Microporous magnets compose a class of multifunctional molecule-based materials where desolvation-driven structural transformation leads to the switching of magnetic properties. Herein, we present a special type of microporous magnet where a dehydration–hydration process within a bimetal coordination framework results in the switching of emissive DyIII single-molecule magnets (SMMs). We report a three-dimensional (3-D) cyanido-bridged coordination polymer, {[DyIII(H2O)2]­[CoIII(CN)6]}·2.2H2O (1), and its dehydrated form of {DyIII[CoIII(CN)6]} (2), which was obtained through a reversible single-crystal-to-single-crystal transformation. Both phases are composed of paramagnetic DyIII centers alternately arranged with diamagnetic hexacyanidocobaltates­(III). The hydrated phase contains eight-coordinated [DyIII(μ-NC)6(H2O)2]3– complexes of a square antiprism geometry, while the dehydrated form contains six-coordinated [DyIII(μ-NC)6]3– moieties of a trigonal prism geometry. This change in coordination geometry results in the generation of DyIII single-molecule magnets in 2, whereas slow magnetic relaxation effect is not observed for DyIII sites in 1. The D 4d-to-D 3h symmetry change of DyIII complexes produces also the shift of photoluminescent color from nearly white to deep yellow thanks to the modulation of emission bands of f–f electronic transitions. A combined approach utilizing dc magnetic data and low-temperature emission spectra confirmed an axial crystal field of trigonal prismatic DyIII complexes in 2, which produces an Orbach type of slow magnetic relaxation. Therefore, we present a unique route to the efficient switching of SMM behavior and photoluminescence of DyIII complexes embedded in a 3-D cyanido-bridged framework.

A conjunction of Single‐Molecule Magnet (SMM) behavior and luminescence thermometry is an emerging research line aiming at contactless read‐out of temperature in future SMM‐based devices. The shared working range between slow magnetic relaxation and the thermometric response is typically narrow or absent. We report TbIII‐based emissive SMMs formed in a cyanido‐bridged framework whose properties are governed by the reversible structural transformation from [TbIII(H2O)2][CoIII(CN)6] ⋅ 2.7H2O (1) to its dehydrated phase, TbIII[CoIII(CN)6] (2). The 8‐coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal‐prismatic TbIII complexes in 2, showing the SMM features up to 42 K. They are governed by the combination of QTM, Raman, and Orbach relaxation with the energy barrier of 594(18) cm−1 (854(26) K), one of the highest among the TbIII‐based molecular nanomagnets. Both systems exhibit emission related to the f–f electronic transitions, with the temperature variations resulting in the optical thermometry below 100 K. The dehydration leads to a wide temperature overlap between the SMM behavior and thermometry, from 6 K to 42 K. These functionalities are further enriched after the magnetic dilution. The role of post‐synthetic formation of high‐symmetry TbIII complexes in achieving the SMM effect and hot‐bands‐based optical thermometry is discussed.

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