Precise Control of Intramolecular Charge‐Transport: The Interplay of Distance and Conformational Effects

A new series of donor–bridge–acceptor (D–B–A) compounds consisting of π‐conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron‐donor and a fullerene (C₆₀) electron‐acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono‐ and bi‐fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p‐phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy‐matching between the Fc and the oFL units, which results in strong electronic‐coupling. Hence, intramolecular charge‐transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc–oFL–C₆₀ conjugates results in transient radical‐ion‐pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge‐separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge‐separation rates, resulting in beta values between β_CS 0.08 and 0.19 Å^?1. In contrast, charge recombination depends strongly on the electron‐donor–acceptor distance, but not at all on the linker. A value of β_CR (0.35±0.01 Å^?1) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D–B–A systems.     

Application of 2,4,6-trioxo-pyrimidin-5-ylidene alditols in the synthesis of pyrano[2,3-d]pyrimidines containing a sugar moiety by hetero-Diels–Alder reactions and by conjugate Michael addition–cyclizations

A convenient and efficient procedure for the preparation of 3,4-dihydro-2H-pyran derivatives containing a sugar moiety is described. The reaction sequence is: Knoevenagel condensation of unprotected sugars and CH acids, acetylation of C-glycosides and hetero-Diels–Alder reaction. O-Acetylated 1,3-dimethyl-2,4,6-trioxo-pyrimidin-5-ylidene derivatives were used as new heterodienes in the synthesis of fused uracils—pyrano[2,3-d]pyrimidines with a sugar moiety. Solvent-free hetero-Diels–Alder cycloadditions of O-acetylated pyrimidin-5-ylidene alditols with enol ethers and cyclic enol ether were investigated at room temperature. New, enantiomerically pure cis and trans diastereoisomers of pyrano[2,3-d]pyrimidines and cis–cis, trans–cis diastereoisomers of pyrano[3′,2′:5,6]pyrano[2,3-d]pyrimidines with alditol moiety were obtained. The same pyrimidin-5-ylidene alditols underwent conjugate Michael addition–cyclizations with malononitrile at room temperature to afford optically active diastereoisomers of pyrano[2,3-d]pyrimidine-6-carbonitriles with a sugar moiety.     

Double Deposition and Stripping Steps for Trace Determination of Au(III) Using Anodic Stripping Voltammetry

A new way of decreasing the detection limit ‐ double deposition and stripping steps was proposed to determine trace amounts of gold(III) by anodic stripping voltammetry. Two carbon composite electrodes that differed drastically in their surface areas were used for the measurements. The calibration graph was linear from 1×10^?9 to 1×10^?8 mol L^?1 following deposition time of 300 s at the first and the second electrode. The detection limit was found to be 2.3×10^?10 and 1.4×10^?11 mol L^?1 for deposition time 600 and 2400 s, respectively. It is the lowest detection limit obtained so far for gold(III) determination in stripping voltammetry.     

Chiral dioxovanadium(V) complexes with single condensation products of 1,2-diaminocyclohexane and aromatic o-hydroxycarbonyl compounds: Synthesis,characterization, catalytic properties and structure

New dioxovanadium(V) complexes bearing tridentate products of single condensation of RR(−) and of SS(+)-1,2-diaminocyclohexane with salicylaldehyde and its derivatives, 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone, have been synthesized. The crystal structure of the {RR(−)-1-amino-2-N-[1′-(1″-oxido-κO-2″-naphthyl)ethylidene]aminocyclohexane-κ²N}dioxovanadium(V) hemihydrate hemiethanol solvate, determined by X-ray analysis, is characterized by trigonal bipyramidal coordination geometry of complex molecules which are linked to solvent molecules by hydrogen bonds to form chains. The cyclohexane ring is in chair conformation with a very small distortion towards half-chair and envelope forms. Complexes were characterized by UV–Vis, FTIR, ¹H, ¹³C, ⁵¹V NMR and CD spectroscopies. CD spectra of {RR(−)-1-amino-2-N-[(2′-oxido-κO-5′-nitrophenyl)methylene]aminocyclohexane-κ²N}dioxovanadium(V) and of R(−)-1,2-diaminopropane analogue do not bear mirror image relationship in contrast to V(IV and V), Cu(II) and Ni(II) complexes containing double condensed diamines in the same absolute configurations. ¹H and ¹³C NMR resonance signals of all complexes dissolved in DMSO were assigned. The complexes bearing the methoxy substituent in position 3 or 5 of the aryl group catalyse the oxidation of methyl phenyl sulfide by cumene hydroperoxide to the corresponding sulfoxide with reasonable enantiomeric excesses (19–23%).     

Statistics of the envelope of ultrasonic backscatter from human trabecular bone

The paper describes the investigations intended to compare the results of experimental measurements of backscattering properties of the trabecular bone with the results of computer simulations. Ultrasonic RF echoes were collected using two bone scanners operating at 0.58 and 1.3 MHz. The simulations of the backscattered RF echoes were performed using the scattering model of the trabecular bone that consisted of cylindrical and spherical elements uniformly distributed in water-like medium. For each measured or simulated RF backscatter the statistical properties of the signal envelope were determined. Experimental results suggest deviations of the backscattering properties from the Rayleigh distribution. The results of simulation suggest that deviation from Rayleigh distribution depends on the variation of trabeculae diameters and the number of thin trabeculae. Experimentally determined deviations corresponded well to the deviations calculated from simulated echoes assuming trabeculae thickness variation equaled to the earlier published histomorphometric study results.     

Modeling and analysis of multiple scattering of acoustic waves in complex media: application to the trabecular bone

The integral equations that describe scattering in the media with step-rise changing parameters have been numerically solved for the trabecular bone model. The model consists of several hundred discrete randomly distributed elements. The spectral distribution of scattering coefficients in subsequent orders of scattering has been presented. Calculations were carried on for the ultrasonic frequency ranging from 0.5 to 3 MHz. Evaluation of the contribution of the first, second, and higher scattering orders to total scattering of the ultrasounds in trabecular bone was done. Contrary to the approaches that use the μCT images of trabecular structure to modeling of the ultrasonic wave propagation condition, the 3D numerical model consisting of cylindrical elements mimicking the spatial matrix of trabeculae, was applied. The scattering, due to interconnections between thick trabeculae, usually neglected in trabecular bone models, has been included in calculations when the structure backscatter was evaluated. Influence of the absorption in subsequent orders of scattering is also addressed. Results show that up to 1.5 MHz, the influence of higher scattering orders on the total scattered field characteristic can be neglected while for the higher frequencies, the relatively high amplitude interference peaks in higher scattering orders clearly occur.     

The influence of oxidation with nitric acid on the preparation and properties of active carbon enriched in nitrogen

The effect of oxidation by 20% nitric acid on the properties and performance of active carbons enriched with nitrogen by means of the reaction with urea in the presence of air has been studied. The study has been made on demineralised orthocoking coal and the carbonisates obtained from it at 600 or 700 °C, subjected to the processes of nitrogenation, oxidation and activation with KOH in different sequences. The amount of nitrogen introduced into the carbon with the aid of urea has been found to depend on the stage at which the process of nitrogenation was performed. The process of oxidation of the demineralised coal and the active carbon obtained from the former has been found to favour nitrogen introduction into the carbon structure. In the process of nitrogenation of the carbonisates the amount of nitrogen introduced has inversely depended on the temperature of carbonisation. The modifications of the processes permitted obtaining materials of different textural parameters, different acid-base character of the surface and different iodine sorption capacity.     

Single Machine Scheduling with Major and Minor Setup Times: A Tabu Search Approach

The paper deals with a single machine scheduling problem in which jobs are grouped into families that require major setup times, and where jobs within families require for processing minor setup times. The former are sequence independent and the latter have special triangular structure. The problem is to find a partition of families into batches, sequences of jobs in particular batches and a sequence of batches which minimize a general regular cost function. The tabu search algorithm for finding near-optimal schedules is proposed and results of computational experiments are presented.     

Wave envelopes method for description of nonlinear acoustic wave propagation

A novel, free from paraxial approximation and computationally efficient numerical algorithm capable of predicting 4D acoustic fields in lossy and nonlinear media from arbitrary shaped sources (relevant to probes used in medical ultrasonic imaging and therapeutic systems) is described. The new WE (wave envelopes) approach to nonlinear propagation modeling is based on the solution of the second order nonlinear differential wave equation reported in [J. Wójcik, J. Acoust. Soc. Am. 104 (1998) 2654-2663; V.P. Kuznetsov, Akust. Zh. 16 (1970) 548-553]. An incremental stepping scheme allows for forward wave propagation. The operator-splitting method accounts independently for the effects of full diffraction, absorption and nonlinear interactions of harmonics. The WE method represents the propagating pulsed acoustic wave as a superposition of wavelet-like sinusoidal pulses with carrier frequencies being the harmonics of the boundary tone burst disturbance. The model is valid for lossy media, arbitrarily shaped plane and focused sources, accounts for the effects of diffraction and can be applied to continuous as well as to pulsed waves. Depending on the source geometry, level of nonlinearity and frequency bandwidth, in comparison with the conventional approach the Time-Averaged Wave Envelopes (TAWE) method shortens computational time of the full 4D nonlinear field calculation by at least an order of magnitude; thus, predictions of nonlinear beam propagation from complex sources (such as phased arrays) can be available within 30-60 min using only a standard PC. The approximate ratio between the computational time costs obtained by using the TAWE method and the conventional approach in calculations of the nonlinear interactions is proportional to 1/N2, and in memory consumption to 1/N where N is the average bandwidth of the individual wavelets. Numerical computations comparing the spatial field distributions obtained by using both the TAWE method and the conventional approach (based on a Fourier series representation of the propagating wave) are given for circular source geometry, which represents the most challenging case from the computational time point of view. For two cases, short (2 cycle) and long (8 cycle) 2 MHz bursts, the computational times were 10 min and 15 min versus 2 h and 8 h for the TAWE method versus the conventional method, respectively.     

Synthesis and Evaluation of a 2,11‐Cembranoid‐Inspired Library

The 2,11‐cembranoid family of natural products has been used as inspiration for the synthesis of a structurally simplified, functionally diverse library of octahydroisobenzofuran‐based compounds designed to augment a typical medicinal chemistry library screen. Ring‐closing metathesis, lactonisation and SmI₂‐mediated methods were exemplified and applied to the installation of a third ring to mimic the nine‐membered ring of the 2,11‐cembranoids. The library was assessed for aqueous solubility and permeability, with a chemical‐space analysis performed for comparison to the family of cembranoid natural products and a sample set of a screening library. Preliminary investigations in cancer cells showed that the simpler scaffolds could recapitulate the reported anti‐migratory activity of the natural products.