Excited-state processes in first-generation phenyl-cored thiophene dendrimers

First generation dendrimers with three oligothiophene arms (meta-arranged, 3G1-nS) and four arms (ortho- and para-arranged, 4G1-nS) connected to a central phenyl core were investigated spectroscopically in solution. In all dendrimers, on an ultrafast time scale (<10 ps), two "cooling" processes convert the initially generated, "hot" exciton into the geometrically relaxed, "cold" exciton. A decrease in the triplet yield, particularly evident for the 4-arm dendrimers; intersystem crossing rate; and nonradiative triplet decay time with increasing number of bridging thiophene units n all meet with expectations from prior studies on linear oligothiophenes. A relatively fast internal conversion process (>0.6 ns(-1)) is observed in both dendrimer series, possibly due to increased twisting about the phenyl core that reduces the triplet yields considerably with respect to oligothiophenes. An anomalous shifting of the triplet-triplet absorption spectra characterizes the 4G1-nS dendrimers as unique from the 3G1-nS series in terms of the hindrance of torsional motion and confinement of excited states enforced by the arrangement of dendrons.     

Microsecond charge recombination in a linear triarylamine-Ru(bpy)3(2+)-anthraquinone triad

Linear triads with ruthenium photosensitizers are frequently based on the Ru(terpyridine)(2)(2+) unit. We report on vectorial photoinduced electron transfer in a linear triad based on the Ru(bipyridine)(3)(2+) photosensitizer. Electron-hole separation over a 22 ?-distance is established with a quantum yield greater than 64% and persists for 1.3 μs in acetonitrile.     

Evaluation of methods to predict reactivity of gold nanoparticles

Several methods have appeared in the literature for predicting reactivity on metallic surfaces and on the surface of metallic nanoparticles. All of these methods have some relationship to the concept of frontier molecular orbital theory. The d-band theory of Hammer and N?rskov is perhaps the most widely used predictor of reactivity on metallic surfaces, and it has been successfully applied in many cases. Use of the Fukui function and the condensed Fukui function is well established in organic chemistry, but has not been so widely applied in predicting the reactivity of metallic nanoparticles. In this article, we will evaluate the usefulness of the condensed Fukui function in predicting the reactivity of a family of cubo-octahedral gold nanoparticles and make comparison with the d-band method.     

Poly[tetrakis(n‐propylammonium) [octa‐μ‐chlorido‐dichloridotrinickelate(II)]]: a hybrid organic–inorganic layer compound in the Cs4Mg3F10 structure type

The title compound, {(C₃H₁₀N)₄[Ni₃Cl₁₀]}_n, contains zigzag layers of tri‐μ‐chlorido‐bridged linear 2/m‐symmetric Ni₃Cl₁₂ segments, linked by mono‐μ‐chlorido quasi‐linear bridges to two other segments at each end. These inorganic layers are interleaved with interdigitated bilayers of mirror‐symmetric n‐propylammonium cations, the ammonium head groups of which are directed into the inorganic layers to form multiple N—H...Cl hydrogen bonds, while the propyl tail groups pack together in a tongue‐and‐groove manner in the center of the bilayer. The propyl groups are ordered at 100 K but disordered with opposite conformations on the mirror plane at 240 K.     

Solid-Phase Total Synthesis of Cyclosporine Analogues

Syntheses of cyclosporine analogues are reported wherein the peptide couplings were achieved in solid phase. The Wang resin was used as the solid support, and the peptide couplings commenced with the residue 11 of the cyclosporine skeleton. The couplings proceeded in a stepwide manner up to the residue MeBmt¹, using symmetric anhydrides. The peptides were then cleaved off the resin, and the cyclization was achieved in solution using Castro's reagent. The solid-phase synthesis described herein offers a very efficient method for the rapid synthesis of structurally diverse cyclosporine analogues in small quantities. The biological activities of the synthetic cyclosporine analogues were evaluated in two in vitro assays, including the IL-2 reporter gene assay and the cyclophilin binding assay. The structure-activity relationship is discussed.     

Raman scattering and fluorescence emission in a single nanoaperture: Optimizing the local intensity enhancement

We investigate the potential of a single subwavelength aperture milled in an aluminium film to enhance the local electromagnetic field. We compare the Raman scattering of unadsorbed chlorobenzene molecules and the fluorescence emission of Cyanine-5 dyes, having the same excitation and collection setup for both experiments. For the optimal nanoaperture diameter, we report a clear enhancement factor of about 5 of the Raman scattering intensity per unit volume. Since Raman scattering probes the molecular vibrational levels and avoids the resonant pumping of a real excited state, the observed Raman enhancement is disconnected from the effects of the molecular energy levels alteration previously reported for fluorescent dyes. The observations are similar for both Raman and fluorescence experiments, and stand in good agreement with numerical electromagnetic computations of the excitation intensity inside the nanoaperture.     

An infrared study of the chemistry of methyl species on Pt(111) formed by the decomposition of dimethylmercury

The chemistry of dimethyl mercury on a Pt(111) single crystal surface has been investigated by reflection-absorption infrared spectroscopy (RAIRS). Dimethyl mercury appears to be highly reactive on Pt(111) and readily decomposes on the surface at temperatures of 100 K and above. Adsorption at 100 K initially occurs in a dissociative manner to produce CH₃ and CH₃Hg species on the surface, both of which are identified as having C_3v local symmetry. At higher exposures, molecular adsorption dominates with the Hg---C---Hg axis initially oriented parallel to the surface. This preferred orientation, however, does not persist into the multilayer. Thermal treatment of the surface layer results in multilayer desorption between 130 and 135 K, and no parent molecular species are observed beyond 160 K. Adsorption at 200 and 300 K produces an overlayer consisting primarily of CH₃Hg species, which are thermally stable to about 350 K. Subsequent heating to 400 K results in the formation of ethylidyne species which are characterised by RAIRS. Adsorption at 400 K results in the direct formation of an ethylidyne layer estimated to be about 85% of saturated coverage.     

Orientation of O(3) and SU(2)⊗CI representations in cubic point groups (Oh,Td) for application to molecular spectroscopy

We propose a detailed method for the symmetrization of the standard O(3) or SU(2)⊗C_I basis |j_τ,m〉 (τ=g or u) into the O_h or T_d point group. This is realized by means of an orientation matrix called G. The oriented basis obtained in this way allows matrix element calculations for rovibronic spectroscopic problems concerning octahedral or tetrahedral molecules. Particular attention has been put on careful phase choices. A numerical calculation of all the G matrix elements for both integer and half-integer j values up to 399/2 has been performed. Such high angular momentum values are necessary for the case of heavy molecules with high rotational excitation. To calculate the G coefficients with high precision at high j values we pre-calculated the necessary Wigner functions using symbolic MAPLE software and made then the numerical calculations with quadruple precision. The complete list of these coefficients can be obtained freely at the URL: http://www.u-bourgogne.fr/LPUB/group.html. As an illustration, we also present briefly an application to two typical spectroscopic calculations: the pure rotational levels of SF₆ in its ground vibrational state and the ν₃ band of ReF₆ (an open-shell molecule with an odd number of electrons and a fourfold degenerate electronic ground state).