Stimulus-responsive Pyridine-containing Polydiacetylene Nanotubes

Abstract

Polydiacetylene (PDA), a family of conjugated polymers, is essentially an ordered supramolecular structure and can be produced by integrating selfassembled diacetylene monomers using UV or gamma-ray induced polymerization. PDAs are typically chromophoric due to the extended ᴨ-conjugated network and absorb visible light. PDA has a unique color transition property from blue (lmax: ~640 nm) to red (lmax: ~550 nm) accompanied by the generation of fluorescence. These optical properties were induced when their p-conjugated backbone is distorted due to various external stimuli such as heat, solvent, pH, and mechanical stress. Conventional tubular structures often lack stability as non-covalent molecular interactions are responsible for morphological integrity. In this dissertation research, we report the development of covalently linked chromogenic organic nanotubes prepared using topochemical polymerization of three different types of macrocyclic diacetylenes (MCDAs) via self-assembly approach. Firstly, we designed PyMCDA with bis-aminopyridine to construct the supramolecular structure of the cationdirected self-assembly process. Owing to the metal coordination affinity of bisaminopyridine and the π-π stacking characteristic of diacetylene template and complementary to the cyclic molecular framework, Cs + -directed organic nanotubes are generated via unidirectional self-assembly of PyMCDA. The monomeric PyMCDA nanotubes are transformed into the covalently cross-linked chromogenic polydiacetylene nanotubes (PyMCPDA-Cs +) by UV irradiation. Interestingly,PyMCDA-Cs+ nanotubes exhibit thermochromic property with a brilliant blue-tored color transition. Secondly, a new macrocycle, PMCDA, was constructed from a more flexible polymerizable diacetylene template and pyridine rings. Owing to the proton receptor nature of pyridine ring and π−π stacking characteristic of DA template, protonation–induced tubular structures are generated through the columnar assembly of PMCDA-H + . Upon UV irradiation, the monomeric PMCDAH + are transformed into the robust covalently cross-linked blue-phase polydiacetylene (PMCPDA-H + ). Very interestingly, the blue-phase PMCPDA-H + displays multi-stimuli-responsive colorimetric sensory responses: reversible thermochromism, selective solvatochromism for DMSO and DMF, and organic/inorganic base sensing. Lastly, a novel supramolecular gel was fabricated through the protonation-triggered unidirectional self-assembly of pyridine attached macrocyclic diacetylene (PyMCDA). Basic nitrogen of pyridine ring with a strong affinity towards proton transforms the neutral PyMCDA into gelator in its protonated pyridinium salt form (PyMCDA-H + ), which further evolve to nano fibrillar networks to yield a supramolecular gel. Under the irradiation of UV light, the white color gel turned to robust covalently cross-linked blue-phase PDA gel (PyMCPDA-H+). Significantly, polymeric PyMCPDA-H+gel exhibits a naked-eye detectable reversible blue-red colorimetric response for alternating acid/base(H2SO4/NH4OH) and selective sensitivity to CH3COO–and CN–ions.