SCMs possess bistable states capable of interchanging between two magnetic polarization directions 14, 15, 16, 17, 18. However, no one has so far succeeded in introducing light-responsive SCO units into molecular nanomagnets, such as single-chain magnets (SCMs) and single-molecule magnets, to switch the bistable states of the nanomagnets. Moreover, SCO units and other paramagnetic ions have been linked via a short bridge to control intramolecular magnetic interactions 25, 26, 27, 28, 29, 30, 31, 32, 33 and generate magnetic ordering 34 via LIESST. LIESST effects have been observed in various Fe II (or a few Fe III) mononuclear complexes 19, 20, 21, 22, 23, 24. In particular, the light-induced excited spin-state trapping (LIESST) 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, which induces conversion of a LS state to a HS state by means of light irradiation, has attracted considerable interest.
SCO complexes show reversible transformation between a high-spin (HS) and a low-spin (LS) state 1, 2, 3, 4, 5, 6, 7. In general, spin-crossover (SCO) complexes 1, 2, 3, 4, 5, 6, 7 and molecular nanomagnets 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 show such magnetic bistable states. As a result, molecules possessing magnetic bistable states are attracting a great deal of attention 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, because these bistable states can act as elementary binary units (bits) used for information storage, providing potential applications in high-density information storage 3, 5, quantum computing 10 and spintronics 13. It is a challenge to reduce the size of electronic devices and increase information density in materials area. The result provides a strategy to switch the bistable state of molecular nanomagnets using external stimuli such as light and heat, with the potential to erase and write information at a molecular level. Single-chain magnet behaviour is actuated accompanying the synergy between light-induced excited spin-state trapping at the iron(II) sites and ferromagnetic interactions between the photoinduced high-spin iron(II) and low-spin iron(III) ions in the chain. The chain exhibits thermally induced reversible spin-crossover and light-induced excited spin-state trapping at the iron(II) sites. Here we report an iron(II) spin-crossover unit and paramagnetic iron(III) ions that are incorporated into a well-isolated double-zigzag chain. It is a challenge to introduce spin-crossover units into molecular nanomagnets to switch the bistable state of the nanomagnets through external stimuli-tuned spin crossover. Both spin-crossover complexes and molecular nanomagnets display bistable magnetic states, potentially behaving as elementary binary units for information storage.