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Abstract:
Octa-alkyloxy tetrabenzo-orthocyclophanes (TBC-n where n is the number of carbons in the side chains) exhibit columnar mesophases of various symmetries, depending on the length of the side chains. The conformation of the orthocylophane core in these mesophases is that of a sofa, with the molecules stacking on top of each other to form columns. Within these columns, the molecules undergo reorientation via a combined pseudorotation/rotation mechanism. In the present work, carbon-13 NMR is used to study the mechanism and the kinetic parameters of this process, as well as the ordering characteristics in two homologues of the series, viz. TBC-9, which exhibits a single rectangular mesophase (Dr), and TBC-12, which is polymorphic with two mesophases, a low-temperature Dr and a high-temperature hexagonal columnar phase, Dh. Using 1D and 2D dynamic carbon-13 MAS NMR, the pseudorotation/rotation mechanism suggested earlier by Kuebler et al. on the basis of deuterium NMR studies (Liq. Cryst. 1995, 18, 300) is confirmed and kinetic parameters for the process are derived. Where comparable, the results are consistent with the earlier measurements. The results also indicate the presence of fast high-amplitude librations of the sofa conformation within the columns. The NMR lines of the MAS spectra within the mesophases are inhomogeneously broadened due to the distribution of isotropic chemical shifts. The latter probably reflects packing heterogeneity along the columns. Dynamic measurements of the sofa-sofa pseudorotation of TBC-9 in chloroform solutions are also reported.
