Twisting Crystals By Temperature

How tightly a helical organic crystal twists can be dictated by temperature, reports a Chinese research team led by Jian Pei of Peking University and Jian Wang of South China University of Technology (Langmuir 2009, 25, 5459). Helical micro- and nanostructures have potential applications in optoelectronic devices. But precise control over the helical pitch, or how tightly a crystal is coiled, is challenging and has typically been determined by the chemical structure of the compounds involved. Pei, Wang, and colleagues built the achiral, π-conjugated, X-shaped compound shown that crystallizes into a tightly wound helix at lower temperature (15 °C) and a more loosely wound form at higher temperature (35 °C). The researchers postulate that an imbalance in the crystal growth rate between the center and the edge of the helix is the driving force for the twisting: At lower temperature, the difference between the two regions is larger, leading to tighter helical twist.