Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet‐DNP) has great potential for room‐temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced‐fit‐type structural transformation of a crystalline yet flexible metal–organic framework (MOF). In contrast to the usual hosts, 1H spin‐lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5‐fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host–guest interactions. Combined with triplet‐DNP and cross‐polarization (CP), this system realizes an enhanced 19F NMR signal of guest target molecules.
Dynamic nuclear polarization (DNP) using transient electron spin polarization generated by photoexcitation can improve nuclear magnetic resonance (NMR) sensitivity far beyond the thermal equilibrium limit for analysis in life science and drug discovery. However, DNP of liquid water at room temperature remains an important challenge. Here, we propose a new method called hyperpolarization relay, in which the nonequilibrium polarization of electron spins is transferred to proton spins in the nanocrystals and then to proton spins in bulk water. Molecular nanocrystals doped with a polarizing agent that generates a highly polarized photoexcited triplet are synthesized by a reprecipitation method while controlling the size of the nanocrystals. The triplet-DNP sequence of repeated laser and microwave irradiation enhances the NMR signal of bulk water as well as nanocrystals. The smaller size of the nanocrystals increases the efficiency of polarization transfer from the nanocrystals to water due to the increased surface area. A series of control experiments and simulations based on Solomon equations confirmed the hyperpolarization relay mechanism.
Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet‐DNP) has great potential for room‐temperature hyperpolarization of nuclear spins. However, the polarization transfer to molecules of interest remains a challenge due to the fast spin relaxation and weak interaction with target molecules at room temperature in conventional host materials. Here, we demonstrate the first example of DNP of guest molecules in a porous material at around room temperature by utilizing the induced‐fit‐type structural transformation of a crystalline yet flexible metal–organic framework (MOF). In contrast to the usual hosts, 1H spin‐lattice relaxation time becomes longer by accommodating a pharmaceutical model target 5‐fluorouracil as the flexible MOF changes its structure upon guest accommodation to maximize the host–guest interactions. Combined with triplet‐DNP and cross‐polarization (CP), this system realizes an enhanced 19F NMR signal of guest target molecules.
The photoexcited triplet state of dyes can generate highly polarized electron spins for sensing and dynamic nuclear polarization. However, while triplets exhibit long spin− lattice relaxation times (T 1 ) on the microsecond scale in solids, the polarization quickly relaxes on the nanosecond scale in solution due to the rotational motion of chromophores. Here, we report that the immobilization of dye molecules on a solid surface allows molecular contact with a liquid while maintaining high polarization and long T 1 as in a solid. By adsorbing anionic porphyrins on cationic mesoporous silica gel, porphyrin triplets exhibit high polarization and long T 1 at the solid−liquid interface of silica and toluene. Furthermore, porphyrin triplets on the solid surface can exchange spin polarization with TEMPO radicals in solution. This simple and versatile method using the solid−liquid interface will open the door for utilizing the photoinduced triplet spin polarization in solution, which has been mainly limited to the solidstate.
Dynamic nuclear polarization (DNP) using transient electron spin polarization generated by photoexcitation can improve nuclear magnetic resonance (NMR) sensitivity far beyond the thermal equilibrium limit for analysis in life science and drug discovery. However, DNP of liquid water at room temperature remains an important challenge. In previous studies, polarization has been transferred directly from the electron spins in the solid to the nuclear spins of the target, and this has been limited to near-surface solid or highly-viscous targets. Here, we propose a new method called hyperpolarization relay, in which the polarization of electron spins is transferred to proton spins in the nanocrystals and then to proton spins in bulk water by the nuclear Overhauser effect (NOE). Molecular nanocrystals doped with a polarizing agent that generates a highly-polarized photoexcited triplet were synthesized by a reprecipitation method while controlling the size of the nanocrystals. As the size of the nanocrystals decreases, the efficiency of polarization transfer from nanocrystals to water was improved due to the increase in the surface area.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.