ChemPhysChem

The cover picture illustrates the dual fluorescence when 2-aminopurine, a fluorescent analogue of adenine, is substituted for a natural base in DNA. Excitation at 300 nm produces the well-known 370 nm emission band, but excitation at 360 nm produces a second, red-shifted emission band at 450 nm. The long-wavelength emission is attributed to conformational states of the duplex in which 2-aminopurine forms a highly overlapped, π-stacked heterodimer with an adjacent base. It is proposed by Bonnist and Jones (page 1121) that the essential difference between the short-wavelength and long-wavelength emitting conformations is the size of the helical twist between 2-aminopurine and its flanking bases.

Photons in, energy out: At the Solar′08 international conference (February 24–28, 2008), 120 scientists from 41 countries gathered to discuss the field of photochemistry, ranging from fundamental photophysical properties of systems to their applications. S. Lacombe presented work on silica xerogel monoliths (see figure) which efficiently produce singlet oxygen when irradiated under visible light.

The outstanding difference between the reactivities of saturated and unsaturated hydrocarbon radicals can be rationalized in terms of the characteristic geometry of the tight transition state and high forward and reverse activation barriers along the reaction coordinate on the potential energy surface, as shown in the schematic representation.

Efficient recoupling: A new experimental procedure using a broadband variant of rotary resonance recoupling (B²R³, see figure) improves ¹³C–¹³C transfer of magnetization in biological molecules. The robustness of the method, combined with its simplicity, results in improved structural analysis of crystalline and non-crystalline systems.

Regioselective H/D exchange at the side-chain methyl group of ethylbenzene on dealuminated zeolite H-Y and formation of carbenium ions are observed for the first time by the novel in situ pulsed-flow ¹H MAS NMR-UV/Vis technique. Hydride abstraction on Lewis acid sites lead to the formation carbenium ions which are protonated following Markovnikov's rule (see scheme).

Members only: An amphiphilic copolymer, poly(L-leucine)-grafted polyallylamine, forms a stable and regular molecular membrane having a β-sheet structure at the water–water interface under acidic conditions. The permeability of L-leucine through the molecular membrane is larger than that of D-leucine by a factor of 26 (see picture).

Gold-borne: Phthalocyanine (F₁₆CuPc) assembles into multiwalled nanotubes by vapor deposition onto SiO₂ surfaces functionalized by Au nanodots. Their length can be tuned over a large range. The picture shows the wall spacing d* of a F₁₆CuPc nanotube as deduced from the first diffraction fringe of the electron diffractogram.

No independent rotation: Detailed analysis of the PES for triptycyl[3]helicene (see figure) shows that rotation around the helicene–triptycyl bond cannot be considered independently from other degrees of freedom when analyzing its stereodynamic behavior. The possibility of enantiomerization of the helicene pawl results in more complex dynamics than previously expected.

Illuminating interactions: A fluorescent analogue of adenine illuminates interbase interactions in DNA. Long-wavelength spectra arise from the formation of a ground-state heterodimer with an adjacent π-stacked, natural base, in addition to the familiar short-wavelength spectra (see figure).

Iodine intricacy: The halogen fragments (see the picture of Br) in the photodissociation of bromobenzene and iodobenzene at 266 nm are compared. The bromine fragmentation stems from excitation of the lowest excited singlet state followed by predissociation along a repulsive triplet state. When iodine is substituted, the iodine effect stabilizes the repulsive states associated with the IC₆H₅ bond rupture.

Vibrationally hot: Bromine elimination during the photolysis of 1,2-dibromoethylene (see structure) is studied using cavity ring-down absorption spectroscopy (CRDS). The Br₂ fragment is produced in hot vibrational states. Temperature-dependent measurements indicate that the Br₂ elimination pathway proceeds via the electronic ground state by internal conversion.

Pressure-dependent populations of different species in the ground state form the basis of a kinetic model to explain the dramatic enhancement of emission intensity, due to the adopted intramolecular charge-transfer state (ICT), of dual-fluorescent dicyanovinyl-substituted aromatic compounds with increasing pressure, when dispersed in a polymer medium (see figure).

Nonlinear kinetics: Owing to the quadratic autocatalysis of protons, the cubic autocatalysis of iodide and the negative feedback of protons, the iodate/sulfite/thiosulfate reaction system displays pH waves (front, circular pulse and target patterns, • in figure) and iodine waves (▪). Transitions of different waves are obtained by adjusting the concentrations of reactants.

Linear nanorod heterostructures are prepared by growing CdTe epitaxially onto the tips of CdSe nanorods (see image). The type II band offset and the limited interface between the domains lead to interesting optical properties. The structures of the linear heterostructures and their photoluminescence properties are characterized between 5 and 285 K.

Two are better than one: The deposition of a two-component film onto a carbon electrode is achieved by electrochemical reduction of a binary mixture of two aryl diazonium salts in acetonitrile. The relative composition (see scheme) of the grafted film is subsequently determined for the first time by X-ray photoelectron spectroscopy.

Less is more: The structural and electrochemical properties of nano-CoSn (19 nm) are compared to those of micro-CoSn (319 nm). The crystalline structure (see figure) of micro-CoSn is preserved after electrochemical reaction with lithium, but nano-CoSn becomes amorphous upon cycling and forms a LiCoSn nano-alloy. Thus, nano-CoSn exhibits a higher capacity to react with lithium than micro-CoSn.

Molecular interactions in mixtures of water and amphiphilic compounds are investigated by using new model-based adjusted correlation coefficients for pairwise volume fluctuations (see graph). A surprisingly high limiting adjusted correlation coefficient is found for water in neat amino alcohols and alcohols possessing a tertiary functional group.

Local interactions in alkali co-adsorption: Short-range (3–4 Å) interactions between Na with CO and O on Ni(111) occur. The loss spectrum shows two different features as a consequence of different local [CO]:[Na] and [O]:[Na] stoichiometries (see figure). This is in contrast with recent theoretical predictions which overestimate the nonlocal alkali-induced effects.

Radical reactions: Five mechanisms leading to the deamination of cytosine (to uracil) after formation of its deprotonated radical cation (see figure) are investigated theoretically in the gas phase and in bulk aqueous solvent. The most favourable pathway involves hydrogen-atom transfer from a water molecule to the cytosine derivative as the rate-determining step.