Physics of liquid crystals of bent-shaped molecules

Thermotropic liquid crystals can be formed by various molecular shapes, some discovered over 125 years ago. The simplest and most-studied liquid crystals are made of rod-shaped molecules and led to today's omnipresent liquid crystal displays. While applied scientists and engineers have been perfecting liquid crystal displays, a large group of liquid crystal scientists have become excited about liquid crystals of bent-shaped (banana-shaped) molecules. These compounds were first reported 20 years ago and since then have taken center stage in current liquid crystal science. The banana mania is due to the fact that even a small kink in the molecular shape leads to fundamentally new properties and phases. This review summarizes the large variety of novel structures and physical properties and describes the underlying physics. The dependence of macroscopic properties on both the shape of the molecules and the flexibility of the central core is emphasized. Most rigid bent-core molecules form smectic and sometimes columnar structures; only a minority forms nematic phases. By contrast, most flexible bent-core molecules form nanostructured nematic phases, including the twist-bend nematic phase discovered very recently.