Molecular switches are molecules that can transform between two states or more as a result of external stimuli, usually light, heat and chemical input. However, very rarely can hydrostatic pressure become a trigger.

Researchers led by Sean Xiao-An Zhang, Minjie Li, and Bo Zou, Jilin University, Changchun, China, have discovered that the crystal of a benzo[1,3]oxazine derivative undergoes reversible gradual color changes from yellow to red when pressure increases from 0 to 10 GPa. Detailed analysis reveals the dominant species in the equilibrium shifts from the yellow neutral form to a red zwitterionic form caused by a pressure-dependent tautomerization, where the relative energy levels of the two isomers are reversed.

In addition to discovering a promising pressure sensor, this study may shed light on the mechanism of tactile sensing in biological systems, where mechanical forces can break piezoprotein hydrogen-bond networks into zwitterionic residues.

• Dynamic Behavior of Molecular Switches in Crystal under Pressure and Its Reflection on Tactile Sensing,
  Yi Wang, Xiao Tan, Yu-Mo Zhang, Shaoyin Zhu, Ivan Zhang, Binhong Yu, Kai Wang, Bing Yang, Minjie Li, Bo Zou, Sean Xiao-An Zhang,
  J. Am. Chem. Soc. 2015.
  DOI: 10.1021/ja511499p