What role has organic chemistry played in the development of molecule-based magnets?

Abstract

Contributions of organic chemistry to the interdisciplinary science of molecule-based magnets are discussed in reference to transition metal coordination chemistry carried out successfully by Olivier Kahn and other scientists. Typical organic molecules have closed-shell structures and therefore organic compounds are usually diamagnetic. Organic chemists had to start with the design and construction of more-or-less stable open-shell organic molecules. Only then it became possible to study how to let their electron spins exchange-couple strongly by means of spin delocalization and polarization within a molecule and among molecules. Non-Kekulé π-systems were chosen to make the triplet carbene species to interact ferromagnetically. Approaches by means of synthetic organic chemistry proved effective. Magnetic susceptibility measurements revealed that such tetracarbene and nonacarbene have ground S=4 and S=9 states, respectively. Thus, the open-shell hydrocarbon molecules have been shown to have higher magnetic moments than those of S=5/2 Fe(III) and S=7/2 Gd(III) ions. Stable aminoxyl radicals played various roles in generating organic ferromagnets by controlling the exchange coupling between molecules in crystal, or by forming the metal/aminoxyl radical hybrid systems that were made on the basis of the supramolecular approaches and led to a number of ferro- and ferrimagnets. Metal/pyridylcarbene hybrid systems generated photochemically not only revealed the mode of the exchange coupling between the spins of metal d-orbital and those residing in organic π-orbital, but also suggested the route to novel photoresponsive magnetic materials. Most of the organic and metal/organic magnets discussed in the 80s and early 90s were trying to mimic conventional magnets by molecular materials. It is only after the advent of the single-molecule magnets when molecule-based magnets became molecular materials in their own right in the area of nano-science. Here again, novel organic molecules are used as useful, functional ligands for making single ion magnets and single chain magnets.