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 homologous series of dimers, alkandioic acid bis[pentakis( n −heptanoyloxy)phenyl] esters [(C6 H13 COO)5 C6 OC(O)(CH2 )(n−2)/2]2 with n= 6, 8, 12 and 14, are reported. They are henceforth referred to as T n. Between −100 C and the isotropic liquid (I) the four compounds exhibit, with increasing temperature, the phase sequence Cr P I, where 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 of T12 and T14 are completely miscible with that of the corresponding monomer, hexaheptanoyloxybenzene (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, while the benzene core is rigid. In the P(Col) mesophases of T12 and T14, the side chains are also disordered, but in addition the benzene cores undergo high amplitude planar librations. Analysis of the carbon−13 spectra of the carboxyl groups in the latter compounds shows a distribution of librational amplitudes around a mean value of about one rad with a variance of 0.14 rad2. The structure of the mesophases can be visualized as columns consisting of stacked monomeric units. The monomers are linked via spacers to their counterparts in neighbouring columns. The stacking level of the monomeric units in neighbouring columns is not correlated. Some pairs are stacked at the same level, resulting in loose spacers and high librational amplitudes, while other pairs are stacked at different levels with a corresponding tightening of the spacers and reduced librational amplitude