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

Abstract

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 and mechanical strength of the resultant nanotubes. Imine‐linked macrocycles were recently found to assemble into high‐aspect ratio (>103), 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 CF3CO2H per macrocycle. Analysis by gel permeation chromatography and fluorescence spectroscopy revealed a cooperative self‐assembly mechanism. The low acid concentrations needed to induce assembly enabled nanofibers to be obtained by touch‐spinning, which exhibit higher Young's moduli (1.33 GPa) than many synthetic polymers and biological filaments. These findings represent a breakthrough in the design of inverse chromonic liquid crystals, as assembly under such mild conditions will enable the design of structurally diverse and mechanically robust nanotubes from synthetically accessible macrocycles.

Supramolecular polymerization provides non-equilibrium product distributions of imine-linked macrocycles

“…2C, D, F and G) (see ESI †). 31,33 The assembly of MC 2 into nanotubes under conditions typical for its synthesis was also observed in the presence of other acids (HCl, p-toluenesulfonic acid, and methanesulfonic acid) capable of protonating the pyridine ring ( Fig. S40 †).…”

“…[28][29][30] Using the traditional two-step approach, we have previously isolated imine-linked macrocycles derived from aromatic dialdehydes and a bifunctional aryl amine (DAPB); and studied their aptitude to undergo acid-mediated supramolecular polymerization into high-aspect ratio nanotubes. 31,32 In the case of macrocycles derived from simple aromatic dialdehydes such as terephthaldehyde and isophthalaldehyde (IDA, MC 1), high concentrations of CF 3 CO 2 H (>2000 equiv) were required to protonate the imine linkages and drive assembly, while lower acid concentrations catalyzed macrocycle hydrolysis. 31,32 However, including pyridine moieties (MC 2), which are more basic than the imine linkages allowed macrocycle assembly to occur via electrostatic attractions upon pyridinium formation, even in the presence of sub-stoichiometric acid loadings.…”

“…31,32 In the case of macrocycles derived from simple aromatic dialdehydes such as terephthaldehyde and isophthalaldehyde (IDA, MC 1), high concentrations of CF 3 CO 2 H (>2000 equiv) were required to protonate the imine linkages and drive assembly, while lower acid concentrations catalyzed macrocycle hydrolysis. 31,32 However, including pyridine moieties (MC 2), which are more basic than the imine linkages allowed macrocycle assembly to occur via electrostatic attractions upon pyridinium formation, even in the presence of sub-stoichiometric acid loadings. 31 Because of the low concentrations of CF 3 CO 2 H needed for supramolecular polymerization, we hypothesized that pyridinecontaining macrocycles would form nanotubes during their covalent synthesis.…”

“…31,32 However, including pyridine moieties (MC 2), which are more basic than the imine linkages allowed macrocycle assembly to occur via electrostatic attractions upon pyridinium formation, even in the presence of sub-stoichiometric acid loadings. 31 Because of the low concentrations of CF 3 CO 2 H needed for supramolecular polymerization, we hypothesized that pyridinecontaining macrocycles would form nanotubes during their covalent synthesis. In this way, the process of supramolecular polymerization and imine-exchange amongst pyridine containing species may interact and inuence each other.…”

Supramolecular Polymerization Provides Non-Equilibrium Product Distributions of Imine-Linked Macrocycles

“…[28][29][30] Using the traditional two-step approach, we have previously isolated imine-linked macrocycles derived from aromatic dialdehydes and a bifunctional aryl amine (DAPB); and studied their aptitude to undergo acid-mediated supramolecular polymerization into high-aspect ratio nanotubes. [31][32] In the case of macrocycles derived from simple aromatic dialdehydes such as terephthaldehyde and isophthalaldehyde (IDA, MC 1), high concentrations of CF3CO2H (>2000 equiv) were required to protonate the imine linkages and drive assembly, while lower acid concentrations catalyzed macrocycle hydrolysis. [31][32] However, including pyridine moieties (MC 2), which are significantly more basic than the imine linkages allowed macrocycle assembly to occur upon pyridinium formation, even in the presence of sub-stoichiometric acid loadings.…”