The PhD work of Olivier KAHN was dealing with carbonyl derivatives containing bonds between transition metals and elements of the column XIV (C to Pb). The main objective was to compare the electronic structures of isostructural compounds of the type (CO)4Fe-PR3 and (CO)4Mn-SiR3. This work involved synthesis and characterization of organometallics complexes, spectroscopic studies, measurements of dipole moments as well as an attempt to rationalize the results using the quantum mechanical concepts.
For the first time in his scientific carrier Olivier KAHN approached a problem from both experimental and theoretical view points.
Olivier KAHN discovered the beauty and the complexity of the open-shell molecules during a post-doctoral stay at the University of East Anglia at Norwich. During 16 months, he worked on the role of the vibronic coupling on the spectroscopic and magnetic properties of transition metal compounds with an orbitally quasi-degenerate ground state.
His main result concerned the orbital reduction arising from the vibronic coupling. He developped a theoretical formalism along this line, then tested it by studying the electronic structure of several systems. One of them is V(CO)6, with 2T2g ground state.
He demonstrated that in the solid state, this peculiar metal carbonyl derivative undergoes a phase transition at 66 K, with a dimerization, a behavior which was driven by the orbital degeneracy of the ground state.
In October 1975, he joined the University of Paris Sud as a Professor of Chemistry, and founded the Laboratoire de Chimie Inorganique, then the Institut de Chimie Moléculaire d’Orsay.
Since that time, it has probably been more convenient to present his scientific achievements by theme of research rather than of follow the chronology.
Olivier KAHN’s objective was to design polymetallic compounds exhibiting predictable magnetic properties. This requires to be able to control the magnetic interaction between the metal ions in both sign and magnitude. For that, he proposed and orbital model of the magnetic interaction phenomenon in which the relative symmetries of the singly-occupied interacting orbitals played the key role, and developped an extensive synthetic chemistry to test this model.
Among other results, he reported on the first example of strict orthogonality of the magnetic orbitals affording a ferromagnetic coupling, and he showed how it was possible to fine tune the interaction between two copper(II) ions through a bisdidentate ligand. He also defined the conditions to enhance the magnitude of the interaction between two spin carriers far apart from each other.
In most molecular compounds containing several spin carriers - metal ions or organic radicals - the interactions are antiferromagnetic, i.e. of the up-down type, which affords a low-spin ground state. That is why it may be asserted that molecular ferromagnetism is a challenge.
_Olivier KAHN decided to tackle this problem, and to explore the various strategies leading to high-spin ground states, and to genuine molecule-based magnets, i.e. to molecular materials exhibiting a spontaneous magnetization below a critical temperature Tc.
Such an approach leads him to introduce several new concepts like the irregular spin state structure or the one-dimensional ferrimagnetism and in each case to create new objects in order illustrate these concepts.
Eventually, in 1986 he reported on one of the very first molecule-based magnets with Tc = 4.6 K. Since this pionnering work, he has synthesized many other compounds of that kind with increasing critical temperatures.
Olivier KAHN has also been interested by the rationale design of hard magnets, exhibiting wide magnetic hysteresis loops. He succeded to synthesize such compounds whose coercive field may be as large as 2.5 Tesla. This is by far the record for molecule-based magnetic materials. Meanwhile, he rationalized the conditions required to achieve such strong coercivities.
More recently he focused on the physical properties of these rather exotic materials, in particular on the synergy between optical and magnetic properties. He is now involved in the study of the photomagnetic properties of molecule-based magnetic materials.
In the meantime, he systematically explored from a theoretical point of view the various orbital patterns favoring a parallel spin alignment, and rationalized a broad spectrum of experimental data reported by other groups.
For a long time, Olivier KAHN has been interested in chemical topology. More precisely, he looked for synthesizing molecular lattices in which the interactions between active sites would be controlled along the three directions of space. Along this line, he described a family of fully interlooked three-dimensional magnets with an infinite number of knotes. The general formula is (rad)2M2[Cu(opba)]3 where MII may be MnII or CoII, rad is a nitronyl nitroxide radical cation, and opba stands for the ortho-phenylenebis(oxamato). These magnets contain kinds of spin carriers. A schematic view of one of them is shown below :
The spin density map for a molecule-based magnetic materials is certainly the description providing the largest amount of information on the ground state of this compound. Such maps may be obtained experimentally from polarized neutron diffraction, and calculated theoretically using quantum chemistry.
Olivier KAHN has been involved in both experimental and theoretical determinations of spin density maps, and has obtained a series of fundamental results concerning the phenomena of spin delocalization and polarization.
One of Olivier KAHN’s lines of research concerns the synthesis and the study of the physical properties of molecular compounds containing both lanthanides and 3d metal ions. Compounds with quite unexpected structures were obtained. A qualitative model concerning the magnetic behavior of these compounds has been proposed, in which the unquenched orbital momentum of the rare earth is explicitely taken into account.
Professor KAHN is also concerned by the optical properties of this new class of molecular materials.
One of the very exciting perspectives in molecular chemistry deals the use of assemblies of molecules in electronics circuits and devices. The main concern Olivier KAHN in this field is the possibility to obtain a hysteresis effect. It is indeed clear that such a hysteresis in the context of molecular chemistry.
These conditions can be realized for assemblies of weakly interacting molecules. The question at hand is to know whether an isolated molecule as well can exhibit such a hysteresis. This crucial problem as far as the status of molecular electronics is concerned is investigated in close contact with theoretical physicists an biophysicists.
The most spectacular example of molecular bistability is probably provided by the phenomenon of high spin low spin transition. When the cooperativity within the crystal lattice is large enough, the transition may be very abrupt with a large thermal hysteresis effect. To increase the cooperativity, Olivier KAHN explored two approaches, namely the supramolecular chemistry approach and the polymeric approach.
This allowed him to synthesize several families of compounds for which the transition occurs around room temperature, and is associated with a spectacular color change. Such compounds can then be used in genuine devices for display and data recording.
In collaboration with Philips Company, then France Telecom and Motorola he designed such devices, and started exploring their range of applicability. He is also interested by the use of those compounds in various kinds of sensors.
What is crucial for observing a hysteresis in spin transition compound is the cooperativity. Macroscopic models accounting for the observed behaviors have been proposed.
Olivier KAHN is presently working on the microscopic origin of the cooperativity. The goal is, of course, to be capable to predict what compound must be designed to obtain a spin transition regime corresponding to the type of application which is looked for.
Olivier KAHN has written a text book entitled "Molecular Magnetism" that appeared in 1993. This book was edited by the American branch of Verlag-Chemie. It covers the field from the fundamental bases like the interaction between a molecular assembly and a magnetic field up to the lattest developments. Both the theoretical and experimental facets of the field are emphasized, as well as its relations with the emerging field of molecular electronics. The first edition has been sold out within 18 months. In 1996, Olivier KAHN has edited an important highly quoted multi-authored book intitled "Magnetism : a supramolecular function".