Dynamics and Ordering in the Columnar Mesophases of Octa-alkyloxy Orthocylophane:  A Carbon-13 NMR Investigation

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.

Novel Luminescent Diazafluorenone Liquid Crystals

“…Therefore, the presence of an ordered rectangular columnar (Col ro ) mesophase at ambient temperature but a disordered rectangular columnar (Col rd ) mesophase at higher temperatures seems to be reasonable. The observed phase sequence of one (or two) rectangular columnar phases at lower temperatures followed by a hexagonal columnar phase at higher temperatures is in good agreement with similar phase behavior of nondiscoid − and discoid mesogens , and has been examined theoretically. − However, the lattice parameters changed slightly during the Col ro –Col rd transition (Δ a = 0.7 Å, Δ b = 0.2 Å), although a small but distinct endothermal peak was visible in the DSC curve (Figure c, Δ H = 1.3 kJ mol –1 ). Wróbel reported similar lattice parameter changes (Δ a = 0.9 Å, Δ b = 0.3 Å) for vanadyl salene complexes but could not observe the Col ro –Col rd transition peak in the DSC, while rod-like diacetylenes published by Matsumoto revealed a small endothermal DSC peak (Δ H = 0.03–0.14 kJ mol –1 ) and larger changes of the a - and b -values (Δ a = 3.05 Å, Δ b = 1.26 Å) …”

“…63 Therefore, the presence of an ordered rectangular columnar (Col ro ) mesophase at ambient temperature but a disordered rectangular columnar (Col rd ) mesophase at higher temperatures seems to be reasonable. The observed phase sequence of one (or two) rectangular columnar phases at lower temperatures followed by a hexagonal columnar phase at higher temperatures is in good agreement with similar phase behavior of nondiscoid 66−70 and discoid mesogens 71,72 and has been examined theoretically. 73−75 However, the lattice parameters changed slightly during the Col ro −Col rd transition (Δa = 0.7 Å, Δb = 0.2 Å), although a small but distinct endothermal peak was visible in the DSC curve (Figure 5c, ΔH = 1.3 kJ mol −1 ).…”

Conformational Changes at Mesophase Transitions in a Ferroelectric Liquid Crystal by Comparative DFT Computational and 13C NMR Study

“…Owing to the relatively dense molecular packing in the LC phases, the shape of the molecules determines the anisotropy of their local coordination environment. It is well-known, for instance, that molecules consisting of rigid flat cores with symmetrically distributed six or eight lateral flexible chains most likely form discotic LC phases and that, depending on the most stable conformation of the rigid aromatic core, columnar mesophases of different symmetries may be formed. − The difference in the local anisotropy of the calamitic and discotic molecules in the LC phases, along with their structural−chemical features, accounts for the fact that the conformational states of side chains of discotic molecules, for instance, in the discotic nematic phase, have a more profound effect on the order parameter S and on the temperature dependence S ( T ) as compared to that in the calamitic nematic phase . In general, the thermal conformational mobility of the chains results in a decrease in their contribution to the energy of the anisotropic intermolecular interaction and has a strong disordering effect on the molecular cores .…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…In fact, molecular models show that the sofa conformation is highly flexible, so that distorted forms could readily be obtained by right and left twists of the C 2h conformation about the molecular z axis into other conformations of C i symmetry. The second motion has been confirmed in TBC-9 and TBC-12 by Luz et al using 13 C MAS 1D and 2D-exchange spectra [309]. By analyzing the dynamic line broadening of experimental 13 C MAS spectra, rate constants were determined over wide dynamic ranges.…”

“…The TBC-9 discotogen exhibits a single columnar rectangular mesophase, D r with a phase sequence I -147.9°C -D r -109.4°C -Cr, while the TBC-12 shows a low temperature D r and a high temperature D h phase. Both 2 H NMR [308] and 13 C NMR [309] studies have shown that the conformation of the orthocylophane core in the mesophase is that of a sofa, with the molecules stacking on top of each other to form columns. In addition, on the basis of the DNMR spectra and molecular dynamics simulations [308], two modes of motions within the columns have been identified: one consists of rapid (on the NMR time scale) high-amplitude librations involving two (or more) distorted sofa forms, and the other corresponds to an axial reorientation of molecules involving a concerted pseudorotation/rotation mechanism without actually changing their stacking order.…”