Pyrogallol[4]arenes in Self-Assembly

Abstract

A “macrocycle” is defined by IUPAC as “a cyclic macromolecule or a macromolecular cyclic portion of a macromolecule.” The resulting cyclic entity is a preorganized ensemble with a hydrophobic core and a hydrophilic upper/lower rim that allows for either hydrogen bonding or metal coordination. Such coordination assists in the formation of self-assembled supramolecular complexes, which are often either bilayer/cocrystals or discrete nanoassemblies, which mimic Platonic or Archimedean solids. The discrete nanoassemblies often possess cavities, which are capable of hosting a wide variety of guest species. In this article, we will focus on discussing cyclic oligomer of 1,2,3-trihydroxybenzene, commonly known as pyrogallol[4]arene. C-alkylpyrogallol[4]arene (PgCn, where n=alkyl tail length) macrocycles are synthesized by an acid condensation reaction between pyrogallol and aldehyde. The four alkyl groups radiate from the linker carbon atoms forming the lower rim of the macrocycle, while the 12 hydroxyl groups form the upper rim of the macrocycle. Pyrogallol[4]arene has been shown to complex with a variety of metals, solvents, and crown ethers to form a plethora of varied supramolecular complexes. Most commonly, pyrogallol[4]arenes have been shown to form bilayers, nanocapsules, nanotubes, supramolecular organic frameworks (SOFs), and tethered MOF-like structures. Pyrogallol[4]arenes-based bilayers are primarily hydrogen-bonded, which self-assemble through macrocycle–solvent–guest interactions. Nanocapsules and nanotubes of pyrogallol[4]arenes are either assembled through hydrogen bonding or through metal coordination. SOFs and tethered-MOF-like structures are another category of complexes that are constructed from divergent spacer ligands.