Magnetometer-detected Nuclear Magnetic Resonance of Photochemically Hyperpolarized Molecules

Abstract

Photochemically induced dynamic nuclear polarization (photo-CIDNP) enables nuclear spin ordering by irradiating samples with light. Polarized spins are conventionally detected via high-field chemical shift-resolved NMR (above 0.1 T), however, of particular importance are CIDNP processes occurring at relatively low fields (<50 mT). In this paper, we demonstrate in situ low-field photo-CIDNP measurements using shielded fast-field-cycling NMR combined with audio-frequency detection of Larmor precession via atomic magnetometers. A model system is used comprising tetraphenyl porphyrin as a photosensitizer, and 1,4-benzoquinone as a quencher. For solutions comprising sub-mM concentrations of the photosensitizer and mM concentrations of the quencher, hyperpolarized 1H magnetization is detected by pulse-acquire NMR spectroscopy at 170 nT and 2 μT fields. The observed NMR linewidths are about 5 times narrower than normally anticipated in high-field NMR and are systematically affected by light irradiation during the acquisition period, reflecting a reduction of the transverse relaxation time constant, T_{2}*. Magnetometer-detected photo-CIDNP spectroscopy enables straightforward observation of spin-chemistry processes in the ambient field range of a few nT to tens of mT and may help resolve open questions regarding the nature of avian magneto sensing.