Phosphorescent resonant energy transfer between iridium complexes

The mechanism for triplet energy transfer from the green-emitting fac-tris[2-(4'-tert-butylphenyl)pyridinato]iridium (Ir(tBu-ppy)3) complex to the red-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3')(acetylacetonato)iridium (Ir(btp)2(acac)) phosphor has been investigated using steady-state and time-resolved photoluminescence spectroscopy. [2,2';5,'2' ']Terthiophene (3T) was also used as triplet energy acceptor to differentiate between the two common mechanisms for energy transfer, i.e., the direct exchange of electrons (Dexter transfer) or the coupling of transition dipoles (F?rster transfer). Unlike Ir(btp)2(acac), 3T can only be active in Dexter energy transfer because it has a negligible ground state absorption to the 3(pi-pi*) state. The experiments demonstrate that in semidilute solution, the 3MLCT state of Ir(tBu-ppy)3 can transfer its triplet energy to the lower-lying 3(pi-pi*) states of both Ir(btp)2(acac) and 3T. For both acceptors, this transfer occurs via a diffusion-controlled reaction with a common rate constant (ken = 3.8 x 10(9) L mol-1 s-1). In a solid-state polymer matrix, the two acceptors, however, show entirely different behavior. The 3MLCT phosphorescence of Ir(tBu-ppy)3 is strongly quenched by Ir(btp)2(acac) but not by 3T. This reveals that under conditions where molecular diffusion is inhibited, triplet energy transfer only occurs via the F?rster mechanism, provided that the transition dipole moments involved on energy donor and acceptor are not negligible. With the use of the F?rster radius for triplet energy transfer from Ir(tBu-ppy)3 to Ir(btp)2(acac) of R0 = 3.02 nm, the experimentally observed quenching is found to agree quantitatively with a model for F?rster energy transfer that assumes a random distribution of acceptors in a rigid matrix.     

Accumulation of Four Electrons on a Terphenyl (Bis)disulfide

The activation of N₂, CO₂ or H₂O to energy-rich products relies on multi-electron transfer reactions, and consequently it seems desirable to understand the basics of light-driven accumulation of multiple redox equivalents. Most of the previously reported molecular acceptors merely allow the storage of up to two electrons. We report on a terphenyl compound including two disulfide bridges, which undergoes four-electron reduction in two separate electrochemical steps, aided by a combination of potential compression and inversion. Under visible-light irradiation using the organic super-electron donor tetrakis(dimethylamino)ethylene, a cascade of light-induced reaction steps is observed, leading to the cleavage of both disulfide bonds. Whereas one of them undergoes extrusion of sulfur to result in a thiophene, the other disulfide is converted to a dithiolate. These insights seem relevant to enhance the current fundamental understanding of photochemical energy storage.     

Generic irreducibilty of Laplace eigenspaces on certain compact Lie groups

If G is a compact Lie group endowed with a left invariant metric g, then G acts via pullback by isometries on each eigenspace of the associated Laplace operator \(\Delta _g\). We establish algebraic criteria for the existence of left invariant metrics g on G such that each eigenspace of \(\Delta _g\), regarded as the real vector space of the corresponding real eigenfunctions, is irreducible under the action of G. We prove that generic left invariant metrics on the Lie groups \(G={ SU}(2)\times \cdots \times { SU}(2)\times T\), where T is a (possibly trivial) torus, have the property just described. The same holds for quotients of such groups G by discrete central subgroups. In particular, it also holds for \({ SO}(3)\), \({ U}(2)\), \({ SO}(4)\).     

Integrability of geodesic flows and isospectrality of Riemannian manifolds

We construct a pair of compact, eight-dimensional, two-step Riemannian nilmanifolds M and M′ which are isospectral for the Laplace operator on functions and such that M has completely integrable geodesic flow in the sense of Liouville, while M′ has not. Moreover, for both manifolds we analyze the structure of the submanifolds of the unit tangent bundle given by two maximal continuous families of closed geodesics with generic velocity fields. The structure of these submanifolds turns out to reflect the above (non)integrability properties. On the other hand, their dimension is larger than that of the Lagrangian tori in M, indicating a degeneracy which might explain the fact that the wave invariants do not distinguish an integrable from a nonintegrable system here. Finally, we show that for M, the invariant eight-dimensional tori which are foliated by closed geodesics are dense in the unit tangent bundle, and that both M and M′ satisfy the so-called Clean Intersection Hypothesis. The author was partially supported by DFG Sonderforschungsbereich 647.     

Surface oxidation of basalt glass/liquid

To evaluate surface oxidation of Fe²⁺-rich multi-component silicate glass, powder and pieces of natural basalt glass are heat-treated in Ar and subsequently investigated by Mössbauer spectroscopy to monitor the increase of Fe³⁺. Glass pieces show no increase in oxidation with time or temperature, suggesting that the oxygen potential between glass and Ar is insufficient to cause volume oxidation. In contrast, glass powder oxidizes readily to a degree comparable with that of powder oxidation in air, suggesting that surface oxidation does not depend on the oxygen potential. No cation diffusion to the glass surface is detected in Ar, though it is observed upon heat treatment in air; cation diffusion is therefore unlikely to be involved in oxidation. We suggest the following mechanism for surface oxidation: (1) adsorption of water on the glass surface as OH⁻, by exposure to air and (2), a concomitant reaction, i.e., oxidation with the glass, upon heating (chemisorption). Hereby, either oxygen of air or residual oxygen in Ar would react with the hydrogen of the -OH, liberating the oxygen for oxidation of iron. Heat treatment in vacuum of 10⁻⁸ mbar does not result in any oxidation, and we assume that the adsorbed OH is exhausted from the glass surface.     

Atomic decompositions of function spaces with Muckenhoupt weights,and some relation to fractal analysis

This paper deals with atomic decompositions in spaces of type B^s_p,q (?ⁿ , w), F^s_p,q (?ⁿ , w), 0 < p < ∞, 0 < q ≤ ∞, s ∈ ?, where the weight function w belongs to some Muckenhoupt class A_r. In particular, we consider the weight function w^Γ_κ (x) = dist(x, Γ)^κ, where Γ is some d ‐set, 0 < d < n, and κ > –(n – d). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resistive switching in ceramic multiferroic Bi0.9Ca0.1FeO3

We report resistive switching effects in polycrystalline samples of the multiferroic Bi_0.9Ca_0.1FeO₃ with silver electrodes. Mössbauer spectroscopy shows that upon Ca-doping the Fe remains in a 3+ valence state, suggesting charge compensation through the creation of large amounts of oxygen vacancies. Electrical characterization shows that the oxide/metal resistance can be switched between high and low resistance states by applying voltage pulses. This process was shown to be forming free and a strong relaxation after switching was found. We rationalize our results by considering oxygen vacancies migration to and from the metal–oxide interface, resulting in variations of the Schottky potential barrier height that modulate the interface resistance.