Volume 3 Issue 1, January 2011

Using chemical intuition often allows one to predict what might transpire on throwing a batch of chemicals into a beaker, but sometimes the unexpected can occur. Bruce C. Gibb discusses how you define an 'emergent phenomenon', recognizing that it's not a simple exercise and can actually be different for each of us.

The mutual and specific recognition that can be exhibited between 'host' and 'guest' molecules occurs over very small length scales, but this phenomenon has now been demonstrated using macroscopic gels that self-assemble before your eyes.

Rational synthesis of large polycyclic molecules is on its way. A bottom-up strategy for molecular assembly to tailor-make new molecules has been applied to fabricate nanographenes.

Helical macromolecules are ubiquitous in nature, and almost always adopt a one-handed conformation. Synthetic systems, in contrast, are typically obtained in racemic right- and left-handed mixtures. A helical phenylene oligomer has now been prepared that forms a non-racemic mixture on crystallization, and on oxidation locks one conformation in.

The self-assembly of molecules into porous two-dimensional networks on surfaces has been well studied in recent years, but now the concept has been extended to three dimensions with a little help from fullerene molecules.

Transition-metal carbenes and alkylidenes are sometimes considered similar species with subtly different bonding and reactivities. Investigations into scandium and yttrium carbenes have raised questions about our understanding — and definition — of these widely used compounds.

Principles based on overlaps and interactions between bonding and antibonding orbitals are known to control chemical reactivity. This Perspective discusses how, for reactions and kinetics of bioinorganic species, particular pathways are also exchange-enhanced — that is, favoured by an increase in the number of unpaired and spin-identical electrons on a metal centre.

Larmor precession of a quantum mechanical angular momentum vector about an applied magnetic field forms the basis for NMR spectroscopy, MRI and a range of other important analytical techniques. This precessional motion has now been imaged for the first time, using velocity-map imaging in a model system of strongly polarized oxygen atoms.

Specific molecular-recognition interactions are often used to build supramolecular architectures on very small length scales — typically those of molecules or macromolecules. Now, it has been shown that a host-guest system based on cyclodextrin rings and hydrocarbon groups can be used to direct the self-assembly of objects from macroscopic gel-based building blocks.

Water is the most abundant oxygen source, and is used as such by nature in photosynthesis. Now, it has been shown that photocatalytic oxygenation of organic substrates proceeds efficiently with water as the oxygen source, a manganese(III) porphyrin oxygenation catalyst, and a ruthenium complex as a photocatalyst.

Benzene-1,3,5-tricarboxamides are known to self-assemble via intramolecular hydrogen bonding into helical columnar aggregates. Here it is shown that the introduction of a stereocentre by an isotopic substitution — replacing hydrogen for deuterium on the methylene groups next to the amide — is sufficient to direct the helicity of the formed aggregate.

Oxynitrides of transition metals are emerging materials with useful properties and improved stability over corresponding nitrides, but a full understanding of their anion ordering has been lacking. Now, a neutron and electron diffraction study of the perovskites SrNbO₂N and SrTaO₂N reveals the chemical principles for anion order and their potential influence on materials properties.

Biological rotary motors can alter their mechanical function by changing the direction of rotary motion. Now, researchers have designed a synthetic light-driven rotary motor in which the direction of rotation can be reversed on command by changing the chirality of the molecular motor through base-induced epimerization.

Large polycyclic aromatic hydrocarbons or nanographenes have huge potential for organic electronics applications, but it is challenging to synthesize them in a controlled way. Now, a surface chemical route has been used to produce tailored nanographenes with atomically precise control over the final structure.

Phenylene oligomers are shown to form left- and right-handed helices in solution, but a chiral symmetry-breaking process occurs on crystallization to give a non-racemic mixture of crystals that each contains only one enantiomer. One-electron oxidation of the oligomers suppresses the interconversion of the mirror-image helices in solution, locking-in one conformation and leading to chiral memory effects.

The self-assembly of planar molecules at interfaces can produce porous nanostructured surfaces that allow the selective trapping of guest molecules. By careful choice of both network and guest molecule it is possible to promote controlled, reversible growth perpendicular to the surface in the form of a molecular bilayer.

Ceramic preparation of spinels — materials useful for a wide range of applications — requires complicated procedures and heat treatment over long periods. Now, it is shown that rapid synthesis of nanocrystalline Co–Mn–O spinels can be achieved under ambient conditions, and the resulting nanoparticles exhibit considerable catalytic activity towards the electrochemical oxygen reduction/evolution reactions.

Chain-reactions could provide an alternative method for surface patterning. Now the chain reaction of CH₃Cl molecules on a silicon surface has been observed to create lines that are made up of alternating CH₃ groups and Cl atoms. The reactions are propagated through surface-mediated charge-transfer and have been studied using microscopy and ab initio theory.

Jonathan R. Nitschke considers how the story of phosphorus, an element that glows without fire, nicely illustrates the pursuit of scientific knowledge — including how such knowledge goes on to serve many purposes, for better or for worse.