Electron spin polarization transfer from photogenerated spin-correlated radical pairs to a stable radical observer spin

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 (2J_DA) 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 |2J_DA| 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 (d_DA) remains nearly constant. Given this systematic variation in |2J_DA|, 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 |2J_DA|.