Structure and Dynamics of Pyramidic Liquid Crystals by Deuterium MNR and X-Ray Diffraction

Article abstract of DOI:10.1021/ja00198a018

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 P_A 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 D_R(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 P_A mesophase.The high-temperature mesophase of series II-n, P_C, is very similar to the P_A 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 P_A mesophase with an orientational order parameter of S = 0.93 (independent of temperature), and D_R(373 K) = 1.4E5 rad²/s with ΔE = 112 kJ/mol.The low-temperature mesophase of this series, P_D, is biaxial and appears to be rectangular or very nearly so.It is much less mobile than the P_C 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 P_C and P_D mesophases and to some extent even in the solid.The line shape of the α-methylene deuterons in the P_D mesophase suggest a two-site jump process, while in the high-temperature P_C 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.