title: A general formulation for magnetic exchange coupling within long-distance radical ion pairs.
abstract: A Hubbard model-based expression for the magnetic exchange interaction, 2J, between the spins of a radical ion pair is derived for a generic donor-bridge-acceptor (D-B(n)-A) system in terms of the superexchange coupling, t, between molecular orbitals of the donor, bridge, and acceptor and the energies, epsilon, and same-site Coulomb repulsions, U, of these orbitals. The formulation is used to predict 2J for the photophysically well-characterized series phenothiazine-p-(phenylene)n-perylene diimide, n = 2,3,4,5, where the superexchange coupling is calculated with multipathway, second- and third-order perturbation theory coupled with the extended Huckel formalism. U and epsilon are estimated through experimentally determined electrochemical potentials. Due to multiple interactions arising from the large distance between magnetic centers in these donor-bridge-acceptor systems, agreement with experiment was substantially improved when third-order corrections were included in the calculation of t and 2J.
authors.affiliation: Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly, Northwestern University, Evanston, Illinois 60208-3113, USA. eweiss@chem.northwestern.edu
authors.forename: Emily A
authors.initials: EA
authors.lastname: Weiss
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner