Investigating room temperature ferroelectric nematogens and their structure-property relationships

Abstract

The recent discovery of proper ferroelectricity in low molar mass liquid crystals has made the quest for identifying materials which exhibit the phase transition at room temperatures of upmost importance. Here, the synthesis and characterization of twelve different series of ferroelectric nematogens are reported based around the RM734 template structure. These compounds are varied by changing the terminal group; the position of the lateral alkyloxy chain along the mesogenic core; the number of lateral alkyloxy groups present; the degree of fluorination; and the length of the lateral alkyloxy chain. Such systematic modifications allow for a detailed analysis of the structure-property relationships for the ferroelectric nematic phase. All 70 of the compounds reported exhibit the ferroelectric nematic phase, with most exhibiting direct NF-I transitions. Despite these transitions in general being monotropic, the vast majority of these compounds can be supercooled to room temperature with many showing a high degree of stability. Most notably, however, 19 of the reported compounds exhibit a transition to the NF phase below 30 °C. Only a single pure compound to date has exhibited a transition to the NF phase in this temperature range, UUQU-4N, so this is a dramatic increase in the library of compounds which exhibit the NF transition around room temperature. By understanding the molecular structure features leading to lowering these I-NF transition temperatures we move closer to an enantiotropic room temperature ferroelectric nematogen suitable for devices. Together the phase stability and the viscosity of ferroelectric materials, which governs polarization switching dynamics, are tuned by molecular design, allowing control over the balance between switching speed and long-term retention of the polarized state.