In the drive for smaller electronic components, chemists are thinking on a molecular scale. By combining two simple molecules, a hybrid has been produced that is both magnetic and an electrical conductor.
As we approach the physical limits of conventional silicon-based electronics, there is a clear need for entirely new types of materials that can deliver smaller and smaller devices. A strong candidate in this field is molecular electronics, which uses assemblies of individual molecules to mimic larger, conventional structures such as switches or semiconductors. To be effective, full control over the composition, size and function of these molecules is needed.
Over the past 40 years, researchers have created some of the basic building blocks needed for molecular electronics, including metals, semiconductors, superconductors and magnets, as well as the so-called single-molecule magnets. On page 447 of this issue Coronado et al.1 describe a hybrid material that opens a new frontier in molecular electronics. They have created a molecular material that can behave simultaneously as a magnet and an electrical conductor. Whereas conducting magnets, such as nickel or iron, are common among metals and alloys, this is the first reported example in molecular materials. Because the electronic peculiarities of individual molecules are different from those of bulk metals, a conducting molecular magnet is likely to have unexpected properties.
Materials whose properties are based on their component molecules are more versatile than those whose properties derive from their component atoms. Thus the bulk properties of molecules — whether optical, magnetic or electrical — can be controlled using conventional syntheses, such as those used in the pharmaceutical industry. This means that they are tuneable and can more readily be tailored in response to the changing demands of technology. By combining an organic conductor and a magnetic complex, Coronado et al . have now introduced the possibility of materials with multiple functions.
The ability of small organic molecules to exhibit metal-like electrical conductivity was first shown in 1965 in an electron-transfer salt known as TCNQ2. These studies showed that metal-like conductivity as well as metal-like optical properties could be seen in soluble organic materials. This work led to the development of electrically conducting polymers — essentially plastic conductors — for which Alan MacDiarmid, Alan Heeger and Hideki Shirakawa received this year's Nobel Prize for Chemistry. In addition to conducting polymers, other charge-transfer salts were discovered and studied. Of these, the molecule [TTF][TCNQ], which led to the first examples of organic superconductors, is probably the best known. Several organic superconductors3 and organic metals were made from a related molecule called BEDT-TTF (Fig. 1a). Coronado et al. make use of the conducting properties of this molecule in their new material.
Ferromagnetism is the parallel alignment of all the magnetic moments in a material, whether atomic or molecular, induced by applying a weak external magnetic field. This leads to a spontaneous magnetization, which may remain once the external field is removed, as is found in the magnets used on doors, or it may disappear, as in modern current-transformers. Molecule-based ferromagnetism was first reported in 1972 for an iron chloride coordination compound4,5. Then in 1986 ferromagnetism was discovered in an organic-based material6. These materials became ferromagnets at extremely low temperatures (below 5 K) and were soluble in conventional organic solvents. In 1992 Okawa and co-workers surprised researchers when they reported a layered bimetallic compound that behaved as a two-dimensional ferromagnet7. Although insoluble, these magnets are crystalline and their molecular components self-assemble into a layered structure in aqueous solutions.
Coronado et al.1 created their material using controlled self-assembly of alternating single layers of a bimetallic ferromagnet (Fig. 1b) and BEDT-TTF molecules. Precise control of the assembly is possible because the BEDT-TTF layers are positively charged and the magnetic layers are negatively charged. The alternating monolayers of BEDT-TTF and the ferromagnet are just 1.3 and 0.36 nm thick, respectively ( Fig. 2). So the magnetic layers develop bulk ferromagnetic ordering, but this remains independent of the current flowing in the organic layers. The authors find no evidence that the ferromagnetic and conducting subsystems interact with each other, except when an external magnetic field is applied perpendicular to the layers.
The synthesis of molecular materials that can deliver technologically important physical or chemical properties, or a combination of these properties, is now a major goal for chemists. Coronado et al.'s results1 show that self-assembly allows multifunctional materials to be made while keeping precise control over their composition and structure at the nanoscale. In the past few years we have witnessed the birth of 'spintronics' — electronic devices that are based on interactions between the 'spins' of electrons rather than their charge. A popular example is a so-called spin valve. This consists of two ferromagnetic layers separated by a weaker diamagnetic layer, which can be either an insulator or a metallic conductor only a few atomic layers thick. The ferromagnetic layers communicate with each other through electron- tunnelling effects or through the mobile electrons in the metallic layer. The material reported by Coronado's group suggests that molecule-based materials with these properties are on the horizon. This could make these devices easier to process and lead to a substantial reduction in production costs.
In conventional metallic ferromagnets, the mobile electrons play a crucial role in both the magnetic interactions and the conductivity. But in Coronado et al.'s system, the conducting electrons in the organic layer do not appear to interact with the magnetic moments of the ferromagnetic layer. This unique feature, which is only possible because of the molecular nature of the system, may yet yield unforeseen physical behaviour. It will also be desirable to develop hybrid molecular materials in which the conducting and magnetic subsystems do interact with each other — these could then be used to develop new electronic devices that operate at the nanoscale. It is easy to imagine other hybrid materials that combine magnetism with nonlinear optical properties, or ferromagnetism with superconductivity. The latter will be useful for exploring the interplay between superconductivity and magnetism, which, like oil and water, usually do not mix.
Coronado, E., Galán-Mascarós, J. R., Gómez-García, C. J. & Laukhin, V. Nature 408, 447–449 (2000).
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Two tris(oxalato)ferrate(III) hybrid salts with pyridinium derivative isomers as counter cations: synthesis, crystal structures, thermal analyses, and magnetic properties
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