MESOMORPHIC AND DYNAMIC PROPERTIES OF DISCOTIC ALKANOYLOXYBENZENE DIMERS AS STUDIED BY X-RAY AND NMR - THE EFFECT OF SPACER LENGTH

The preparation and mesomorphic properties of four members of the homo logous series of dimers, alkandioic acid bis[pentakis(n-hepranoyloxy)p henyl] esters [(C6H13COO)(5)C6OC(O)(CH2)((n-2)/2)](2) with n = 6, 8, 1 2 and 14, are reported. They are henceforth referred to as T n. Betwee n -100 degrees C and the isotropic liquid (I) the four compounds exhib it, with increasing temperature, the phase sequence Cr --> P --> I, wh ere Cr is a solid and P is an intermediate phase which is crystalline for T6 and T8, and columnar discotic for T12 and T14. The mesophases o f T12 and T14 are completely miscible with that of the corresponding m onomer, hexaheptanoyloxy-benzene (BHA7) and thus are identified as of type Col(rd). This assignment is confirmed by X-ray measurements which also provide the lattice parameters for the two-dimensional columnar array. Deuterium and carbon-13 NMR measurements indicate that the side chains in the P(Cr) phases of T6 and T8 are dynamically disordered, w hile the benzene core is rigid. In the P(Col) mesophases of T12 and T1 4, the side chains are also disordered, but in addition the benzene co res undergo high amplitude planar librations. Analysis of the carbon-1 3 spectra of the carboxyl groups in the latter compounds shows a distr ibution of librational amplitudes around a mean value of about one rad with a variance of similar to 0.14 rad(2). The structure of the mesop hases can be visualized as columns consisting of stacked monomeric uni ts. The monomers are linked via spacers to their counterparts in neigh bouring columns. The stacking level of the monomeric units in neighbou ring columns is not correlated. Some pairs are stacked at the same lev el, resulting in loose spacers and high librational amplitudes, while other pairs are stacked at different levels with a corresponding tight ening of the spacers and reduced librational amplitudes.