Hyperpolarization of cis-¹⁵ N₂ -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 ¹⁵ N₂ -azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized through 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 ¹⁵ N spins and up to 30 for the ¹ H spins. We compare two approaches to observe either hyperpolarized magnetization of ¹⁵ N/¹ H spins, or hyperpolarized singlet order of the ¹⁵ N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis-¹⁵ N₂ -azobenzene is switched by light to trans-¹⁵ N₂ -azobenzene for storing the produced hyperpolarization in the long-lived spin state of the ¹⁵ N pair of trans-¹⁵ N₂ -azobenzene.