Photo‐SABRE: Nuclear Spin Hyperpolarization of cis‐trans Photoswitchable Molecules by Parahydrogen**

Abstract This work reports the 23000‐ and 9200‐fold enhancement of the 15 N nuclear spin polarization of cis and trans isomers of azobenzene, correspondingly, as compared to the thermal NMR signals at 9.4 T by exploiting signal amplification by reversible exchange (SABRE) and parahydrogen molecules at 400 nT simultaneously with light irradiation. Only cis ‐azobenzene gains substantial hyperpolarization of its 15 N spins directly from parahydrogen in SABRE through coherent polarization transfer. Consequently cis‐trans photoisomerization at ultralow magnetic field preserves the SABRE‐derived nuclear hyperpolarization of cis ‐azobenzene resulting in hyperpolarization of trans ‐azobenzene as well, despite its direct coordination with the SABRE Ir‐complex being sterically hindered. Moreover, the proposed approach, which we called photo‐SABRE, allows to hyperpolarize the long‐lived spin order of trans ‐isomer 15 N spins with a lifetime of about 25 minutes, which greatly exceeds their relaxation times at high (10 seconds) and low (200 seconds) magnetic field. Since this spin order in 15 N 2 ‐ trans ‐azobenzene is collectively formed by nuclei of different kinds, it can be detected by both 15 N or 1 H NMR spectroscopy.