title: Magnetic field-induced switching of the radical-pair intersystem crossing mechanism in a donor-bridge-acceptor molecule for artificial photosynthesis.
abstract: A covalent, fixed-distance donor-bridge-acceptor (D-B-A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin-spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highest-energy RP eigenstate (T(+1)) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D-B-A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of (3*)D-B-A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of (3*)D-B-A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T(0) eigenstate.
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Colvin
authors.forename: Annie Butler
authors.initials: AB
authors.lastname: Ricks
authors.forename: Amy M
authors.initials: AM
authors.lastname: Scott
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Smeigh
authors.forename: Raanan
authors.initials: R
authors.lastname: Carmieli
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Miura
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski