Chiralité dans les Précurseurs Electroactifs et Matériaux Moléculaires Multifonctionnels – CHIRAFUN

The concept of chirality is present in many chemistry fields, amongst which the materials science one is very much concerned. The influence of the chirality on the physical properties, such as magnetism and conductivity, of molecular materials constitutes a relatively recent challenge. This feature is in line with the quest for multifunctional molecular materials, a trend of much current interest in contemporary materials science, aiming at combining in the solid state at least two physical properties. It is well known that TTF and its derivatives have been successfully employed as building blocks for organic charge transfer or mixed valence salts having conducting or superconducting properties. If the conductivity is combined with another property within multifunctional materials, for which TTF derivatives proved to be valuable precursors, there is the possibility to have either coexistence or synergy between the properties. In this respect, the combination of chirality and conducting properties has been recently highlighted through experimental evidences of a new phenomenon, called electrical magneto-chiral anisotropy (eMChA). In non-chiral conductors the resistance has a quadratic dependence on the applied external field, which is known as magneto-resistance, whereas in a chiral conductor a new term appears, as a product between the current I through the conductor, the external magnetic field H and a parameter ', denoting the handedness of the chiral conductor and having the same value, but opposite signs, for the two enantiomers ('D = -'L). As a consequence, the electrical resistance R of one enantiomeric conductor is expected to have different values upon reversal of the direction of the current and that of the external field, i.e. when I changes to 'I, or H to 'H, respectively. Thus, this effect should be observed in the magneto-resistance of a chiral material, yet, its very low magnitude and also the relatively rare examples of enantiopure conductors, be they molecular or classical metals, precluded its observation until recently. Indeed, the group of G. Rikken, one of the partners of this project, described the first experimental evidences of this effect in the case of metallic bismuth and single walled nanotubes. Although the effects observed so far are quite small, it has been pointed out that they may be interesting for spintronics, since in chiral conductors electrical resistance depends not only on the magnitude of spin polarization, but also on its direction. Increasing efforts are currently dedicated to spintronic devices, and particularly the organic ones such as the organic spin-valves, and their applications in electronic and computer industries. It is thus clear that a library of chiral precursors, in which the chiral information is addressed in different manners, is needed in order to prepare chiral conducting materials, which are potential candidates for the experimental evidence of the electrical magneto-chiral anisotropy effect. We therefore envisage to introducing, within several families of precursors, either stereogenic centers at the carbon atom, through oxazoline heterocycles or alkyl chains, or at the sulfur atom through the sulfoxide group, or helical supramolecular chirality, or a combination of both. Another field of applications in which we became interested and envisage to exploring with some of our compounds is that of organic field-effect transistors (OFET), which have attracted much attention over the past few years due to their unique processing characteristics and improved electronic mobility. Interestingly, very good FET performances have been reported recently for different TTF derivatives, in crystalline form or as films, which make them good candidates for this type of electronic devices. Note that the first results concerning OFET devices based on TTFs have been published by M. Mas-Torrent, who is partner in this project. Nevertheless, to our knowledge, no chiral TTF derivative has been investigated within a FET configuration, which encourages us to undertake a systematic study in this direction, with all the synthesized chiral TTFs. The second redox active motif we envisage as precursor for chiral molecular conductors and field-effect transistors (FET) concerns the TTT derivatives, which have been studied at a much lesser extent than the TTF derivatives, yet a certain number of conducting salts and charge transfer compounds have been reported to date. Moreover, no tetrathia-tetracene (TTT) derivative, be it chiral or not, has been studied so far as FET, despite its promising potential. The ultimate goal of this approach, besides the investigation of the FET properties of our derivatives, is to analyze the influence of the chirality on the mobility of the charge carriers, either as a consequence of the crystal packing, or as an effect of a magneto-chiral anisotropy.