Well structured: pyrrole as building block for soft materials

Ezine

Published: Jan 5, 2011
Author: David Bradley
Channels: UV/Vis Spectroscopy

thumbnail image: Well structured: pyrrole as building block for soft materials

Stacking up pyrroles

Japanese scientist Hiromitsu Maeda of Risumeikan University and his colleagues have turned to the well-known molecular motif of the pyrrole to make a new class of structured materials. By combining planar pyrrole-containing negatively charged complexes with similarly planar, positively charged organic ions they can generate fibres and soft materials, such as supramolecular gels and liquid crystals based on these organic salts.

At first glance, the pyrrole molecule is nothing more than a simple heterocyclic aromatic organic compound, four-carbons and a nitrogen atom in an unsaturated ring. It is a colourless and volatile liquid that goes dark when exposed to air. But, the pyrrole unit was recruited by nature millions of years ago as a chemical building block for a wide range of molecules including the red pigment at the oxygen-carrying heart of haemoglobin and the solar-energy converting green chlorophyll in plants. As such, this superficially innocuous molecule is somehow chemically privileged and has courted researchers looking to exploit its eclectic properties for decades.

Now, Maeda and colleagues at Japan Synchrotron Radiation Research Institute (JASRI), in Sayo, Kyoto Institute of Technology and Tokyo Institute of Technology, have used it to construct a new range of nanostructured materials. Essentially, they have combined planar pyrrole-containing negatively charged complexes with similarly planar, but positively charged organic ions. They describe some of the fibrous, gelatinous and supramolecular liquid crystals that emerged from this work in the journal Angewandte Chemie. They describe the properties of their materials under UV light and report their fluorescence spectra as well as synchrotron X-ray data.

Common salt, not!

Commonly, we also think of salts as rather simple substances. A cation and an anion, positively and negatively charged particles locked together in crystalline solid or floating free in solution. But, salts can also exist as liquids at everyday temperatures, so-called room temperature ionic liquids are nothing more than salts with ions so bulky that their energy of crystallisation is too high for that process to occur at close to room temperature and they remain in the molten, disordered, state. There is yet another class of materials comprising ionic particles: ionic liquid crystals, which are liquids under normal conditions but can be nudged into a more ordered state by external conditions. Conversely, some materials, gels, are somewhat ordered but can be nudged into a much more disordered state by chemical or physical means. All manner of these materials have potential in a wide range of technologically applications, such as the development of ferroelectric memory devices.

The Japanese team has demonstrated that planar ions can be built up into self-organized stacks with charged components alternating. The first component in their materials is a planar complex made from a small inorganic ion and an organic receptor (a receptor-anion complex). The critical structural element of this receptor component contains two pyrroles bound into a pi-conjugated environment. The pi conjugation allows some of the electrons to move freely in an electron cloud across a large area of the molecule. The ligand surrounds the anion on three sides. The second component is a disk-shaped organic cation comprising an aromatic ring system, with its own electron cloud. The electrostatic attraction between these oppositely charged ions works synergistically with the attractive interactions between the electron clouds causing the anions and cations to stack into alternating columnar units.

Side-chain adjustments

By adjusting any additional side-groups on each of the components, the team can make the columns organize into the more complex structures of fibres, supramolecular gels, or liquid crystals. "Various commercially available or synthesized planar cations may also be fabricated into ordered structures on the basis of a charge-by-charge assembly. The creation of a library of charge-by-charge assemblies, covering various combinations of planar ionic species, would provide promising functional soft materials as oriented salts for future application," the team concludes.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.