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(+center dot)-Ph-n-PDI-center dot). 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 (RPISC) 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- (PDI)-P-3*, 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(+center dot)-Ph-n-PDI-center dot)-> PTZ-Phn-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.