Fig. 4.

Bulky side chain rotation triggers molecular cooperativity and martensitic transition. a Rotation angles of four side chains as a function of temperature from 100→600→100 K. The side chain conformations at 150 and 350 K are illustrated beneath, created by superimposing all conformations observed at the corresponding temperatures. The result indicates freely rotating side chains in the HT form. Vertical dashed lines indicate onset of fluctuations in the side chain rotation upon heating and cooling, which match closely with the observed and measured transition temperature. b Temperature dependence of the average correlation coefficient between rotations of neighboring ditBu side chains. c Inferred molecular pathway for forming the phase boundary line shown in Fig. 1a Type I. The center carbon distances of the tert-butyl groups between molecular pairs were calculated from the crystal structures for both the LT and HT forms. The red arrows indicate the shortest intermolecular distance in each form. Extension along the shortest intermolecular distance for both side chains on a single molecule constitutes a possible molecular pathway that forms the phase boundary line observed in Type I transition