Radical Pair Mechanism
The generation of CIDNP in a typical photochemical system (target + photosensitizer, flavin in this example) is a cyclic photochemical process shown schematically in Figure 1. The chain of reactions is initiated by a blue light photon, which excites the flavin mononucleotide (FMN) photosensitizer to the singlet excited state. The fluorescence quantum yield of this state is rather low, and approximately half of the molecules undergo intersystem crossing into the long-lived triplet state. Triplet FMN has a remarkable electron affinity. If a molecule with a low ionization potential (e.g. phenols, polyaromatics) is present in the system, the diffusion-limited electron transfer reaction forms a spin-correlated triplet electron transfer state – a radical pair. The actual kinetics are rather complicated and may involve multiple (de)protonations and hence exhibit pH dependence.
The radical pair may either cross over to a singlet electron state and then recombine, or separate and perish in side reactions. The relative probability of these two pathways for a given radical pair depends on the nuclear spin state and leads to the nuclear spin state sorting and observable nuclear polarization.
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