Link between molecular mobility and order parameter during liquid–liquid transition of a molecular liquid

Significance Contrary to a common sense that a liquid state is unique for a pure system, there have been a number of indications for the presence of more than two liquid states. The transition between the liquid states is called “liquid–liquid transition.” There has so far been little information on how local molecular dynamics changes during the transformation. Here, we reveal the correlation between the order parameter and molecular dynamics by real-time dielectric spectroscopy measurements during the transformation from liquid I to liquid II in a molecular liquid, triphenyl phosphite. Our results suggest that the local order parameter controls local dynamics. Our finding sheds light on the nature of liquid–liquid transition from a perspective of molecular-level dynamics.