Photo-SABRE: hyperpolarization of cis-trans photoswitchable molecules by parahydrogen

Over the past decade, azobenzene-based molecular photoswitches have emerged as promising control devices in a range of fields, including chemistry, biology, materials science, physics, energy storage and pharmacology. Previous studies revealed that cis isomer of azobenzene gains strong nonequilibrium polarization (called hyperpolarization) of 15N nuclear spins through interaction with parahydrogen molecules (i.e., a dihydrogen isomer with protons having zero total spin, pH2) in the reversible exchange with Ir-complex. This technique, known as SABRE (Signal Amplification by Reversible Exchange), enhances inherently weak NMR signals by several orders of magnitude at relatively low operational cost. We demonstrate that performing SABRE in the presence of light irradiation allows to hyperpolarize trans-azobenzene, which direct coordination with the SABRE Ir-complex is sterically hindered. The proposed approach, which we called photo-SABRE, is robust and efficient, as well as non-destructive and reproducible. It combines coherent polarization transfer from pH2 to cis-azobenzene with the reversible cis-trans-photoisomerization. Moreover, using photo-SABRE, it is possible to hyperpolarize the long-lived spin order of 15N spin pair in trans-azobenzene, with a lifetime of about 25 minutes, which greatly exceeds the ordinary relaxation times T1 of its 15N nuclei at high (around 10 s) and low (around 200 s) magnetic fields. Photo-SABRE amplification of the NMR signals of cis-trans photoswitchable compounds has a potential to become a valuable tool in the ascending field of photopharmacology and novel light-controlled materials.