title: Time-resolved EPR studies of photogenerated radical ion pairs separated by p-phenylene oligomers and of triplet states resulting from charge recombination.
abstract: Photoexcitation of a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Phn), n = 1-5, and A= perylene-3,4:9,10-bis(dicarboximide) (PDI) results in rapid electron transfer to produce 1(PTZ+*-Phn-PDI-*). Time-resolved EPR (TREPR) studies of the photogenerated radical pairs (RPs) show that above 150 K, when n = 2-5, the radical pair-intersystem crossing mechanism (RP-ISC) produces spin-correlated radical ion pairs having electron spin polarization patterns indicating that the spin-spin exchange interaction in the radical ion pair is positive, 2J > 0, and is temperature dependent. This temperature dependence is most likely due to structural changes of the p-phenylene bridge. Charge recombination in the RPs generates PTZ-Phn-3*PDI, which exhibits a spin-polarized signal similar to that observed in photosynthetic reaction-center proteins and some biomimetic systems. At temperatures below 150 K and/or at shorter donor-acceptor distances, e.g., when n = 1, PTZ-Phn-3*PDI is also formed from a competitive spin-orbit-intersystem crossing (SO-ISC) mechanism that is a result of direct charge recombination: 1(PTZ+*-Phn-PDI-*) --> PTZ-Phn-3*PDI. This SO-ISC mechanism requires the initial RP intermediate and depends strongly on the orientation of the molecular orbitals involved in the charge recombination as well as the magnitude of 2J.
authors.affiliation: Department of Chemistry and International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Zachary E X
authors.initials: ZE
authors.lastname: Dance
authors.forename: Qixi
authors.initials: Q
authors.lastname: Mi
authors.forename: David W
authors.initials: DW
authors.lastname: McCamant
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Ahrens
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
mesh_terms: D001713:Biphenyl Compounds / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005609:Free Radicals / Q000737:chemistry / Q000528:radiation effects; D007094:Imides / Q000737:chemistry; D007834:Lasers; D008280:Magnetics; D015394:Molecular Structure; D010569:Perylene / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000528:radiation effects; D010777:Photochemistry; D012680:Sensitivity and Specificity; D013696:Temperature; D013997:Time Factors
chemical_list: D001713:Biphenyl Compounds; D005609:Free Radicals; D007094:Imides; D010569:Perylene