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

Field dependent chemically induced dynamic nuclear polarization (CIDNP) spe ctroscopy has been used to measure the exchange interaction, J, in photoche mically generated radical ion pairs of rigid donor-bridge-acceptor molecule s in benzene and dioxane solution. The positive and surprisingly large valu es of J show an exponential dependence on the bridge length. The magnetic f ield dependence of CIDNP is explained via the radical pair theory and yield s, besides J, the rate constants for the back electron transfer from radica l 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. (C) 2001 Pub lished by Elsevier Science B.V.