Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Room-temperature hyperpolarization of nuclear spins in bulk†
K. Tateishi,∗1 M. Negoro,∗2 S. Nishida,∗3 A. Kagawa,∗2 Y. Morita,∗3 and M. Kitagawa∗2
Dynamic Nuclear Polarization (DNP) is a means oftransferring spin polarization from electrons to nuclei.
As a method for enhancing bulk nuclear spin polarization, DNP has been successfully applied to areas
ranging from fundamental physics to materials science,
biology, and medical science. However, as long as electron spins in thermal equilibrium are used as polarizing
agents, the upper limit of the polarization enhancement will be 660 for 1 H spin and cryogenic temperatures of around 4.2 K will be required for hyperpolarization in the order of 10% even under the strong magnetic ﬁelds used for NMR.1) One approach for overcoming the upper limit of the enhancement factor is touse non-thermalized electron spins. DNP with electron
spins in the photo-excited triplet state (triplet-DNP)
can achieve hyperpolarization independent of the magnetic ﬁeld strength and temperature.2) We report 34%
1H spin polarization in 0.40 T at room temperature.
We employed pentacene as a polarizing agent inwhich the excited electron spins polarize 73%, and pterphenyl as a host material because of its stability atroom temperature and large pentacene capacity. The
curve obtained using ThPh in Fig. 1 is the buildup
curve of 1 H spin polarization by triplet-DNP in a single crystal of p-terphenyl-h14 doped with pentacene-h14
0.05 mol%. We attained a 1 H spin polarization of 14%.
The key breakthrough in the present work for attaining higher polarization at room temperature is thesuppression of the spin-lattice relaxation by stableisotope labeling of the constituent molecules. The 1 H
spin-lattice relaxation in p-terphenyl-h14 was mainly
due to the pendulum motion of the central benzene
ring, which modulates the local dipolar ﬁeld of the1H spins in and near the central ring.3) To suppress
the spin-lattice relaxation, we synthesized p-terphenyl2’,3’,5’,6’-d4 , with which the 1 H spin-lattice relaxation
time was increased from 11 min to 37 min. The
attainable polarization was increased to 16% in theregioselectively-deuterated host doped with pentaceneh14 (TdPh in Fig. 1).
There is another source of 1 H spin-lattice relaxationthat aﬀects DNP. The triplet electrons play the roleof a polarizing agent as well as contribute to 1 H spinlattice relaxation through a perturbation of the local
ﬁeld of the 1 H spins in the vicinities. To suppress
the spin-lattice relaxation, we used pentacene-d14 as
the polarizing agent. The attainable polarization was†
∗1
∗2
∗3

Condensed from the article in Proc. Natl. Acad. Sci. USA111, 7527 (2014)
RIKEN Nishina Center
Department of Electronics and Materials Physics, OsakaUniversity
Department of Chemistry, Osaka University- 193 -Fig. 1. Polarization buildup curves. The blue, green, yellow, and red curves denote the polarization buildup
curves using ThPh, TdPh, ThPd, and TdPd, respectively. The polarizations and enhancement factors wereestimated by comparing the intensities of the hyperpolarized signals and the thermal ones in 0.40 T at room
temperature.

increased to 18% in the p-terphenyl-h14 doped withpentacene-d14 (ThPd in Fig. 1).
Suppressing either of the two relaxation sources
was not suﬃcient. By using p-terphenyl-2’,3’,5’,6’d4 doped with pentacene-d14 (TdPd in Fig. 1), weachieved a bulk 1 H spin polarization of 34% at room
temperature in 0.40 T, which results in an enhancement factor of 250,000.
Room-temperature hyperpolarization techniques using photoexcited triplet electrons simplify DNP experiments. The NMR sensitivity of samples that prefer ambient temperatures can be boosted signiﬁcantly.
Bulk nuclear hyperpolarization in such low magneticﬁelds is also desirable for the polarized target for RI
beams4) and the polarized ﬁlter for neutron beams.5)
References
1) T. Maly et al.: J. Chem. Phys. 128 052211 (2008)
2) K. Takeda: Triplet State Dynamic Nuclear Polarization(VDM Verlag, 2009).
3) K. Kahda et al.: J. Phys. Soc. Jpn. 51 3936 (1982).
4) T. Uesaka et al.: Phys. Rev. C 82 021602(R) (2010).
5) M. Haag et al.: Nucl. Instrum. Methods Phys. Res.,
Sect. A 678 91 (2012).