Hyperpolarized solution-state NMR spectroscopy with optically polarized crystals

Abstract

Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross relaxation at room temperature and moderate magnetic fields (1.45 T). This makes it possible to exploit the high spin polarization of optically polarized crystals while mitigating the challenges of its transfer to external nuclei, particularly of the large distances and prohibitively weak coupling between source and target nuclei across solid-solid or solid-liquid interfaces. With this method, here we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target 1 H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data in order to obtain more conventional NMR spectra, and extract the target nuclei polarization. With the entire process occurring on a timescale of one minute, we observe NMR signals enhanced by factors between -200 and -1730 at 1.45 T for a range of small molecules.

Macroscopic Hyperpolarization Enhanced with Quantum Optimal Control

“…While this is the limit for the lifetime during polarization transfer, the polarization can be stored in the sample afterwards for much longer. Lifetimes between 50 hours [15] and 800 hours [12] have been reported for similar crystals at different temperatures and magnetic fields.…”

“…This is not the case for the "Optimal (Sin.)" sequence which, due to its increased efficiency, has been successfully applied on different crystals with varying spin relaxation times across an extended period of time [15]. This is now the goldstandard pulse in the lab.…”

“…The combination of a 15% faster polarization rate and a 26% higher polarization level provides a factor of 2.6 reduction in the time taken to polarize crystals to within the margin of error of the previous method. This allows for multiple crystals to be polarized per day to be used in external hyperpolarization experiments [15]. Furthermore, these improvements lead to higher levels of polarization in a shorter time, resulting in an overall polarization within the crystal of about 25% [15].…”

“…Due to its increased performance, the "Optimal (Sin.)" pulse was also applied as the hyperpolarization method of choice by Eichhorn et al [15]. In that paper, a bulk crystal polarization of 25% is achieved using the optimized pulse.…”

“…Together with surrounding nuclear spins, this forms a central spin system that resembles other wellknown systems like NV centers in diamond or phosphorous in silicon. This quantum resource for DNP leads to record values of 80% proton polarization [13], which amounts to a polarized proton concentration of 50 M. Exemplary applications of these nuclear spin polarized crystals are portable neutron spin filters in neutron scattering experiments [12,14] and polarization agents for NMR spectroscopy [15,16]. Under typical operating conditions (e.g., high magnetic field), electron and nuclear spins are mutually off resonant, prohibiting direct polarization transfer.…”