Cooperative Self-Assembly of Pyridine-2,6-Diimine-Linked Macrocycles into Mechanically Robust Nanotubes

Abstract

<p>Nanotubes assembled from macrocyclic precursors offer a unique combination of low dimensionality, structural rigidity, and distinct interior and exterior microenvironments. Usually the weak stacking energies of macrocycles limit the length or strength of the resultant nanotubes. Imine-linked macrocycles were recently found to assemble into high-aspect ratio (>10<sup>3</sup>), lyotropic nanotubes in the presence of excess acid. Yet these harsh conditions are incompatible with many functional groups and processing methods, and lower acid loadings instead catalyze macrocycle degradation. Here we report pyridine-2,6-diimine-linked macrocycles that assemble into high-aspect ratio nanotubes in the presence of less than 1 equiv of CF<sub>3</sub>CO<sub>2</sub>H per macrocycle. Analysis by gel permeation chromatography and fluorescence spectroscopy revealed a cooperative self-assembly mechanism. Nanofibers obtained by touch-spinning the pyridinium-based nanotubes exhibit Young’s moduli of 1.48 GPa, which exceeds that of many synthetic polymers and biological filaments. These findings will enable the design of structurally diverse nanotubes from synthetically accessible macrocycles. </p>

Diverse Proton-Conducting Nanotubes via a Tandem Macrocyclization and Assembly Strategy

“…Unassembled macrocycles do not produce observable diffraction. 32 However, when macrocycles assemble into nanotubes, the emergence of a strong diffraction signal, with a characteristic peak at low values of q related to the macrocycle size (between 0.1 and 0.2 Å -1 ) [31][32][33] is observed.…”

“…[27][28][29][30] We recently found that the protonation of imine-linked macrocycles triggers strong electrostatic and solvophobic interactions that drive the formation of high-aspect ratio nanotubes. [31][32][33] However, macrocycle assembly that relied exclusively on imine protonation only assembled in the presence of excess acid, whereas lower acid concentrations accelerated macrocycle hydrolysis. [31][32] We subsequently identified a single macrocycle based on pyridine-2,6-dicarboxaldehyde that assembled using sub-stoichiometric quantities of acid, giving rise to robust assemblies that formed fibers with mechanical properties comparable to covalent linear polymers (Figure 1).…”

“…[31][32][33] However, macrocycle assembly that relied exclusively on imine protonation only assembled in the presence of excess acid, whereas lower acid concentrations accelerated macrocycle hydrolysis. [31][32] We subsequently identified a single macrocycle based on pyridine-2,6-dicarboxaldehyde that assembled using sub-stoichiometric quantities of acid, giving rise to robust assemblies that formed fibers with mechanical properties comparable to covalent linear polymers (Figure 1). [32][33] Here, we dramatically expand the modularity of this design by including the pyridine heterocycle and hydrophobic solubilizing groups in a single monomer, thereby enabling the efficient incorporation of many dialdehydes into macrocycles that assemble into high-aspect ratio nanotubes under mild conditions ( Figure 1).…”

“…[31][32] We subsequently identified a single macrocycle based on pyridine-2,6-dicarboxaldehyde that assembled using sub-stoichiometric quantities of acid, giving rise to robust assemblies that formed fibers with mechanical properties comparable to covalent linear polymers (Figure 1). [32][33] Here, we dramatically expand the modularity of this design by including the pyridine heterocycle and hydrophobic solubilizing groups in a single monomer, thereby enabling the efficient incorporation of many dialdehydes into macrocycles that assemble into high-aspect ratio nanotubes under mild conditions ( Figure 1). Through this approach, ten distinct macrocycles and nanotubes with a range of structural features including different shapes, channel sizes, and chemical functionalities were realized.…”

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR

“…In addition to 7% of carbon in trapped, partially mobile furan, 18% of alkene C was detected. Two-dimensional (2D) 13 C- 13 C and 1 H- 13 C NMR identified 12% C in asymmetric O-CH=CH-CH-CH-and 24% in symmetric O-CH-CH=CH-CH-rings. While the former represented defects or chain ends, some of the latter appeared to form repeating thread segments.…”