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The Scientific Case 67 5 5. The Scientific Case 67 5. Chemistry 5.1 Introduction whereas, including cooperative Panel: magnetic interactions, the U.E. Steiner (Konstanz) In chemistry, the application of modern field of molecular (Chairman) magnetic fields has a long magnetism offers new possibilities R. Bittl (Berlin) tradition and has led to well for structural control by magnetic M. Ernst (Nijmegen) established disciplines and fields. In the following sections O. Kahn (Bordeaux) techniques. Two main purposes in these several areas are discussed using high magnetic fields may be from the point of view of the distinguished; the probing of likely impact of intense magnetic structural and dynamical fields. properties and the controlling of chemical processes and structures. 5.2 Molecular magnetism The ability of a magnetic field to function as a probe is usually based Molecular magnetism is a rather on the exploitation of the field new field of research which has dependence of some property of emerged during the last decade or the chemical substance or system, so. It deals with the physics and including its interaction with chemistry of open-shell radiation; i.e. various kinds of molecules and of molecular spectroscopies, of which NMR is assemblies involving open-shell the most vital and powerful one. units. What characterizes this field Thereby detailed qualitative and of research is its deeply quantitative information can be multidisciplinary character. It gained as to chemical composition brings together organic, inorganic, (chemical analysis) as well as and organometallic synthetic structural and dynamical chemists along with theoreticians, properties. Structural aspects solid state physicists, materials pertain to the characterization of and life scientists. The heart of the details of molecular geometry the discipline nowadays concerns as well as of the energetics and the design and the investigation spatial characteristics of the of the physical properties of valence electrons, while dynamical molecular assemblies exhibiting aspects pertain to internal motions bulk properties such as long-range of the probed molecules as well as magnetic ordering or molecular their molecular environment and bistability. to the rates at which elementary chemical processes take place. The first factor explaining the development of molecular At the molecular level, magnetic magnetism, at the expense of the control of chemical processes is traditional field of possible by the mechanisms of magnetochemistry, is the shift of spin chemistry, applying in interest in molecular chemistry particular to chemical reactions from isolated to collectively with radical pair intermediates, organized molecules 68 5. The Scientific Case Chemistry (supramolecular chemistry). The exciting perspectives in molecular initial successes in the synthesis chemistry is the use of isolated of molecular conductors and molecules or assemblies of superconductors prompted some molecules in electronic circuits or scientists to try to synthesize devices. It is probably a rather molecular magnets, with a shift long-term prospect, and the of interest from simple challenge is not to replace paramagnetic behavior to traditional (silicon) electronics, collective behavior. The but to use molecule-based systems development of supramolecular to perform functions that are not chemistry has some of its roots in possible with silicon. Some ideas the challenge of imitating have already been proposed, with biological molecules, in which some advances made in switching, properly organized building blocks amplification, information form macromolecules capable of storage, and signal processing. highly selective functions. Similarly, in the area of 5.2.1 Molecule-based magnets magnetism, nature sometimes For more than half a century it uses molecular techniques to has been a dream for chemists to develop complex magnetic design molecule-based magnets. behavior patterns. Thus, the Today such exotic materials exist. second important factor behind The first ones were reported in the emergence of molecular 1986, both in America and in magnetism was the new Europe. Ten years later, a few tens understanding of the properties of molecule-based compounds of complex biological systems exhibiting a spontaneous ranging from heme proteins to magnetization below a certain iron-sulfur proteins, from temperature are known. In superoxide dismutase to ferritin, contrast with the classical and thus the mechanism of magnets which are opaque, the biomineralization. A third factor molecule-based magnets are promoting renewed interest in usually weakly colored, so that molecular magnetism was the one of the main issues concerns interest of solid state physicists in the synergy between magnetic low-dimensional physics. During and optical or photophysical the last few years molecular properties. A very appealing magnetism has offered to the perspective would be to design physicists quite a few exotic low- molecule-based magnets whose dimensional magnetic systems magnetic properties could be fine- with unprecedented spin tuned through light irradiation at topologies. The final factor was given wavelengths. the interest in new classes of advanced materials and the The spin topology of most of the appearance of molecular molecule-based magnets is rather electronics. One of the most complex, so that the effect of a 69 high magnetic field is not as this minimum should disappear in straightforward as for classical high fields somewhere between 30 magnets. In many cases, even at and 80 T. There is a strong interest 20 or 30 T, saturation is not in the community about this reached. This situation is due to prediction, and only a large field the fact that the compounds may facility will allow confirmation or present both ferromagnetic and rejection of this conjecture. antiferromagnetic interactions between uncompensated spins. At high enough fields, some decoupling of the antiferromagnetically coupled spins may occur, leading to unusual magnetization versus magnetic field curves (Fig. 1). In Fig. 1: Magnetization (in Bohr magnetons per molecular unit) versus other respects, in purely molecule- magnetic field curves for two molecular-based magnetic compounds showing long range magnetic ordering below 15 K based ferrimagnets, when the field (A) and 22.5 K (A). The magnetization curves were measured at 5 is not too high, the temperature K (A) and 4.2 K (B), respectively. In compound A the dependence of the product of the ferrimagnetically coupled magnetic centers MnII and CuII are magnetic susceptibility exhibits a connected in a two-dimensional network whereas in compound B, where a third spin carrier rad+ replaces the diamagnetic cation minimum at a certain + NBu4 in A, the network becomes three-dimensionaly ordered. In A, temperature. This minimum is full saturation of magnetization is attained within a few tesla, whilst considered as the fingerprint of in B, after a fast rise of magnetization at low fields, a slower rise the ferrimagnetic regime. It has follows which is not even saturated at 20T. This field dependence, the full analysis of which still needs measurements up to several been suggested very recently from times higher fields, bears important clues to the role of the magnetic density matrix renormalization coupling between the spin subsystems, the understanding of which group (DMRG) calculations that is essential for developing new magnetic materials. 70 5. The Scientific Case Chemistry 5.2.2 Molecular bistability and 5.2.3 Nanoscale magnetic spin transition materials materials The most spectacular example of A third exciting area in molecular molecular bistability is certainly magnetism, which was developed provided by the phenomenon of more recently, concerns molecular spin crossover, or spin transition. clusters of nanoscale dimensions. Some transition metal molecular The interest here derives from the compounds can present a same ideas which result in the crossover between a low-spin (LS) miniaturization of electronic and a high-spin (HS) state. This devices. Matter on the nanoscopic crossover may be induced by a scale has different properties, variation of temperature, which could open up many new pressure, or by irradiation with perspectives. One of these light. In some cases, the perspectives is to detect a thermally induced transition transition from the quantum between LS and HS states is regime of microscopic particles to cooperative, occurring with well that of the thermodynamical pronounced thermal hysteresis. In regime, which applies to bulk the family of Fe(II)-1,2,4-triazole magnets. Another appealing issue compounds, for instance, the here is to demonstrate the effects transitions occur around room of quantum tunnelling. Such temperature, with thermal effects have been found in a Mn12 hysteresis widths which may molecular cluster. This cluster has reach 50 K. Furthermore, the a ground state with spin S=10 and transitions are accompanied by a a large magnetic anisotropy. When spectacular change of colour, the cluster is in its ground state, from violet in the LS state to the magnetization is preferentially white in the HS state. Several of oriented parallel to a tetragonal these compounds have already symmetry axis. It can be oriented been used as active elements of either up or down, the two display devices. Of course, the orientations having the same spin transition regime may be energy. For the system to reorient, influenced by a magnetic field it must overcome an energy which shifts in energy the barrier between
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