Singlet fission (SF), which forms two triplet excitons from one single singlet exciton, has been widely investigated for applications in optoelectronics. SF is also known to generate spin-polarized quintet states in organic molecular systems, which has been utilized for elucidating the mechanism of SF. However, applications using spin degrees of freedom in SF have not been achieved. Our group has controlled the discrete assembly of pentacene derivative in aqueous media by supramolecular complexation with cyclodextrins (CD) to cause SF. In addition, we have enhanced 1H NMR signal intensity of water molecules by transferring the spin polarization of the SF-derived quintet state to the 1H nuclear spins of the water molecule using dynamic nuclear polarization (DNP) (Figure 1).
The present study is interdisciplinary, bridging the different fields of SF and DNP, which was inspired by our group's previous works in the two fields of triplet-triplet annihilation-based photon upconversion (TTA-UC) and DNP using triplets1. In our TTA-UC studies, we have been controlling triplet generation and diffusion based on the control of chromophore assembly structure. Since SF is the reverse process of TTA-UC and they are closely related, our group has recently been active in the study of SF. On the other hand, our group has also focused on the spin degrees of freedom of triplets and their application to DNP to increase the sensitivity of NMR/MRI measurements of biomolecules. Since our group has been exploring different research themes with triplets as a common keyword, it was natural for us to merge them. In addition, this study brought together the synthesis, ESR, and DNP measurements of our group with the ultrafast spectroscopy of Miyata's group, MD simulations of Watanabe's group, and ESR simulations of Kobori's group, and was made possible by this excellent collaborative research team.
It is important to increase the NMR sensitivity of water molecules because of its potential application in protein NMR and MRI diagnosis. Triplet-DNP uses acene molecules such as pentacene as a polarizing agent, but it has been a major challenge how to transfer the triplet electron spin polarization of the hydrophobic acenes to water. Our group has so far succeeded in hyperpolarization of water molecules by dispersing polarizing agents in water using various approaches such as dispersion of nanoparticles in water1, 2 and ion pairing formation with bulky hydrophilic counterions3. In such a context, we mixed cyclodextrin (CD) and water-soluble pentacene derivative, NaPDBA4, and controlled the aggregation state of pentacene chromophores by changing the inner diameter of CD. Bare NaPDBA and NaPDBA-γCD formed pentacene dimers and showed SF in aqueous media (Figure 2).
DNP experiments were performed by dispersing the pentacene aggregates into water-glycerol glass to transfer the electron spin polarization of the quintet state to the 1H nuclear spins of the water molecules. As a result, we succeeded in enhancing the NMR sensitivity of water by a factor of 20 (Figure 3). Since the Rabi frequency of the quintet spin state is higher than that of the triplet, we also demonstrated that polarization transfer can be achieved by a weaker microwave power with quintets than the conventional DNP with triplets.
This research pioneered SF as a polarized spin generator for quantum biotechnology. The enhanced NMR sensitivity of water molecules can be applied to the structural analysis of proteins and in vivo imaging with MRI. We believe that ‘quintet-DNP’ is particularly effective for improving NMR sensitivity in biology-relevant conditions since it can transfer spin polarization with a weaker microwave power.
2 Nishimura, K. et al. Triplet dynamic nuclear polarization of nanocrystals dispersed in water at room temperature. Phys. Chem. Chem. Phys. 21, 16408-16412, doi:10.1039/c9cp03330k (2019).
3 Kouno, H. et al. Triplet dynamic nuclear polarization of crystalline ice using water-soluble polarizing agents. Chem. Commun. 56, 3717-3720, doi:10.1039/d0cc00836b (2020).
4 Matsumoto, N. et al. Proton Hyperpolarization Relay from Nanocrystals to Liquid Water. J. Am. Chem. Soc. 144, 18023-18029, doi:10.1021/jacs.2c07518 (2022).
5 Fujiwara, S. et al. Dynamic Nuclear Polarization of Metal-Organic Frameworks Using Photoexcited Triplet Electrons. J. Am. Chem. Soc. 140, 15606-15610, doi:10.1021/jacs.8b10121 (2018).
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