Generating and sustaining long-lived spin states in 15N,15N′-azobenzene

Abstract

Long-Lived spin States (LLSs) hold a great promise for sustaining non-thermal spin order and investigating various slow processes by Nuclear Magnetic Resonance (NMR) spectroscopy. Of special interest for such application are molecules containing nearly equivalent magnetic nuclei, which possess LLSs even at high magnetic fields. In this work, we report an LLS in trans-15N,15N′-azobenzene. The singlet state of the 15N spin pair exhibits a long-lived character. We solve the challenging problem of generating and detecting this LLS and further increase the LLS population by converting the much higher magnetization of protons into the 15N singlet spin order. As far as the longevity of this spin order is concerned, various schemes have been tested for sustaining the LLS. Lifetimes of 17 minutes have been achieved at 16.4 T, a value about 250 times longer than the longitudinal relaxation time of 15N in this magnetic field. We believe that such extended relaxation times, along with the photochromic properties of azobenzene, which changes conformation upon light irradiation and can be hyperpolarized by using parahydrogen, are promising for designing new experiments with photo-switchable long-lived hyperpolarization.

Generalised magnetisation-to-singlet-order transfer in nuclear magnetic resonance

“…[13][14][15][16][17] For the field of hyperpolarisation NMR long-lived spin modes represent promising candidates for the ''storage'' of an enhanced magnetic response, and its readout at convenient times. [18][19][20][21][22][23][24][25][26][27][28] More recently, techniques of this type have been applied to the study of bio-molecular markers and their intricate interactions with their surroundings. [29][30][31][32][33][34] Many experiments in singlet-assisted NMR exploit nearequivalent spin-1/2 pairs, meaning that the difference in chemically shifted resonance frequencies for the two spins D is much smaller than the scalar coupling constant J (both D and J are defined in Hz).…”

“…37,38 We therefore anticipate its incorporation into a wide class of singlet-assisted NMR experiments. [15][16][17][25][26][27][28][31][32][33][34] Further extensions to the case of heteronuclear spin systems are feasible, for example in the context of parahydrogenenhanced NMR. [63][64][65]…”

Representation of population exchange at level anti-crossings

“…A related field is singlet-state NMR ; Carravetta and , dealing with slowly relaxing symmetry-protected spin states, which can be used to probe various slow processes and to store nonequilibrium spin polarization. In many molecules Stevanato et al, 2015;Sheberstov et al, 2019;Zhou et al, 2017;Wang et al, 2017;Buratto et al, 2014;Vasos et al, 2009;Zhang et al, 2015;Franzoni et al, 2012;DeVience et al, 2013) singlet-order relaxes much longer than spin magnetization for the reason that it is immune to some relaxation mechanisms, for instance, in a two-spin system dipolar relaxation cannot drive singlettriplet transitions because the dipole-dipole interaction is invariant to the exchange of the two spins (Pileio, 2010). In singlet-state NMR experiments, spin magnetization is converted into singlet order by a suitable pulse sequence; singletstate readout is also done by singlet-to-magnetization conversion using special pulse sequences.…”

“…In the cases of PHIP and singlet-state NMR a consideration of LACs often becomes important, in particular, in molecules with pairs of nearly equivalent spins (Ivanov et al, 2014;Pravdivtsev et al, 2013b;Franzoni et al, 2013Franzoni et al, , 2012Sheberstov et al, 2019;Stevanato et al, 2015;De-Vience et al, 2013;Theis et al, 2014a), such that the symmetry breaking is due to a very small chemical shift difference of the nuclei or due to their magnetic non-equivalence, i.e., due to slightly different couplings to other spins. Such symmetry breaking is usually a minor effect, giving rise to spin mixing only under special conditions, which correspond to LACs.…”

Representation of population exchange at level anti-crossings

“…A related field is singlet-state NMR (Levitt, 2012;, dealing with slowly relaxing symmetry-protected spin states, which can be used to probe various slow processes and to store non-equilibrium spin polarization. In many molecules (Levitt, 2012;Carravetta 50 and Stevanato et al, 2015;Sheberstov et al, 2019;Zhou et al, 2017;Wang et al, 2017;Buratto et al, 2014;Vasos et al, 2009;Zhang et al, 2015;Franzoni et al, 2012;Kiryutin et al, 2019;DeVience et al, 2013) singlet-order relaxes much longer than spin magnetization for the reason that it is immune to some relaxation mechanisms, for instance, in a two-spin system dipolar relaxation cannot drive singlet-triplet transitions because the dipole-dipole interaction is invariant to 55 exchange of the two spins (Pileio, 2010). In singlet-state NMR experiments, spin magnetization is converted into singlet order by a suitable pulse sequence; singlet-state readout is also done by singlet-tomagnetization conversion using special pulse sequences.…”