Revisiting random walks in fractal media: on the occurrence of time discrete scale invariance

This paper addresses the kinetic behavior of random walks in fractal media. We perform extensive numerical simulations of both single and annihilating random walkers on several Sierpinski carpets, in order to study the time behavior of three observables: the average number of distinct sites visited by a single walker, the mean-square displacement from the origin, and the density of annihilating random walkers. We found that the time behavior of those observables is given by a power law modulated by soft logarithmic-periodic oscillations. We conjecture that logarithmic-periodic oscillations are a manifestation of a time domain discrete scale iNvariance (DSI) that occurs as a consequence of the spatial DSI of the substrate. Our conjecture implies that the logarithmic periods of oscillations in space and time domains are linked by a dynamic exponent z, through z=log(tau)/log(b(1)), where tau and b(1) are the fundamental scaling ratios of the DSI symmetry in the time and space domains, respectively. We use this relationship in order to compute z for different observables and fractals. Furthermore, we check the values obtained with independent measurements provided by the power-law behavior of the mean-square displacement with time [R(2)(t) proportional variant t(2/z)]. The very good agreement obtained between both computations of the z exponent gives strong support to the idea of an intimate interplay between spatial and time symmetry properties that we expect will have a quite general scope. We expect that the application of the outlined concepts in the field of dynamic processes in fractal media will stimulate further research.     

A Hexameric Cationic Copper(II) Metallacrown as a Pertechnetate and Perrhenate Scavenger

Materials based on the cationic copper(II) hexanuclear 18‐membered metallacrown [18‐MC ‐6]⁶⁺ (2phH=2‐piconyl hydrazide) and tetrafluoroborate, perchlorate, nitrate, sulfate, and perrhenate anions were prepared by an easy method in aqueous medium. Single‐crystal X‐ray characterization of six members of this new family of complexes showed that the anions are attached to the metallacrown by direct coordination to a copper cation or by hydrogen‐bonding interaction with the center of the hexamer. The stable cationic nature of the complexes and their ability to bind different anions allows them to adsorb and immobilize environmentally relevant anions such as MO₄^? (M=Tc, Re). The MO₄^? trapping capacities suggest that these materials would be useful in the treatment of oxoanionic contaminants in water.     

Shielding of Electron Acceptors Coordinated to Rhenium(I) by Carboxylato Groups. Intraligand and Charge‐Transfer Excited‐State Properties of fac‐(‘Anthraquinone‐2‐carboxylato')(2,2′‐bipyridine)tricarbonylrhenium (fac‐[ReI(aq‐2‐CO2)(2,2′‐bipy)(CO)3])

The photophysical and photochemical properties of (OC‐6‐33)‐(2,2′‐bipyridine‐κN¹,κN¹′)tricarbonyl(9,10‐dihydro‐9,10‐dioxoanthracene‐2‐carboxylato‐κO)rhenium (fac‐[Re^I(aq‐2‐CO₂)(2,2′‐bipy)(CO)₃]) were investigated and compared to those of the free ligand 9,10‐dihydro‐9,10‐dioxoanthracene‐2‐carboxylate (=anthraquinone‐2‐carboxylate) and other carboxylato complexes containing the (2,2′‐bipyridine)tricarbonylrhenium ([Re(2,2′‐bipy)(CO)₃]) moiety. Flash and steady‐state irradiations of the anthraquinone‐derived ligand (λ_exc 337 or 351 nm) and of its complex reveal that the photophysics of the latter is dominated by processes initiated in the Re‐to‐(2,2′‐bipyridine) charge‐transfer excited state and 2,2′‐bipyridine‐ and (anthraquinone‐2‐carboxylato)‐centered intraligand excited states. In the reductive quenching by N,N‐diethylethanamine (TEA) or 2,2′,2″‐nitrilotris[ethanol] TEOA, the reactive states are the 2,2′‐bipyridine‐centered and/or the charge‐transfer excited states. The species with a reduced anthraquinone moiety is formed by the following intramolecular electron transfer, after the redox quenching of the excited state: [Re^I(aq−2−CO₂)(2,2′‐bipy^.)(CO)₃]⁻⇌[Re^I(aq−2−CO₂^.)(2,2′‐bipy)(CO)₃]⁻ The photophysics, particularly the absence of a Re^I‐to‐anthraquinone charge‐transfer excited state photochemistry, is discussed in terms of the electrochemical and photochemical results.     

Non-Covalent Interactions in a Nickel(II) Complex with Benzilate and 1,10-Phenanthroline

Abstract  

A new mixed-ligand nickel(II) coordination compound [Ni(bz)₂(phen)₂] (1) (bz = benzilate and phen = 1,10-phenanthroline) has been synthesized by refluxing nickel(II) acetate with benzilic acid and 1,10-phenantholine. Compound 1 has been characterized by elemental analysis, IR and electronic absorption spectroscopy, magnetic measurements, thermogravimetric analysis and single-crystal X-ray diffraction The crystal of 1 belongs to monoclinic crystal system, space group C2/c and its unit cell parameters are a = 25.881(3), b = 11.1728(15), c = 17.609(2) ? and β = 124.104(3)°. The effect of non-covalent interactions, such as π···π and C–H···π, on the 3D supramolecular organization of this molecular complex is analyzed.     

Distinct dynamic behaviors of water molecules in hydrated pores

Water molecules confined inside narrow pores are of great importance in understanding the structure, stability, and function of water channels. Here we report that besides the H-bonding water that structures the pore, the permanent presence of a significant, fast-moving fraction of incompletely H-bonded water molecules inside the pore should control the free entry and exit of water. This is achieved by means of complementary DSC and solid-state NMR studies. We also present compelling evidence from X-ray diffraction data that the cluster formed by six water molecules in the most stable cage-like structure is sufficiently hydrophobic to be stably adsorbed in a nonpolar environment.     

Aggregation in nanobundles and the effect of diverse environments on the solution-phase photochemistry and photophysics of -Re(CO)3L+ (L = 1,10-phenanthroline, 2,2'-bipyridine) pendants bonded to poly(4-vinylpyridine)

The UV-vis spectroscopy and photochemical properties of {(vpy-[Re(CO)3(2,2'-bpy)])m(vpy-[Re(CO)3(phen)])n(vpy)p}(CF3SO3)(m+n)}, vpy = 4-vinylpyridine, m = 131, n = 131 or m = 200, n = 150, and m + n + p = 600, were investigated in solution phase. The polymers exist in solution as aggregates of polymer strands with radii as large as approximately 10(2) nm. Given the size of the poly-vpy backbone, the aggregates must contain a large number of strands. The luminescence spectrum exhibits a strong resemblance to the emission spectrum of {(vpy-[Re(CO)3(phen)])200(vpy)400}(CF3SO3)200. The existence of Re(I) chromophores in diverse environments was shown by the intrinsic kinetics of the luminescence, the decay kinetics of the MLCT excited states observed by time resolved-absorption spectroscopy, and the quenching of the luminescence by various quenchers. Redox reactions of the MLCT excited states with the quenchers were responsible for the luminescence quenching. While static quenching resulted when Cu(II) and Fe(III) EDTA complexes were the quenchers, a dynamic quenching resulted with Fe(CN)6(4-) or 2,2',2' '-triethanolamine, TEOA. The photochemical and photophysical properties of the mixed-pendant polymers have been discussed in terms of arrays of MLCT excited states whose energies are determined by the diverse environments of the Re(I) chromophores. Conversions (with and without radiation) of the upper-energy MLCT excited states to the ground state and lower-energy MLCT excited states and the latter excited state to the ground state account for the experimental results.     

Mass spectrometry of rhenium complexes: a comparative study by using LDI‐MS,MALDI‐MS,PESI‐MS and ESI‐MS

A group of rhenium (I) complexes including in their structure ligands such as CF₃SO₃‐, CH₃CO₂‐, CO, 2,2′‐bipyridine, dipyridil[3,2‐a:2′3′‐c]phenazine, naphthalene‐2‐carboxylate, anthracene‐9‐carboxylate, pyrene‐1‐carboxylate and 1,10‐phenanthroline have been studied for the first time by mass spectrometry. The probe electrospray ionization (PESI) is a technique based on electrospray ionization (ESI) that generates electrospray from the tip of a solid metal needle. In this work, mass spectra for organometallic complexes obtained by PESI were compared with those obtained by classical ESI and high flow rate electrospray ionization assisted by corona discharge (HF‐ESI‐CD), an ideal method to avoid decomposition of the complexes and to induce their oxidation to yield intact molecular cation radicals in gas state [M]^+. and to produce their reduction yielding the gas species [M]^–.. It was found that both techniques showed in general the intact molecular ions of the organometallics studied and provided additional structure characteristic diagnostic fragments. As the rhenium complexes studied in the present work showed strong absorption in the UV–visible region, particularly at 355 nm, laser desorption ionization (LDI) mass spectrometry experiments could be conducted. Although intact molecular ions could be detected in a few cases, LDI mass spectra showed diagnostic fragments for characterization of the complexes structure. Furthermore, matrix‐assisted laser desorption ionization (MALDI) mass spectra were obtained. Nor‐harmane, a compound with basic character, was used as matrix, and the intact molecular ions were detected in two examples, in negative ion mode as the [M]^–. species. Results obtained with 2‐[(2E)‐3‐(4‐tert‐buthylphenyl)‐2‐methylprop‐2‐enylidene] malononitrile (DCTB) as matrix are also described. LDI experiments provided more information about the rhenium complex structures than did the MALDI ones. Copyright © 2012 John Wiley & Sons, Ltd.     

Combined PIV/PLIF measurements of a steady density current front

A novel method for combined particle image velocimetry and laser induced fluorescence is described and results from an experiment in a stratified flow are presented. A standard two-dimensional, one camera particle image velocimetry configuration is used, acquiring images of the seeding particles and the dye marking the current simultaneously and separating the two fields digitally. The implementation of the postprocessing method, its capabilities and the necessary conditions for its use are discussed in detail. The proposed method is applied to an arrested density current front. The front is made stationary by opposing a uniform velocity profile, obtained from the combination of a moving floor and the recirculation of fresh water in the channel. To improve the quality of the images, the current is made optically homogeneous by matching the refractivity index throughout the domain. Instantaneous and time averaged fields are obtained for both velocity and density. Simultaneous measurements of these fields provide insight in the mixing processes at the front of the density current. In particular persistent billow generation, similar to that found in shear layers and associated with Kelvin–Helmholtz instabilities, is observed. Support for this work was partially provided by the US Office of Naval Research through the Coastal Geosciences Program and by Exxon Mobil Exploration Company. The authors would like to thank Dr. Wing Lai of TSI, Inc, for loaning part of the PIV equipment used in this study.     

Extended Alkenyl and Alkynyl Benzotriazoles with Enhanced Two-Photon Absorption Properties as a Promising Alternative to Benzothiadiazoles

A series of donor–π–acceptor–π–donor (D -π-A-π-D) benzoazole dyes with 2H-benzo[d][1,2,3]triazole or BTD cores have been prepared and their photophysical properties characterized. The properties of these compounds display remarkable differences, mainly as a result of the electron-donor substituent. Dyes with the best properties have visible-light absorption over λ=400 nm, large Stokes shifts in the range of about 3500–6400 cm⁻¹, and good fluorescence emission with quantum yields of up to 0.78. The two-photon absorption properties were also studied to establish the relationship between structure and properties in the different compounds synthesized. These results provided cross sections of up to 1500 GM, with a predominance of S₂←S₀ transitions and a high charge-transfer character. Time-dependent DFT calculations supported the experimental results.     

Rapid amplitude-modulation of a diarylethene photoswitch: en route to contrast-enhanced fluorescence imaging

A water soluble diarylethene (DAE) derivative that displays exceptionally intense fluorescence from the colorless open form has been synthesized and characterized using UV/vis spectroscopy and fluorescence microscopy. We show that the bright emission from the open form can be rapidly switched using amplitude modulated red light, that is, by light at wavelengths longer than those absorbed by the fluorescent species. This is highly appealing in any context where undesired background fluorescence disturbs the measurement, e.g., the autofluorescence commonly observed in fluorescence microscopy. We show that this scheme is conveniently applicable using lock-in detection, and that robust amplitude modulation of the probe fluorescence is indeed possible also in cell studies using fluorescence microscopy.

A water soluble diarylethene derivative displaying exceptionally bright fluorescence in the open isomeric form has been used for emission amplitude-modulation. We apply this scheme in fluorescence microscopy, aiming to suppress undesired background.