Protonation-Triggered Supramolecular Gel from Macrocyclic Diacetylene: Gelation Behavior, Topochemical Polymerization, and Colorimetric Response.

Supramolecular gels originating from the hierarchical self-assembly of low molecular weight organic molecules is a strongly emerging field of advanced material research for the fabrication of soft functional materials. Herein, a novel supramolecular gel was fabricated through the protonation-triggered unidirectional self-assembly of pyridine-attached macrocyclic diacetylene (PyMCDA). Basic nitrogen of a pyridine ring with a strong affinity toward proton transforms the neutral PyMCDA into gelator in its protonated pyridinium salt form (PyMCDA-H+), which further evolves to nano-fibrillar networks to yield a supramolecular gel. Under the irradiation of UV light, the white color gel turned to a robust covalently cross-linked blue-phase PDA gel. Interestingly, polymeric PyMCPDA-H+ gel exhibits a naked-eye detectable reversible blue-red colorimetric response for alternating acid/base (H2SO4/NH4OH) and colorimetric sensitivity toward selected anions: CH3COO-, CN-, HCOO-, and CH3CH2COO-. It is with the hope that this work point toward the utility and versatility of macrocyclic PDAs for constructing chromogenic supramolecular gels for their possible use in sensing systems.