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+•−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.