Volume 2 Issue 8, August 2010

Michelle Francl reminds us that even a rigorous scientific discipline such as chemistry has its own myths and legends — and explains why this isn't such a bad thing.

Proteins are most at home in water, although it has been known for some time that they can remain functional in non-aqueous environments. Researchers have now shown that in solvent-free melts, the oxygen-binding protein myoglobin adopts a near-native structure and retains its biological activity.

The absorption of a single photon can cause the excitation of more than one electron, but the mechanism of this 'multi-exciton generation' process is elusive. Now, calculations on pentacene show that geometrical distortions and intermediate excited states assist in producing two excited electrons from one photon.

The behaviour of molecules in confined environments is typically studied in synthetic systems by trapping them inside molecular cages or porous materials. Now, the properties of photosensitive metal–diimine complexes incorporated in the scaffold of a metal–organic framework, rather than in its cavities, have been investigated.

The concept of encoding information into a synthetic polymer through its monomer sequence promises very-high-density storage, but decoding such data is a significant challenge. Now, it has been shown that small molecular tweezers can recognize different triplet sequences in oligoimides, offering a glimpse of how to read out information from polymer codes.

Pattern mapping is a synthetic tactic that is complementary to standard retrosynthetic analysis. When combined with the concept of traceless stereochemical guidance, it leads to an efficient synthesis of the steroidal natural product (±)-aplykurodinone-1.

The active sites of enzymes have been widely used as the inspiration for the preparation of self-assembled catalysts. This Perspective describes a more recently adopted approach to catalyst assembly that makes use of the same interactions, but takes its inspiration from more traditional organometallic and organocatalytic approaches.

Freeze-drying of aqueous myoglobin–polymer surfactant nanoconjugates affords a water-free solid that melts at room temperature to produce a viscous solventless liquid protein that exhibits near-native secondary structure and reversible dioxygen binding. The results challenge the accepted role of solvent molecules in mediating protein structure and function, and offer new opportunities in protein-based nanoscience and bionanotechnology.

Direct experimental probing of conical intersections is rare but here, in studies of the photodissociation of thioanisole, a striking dependence of the non-adiabatic transition probability on photoexcitation energy has been observed, revealing the nuclear configuration of the conical intersection and its dynamic role in such transitions.

Porous coordination polymers can form materials that are both crystalline and flexible, creating regular yet dynamic channels that are promising for guest sorption. Guest selectivity is difficult to achieve, however, and typically relies on size- or shape-recognition. A framework has now been assembled that combines charge-transfer interactions and structural flexibility to only accommodate O₂ and NO.

A convergent route to stereodefined skipped polyenes, which proceeds through the direct union of vinylcyclopropanes with alkynes, is described. Overall, C?C bond formation occurs in concert with the establishment of up to three stereodefined alkenes, through a tandem stereoselective metallacycle-mediated coupling/stereospecific fragmentation sequence.

Nitric oxide, NO, has an unpaired electron and is widely used and studied in such diverse fields as biochemistry and atmospheric chemistry. Its radical nature means that singly charged species are common, but now two electrons have been added to give a radical dianion in an yttrium complex.

A fundamental mechanism for singlet fission, a process that may govern instances of multi-exciton generation, has yet to be described. Sophisticated calculations now show that singlet fission in pentacene proceeds through the conversion of a photoexcited state into a dark state of multi-exciton character that subsequently splits into two triplets.

Monomer-sequence information in synthetic copolyimides can be recognized by tweezer-type molecules binding to adjacent triplet-sequences through complementary aromatic π–π-stacking, with different tweezer molecules showing different sequence-selectivities. Single-crystal X-ray analysis of tweezer–oligomer complexes confirms the tweezer-binding selectivity, and NMR spectroscopy reveals a new mechanism (‘frameshift-reading’) for tweezer-based sequence recognition in chain-folding copolyimides.

Usable electronic devices exploiting the attractive properties of graphene will require narrow ‘nanoribbons’ of the atom-thin carbon sheets. Ribbons narrower than 5 nm are desirable for effective devices, but conventional lithography is limited to 20 nm. Now, a gas-phase chemical approach for etching graphene from the edges has produced graphene nanoribbons below 5 nm.

The anomeric effect, which influences the position of polar substituents in the chair conformation of various heterocycles, is commonly rationalized using hyperconjugation. Now, by theoretically studying molecules that display the anomeric effect, strong evidence is provided that hyperconjugation is not responsible and it is better interpreted in terms of electrostatics.

Covalent organic frameworks (COFs) organize organic subunits into predictable and precise networks with long-range order. The limited generality of methods for COF synthesis has thus far precluded the incorporation of complex building blocks into these emerging materials. Now, a new Lewis acid-catalysed protocol for boronic ester formation provides a two-dimensional COF containing stacked phthalocyanine chromophores.

Metal-containing fullerene cages are widely known, but hard to characterize because of their reactivity towards empty cages. Now the molecular and crystal structures of lithium-containing C₆₀ molecules have been determined.

The addition of a single chemical signal can trigger multiple disassembly–reassembly events in a dynamic self-assembling system that is based on the formation and exchange of both imine and metal–ligand bonds. Different metal-helicate superstructures are either created or destroyed in response to the signal as the system seeks thermodynamic equilibrium following perturbation.

Metal–organic frameworks (MOFs) are typically built by linking metal centres with organic bridges. By using metal–diimine complexes as linkers, researchers have now immobilized these photoresponsive moieties into a MOF scaffold, which enabled them to observe a different excited state from that occurring in solution, and study a photoreaction crystallographically.

Jean-Claude Bünzli sheds light on why europium — an element that is neither abundant in the Earth's crust nor involved in biological processes — has nevertheless attracted a great deal of interest from chemists.