Vibrational circular dichroism studies of exceptionally strong chirality inducers in liquid crystals.

7,7'-Disubstituted 2,2'-methylenedioxy-1,1'-binaphthyls are highly efficient chirality inducers in nematic liquid crystals. The absolute configuration of these compounds is, however, hard to determine as they only crystallize as racemic mixtures. In this work a Vibrational Circular Dichroism (VCD) study is reported that provides an unambiguous determination of the absolute configuration of these compounds. An in-depth General Coupled Oscillator (GCO) analysis of the source of the VCD signal reveals that the unusual structure of these binaphthyl compounds inherently leads to strong and robust VCD bands. Combined with linear transit calculations, our VCD studies allow for the determination of key structural parameters.

Inverted Carbon Geometries: Challenges to Experiment and Theory.

Disproving a long C-C-bond textbook example: The reported 1.643 Å C-C bond in 5-cyano-1,3-dehydroadamantane was redetermined and "only" amounts to 1.584 Å. While this value is well reproduced with ab initio methods, some common DFT approaches perform poorly and are only consistent with CCSD(T)/cc-pVTZ optimizations for noninverted carbons. Large deviations from experiment were also found for other molecules with atypical electron density distributions, e.g., cubane, bicyclo[2.2.0]hexane, and bicyclo[2.1.0]- and bicyclo[1.1.1]pentane, thereby presenting challenging structures for some DFT implementations.

Crystal design using multipolar electrostatic interactions: A concept study for organic electronics.

Using a simple synthetic protocol, heterohexacene analogues with a quadrupolar distribution of partial charges are readily available. In contrast to most other acenes, these compounds crystallize with a slipped-stack, brickwork-like packing which is mainly controlled by electrostatic interactions. This type of packing offers an advantage for organic semiconductors, because it allows more isotropic charge transport compared to the "herring bone" stacking observed for other acenes.

Interplay between Long-Range Crystal Order and Short-Range Molecular Interactions Tunes Carrier Mobility in Liquid Crystal Dyes.

We investigated the influence of molecular packing on the optical and electrical properties of the liquid crystalline dye 4,7-bis[5-(2-fluoro-4-pentyl-phenyl)-2-thienyl]-2,1,3-benzothiadiazole (FPPTB). FPPTB is crystalline at room temperature, exhibits a nematic phase at temperatures above 149 °C and is in an isotropic melt at temperatures above 230 °C. Solution processed FPPTB films were subject to thermal annealing through these phase transition temperatures and characterized with X-ray diffraction and polarized optical microscopy. Cooling FPPTB films from the nematic and isotropic phases increased crystal domain size, but also induced local structural variations in the molecular packing of crystalline FPPTB. The decrease in long-range order was correlated with an increase in short-range π-π interactions, leading to changes in molecular aggregation which persisted even when the FPPTB films were cooled to room temperature. Annealing-induced changes in molecular aggregation were confirmed with optical spectroscopy. The carrier mobility in FPPTB films increased over 2 orders of magnitude from (2.2 ± 0.4) × 10-5 cm2 V-1 s-1 in as-spun films to µ = (5.0 ± 0.8) × 10-3 cm2 V-1 s-1 in films cooled from the isotropic melt. We discuss the relationship between thermal stability and high carrier mobility values in terms of the interplay between long-range molecular order and increased π-π interactions between molecular pairs in the FPPTB film.

Silica- and alkoxysilane-coated ultrasmall superparamagnetic iron oxide particles: a promising tool to label cells for magnetic resonance imaging.

In this study silica- and alkoxysilane-coated ultrasmall superparamagnetic iron oxide (USPIO) particles were synthesized, and their ability to label immortalized progenitor cells for magnetic resonance imaging (MRI) was compared. USPIO particles were synthesized by coprecipitation of ferric and ferrous salts. Subsequently, the particles were coated with silica, (3-aminopropyl)trimethoxysilane (APTMS), and [N-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (AEAPTMS). The size of the USPIO particles was about 10 nm without a significant increase in diameter after coating. The highest T2 relaxivity was achieved for silica-coated USPIO particles, 339.80 +/- 0.22 s-1 mM-1, as compared with APTMS- and AEAPTMS-coated ones, reaching 134.40 +/- 0.01 and 84.79 +/- 0.02 s-1 mM-1, respectively. No toxic effects on the cells could be detected by trypan blue, TUNEL, and MTS assays. Uptake of USPIO particles was evaluated by Prussian blue staining, transmission electron microscopy, T2-MR relaxometry, and mass spectrometry. It was found that cell uptake of the different USPIO particles increased for longer incubation times and higher doses. Maximum cellular iron concentrations of 42.1 +/- 4.0 pg/cell (silica-coated USPIO particles), 37.1 +/- 3.5 pg/cell (APTMS-coated USPIO particles), and 32.7 +/- 4.0 pg/cell (AEAPTMS-coated USPIO particles) were achieved after incubation of the cells with USPIO particles at a dose of 3 micromol/mL for 6 h. The decrease of the T2 relaxation time of the cell pellets was most pronounced for cells incubated with silica-coated USPIO particles followed by APTMS- and AEAPTMS-coated particles, respectively. In gelatin gels even small clusters of labeled cells were detected by 1.5 T MRI, and significant changes in the T2 relaxation times of the gels were determined for 10000 labeled cells/mL for all particles. In summary, as compared with APTMS- and AEAPTMS-coated particles, silica-coated USPIO particles provide the highest T2 relaxivity and most effectively reduce the T2 relaxation time of immortalized progenitor cells after internalization. This suggests silica-coated USPIO particles are most suited for cell labeling approaches in MRI.