Mesomorphic properties of short chains substituted heteroaromatic salts

Abstract

The synthesis of triaryl-2,4,6 pyrylium tetrafluoroborates substituted by six short (ethyloxy : C2, propyloxy : C3) alcoxy chains has been performed. Classical characterization methods (DSC and optical microscopy) show that these two new compounds present the same columnar phase Dh as that displayed by the other members of the homologous series (C4 to C12). However, these two new compounds are subject to chemical decomposition when heated for a few hours in the mesophase domain of stability but close to Tc. The existence of such a mesophase implies that neighbouring columns be discorrelated. Disordered alcoxy chains are usually assumed to play this role which is however questionable in the case of chains as short as n = 2 or 3. In this case, the BF- 4 anions should play a role in the mesophase formation. In order to obtain information about the behaviour of these anions, we have first performed a comprehensive X ray diffraction study of the whole homologous series. From the intensities of the 2D lattice reflections for all members of the series, we derived a simple model of the molecular organization of this mesophase in which the BF-4 anions are spatially disordered but still located close to the aromatic cores of the cations. We also performed a 19F NMR study of compounds C4, C 5, C8 and C12, which shows that the BF- 4 anions undergo several types of motion in the mesophase. At high temperatures, the spherical reorientation motion of the BF- 4 anions is completed. The onset of this motion may be mainly responsible for discorrelating neighbouring columns and therefore for inducing the Dh phase displayed at high temperatures by compounds C2 and C3