Jumping Crystal of a Hydrogen-Bonded Organic Framework Induced by the Collective Molecular Motion of a Twisted π System.

There is a limited number of reports on mechanically responsive molecular crystals, including thermo-responsive and light-responsive crystals. Rigid ordered molecular crystals with a close-packing structure are less able to accept distortion, which hampers the development of such molecular crystals. The thermosalient effect, or "crystal jumping", refers to a thermo-responsive system that converts heat into mechanical force by thermally induced phase transition. While they have recently attracted attention as potential highly efficient molecular actuators, less than two dozens of thermosalient molecular crystals have been reported to date, and the design of such molecules as well as how they assemble to express a thermosalient effect are unknown. Herein, we demonstrate how the cooperative molecular motion of twisted π units could serve to develop a thermo-responsive jumping molecular crystal with a hydrogen-bonded organic framework (HOF) of tetra[2,3]thienylene tetracarboxylic acid (1). The cooperative change in the molecular structure triggered by the desolvation of THF in the channel of the HOF structure induced not only a change in the structure of HOF but also mechanical force. Hydrogen bonding interactions contributed significant thermal stability to maintain the HOF assembly even with a dynamic structural change.