Photoinduced Switching-on of Intermolecular Ferromagnetic Exchange in a Cobalt–Viologen Donor–Acceptor Assembly

Abstract

Crystal engineering of anisotropic molecular solids that translate photoinduced charge transfer into switchable magnetic exchange offers a route to optically addressable exchange-correlated magnetic states. Here, we show that (MV)[Co(pdms)2](MeOH)2(MeCN), a donor–acceptor (D–A) crystal incorporating Co(II) single-molecule magnet (SMM) units, converts photoinduced charge redistribution into reversible switching of intermolecular magnetic exchange (MV = methyl viologen, H2pdms = 1,2-bis(methanesulfonamido)benzene). Single-crystal, electrochemical, and natural transition orbital analyses establish that MV2+ remains the acceptor, whereas [Co(pdms)2]2– serves as the donor in a preorganized yet charge-localized one-dimensional zigzag D–A array that provides a through-space pathway for photoinduced intermolecular charge transfer. Upon 365 nm irradiation, this D–A lattice forms a metastable charge-transfer/radical photostate that reversibly switches it from a weakly coupled Co(II)-SMM assembly into a single-chain-magnet-like exchange-correlated state, as evidenced by the enhanced χmT response, the Weiss temperature change from −0.9 to +29.3 K, and magnetic hysteresis persisting up to 6 K. Dynamic magnetic measurements further reveal exchange-correlated relaxation with an effective correlation energy of Δξ/kB = 53 K, corresponding to a photoinduced ferromagnetic exchange of Jeff = 5.9 K. These results establish preorganized D–A solids as a platform for coupling photoinduced charge redistribution with local magnetic anisotropy to access optically switchable exchange-correlated molecular magnetic states