Two-Step Thermoinduced Metal-to-Metal Electron Transfer and ON/OFF Photoswitching in a Molecular [Fe2Co2] Square Complex

A cyanide-bridged [Fe(2)Co2] molecular square complex, {[Fe(Tp)(CN)3]2[Co(L)2]2}(BF4)2.2CH3CN.6H2O [1; Tp = hydrotris(pyrazol-1-yl)borate and L = bis(1-ethyl-1H-imidazol-2-yl)ketone], has been synthesized and characterized fully by single-crystal X-ray diffraction, (photo)magnetic measurements, optical reflectivity, and other physical measurements. 1 exhibits a two-step metal-to-metal electron-transfer (MMET)-induced spin transition accompanied by thermal hysteresis (T1/2? = 332 and 407 K and T1/2? = 320 and 405 K, respectively), converting the low-temperature diamagnetic {FeCNCo} ground state into the high-temperature paramagnetic {FeCNCo} state. Additionally, 1 displays reversible photoinduced MMET under light irradiation (ON mode using 808 nm laser light and OFF mode using 532 nm laser light), as confirmed by optical reflectivity and (photo)magnetic measurements. The photoinduced paramagnetic metastable state relaxes back to the diamagnetic ground state at 91 K (TLIESST = 91 K). Astonishingly, 1 also exhibits a 27 K wide light-induced thermal hysteresis below 100 K. The overall results show that 1 is a multistimuli-responsive bistable material that exhibits reversible switching between the diamagnetic state, {FeCNCo}, and the paramagnetic state, {FeCNCo}, under the application of temperature and light.