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+dot;-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-Ph n-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-Ph n-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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry