Spin-selective charge transport pathways through p-oligophenylene-linked donor-bridge-acceptor molecules

A series of donor-bridge-acceptor (D-B-A) triads have been synthesized in which the donor, 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An), and the acceptor, naphthalene-1,8:4,5-bis(dicarboximide) (NI), are linked by p-oligophenylene (Ph_n) bridging units (n = 1-5). Photoexcitation of DMJ-An produces DMJ^+•-An^-• quantitatively, so that An-^-• acts as a high potential electron donor, which rapidly transfers an electron to NI yielding a long-lived spin-coherent radical ion pair (DMJ^+•-An-Ph_n-NI^-•). The charge transfer properties of 1-5 have been studied using transient absorption spectroscopy, magnetic field effects (MFEs) on radical pair and triplet yields, and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The charge separation (CS) and recombination (CR) reactions exhibit exponential distance dependencies with damping coefficients of β = 0.35 Å_-1 and 0.34 Å_-1, respectively. Based on these data, a change in mechanism from superexchange to hopping was not observed for either process in this system. However, the CR reaction is spin-selective and produces the singlet ground state and both ^3*An and ^3*NI. A kinetic analysis of the MFE data shows that superexchange dominates both pathways with β = 0.48 Å^-1 for the singlet CR pathway and β = 0.35 Å^-1 for the triplet CR pathway. MFEs and TREPR experiments were used to measure the spin-spin exchange interaction, 2J, which is directly related to the electronic coupling matrix element for CR, V_CR². The magnitude of 2J also shows an exponential distance dependence with a damping coefficient α = 0.36 Å^-1, which agrees with the β values obtained from the distance dependence for triplet CR. These results were analyzed in terms of the bridge molecular orbitals that participate in the charge transport mechanism.