Self-Assembly of Amphoteric Azopyridine Carboxylic Acids II: Aspect Ratio Control of Anisotropic Self-Assembled Fibers By Tuning the π–π Stacking Interaction

Abstract

Abstract Two ways to control the macroscopic morphology of fibrous organizates of amphoteric azopyridine carboxylic acids by tuning the strength of π–π stacking among the component molecules are presented. The self-organization of the azopyridine carboxylic acids (1–5) from aqueous solutions is governed not only by carboxyl/pyridyl hydrogen bondings, but also by dipole–dipole and π–π stacking interactions. The level of the latter could be tuned by the substituent structure at the phenyl ring. As a result, whereas leaflet crystals were formed from non-substituted derivative 1, 3 with a propyl substituent gave fibrous assemblages, and needle-like assemblages were obtained from 2, 4, and 5 with methyl, s-butyl, and ethoxy substituents, respectively. The propyl-substitution is likely to enhance an anisotropic growth rate of the intermolecular hydrogen bonds due to efficient suppression of π–π stacking among aromatic cores, leading to the appearance of microfibers with the highest aspect ratio. The second way to control the organization morphology is based on “supramolecular copolymerization”, which is attained by mixing 1 and 3 to modulate the strength of the π–π stacking. The aspect ratio of fibrous materials was significantly influenced by the mixing ratio.