Self-assembly of amphoteric azopyridine carboxylic acids: Organized structures and macroscopic organized morphology influenced by heat, pH change, and light

In this paper, we describe the synthesis of novel amphoteric azopyridine carboxylic acids (2b, 3a, 3b, 4a, and 4b), having both a carboxyl group as a hydrogen donor and a pyridyl group as an acceptor at each molecular terminus, and their self-organization, which is markedly affected by external stimuli including heat, pH changes, and light. The amphoteric compounds form intermolecular hydrogen bonds between pyridyl and carboxyl groups in a head-to-tail manner in the solid state to give linear pseudopolymer structures, as supported by FT-IR analysis. Heating and cooling across their melting points induced thermoreversible supramolecular depolymerization to and polymerization from small molecular components of monomers and the corresponding carboxylic acid dimers. In alkaline aqueous media, these amphoteric compounds, dissolved as carboxylate anions, were gradually neutralized by atmospheric carbon dioxide, leading to their deposition as novel fibrous materials from 3b and 4b, substituted with a propyl group at the phenyl ring, and as leaflet crystals from 3a and 4a bearing no substituent. FT-IR and X-ray diffraction measurements supported the conclusion that the formation of fibrous materials from 3b and 4b arises from their intermolecular hydrogen bonding in a head-to-tail manner as well as the suppressive effect of propyl substitution on the pi-pi stacking of the molecules. UV irradiation of alkaline solutions of 4b resulted in the modification of the morphology of fibrous materials, probably because the photoisomerized Z-isomer of 4b affected the nucleation process in the fibrous formation. These results suggest that morphological properties of these macroscopic self-assemblages are tunable by appropriate choices of environmental stimuli such as heat, pH, and light.