Ultrafast geminate recombination after photodetachment of aqueous hydroxide

The photodetachment of aqueous hydroxide (OH(?)(aq) and OD(?)(aq)) is studied using femtosecond pump?probe and pump?repump?probe spectroscopy. The electron is detached after excitation of the hydroxide ion to a charge-transfer-to-solvent (CTTS) state at 202 nm. An early intermediate is observed that builds up within 160 fs and is assigned to nonequilibrated OH?electron pairs. The subsequent dynamics are governed by thermalization, partial recombination, and dissociation of the pairs, yielding the final hydrated electrons and hydroxyl radicals. An additional pulse at 810 nm is used for secondary excitation of the intermediate species so that more insight is gained into the recombination process(es). Using this technique we observe a novel geminate recombination channel of OH with adjacent hydrated electrons. This channel leads to ultrafast quenching (700 fs) of almost half the initial number of radicals. The fast mechanism displays an isotope effect of 1.4 (for OD(?)(aq) quantum yield 35%, time constant 1.0 ps). This process was not observed in similar experiments on aqueous bromide and seems to be related to the special properties of the hydroxide ion and its local H-bonding environment. Our findings underline the high reactivity of the prehydrated electron.     

NIR‐Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free‐Radical Photopolymerization

Cyanines comprising either a benzo[e]‐ or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR‐LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso‐position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.     

Application of PEDOT‐CNT Microelectrodes for Neurotransmitter Sensing

In this work, composite microelectrodes from poly(3,4‐ethylenedioxythiophene) (PEDOT) and carbon nanotubes (CNT) are characterized as electrochemical sensing material for neurotransmitters. Dopamine can be detected using square wave voltammetry at these microelectrodes. The CNTs improve the sensitivity by a factor of two. In addition, the selectivity towards dopamine in the presence of ascorbic acid and uric acid was examined. While both electrodes, PEDOT and PEDOT‐CNT are able to detect all measured concentrations of dopamine in the presence of uric acid, small concentrations of dopamine and ascorbic acid are only distinguishable at PEDOT‐CNT electrodes. Changing the pH has a strong influence on the selectivity. Moreover, it is possible to detect concentrations as low as 1 µM dopamine in complex cell culture medium. Finally, other catecholamines like serotonin, epinephrine, norepinephrine and L ‐dopa are also electrochemically detectable at PEDOT‐CNT microelectrodes.     

Synthesis,Structure, and Frequency‐Doubling Effect of Calcium Cyanurate

Calcium cyanurate is synthesized by reacting calcium chloride with potassium cyanate following a solid‐state reaction. The formation of the new compound Ca₃(O₃C₃N₃)₂ (CCY), which occurs by the cyclotrimerization of cyanate ions, was examined thermoanalytically and the crystal structure was determined by single‐crystal structure analysis. The structure of CCY is closely related to the structure of the well‐known oxoborate β‐BaB₂O₄ (BBO). Second harmonic generation (SHG) measurements on crystal powders show a higher SHG efficiency for CCY than for BBO by about one order of magnitude.     

Effect of microfibril twisting on theoretical powder diffraction patterns of cellulose Iβ

Previous studies of calculated diffraction patterns for cellulose crystallites suggest that distortions that arise once models have been subjected to molecular dynamics (MD) simulation are the result of both microfibril twisting and changes in unit cell dimensions induced by the empirical force field; to date, it has not been possible to separate the individual contributions of these effects. To provide a better understanding of how twisting manifests in diffraction data, the present study demonstrates a method for generating twisted and linear cellulose structures that can be compared without the bias of dimensional changes, allowing assessment of the impact of twisting alone. Analysis of unit cell dimensions, microfibril volume, hydrogen bond patterns, glycosidic torsion angles, and hydroxymethyl group orientations confirmed that the twisted and linear structures collected with this method were internally consistent, and theoretical powder diffraction patterns for the two were shown to be effectively indistinguishable. These results indicate that differences between calculated patterns for the crystal coordinates and twisted structures from MD simulation can result entirely from changes in unit cell dimensions, and not from microfibril twisting. Although powder diffraction patterns for models in the 81-chain size regime were shown to be unaffected by twisting, suggesting that a modest degree of twist is not inconsistent with available crystallographic data, it may be that other diffraction techniques are capable of detecting this structural difference. Until such time as definitive experimental evidence comes to light, the results of this study suggest that both twisted and linear microfibrils may represent an appropriate model for cellulose Iβ.     

Chromophores in cellulosics,XI: isolation and identification of residual chromophores from bacterial cellulose

In the present work, bacterial cellulose (BC) was analyzed for its chromophore content with the chromophore release and identification (CRI) method. In aged BC, seven chromophores were unambiguously identified, despite their very low (ppb) presence. The compounds contain 2-hydroxy-[1,4]benzoquinone, 2-hydroxyacetophenone and 5,8-dihydroxy-naphthoquinone moieties. These three classes of key chromophores had also been found in other celluloses such as bleached pulp, fiber and derivatives. The detection of chromophores in lignin-free BC confirms that the chromophores are formed from oxidized and degraded carbohydrate material rather than lignin fragments. Samples that were insufficiently purified before the aging step also yielded three N-heterocyclic compounds. They originated from tryptophan moieties in residual adhering protein and protein degradation products. N-Heteropentacycles 9 and 10 showed strong adsorption onto cellulosic surfaces, and that adsorption aided oxidation of cyclohexadiene derivative 9 into the corresponding aromatic and intensely yellow system 10, consistent with the strain-induced bond localization theory. In purified fresh BC, no chromophores were detectable by the CRI method. The present contribution is the first report on defined chromophoric structures isolated from BC.     

Monitoring of potential cytotoxic and inhibitory effects of titanium dioxide using on-line and non-invasive cell-based impedance spectroscopy

Titanium dioxide (TiO₂) nanoparticles (NPs) with different sizes and structures were probed for plausible cytotoxicity using electric cell-substrate impedance sensing (ECIS), a non-invasive and on-line procedure for continuous monitoring of cytotoxicity. For insect cells (Spodoptera frugiperda Sf9), the ECIS₅₀ values, i.e., the concentration required to achieve 50% inhibition of the response, differed depending on the size and shape of the TiO₂ nanostructure. The lowest ECIS₅₀ value (158 ppm) was observed for the needle shaped rutile TiO₂ (10 nm × 40 nm, 15.5 nm nominal particle size), followed by 211 ppm for P-25 (34.1 nm, 80% anatase and 20% rutile), 302 ppm for MTI5 (5.9 nm, 99% anatase) and 417 ppm for Hombitan LW-S bulk TiO₂ (169.5 nm, 99% anatase). Exposure of TiO₂ NPs to UV light at 254 nm or 365 nm exhibited no significant effect on the ECIS₅₀ value due to the aggregation of TiO₂ NPs with diminishing photocatalytic activities. Chinese hamster lung fibroblast V79 cells, exhibited no significant cytotoxicity/inhibition up to 400 ppm with P25, MTI₅ and bulk TiO₂. However, a noticeable inhibitory effect was observed (ECIS₅₀ value of 251 ppm) with rutile TiO₂ as cell spreading on the electrode surface was prevented     

The Reaction of Heptachlor on Alumina Impregnated with Sulfuric Acid. A Ready Synthesis of 3-Exophenylchlordene

When heptachlor (la) (I-exo-3,7,8,9,10,10-heptachloro-endo-tricyclo-(5,2,1,0,2,6)-deca-4,8 diene) in benzene is treated with an alumina/sulfuric acid solid matrix up to 90% of 3-exo-phenylchlordene (1b) is obtained as the only isolable product. This easy and efficient route to substituted chlordenes suggest the possibility of incorporating photochemically degradable residues onto the heptachlor skeleton.     

Some remarks on Richardson orbits in complex symmetric spaces

Roger W. Richardson proved that any parabolic subgroup of a complex semisimple Lie group admits an open dense orbit in the nilradical of its corresponding parabolic subalgebra. In the case of complex symmetric spaces we show that there exist some large classes of parabolic subgroups for which the analogous statement which fails in general, is true. Our main contribution is the extension of a theorem of Peter E. Trapa (in 2005) to real semisimple exceptional Lie groups.