA). The polarization profiles, maxima, and build-up times were compared for different concentrations of trityl and BDPA under the constraint that both radicals had the same concentration. This study found that the polarization maxima displayed a sort of competition between two contributors to the intensity: the spin-lattice relaxation time and the number of polarizing centers. As expected based upon the mechanics of DNP, the extrapolated build-up times fell with increasing radical concentration. These results are discussed in the context of thermal mixing DNP mechanism.