Photochemically Induced Dynamic Nuclear Polarization (Photo-CIDNP) Magic-Angle Spinning NMR

Abstract

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is non-Boltzmann nuclear magnetiza- tion which can be observed by NMR spectroscopy as enhanced absorptive (positive) or emissive (negative) signals. In solids, photo-CIDNP has been observed since its discovery in 1994 in various photosynthetic reac- tion centers (RCs) by magic angle spinning (MAS) solid-state NMR. The photo-CIDNP effect in solids can be explained by a combination of several mechanisms, e.g., three-spin mixing (TSM) and differential decay (DD), which transfer the electron spin polarization obtained in the initial singlet radical pair to nuclei via hyperfi ne interaction. Therefore, photo-CIDNP MAS NMR allows to study the electronic structure of the electronic ground-state after the photocycle (chemical shifts) and of the radical pair (NMR intensities), providing insight into the photochemical machinery of RCs at the atomic scale. Recently, nuclear polarizations up to a factor of 10,000 above the Boltzmann equilibrium have been observed, opening new experimental possibilities for solid-state NMR.