Photoinitiated long-lived charge separation with near-unity quantum yield in donor-acceptor₁-acceptor₂ systems for artificial photosynthesis

We report the synthesis and photophysical characterization of isomeric donor-acceptor₁-acceptor₂ (D-A₁-A₂) triads each comprising a perylene donor (D) connected by a xylene bridge to a naphthalene-1,8-dicarboximide primary acceptor (NMI, A₁) that is, in turn, covalently linked to a naphthalene-1,4:5,8-bis(dicarboximide) secondary acceptor (NDI, A₂) through a benzene ring at its ortho, meta or para positions (Per-Xy-NMI-x-NDI). Selective photoexcitation of Per produces both Per^+• and NDI^‒• simultaneously with time constants of ∼50 ps in benzonitrile and ∼80 ps in toluene with a >98% yield. This is much faster than expected for single-step electron transfer from ^1*Per to NDI and is consistent with a two-step mechanism, ^1*Per-Xy-NMI-x-NDI → Per^+•-Xy-NMI^‒•-x-NDI → Per^+•-Xy-NMI-x-NDI^‒•, in which the second step is much faster than the first step. In addition, preserving long-lived charge separation in these systems is found to depend on inhibiting charge recombination by a triplet radical ion pair recombination pathway.