Low field photo-CIDNP in the intramolecular electron transfer of naproxen–pyrrolidine dyads

Photoinduced processes with partial (exciplex) and full charge transfer in donor–acceptor systems are of interest because they are frequently used for modeling drug–protein binding. Low field photo-CIDNP (chemically induced dynamic nuclear polarization) for these processes in dyads, including the drug, (S)- and (R)-naproxen and (S)-N-methyl pyrrolidine in solutions with strong and weak permittivity have been measured. The dramatic influence of solvent permittivity on the field dependence of the N-methyl pyrrolidine 1H CIDNP effects has been found. The field dependences of both (R,S)- and (S,S)-dyads in a polar medium are the curves with a single extremum in the area of the S–T+ terms intersection. Moreover, the CIDNP field dependences of the same protons measured in a low polar medium present curves with several extrema. The shapes of the experimental CIDNP field dependence with two extrema have been described using the Green function approach for the calculation of the CIDNP effects in the system without electron exchange interactions. The article discusses the possible causes of the differences between the CIDNP field dependence detected in a low-permittivity solvent with the strong Coulomb interactions and in a polar solvent.