Direct Observation of the Preference of Hole Transfer over Electron Transfer for Radical Ion Pair Recombination in Donor−Bridge−Acceptor Molecules

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Abstract

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+*-B-A-*) 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+*-B-A-* are primarily involved in charge recombination. We have developed a new series of D-B-A molecules consisting of a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) electron donor linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor via a series of Phn oligomers, where n = 1-4, to give DMJ-An-Phn-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 Phn bridge to produce a spin-coherent singlet RP 1(DMJ+*-An-Phn-NI-*). Subsequent radical pair intersystem crossing yields 3(DMJ+*-An-Phn-NI-*). 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-Phn-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-Phn-3*NI, triplet-triplet energy transfer occurs to produce DMJ-3*An-Phn-NI with rate constants that show a distance dependence consistent with those determined for charge separation and recombination.

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