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^+•-³(A^-•-R^•) and D^+•-¹(A^-•-R^•), and subsequent reduction of R^• only occurs in D^+•-¹(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.