Category Archives: photoexcited triplet state

Materials chemistry of triplet dynamic nuclear polarization #DNPNMR

Nishimura, Koki, Hironori Kouno, Yusuke Kawashima, Kana Orihashi, Saiya Fujiwara, Kenichiro Tateishi, Tomohiro Uesaka, Nobuo Kimizuka, and Nobuhiro Yanai. “Materials Chemistry of Triplet Dynamic Nuclear Polarization.” Chemical Communications 56, no. 53 (2020): 7217–32.

https://doi.org/10.1039/D0CC02258F

This Feature Article overviews the recently-emerged materials chemistry of triplet dynamic nuclear polarization (triplet-DNP) towards biological and medical applications.

Dynamic nuclear polarization with photo-excited triplet electrons (triplet-DNP) has the potential to enhance the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at a moderate temperature. While many efforts have been devoted to achieving a large nuclear polarization based on triplet-DNP, the application of triplet-DNP has been limited to nuclear physics experiments. The recent introduction of materials chemistry into the field of triplet-DNP has achieved air-stable and water-soluble polarizing agents as well as the hyperpolarization of nanomaterials with a large surface area such as nanoporous metal–organic frameworks (MOFs) and nanocrystal dispersion in water. This Feature Article overviews the recently-emerged materials chemistry of triplet-DNP that paves new paths towards unprecedented biological and medical applications.

Conformational control of nonplanar free base porphyrins: towards bifunctional catalysts of tunable basicity #DNPNMR

Roucan, M., M. Kielmann, S. J. Connon, S. S. R. Bernhard, and M. O. Senge. “Conformational Control of Nonplanar Free Base Porphyrins: Towards Bifunctional Catalysts of Tunable Basicity.” Chemical Communications 54, no. 1 (2018): 26–29.

https://doi.org/10.1039/C7CC08099A

For the first time, free base and N-methylated porphyrins have been utilized as bifunctional organocatalysts in Michael additions and it was found that distortion of the macrocycle is a vital prerequisite for their catalytic activity. Conformational design has been used to tailor the properties of nonplanar porphyrins with regards to availability of the N–H units for hydrogen bonding (distortion-dependent hydrogen bonding) and the basicity of the heterocyclic groups. NMR spectroscopic- and catalyst screening studies provided insight into the likely mode of catalyst action. This unprecedented use of free base and N-substituted porphyrins as organocatalysts opens a new functional role for porphyrins.

High-field NMR with dissolution triplet-DNP

Kagawa, Akinori, Koichiro Miyanishi, Naoki Ichijo, Makoto Negoro, Yushi Nakamura, Hideo Enozawa, Tsuyoshi Murata, Yasushi Morita, and Masahiro Kitagawa. “High-Field NMR with Dissolution Triplet-DNP.” Journal of Magnetic Resonance 309 (December 2019): 106623.

https://doi.org/10.1016/j.jmr.2019.106623

Dissolution dynamic nuclear polarization (DNP) has wide variety of important applications such as real-time monitoring of chemical reactions and metabolic imaging. We construct DNP using photoexcited triplet electron spins (Triplet-DNP) apparatus combined with dissolution apparatus for solution NMR in a high magnetic field. Triplet-DNP enables us to obtain high nuclear polarization at room temperature. Solid-state samples polarized by Triplet-DNP are transferred to a superconducting magnet and dissolved by injecting aqueous solvents. The 13C polarization of 0.22 % has been obtained for [caryboxy-13C]benzoic acid-d in the liquid state. Our results show that Triplet-DNP can be applied to real-time monitoring with solution NMR.

Dynamic nuclear polarization with photo-excited triplet electrons using 6,13-diphenylpentacene #DNPNMR

Tateishi, Kenichiro, Makoto Negoro, Hiroshi Nonaka, Akinori Kagawa, Shinsuke Sando, Satoshi Wada, Masahiro Kitagawa, and Tomohiro Uesaka. “Dynamic Nuclear Polarization with Photo-Excited Triplet Electrons Using 6,13-Diphenylpentacene.” Physical Chemistry Chemical Physics 21, no. 36 (2019): 19737–41.

https://doi.org/10.1039/C9CP00977A

Dynamic nuclear polarization with photo-excited triplet electrons (Triplet-DNP) is demonstrated using 6,13-diphenylpentacene (DPPentacene). DPPentacene is soluble in various organic solvents, while pentacene, which is used in most of the triplet-DNP experiments, has limited solubility. An enhancement factor of 81 is obtained for 1H spins in the glass of ethanol-d6 : water = 80 : 20 (w/w) doped with 0.1 mM DPPentacene at 90 K in 0.67 T.

Triplet dynamic nuclear polarization of nanocrystals dispersed in water at room temperature

Nishimura, Koki, Hironori Kouno, Kenichiro Tateishi, Tomohiro Uesaka, Keiko Ideta, Nobuo Kimizuka, and Nobuhiro Yanai. “Triplet Dynamic Nuclear Polarization of Nanocrystals Dispersed in Water at Room Temperature.” Physical Chemistry Chemical Physics 21, no. 30 (2019): 16408–12.

https://doi.org/10.1039/C9CP03330K

While dynamic nuclear polarization using photo-excited triplet electrons (triplet-DNP) can improve the sensitivity of nuclear magnetic resonance at room temperature, it has not been carried out in water. Here, we report the first example of triplet-DNP in water by downsizing the conventional bulk crystals to nanocrystals.

Nonpentacene Polarizing Agents with Improved Air Stability for Triplet Dynamic Nuclear Polarization at Room Temperature

Kouno, Hironori, Yusuke Kawashima, Kenichiro Tateishi, Tomohiro Uesaka, Nobuo Kimizuka, and Nobuhiro Yanai. “Nonpentacene Polarizing Agents with Improved Air Stability for Triplet Dynamic Nuclear Polarization at Room Temperature.” The Journal of Physical Chemistry Letters 10, no. 9 (May 2, 2019): 2208–13.

https://doi.org/10.1021/acs.jpclett.9b00480

Triplet dynamic nuclear polarization (triplet-DNP), a method to enhance the NMR and MRI sensitivity using photo-excited triplet electrons, has a great potential to hyperpolarize nuclear spins at room temperature. Since the first report of room-temperature triplet-DNP in 1990, pentacene has been the only and best option of triplet polarizing agents. However, the poor airstability of pentacene has severely limited the applicability of triplet-DNP. Here, we report the first example of polarizing agents with significant air-stability as well as high polarizing ability comparable to pentacene. The introduction of electron-withdrawing diaza-substitution to pentacene and tetracene reduces the LUMO level and endows much-improved stability under the ambient condition. Importantly, the diaza-substituted pentacene and tetracene offer similar, or even slightly better, 1H NMR signal enhancement compared with pentacene in the prototypical triplet-DNP test using p-terphenyl crystals. This work removes one of the largest obstacles towards the application of triplet-DNP for hyperpolarization of biological molecules.

Dynamic Nuclear Polarization of Metal–Organic Frameworks Using Photoexcited Triplet Electrons #DNPNMR

Fujiwara, Saiya, Masanori Hosoyamada, Kenichiro Tateishi, Tomohiro Uesaka, Keiko Ideta, Nobuo Kimizuka, and Nobuhiro Yanai. “Dynamic Nuclear Polarization of Metal–Organic Frameworks Using Photoexcited Triplet Electrons.” Journal of the American Chemical Society 140, no. 46 (November 21, 2018): 15606–10.

https://doi.org/10.1021/jacs.8b10121.

While dynamic nuclear polarization based on photoexcited triplet electrons (triplet-DNP) has the potential to hyperpolarize nuclear spins of target substrates in the low magnetic field at room temperature, there has been no triplet-DNP system offering structural rigidity and substrate accessibility. Here, we report the first example of triplet-DNP of nanoporous metal−organic frameworks. Accommodation of a carboxylate-modified pentacene derivative in a partially deuterated ZIF-8 (DZIF-8) results in a clear 1H NMR signal enhancement over thermal equilibrium.

Room temperature hyperpolarization of nuclear spins in bulk

Tateishi, K., et al., Room temperature hyperpolarization of nuclear spins in bulk. Proc Natl Acad Sci U S A, 2014. 111(21): p. 7527-30.

http://www.ncbi.nlm.nih.gov/pubmed/24821773

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 ( approximately 0.6 mg, approximately 0.7 x 0.7 x 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.

Room temperature hyperpolarization of nuclear spins in bulk

Tateishi, K., et al., Room temperature hyperpolarization of nuclear spins in bulk. Proc. Nat. Aca. Sci. USA, 2014. 111(21): p. 7527-7530.

http://www.pnas.org/content/111/21/7527.abstract

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

Tateishi, K., et al., Dynamic nuclear polarization with photoexcited triplet electrons in a glassy matrix. Angew Chem Int Ed Engl, 2013. 52(50): p. 13307-10.

http://www.ncbi.nlm.nih.gov/pubmed/24249595

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􏰵5 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 1H spins, and therefore, DNP can enhance the 1H spin polarization (hence the sensitivity) by a factor of at most 660.

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