Reevaluating the Solution Photophysics of Tetraphenylethylene at the Origin of their Aggregation-Induced Emission Properties

Although tetraphenylethylene (TPE) and its derivatives have been the most commonly used building blocks in the construction of molecules with aggregation-induced emission (AIE) properties, no absolute consensus exists regarding the mechanisms at the origin of the phenomenon. Restriction of intramolecular rotations (RIR) of peripheral phenyls has historically been a dominant paradigm, which has served as a valuable guideline in the molecular engineering of AIEgens. Yet, an increasing number of recent works have established that photoisomerization or photocyclization may actively participate in the nonradiative dissipation of the excitation energy. In this paper, the first experimental evaluation of the quantum efficiencies of these different processes is reported, and photoisomerization is shown to be by far the dominant photophysical pathway in solution, accounting for virtually all nonradiative decay of the molecule‘s excited state in degassed solution.     

The quantitative effect of microextractor cell geometry on the analytical supercritical fluid extraction efficiencies of environmentally important compounds

Summary The quantitative effect of microextractor cell geometries on supercritical fluid extraction (SFE) efficiencies of polycyclic aromatic hydrocarbons (PAHs) and methoxychlor from octadecyl-bonded sorbents has been evaluated and compared to similar effects seen upon increasing the supercritical fluid density. For the PAHs studied, correlations between the fused ring number and the relative increase in recoveries have been established. SFE recoveries can be increased by greater than a factor of two by decreasing the diameter to length ratio from 120 to 11. The relative recovery increase upon decreasing the diameter to length ratio of the extraction vessel is dependent on the analyte extractability, increasing in proportion to the fused ring number for the PAHs. Recoveries increased linearly as a function of supercritical fluid density for the PAHs studied. The change in the relative recovery upon increasing the supercritical carbon dioxide density again was dependent on the analyte type, decreasing linearly with fused ring number. Although fluid density generally had the greatest effect on achievable SFE recoveries, the cell geometry had effects of a similar order of magnitude, highly dependent on the initial extractability of the analyte.     

Collisional energy transfer in the reactions I + NO + M → INO + M and I + NO2 + M → INO2 + M

The low-pressure recombination rate constants of the reactions I + NO + M → INO + M (with 14 different M) and I + NO₂ + M → INO₂ + M (with 26 different M) have been measured at 330°K by laser flash photolysis. The collision efficiencies β_c are analyzed and compared with other thermal activation systems. Whereas β_c increases in one reaction with an increasing number of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting molecules are considered.     

Ionization efficiencies of C3H6, C4H8, C6H12, C2H6O,and C3H8O isomers

Ionization efficiencies of 14 organic compounds have been measured in the wavelength region from 105 to 134nm using an ionization chamber. The compounds examined are cyclopropane, propylene, l-butene, isobutene, cis-and trans-2-butenes, cyclohexane, 1-hexane, tetramethylethylene, ethyl alcohol, dimethyl ether, n-, and iso-propyl alcohol, and ethyl methyl ether. The ionization efficiencies of cyclopropane and cyclohexane monotonically increase with increasing photon energy, but those for the others show a peak or a shoulder in the wavelength region of the present work.     

Monte Carlo studies of a novel X-ray tube anode design

When energetic electrons are incident on high atomic number absorbers, a substantial fraction is back-scattered. This phenomenon is responsible for several undesirable effects in X-ray tubes, in particular a reduction in the X-ray output. The extent of this shortfall has been estimated by using Monte Carlo simulation to start electrons at increasing depth inside the anode, the results indicating that an output enhancement of nearly 50% could be achieved in principle if the electrons wasted in back-scatter events could be trapped inside a tungsten anode. To test this idea a further set of simulations were done for a novel anode geometry. Results showed that X-ray tube efficiencies might be substantially enhanced by this approach.     

Capillary electrochromatography with macroporous particles.

The performance of macro-porous particles in capillary electrochromatography is studied. Three reversed-phase stationary phases with pore diameters between 500 A and 4000 A have been tested for separation efficiency and mobile phase velocity. With these stationary phases, a large portion of the total flow appears to be through the pores of particles, thereby increasing the separation efficiency through a further decrease of the flow inhomogeneity and through enhancement of the mass transfer kinetics. The effects of pore size and mobile phase composition on the plate height and mobile phase velocity have been studied. With increasing buffer concentrations and larger pore diameters, higher mobile phase velocities and higher separation efficiencies have been obtained. Columns packed with 7 microns particles containing pores with a diameter of 4000 A generated up to 430,000 theoretical plates/m for retained compounds. Reduced plate heights as low as 0.34 have been observed, clearly demonstrating that a significant portion of the flow is through the pores. For the particles containing 4000 A pores no minimum was observed in the H-u plot up to linear velocities of 3.3 mm/s, suggesting that the separation efficiency is dominated by axial diffusion. On relatively long (72 cm) columns, efficiencies of up to 230,000 theoretical plates/column have been obtained under non-optimal running conditions. On short (8.3 cm) columns fast separations could be performed with approximately 15,000 theoretical plates generated in less than 30 s.     

Photon trapping by the internal Bragg reflection of colloidal crystals

Fluorescence light emitted from photoexcited rhodamine 6G (R6G) doped in colloidal crystals of exhaustively deionized colloidal silica suspension is partially trapped within a crystal cage. This photon trapping is caused by Bragg reflection in crystal lattices. The photon trapping efficiencies were quantitatively examined as a function of the thickness of measurement cell. The efficiency increased from about 40 to 60% as the cell thickness increased from 1 to 10 mm for an R6G concentration of 5×10⁻⁶ mol/L. This result is attributed to an increase in the number of crystal layers perpendicular to the observation direction; these are formed in the cell with a large optical path length. On the other hand, the trapping efficiencies were constant irrespective of the angle between the incident and observed light of the cylindrical cells. The constant efficiencies are attributed to the fact that the heterogeneous crystal layers around the inner cell wall have the same thickness.     

Efficient photosensitized splitting of the thymine dimer/oxetane unit on its modifying beta-cyclodextrin by a binding electron donor

Two modified beta-cyclodextrins (beta-CDs) with a thymine dimer and a thymine oxetane adduct respectively, TD-CD and Ox-CD, have been prepared, and utilized to bind an electron-rich chromophore, indole or N,N-dimethylaniline (DMA), to form a supramolecular complex. We have examined the photosensitized splitting of the dimer/oxetane unit in TD-CD/Ox-CD by indole or DMA via an electron-transfer pathway, and observed high splitting efficiencies of the dimer/oxetane unit. On the basis of measurements of fluorescence spectra and splitting quantum yields, it is suggested that the splitting reaction occurs in a supramolecular complex by an inclusion interaction between the modified beta-CDs and DMA or indole. The back electron transfer, which leads low splitting efficiencies for the covalently-linked chromophore-dimer/oxetane compounds, is suppressed in the non-covalently-bound complex, and the mechanism has been discussed.     

Resolution of positional isomers by capillary electrochromatography

Electroseparations have been very successful in increasing efficiencies and reducing analysis times. The analytical technique originally applied to open tube capillaries (capillary electrophoresis) has been used as a basis to develop renewed interest in electrochromatography. This paper describes the use of capillary electrochromatography to separate two positional isomers and describes a comparison between gas chromatography (GC), capillary electrochromatography (CEC) and nano-HPLC. The resolution of these isomers is quite crucial, since one of the isomers is the impurity in a pharmaceutically active drug.     

A contactless conductivity detector for capillary electrophoresis: effects of the detection cell geometry on the detector performance

The effect of the gap between the electrodes and of their width on the behavior of a capacitively wired contactless conductivity detector was studied. The results obtained have indicated that the detector response can be qualitatively described by a model based on the concept of the effective electrode width which is a complex parameter determined by the gap between the electrodes, the frequency of the input signal and the conductivity of the test solution. The detector sensitivity and the effect on the separation efficiency depend on the difference between the effective and geometric electrode widths. Higher detection sensitivities have been attained for detectors with wide electrodes operating at lower frequencies, however, better separation efficiencies have been achieved using detectors with narrow electrodes and higher operational frequencies. The noise increases with decreasing gap between the electrodes and increasing frequency, especially with detectors employing narrow electrodes.     

Linear relations for efficiency calibration of γ-spectrometric measurements of bulk samples

The self-attenuation correction factor is used to relate the efficiency for a sample with a given matrix to the efficiency for an ideal sample with identical geometry but negligible photon attenuation. A certain linear relation for the efficiency for a given sample as a function of the efficiencies for a number of subsamples into which the original sample can be decomposed is established and experimentally validated. This relation can be used also in the case when the sample and the subsamples have different matrices. In this way the efficiency for volume samples with arbitrary compositions and densities can be constructed on the basis of the efficiencies (independently measured) for a number of basic geometries. Also a possibility to check the consistency of efficiency calibrations carried out with different standard sources (with different matrices) is provided.     

Influence of the extraction methodology on the analysis of polycyclic aromatic hydrocarbons in pasture vegetation

Pasture vegetation plays an important role in the air-surface exchange and food chain transfer of polycyclic aromatic hydrocarbons (PAHs). Therefore, considerable research has been focused towards measuring PAHs in vegetation using different analytical methods. However, in most cases information on the efficiencies of the different extraction methods employed is missing. This complicates data interpretation and inter-study comparisons. To address this deficiency, the extraction efficiencies of two commonly used pasture vegetation extraction techniques (sonication and soxhlet) and different solvents (hexane, DCM and hexane:acetone [4:1, v/v]) were compared. The completeness of the extraction was investigated using alkaline saponification in methanol. Soxhlet extraction was able to access between 60 and 90% of the total amount of PAHs in the pasture vegetation. Sonication was less efficient, only being able to extract between 10 and 50% of the PAHs. Extraction efficiencies were found to increase with increasing PAH molecular weight. The implications of these findings on data interpretation are discussed.     

新型单壁碳纳米管采样吸附剂性能的评价

研究了单壁碳纳米管(SWCNTs)作为新型采样吸附剂的性能和效果,并应用于空气中挥发性有机化合物的分析测定。结果表明,单壁碳纳米管具有较大的比表面积,与经典Tenax TA吸附剂相比,对低碳数挥发性强的有机化合物回收率高,有更强的吸附能力;空白实验表明,SWCNTs易获得较低本底,具有化学惰性和疏水特性,采样时水的干扰小。当湿度增加时在误差允许的范围内准确度不受影响;实验测定具有较大的穿透容量和安全采样体积。将单壁碳纳米管吸附剂实际应用于大气中挥发性有机化合物的测定,通过与经典吸附剂Tenax TA相比,更适于采集大气中的挥发性有机化合物。     

Cerenkov counting rates of radionuclides for activation analysis purposes

Cerenkov counting efficiencies for a large number of radionuclides formed by (n, γ) reactions and used in the field of activation analysis have been obtained by two different experimental procedures. A comparison between the Cerenkov counting technique and scintillation low-resolution gamma-ray spectrometry is also presented. Results are summarized in tables and briefly discussed.     

ARYL AZIDES AS PROTEIN PHOTOLABELS: ABSORPTION SPECTRAL PROPERTIES AND QUANTUM YIELDS OF PHOTODISSOCIATION

Abstract A series of ring-substituted phenyl and naphthyl azides have been prepared. Electronic absorption spectral properties and quantum yields of disappearance have been determined in aqueous solution. For phenyl azide derivatives, the wavelengths of maximum absorption vary systematically, correlating with electronic effects of the ring substituents. Quantum yield values of 0.1–0.7 are obtained and are independent of a number of experimental conditions, including concentration. There is no simple correlation of the quantum yields of azide disappearance with electronic effects, except that the naphthyl azides have higher photochemical efficiencies than the phenyl azide derivatives. A series of potential protein photolabels has been identified based on electronic absorption characteristics and photochemical efficiencies.     

Quantifying the Overall Efficiency of Circularly Polarized Emitters

An increasing number of circularly polarized luminescence (CPL) molecular emitters has been developed in recent years and many of them are intended for applications in which high overall CPL efficiencies are required. In order to have a complete picture of the efficiency of a CPL emitter, dissymmetry factor (g_lum) is not enough. In the following we propose a new quantity, named CPL brightness (B_CPL), which takes into account absorption extinction coefficient and quantum yield along with the g_lum factor. We calculated B_CPL value for more than 180 compounds reported in the literature and we analyse data distribution for the main classes of CPL molecular emitters. This tool can be employed to put into context new CPL active compounds and to direct the choice of molecular systems for specific CPL applications.     

Recent trends in nanomaterial-based microanalytical systems for the speciation of trace elements: A critical review

Trace element speciation in biomedical and environmental science has gained increasing attention over the past decade as researchers have begun to realize its importance in toxicological studies. Several nanomaterials, including titanium dioxide nanoparticles (nano-TiO₂), carbon nanotubes (CNTs), and magnetic nanoparticles (MNPs), have been used as sorbents to separate and preconcentrate trace element species prior to detection through mass spectrometry or optical spectroscopy. Recently, these nanomaterial-based speciation techniques have been integrated with microfluidics to minimize sample and reagent consumption and simplify analyses. This review provides a critical look into the present state and recent applications of nanomaterial-based microanalytical systems in the speciation of trace elements. The adsorption and preconcentration efficiencies, sample volume requirements, and detection limits of these nanomaterial-based speciation techniques are detailed, and their applications in environmental and biological analyses are discussed. Current perspectives and future trends into the increasing use of nanomaterial-based microfluidic techniques for trace element speciation are highlighted.     

Electrochemical synthesis of copper(II) carboxylates

A number of copper(II) carboxylates have been synthesized by anodic oxidation of copper. The reactions were found to proceed with current efficiencies of 0.95 to 1.00 eq./Faraday. Reaction products have been characterized by infrared spectral studies and analysis of copper.     

Comparison of atomic fluorescence power efficiencies for the helium-oxygen-acetylene and air-acetylene flames

Power efficiencies for five elements have been measured for the helium-oxygen-acetylene and air-acetylene flames. The increased power efficiencies found in this study for the helium-diluted flame, coupled with its enhanced atom-formation capabilities, suggest that lower atomic fluorescence detection limits should exist. However, in a comparison study with an air-acetylene flame using identical experimental conditions, a decreased atomic fluorescence signal-to-noise ratio was found for most elements in the helium-diluted flame. This decrease is ascribed to greater background emission noise in the hotter helium-diluted flame and decreased nebulization efficiency caused by the low density of the helium-containing nebulizer gas. A comparison of flame emission detection limits for the two flames confirms the increased sensitivity of the hotter helium-oxygen-acetylene flame, despite its lower nebulization efficiency.     

Comparison of ionization behaviors of ring and linear carbohydrates in MALDI-TOFMS

Ionization efficiencies of cyclodextrins and their linear compounds in matrix-assisted laser desorption and ionisation (MALDI) analysis were compared, and differences in the ionization efficiencies of α- and β-cyclodextrins were also studied. The mass spectra showed a series of the [M+cation]⁺ ions but not the [M+H]⁺ ions. Alkali metal salts of Li⁺, Na⁺, K⁺, and Cs⁺ were used as the cationizing agents to enhance the ionization efficiency. Relative ion intensities of the ring compounds (α- and β-cyclodextrins) were much larger than those of the linear ones (maltohexaose and maltoheptaose), and the difference showed an increasing trend with the size of the alkali metal cation. β-Cyclodextrin had higher ionization efficiency than α-cyclodextrin and the difference increased by increasing the size of the alkali metal cation. It was also found that the ionization efficiency was affected by the counter anion of the salt. The higher ionization efficiencies of cyclodextrins were explained with the number of coordination sites and the binding energies.