Time-resolved CIDNP from photochemically generated radical ion pairs in rigid bichromophoric systems

Laser pulse photoexcitation of rigid donor-spacer-acceptor molecules in var ious solvents leads to chemically induced dynamic nuclear polarization (CID NP) of the ground state species. This polarization is created during the si multaneous charge recombination of the intermediate ion pairs via singlet a nd triplet pathways. The different channels yield polarizations with opposi te signs and equal magnitudes and are separated in time because the triplet contribution appears delayed. Their time dependence yields the tripler dec ay parameters. The CIDNP intensities depend strongly on the spacer length a nd on the dielectric constant of the medium. They are explained in terms of the radical pair theory combined with the known dependence of rate constan ts on energy gaps, and provide the exchange interaction in the ion pairs. ( C) 1999 Elsevier Science B.V. All rights reserved.