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.

Rare-earth (RE) metal complexes exhibit attractive magnetic and optical properties related to their strong magnetic anisotropy and efficient photoluminescence. Due to the accessibility of high coordination numbers, they are useful molecular building blocks for the construction of bimetallic coordination systems. Therefore, they can be readily combined with hexacyanidometallate ions of transition metals into the d–f cyanido-bridged frameworks. This leads to the rich family of molecule-based materials varying from purely inorganic 3-D RE^III–[M^II/III(CN)₆]^4−/3− (RE = La–Lu, Y; M = Cr, Fe, Ru, Co) networks, through coordination polymers of lower dimensionality obtained with supporting organic ligands, up to strictly molecular species. The past decade brought intensive interest in the design and synthesis of rare-earth hexacyanidometallate frameworks with particular attention paid to their magnetic, optical, electronic, and mechanical properties. In this review, we discuss the most important results of this research area, including remarkable thermal expansion properties, magnetic ordering effects, single-molecule magnetism, luminescent functionalities, electrochemical activity for applications in alkali ion batteries and electroanalytical sensing, and efficient NMR relaxivity for magnetic resonance imaging, as well as photo- and pressure-induced switching of magnetic and optical characteristics. We focus on the recent advances, emphasizing also the general advantages and the future perspectives in the combination of RE(III) complexes and hexacyanidometallates within the d–f coordination frameworks towards the generation of diverse physical functionalities.

Dicyanidobis(aromatic α , α ′-diimine) Fe( ii ) complexes efficiently constrain the equatorial plane of the Dy( iii ) centre which opens a pathway for controlling the lanthanide magnetic anisotropy by placing organic N -oxides at axial positions.

[Re(CN)₈]³⁻ and [W(CN)₈]³⁻ compete in shaping the phase transition and magnetic interactions in trimetallic {Fe₉[Re(CN)₈]_6−x[W(CN)₈]_x} clusters via tuning of the valence electron distribution and mobility.