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Free keywords:
Liquid crystals; Columnar liquid crystals; Discotic liquid crystals; 13C NMR; CSA; Heteronuclear dipolar coupling; Heteronuclear dipolar decoupling; Heteronuclear dipolar recoupling; SLF; PDLF; PISEMA; Magic-angle spinning; MAS; CP; LG-CP; FSLG; FSLG-CP; R-type sequences
Abstract:
Many materials composed of disc-shaped molecules exhibit thermotropic liquid crystalline phases. Most of the mesophases formed by these molecules display an architecture where the molecules are stacked into columns, which in turn form two-dimensional arrays. These columnar phases, which have gained much attention due to the potentially important practical applications, particularly in electronics and display technology, are currently under active studies by various experimental techniques, including nuclear magnetic resonance (NMR). Carbon-13 NMR spectroscopy is an increasingly important tool in studies of columnar liquid crystals. The method is a useful complement to the traditional 2H NMR approach, which requires isotopic labeling and may be difficult to realize due to problems with spectral assignment and overlap. By employing advanced 13C NMR methods developed during the last decade, it is now possible to extract very detailed information on molecular structure, order and dynamics from multi-dimensional 13C NMR experiments in oriented as well as in unoriented mesophases under both static and magic-angle spinning (MAS) conditions. In this chapter, we demonstrate applications of recently developed NMR methods to columnar liquid crystals. Several techniques are considered; in particular we describe experimental methods for signal enhancement, spectral assignment, determination of the chemical shift anisotropies, and measurement of dipolar couplings. The efficiency and accuracy of the various approaches are also discussed.
