For solid-state DNP-NMR spectroscopy very large enhancement factors can be achieved when employing the cross-effect, which is especially sufficient when using bi-radicals (e.g. TOTAPOL,bTbk ...). Bi-radicals (or Di-radicals) were not invented for DNP-NMR spectroscopy and a huge amount of EPR literature is available describing their magnetic resonance related properties.
The article cited below gives a comprehensive overview and can a source of new inspiration in the hunt for more efficient polarizing agents for DNP-NMR spectroscopy.
Abe, M., Diradicals. Chem. Rev., 2013. 113(9): p. 7011-7088.
Unfortunately no abstract available.