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 1 H 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 1 H 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 mm 3 ) containing >10 19 1 H 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.N uclear spin is a useful probe for noninvasive analysis of bulk materials such as chemical compounds, industrial products, biological samples, and human bodies. The signal from a spin ensemble is proportional to the polarization P. In thermal equilibrium in a magnetic field B at temperature T, P for spin-1/2 particles is given bywhere Z is the Planck constant, k is the Boltzmann constant, and γ is the gyromagnetic ratio. In a magnetic field of several teslas at room temperature, the nuclear spin energy ZγB=2 is much smaller than the thermal energy kT, so nuclear spins are only slightly polarized. This is the major reason why the sensitivity of NMR spectroscopy and MRI is so low. Dynamic nuclear polarization (DNP) is a means of transferring spin polarization from electrons to nuclei. As a method to enhance the bulk nuclear polarization, DNP has been successfully applied to areas ranging from fundamental physics (1-3) to materials science (4), biology (5-7), and medical science (8), since it was discovered 60 y ago (9, 10). As long as electron spins in thermal equilibrium are used as polarizing agents, the upper limit of the polarization enhancement is 660 for 1 H spins and cryogenic temperatures of around 4.2 K are required for hyperpolarization in the order of 10% even in the strong magnetic fields used for NMR. Hyperpolarization at room temperature will simplify instrumentation and expand the sample variety to materials that prefer ambient temperatures. Other techniques such as optical pumping in semiconductors (11) and the Haupt effect (12) also require cryogenic temperature for increasing bulk polarization beyond 10%.One solution for overcoming the upper limit of the enhancement factor of the conventional DNP, γ e =γ n , is to use nonthermalized electron spins as polarizing agents. A number of organic molecules have photo-excited triplet states where, due to the selection rule in the intersystem crossing from the excited singlet state to the triplet state, the population distribution is highly biased. DNP using electron spins in the photo-excited triplet state can achieve hyperpolarization independent of the magnetic field strength and temperature (13)(14)(15)(16). In this work, we have achieved a bulk hy...

NMR spectroscopy and MRI are powerful methods for the non-destructive analysis of microscopic structures inside bulk materials and human bodies. As a method to enhance their sensitivities, dynamic nuclear polarization (DNP) has attracted great attention. The intensity of a signal from nuclear spins is proportional to the spin polarization. In magnetic fields conventionally used for NMR spectroscopy and MRI, thermal polarization of nuclear spins at room temperature is in the order of 10 À5 or less. DNP is a means of transferring spin polarization from electrons to nuclei.[1] The thermal polarization of electron spins is 660 times larger than that of 1 H spins, and therefore, DNP can enhance the 1 H spin polarization (hence the sensitivity) by a factor of at most 660.With a lower temperature, electron spins are more highly polarized in thermal equilibrium and therefore a higher nuclear spin polarization can be achieved with DNP. (Note that, even at lower temperature, the upper limit of the polarization enhancement factor with DNP, which is defined as the ratio of the polarization after/before DNP at the same temperature herein, is not increased.) For example, at 4.2 K in 2.5 T, the bulk nuclear polarization can reach the order of 10 %. Developing special peripheral equipment, such as a sample transfer system, [2,3] a temperature jump system, [2][3][4] and a cryogenic magic angle spinning system, [5,6] we are able to combine hyperpolarization at cryogenic temperatures around liquid helium temperature with high-resolution NMR spectroscopy or MRI. Until now, DNP has succeeded in hyperpolarizing various materials, such as trans-membrane proteins, [7] nanocrystals of amyloid, [8] and interfaces of porous media, [9] as well as injectable molecules for MRI.[3] The sample preparation method using a glassy matrix is one of the most important factors with regards to the versatility of DNP.[5] In the above-mentioned study, [2][3][4][5][6][7][8][9] materials of interest are codoped into a glassy matrix together with free radicals.If hyperpolarization can be achieved above liquid nitrogen temperature, the peripheral equipment and the experiments will be simplified and the application field will be broadened. There are many samples of interest for which a higher temperature is preferable. One solution for overcoming the upper limit (660) of the polarization enhancement factor achieved by conventional DNP is to use non-equilibrated electron spins as polarizing agents. A number of molecules, such as pentacene, have photoexcited triplet states where the population distribution over the triplet spin sublevels is highly biased owing to intersystem crossing regardless of temperature and magnetic field strength. [10] With single crystal samples doped with pentacene, DNP utilizing the photoexcited triplet electrons as polarizing agents [11][12][13] has achieved bulk 1 H spin polarizations of 70 % at 105 K [14] and 34 % at room temperature.[15] Herein, we report the first demonstration of DNP using photoexcited triplet electrons in a gl...

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.

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