Institute for Soldier Nanotechnologies, Cambridge, Massachusetts 02139, United States.
J Am Chem Soc. 2021 Dec 8;143(48):20281-20290. doi: 10.1021/jacs.1c09406. Epub 2021 Nov 23.
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 H incorporation of >93%. The fluorene moieties were subsequently used to synthesize two octadeuterated BDPA radicals, 1,3-[α,γ-]-BDPA and 1,3-[β,δ-]-BDPA, and a BDPA radical with perdeuterated fluorene moieties, 1,3-[α,β,γ,δ-]-BDPA. In contrast to the strong positive OE enhancement observed in degassed samples of fully protonated -BDPA (ε ∼ +70), perdeuteration of the fluorenes results in a negative enhancement (ε ∼ -13), while selective deuteration of α- and γ-positions ( ∼ 5.4 MHz) in BDPA results in a weak negative OE enhancement (ε ∼ -1). Furthermore, deuteration of β- and δ-positions ( ∼ 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 H 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.
奥弗豪泽效应（OE）在液体和导电固体的 NMR 谱中普遍存在，最近在掺杂自由基 1,3-双二苯乙烯-2-苯丙烯（BDPA）的绝缘固体中被发现。然而，OE-DNP 在绝缘体中极化转移的机制尚未建立，但自由基与 BDPA 中质子的超精细耦合已被提出。在本文中，我们通过为此目的合成的一些选择性氘代 BDPA 自由基的 EPR 和 DNP 测量，研究了超精细耦合的作用。新开发的合成路线可实现对位或全氘代芴基的选择性氘代，氘代率 >93%。芴基随后用于合成两个十八氘代 BDPA 自由基，1,3-[α,γ-]-BDPA 和 1,3-[β,δ-]-BDPA，以及一个全氘代芴基的 BDPA 自由基，1,3-[α,β,γ,δ-]-BDPA。与完全质子化的 -BDPA（ε∼+70）在除气样品中观察到的强烈正 OE 增强形成对比，芴基的全氘代导致负增强（ε∼-13），而 BDPA 的α-和γ-位（∼5.4 MHz）的选择性氘代导致弱负 OE 增强（ε∼-1）。此外，β-和δ-位（∼1.2 MHz）的氘代导致正 OE 增强（ε∼+36），尽管与完全质子化的 BDPA 相比，增强幅度减小。我们的结果清楚地表明，BDPA 自由基中超精细耦合的α和γ H 自旋在确定零和双量子交叉弛豫途径以及极化转移机制对基体的主导作用方面的作用。
