Self-stretching molecular monolayers with autonomous folding and mechanical adaptation.

Molecular monolayers at fluid interfaces are generally considered mechanically fragile, collapsing under lateral compression. Here we show that a rationally designed amphiphilic π-conjugated molecule instead forms a monolayer that adapts to mechanical stress. Langmuir films of 3-hydroxyphenyl-7-decyl-[1]benzothieno[3,2-b][1]benzothiophene (HP-BTBT-C10) accommodate and store compressive stress without collapse. In situ optical imaging reveals that lateral compression converts into reversible out-of-plane deformation via anisotropic molecular reorganization, producing ordered wrinkles. Electron diffractometry and high-resolution electron microscopy show localized multilayer folding while retaining local molecular packing. Intermolecular interaction calculations suggest that hydroxy substitution helps stabilize both the Langmuir monolayer on water and folded multilayer wrinkles, while preserving the strong layer-forming tendency of alkyl-substituted extended π-core molecules. Dynamic measurements based on barrier-oscillation experiments further demonstrate repeatable viscoelastic stress responses mediated by sequential folding and wrinkle reversibility, establishing mechanically adaptive molecular monolayers that bridge conventional Langmuir films and biologically inspired membranes.