Time-resolved EPR studies of photogenerated radical ion pairs separated by p-phenylene oligomers and of triplet states resulting from charge recombination

Photoexcitation of a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Ph_n), n = 1-5, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI) results in rapid electron transfer to produce ¹(PTZ^+dot;-Ph_n-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-Ph_n-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 ^+̇-Ph_n-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.