Light-induced contraction and extension of single macromolecules on a modified graphite surface.

Synthetic rigid-rod polymers incorporating multiple azobenzene photoswitches in the backbone were deposited from solution onto a monolayer of octadecylamine covering the basal plane of graphite. Large contractions and extensions of the single macromolecules on the surface were induced by irradiation with UV and visible light, respectively, as visualized by scanning force microscopy. Upon contraction, the single polymer chains form more compact nanostructures and also may move across the surface, resembling a crawling movement. We attribute the efficiency of these processes to the low mechanical and electronic coupling between the surface and polymers, the high density of azobenzenes in their backbones, and their rigidity, allowing for maximized photodeformations. The visualization of on-surface motions of single macromolecules directly induced by light, as reported herein, could help promote the development of optomechanical nanosystems.