Thermally driven polymorphic transition prompting a naked-eye-detectable bending and straightening motion of single crystals

The amplification of molecular motions so that they can be detected by the naked eye (10⁷-fold amplification from the ångström to the millimeter scale) is a challenging issue in the development of mechanical molecular devices. In this context, the perfectly ordered molecular alignment of the crystalline phase has advantages, as demonstrated by the macroscale mechanical motions of single crystals upon the photochemical transformation of molecules. In the course of our studies on thermoresponsive amphiphiles containing tetra(ethylene glycol) (TEG) moieties, we serendipitously found that thermal conformational changes of TEG units trigger a single-crystal-to-single- crystal polymorphic phase transition. The single crystal of the amphiphile undergoes bending and straightening motion during both heating and cooling processes at the phase-transition temperatures. Thus, the thermally triggered conformational change of PEG units may have the advantage of inducing mechanical motion in bulk materials.