Wire-like charge transport at near constant bridge energy through fluorene oligomers

The study of photoinitiated electron transfer in donor-bridge-acceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FL_n) 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 rate constants for both charge separation and recombination as well as the energy levels of the relevant FL_n ⁺asterisk inside a circle sign bridge states for n = 1-4 are only weakly distance dependent. After the initial photogeneration of ¹(PTZ⁺asterisk inside a circle sign-FL_n- PDI^-asterisk inside a circle sign), radical pair intersystem crossing results in formation of ³(PTZ⁺asterisk inside a circle sign-FL_n-PDI^-asterisk inside a circle sign) that recombines to give ³* PDI. The dependence of the 3*PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FL_n is dominated by incoherent hopping at long distances.