Optimization of 1H spin density for dynamic nuclear polarization using photo-excited triplet electron spins

Kagawa, Akio; Murokawa, Yu; Takeda, Kazuyuki; Kitagawa, Masahiro

doi:10.1016/j.jmr.2008.11.009

Cited by 32 publications

(29 citation statements)

References 21 publications

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“…1C) (see SI Text for details of organic synthesis). T 1n of the regioselectively deuterated sample with the deuteration ratio 4/14 = 28.6% is four times longer than the nondeuterated sample, whereas T 1n of the sample blending 30% fully deuterated p-terphenyl-d14 and 70% nondeuterated p-terphenyl-h14 remains almost the same as the nondeuterated sample (21). Therefore, T 1n was increased owing to the regioselective deuteration.…”

Section: Significancementioning

confidence: 93%

Room temperature hyperpolarization of nuclear spins in bulk

Tateishi

Negoro

Nishida

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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129

161

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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...

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Section: Significancementioning

confidence: 93%

Room temperature hyperpolarization of nuclear spins in bulk

Tateishi

Negoro

Nishida

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

129

161

View full text Add to dashboard Cite

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“…With sufficient microwave power to overcome the unfavorable field dependence, SE can generate large DNP enhancements as shown in a trityl-doped solution in a capillary tube held in a cylindrical microwave resonator [31*]. Despite unfavorable field dependence, the option of SE can be exclusive to highly diluted paramagnetic species, such as naturally occurring electron centers in diamonds [117; 118; 119; 120], γ-ray irradiated organic solids [121] and sparsely dye-doped systems in which non-Boltzmann electron spin polarization can be obtained from photoexcited triplets [122; 123]. …”

Section: Polarizing Agents For Dnp In Dielectric Solidsmentioning

confidence: 99%

Polarizing agents and mechanisms for high-field dynamic nuclear polarization of frozen dielectric solids

2011

Solid State Nuclear Magnetic Resonance

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This article provides an overview of polarizing mechanisms involved in high-frequency dynamic nuclear polarization (DNP) of frozen biological samples at temperatures maintained using liquid nitrogen, compatible with contemporary magic-angle spinning (MAS) nuclear magnetic resonance (NMR). Typical DNP experiments require unpaired electrons that are usually exogenous in samples via paramagnetic doping with polarizing agents. Thus, the resulting nuclear polarization mechanism depends on the electron and nuclear spin interactions induced by the paramagnetic species. The Overhauser Effect (OE) DNP, which relies on time-dependent spin-spin interactions, is excluded from our discussion due the lack of conducting electrons in frozen aqueous solutions containing biological entities. DNP of particular interest to us relies primarily on time-independent, spin interactions for significant electron-nucleus polarization transfer through mechanisms such as the Solid Effect (SE), the Cross Effect (CE) or Thermal Mixing (TM), involving one, two or multiple electron spins, respectively. Derived from monomeric radicals initially used in DNP experiments, bi- or multiple-radical polarizing agents facilitate CE/TM to generate significant NMR signal enhancements in dielectric solids at low temperatures (< 100 K). For example, large DNP enhancements (~300 times at 5 T) from a biologically compatible biradical, 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)propan-2-ol (TOTAPOL), have enabled high-resolution MAS NMR in sample systems existing in submicron domains or embedded in larger biomolecular complexes. The scope of this review is focused on recently developed DNP polarizing agents for high-field applications and leads up to future developments per the CE DNP mechanism. Because DNP experiments are feasible with a solid-state microwave source when performed at <20 K, nuclear polarization using lower microwave power (< 100 mW) is possible by forcing a high proportion of biradicals to fulfill the frequency matching condition of CE (two EPR frequencies separated by the NMR frequency) using the strategies involving hetero-radical moieties and/or molecular alignment. In addition, the combination of an excited triplet and a stable radical might provide alternative DNP mechanisms without the microwave requirement.

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“…They judged , as has been studied in connection with realizations of both dynamic nuclear polarization 25 and electron spin resonance of single molecules 12 . Upon S 0 → S 1 photoexcitation with yellow pump light (see Figure 1), ISC provides a strongly inverted spin population between the uppermost (X) and lowest (Z) spin sub-levels of pentacene's triplet ground state 11,12 .…”

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confidence: 99%