Engineering Magnetic Bistability: Solvent-Driven Photoresponsive Single-Chain Magnet Behavior in Fe ₂ Co Coordination Polymer

Single-chain magnets (SCMs), characterized by slow magnetic relaxation, hold immense promise for future data storage and advanced materials. Integrating photoswitchable electron transfer coupled spin transition (ETCST) or spin crossover (SCO) exhibiting units has been a strategy to achieve photoresponsive SCMs. However, achieving the ETCST behavior, alongside photoresponsive switching, is challenging, and even if accomplished, it does not always guarantee SCM behavior. Here, we demonstrate solvent-directed control of magnetic behavior in isostructural cyanide-bridged Fe₂Co 1D chains of the type {[Fe-(Tp)-(CN)₃]₂[Co-(L)₂]·xA·yB}, where L = 4-nitrophenylimidazole, Tp = hydrotris-(pyrazol-1-yl)-borate, and A/B are solvent molecules. Despite identical Fe₂Co cores, the complexes exhibit distinct magnetic properties. Notably, <b>1·5H</b> _<b>2</b> <b>O</b> (<i>x</i> = 0; <i>y</i> = 5, B = H₂O) and <b>2</b> ^<b>a</b> <b>·EtOH·1.5H</b> _<b>2</b> <b>O</b> (<i>x</i> = 1, A = EtOH; <i>y</i> = 1.5, B = H₂O) exhibit both thermal and light-induced bistability, displaying photoactivated SCM behavior, while <b>3·</b> ^<b>i</b> <b>PrOH·1.5H</b> _<b>2</b> <b>O·0.5L</b> (<i>x</i> = 1, A = ⁱPrOH; <i>y</i> = 1.5, B = H₂O) remains a nonresponsive SCM due to intrinsic paramagnetism. Intrachain ferromagnetic interactions lead to high energy barriers for reversal of magnetization, magnetic coercivity, and the highest number of correlated units reported for SCMs. These findings highlight the influence of solvent-driven crystal packing and intermolecular interactions (hydrogen bonding (HB), π···π, and CH···π interactions) in tuning photoresponsive magnetic properties, offering a pathway for designing next-generation stimuli-responsive molecular magnets.