Triplet
dynamic nuclear polarization (triplet-DNP) achieves nuclear
spin polarization at moderate temperatures by using spin polarization
of photoexcited triplet electrons. The applications of triplet-DNP
for biomolecules have been hampered because acenes, the only polarizing
agents used so far, tend to aggregate and lose their polarization
in biomolecular matrices. Here, we report for the first time use of
porphyrins as polarizing agents of triplet-DNP and propose a new concept
of aggregation-tolerant polarizing agents. Sodium salts of tetrakis(4-carboxyphenyl)porphyrin
(TCPPNa) can be dispersed in amorphous as well as crystalline biomolecular
matrices, and importantly, it can generate polarized triplet electrons
even in a slightly aggregated state. Triplet-DNP of crystalline erythritol
containing slightly aggregated TCPPNa can achieve more than 120-fold
signal enhancement. Because TCPPNa is also the first biocompatible
triplet-DNP polarizing agent, this work provides a crucial step forward
for the biological and medical applications of triplet-DNP.
This Feature Article overviews the recently-emerged materials chemistry of triplet dynamic nuclear polarization (triplet-DNP) towards biological and medical applications.
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.
Visible-to-UV TTA-based photon upconversion in aerated water is achieved for the first time by utilizing oxygen blocking ability of dense multicomponent supramolecular co-assemblies.
Singlet fission (SF), converting a singlet excited state into a spin-correlated triplet-pair state, is the sole way to generate a spin quintet state in organic materials. Although its application to photovoltaics as an exciton multiplier has been extensively studied, use of its unique spin degree of freedom is largely unexplored. Here, we demonstrate that the spin polarization of the quintet multiexcitons generated by SF improves the sensitivity of biological magnetic resonance through dynamic nuclear polarization (DNP). We form supramolecular assemblies of a few pentacene chromophores and use SF-born quintet spins to achieve DNP of water-glycerol, the most basic biological matrix, at lower microwave intensities than for conventional triplet-based DNP. Our demonstration opens a new use of SF as a “polarized spin generator” in bio-quantum technology.
The
spin-polarized triplet state generated by light irradiation
has potential for applications such as triplet dynamic nuclear polarization
(triplet-DNP). Recently, we have reported free-base porphyrins as
versatile and biocompatible polarizing agents for triplet-DNP. However,
the electron polarization of free-base porphyrins is not very high,
and the dilemma is that the high polarization of metalloporphyrins
is accompanied by a too short spin–lattice relaxation time
to be used for triplet-DNP. We report here that the introduction of
electron-withdrawing fluorine groups into Zn porphyrins enables a
long enough spin–lattice relaxation time (>1 μs) while
maintaining a high polarization (P
x:P
y:P
z = 0:0:1.0)
at room temperature. Interestingly, the spin–lattice relaxation
time of Zn porphyrin becomes much longer by introducing fluorine substituents,
whereas the spin–lattice relaxation time of free-base porphyrin
becomes shorter by the fluorine substitution. Theoretical calculations
suggest that this is because the introduction of the electron-withdrawing
fluorine substituents reduces the spin density on Zn atoms and weakens
the spin–orbit interaction.
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