Catalytic reactions of chlorofluoroethanes at fluorinated alumina and chromia aerogels and xerogels: A comparison of reaction pathways in alumina- and chromia-based catalysts

The differences in the dynamic behaviour of 1,1,2-trichlorotrifluoroethane over fluorinated alumina or chromia catalysts at moderate temperatures are significant. Alumina-based catalysts favour the isomerization of CCl₂FCClF₂ followed by the dismutation of CCl₃CF₃ so formed. Fluorinated chromias are less selective since halogen exchange and isomerization reactions both occur. An explanation in terms of the differences in Al---X, X=Cl or F, bond energies compared with their Cr---X counterparts is suggested. The identity of the catalyst precursor has little effect in the chromium case and no effect in the case of aluminium.     

Bistable photoswitching in poly(N-isopropylacrylamide) with spironaphthoxazine hydrogel for optical data storage

A bistable switching photochromic poly(N-isopropylacrylamide) with spironaphthoxazine hydrogel copolymer (PNIPA-SPO-BIS) has been designed and synthesized by radical polymerization. The PNIPA-SPO-BIS copolymer is identified by ¹H NMR spectroscopy, FIT-IR spectroscopy and gel permeation chromatography (GPC). The morphology of the internal microstructures of the PNIPA-SPO-BIS hydrogels was observed by scanning electron microscopy (SEM). The PNIPA-SPO-BIS copolymer showed excellent photochromic behavior in water solution and in gel state. In addition, erasable and rewritable (EARW) photoimaging on the PNIPA-SPO-BIS hydrogel was successfully demonstrated. A novel optical data storage materials based on photochromic hydrogel was developed. These developments are crucial for fundamental studies and eventual technical application for all-photo mode high-density optical data storage.     

Changes in the structure of composite colloid-polymer xerogels after cold isostatic pressing

Monolithic gels, prepared from different mixtures of colloidal silica in a sol solution containing tetraethoxysilane under powerful ultrasonic agitation (sonosols), were compacted at an isostatic pressure of 390 MPa. Then N₂ adsorption-desorption data were used to construct structural models of the gels using Monte-Carlo calculations on the basis of random close-packing (RCP) premises. Structural information on these composites obtained before compaction indicates that the characteristic uniform structure of silica colloid gel undergoes profound modification when it is mixed with silica sonogel. From a structural point of view, the behaviour under compaction of the sonogel phase, which exhibits a significant degree of microporosity, depends on the relative concentration of the colloidal phase. Two hierarchic levels of micropores were discerned. After compression, the size of the elementary particles—and their aggregates—of the sonogel phase increases from 1.6 to 2.1 nm radius when the colloidal phase content is increased from 30 to 82% by weight. For an intermediate content, 50% of the volume reduction is caused by compression of the sonogel phase at the micropore level.     

Effect of adsorbed concentration on the radiative rate enhancement of photoexcited molecules embedded in single microspheres

The variation of the molecular density in a single microcavity and its influence on the radiative rate enhancement (RRE) are reported here. The quality factors of the observed morphology-dependent resonances (MDRs) of the microcavity remain unchanged in the absence of any absorbing effects. In contrast, the MDRs tend to disappear in the presence of strong absorption even due to the self-absorption by the molecule. Time-resolved fluorescence studies reveal the fact that the value of RRE decreases with an increase in the adsorbed concentration of the molecules. The results have been explained in terms of a detuning parameter, which is a function of the refractive index of the microcavity. The increased dispersing capability of the microsphere upon increasing its molecular density has been found to be responsible for the observed decrease in RRE.     

Substituent Effect on the Stability and 14N NQR Parameters of Linkage Isomers of Nitriles in a Rhodium Half‐Sandwich Metallacycle: A Theoretical Study

In this work, using the MPW1PW91 method, the substituent effect on the stability and on the ¹⁴N NQR parameters of linkage isomers of nitriles in a rhodium half‐sandwich metallacycle is illustrated. After determination of the corresponding isomerization transition state (TS), the substituent effect on the barrier energy and on the activation thermodynamic parameters (ΔG^‡ and ΔH^‡) of isomerization is explored. The electric field gradient tensor, nuclear quadrupole coupling constant, asymmetry parameter, and nuclear quadrupole resonance frequencies of the studied isomers are calculated. Also, linear correlations between these parameters and Hammett constant of the substituent are explored.     

Recent advances in optical manipulation of cells and molecules for biological science

Noninvasive and nondestructive techniques for monitoring and manipulating cells or biomolecules are essential for understanding biological processes. Optical methodologies have been used for the noninvasive and nondestructive monitoring of intracellular molecules and manipulation of cellular activities to elucidate the localization and interactions of these biomolecules. Since the pioneering work of Ashkin, optical trapping has been used to study cellular elasticity and mechanical characteristics of intracellular molecules. In recent years, there has been a substantial amount of research on the optical manipulation of nanometer-sized objects, including the manipulation of the assembly of nanomaterials and the enhancement of optical forces with optical resonance effects. In the study of biomolecular manipulation by optical forces, the functions and roles of biomolecules have been clarified by analyzing the changes in cellular functions induced by manipulation. In this review, we focus on recent studies on optical trapping for the manipulation of living cells or biomolecules and introduce techniques for the manipulation of cellular functions using optical forces.     

Spectral characterization of chlorophyll fluorescence in extract of barley leaves embedded in silica xerogel matrix

We have used the sol-gel method to prepare SiO₂ based matrix containing barley leaves extracts and studied the spectral characteristics of chlorophyll fluorescence of the glass under structural evolution promoted by heat treatment. One primary effect on the fluorescence for barley leaves embedded in glass is that PSII chlorophyll fluorescence transients are not present. We obtain a higher PSII thermostability for leaves embedded in xerogel matrix than in the green barley leaves. We observed for high temperatures that fluorescing aggregates are formed. The behavior of the PSII fluorescence under heat treatment will be used in subsequent works to study the microstructural evolution during the silica-gel-glass conversion and their optical properties.     

Distribution of single molecules in polymer thin films

Distribution of fluorescent dye molecules in polymer thin (100 nm) films was investigated using far-field single-molecule video microscopy, by varying concentrations of dye molecules mixed in the polymer. Histograms of fluorescence photocounts of individual fluorescent spots showed wide distribution, varying in the number of fluorescent spots composed of one, two, three or group of molecules. The number of the molecules present in the fluorescent spots was also ascertained by fluorescence photobleaching experiments. Photocounts associated with maxima of the histograms were found to be independent of the concentrations; however, the number of occurrences associated with more than one molecule decreased with decreasing concentration. By reducing concentration as well as by mixing dye molecules into a polymer solution, fluorescent spots grouping more than one molecule were separated considerably into fluorescent spots including a single-molecule.     

Indirect micromanipulation of single molecules in water-in-oil emulsion

Based on real-time observation and micromanipulation, analytical methods for single DNA molecules have been under development for some time. Precise manipulation, however, is still difficult because single molecules are too small for conventional techniques. We have developed a chemical reaction system that uses water droplets in oil as containers of materials. The water droplets can be manipulated by optical force. The manipulation of the water droplets permits the fusion of two selected droplets. This process corresponds to mixing of different samples. We designate this system as "w/o (water-in-oil emulsion) microreactor system", and each droplet can be thought of as a "microreactor". In this system, single molecules can be manipulated readily, as a molecule can be contained in a microm-sized microreactor. The microreactor utilizes extremely small quantities of samples, therefore, reactions are rapid, as diffusion times in the microreactor are very short. The manipulation technique of the microreactors based on optical force has been applied to induce fusion between microreactors loaded with DNA and YOYO, a fluorescent dye that binds to DNA. This fusion induced a rapid binding of YOYO.     

Orientation of anthracene molecules in naphthalene single crystals

The orientation of anthracene in naphthalene matrix has been determined by EPR of the triplet state of the guest molecule. It differs from the host molecule orientation mainly by a rotation of 25° about the z axis.     

Optical imaging of excited-state tautomerization in single molecules

Tautomerism process of single fluorescent molecules was studied by means of confocal microscopy in combination with azimuthally or radially polarized laser beams. During a tautomerism process the transition dipole moment (TDM) of a molecule changes its orientation which can be visualized by the fluorescence excitation image of the molecule. We present experimental and theoretical studies of two porphyrazine-type molecules and one type of porphyrin molecule: a symmetrically substituted metal-free phthalocyanine and porphyrin, and nonsymmetrically substituted porphyrazine. In the case of phthalocyanine the fluorescence excitation patterns show that the angle between the transition dipole moments of the two tautomeric forms is near 90°, in agreement with quantum chemical calculations. For porphyrazine we find that the orientation change of the TDM is less than 60° or larger than 120°, as theoretically predicted. Most of the porphyrin molecules show no photoinduced tautomerization, while for 7% of the total number of investigated molecules we observed excitation patterns of two different trans forms of the same single molecule. We demonstrate for the first time that a molecule, undergoing a tautomerism process stays in one tautomeric trans conformation during a time comparable with the acquisition time of one excitation pattern. This allowed us to visualize the existence of each of the two trans forms of one single porphyrin molecule, as well as the sudden switching between these tautomers.     

Study of strain-transfer of FBG sensors embedded in unidirectional composites

Optical fibre Bragg grating (FBG) sensors are now quite established and widely used in strain measurements in composites. However, insufficient understanding of the limitations of the embedment and measuring techniques often leads to inaccurate results.     

Elongation method for calculating excited states of aromatic molecules embedded in polymers

Photophysical properties of polyethylene structures embedding aromatic fragments (benzene, anthracene, 4‐dicyanomethylene‐4H‐pyran, tryptophan, and estradiol) responsible for existence lowest electronically excited states were studied by new technique involving the elongation method applied to quantum‐chemical calculations. Absorption spectra and some photophysical properties were obtained. The comparison between the elongation and the conventional calculations was made, and it is shown that the elongation method is a powerful tool to determine the excited states as well as optical properties for large systems. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Benefits of mesopores in sulfated zirconia catalyst

Activities of two catalysts obtained from hydroxide and alkoxide precursors were correlated to their different structural and textural properties. Activity is mainly determined by the presence of active tetragonal phase, which is more stable at high temperatures in the catalyst of alkoxide origin than in one from hydroxide. An appropriate structure must be accompanied with beneficial pore structure, as is the case of mesopores of the catalyst from alkoxide. In the case of very small pores of the catalyst from hydroxide precursor, activity is negligible due to pores plugged by coke.     

CTAB-assisted hydrothermal synthesis of YVO4:Eu powders in a wide pH range

Rhombus-, rod-, soya bean- and aggregated soya bean-like YVO₄:Eu³⁺ micro- and nanostructures were synthesized by a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method at 180 °C for 24 h in a wide pH range. The as-synthesized powders were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). The XRD results confirmed the formation of phase-pure YVO₄:Eu³⁺ powders with tetragonal structure under hydrothermal process in a wide pH range. Electron microscopic observations evidenced the morphological transformation of YVO₄:Eu³⁺ powders from rhombus-like microstructure to rod-, soya bean, and aggregated soya bean-like nanostructures with an increase in the pH of the synthesis solution. The results from the PL measurements revealed that the intensities of PL emission peaks were significantly affected by the morphologies and crystallinity of samples due to the absence of an inversion symmetry at the Eu³⁺ lattice site, and the highest luminescence intensity was observed for rod-like YVO₄:Eu³⁺ powders.     

Combining silica-based adsorbents and SPME fibers in the extraction of the volatiles of beer: an exploratory study

A series of silica-based materials were employed as sorbents within solid-phase microextraction vials. The aim of the study was to evaluate the effect of an additional phase on the distribution of the volatile and less volatile analytes. The adsorption of six probe molecules, namely isoamyl acetate, ethyl hexanoate (ethyl caproate), phenylethyl alcohol, ethyl octanoate (ethyl caprilate), 2-phenylethyl acetate, and ethyl decanoate, was monitored by detecting the desorbed amount on a DVD–CAR–PDMS fiber from Pilsen beer. The microextraction process involved the presence of different silica-based phases produced via different methods: xerogel produced by hydrolytic and non-hydrolytic routes, aerogel, pyrogenic, and precipitated silica. The resulting data are discussed in correlation with sorbent texture properties (specific area and pore diameter). The modification of silica with alkyl groups also affects the preconcentrated amount of the target molecules in the headspace. The presence of sorbents was shown to affect the analyte signal more than the addition of NaCl or the use of ultrasound. Analyte’s equilibrium between fiber and sorbent     

Structure-dependent photophysics studied in single molecules of polythiophene derivatives.

We report on the photophysical characterization at the single-molecule level of a graft copolymer consisting of a polythiophene backbone and long polystyrene branches. The presence of the branches prevents the polymer chain from forming a collapsed conformational state. The photophysical properties of the resulting solution-like conformation are studied by measuring single-molecule photobleaching dynamics, emission polarization anisotropy and emission spectra. The results are compared with those obtained on the same polythiophene derivative without the branches. It is found that the presence of the branches is a decisive factor in determining the photophysical properties of the polymers on the single-molecule level.     

Power-law blinking in the fluorescence of single organic molecules.

The blinking behavior of perylene diïmide molecules is investigated at the single‐molecule level. We observe long‐time scale blinking of individual multi‐chromophoric complexes embedded in a poly(methylmethacrylate) matrix, as well as for the monomeric dye absorbed on a glass substrate at ambient conditions. In both these different systems, the blinking of single molecules is found to obey analogous power‐law statistics for both the on and off periods. The observed range for single‐molecular power‐law blinking extends over the full experimental time window, covering four orders of magnitude in time and six orders of magnitude in probability density. From molecule to molecule, we observe a large spread in off‐time power‐law exponents. The distributions of off‐exponents in both systems are markedly different whereas both on‐exponent distributions appear similar. Our results are consistent with models that ascribe the power‐law behavior to charge separation and (environment‐dependent) recombination by electron tunneling to a dynamic distribution of charge acceptors. As a consequence of power‐law statistics, single molecule properties like the total number of emitted photons display non‐ergodicity.