title: Electron spin polarization transfer from photogenerated spin-correlated radical pairs to a stable radical observer spin.
abstract: A series of donor-chromophore-acceptor-stable radical (D-C-A-R(•)) molecules having well-defined molecular structures were synthesized to study the factors affecting electron spin polarization transfer from the photogenerated D(+•)-C-A(-•) spin-correlated radical pair (RP) to the stable radical R(•). Theory suggests that the magnitude of this transfer depends on the spin-spin exchange interaction (2JDA) of D(+•)-C-A(-•). Yet, the generality of this prediction has never been demonstrated. In the D-C-A-R(•) molecules described herein, D is 4-methoxyaniline (MeOAn), 2,3-dihydro-1,4-benzodioxin-6-amine (DioxAn), or benzobisdioxole aniline (BDXAn), C is 4-aminonaphthalene-1,8-dicarboximide, and A is naphthalene-1,8:4,5-bis(dicarboximide) (1A,B-3A,B) or pyromellitimide (4A,B-6A,B). The terminal imide of the acceptors is functionalized with either a hydrocarbon (1A-6A) or a 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (R(•)) (1B-6B). Photoexcitation of C with 416-nm laser pulses results in two-step charge separation to yield D(+•)-C-A(-•)-(R(•)). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy using continuous-wave (CW) microwaves at both 295 and 85 K and pulsed microwaves at 85 K (electron spin-echo detection) was used to probe the initial formation of the spin-polarized RP and the subsequent polarization of the attached R(•) radical. The TREPR spectra show that |2JDA| for D(+•)-C-A(-•) decreases in the order MeOAn(+•) > DioxAn(+•) > BDXAn(+•) as a result of their spin density distributions, whereas the spin-spin dipolar interaction (dDA) remains nearly constant. Given this systematic variation in |2JDA|, electron spin-echo-detected EPR spectra of 1B-6B at 85 K show that the magnitude of the spin polarization transferred from the RP to R(•) depends on |2JDA|.
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Colvin
authors.forename: Raanan
authors.initials: R
authors.lastname: Carmieli
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Miura
authors.forename: Sabine
authors.initials: S
authors.lastname: Richert
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Gardner
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Smeigh
authors.forename: Scott M
authors.initials: SM
authors.lastname: Dyar
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Conron
authors.forename: Mark A
authors.initials: MA
authors.lastname: Ratner
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski