Kenichiro Tateishi

Room temperature hyperpolarization of nuclear spins in bulk

Significance Nuclear spins are only slightly aligned even in the strong magnetic fields of superconducting magnets because the magnetic energy of nuclear spin is much smaller than thermal energy. This is the major reason for the low sensitivity of NMR spectroscopy. Using electron spins in thermal equilibrium, which have 660 times higher magnetic energy, the sensitivity can be enhanced by at most this factor through a method called dynamic nuclear polarization. Utilizing photo-excited nonthermalized electrons instead, we demonstrate an enhancement factor of 250,000 at room temperature, which can be applied to a wide range of fields including NMR, MRI, and physics. Dynamic nuclear polarization (DNP), a means of transferring spin polarization from electrons to nuclei, can enhance the nuclear spin polarization (hence the NMR sensitivity) in bulk materials at most 660 times for 1H spins, using electron spins in thermal equilibrium as polarizing agents. By using electron spins in photo-excited triplet states instead, DNP can overcome the above limit. We demonstrate a 1H spin polarization of 34%, which gives an enhancement factor of 250,000 in 0.40 T, while maintaining a bulk sample (∼0.6 mg, ∼0.7 × 0.7 × 1 mm3) containing >1019 1H spins at room temperature. Room temperature hyperpolarization achieved with DNP using photo-excited triplet electrons has potentials to be applied to a wide range of fields, including NMR spectroscopy and MRI as well as fundamental physics.

Dynamic Nuclear Polarization with Photoexcited Triplet Electrons in a Glassy Matrix

In this decade, dynamic nuclear polarization (DNP) using equilibrated electron spin has attracted considerable attention in the fields of NMR spectroscopy and MRI as a method to enhance sensitivity. The intensity of a signal from nuclear spins is proportional to the spin polarization. DNP is a means of transferring spin polarization from electrons to nuclei, and the equilibrated polarization of electron spins is 660 times larger than that of H spins. Developing special peripheral equipment, we are able to combine hyperpolarization at cryogenic temperatures around liquid helium temperature with high-resolution NMR spectroscopy or MRI. For such applications, the sample preparation method which materials of interest are codoped into a glassy matrix together with free radicals is one of the most important factors in terms of versatility. On the other hand, by using single crystal of organic molecules, we have developed a polarized solid-state target with DNP using photoexcited triplet electron spin (triplet-DNP) of pentacene. The polarization of such non-equilibrated electron spins is more than 70% independent of temperature and magnetic field. Using this method, we can overcome the upper limit (660) of the polarization enhancement factor achieved by conventional DNP. Herein, we report the first demonstration of triplet-DNP in a glassy matrix for application in NMR spectroscopy and MRI. We applied two types of host molecules that have higher glass transition temperature than conventionally used glasses. One is a non-polar molecule, oterphenyl (OTP). The other is a polar molecule, benzophenone (BZP). Using partially deuterated OTP and BZP as host materials, we obtained 1.5% and 0.7% H spin polarization under 0.4 T at 120 K, respectively (Fig. 1). The enhancement factor for OTP and BZP was 4,250 and 1,900, respectively. We have also succeeded in polarizing third molecules, 2, 3, 4trifluorobenzoic acid and 5-fluorouracil, codoped into a glassy matrix with polarizing agent (Fig. 2). F spin in the thrid molecules were polarized using the field cycling method. The use of photoexcited triplet electrons is a promising method to extend the limitation of DNP to higher temperatures. If hyperpolarization can be achieved above liquid nitrogen temperature, the peripheral equipment and the experiments for spectroscopy will

Scalable spin amplification with a gain over a hundred.

We propose a scalable and practical implementation of spin amplification which does not require individual addressing nor a specially tailored spin network. We have demonstrated a gain of 140 in a solid-state nuclear spin system of which the spin polarization has been increased to 0.12 using dynamic nuclear polarization with photoexcited triplet electron spins. Spin amplification scalable to a higher gain opens the door to the single spin measurement for a readout of quantum computers as well as practical applications of nuclear magnetic resonance spectroscopy to infinitesimal samples which have been concealed by thermal noise.

H2-decoupling-accelerated H1 spin diffusion in dynamic nuclear polarization with photoexcited triplet electrons

In dynamic nuclear polarization (DNP) experiments applied to organic solids for creating nonequilibrium, high (1)H spin polarization, an efficient buildup of (1)H polarization is attained by partially deuterating the material of interest with an appropriate (1)H concentration. In such a dilute (1)H spin system, it is shown that the (1)H spin diffusion rate and thereby the buildup efficiency of (1)H polarization can further be enhanced by continually applying radiofrequency irradiation for deuterium decoupling during the DNP process. As experimentally confirmed in this work, the electron spin polarization of the photoexcited triplet state is mainly transferred only to those (1)H spins, which are in the vicinity of the electron spins, and (1)H spin diffusion transports the localized (1)H polarization over the whole sample volume. The (1)H spin diffusion coefficients are estimated from DNP repetition interval dependence of the initial buildup rate of (1)H polarization, and the result indicates that the spin diffusion coefficient is enhanced by a factor of 2 compared to that without (2)H decoupling.

Kinetic Parameters of Photo-Excited Triplet State of Pentacene Determined by Dynamic Nuclear Polarization

The lifetimes and spin–lattice relaxation time of photo-excited triplet electron of pentacene doped in p-terphenyl at room temperature have been investigated. Values of spin–lattice relaxation time previously reported in ESR studies are inconsistent with each other. In this paper, we determined these time constants based on proton signals enhanced by dynamic nuclear polarization using the electrons (Triplet-DNP). The combined analysis of dependences of proton signal intensities on the delay time of polarization transfer and laser pulse structure allows us to disentangle contributions of the lifetimes and spin–lattice relaxation time. The lifetimes of triplet sublevels with ms = 0 and ±1 were determined to be 22.3 and 88 µs, respectively. The spin–lattice relaxation time was found to be longer than 300 µs, hence the time evolution of the electron population in the triplet state is governed by the lifetimes. It was also found that the proton signal enhancement is limited at a high repetition rate by the par...

Hyperpolarization of Thin Films with Dynamic Nuclear Polarization Using Photoexcited Triplet Electrons

With dynamic nuclear polarization using the photoexcited triplet electron spin (triplet-DNP) of pentacene, nuclear spins can be hyperpolarized even in a low magnetic field at room temperature. Several demonstrations have been performed using bulk crystals. Hyperpolarization in a thin film with triplet-DNP enables new applications, such as general NMR spectroscopy and the polarized target of unstable nuclei. In this work, we succeeded in polarizing 1H spins in a thin film fabricated by the cell method. We obtained a 1H spin polarization of 12.9% using a 7-μm-thick film of \(p\)-terphenyl doped with pentacene in 0.4 T at room temperature. We also obtained a 1H spin polarization of 3.9% in 0.4 T at 150 K using a 60-μm-thick film of trans-stilbene doped with pentacene, whose single crystal cannot be made easily by conventional methods.

Dissolution Dynamic Nuclear Polarization at Room Temperature Using Photoexcited Triplet Electrons

Dissolution dynamic nuclear polarization (DNP) has recently gained attention as a method to enhance the sensitivity of liquid NMR spectroscopy and MRI. We demonstrate dissolution of the sample hyperpolarized by DNP using photoexcited triplet electrons in 0.38 T at room temperature. The achieved polarization of 0.8% is 6100 times as high as that at thermal equilibrium under the condition. The result is an important step for DNP using photoexcited triplet electrons to become widely used in chemical and biomedical research.

Studies of Unstable Nuclei with Spin-Polarized Proton Target

Roles of spin-dependent interactions in unstable nuclei have been investigated via the direct reaction of radioactive ions with a solid spin-polarized proton target. The target has a unique advantage of a high polarization of 20–30% under low magnetic field of 0.1 T and at a high temperature of 100 K, which allow us to detect recoil protons with good angular resolution. Present status of on-going experimental studies at intermediate energies, such as proton elastic scattering and (p, 2p) knockout reaction, and new physics opportunities expected with low-energy RI beams are overviewed.