Overhauser Dynamic Nuclear Polarization with Selectively Deuterated BDPA Radicals

The Overhauser effect (OE), commonly observed in NMR spectra of liquids and conducting solids, was recently discovered in insulating solids doped with the radical 1,3-bisdiphenylene-2-phenylallyl (BDPA). However, the mechanism of polarization transfer in OE-DNP in insulators is yet to be established, but hyperfine coupling of the radical to protons in BDPA has been proposed. In this paper we present a study that addresses the role of hyperfine couplings via the EPR and DNP measurements on some selectively deuterated BDPA radicals synthesized for this purpose. Newly developed synthetic routes enable selective deuteration at orthogonal positions or perdeuteration of the fluorene moieties with ²H incorporation of >93%. The fluorene moieties were subsequently used to synthesize two octadeuterated BDPA radicals, 1,3-[α,γ-d₈]-BDPA and 1,3-[β,δ-d₈]-BDPA, and a BDPA radical with perdeuterated fluorene moieties, 1,3-[α,β,γ,δ-d₁₆]-BDPA. In contrast to the strong positive OE enhancement observed in degassed samples of fully protonated h₂₁-BDPA (ε ∼ +70), perdeuteration of the fluorenes results in a negative enhancement (ε ∼ -13), while selective deuteration of α- and γ-positions (a_iso ∼ 5.4 MHz) in BDPA results in a weak negative OE enhancement (ε ∼ -1). Furthermore, deuteration of β- and δ-positions (a_iso ∼ 1.2 MHz) results in a positive OE enhancement (ε ∼ +36), albeit with a reduced magnitude relative to that observed in fully protonated BDPA. Our results clearly show the role of the hyperfine coupled α and γ ¹H spins in the BDPA radical in determining the dominance of the zero and double-quantum cross-relaxation pathways and the polarization-transfer mechanism to the bulk matrix.