“Golden” Cascade Cyclization to Benzo[c]-Phenanthridines

Herein, we describe a gold-catalyzed cascade cyclization of Boc-protected benzylamines bearing two tethered alkyne moieties in a domino reaction initiated by a 6-endo-dig cyclization. The reaction was screened intensively, and the scope was explored, resulting in nine new Boc-protected dihydrobenzo[c]phenanthridines with yields of up to 98 %; even a π-extension and two bidirectional approaches were successful. Furthermore, thermal cleavage of the Boc group and subsequent oxidation gave substituted benzo[c]phenanthridines in up to quantitative yields. Two bidirectional approaches under the optimized conditions were successful, and the resulting π-extended molecules were tested as organic semiconductors in organic thin-film transistors.     

Tetracyanoquinodimethane Reduction by Complexed Guanidinyl‐Functionalized Aromatic Compounds

In this work, we report on the reduction of tetracyanoquinodimethane (TCNQ) with dicationic complexes of guanidinyl‐functionalized aromatic (GFA) electron donors. In contrast to reduction with free GFAs, milder reduction conditions were achieved, and this led to semiconducting materials with extended TCNQ π stacking. The charge on the TCNQ units was estimated from the structural data obtained by single‐crystal X‐ray diffraction analysis and from IR spectroscopic data. The electrical conductivity was studied and the activation energy of the semiconducting materials was estimated from the temperature dependence of the conductivity.     

Heterogeneous freezing on pyroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin films

Active deicing of technical surfaces, such as for wind turbines and heat exchangers, currently requires the usage of heat or chemicals. Passive coating strategies that postpone the freezing of covering water would be beneficial in order to save costs and energy. One hypothesis is that pyroelectric active materials can achieve this because of the surface charges generated on these materials when they are subject to a temperature change. High-quality poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films with a high crystallinity, prefererd edge-on orientation, low surface roughness, and comprised of the β-analogous ferroelectric phase were deposited by spin-coating. Freezing experiments with a cooling rate of 1 K min⁻¹ were made on P(VDF-TrFE) coatings in order to separate the effect of different parameters such as the poling direction, film thickness, used solvent, deposition process, underlying substrate, and annealing temperature on the achievable supercooling. The topography and the underlying substrate significantly changed the distribution of freezing temperatures of water droplets in contact with these thin films. In contrast, no significant effect of the thickness, morphology, or pyroelectric effect of the as-prepared domain-state on the freezing temperatures was found.     

Identification of Semiconductive Patches in Thermally Processed Monolayer Oxo‐Functionalized Graphene

The thermal decomposition of graphene oxide (GO) is a complex process at the atomic level and not fully understood. Here, a subclass of GO, oxo‐functionalized graphene (oxo‐G), was used to study its thermal disproportionation. We present the impact of annealing on the electronic properties of a monolayer oxo‐G flake and correlated the chemical composition and topography corrugation by two‐probe transport measurements, XPS, TEM, FTIR and STM. Surprisingly, we found that oxo‐G, processed at 300 °C, displays C?C sp³‐patches and possibly C?O?C bonds, next to graphene domains and holes. It is striking that those C?O?C/C?C sp³‐separated sp²‐patches a few nanometers in diameter possess semiconducting properties with a band gap of about 0.4 eV. We propose that sp³‐patches confine conjugated sp²‐C atoms, which leads to the local semiconductor properties. Accordingly, graphene with sp³‐C in double layer areas is a potential class of semiconductors and a potential target for future chemical modifications.     

Preparation and properties of isotactic and syndiotactic poly(methyl methacrylate)s,uniform with respect to molecular weight

Highly isotactic (it-) and highly syndiotactic (st-) poly(methyl methacrylate)s (PMMAs) uniform with respect to molecular weight (stereoregular, uniform PMMAs) were isolated up to the 100mer from it- and st-PMMAs by supercritical fluid chromatography (SFC) and characterized by NMR and mass spectroscopies. Glass transition temperatures (Tg's) of the uniform it- and st-PMMAs were higher than those of the corresponding PMMAs with MWD and with similar molecular weight on average. The Tg values of the uniform it-and st-PMMAs series obeyed the relationship, Tg = Tg∞ - K/M, where M and Tg∞ denote molecular weight and Tg at infinite M, respectively. The reciprocal melting points (1/Tm) of uniform it-PMMA (degree of polymerization, DP = 28–44), obtained from methanol solutions by evaporating the solvent, increased linearly with increasing 1/DP. Extrapolation of the linear relation to 1/DP = 0 gave the Tm of it-PMMA with infinite DP; Tm∞ = 171.1°C. Thermal degradation behavior was studied by thermogravimetry and by SFC analysis of degradation products. In gel-permeation chromatography (GPC) measurements, the it-50mer eluted faster than the st-50mer. Calibration curves for it- and st-PMMAs could be obtained using series of the uniform PMMAs. The instrumental spreading factor determined using it-25mer, it-50mer, st-25mer and st-50mer for our GPC chromatograph was 0.070 ml when the volume of the sample solution was 0.003ml. GPC analysis of a mixture of the it- and st-50mers in tetrahydrofuran indicated formation of a stereocomplex in the solution, and was found quite useful to elucidate the minimum sequence length required for complex formation.     

CE characterization of semiconductor nanocrystals encapsulated with amorphous silicium dioxide

The electrophoretic mobility of silica-encapsulated semiconductor nanocrystals (quantum dots) dependent on the pH and the ionic strength of the separation electrolyte has been determined by CE. Having shown the viability of the approach, the electrophoretic mobility mu of the nanoparticles investigated is calculated for varied zeta potential zeta, particle radius r, and ionic strength I employing an approximate analytical expression presented by Ohshima (J. Colloid Interface Sci. 2001, 239, 587-590). The comparison of calculated with measured data shows that the experimental observations exactly follow what would be expected from theory. Within the parameter range investigated at fixed zeta and I there is an increase in mu with r which is a nonlinear function. This dependence of mu on size parameters can be used for the size-dependent separation of particles. Modeling of mu as function of I and zeta makes it possible to calculate the size distribution of nanoparticles from electrophoretic data (using the peak shape of the particle zone in the electropherogram) without the need for calibration provided that zeta is known with adequate accuracy. Comparison of size distributions calculated via the presented method with size histograms determined from transmission electron microscopy (TEM) micrographs reveals that there is an excellent matching of the size distribution curves obtained with the two independent methods. A comparison of calculated with measured distributions of the electrophoretic mobility showed that the observed broad bands in CE studies of colloidal nanoparticles are mainly due to electrophoretic heterogeneity resulting from the particle size distribution.     

Light emission from nanocrystalline silicon clusters embedded in silicon dioxide: Role of the suboxide states

Silicon clusters embedded in a silicon dioxide matrix were prepared by ultrasound-assisted implantation resulting in a modified concentration of suboxide states as revealed by high-resolution photoelectron spectroscopy. It is suggested that ultrasound treatment results in formation of different interface structure between silicon cluster and silicon dioxide matrix which is characterized by a distinctly reduced concentration of the suboxide states. It is observed that photoluminescence properties are strongly correlated with the concentration of the suboxide states thereby providing an evidence that besides a quantum confinement effect a closer look at the chemical composition of the nc-Si/SiO₂ system is important.     

Systematic optimisation of high-performance liquid chromatographic separation by varying the temperature, gradient, and stationary phase

Optimisation of the separation of a synthetic drug mixture by HPLC is performed by changing both continuous variables, i.e. mobile phase composition and temperature, and categorical variables, here the stationary phase. The retention of solutes is described on the basis of a general linear model in which the different columns are modelled by indicator variables. From the solute-specific retention models the global separation optimum is evaluated on the basis of multidimensional window diagrams using relative retentions of all peak pairs as the figure-of-merit.