Self-assembly of water-filled molecular saddles to generate diverse morphologies and high proton conductivity

The design of single-component organic compounds acting as efficient solid-state proton conduction (SSPC) materials has been gaining significant traction in recent times. Molecular design and controlled self-assembly are critical components in achieving highly efficient SSPC. In this work, we report the design, synthesis, and self-assembly of an organic macrocyclic aza-crown-type compound, P2Mac, which complements synthetic ease with efficient SSPC. P2Mac is derived from the pyridine-2,6-dicarboxamide (PDC) framework and contains polar amide and amine residues in its inner region, while aromatic residues occupy the periphery of the macrocycle. The crystal structure analysis revealed that P2Mac adopts a saddle-shaped geometry. Each P2Mac molecule interacts with one water molecule that is present in its central polar cavity, stabilized by a network of five hydrogen bonds. We could self-assemble P2Mac in a variety of unique, aesthetically pleasing morphologies such as micron-sized octahedra, hexapods, as well as hollow nanoparticles, and microrods. The water-filled polar channels formed through the stacking of P2Mac allow attaining a high proton conductivity value of 21.1 mS cm^-1 at 27 °C under a relative humidity (RH) of 95% in the single crystals of P2Mac, while the as-prepared P2Mac pellet sample exhibited about three-orders of magnitude lower conduction under these conditions. The low activation energy of 0.39 eV, calculated from the Arrhenius plot, indicates the presence of the Grotthus proton hopping mechanism in the transport process. This report highlights the pivotal role of molecular design and self-assembly in creating high-performance SSPC organic materials.