60658-40-4

Pyramidic liquid crystals are mesogens derived from the cone-shaped tribenzocyclononene (TBCN) core, substituted with appropriate side chains.The pyramidic mesophases consist of two-dimensional columnar structures in which the rigid cores of the mesogen molecules are stacked in parallel on top of each other.In the present work X-ray and deuterium NMR measurements on the solid and mesomorphic phases of homologues of the alkyloxy (I-n) and alkanoyloxy series (II-n) are reported.In the PA mesophase of series I-n, the columns for a two-dimensional hexagonal array with a single molecule per unit cell.Deuterium NMR measurements on three specifically deuterated isotopomers of I-8 (including species deuterated at the unsubstituted aromatic sites, crown-ring methylenes and α-methylene of the side chain) indicate that within this mesophase two types of uncorrelated motions with very different correlation times can be distinguished: (i) fast fluctuations of the molecular C3 axis, which results in an effective orientational order of S = 0.85, essentially independent of temperature; (ii) planar reorientation about the C3 axis.The rate of the latter process is within the dynamic range of deuterium NMR, allowing its determination by line-shape analysis.Assuming this motion to correspond to planar diffusion, a diffusion constant at room temperature of DR(300 K) = 6.8E5 rad2/s with an activation energy of ΔE = 63 kJ/mol is derived from the experimental spectra.At least the first three atoms in the alkyloxy side chains do not participate in fast (on the NMR time scale) conformational isomerization in the solid or in the PA mesophase.The high-temperature mesophase of series II-n, PC, is very similar to the PA mesophase of I-n.It has an hexagonal structure with, however, four columns per unit cell.Deuterium line-shape analysis on specifically deuterated II-13 species indicates the same type of averaging motions as in the PA mesophase with an orientational order parameter of S = 0.93 (independent of temperature), and DR(373 K) = 1.4E5 rad2/s with ΔE = 112 kJ/mol.The low-temperature mesophase of this series, PD, is biaxial and appears to be rectangular or very nearly so.It is much less mobile than the PC mesophase, as evidenced by lack of planar reorientation, although there are still fast fluctuations of the molecular C3 axis (giving S = O.93).Unlike in the alkyloxy (I-n) series, in the alkanoyloxy (II-n) series there is fast side-chain isomerization in the PC and PD mesophases and to some extent even in the solid.The line shape of the α-methylene deuterons in the PD mesophase suggest a two-site jump process, while in the high-temperature PC phase, additional motions appear to set in, resulting in further narrowing of the spectrum width.Additional deuterium NMR measurements on deuterated I-8, II-11, and II-13 dissolved in nematic solvents in order to determine molecular structural parameters are reported.