Fluorescence spectroscopy of semiconductor CdTe nanocrystals: preparation effect on photostability

We report on continuous-wave and time-resolved fluorescence spectroscopy studies of CdTe water-soluble nanocrystals at room temperature. For nanocrystals spread directly on the substrate we observe large variation both in fluorescence maximum energy and fluorescence lifetime. We attribute this to the influence of the surface of the nanocrystals on the stability of excitations in the nanocrystals. As the fluorescence lifetime of the nanocrystals is monitored, we find it increases with time from 6 to 18 ns and then saturates. Placing the nanocrystals in a polymer matrix remarkably improves the photostability and all the above-mentioned effects are diminished. Upon mixing the nanocrystals with gold spherical nanoparticles we observe a decrease of the fluorescence intensity due to efficient energy transfer to the nanoparticles.     

Mechanism of migration of the trimethylsilyl group during reactions of methoxy[(trimethylsilyl)ethoxy]carbene with N-phenylmaleimide and C(60).

A novel migration of the trimethylsilyl group during reaction of methoxy[(trimethylsilyl)ethoxy]carbene with N-phenylmaleimide (NPM) and with C(60), reported earlier, was examined by means of deuterium labeling of the carbene. For the NPM case it was found that the CD(2)CH(2)SiMe(3) group, initially bound to oxygen, became the CH(2)CD(2)SiMe(3) group bound to carbon in the end product. Not only had the trimethylsilylethyl group moved from oxygen to carbon, but the TMS group had also migrated 1,2 along the ethyl chain. For the C(60) case, complete scrambling of the CD(2) group was observed, strongly implying the involvement of a silacyclopropane carbocation responsible for product formation. The labeling study supports the mechanism that was tentatively advanced earlier for addition to NPM and one of the possibilities suggested for addition to C(60).     

Dynamical properties of potassium ion K+ trapped in a fullerene C60 cage: An MD simulation

We have simulated a system composed of endohedral fullerene K⁺@C₆₀ molecules. Atomically detailed MD simulations have allowed us to analyze the dynamics of a potassium ion inside a fullerene cage and the motion of K⁺@C₆₀ molecules. The mean square displacement, the translational velocity correlation functions and their Fourier transforms, and the librational frequency of a K⁺ ion inside a C₆₀ cage have been calculated for several temperatures of the studied system. The librational frequency of a potassium ion inside a fullerene cage has been estimated. The solid-fluid phase transition has been observed in the system.     

Learning from Own and Foreign Experience: Technological Adaptation by Imitating Firms

In this paper we study the adaptive behavior of firms which repeatedly have to make a production decision. In a single good market the firms use own experience as well as information gathered by observing competitors to iteratively choose a production technology out of a given set. The adaptive learning of the firms is described in a dynamic model and analyzed in a simulation framework. We show that a small but positive propensity to imitate is optimal for the firms and yields production efficiencies above 95% of the maximal value. Furthermore, we observe that in a competitive situation firms using optimal propensities to imitate outmatch pure imitators and nonimitators in production efficiency as well as in profits. This revised version was published online in August 2006 with corrections to the Cover Date.     

The geometry of proper scoring rules

A decision problem is defined in terms of an outcome space, an action space and a loss function. Starting from these simple ingredients, we can construct: Proper Scoring Rule; Entropy Function; Divergence Function; Riemannian Metric; and Unbiased Estimating Equation. From an abstract viewpoint, the loss function defines a duality between the outcome and action spaces, while the correspondence between a distribution and its Bayes act induces a self-duality. Together these determine a “decision geometry” for the family of distributions on outcome space. This allows generalisation of many standard statistical concepts and properties. In particular we define and study generalised exponential families. Several examples are analysed, including a general Bregman geometry.     

Octacyanidorhenate(V) Ion as an Efficient Linker for Hysteretic Two‐Step Iron(II) Spin Crossover Switchable by Temperature,Light, and Pressure

A two‐step hysteretic Fe^II spin crossover (SCO) effect was achieved in programmed layered Cs{[Fe(3‐CNpy)₂][Re(CN)₈]}?H₂O ( 1 ) (3‐CNpy=3‐cyanopyridine) assembly consisting of cyanido‐bridged Fe^II‐Re^V square grid sheets bonded by Cs⁺ ions. The presence of two non‐equivalent Fe^II sites and the conjunction of 2D bimetallic coordination network with non‐covalent interlayer interactions involving Cs⁺, [Re^V(CN)₈]^3? ions, and 3‐CNpy ligands, leads to the occurrence of two steps of thermal SCO with strong cooperativity giving a double thermal hysteresis loop. The resulting spin‐transition phenomenon could be tuned by an external pressure giving the room‐temperature range of SCO, as well as by visible‐light irradiation, inducing an efficient recovery of the high‐spin Fe^II state at low temperatures. We prove that octacyanidorhenate(V) ion is an outstanding metalloligand for induction of a cooperative multistep, multiswitchable Fe^II SCO effect.     

Illuminating the origins of two-photon absorption properties in fluorescent protein chromophores

We provide here a structural impact on two-photon absorption cross-section (σ^TPA) for 22 distinct fluorescent protein (FP) chromophores. By employing time-dependent density functional theory, we gain insight into two-photon absorption (TPA) process by investigating relationship between σ^TPA and one-photon electronic transition dipole moment and permanent dipole moment change (Δμ) upon transition. Our results reveal that for the S₁ excited state, σ^TPA is proportional to (Δμ)² in agreement with two-state model of TPA process. On the contrary, the TPA spectroscopy of higher excited states (S _n, n > 1) is much more complex. We do not find a main driving force of large σ^TPA that would be common for investigated chromophores. Instead, it seems that channel interference between one-photon transition dipole moment vectors is responsible for enhancement or diminishment of σ^TPA. Our in vacuo results may serve as a benchmark to investigate a role of chromophore-protein interaction in shaping TPA spectra of FPs.     

Control of the Single‐Molecule Magnet Behavior of Lanthanide‐Diarylethene Photochromic Assemblies by Irradiation with Light

Lanthanide‐based extended coordination frameworks showing photocontrolled single‐molecule magnet (SMM) behavior were prepared by combining highly anisotropic Dy^III and Ho^III ions with the carboxylato‐functionalized photochromic molecule 1,2‐bis(5‐carboxyl‐2‐methyl‐3‐thienyl)perfluorocyclopentene (H₂dae), which acts as a bridging ligand. As a result, two new compounds of the general formula [{Ln^III₂(dae)₃(DMSO)₃(MeOH)} ? 10 M eOH]_n (M=Dy for 1 a and Ho for 2 ) and two additional pseudo‐polymorphs [{Dy^III₂(dae)₃(DMSO)₃(H₂O)} ? x MeOH]_n ( 1 b ) and [{Dy^III₂(dae)₃(DMSO)₃(DMSO)} ? x MeOH]_n ( 1 c ) were obtained. All four compounds have 2D coordination‐layer topologies, in which carboxylate‐bridged Ln₂ units are linked together by dae^2? anions into grid‐like frameworks. All four compounds exhibited a strong reversible photochromic response to UV/Vis light. Moreover, both 1 a and 2 show field‐induced SMM behavior. The slow magnetic relaxation of 1 a is influenced by the photoisomerization reaction leading to the observation of the cross‐effect: photocontrolled SMM behavior.     

Dielectric relaxation in water–cholesterol mixture cluster: molecular dynamics simulation

Molecular dynamics (MD) studies of the cluster composed of cholesterol (C₂₇H₄₅OH) and water molecules are presented. We have investigated several dynamical quantities of cholesterol as a function of its concentration in the mixture cluster and the temperature. The main attention was focused on the temperature and concentration dependence of the calculated total dipole moment autocorrelation function and dielectric loss of the cluster.     

Biphasic ethylene polymerisation using 1-n-alkyl-3-methylimidazolium tetrachloroaluminate ionic liquid as a medium of the Cp2TiCl2 titanocene catalyst

A systematic analysis was performed on a series of 1-n-alkyl-3-methylimidazolium tetrachloroaluminates (where alkyl = ethyl, butyl, hexyl, and octyl), applied as a medium of the Cp₂TiCl₂ titanocene catalyst, to evaluate the influence of the physical properties of the ionic liquids on the polymerisation reaction carried out in the biphasic ionic liquid/hexane mode. Two alkylaluminium compounds, AlEtCl₂ and AlEt₂Cl, were used as activators. The influence of the activator/catalyst molar ratio on the performance of the ethylene polymerisation was determined for each ionic liquid studied. The best results were obtained using 1-n-octyl-3-methylimidazolium tetrachloroaluminate. For the titanocene catalyst immobilised in the ionic liquid, AlEtCl₂ turned out to be a better activator than AlEt₂Cl in our studies. The properties of the polyethylene product have also been presented.