Hyperpolarization of cis-15N2-azobenzene by parahydrogen at ultralow magnetic fields.

The development of nuclear spins hyperpolarization, and the search for molecules that can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. In this work we present a detailed study of SABRE SHEATH (Signal Amplification By Reversible Exchange in Shield Enabled Alignment Transfer to Heteronuclei) experiments on 15N2-azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized via transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process. Azobenzene exists in two isomers trans- and cis-. We show that all nuclear spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4 T) reach up to 3000 for 15N spins and up to 30 for the 1H spins. We compare two approaches to observe either hyperpolarized magnetization of 15N/1H spins, or hyperpolarized singlet order of the 15N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis-15N2-azobenzene is switched by light to trans-15N2-azobenzene for storing the produced hyperpolarization in the long-lived spin state of the 15N pair of trans-15N2-azobenzene.