Mesomorphism in columnar phases studied by solid-state nuclear magnetic resonance

In this paper, we present ${}^{13}\mathrm{C}$ and ${}^1\mathrm{H}$ NMR investigations of 2, 3, 6, 7, 10, 11-hexahexyl-thiotriphenylene (HHTT). The measurements were carried out under both static and magic-angle spinning conditions. The phase diagram of HHTT is $K\leftrightarrow H\leftrightarrow D_{hd}\leftrightarrow I$, where $H$ is a helical phase and $D_{hd}$ is a columnar liquid crystal. The motivation was to characterize the molecular order and dynamics and to investigate differences at the molecular level between the two mesophases: $H$ and $D_{hd}$. It is shown that $D_{hd}$ is a conventional columnar liquid crystal, where the molecular core undergoes fast rotation about the symmetry axis. The orientational order in this mesophase is lower and the temperature dependence of the order parameter is steeper than in other triphenylene-based compounds. On the other hand, in the helical phase the core, similarly to the solid phase, is essentially rigid. The difference between the solid and helical phases is mainly manifested in an increased mobility of the aliphatic chains observed in the latter phase. In addition, the sample exhibits thermal history effects, which are observed in the different behavior upon cooling and heating.

Figure 1

Schematic of 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT).

Figure 2

Differential scanning calorimetry (DSC) thermograms of HHTT.

Figure 3

Carbon-13 NMR spectra of the static HHTT sample as a function of temperature. The NOE and CP enhancement techniques were employed for recording the spectra in the isotropic and other phases, respectively.

Figure 4

Temperature dependence of ${}^1\mathrm{H}$ NMR spectra of the static HHTT sample.

Figure 5

${}^1\mathrm{H}\text{−}{}^{13}\mathrm{C}$ 2D PDLF spectrum in an oriented sample of HHTT at $90°\mathrm{C}$. The cross sections in the dipolar dimension are shown to the right.

Figure 6

Temperature dependence of the ${}^1\mathrm{H}\text{−}{}^{13}\mathrm{C}$ dipolar splittings in the HHTT columnar phase.

Figure 7

Carbon-13 MAS NMR spectra obtained at a spinning speed of $8\mathrm{kHz}$. Spectra in the left column are recorded on heating. The spectrum obtained in the helical phase by cooling from the columnar liquid crystal is shown to the right. Further cooling to the solid phase resulted in the spectra coinciding with the solid-state spectra in the left column. Note that the signals at $\sim 200$ and $\sim 45\mathrm{ppm}$ in the solid and helical phases are the side bands.

Figure 8

${}^{13}\mathrm{C}$ MAS NMR spectra recorded at a spinning frequency of $4\mathrm{kHz}$. Asterisks indicate sidebands.

Figure 9

${}^1\mathrm{H}$ MAS NMR spectra obtained at 8 (left) and $4\mathrm{kHz}$ (right) spinning frequency.

Figure 10

${}^{13}\mathrm{C}\text{−}{}^1\mathrm{H}$ FSLG-CP dipolar spectra obtained in the solid phase (first column), helical phases (second and third column), and columnar phase (last column). Spinning frequency is $8\mathrm{kHz}$. The assignment of the aliphatic signals in the solid and helical phases is tentative, based on the strength of the dipolar splittings. The aliphatic signals are shown in the order of the segment number along the chains.