Thermosalience of 1,2,4,5-Tetrachlorobenzene

Crystals undergoing a thermosalient phase transition are capable of small and cooperative rearrangement of their structural units that is converted into rapid and readily observable crystal shape transformation, ballistic events, and/or progression in space. This amplification of dynamics from molecular to macroscopic scale is attractive for fast and efficient conversion of heat into mechanical work in devices such as micromechanical actuators. By using microscopic, thermal and structural analysis here we demonstrate the thermosalient effect in single crystals of 1,2,4,5-tetrachlorobenzene (TCB), an analogue of the well-known thermosalient tetrabromo compound. It is shown that TCB crystals are reversibly thermosalient when they transition between two solid phases, labeled forms I and II, however contrary to the tetrabromo compound which is thermosalient above room temperature, the effect occurs below room temperature. Comparison of the structures of the two phases shows that, similar to other thermosalient solids, the two phases of TCB feature identical sheet motif of molecules in their structures, and the transition is governed by subtle changes in the orientation of molecules which are held together by π-π and weak Cl⋅⋅⋅Cl interactions. This structural perturbation generates strain within the crystal and the sudden release of the accumulated strain results in crystal motion.