Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors.