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.
Highlights
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
As far as the signal intensity is concerned, it is optimal to populate the singlet order from the 10-fold higher thermal 1H polarization: this gives rise to a ca. 10-fold enhancement of the long-living spin order and the resulting NMR signal
The LLS-derived signal is even stronger than the 15N signal coming from the thermal polarization, reaching up to 147% of the thermal 15N signal (Fig. 4b)
Summary
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. Previous works report very large TS/T1 ratios and TS relaxation times reaching minutes for 1H spins[20,21], tens of minutes[22,23] or even more than an hour[24] for pairs of 15N or 13C spins All these examples of extremely long singlet state lifetimes have been found at low magnetic fields. At magnetic fields typical for detection for most nuclei (e.g. 13C, 15N, 19F) Chemical Shift Anisotropy (CSA) gives the dominant contribution to relaxation In this situation, extended lifetime of the singlet order is expected when the CSA tensors of the spin pair have the same principal axes systems and the same principal values[25].
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