Spin-Selective Photoreduction of a Stable Radical within a Covalent Donor-Acceptor-Radical Triad.

Controlling spin-spin interactions in multispin molecular assemblies is important for developing new approaches to quantum information processing. In this work, a covalent electron donor-acceptor-radical triad is used to probe spin-selective reduction of the stable radical to its diamagnetic anion. The molecule consists of a perylene electron donor chromophore (D) bound to a pyromellitimide acceptor (A), which is, in turn, linked to a stable α,γ-bisdiphenylene-β-phenylallyl radical (R) to produce D-A-R. Selective photoexcitation of D within D-A-R results in ultrafast electron transfer to form the D-A-R triradical, where D-A is a singlet spin-correlated radical pair (SCRP), in which both SCRP spins are uncorrelated relative to the R spin. Subsequent ultrafast electron transfer within the triradical forms D-A-R, but its yield is controlled by spin statistics of the uncorrelated A-R radical pair, where the initial charge separation yields a 3:1 statistical mixture of D-(A-R) and D-(A-R), and subsequent reduction of R only occurs in D-(A-R). These findings inform the design of multispin systems to transfer spin coherence between molecules targeting quantum information processing using the agency of SCRPs.