Direct observation of the preference of hole transfer over electron transfer for radical ion pair recombination in donor-bridge-acceptor molecules

Understanding how the electronic structures of electron donor-bridge-acceptor (D-B-A) molecules influence the lifetimes of radical ion pairs (RPs) photogenerated within them (D+ center dot-B-A-center dot) is critical to designing and developing molecular systems for solar energy conversion. A general question that often arises is whether the HOMOs or LUMOs of D, B, and A within D+ center dot -B-A-center dot are primarily involved in charge recombination. We have developed a new series of D-B-A molecules consisting of a 3,5-dimetho4-(9-anthracenyl)julolidine (DMJ-An) electron donor linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor via a series of Ph-n oligomers, where n = 1.4, to give DMJ-An-Ph-n-NI. The photoexcited charge transfer state of DMJ-An acts as a high-potential photoreductant to rapidly and nearly quantitatively transfer an electron across the Ph-n bridge to produce a spin-coherent singlet RP (1)(DWJ(+) center dot-An-Ph-n-NI-center dot 0). Subsequent radical pair intersystem crossing yields 3(DMJ(+) center dot-An-Ph-n-NI-center dot). Charge recombination within the triplet RP then gives the neutral triplet state. Time-resolved EPR spectroscopy shows directly that charge recombination of the RP initially produces a spin-polarized triplet state, DMJ-An-Ph-n-3(+)NI, that can only be produced by hole transfer involving the HOMOs of D, B, and A within the D-B-A system. After the initial formation of DMJ-An-Ph-n-3(+)NI, triplet-triplet energy transfer occurs to produce DMJ-3(+)An-Ph-n-NI with rate constants that show a distance dependence consistent with those determined for charge separation and recombination.