Chiral nanoscale systems: preparation, structure, properties and function

David Amabilino

David Amabilino (Welwyn Garden City, Herts, UK, 1966) is a researcher at the Materials Science Institute (CSIC) near Barcelona. He worked as a postdoc on self-assembly of interlocked molecules in Fraser Stoddart's group (then in Birmingham, UK), on porphyrin-containing assemblies in Jean-Pierre Sauvage's group (ULP, Strasbourg) and chiral polymers in Jaume Veciana's group (ICMAB, CSIC) after receiving both a BSc (hons.) and a PhD from Royal Holloway and Bedford New College (University of London). His current interests include the influence of supramolecular and stereochemical effects on molecular materials.

Pasteur’s historic separation of enantiomorphic hemihedral crystals of sodium ammonium tartrate—which led to so many important theories, findings and inventions in stereochemistry¹—is an enigmatic example of a macroscopic resolution which has been emulated in a nanoscale system. While repeating macroscopic experiments in tiny dimensions is a remarkable feat, there are also new phenomena being uncovered which over the course of time may prove significant in a wider context.²

Chiral chemical systems with nanometre scale features—the effects of chirality on structuring at the nanoscale, how chirality is recognised and transferred at the molecular and supramolecular levels, how chirality is expressed in terms of chemical interactions and physical properties—are of vital significance to society as a whole because of the socio-economic areas where chiral products are the merchandise. Chirality in general is recognised widely as an area of great scientific and commercial interest. Pharmaceutical science depends largely on chiral recognition, it is important for catalysis for the fine chemicals industry, and in liquid crystal displays. There are even suggestions that studies at this level could point to how the homochirality of biological systems originated. Furthermore, it is an area of intense interest and growth. This themed issue of Chemical Society Reviews is therefore important and timely for science and technology.

This issue contains reviews by experts who deal with everything from the theory of chiral systems, to their synthesis, self-assembly and processing; from characterisation using the most up to date techniques to their use in separation of enantiomers and their behaviour as materials.

The preparation of chiral systems has traditionally been done by exploiting diastereomeric relationships between reagents, but the passage of these conformations over tens of Ångstroms is a completely different challenge. Jonathan Clayden (DOI: 10.1039/b801639a) explains how chiral conformations can help to achieve this important objective. In the field of coordination complexes, where point chirality can be transferred from the conformations of ligands when they are bound to metal ions, the passage of chirality is explained by Jeanne Crassous (DOI: 10.1039/b806203j). These complexes are of undoubted importance in the areas of catalysis and materials in general, and therefore this review gives an important perspective on this area.

The preparation of systems which can be interconverted between different states is a very timely topic, and one which James Canary (DOI: 10.1039/b800412a) explains for the case of compounds which can be oxidised and reduced. He shows how coordination complexes and organic compounds can behave as redox-triggered chiroptical molecular switches.

The interaction of chiral molecules with metals is a potentially important area, for both catalysis and other types of materials chemistry. Cecilia Noguez and Ignacio Garzón (DOI: 10.1039/b800404h) describe the optical activity of metal nanoparticles with chiral ligands at their periphery. The article raises interesting and intriguing questions concerning the origin of the Cotton effects displayed by these systems. On flat metal surfaces, a number of techniques can be used to probe the structural chirality at the scale of nanometres, as Rasmita Raval (DOI: 10.1039/b800411k) explains. She pays special attention to the use of surface science techniques for tracking the transfer of chirality molecule by molecule.

The chirality of monolayers physisorbed at surfaces is an area where a great deal of information can be gleaned concerning the appearance, induction and transfer of chirality. Steven De Feyter and colleagues (DOI: 10.1039/b800403j) show how scanning tunnelling microscopy can give sub-molecular information on these phenomena. Of course, molecular modelling is an almost essential part of the interpretation in nanoscale phenomena, and Roberto Lazzaroni and co-workers (DOI: 10.1039/b801638k) teach us which theoretical tools can be used, and what information they can provide for the explanation of chiral nanostructures.

Teresa Sierra and colleagues (DOI: 10.1039/b800408k) describe the transfer of chirality in thermotropic and lyotropic liquid crystal systems of columnar nature, where non-covalent forces help to determine the organisation of the stacked structures. Keeping the materials theme, Albertus Schenning and colleagues (DOI: 10.1039/b800407m) describe how chirality can be passed up in scale into helical nanofibres. The self-assembly in this kind of fibrous morphology can be very nicely explored in gel-phase materials, as David K. Smith (DOI: 10.1039/b800409a) explains, and in which he answers the question: is chirality “Lost in translation”? The imaging of chiral fibres can be tremendously challenging, and Jiro Kumaki, Shin-ichiro Sakurai and Eiji Yashima (DOI: 10.1039/b718433f) show how perhaps the ultimate nanofibres—single polymer chains—can have their chiral structure probed using high-resolution atomic force microscopy.

In terms of the use of chiral systems, surfaces are potentially useful for crystallisation of optically active compounds. Yitzhak Mastai (DOI: 10.1039/b812587m) informs us of the few but exciting results in this area, as well as describing how the chirality of the systems can be probed using a new analytical technique: the polarised near-field scanning optical microscope. The separation of enantiomers is very often achieved by chromatographic methods, an area which has traditionally focused on an empirical approach towards large scales, but recently nanostructuring and miniaturisation are taking on increasing importance. Raquel Sancho and Cristina Minguillón (DOI: 10.1039/b718359n) show how separations can be achieved and improved by nanoscale design, and identify the possibilities for this area of science which is making the most of recent advances in microfluidics and the preparation of nanostructured materials.

Heterogeneous catalysis is perhaps one of the most immediate applications of chiral nanoscience, and in this context chiral complexes on nanostructured supports is particularly promising. José Fraile, José Mayoral and co-workers (DOI: 10.1039/b806643b) describe the principles and state of the art in this exciting area of research, including the immobilisation of the catalytic species and their performance.

This themed issue could not cover all aspects of chirality in nanoscale systems. Many of the articles one will stumble upon while researching chirality in nanoscale systems are related to the structure of carbon nanotubes, where the chirality is only defined on very few occasions. There is no doubt that the topology of the tubes plays a role in their properties, but the precise nature of the relationship is a gold-mine awaiting any prospector brave enough to separate these intriguing materials.

While some of the reviews introduced in this issue are treated more comprehensively in a forthcoming book,³ the present collection is a uniquely broad and instructive body of work which helps define the area and raise important questions and opportunities in it.

The entry route for me into the area of chirality at the nanoscale was greatly facilitated by the Marie Curie Research Training Network CHEXTAN (MRTN-CT-2004-512161) which originated through a COST working group (D19/004/01), and I am indebted to the European Commission for the funding of the project and to all of the people who contributed to it.

Most importantly, I am very grateful to all the authors for the effort they put into writing the excellent reviews which this issue of Chemical Society Reviews contains. The pressures of writing nowadays make it hard to dedicate time to this kind of article, but they collaborated wonderfully and the issue is one which I hope you, the readers, will enjoy and learn from.

References

1. R. G. Kostyanovsky, Mendeleev Commun., 2003, 13, 85.
2. G. Wagniere, On Chirality and the Universal Asymmetry—Reflections on Image and Mirror Image, HCA-Wiley/VCH, Zurich, 2007.
3. Chirality at the Nanoscale, Nanoparticles, Surfaces, Materials and more, ed. D. B. Amabilino, Wiley-VCH, Weinheim, 2009.

Footnote

† Part of the nanoscale chirality themed issue.

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