Fixing the Chirality and Trapping the Transition State of Helicene with Atomic Metal Glue

By combining the intriguing geometrical properties of two classes of well‐established molecules, the metallocenes and the helicenes, we propose a hybrid class of structures—the metallohelicenes. In these, the outer most aryl groups of a specific helicene are glued together by a complexing metal atom. This effectively fixes the chirality of the parent helicene, which otherwise easily undergoes thermal racemization. The fixed chirality suggests several interesting applications, ranging from building blocks of stable molecules with high circular dichroism and optical activity to chiral ligands and catalysts. Alternatively, the metal glue can trap the non‐chiral transition state structure of helicene. High‐level quantum chemical calculations show the readiness of formation and stability of the proposed complexes.     

Combined vehicle routing and scheduling with temporal precedence and synchronization constraints

We present a mathematical programming model for the combined vehicle routing and scheduling problem with time windows and additional temporal constraints. The temporal constraints allow for imposing pairwise synchronization and pairwise temporal precedence between customer visits, independently of the vehicles. We describe some real world problems where in the literature the temporal constraints are usually remarkably simplified in the solution process, even though these constraints may significantly improve the solution quality and/or usefulness. We also propose an optimization based heuristic to solve real size instances. The results of numerical experiments substantiate the importance of the temporal constraints in the solution approach. We also make a computational study by comparing a direct use of a commercial solver against the proposed heuristic, where the latter approach can find high quality solutions within specific time limits.     

Comprehensive two-dimensional gas chromatography of the 209 polychlorinated biphenyls

Comprehensive two-dimensional gas chromatography (GC x GC) of the 209 polychlorinated biphenyls (CBs) was carried out using a longitudinally modulated cryogenic system (LMCS) and liquid carbon dioxide as cryogen. The effluent from a non-polar column was modulated and further separated on either a polar or a shape-selective second-dimension column. Five GC x GC column combinations were evaluated, with DB-XLB as the first column in each case. DB-XLB separates more congeners than any other GC column currently available. When combined with a biscyanopropyl siloxane (SP-2340 or BPX70) or smectic liquid crystal (LC-50) second-dimension column in a GC x GC system many additional CBs can be separated. In total, 176 and 181 of the 209 congeners were separated (Rs = 0.5) on the column combinations DB-XLB/SP-2340 and DB-XLB/LC-50, respectively. Of the 136 CBs present in any Aroclor mixture at concentrations greater than 0.05% (w/w), 126 were resolved using either of these two column combinations. The seven frequently measured CBs 28, 52, 101, 118, 138, 153, 180, and the WHO-PCBs 77, 81, 105, 114, 118, 123, 126, 156. 157, 167, 169 and 189 were all separated from Aroclor CBs on the DB-XLB/LC-50 column set, whereas CBs 118 and 131 coeluted on the DB-XLB/SP-2340 column set. In addition, three technical CB formulations (Aroclors 1232, 1248 and 1260) and a seal blubber sample (Halichoerus grypus) from the Baltic Sea were analyzed. Similar peak patterns were found for Aroclor 1260 and the CBs in the seal blubber, facilitating use of this technical CB formulation to identify the CBs in the blubber by GC x GC. Individual CBs in environmental samples, such as seal blubber, may be identified semi-automatically by matching the samples GC x GC profiles to a template generated using a standard containing all 209 CBs. Using such a template, 64 CBs were identified in the grey seal blubber sample.     

All alkoxide route to manganate and coboltate perovskite films and powders: effects of processing parameters

Manganate and cobaltate perovskites having the general formula ABO₃ have many technologically important applications. Here we present all alkoxide based routes to manganate and coboltate perovskite films and nano-phase powders of the compositions; LCMO (La_0.67Ca_0.33MnO₃), LSMO (La_0.75Sr_0.25MnO₃), LNCMO (La_0.33Nd_0.33Ca_0.33MnO₃), LBSMO (La_0.75Ba_0.125Sr_0.125MnO₃) and LSCO (La_0.50Sr_0.50CoO₃). The Mn and Co (oxo) methoxy-ethoxide precursors were prepared by reaction of MnCl₂ or CoCl₂ with 2 Kmoe in methoxy-ethanol-toluene. After hydrolysis of the alkoxide solutions by atmospheric air all systems produced X-ray and electron diffraction amorphous gels of high elemental homogeneity, and the IR spectra showed that they consisted of hydrated oxo-carbonates. Heating in air resulted in similar weight-loss curves for all studied gels passing; loss of H₂O in the range 20–300 °C, decomposition of carbonate groups into oxide and CO₂ in the temperature range 300–700 °C, and in some cases loss of a small amount of oxygen in the temperature range 700–1,000 °C. The pure perovskites were obtained at 690–770 °C with heat rates of typically 5–20 °C min^?1 without annealing. Perovskites could also be obtained at 550 °C by annealing, but these perovskites are prone to be A-site ion inhomogeneous according to the TEM EDS studies, which was not the case for the perovskites heat-treated to at least the carbonate decomposition temperature. This A-site inhomogeneity is ascribed to sequential decomposition of carbonates due to their different thermal stabilities, which is probably a general feature also with other sol–gel precursors and low temperature annealing. High quality polycrystalline films were prepared on Si/SiO₂/TiO₂/Pt substrates with all compositions and high quality epitaxial films were prepared of LCMO (on 100 LaAlO₃) and LSCO (on 100 SrTiO₃). The colossal magneto resistance (CMR) of the epitaxial LCMO films of 32% (246 K) is in parity with PLD derived films. The conductivities of the epitaxial and polycrystalline LSCO films were 1.9 mΩcm (on STO) and 1.7 mΩcm (on α-Al₂O₃, respectively).     

Triplet‐State Aromaticity of 4nπ‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function

The π contribution to the electron localization function (ELF) is used to compare 4nπ‐ and (4n+2)π‐electron annulenes, with particular focus on the aromaticity of 4nπ‐electron annulenes in their lowest triplet state. The analysis is performed on the electron density obtained at the level of OLYP density functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all‐carbon annulenes are set up: the span in the bifurcation values ΔBV(ELF_π) should be small, ideally zero, and the bifurcation value for ring closure of the π basin RCBV(ELF_π) should be high (≥ 0.7). On the basis of these criteria, nearly all 4nπ‐electron annulenes are aromatic in their lowest triplet states, similar to (4n+2)π‐electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D_4h and D_8h symmetry, respectively, the RCBV(ELF_π) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of α‐ and β‐electrons.     

Deciphering the Electronic Transitions of Thiophene-Based Donor-Acceptor-Donor Pentameric Ligands Utilized for Multimodal Fluorescence Microscopy of Protein Aggregates

Anionic pentameric thiophene acetates can be used for fluorescence detection and diagnosis of protein amyloid aggregates. Replacing the central thiophene unit by benzothiadiazole (BTD) or quinoxaline (QX) leads to large emission shifts and basic spectral features have been reported [Chem. Eur. J. 2015 , 21, 15133-13137]. Here we present new detailed experimental results of solvent effects, time-resolved fluorescence and examples employing multi-photon microscopy and lifetime imaging. Quantum chemical response calculations elucidate how the introduction of the BTD/QX groups changes the electronic states and emissions. The dramatic red-shift follows an increased conjugation and quinoid character of the π-electrons of the thiophene backbone. An efficient charge transfer in the excited states S₁ and S₂ compared to the all-thiophene analogue makes these more sensitive to the polarity and quenching by the solvent. Taken together, the results guide in the interpretation of images of stained Alzheimer disease brain sections employing advanced fluorescence microscopy and lifetime imaging, and can aid in optimizing future fluorescent ligand development.     

Electrochemical detection of microcystins, cyanobacterial peptide hepatotoxins, following high-performance liquid chromatography

A novel amperometric HPLC detection method for the cyanobacterial (blue–green algal) peptide toxins microcystin-LR, -YR and -RR was developed. Purified microcystins and cyanobacterial extracts were chromatographed using an internal surface reversed-phase column with acetate- and phosphate-based mobile phase systems. Electrochemical oxidation reactions at 1.20 V vs. Ag/AgCl (glassy carbon working electrode) were shown to originate in arginine and tyrosine residues of microcystins.     

The primary electroviscous effect of prolate silica sols

The intrinsic viscosity and the dynamic mobility of four silica sols have been measured as a function of the ionic strength. It was found that intrinsic viscosity decreased with increasing ionic strength, which we attribute to the primary electroviscous effect. The geometry and the charge of the particles were fitted using experimental viscosity, light scattering, and dynamic mobility data, where the intrinsic viscosity measured at the highest ionic strength for a given sol was used as input data in our analysis. Further, the boundary element (BE) method was used to calculate the primary electroviscous effect and electrophoretic mobility of charged prolate ellipsoids. These calculations were then compared with experimental data, and the primary electroviscous effect was subtracted from the intrinsic viscosity at a given ionic strength, which led to a slightly altered geometry of the particles. This revised geometry was used as input data using the BE method, and the procedure was repeated iteratively until agreement was obtained at high ionic strength. In general, good agreement between theory and experiment was found.     

Electron and energy transfer in donor-acceptor systems with conjugated molecular bridges

Electron and energy transfer reactions in covalently connected donor-bridge-acceptor assemblies are strongly dependent, not only on the donor-acceptor distance, but also on the electronic structure of the bridge. In this article we describe some well characterised systems where the bridges are pi-conjugated chromophores, and where, specifically, the interplay between bridge length and energy plays an important role for the donor-acceptor electronic coupling. For any application that relies on the transport of electrons, for example molecule based solar cells or molecular scale electronics, it will be imperative to predict the electron transfer capabilities of different molecular structures. The potential difficulties with making such predictions and the lack of suitable models are also discussed.     

Fulvenes, fulvalenes, and azulene: are they aromatic chameleons?

On the basis of the theory of Baird on reversal of Hückel's rule for aromaticity and antiaromaticity of annulenes when going from the electronic ground state (S0) to the lowest pipi* triplet state (T1) (J. Am. Chem. Soc. 1972, 94, 4941), we argue that fulvenes, fulvalenes, and azulene are "aromatic chameleons". The dipole moments of fulvenes in T1 should be of comparable magnitude to those of S0, but due to the reversal of Hückel's aromaticity rule in T1, their dipole should be in the opposite direction. Thereby, they are capable of adopting some aromaticity in both the T1 and S0 states as they adapt their dipolar resonance structures. The same applies to fulvalenes and azulene in their lowest quintet states (Q1) when compared to S0. Our hypothesis on chameleon behavior is supported by quantum chemical OLYP, CASSCF, and CASPT2 calculations of dipole moments, pi-orbital populations, and energies.