Research on Creating Magnets with Molecular Crystals
What comes to mind when you hear the word “magnet”? You might picture a bar magnet or a magnet on a whiteboard. Humanity's encounter with magnets is ancient, with records of their use for determining north and south dating back to before the common era. Magnetic materials can be considered one of the earliest known functional materials. The magnetic materials around us, such as iron, cobalt, nickel, rare earth elements, or metal oxides, invariably contain metal ions. In contrast to this atomic magnetism based on inorganic substances, research into molecular magnetism, composed of complexes and organic radical molecules, has been developing since the 1980s. Molecular magnetism is considered a highly flexible form of magnetism that can be easily combined with other functions, such as electronic and optical properties, by leveraging its molecular characteristics. Analytical methods, including magnetic measurements, single-crystal structure analysis, and molecular orbital calculations, have also advanced rapidly, making it possible to conduct material synthesis, molecular structure analysis, and electronic structure analysis in parallel.
In our laboratory, we construct magnetic molecular assemblies by controlling the arrangement of molecules with unpaired electrons through a crystal engineering approach. The orientation of magnetic moments between molecules changes significantly due to the interactions between molecular orbitals as the molecules assemble. By conducting parallel research on organic radicals and transition metal complexes, we can observe various differences even within the same molecule.
Radical unpaired electrons in π-orbitals tend to delocalize over the entire molecule. The extent of this delocalization can be measured from Electron Spin Resonance (ESR) spectra. However, by “visualizing” the spread of even minute spin densities, which are undetectable by ESR, through molecular orbital calculations, it has become clear that this has a significant impact on the magnetic properties of the entire crystal (Figure 1).
In the world of electronic properties, the dimensionality of interactions is crucial; simply arranging molecules in one dimension will not cause a magnetic transition. It is necessary to stack “molecular building blocks” that possess spin while considering the dimensionality of their interactions.
While placing a large number of unpaired electrons within a single “molecular building block” could theoretically produce stronger magnetic properties, increasing the number of unpaired electrons reduces chemical stability. The theory of aligning the magnetic moments of unpaired electrons within the same molecule has so far only been applied to π-conjugated molecules. Recently, our laboratory has discovered that quite strong interactions also exist between unpaired electrons that are not mediated by a π-conjugated system. In the future, by experimentally verifying this with a wider range of substances, we aim to design “molecular building blocks” with large magnetic moments while maintaining chemical stability, thereby advancing our research on creating magnets with molecular crystals.