Challenges in distinguishing superexchange and hopping mechanisms of intramolecular charge transfer through fluorene oligomers

The temperature dependence of intramolecular charge separation in a series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene FL_n (n = 1-4) bridges, and perylene-3,4:9,10- bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FL_n bridge. In toluene, the temperature dependence of the charge separation rate constants for PTZ-FL_n-PDI, (n = 1-4) is relatively weak and is successfully described by the semiclassical Marcus equation. The activation energies for charge separation suggest that bridge charge carrier injection is not the rate limiting step. The difficulty of using temperature and length dependence to differentiate hopping and superexchange is discussed, with difficulties in the latter topic explored via an extension of a kinetic model proposed by Bixon and Jortner.