ChemPhysChem

From houses, to vehicles, and to mobile telephones: the fuel cell may replace the energy requirements of heating, internal combustion engines, and chemical batteries. This Review covers the basic physicochemical principles on which a fuel cell operates. The specific requirements from the materials to the systems in various fueld cells are discussed and concludes with actual applications. The image shows the main components of a Siemens alkaline fuel cell stack, namely the bipolar plates, the gas-permeable backings, and the electrode–electrolyte assembly.

A guanine radical cation G induces electron transfer from a distal guanine G_b through DNA over long adenine:thymine (A:T)_n bridges according to Equation (1). For n≤3, only guanine bases are charge carriers and the electron transfer rate k_CT depends strongly upon the distance. For n≥4, not only guanines but also adenines are charge carriers and the charge transfer rate from G_a to G_b only slightly decreases with a further increase in n. This effect is shown in the graph. The yields of the water-trapped products are described by the Curtin–Hammett principle.

Partitioning at the intersection between the product- and the ground state potential energy surfaces is the reason why photoinduced dissociative electron transfers are not necessarily endowed with a unity quantum yield. The photoinduced reductive cleavage of CCl₄ provides a clear illustration of this effect, in accordance with a recent theoretical prediction. The picture shows the potential energy curves involved in the photoinduced electron transfer from a donor D to CCl₄.

Using a derivative of the blue-emitting polyfluorene (see the graph for the output spectrum and the inset picture for the color), efficient devices exhibiting a power efficiency close to 2.7 cd A⁻¹ at 100 cd m⁻² light output were realized.

Intracrystalline diffusion kinetics of two cationic dyes in an aqueous medium were studied by means of energy transfer from an electronically excited donor to an acceptor located in one-dimensional crystalline channels. Different stages of the diffusion processes were visualized by using fluorescence microscopy. The picture illustrates the counter diffusion of the dye (hatched block) and water molecules (circles) in such a one-dimensional channel; the diffusion of the water molecules past the dye (1→2) is the rate-determining step rather than the period between encountering different dyes (2→3).

Spin-state selective coherence transfer between single-transition coherences in a scalar-coupled two-spin system may be achieved by employing highly selective, constant amplitude radio frequency fields. We extend this method by employing amplitude-modulated pulses to achieve an adiabatic transfer, as shown in the picture (I=intensity). Different limiting cases are considered. The imperfect reversibility of the transfer reveals deviations from adiabaticity.

A chiral host, cryptophane-A (1), makes even a monoatomic noble gas chiral. The interaction of xenon and 1 was monitored by ¹²⁹Xe NMR and in the presence of a chiral chemical shift reagent.

A fourth generation POPAM dendrimer functionalized at the periphery with 32 dansyl groups (circles in the picture) can host up to six eosin dye molecules (black ovals) in two different sites (triangle, square). Efficient energy transfer processes from the dansyl to eosin molecules occur. The picture shows some characteristic wavelengths and lifetimes of these transitions; excitation, emission, and energy transfer processes are represented by dotted, dashed, and full arrows, respectively.