The preparation and mesomorphic properties of a new discotic dimer, bis [pentakis(pentyloxy)triphenylenyloxy]- decane (DTHE5) is described. This molecule is a perfect twin of the monomeric hexakis(pentyloxy) triphenylene (THE5) in that both the spacer and the free alkyl chains are linked via ether bonds and the spacer is exactly twice the length of the free side chains. DTHE5 exhibits a columnar discotic phase which is completely miscible with that of THE5 and is thus classified as D-A, i.e. ordered hexagonal (D-ho. X-ray diffraction measurements confirm the assignment and provide geometrical lattice parameters (a = 20.2 Angstrom; stacking distance of d = 3.48 Angstrom at room temperature) as well as correlation lengths for both (345 and 125 Angstrom, respectively). Deuterium NMR measurements of specifically deuterated DTHE5 in the mesophase region exhibit dynamic line shapes consistent with restricted, high-amplitude, planar librations of the monomeric subunits within the columns. A theory which considers the effect of this motion on the NMR line shape in terms of a diffusion equation in a restricting potential is developed and used to simulate the experimental spectra. The results of the analysis provide kinetic and model related parameters.