Guest-Triggered Charge Transfer for Magnetic Change in Redox-Active MOF Magnets

ConspectusThe synergistic control of lattice properties and porosity with mass transport is a defining feature of molecular lattices known as metal-organic frameworks (MOFs), a capability largely absent in conventional rigid inorganic materials. These open frameworks provide adaptive chemical environments in which guest inclusion can directly reorganize electronic structure and magnetic order. In redox-active MOFs, guest insertion, ejection, and transport, collectively referred to as guest dynamics, can reversibly modulate charge distribution and spin states, enabling electronic and magnetic phase switching. Such dynamic coupling between framework, space, and closely spaced electronic states establishes porous magnets as a versatile platform for stimulus-responsive molecular materials with potential applications in information storage and chemical sensing. This Account summarizes our efforts to develop redox-active MOF magnets based on donor-acceptor (D/A) architectures. These systems integrate redox-active paddlewheel-type diruthenium (II,II) complexes ([Ru₂II,II]; donors, D) with π-acidic TCNQ derivatives (acceptors, A), forming layered D₂A frameworks that support multiple, closely spaced electronic states. Because these states lie in delicate energetic balance, subtle structural perturbations such as guest adsorption can trigger charge transfer and reorganize magnetic ground states. To enable guest-induced magnetic switching, we have developed two key mechanisms: (1) on-host charge transfer (CT), where neutral guests modulate the electronic state of the host framework, and (2) host-guest CT, where redox-active guests directly exchange electrons with the framework. Whereas host-guest CT is limited to strongly redox-active guests, on-host CT exploits the intrinsic energetic competition between donor and acceptor units, amplified by lattice electrostatics. The central question addressed in this Account is how to rationally design D/A-MOFs poised at electronic instability, such that minor external stimuli can tip the balance between competing charge states. We outline two guiding strategies: positioning donor-acceptor pairs at the boundary of multiple electronic states and targeting systems that display emergent electronic configurations beyond initial predictions. Guided by these principles, five representative systems showing guest-induced on-host CT have been discovered. We hope this Account will encourage continued investigation into these multifunctional materials at the interface of magnetism, electronic regulation, and host-guest chemistry.