Modern NMR spectroscopy is eager for spin hyperpolarization, as low sensitivity is usually the bottleneck for detailed studies of complex molecular and materials systems. In the solid state, the most general approach developed so far is microwave-driven dynamic nuclear polarization (DNP), where a suitable polarizing agent – typically an organic biradical – is co-formulated with the system under investigation and microwaves are used to transfer the large thermal electron spin polarization from the polarizing agent to nearby nuclei and successively, by spin diffusion, to the entire nuclear spin network.
In this picture, 1H nuclei represent the ideal species to be polarized by DNP because of the efficient spin diffusion that allows to spread hyperpolarization throughout the sample. Nonetheless, maximum DNP enhancements are tied to the thermal electron spin polarization, and for 1H nuclei they cannot exceed a factor of 658. On the contrary, optically based hyperpolarization methods are not limited by thermal constraints, and nuclear polarizations on the order of unity are in principle feasible even at high temperatures.
One such optical technique is photochemically induced dynamic nuclear polarization (photo-CIDNP), where light is used to excite a suitable donor–acceptor system, creating a radical pair whose evolution drives nuclear hyperpolarization. In solids, up to now only 13C and 15N nuclei have been directly polarized via photo-CIDNP, but their low natural abundance usually traps the hyperpolarization in the vicinity of the polarizing agent, thus limiting the utility for bulk hyperpolarization. Here we report optically enhanced solid-state 1H NMR spectroscopy achieved via direct 1H photo-CIDNP in a frozen solution at 0.3 T and 85 K, where spontaneous 1H-1H spin diffusion relays polarization through the whole sample, yielding a bulk signal enhancement under continuous laser irradiation at 450 nm. Our findings enable a novel strategy for hyperpolarized NMR beyond the limits of microwave-driven DNP.