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Hydrogen bonds and charge–charge interactions have been widely used (either alone or in combination) in the design of efficient and selective synthetic receptors for anions. Intuitively, the interaction between anions and Π-aromatic systems is associated with a repulsive force. Consequently, for many years, anion-Π interactions have been completely neglected as favorable non-covalent interactions for the construction of efficient anion receptors. Recently, however, theoretical studies indicate the existence of an attractive interaction between anions and certain type of Π-acidic aromatic systems. These theoretical studies together with the observation of supramolecular complexes in the solid state in which anions are included in deep aromatic cavities have encouraged an in depth study of the anion-Π interaction. Nowadays, the anion-Π interaction is considered by several researchers a potential non-covalent interaction for the design of anion receptors. This chapter will provide an overview of recent theoretical investigations, performed since 2002, on anion binding involving six-membered aromatic rings. A series of experimental studies, carried out since 2004, also evidencing the existence of a possible attractive interactions betweens anions and six-membered aromatic moieties of host-guest systems in the solid state and in solution is also discussed. [ABSTRACT FROM AUTHOR]

All nature is but art unknown to thee, All chance, direction which thou canst not see; All discord, harmony not understood; All partial evil, universal good; And, spite of pride, in erring reason's spite, One truth is clear, Whatever is, is right. The bonding between atoms is governed by the structure of the atoms themselves, and because all atoms of a given element are identical, we might expect the atomic arrangement in a crystal to be regular. Crystals, and indeed solids in general, were believed to be perfect for many years after the atomic structure of matter had become generally accepted. There are now so many common phenomena whose very existence is the result of a lack of perfection that it seems surprising that the concept of crystal defects developed only recently. The idea emerged in several different guises, and there was an important precursor which, although not specifically related to crystals, played a major role in heralding the new era. The first person to delve into the realm of imperfection was Alan Griffith, in 1920, following studies of the strengths of glass rods and fibres. He found that when the diameter decreases to about 10 μm, the strength of a fibre becomes markedly higher than that of a relatively thick rod. Assuming that such thin fibres must be free of the flaws that plague thicker specimens, Griffith analyzed the energy balance at idealized cracks and its dependence on applied stress. [ABSTRACT FROM AUTHOR]