High aspect ratio nanotubes assembled from macrocyclic iminium salts

Abstract

One-dimensional nanostructures such as carbon nanotubes and actin filaments rely on strong and directional interactions to stabilize their high aspect ratio shapes. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>10), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that, in organic solvent (tetrahydrofuran), are two orders of magnitude stronger than the neutral macrocycles, as explained by physical arguments and demonstrated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by cross-linking their pendant alkenes. As large macrocyclic chromonic liquid crystals, these iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.

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

“…[4] Previously,S hionoya and co-workers and Tanaka and coworkers synthesized macrocycles containing 1,10-phenanthroline and bis(salicylidene)-o-phenylenediamine moieties, respectively,w hich assembled into nanotubes upon binding transition metals. [10,11] One of the unique aspects of this system is that al arge excess of CF 3 CO 2 Hi nduced assembly,y et lower acid loadings catalyzed macrocycle hydrolysis.H owever, these characteristics are detrimental to accessing functionalized structures,a sw ell as processing and aligning the nanotubes. [10,11] One of the unique aspects of this system is that al arge excess of CF 3 CO 2 Hi nduced assembly,y et lower acid loadings catalyzed macrocycle hydrolysis.H owever, these characteristics are detrimental to accessing functionalized structures,a sw ell as processing and aligning the nanotubes.…”

“…[11] Touch-spinning of nanotube solutions provided nanofibers with Youngsmoduli of 1.33 GPa, which exceeds those of several synthetic polymers and actin filaments. [11] Touch-spinning of nanotube solutions provided nanofibers with Youngsmoduli of 1.33 GPa, which exceeds those of several synthetic polymers and actin filaments.…”

“…[16,17] Then anotubes reported here fulfill both of these criteria, while at the same time developing ag eneral synthetic strategy to form nanostructures with designed and dynamic emergent functions. [10,11] Both macrocycles exhibit single narrow peaks in the gel permeation chromatography (GPC) traces,aswell as peaks in the matrix assisted laser desorption ionization mass-spectrometry (MALDI-MS) spectra corresponding to the desired [3+ +3] product (See Supporting Information). [10,11] Both macrocycles exhibit single narrow peaks in the gel permeation chromatography (GPC) traces,aswell as peaks in the matrix assisted laser desorption ionization mass-spectrometry (MALDI-MS) spectra corresponding to the desired [3+ +3] product (See Supporting Information).…”

“…[11] To demonstrate the protection of the imine linkages of MC 1 upon the addition of 1-2000 equiv of CF 3 CO 2 H, MALDI-MS spectra were taken following acid neutralization, which showed recovery of the initial macrocycle,with no discernible peaks corresponding to macrocycle degradation products ( Figures 2C and S49-S52). [11] To demonstrate the protection of the imine linkages of MC 1 upon the addition of 1-2000 equiv of CF 3 CO 2 H, MALDI-MS spectra were taken following acid neutralization, which showed recovery of the initial macrocycle,with no discernible peaks corresponding to macrocycle degradation products ( Figures 2C and S49-S52).…”

“…[11] To demonstrate the protection of the imine linkages of MC 1 upon the addition of 1-2000 equiv of CF 3 CO 2 H, MALDI-MS spectra were taken following acid neutralization, which showed recovery of the initial macrocycle,with no discernible peaks corresponding to macrocycle degradation products ( Figures 2C and S49-S52). [11] (Right) [3+ +3] macrocycles, constructed from aromatic 1,3-dialdehydes, which assemblei nto nanotubesi nthe presence of as little as 0.4 equiv of CF 3 with the lack of extended structures found by AFM or GI-WAXS,demonstrates the inability of MC 2 to assemble under low acid loadings ( Figures S24 and S44). Theh ydrolysis of MC 2 under low acid loadings,a long ] macrocycles constructed from bifunctionala ryl amine and terepthalaldehyde, which assemblei nto high-aspectratio nanotubesi nthe presence of alarge excess of acid, but are unstable to lower amounts of added acid.…”