S. Holger Eichhorn

Application of solid-state 35Cl NMR to the structural characterization of hydrochloride pharmaceuticals and their polymorphs.

Solid-state (35)Cl NMR (SSNMR) spectroscopy is shown to be a useful probe of structure and polymorphism in HCl pharmaceuticals, which constitute ca. 50% of known pharmaceutical salts. Chlorine NMR spectra, single-crystal and powder X-ray diffraction data, and complementary ab initio calculations are presented for a series of HCl local anesthetic (LA) pharmaceuticals and some of their polymorphs. (35)Cl MAS SSNMR spectra acquired at 21.1 T and spectra of stationary samples at 9.4 and 21.1 T allow for extraction of chlorine electric field gradient (EFG) and chemical shift (CS) parameters. The sensitivity of the (35)Cl EFG and CS tensors to subtle changes in the chlorine environments is reflected in the (35)Cl SSNMR powder patterns. The (35)Cl SSNMR spectra are shown to serve as a rapid fingerprint for identifying and distinguishing polymorphs, as well as a useful tool for structural interpretation. First principles calculations of (35)Cl EFG and CS tensor parameters are in good agreement with the experimental values. The sensitivity of the chlorine NMR interaction tensor parameters to the chlorine chemical environment and the potential for modeling these sites with ab initio calculations hold much promise for application to polymorph screening for a wide variety of HCl pharmaceuticals.

Hierarchy of Order in Liquid Crystalline Polycaps

When outfitted with long alkyl chains, polycaps, capsules along a polymer chain, spontaneously organize themselves into a two-dimensional liquid crystalline phase. Further organization results from shearing or pulling the liquid crystalline samples, producing three-dimensional assemblies of micrometer-wide, infinitely long fibers (see schematic representation).

Mesomorphic [2]Rotaxanes: Sheltering Ionic Cores with Interlocking Components

Two types of liquid crystalline [2]rotaxanes based on a conventional tetracatenar motif (a rod-shaped molecule with two side chains at each end) have been prepared. Dicationic compounds with ester stoppers and tetracationic materials with pyridinium stoppers are compared to each other and their dumbbell shaped analogs. Since the ionic core contributes about 70% to the overall length and molecular weight of the molecules, sheltering the ionic cores with an interlocked neutral macrocycle has considerable effect on the mesomorphism and thermal stability of the materials. The influence of the sheltering macrocycle, the numbers of charges on the core and the size and nature of the side chains (aliphatic vs siloxane) were probed. [2]Rotaxanes with linear side chains and minimum ratios of chain-to-core volumes of about 0.35 and 0.30 for tetra- and dicationic compounds, respectively, display smectic liquid crystal phases. Larger ratios increase the temperature range of the smectic A phases beyond the decomposition temperatures; a disadvantage for processing because no stable isotropic liquid phase is available. The change from tetra- to dicationic [2]rotaxanes increased not only the fluidity of their smectic A phases but also their thermal and chemical stability. Branched side chains (2-hexyldecyl) disfavor the formation of lamellar mesophases and, instead, induce higher ordered soft crystal phases. No liquid crystal phases but soft crystal phases are observed for the analogous di- and tetracationic compounds without an ion sheltering interlocked macrocycle (dumbbells).

Experimental and Computational Insights into the Stabilization of Low-Valent Main Group Elements Using Crown Ethers and Related Ligands

A series of tin(II) triflate and chloride salts in which the cations are complexed by either cyclic or acyclic polyether ligands and which have well-characterized single-crystal X-ray structures are investigated using a variety of experimental and computational techniques. Mössbauer spectroscopy illustrates that the triflate salts tend to have valence electrons with higher s-character, and solid-state NMR spectroscopy reveals marked differences between superficially similar triflate and chloride salts. Cyclic voltammetry investigations of the triflate salts corroborate the results of the Mössbauer and NMR spectroscopy and reveal substantial steric and electronic effects for the different polyether ligands. MP2 and DFT calculations provide insight into the effects of ligands and substituents on the stability and reactivity of the low-valent metal atom. Overall, the investigations reveal the existence of more substantial binding between tin and chlorine in comparison to the triflate substituent and provide a rationale for the considerably increased reactivity of the chloride salts.

Alignment of Discotic Liquid Crystals

Alignment techniques for calamitic liquid crystals are well developed and play a crucial role in liquid crystal devices. Comparatively little attention has been paid to the alignment of discotic liquid crystals. Only recently the integration of discotic liquid crystals into optical and electronic devices stimulated the development of new alignment techniques for discotic nematic and columnar mesophases. This mini-review attempts a brief summary of the challenges and recent success stories. Two new approaches to the alignment of discotic columnar mesophases will also be presented.

Halide effect in electron rich and deficient discotic phthalocyanines

A series of discotic octa- and tetra-alkylthio substituted phthalocyanines containing four Cl, Br, I atoms or twelve fluorine atoms have been prepared. All compounds display columnar mesomorphism as confirmed by polarized optical microscopy, thermal analysis, and variable temperature X-ray diffraction. Phthalocyanines containing halide atoms do not crystallize but form glassy or partially crystalline hexagonal columnar phases. Glass transition temperatures increase with increasing size of the halide atoms and with decreasing length of the alkyl chains. In contrast, all octa- and tetra-alkylthio substituted phthalocyanines crystallize and octa-substituted derivatives with aliphatic chain lengths of C5–7 exhibit tilted (rectangular) columnar mesophases. Cyclic and differential pulse voltammetry, UV-Vis spectroscopy, and quantum chemical calculations at the DFT level have been employed to determine frontier orbital energies of all synthesized and some reference phthalocyanines. Octa- and tetra-alkylthio substituted phthalocyanines are typical p-type semiconductors and the introduction of four Cl, Br, or I atoms lowers frontier orbital energies by only up to 0.1 eV and the optical gap by up to 0.03 eV. A significant decrease in LUMO energy by 0.5 eV to about −4 eV is observed for the fluorinated phthalocyanine, which is a value right at the border of organic n-type semiconductors that may be stable in air.

Synthesis, mesomorphism and electronic properties of nonaflate and cyano-substituted pentyloxy and 3-methylbutyloxy triphenylenes

Despite recent advances in the application of discotic liquid crystals (DLC) in organic electronic devices a better understanding of structure–property relationships is required for designing commercially viable discotic semiconductors. This is a complex task because a single structural change to the DLC may affect many relevant properties of the discotic material such as chemical stability, mesomorphism, alignment, and electronic properties. Presented here is a comprehensive study of hexa-substituted triphenylene derivatives containing pentyloxy and 3-methylbutyloxy groups as well as one or three nonaflate or cyano groups. The compounds are synthesized by both established and new synthetic approaches and are isolated as pure isomers. Their structure–mesomorphism relationship is studied by optical polarized microscopy, thermal analysis, and variable temperature X-ray diffraction and their relationships between structure and frontier orbital energies are investigated by cyclic voltammetry in solution, UV-Vis spectroscopy and computational studies at the DFT level. Both mesomorphism and frontier orbital energies of the compounds depend on the number and positions of nonaflate and cyano groups while only their mesomorphism is affected by a change from linear pentyloxy to branched 3-methylbutyloxy chains. To our surprise lowering the symmetry of the compounds is equally effective in lowering HOMO–LUMO gaps than the attachment of strongly π-electron withdrawing cyano groups. Consequently, the non-symmetrically substituted triphenylenes with three cyano groups are the most electron deficient and have the smallest HOMO–LUMO gap of the compounds prepared here but their ELUMO of about −2.8 eV is still high in comparison to typical electron acceptors (ELUMO below −3.5 eV). The computational approach established for the prepared compounds is also employed for the prediction of frontier orbital energies of new electron deficient discotic triphenylene derivatives.

The Effect of Incorporating Fréchet Dendrons into Rotaxanes and Molecular Shuttles Containing the 1,2‐Bis(pyridinium)ethane⊂[24]Crown‐8 Templating Motif

Fréchet-type dendrons (G0-G3) were added as both axle stoppering units and cyclic wheel appendages in a series of [2]rotaxanes, [3]rotaxanes, and molecular shuttles that employ 1,2-bis(pyridinium)ethane axles and 24-membered crown ethers wheels. The addition of dendrimer wedges as stoppering units dramatically increased the solubility of simple [2]rotaxanes in nonpolar solvents. The X-ray structure of a G1-stoppered [2]rotaxane shows how the dendritic units affect the structure of the interlocked components. Increased solubility allows observation of how the interaction of dendritic units on separate components in interlocked molecules influences switching properties and molecular size. In a series of [2]rotaxane molecular shuttles incorporating two recognition sites, it was demonstrated that an increase in generation on either the stoppering unit or cyclic wheel could influence both the rate of shuttling and the site preference of the wheel on the axle.

Metal ion mediated mesomorphism and thin film behaviour of amphitropic tetraazaporphyrin complexes

Octa-substituted tetraazaporphyrins with amphiphilic 3,6-dioxaheptylthio and 3,6,9-trioxadecylthio chains and their metal complexes (Co(II), Ni(II), Cu(II), Zn(II)) were synthesised in order to compare their thermotropic and lyotropic mesomorphism as well as their thin film properties with related tetraazaporphyrins, phthalocyanines and triphenylenes, which have been reported previously. The Co, Ni, and Cu complexes melted into hexagonal columnar mesophases, whereas the Zn(II) complexes and the free-base porphyrins did not display mesomorphism. In fact, the zinc complexes did not crystallise on cooling, rather giving highly viscous isotropic oils at room temperature that solidified below −50 °C to glasses. This unusual behaviour may be caused by axial interactions between the zinc ion and the oxygen of the polyether groups. All 3,6,9-trioxadecylthio substituted derivatives solidified far below 0 °C giving rise to columnar hexagonal mesophases at room temperature for the Co(II), Ni(II), and Cu(II) complexes. The columnar hexagonal mesophases of the long chain derivatives spontaneously align homeotropically when sandwiched between two substrates, in contrast to the short chain analogues. Both the long chain and the short chain Co(II) complexes displayed a rather disordered hexagonal columnar packing as revealed by X-ray diffraction. Binary mixtures of the short chain derivatives with non-polar or polar organic solvents did not display additional lyotropic mesophases. However, the transition temperatures and enthalpies, as well as the texture of the hexagonal columnar mesophase were affected, assuming a solubility of the solvent in the thermotropic mesophase as shown for the Cu(II) derivative. All long chain derivatives were also soluble in water but, again, lyotropic mesomorphism was observed neither with water nor with organic solvents. The film forming properties of some derivatives were investigated by spin-coating and the Langmuir–Blodgett (LB) technique. Homogeneous films of the mesomorphic short chain copper complex were obtained by the spin-coating method. The films displayed a layer structure with edge-on orientation at ambient temperature, although they were of crystalline morphology, as shown by X-ray reflectivity measurements and polarised UV/VIS spectroscopy. Only monolayer films of short and long chain derivatives could be transferred by the LB method. Again, determination of the thickness of the monolayer by X-ray agreed with an edge-on orientation of the tetraazaporphyrins. In contrast, the extrapolated molecular areas on the water surface suggested a flat-on orientation of the macrocycle with the oligo(oxyethylene) chains being dissolved in the sub-phase.

Thermal conductivity of polyurethane composites containing nanometer- and micrometer-sized silver particles

Polyurethane composites containing spherical and flake-shaped silver fillers of micrometer and nanometer sizes were prepared by reacting suspensions of the silver filler in tetraethylene glycol with Desmodur® HL BA. Both the thermal conductivity and the stability of the silver composites are increased in comparison with a reference polyurethane sample without filler. Unexpectedly, the largest increases in thermal conductivity and stability are observed for the spherical silver particles of micrometer size but not for the silver nanoparticles, which is reasoned with larger aggregates of silver particles and a higher degree of crystallinity in the sample containing micrometer-sized silver particles.