Inhomogeneity of electron injection rates in dye-sensitized TiO2: comparison of the mesoporous film and single nanoparticle behavior

As it has been shown by pump-probe experiments electron injection at the interface between a dye molecule and mesoporous TiO2 proceeds with rates exceeding 1 x 10(13) s(-1). However, similar dye-TiO2 systems exhibit residual dye emission with lifetimes extending into the long nanosecond range. To address this inhomogeneity of injection rates time-correlated single photon counting microscopy was used to compare the emission behavior of dye-sensitized mesoporous films of TiO2 with that of individual anatase nanoparticles that had undergone extensive dialysis. The sensitized films produce intense residual emission with multiexponential decay components as long as 220 ns. The channels of mesoporous films contain physisorbed and trapped dye, which is the dominant source of the emission. It is likely that the wide range of lifetimes reflects the distribution of mean free paths experienced by the loose dye molecules diffusing within the film prior to undergoing oxidative quenching. In contrast, the intensity of emission from individual nanoparticles from which the loose dye was removed by dialysis is orders of magnitude lower. The lifetimes obtained from such particles are much shorter, with the primary component on a sub-nanosecond time scale. The presence of residual emission with a 230 ps lifetime shows that even on the surfaces of dialyzed nanoparticles there is a fraction of sensitizer molecules that do not inject electrons with the same high rate as is observed in ultrafast pump-probe experiments on films. Since the physisorbed dye was removed from these samples by dialysis, the residual emission is likely to originate from dye molecules bound to surface defects. Unusual collective emission bursts were observed in some of the measurements on sensitized nanoparticles. We attribute this behavior to stimulated emission from individual nanocrystallites.     

A biohybrid dynamic random access memory

We report that GABA(A) receptors in a patch-clamped biological cell form a short-term memory circuit when integrated with a scanning-probe microfluidic device. Laminar patterns of receptor activators (agonists) provided by the microfluidic device define and periodically update the data input which is read and stored by the receptors as state distributions (based on intrinsic multistate kinetics). The memory is discharged over time and lasts for seconds to minutes depending on the input function. The function of the memory can be represented by an equivalent electronic circuit with striking similarity in function to a dynamic random access memory (DRAM) used in electronic computers. Multiplexed biohybrid memories may form the basis of large-scale integrated biocomputational/sensor devices with the curious ability to use chemical signals including odorants, neurotransmitters, chemical and biological warfare agents, and many more as input signals.     

Absence of reverse anomeric effect in furanosides

A series of conformationally restricted N-"furanosides" has been synthesized, where the carbons of the tetrahydrofuran ring are kept in one plane by a rigid norbornane skeleton, permitting only the ring oxygen to move above or below the tetrahydrofuran ring plane. This causes the substituents of the anomeric carbon to occupy a pseudoaxial or a pseudoequatorial position. On protonation of these "norbornane-furanosides" with trifluoromethanesulfonic acid, all three compounds exhibited decreasing coupling constants for the anomeric proton, indicating a shift toward the pseudoaxial conformation. The coupling constant measurements were supported by volume integration of NOESY cross-peaks, which also showed a change toward the pseudoaxial conformation upon protonation of the nitrogen. These results provide no evidence for the so-called reverse anomeric effect; on the contrary they are in full agreement with a small normal anomeric effect.     

Photodegradation of 5-methyltetrahydrofolate: biophysical aspects

5-methyltetrahydrofolate (5MTHF) absorbs UV radiation and has an absorption coefficient of 24250+/-1170 M(-1) cm(-1) at 290 nm. It has a weak fluorescence emission in the wavelength region around 360 nm. Our data demonstrated induction of 5-methyldihydrofolate by exposure to UVB and, after continues irradiation, p-aminobenzoyl-L-glutamic acid was found. The photodegradation of 5MTHF follows a first order kinetic with a degradation rate constant of 9.2 x 10(-3) min(-1) under our conditions (fluence rate of 2.15 mW cm(-2), exposure wavelengths from 280 to 350 nm). Our results indicate that a direct degradation of 5MTHF by UV exposure in humans in vivo is rather unlikely. 5MTHF mainly absorbs, and is degraded by, UVB and UVC, radiation that does not penetrate the earth's atmosphere and the human skin well.     

Rhodium-Catalyzed Asymmetric Conjugate Additions of Boronic Acids to enones using DIPHONANE: a novel chiral bisphosphine ligand

[reaction: see text] The synthesis of a novel enantiopure C2-symmetric bisphosphine, DIPHONANE, was accomplished starting from 2,5-norbornadione, utilizing (R,R)- and/or (S,S)-(2,3-O-di[(phenylamino)carbonyl]tartaric acid for the resolution of an intermediate phosphineoxide. The application of this ligand in the rhodium-catalyzed asymmetric conjugate addition of boronic acids to cyclic enones provides the 1,4-addition products in good yields (69-98%) and high ee's (78-95% ee). A byproduct arising from a consecutive 1,4-addition and 1,2-addition was also observed.     

Green one-pot multicomponent synthesis of pyrrolidinones using planetary ball milling process under solvent-free conditions

This paper presents a novel and efficient protocol for the synthesis of pyrrolidinones using catalytic loading of 1,1'-butylenebis(3-sulfo-3H-imidazol-1-ium) hydrogen sulfate as a recyclable Brönsted acid ionic liquid through ball milling process at room temperature under solvent-free conditions. The developed method provides good to excellent yields of various pyrrolidinones in environmentally friendly conditions. Furthermore, this efficient protocol displays a combination of the synthetic advantage of one-pot multicomponent reaction with ecological benefits and convenience of a mechanochemical procedure.     

Spectral moments of polycyclic aromatic hydrocarbons. Solution of a kinetic problem

The relationship between the rate of supercritical fluid extraction of polycyclic aromatic hydrocarbons (PAHs) from coal-tar pitch and some topological invariants are examined. The aim is to explain the appearance of a minimum value on the activation energy/molar mass curve of the PAHs. Rate constants are approximated using spectral moment expansion. It is shown that the size of the PAHs determines more than 95% of the extraction rate. Activation energy for the extraction of the PAHs was found to increase with increasing molar mass. The appearance of a minimum value on the activation energy/molar mass curve is the consequence of experimental difficulties resulting from the tendency of lower members of the PAHs to sublime.