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

Controlling spin-spin interactions in multi-spin 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 alpha,gamma-bisdiphenylene-beta-phenylally1 radical (R-center dot) to produce D-A-R-center dot. Selective photoexcitation of D within D+center dot-A(-center dot)-R-center dot. results in ultrafast electron transfer to form the triradical, where D+center dot-A(-center dot) is a singlet spin-correlated radical pair (SCRP), in which both SCRP spins are uncorrelated relative to the R-center dot spin. Subsequent ultrafast electron transfer within the triradical forms D+center dot-A-R-, but its yield is controlled by spin statistics of the uncorrelated A(-center dot)-R-center dot radical pair, where the initial charge separation yields a 3:1 statistical mixture of D+center dot-(3)(A(-center dot)-R-center dot) and D+center dot-(1)(A(-center dot)-R-center dot), and subsequent reduction of R-center dot only occurs in D+center dot-(1)(A(-center dot)-R-center dot). These findings inform the design of multispin systems to transfer spin coherence between molecules targeting quantum information processing using the agency of SCRPs.