Spin crossover and photomagnetic behaviors in one-dimensional looped coordination polymers

Three one-dimensional looped coordination polymers with the formula of [Fe(dpds)2{C(CN)3}2]·dpds (1, dpds = 4,4′-dipyridyl disulfide), [Fe(dpds)2(NCBH3)2] (2) and [Fe(dpds)2(NCSe)2]·3.5H2O (3) have been synthesized and characterized by single-crystal X-ray crystallography, Mössbauer spectroscopy and magnetic measurements. The spin-crossover (SCO) behaviors of these compounds depend significantly on axial ligands: compound 1 exhibited thermal-induced complete one-step SCO behavior with a T1/2 of 170 K, and 2 showed incomplete SCO behavior with T1/2 being 130 K, whereas 3 was paramagnetic. Meanwhile, both compounds 1 and 2 displayed light-induced excited spin-state trapping (LIESST) effects, featuring TLIESST values of 45 K and 61 K for 1 and 2, respectively, which were independent of exciting laser wavelengths. These results suggest that anions such as C(CN)3- and NCBH3- with much stronger ligand fields are needed in order to realize spin-state switching, if a ligand such as dpds is adopted to synthesize SCO compounds.