Thiophene- and selenophene-based heteroacenes: combined quantum chemical DFT and spectroscopic Raman and UV-Vis-NIR study

In this article, we report the characterization of a series of thiophene- and selenophene-based heteroacenes, materials with potential applications in organic electronics. In contrast to the usual alpha-oligothiophenes, these annelated oligomers have a larger band gap than most semiconductors currently used in the fabrication of organic field-effect transistors (OFETs) and therefore they are expected to be more stable in air. The synthesis of these fused-ring molecular materials was motivated by the notion that a more rigid and planar structure should reduce defects (such as torsion about single bonds between alpha-linked units or S-syn defects) and thus improve pi-conjugation for better charge-carrier mobility. The conjugational properties of these heteroacenes have been investigated by means of FT-Raman spectroscopy, revealing that pi-conjugation increases with the increasing number of annelated rings. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G** level, to assess information regarding the minimum-energy molecular structure, topologies, and absolute energies of the frontier molecular orbitals around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.     

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite-water interface

The surface properties of a well-crystallized synthetic goethite have been studied by acid-base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3+/-0.1 and 9.3+/-0.2, respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO(3) solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite-water interface.     

Understanding the Driving Mechanisms of Enhanced Luminescence Emission of Oligo(styryl)benzenes and Tri(styryl)-s-triazine

This work is focused on unraveling the mechanisms responsible for the aggregation-induced enhanced emission and solid-state luminescence enhancement effects observed in star-shaped molecules based on 1,3,5-tris(styryl)benzene and tri(styryl)-s-triazine cores. To achieve this, the photophysical properties of this set of molecules were analyzed in three states: free molecules, molecular aggregates in solution, and the solid state. Different spectroscopy and microscopy experiments and DFT calculations were conducted to scrutinize the causative mechanisms of the luminescence enhancement phenomenon observed in some experimental conditions. Enhanced luminescence emission was interpreted in the context of short- and long-range excitonic coupling mechanisms and the restriction of intramolecular vibrations. Additionally, we found that the formation of π-stacking aggregates could block E/Z photoisomerization through torsional motions between phenylene rings in the excited state, and hence, enhancing the luminescence of the system.     

Electrochemical studies of nafion–trimethylsilyl and nafion–trimethylsilyl/Ru complex-modified electrodes

In this work, we report the electrochemical properties of the nafion–trimethylsilyl (Naf–TMS) polymer. First, we introduce a procedure to dissolve Naf–TMS polymer and the incorporation of ruthenium catalyst complexes into it. The inclusion of the catalysts involved two strategies. The first one concerned the direct formation of a Naf–TMS/Ru complex solution. The second one consists of depositing Naf–TMS solution on a glassy carbon electrode, followed by the incorporation of Ru complexes under potentiodynamic conditions. Electrochemical studies showed the good ion permeation capability of Naf–TMS membranes and its use as a good alternative approach to Nafion ion-conducting membranes. The analytical capabilities of Naf–TMS- and Naf–TMS/Ru-modified glassy carbon electrodes have been tested for the detection of dopamine in standard solutions. Detection limits in the order of nanomolar have been achieved with working ranges extending over three decades in concentration at pH 7.2. Further enhancement in the dopamine oxidation current was achieved by the incorporation of Ru complexes into the Naf–TMS polymer. This study offers a new insight into the investigation of Naf–TMS resin as an ion-conducting polymer.     

Aporphine alkaloid contents increase with moderate nitrogen supply in Annona diversifolia Saff. (Annonaceae) seedlings during diurnal periods

Aporphine alkaloids are secondary metabolites that are obtained in low levels from species of the Annonaceae family. Nitrogen addition may increase the alkaloid content in plants. However, previous studies published did not consider that nitrogen could change the alkaloid content throughout the day. We conducted this short-term study to determine the effects of nitrogen applied throughout the diurnal period on the aporphine alkaloids via measurements conducted on the roots, stems and leaves of Annona diversifolia seedlings. The 60-day-old seedlings were cultured with the addition of three levels of nitrogen (0, 30 and 60 mM), and alkaloid extracts were analysed using high-performance liquid chromatography. The highest total alkaloid content was measured in the treatment with moderate nitrogen supply. Further, the levels of aporphine alkaloids changed significantly in the first few hours of the diurnal period. We conclude that aporphine alkaloid content increased with moderate nitrogen supply and exhibited diurnal variation.     

A Versatile Synthesis of β‐Lactam‐Fused Oxacycles through the Palladium‐Catalyzed Chemo‐, Regio‐, and Diastereoselective Cyclization of Allenic Diols

Chemo‐, regio‐ and stereocontrolled palladium‐catalyzed preparations of enantiopure morpholines, oxocines, and dioxonines have been developed starting from 2‐azetidinone‐tethered γ,δ‐, δ,ε‐, and ε,ζ‐allendiols. The palladium‐catalyzed cyclizative coupling reaction of γ,δ‐allendiols 2 with allyl bromide or lithium bromide was effective as 8‐endo cyclization by attack of the primary hydroxy group to the terminal allene carbon to afford enantiopure functionalized oxocines; whereas the palladium‐catalyzed cyclizative coupling reaction of 2‐azetidinone‐tethered ε,ζ‐allendiols 4 furnished dioxonines 16 through a totally chemo‐ and regioselective 9‐endo oxycyclization. By contrast, the palladium‐catalyzed cyclizative coupling reaction of 2‐azetidinone‐tethered δ,ε‐allendiols 3 with aryl and alkenyl halides exclusively generated six‐membered‐ring compounds 14 a and 15 a . These results could be explained through a 6‐exo cyclization by chemo‐ and regiospecific attack of the secondary hydroxy group to the internal allene carbon. Chemo‐ and regiocontrol issues are mainly influenced by the length of the tether rather than by the nature of the metal catalysts and substituents. This reactivity can be rationalized by means of density functional theory calculations.     

sp2‐Iminosugar O‐, S‐, and N‐Glycosides as Conformational Mimics of α‐Linked Disaccharides; Implications for Glycosidase Inhibition

The synthesis of mimics of the α(1→6)‐ and α(1→4)‐linked disaccharides isomaltose and maltose featuring a bicyclic sp²‐iminosugar nonreducing moiety O‐, S‐, or N‐linked to a glucopyranoside residue is reported. The strong generalized anomeric effect operating in sp²‐iminosugars determines the α‐stereochemical outcome of the glycosylation reactions, independent of the presence or not of participating protecting groups and of the nature of the heteroatom. It also imparts chemical stability to the resulting aminoacetal, aminothioacetal, or gem‐diamine functionalities. All the three isomaltose mimics behave as potent and very selective inhibitors of isomaltase and maltase, two α‐glucosidases that bind the parent disaccharides either as substrate or inhibitor. In contrast, large differences in the inhibitory properties were observed among the maltose mimics, with the O‐linked derivative being a more potent inhibitor than the N‐linked analogue; the S‐linked pseudodisaccharide did not inhibit either of the two target enzymes. A comparative conformational analysis based on NMR and molecular modelling revealed remarkable differences in the flexibility about the glycosidic linkage as a function of the nature of the linking atom in this series. Thus, the N‐pseudodisaccharide is more rigid than the O‐linked derivative, which exhibits conformational properties very similar to those of the natural maltose. The analogous pseudothiomaltoside is much more flexible than the N‐ or O‐linked derivatives, and can access a broader area of the conformational space, which probably implies a strong entropic penalty upon binding to the enzymes. Together, the present results illustrate the importance of taking conformational aspects into consideration in the design of functional oligosaccharide mimetics.