NMR STUDIES OF THE MESOMORPHISM, STRUCTURE AND DYNAMICS OF SOME NEW PYRAMIDIC LIQUID CRYSTALS

摘要

Pyramidic columnar mesophases lend themselves to a variety of dynamic studies by solid-state NMR methods. In the present work we emphasized the use of ~(13)C MAS and rotor synchronized 2D exchange experiments for measuring molecular reorientation within the columns in columnar structures. Several other applications of ~(13)C MAS NMR to study discotic liquid crystals can be envisioned. One relates to density modulation within columnar mesophases. The stacking of discotic molecules into columnar structure may pose packing problems because of.the incommensurability of aliphatic and aromatic moieties. Under certain conditions this may lead to density modulation within the columns. Such modulations may lead to inhomogeneous line broadening of ~(13)C MAS spectra and thus provide a tool for the investigation of the phenomenon. Related effects on the deuterium NMR of discotic mesophases were found in columnar mesophases related to derivatives of rufigallol. Another application of ~(13)C MAS NMR to dynamics in discotic mesophases relates to dipolar recoupling. In most cases the observed carbons are coupled to nearby protons and strong decoupling of the ~(13)C-~1H dipolar interaction is required to observe narrow lines. In the presence of motion, when the rate of the process is of the order of the decoupling field the effect of the latter is destroyed. This results in lines broadening, thus providing another tool for rate measurements. It is likely that the molecular reorientation rate in many discotic columnar mesophases fall in this time scale.