Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Ho ^III complexes bearing organic luminophores and inorganic metalloligands are an effective tool for achieving the unique conjunction of single-molecule magnetism and thermometric luminescence re-absorption phenomenon.

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 Tb^III‐based emissive SMMs formed in a cyanido‐bridged framework whose properties are governed by the reversible structural transformation from [Tb^III(H₂O)₂][Co^III(CN)₆] ⋅ 2.7H₂O (1) to its dehydrated phase, Tb^III[Co^III(CN)₆] (2). The 8‐coordinated complexes in 1 show the moderate SMM effect but it is enhanced for trigonal‐prismatic Tb^III 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 Tb^III‐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 Tb^III complexes in achieving the SMM effect and hot‐bands‐based optical thermometry is discussed.