9:15 AM - EQ03.09.04
Room Temperature Hyperpolarization of Polycrystalline Samples with Optically Polarized Triplet Electrons—NV Centers versus Pentacene
Koichiro Miyanishi1,Takuya Segawa2,3,Kazuyuki Takeda3,Izuru Ohki4,Shinobu Onoda5,6,Takeshi Ohshima5,6,Hiroshi Abe5,6,Hideaki Takashima3,Shigeki Takeuchi3,Alexander Shames7,Kohki Morita3,Yu Wang3,Frederick So3,5,Daiki Terada3,5,Ryuji Igarashi5,6,8,Akinori Kagawa1,8,Masahiro Kitagawa1,Norikazu Mizuochi3,Masahiro Shirakawa3,5,Makoto Negoro5
Osaka University1,ETH Zürich2,Kyoto University3,Institute for Chemical Research4,Institute for Quantum Life Science, National Institutes for Quantum and Radiological Science and Technology5,Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology6,Ben-Gurion University of the Negev7,JST, PREST8
Dynamic nuclear polarization (DNP), a technique to transfer spin polarization from electrons to nuclei, has been studied since its early discovery  and has opened the way for high sensitive nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging. In "conventional" DNP, which uses thermally-polarized unpaired electrons as the source of polarization, the DNP enhancement factor is limited to γe/γn, where γe(n) are the gyromagnetic ratios of the electron (nuclear) spins. In DNP using the paramagnetic electrons in thermal equilibrium, experiments need to be performed at cryogenic temperature to increase the electron-spin polarization as much as possible. Conversely, the spins of optically created electrons in the triplet state can have much higher polarization than their thermal equilibrium value, DNP using the triplet state, referred to as triplet DNP, can lead to nuclear hyperpolarization beyond the limit of the conventional DNP using thermal electron polarization. Furthermore, experiments can be carried out at room temperature, by irradiating the sample with laser light. Recently, DNP of an ensemble of 13C nuclear spins using negatively charged nitrogen-vacancy (NV−) color centers in a bulk diamond single crystal has been demonstrated at room temperature and the 13C polarization of 6 % has been achieved via the combination of the thermal mixing and the solid effect . DNP using NV− in powdered microdiamonds has been reported by Ajoy et al., who took advantage of the reduced width of the anisotropic electron spin resonance powder pattern of the NV− centers at the magnetic field of ca. 30 mT . However, triplet DNP is much older and achieved a 1H polarization of 34 % at room temperature and a magnetic field of 0.4 T using pentacene in p-terphenyl crystal . Even though triplet DNP in both systems, NV- centers and pentacene, relies on the transfer of spin polarization from optically hyperpolarized triplet electrons to nuclei , there are important differences. While the NV- center has an electronic triplet ground state and is therefore paramagnetic, pentacene has an electronic singlet ground state, is diamagnetic, and only becomes paramagnetic through optical excitation into a triplet state. On the other hand, the zero-field splitting parameter D for pentacene is only half as large as in NV- centers, which is an advantage when disordered powder is used as a sample.
In this work, we compare triplet-DNP of NV− centers in diamond and pentacene doped in [carboxyl-13C] benzoic acid (PBA) in polycrystalline samples at room temperature . In the DNP experiments, the integrated solid effect (ISE) was used to transfer the polarization from electrons to nuclei. The ISE employs microwave irradiation and external magnetic-field sweep, so that the Hartmann–Hahn matching is implemented between the electron spins in the rotating frame and the nuclear spins in the laboratory frame. We study the behavior of the 13C polarization buildup in terms of the polarization efficiency of the transfer from the electron to nuclei, exchange rate, and the 13C spin diffusion. As a result, we obtained the 13C polarization of 0.01 % in the microdiamonds, and of 0.12 % in PBA at room temperature in a magnetic field of 0.4 T by using the integrated solid effect and the obtained exchange rate was 0.87 % for microdiamonds and 3.5 % for PBA. The 13C polarization enhancements for the diamond and the PBA were 300 and 3600 compared to the thermal NMR polarization. Besides the initial polarization transfer from the triplet electron to the nuclei, we also shed light on the process of nuclear spin-spin diffusion, which distributes the hyperpolarization within the sample.
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