Electron transfer across modular oligo-p-phenylene bridges in Ru(bpy)2(bpy-ph(n)-DQ)4+ (n = 1-5) dyads. Unusual effects of bridge elongation

A series of dyads of general formula Ru(bpy)(2)(bpy-ph(n)-DQ)(4+) (n = 1-5), based on a Ru(II) polypyridine unit as photoexcitable donor, a set of oligo-p-phenylene bridges with 1-5 modular units, and a cyclo-diquaternarized 2,2'-bipyridine (DQ(2+)) as electron acceptor unit, have been synthesized. Their spectroscopic and photophysical properties have been investigated in CH(3)CN and CH(2)Cl(2) by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. The experimental study has also been complemented with a computational investigation carried out on the whole series of dyads. The absorption spectra of the dyads show new spectroscopic transitions, in addition to those characteristic of the donor, bridge, and acceptor fragments. DFT calculations suggest the assignment of such bands as bridge-to-acceptor (π ph(n)) → (π* DQ) charge-transfer transitions. This assignment is consistent with the solvatochromic and spectroelectrochemical behavior of the new bands. For all the dyads at room temperature in fluid solution, the typical (3)MLCT luminescence of the Ru(II) polypyridine unit is strongly (>90%) quenched, supporting the occurrence of an efficient intramolecular photoinduced electron transfer. The study has revealed, however, that the photophysical mechanism is actually more complex than presumed on the basis of a simple photoinduced electron-transfer scheme. For n = 1, very fast (few picoseconds) photoinduced electron transfer from the MLCT state localized on the substituted bpy ligand to the DQ unit has been observed, followed by slower interligand hopping and charge recombination. For n = 2-5, MLCT excited-state quenching takes place without transient detection of charge-separated product, indicating that charge recombination is faster than charge separation. This behavior can be rationalized in terms of the superexchange couplings expected through this type of bridges for the two processes. The kinetics of MLCT quenching in the dyads with n = 1-5 does not follow the usual exponential falloff with bridge length: after a regular decrease for n = 1-3, the rate constants become almost insensitive to bridge length for n = 3-5. The rationale of this uncommon behavior, as suggested by DFT calculations, lies in a switch in the MLCT quenching mechanism with increasing bridge length, from oxidative quenching by the DQ acceptor to reductive quenching by the bridge.     

On the solvation of ions in small water droplets

The solvations of positively and negatively charged model ions in water droplets have been studied using Monte Carlo simulations performed with a polarizable intermolecular potential function model. Special focus has been placed on the position of the ion in the water droplet. It was found that the sign of the ionic charge is of minor importance but an increased ionic charge localizes the ion to the central regions of the droplet, whereas a large polarizability and a large ionic radius favor locations close to the surface of the water droplet.     

Enhanced automated structure elucidation by inclusion of two‐bond specific data

The availability of cryogenically cooled probes permits routine acquisition of data from low sensitivity pulse sequences such as inadequate and 1,1‐adequate. We demonstrate that the use of cryo‐probe generated 1,1‐adequate data in conjunction with HMBC dramatically improves computer‐assisted structure elucidation (CASE) both in terms of speed and accuracy of structure generation. In this study data were obtained on two dissimilar natural products and subjected to CASE analysis with and without the incorporation of two‐bond specific data. Dramatic improvements in both structure calculation times and structure candidates were observed by the inclusion of the two‐bond specific data. Copyright © 2010 John Wiley & Sons, Ltd.     

A numerical and experimental study of batch sedimentation and viscous resuspension

A suspension of non‐neutrally buoyant, large, nearly monodisperse spheres is studied both in batch sedimentation and in shear between concentric rotating cylinders. We apply a continuum constitutive equation based on the diffusive flux model augmented with buoyancy terms derived by Acrivos and coworkers and discretize the resulting equation set with the finite element method. We simulate batch sedimentation using this method and obtain a reasonable match with experiment. Next used two‐dimensional NMR imaging to measure the evolution of solid fraction profiles in the same suspension undergoing flow between rotating concentric cylinders with two different initial conditions. Here, both gravity‐induced and shear‐induced particle migration are significant. Under these conditions, we have found that simulating the correct initial condition is critical to matching the experimental results. When this is done, the model results compare well with the experiments. Copyright © 2002 John Wiley & Sons, Ltd.     

Compositional effects and spectroscopy of rare earths (Er, Tm, and Nd) in tellurite glasses

Tellurium oxide glass hosts have exceptionally large solubility for RE-ions. In this paper, the relationship between the host glass composition and spectroscopic properties of Er³⁺-doped and Tm³⁺-doped tellurite glasses and fibres has been examined in detail. In particular, the effect of compositional modification on the line-shapes of the transitions: ⁴I_13/2→⁴I_15/2 in Er³⁺-doped and ³H₄→³F₄ in Tm³⁺-doped tellurite glasses is analysed and discussed. The presence of co-dopants, namely Tb³⁺ and Ho³⁺ ions in Tm³⁺-doped glasses on the lifetimes of the upper (³H₄) and the lower (³F₄) levels have also been discussed for shortening the lifetime of the transition between the ³F₄ level and the ground state in thulium. The spectroscopic properties of Nd³⁺-doped glasses for designing amplifiers operating in the 1330- to 1370-nm region has also been explained for demonstrating a continuous gain spectrum from 1310- to 1610-nm. The fibre loss measurements demonstrate that the Er-doped fibres have less than 880 dB km^–1 attenuation in the 1200- to 1400-nm range. An explanation is given to determine the suitability of such RE-doped tellurite fibres for designing broadband amplifiers.     

Microviscosity in poly(ethylene oxide)‐polypropylene oxide‐poly(ethylene oxide) block copolymers probed by fluorescence depolarization kinetics

Triblock copolymers [poly(ethylene oxide) (PEO) and polypropylene oxide (PPO)], Pluronic F127 with 100 PEO blocks on each end, and 65 blocks of PPO in the center were examined in aqueous solution. The “sol” and “gel” phase diagram was determined as a function of concentration and temperature. For further study, the concentration was fixed at 20 wt %, and the temperature dependence of the dynamic viscosity differed from the temperature dependence of fluorescence emission spectra and the microviscosity probed by the fluorescence depolarization kinetics of rhodamine 123 dye, which was dissolved in the continuous hydrophilic phase. The depolarization measurements used single‐photon counting after two‐photon excitation with a Ti‐sapphire femtosecond laser. Although the viscoelastic modulus increased by an order of magnitude when the sol‐to‐gel transition was crossed, the microviscosity of the hydrophilic continuous medium showed only minor changes. At different temperatures the fluorescence lifetime was the same with a single‐exponential time constant, but the fluorescence depolarization displayed a double‐exponential decay. After comparison with fluorescence depolarization of the dye in PPO melt and PEO whose molecular weight and aqueous concentrations were varied, the relative proportions of faster and slower components of the fluorescence depolarization were tentatively attributed to varying ratios of the dye in free solution and associated with micelles. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2883–2888, 2002