Field dependent CIDNP from photochemically generated radical ion pairs in rigid bichromophoric systems Original Research Article

Field dependent chemically induced dynamic nuclear polarization (CIDNP) spectroscopy has been used to measure the exchange interaction, J, in photochemically generated radical ion pairs of rigid donor-bridge-acceptor molecules in benzene and dioxane solution. The positive and surprisingly large values of J show an exponential dependence on the bridge length. The magnetic field dependence of CIDNP is explained via the radical pair theory and yields, besides J, the rate constants for the back electron transfer from radical ion pair singlet and triplet states. Intersystem crossing is promoted by the weak magnetic interactions of the radical ion pair and hindered by the singlet–triplet energy gap. Therefore, the overall lifetime of the species is dominated by back electron transfer from the singlet state.