Switching the magnetic hysteresis of an [Feii-NC-Wv]-based coordination polymer by photoinduced reversible spin crossover.

Magnetic bistable materials that feature magnetic hysteresis are comparable to elementary binary units and promising for application in switches and memory devices. In this work, we report a material that consists of parallel cyanide-bridged [Feii–Wv] coordination chains linked together through rigid bis(imidazolyl)–benzene ligands and displays multiple magnetic states. The paramagnetic high-spin and diamagnetic low-spin states of the spin-crossover Feii ions can be interconverted by reversible light-induced excited spin state trapping (LIESST) by alternating between light irradiation of 808 and 473 nm. At 1.8 K, under 808-nm-light irradiation, magnetic interactions between the photogenerated paramagnetic high-spin Feii centres and the Wv centres lead to long fragments that exhibit single-chain magnet behaviour, with a wide magnetic hysteresis and a large coercive field of 19 kOe; under a 473 nm light, isolated Feii–Wv fragments behave as single-molecule magnets instead. At 3.3 K, the high-spin form still displays magnetic hysteresis, albeit narrower, whereas the low-spin one does not. Photoinduced spin crossover offers a convenient handle on the spin states and magnetic interactions within a material, which is promising for the development of photoresponsive nanomagnets. Now, a Wv–CN–Feii-based coordination polymer has been prepared that behaves either as a single-chain magnet or as single-molecule magnets under different light irradiations. Its magnetic hysteresis can also be switched on and off.