Densely Packed and Well-Aligned Liquid-Crystalline Scaffolds Drive Controllable Axial Crystal Strain.

While molecular crystals that convert phase transitions into macroscopic shape changes are promising actuators, rational control over crystal-length strain remains challenging. Here, we densely pack a liquid-crystalline (LC) scaffold, nIB (n = 3, 4, 5, 7), within the rigid crystalline lattice of Au-nIB. In all polymorphs, the densely packed and well-aligned nIB moieties form smectic-like layer arrangements in the crystalline lattice, wherein the mesogen layers are separated by the interdigitated alkyl chains of the molecular units. Thermally driven polymorphic transitions reconfigure the periodicity of the mesogen layers to generate programmable uniaxial deformation with a crystal-length ratio ρL of up to 1.52. Single-crystal X-ray diffraction analyses revealed a quantitative, inverse correlation between the macroscopic length ratio ρL and the mesogen-layer thickness ratio (ρm), i.e., ρL ≈ 1/ρm, confirming the establishment of a structural design rule for axial strain. Domain engineering of Au-4IB yielded reconfigurable, multistep elongation-contraction in single crystals. By coupling LC softness with crystalline order, this strategy furnishes a general, chemically and functionally tunable platform for rationally designed crystal actuators and compliant organic devices.