4-Methoxyanilinium Perrhenate 18-Crown-6: A New Ferroelectric with Order Originating in Swinglike Motion Slowing Down

A supramolecular adduct 4-methoxyanilinium perrhenate 18-crown-6 was synthesized, which undergoes a disorder-order structural phase transition at about 153 K ($T_c$) due to slowing down of a pendulumlike motion of the 4-methoxyanilinium group upon cooling. Ferroelectric hysteresis loop measurements give a spontaneous polarization of $1.2\mu \mathrm{C}/{\mathrm{cm}}^2$. Temperature-dependent solid-state nuclear magnetic resonance measurements reveal three kinds of molecular motions existing in the compound: pendulumlike swing of 4-methoxyanilinium cation, rotation of 18-crown-6 ring, and rotation of the methoxyl group. When the temperature decreases, the first two motions are frozen at about 153 K and the methoxyl group becomes rigid at around 126 K. The slowing down or freezing of pendulumlike motion of the cation triggered by temperature decreasing corresponds to the centrosymmetric-to-noncentrosymmetric arrangement of the compound, resulting in the formation of ferroelectricity.

Figure 1

Structural unit of MOAP in (a) the high-temperature phase (293 K, paraelectric phase) showing totally disordered 4-methoxyanilinium cation, and (b) the low-temperature phase (93 K, ferroelectric phase) showing ordered 4-methoxyanilinium cation. (c) shows the transformation of the point group of MOAP from high-temperature paraelectric phase ($Pnma$ with symmetric elements: $E$, $C_2$, $C_2^{\prime }$, $C_2^{\prime \prime }$, $i$, ${\sigma }_h$, ${\sigma }_v$, ${\sigma }_v^{\prime }$) to low-temperature ferroelectric phase ($Pna{2}_1$ with symmetric elements: $E$, $C_2$, ${\sigma }_v$, ${\sigma }_v^{\prime }$). (d) Dielectric hysteresis loops of MOAP recorded along the $c$ axis measured at frequency of 50 Hz and different temperatures.

Figure 2

(a) Temperature dependence of the second-order nonlinear optical coefficient. The inset shows the wavelength dependence of SHG intensity at different temperatures. (b) Temperature dependence of heat capacity (Cp) and DSC obtained on a heating-cooling cycle. (c) Temperature dependence of dielectric constant at different frequencies. The inset shows the Curie-Weiss law fitting of the dielectric constant at a frequency of 100 Hz in the interval of phase transition.

Figure 3

(a) ${}^{13}\mathrm{C}$ MAS NMR spectra of MOAP acquired at different temperatures. (b) ${}^{13}\mathrm{C}\mathrm{\text{−}}{}^1\mathrm{H}$ dipolar couplings of methoxy C19 measured at different temperatures. The dipolar dephasing curves are extracted from the $t_1$ dimension of 2D DIPSHIFT spectra. The true coupling strengths, which were calculated from the simulated values and scaling factor, are indicated. (c) The temperature dependence of dipolar coupling for these nonquaternary carbon atoms.

Figure 4

Potential energy calculations for (a) pendulumlike motion of the cation, (b) the phenyl group rotation, and (c) rotation of the methoxyl group, where the solid lines are fitting lines to the data. The insets show the initial structure for the calculation: (a) the cation moves on the benzene ring plane by fixing the $\mathrm{NH}_3{}^{+}$ group in the crown ether ring center; (b) the rotation along C-N axis with fixed $\mathrm{NH}_3{}^{+}$ moiety and methoxyl group; (c) the rotation along C-O axis with fixed $\mathrm{NH}_3{}^{+}$ and phenyl groups.