Long-Lived, Transportable Reservoir of Nuclear Polarization Used to Strongly Enhance Solution-State NMR Signals

There is a fundamental issue with the use of dynamic nuclear polarization (DNP) to enhance nuclear spin polarization: the same polarizing agent (PA) needed for DNP is also responsible for shortening the lifetime of the hyperpolarization. As a result, long-term storage and transport of hyperpolarized samples is severely restricted and the apparatus for DNP is necessarily located near or integrated with the apparatus using the hyperpolarized spins. In this paper, we demonstrate that naphthalene single crystals can serve as a long-lived reservoir of proton polarization that can be exploited to enhance signals in benchtop and high-field NMR of target molecules in solution at a site 300 km away by a factor of several thousand. The naphthalene protons are polarized using short-lived optically excited triplet states of pentacene instead of stable radicals. In the absence of optical excitation, the electron spins remain in a singlet ground state, eliminating the major pathway of nuclear spin-lattice relaxation. The polarization decays with a time constant of about 50 h at 80 K and 0.5 T or above 800 h at 5 K and 20 mT. A module based on a Halbach array yielding a field of 0.75 T and a conventional cryogenic dry shipper, operating at liquid nitrogen temperature, allows storage and long distance transport of the polarization to a remote laboratory, where the polarization of the crystal is transferred after dissolution to a target molecule of choice by intermolecular cross-relaxation. The procedure has been executed repeatedly and has proven to be reliable and robust.

Automated summary

This paper addresses a fundamental issue in the use of dynamic nuclear polarization (DNP) regarding the short lifetime of hyperpolarization due to the polarizing agent (PA) used. The authors demonstrate that naphthalene single crystals can serve as a long-lived reservoir of proton polarization, which can be utilized to enhance NMR signals of target molecules in solution at a remote site by several thousand fold. The polarization of naphthalene protons is achieved using short-lived optically excited triplet states, providing a solution to the restricted storage and transport of hyperpolarized samples.