title: Large Dipolar Spin-Spin Interaction in a Photogenerated U-Shaped Triradical.
abstract: Transient electron paramagnetic resonance (TREPR) spectroscopy has been used to study the spin-spin interactions in a novel U-shaped electron donor-chromophore-acceptor-radical (D-C-A-R(•)) system in which a xanthene bridge holds a tert-butylphenyl nitroxide (BPNO(•)) radical in close proximity to a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptor. Photoexcitation of the 4-aminonaphthalene-1,8-dicarboximide (ANI) chromophore results in rapid, two-step electron transfer to generate the triradical (D(+•)-C-A(-•)-R(•)). The large through-bond distance between A(-•) and R(•) makes their spin-spin exchange interaction (2JAR) negligibly small, whereas their short through-space distance results in a strong dipolar interaction (DAR), which is observed as a set of broad lines in the TREPR spectra of D(+•)-C-A(-•)-R(•) in solid toluene solution at 85 K. Transient nutation experiments show that these transitions belong to a species with spin S = 1, whereas experiments on D(+•)-C-A(-•)-R(•) in the oriented nematic liquid crystal 4-cyano-4'-n-pentylbiphenyl at 85 K demonstrate the anisotropy of DAR.
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Gardner
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
authors.forename: Hsiao-Fan
authors.initials: HF
authors.lastname: Chen
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Krzyaniak
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski