Orientational ordering of a banana-shaped solute molecule in a nematic calamitic solvent by ${}^2\mathrm{H}$-NMR spectroscopy: An indication of glasslike behavior

The Saupe ordering matrix of a banana-shaped mesogenic molecule as a solute in a common nematic calamitic solvent has been determined by ${}^2\mathrm{H}$-NMR spectroscopy as a function of temperature. The temperature dependence of the Saupe ordering matrix element associated with the principal molecular axis is consistent with a glassy behavior in the reorientational motion of this particular solute molecule. The Haller expression, appropriately modified, provides a good fit to the experimental data.

Figure 1

Molecular structure of the banana-shaped mesogenic solute molecule. The asterisk means that the deuteration at position 5 is less than 10%, and, consequently, it could not be detected. The inset shows quantities used to define the geometry of the central ring and the molecular reference frame.

Figure 2

Spectra at $25°\mathrm{C}$, acquired by ${}^2\mathrm{H}$-NMR QE sequence (a) with and (b) without ${}^1\mathrm{H}$ decoupling. a and b indicate the quadrupolar doublets due to deuterons in positions 2 and 6 of ClPbis11BB-${\mathrm{d}}_3$, respectively. Signals not indicated are due to the ${}^2\mathrm{H}$ signals of natural abundance of the E7 solvent. 100 000 scans have been acquired.

Figure 3

(a) Temperature dependence of the dipolar splittings in absolute value from the ${}^2\mathrm{H}$-NMR spectra of ClPbis11BB-${\mathrm{d}}_3$ dissolved in E7. Experimental error is $\pm 75\mathrm{Hz}$. (b)Temperature dependence of the qudrupolar splittings in absolute value from the ${}^2\mathrm{H}$-NMR spectra of ClPbis11BB-${\mathrm{d}}_3$ dissolved in E7. Full and empty symbols refer to the NMR signal a and b in Fig. 2, respectively. Experimental error is $\pm 100\mathrm{Hz}$ for the inner splittings (a in Fig. 2), and $\pm 150\mathrm{Hz}$ for the outer splittings (b in Fig. 2).

Figure 4

Temperature dependence of the orientational order parameter $S_{zz}$ as obtained from the analysis of the ${}^2\mathrm{H}$-NMR spectra. The dotted line is the fit via Eq. (3), while the solid line is the fit via Eqs. (4, 6).

Figure 5

Schematic representation of the solution formed by a banana-shaped solute and a nematic calamitic solvent. The dotted circles represent the borders of the aggregates of rodlike solvent molecule around a banana-shaped solute molecule, the latter represented by a bent line.