Kinetic photocycle description of photochemically induced dynamic nuclear polarization for dye-sensitized solid-state NMR spectroscopy

Abstract

Solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) enables the amplification of the nuclear magnetic resonance (NMR) signal in linked donor–acceptor (D–A) systems under light irradiation. At high fields, the effect relies on the creation of a transient radical pair having a large initial spin order, which is spontaneously transferred to hyperfine-coupled nuclear spins and converted into polarization during the evolution of the system within the zero-quantum electron spin manifold. Previous quantum mechanical models to quantify the solid-state photo-CIDNP effect were based on a simplified representation of the D–A photocycle and did not account for the possibility of re-exciting the molecule after the radical pair state decays back to the molecular ground state. Here, we present a model for the quantification of solid-state photo-CIDNP using a unified master equation that spans all relevant photocycle states in Liouville space to account for their simultaneous evolution. The model accounts for multiple photocycle states interconnected by kinetic rates, with the various photocycle transitions described as spin-conserving Markovian processes. We first apply the simplified model to identify conditions for solid-state 1H photo-CIDNP at high magnetic fields (here, 9.4 T), and then, based on the information gathered in this way, we explore different cases with the unified model to identify potential design factors for second-generation D–A polarizing agents for dye-sensitized solid-state NMR experiments.