1 An Introduction to Chirality at the Nanoscale

Scientists have been fascinated by chirality, meaning rightor left-handedness, in the structure ofmatter ever since the concept first arose as a result of the discovery, in the early years of the nineteenth century, of natural optical activity in refracting media. The concept of chirality has inspired major advances in physics, chemistry and the life sciences [1, 2]. Even today, chirality continues to catalyze scientific and technological progress in many different areas, nanoscience being a prime example [3–5]. The subject of optical activity and chirality started with the observation by Arago in 1811 of colors in sunlight that had passed along the optic axis of a quartz crystal placed between crossed polarizers. Subsequent experiments by Biot established that the colors originated in the rotation of the plane of polarization of linearly polarized light (optical rotation), the rotation being different for light of different wavelengths (optical rotatory dispersion). The discovery of optical rotation in organic liquids such as turpentine indicated that optical activity could reside in individual molecules and could be observed even when the molecules were oriented randomly, unlike quartz where the optical activity is a property of the crystal structure, because molten quartz is not optically active. After his discovery of circularly polarized light in 1824, Fresnel was able to understand optical rotation in terms of different refractive indices for the coherent rightand left-circularly polarized components of equal amplitude into which a linearly polarized light beam can be resolved. This led him to suggest that optical activity may result from “a helicoidal arrangement of the molecules of the medium, which would present inverse properties according to whether these helices were dextrogyrate or laevogyrate.” This early work culminated in Pasteur’s epochmaking separation in 1848 of crystals of sodiumammoniumparatartrate, an optically inactive form of sodium ammonium tartrate, into two sets that, when dissolved in water, gave optical rotations of equalmagnitude but opposite sign. This demonstrated that paratartaric acid was a mixture, now known as a racemic mixture, of equal numbers of mirror-image molecules. Pasteur was lucky in that his racemic solution crystallized into equal amounts of crystals containing exclusively one or other of the