Hierarchical structures in fibrous assemblies self-organized from nanometre-sized monolayer clusters are elucidated. Langmuir monolayers of a short chain carboxyazobenzene derivative feature mesoscopic fibrous structures, a monolayer thick, 80 nm apparent width, and over 100 μm long when transferred onto mica from the air-water interface. The effects of pH and salts on the large-scale morphology are followed by atomic force microscopy (AFM) and those of molecular-scale orientation are probed by UV-vis spectroscopy. The large morphological changes, despite no evidence of molecular reorientation, indicate that a small monolayer cluster, rather than a single molecule, is the relevant unit making up a fiber. Furthermore, detailed statistical analyses of the images reveal that nearly all fibers actually consist of a pair of strands, each strand 29 nm wide, that are imaged by the AFM tip, having an effective apex size of 21 nm. The results suggest the following hierarchical structure. Molecules aggregate into a monolayer cluster 29 nm wide. These clusters associate one-dimensionally to form a long monolayer strand. Finally, two strands join together to result in a fiber with a true width of 58 nm.
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