DYNAMICS AND STRUCTURE OF A FLEXIBLE COLUMNAR LIQUID-CRYSTAL BASED ONTETRABENZOCYCLODODECATETRAENE

The dynamical and conformational behaviour of a flexible tetrabenzocyc lododecatetraene derivative exhibiting a columnar mesophase has been s tudied by a combination of deuteron solid state NMR spectroscopy and m olecular dynamics (MD) simulations. As shown by two-dimensional (2D) e xchange NMR, the mesophase is characterized by slow axial reorientatio ns (similar to 10(-3) s) of single molecular units where the phenylene rings exhibit a well-defined quasi-fourfold potential, while the 2D s pectra of the core methylene sites are sensitive to the molecular conf ormation and reorientation mechanism. Motional narrowing of one-dimens ional (1D) spectra reveals additional fast librations due to the inter nal flexibility of the mesogenic moiety. The various reorientation pat hways comprising interconversions and pseudo-rotations between differe nt energetically stable conformations are elucidated on a microscopic level by molecular dynamics simulations. The mesophase dynamics is asc ribed to a complex axial motion involving rotational jumps combined wi th a pseudo-rotation between two symmetry related sofa forms. This is confirmed quantitatively by comparing the experimental 2D NMR spectra of the core methylene sites and the simulations which are based on the molecular geometries obtained by MD simulations. The lineshapes of on e- and two-dimensional spectra of magnetically aligned samples specifi c to the orientation behaviour of the sofa conformer are discussed.