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

Abstract

Macrocycles that assemble into nanotubes exhibit emergent properties stemming from their low dimensionality, structural regularity, and distinct interior environments. Here, we report a versatile strategy to synthesize diverse nanotube structures in a single, efficient reaction by using a conserved building block bearing a pyridine ring. Imine condensation of a 2,4,6-triphenylpyridine-based diamine with various aromatic dialdehydes yields chemically distinct pentagonal [5+5], hexagonal [3+3], and diamond-shaped [2+2] macrocycles depending on the substitution pattern of the aromatic dialdehyde monomer. Atomic force microscopy and <i>in solvo </i>X-ray diffraction demonstrate that protonation of the macrocycles under the mild conditions used for their synthesis drives assembly into high-aspect ratio nanotubes. Each of the pyridine-containing nanotube assemblies exhibited measurable proton conductivity by electrochemical impedance spectroscopy, with values as high as 10<sup>-3</sup> S m<sup>-1</sup> (90% R.H., 25°C) that we attribute to differences in their internal pore sizes. This synthetic strategy represents a general method to access robust nanotube assemblies from a universal pyridine-containing monomer, which will enable systematic investigations of their emergent properties.