title: Direct observation of the preference of hole transfer over electron transfer for radical ion pair recombination in donor-bridge-acceptor molecules.
abstract: Understanding how the electronic structures of electron donor-bridge-acceptor (D-B-A) molecules influence the lifetimes of radical ion pairs (RPs) photogenerated within them (D+*-B-A-*) is critical to designing and developing molecular systems for solar energy conversion. A general question that often arises is whether the HOMOs or LUMOs of D, B, and A within D+*-B-A-* are primarily involved in charge recombination. We have developed a new series of D-B-A molecules consisting of a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) electron donor linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor via a series of Phn oligomers, where n = 1-4, to give DMJ-An-Phn-NI. The photoexcited charge transfer state of DMJ-An acts as a high-potential photoreductant to rapidly and nearly quantitatively transfer an electron across the Phn bridge to produce a spin-coherent singlet RP 1(DMJ+*-An-Phn-NI-*). Subsequent radical pair intersystem crossing yields 3(DMJ+*-An-Phn-NI-*). Charge recombination within the triplet RP then gives the neutral triplet state. Time-resolved EPR spectroscopy shows directly that charge recombination of the RP initially produces a spin-polarized triplet state, DMJ-An-Phn-3*NI, that can only be produced by hole transfer involving the HOMOs of D, B, and A within the D-B-A system. After the initial formation of DMJ-An-Phn-3*NI, triplet-triplet energy transfer occurs to produce DMJ-3*An-Phn-NI with rate constants that show a distance dependence consistent with those determined for charge separation and recombination.
authors.affiliation: Department of Chemistry, Argonne-Northwestern Solar Energy Research (ANSER) Center, 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: Michael J
authors.initials: MJ
authors.lastname: Ahrens
authors.forename: Amy M
authors.initials: AM
authors.lastname: Vega
authors.forename: Annie Butler
authors.initials: AB
authors.lastname: Ricks
authors.forename: David W
authors.initials: DW
authors.lastname: McCamant
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski