Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Dynamic nuclear polarization with photoexcited triplet electrons in aglassy matrix†
K. Tateishi,∗1 M. Negoro,∗2 A. Kagawa,∗2 and M. Kitagawa∗2
In this decade, dynamic nuclear polarization (DNP)
using equilibrated electron spin has attracted considerable attention in the fields of NMR spectroscopy andMRI as a method to enhance sensitivity.1) The intensity of a signal from nuclear spins is proportional to thespin polarization. DNP is a means of transferring spinpolarization from electrons to nuclei, and the equilibrated polarization of electron spins is 660 times larger
than that of 1 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 themost important factors in terms of versatility.
On the other hand, by using single crystal of organic
molecules, we have developed a polarized solid-statetarget with DNP using photoexcited triplet electron
spin (triplet-DNP) of pentacene.2) The polarization ofsuch 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 applicationin NMR spectroscopy and MRI.
We applied two types of host molecules that havehigher glass transition temperature than conventionally used glasses. One is a non-polar molecule, oterphenyl (OTP).3) The other is a polar molecule, benzophenone (BZP).4) Using partially deuterated OTP
and BZP as host materials, we obtained 1.5% and0.7% 1 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 havealso succeeded in polarizing third molecules, 2, 3, 4trifluorobenzoic acid and 5-fluorouracil, codoped intoa glassy matrix with polarizing agent (Fig. 2). 19 F
spin in the thrid molecules were polarized using thefield cycling method.5)
The use of photoexcited triplet electrons is a promising method to extend the limitation of DNP to highertemperatures. If hyperpolarization can be achieved
above liquid nitrogen temperature, the peripheral
equipment and the experiments for spectroscopy will†
∗1
∗2

Condensed from the article in Angew. Chem. Int. Ed. 52,13307-13310 (2013)
RIKEN Nishina Center
Department of Electronics and Materials Physics, OsakaUniversityFig. 1. 1 H spin polarization buildup curves for (a) partiallydeuterated OTP ([D14]OTP/OTP=90:10 wt%) and (b)partially deuterated BZP ([D10]BZP/BZP=90:10 wt%)
doped with 0.05 mol% pentacene.

Fig. 2. Polarized 19 F NMR spectra of (a) 2,3,4trifluorobenzoic acid and (b) 5-fluorouracil in glassy
matrices. After DNP for 3 min and field cycling, the 19 F
NMR signals were acquired. The NMR signals of thesamples under 0.40 T at 120 K in thermal equilibrium
are also shown.

be simplified, and the application field will be broadened. There are many samples of interest for which ahigher temperature is preferable. DNP using photoexcited triplet electrons has the potential to significantly
enhance the NMR/MRI sensitivity while the sample is
kept at room temperature.
References
1) D. A. Hall et al.: Science 276, 930 (1997).
2) T. Uesaka et al.: Phys. Rev. C 82, 021602(R) (2010).
3) S. V. Adichtchev et al.: J. Non-Cryst. Solids 353, 1491(2007).
4) L. M. Babkov et al.: J. Mol. Struct. 887, 87 (2008).
5) M. Negoro et al.: Phys. Rev. Lett. 107, 050503 (2011).

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