Two ferroelectric phases of a columnar dibenzopyrene

Abstract

The ferroelectric switching of columnar 1,2,5,6,8,9,12,13-octakis-((S)-2-heptyloxy-propanoyloxy)dibenzo[e,l]pyr-2-heptyloxy-propanoyloxy)dibenzo[e,1]pyrene was studied in detail between 60°C and the clearing point 115°C. The switching angle (optical tilt angle) is ±24.5° up to 10V μm⁻¹ and ±37° at higher field strengths. The electric polarization is 60nC cm⁻² in the low field phase and 180nC cm⁻² in the high field phase. The switching rate has apparent activation energies that increase with voltage from 3 × 10⁻¹⁹ to 10⁻¹⁸ J. It varies roughly with a power of the voltage, the exponent increasing with decreasing temperature from 2 to 5. At equal voltages, switching is faster in the low field than in the high field phase. We tentatively infer the structures of the two columnar lattices from the ratio of polarizations and other data. From the switching angles, we calculate a tilt angle of 44° for the aromatic cores of the disc-like molecules stacked in the columns. Finally, we point out possible advantages of ferroelectric columnar liquid crystals over their smectic counterparts in electro-optical displays.