Die vorliegende Arbeit befasst sich mit der Synthese und Untersuchung V- und brettförmiger Flüssigkristalle zur Realisierung einer biaxialen nematischen Mesophase. Es wurde erfolgreich eine Serie neuer Mesogene mit hockeyschlägerförmiger und V-förmiger Struktur synthetisiert. Zusätzlich wurden Dimere aus einem dieser hockeyschlägerförmigen Verbindungen dargestellt. Als Kernbaustein wurde Benzo[1,2-b:4,3-b']dithiophen verwendet, dessen lokales Kerndipolmoment von 1.0 Debye sich nach theoretischen Vorgaben zusätzlich zum Bindungswinkel (108.9 °) positiv auf die Bildung einer Nb-Phase auswirken soll. Überraschenderweise bilden nur die hockeyschlägerförmigen Moleküle eine uniaxiale, optisch positive nematische Mesophase aus. Alle anderen V-förmigen Verbindungen und sogar die Dimere sind ausschließlich kristallin und keine Flüssigkristalle. Die Einkristallstrukturanalyse eines hockeyschlägerförmigen Mesogens sowie eines V-förmigen Moleküls zeigt bemerkenswerte Ähnlichkeiten auf. Ein Modell des Phasenübergangs wird präsentiert, welches die Abwesenheit der nematischen Mesophase in der Familie der V-förmigen, formstabilen Mesogene mit terminalen aliphatischen Ketten erklärt. Zudem befasst sich die Arbeit mit der Synthese und der Untersuchung brettförmiger Moleküle, welche dem optimalen Seitenverhältnis von 15 : 5 : 3 mit L > B > T zur Bildung biaxialer Mesophasen, relativ nahekommen. Ein Anthrachinon-Kernbaustein wurde dabei mit Armen bestehend aus einem Oligo(phenylenethinylen)-Grundgerüst entsprechender Länge verknüpft. Es konnten verschiedene dachförmige Mesogene dargestellt werden, bei denen die Art und Anzahl der Seitenketten sowie der terminalen Ketten variiert wurde. Thermische sowie mikroskopische Untersuchungen zeigen bei allen Verbindungen eine breite nematische Mesophase. Mittels spezieller Röntgenstreuung im magnetischen Feld kann die Bildung nematischer Domänen mit SmC-artigen biaxialen Aggregaten bestätigt werden. / The present work deals with the synthesis and investigation of V- and board¬¬¬-shaped liquid crystals for the realization of a biaxial nematic mesophase. A series of new mesogens has been succesfully prepared, realizing hockey-stick, hockey-stick dimer and V-shaped molecular topologies. As a core unit, benzo[1,2-b: 4,3-b ']dithiophene was used, whose local core dipole moment of 1.0 Debye in addition to the bond angle (108.9 °) is supposed to have a positive effect on the formation of an Nb-phase according to theoretical guidelines. Surprisingly, only the hockey stick molecules form a uniaxial, optically positive nematic mesophase. All other V-shaped compounds and even the dimers are exclusively crystalline and no liquid crystals. Single crystal structure of a hockey-stick mesogen and a V-shaped molecule shows remarkable similarities. A model of the phase transition is presented, which explains the absence of the nematic mesophase in the family of V-shaped, shape-persistent mesogens with terminal aliphatic chains. In addition, the work is about the synthesis and investigation of board-shaped molecules, which are relatively close to the optimal aspect ratio of 15 : 5 : 3 with L > W > D for the formation of biaxial mesophases. An anthraquinone core building block was linked to arms consisting of an oligo(phenylenethynylene) scaffold of appropriate length. Different roof-shaped mesogens could be presented, in which the type and number of the side chains and the terminal chains as well, were varied. Thermal and microscopic studies show a broad nematic mesophase for all compounds. By means of special X-ray scattering in the magnetic field, the formation of nematic domains with SmC-like biaxial aggregates can be confirmed.

Die vorliegende Dissertation befasst sich mit den Struktur-Eigenschafts-Beziehungen von sternförmigen Mesogenen mit kontrollierbaren Konformationen in den LC-Phasen. Zunächst sollte mithilfe verschiedener Moleküldesigns geklärt werden, wie eine Faltung der Arme verhindert werden kann, und somit, ob sternförmige Konformationen in den kolumnaren Packungen realisiert werden können. Hierzu wurde erfolgreich eine Bibliothek von dreiarmigen Amidsternen, semiflexiblen Oligoestersternen mit hexasubstituiertem Benzolkern und formtreuen hexasubstituierten Benzolen synthetisiert. Die besondere Herausforderung bei der Darstellung letzterer lag in der C3-Symmetrie der Verbindungen und konnte durch Optimierung der Synthesestrategie mittels aufeinander folgender Wittig-Horner- und Suzuki-Reaktionen in einem divergenten Ansatz gemeistert werden. Ein herausragendes Ergebnis ist die Flüssigkristallinität dieser formtreuen hexasubstituierten Strukturen, wenn sie mindestens neun bzw. zwölf periphere Ketten besitzen. Die detaillierte Auswertung der Kolumnendurchmesser mithilfe von äquatorialen Reflexen sowie der Dichte und der meridionalen Beugungsmuster zeigen, dass lediglich für die formtreuen hexasubstituierten Benzolderivate eine Faltung verhindert werden kann. Intrinsische Freiräume (Kävitäten) zwischen den Oligo(phenylenvinylen)-Armen werden durch außergewöhnliche Dimerenbildung und helikale Packung der Moleküle kompensiert. In die Kavitäten der Trispyridylverbindungen können Carbonsäure-funktionalisierte Gäste unter Ausbildung von Wasserstoffbrücken eingelagert werden. Mit zunehmender Gastkonzentration wird die helikale Dimerphase des Wirts kontinuierlich in eine neue kolumnare Phase von monomeren Supermesogenen ohne helikale Struktur umgewandelt. Da die Gäste in den Supermesogenen vollständig von den Oligo(phenylenvinylen)-Armen und den aliphatischen Ketten umschlossen sind, handelt es sich bei der Wirtverbindung erstmals um einen flüssigkristallinen Endorezeptor mit drei Bindungsstellen. Das Sternmesogen mit größeren intrinsischen Freiräumen ermöglicht die Einlagerung von funktionalen Bausteinen wie z.B. Anthracenchromophoren. Aus Untersuchungen mittels Festkörper-NMR- und Fluoreszenzspektroskopie geht hervor, dass sich die Mesophase mit drei Anthracengästen langsam in eine doppelt nanosegregierte Struktur umwandelt, in der intrakolumnar Oligo(phenylenvinylen)-Arme und Anthracene Seite an Seite segregiert stapeln und so segmentierte Kolumnen bilden. Diese Art von doppelter Nanosegregation offenbart das Potential des verwendeten Moleküldesigns im Bezug auf die Entwicklung mesomorpher Multikabelstrukturen. Im Vergleich zu den Supermesogenen weisen die analogen Sternverbindungen mit kovalent gebundenen Pseudogästen um über 100 °C höhere Klärpunkte auf, was unter Berücksichtigung der strukturellen Ähnlichkeit der kolumnaren Phasen und der ähnlichen Mischungsenthalpien in unterschiedlichen Werten der Mischungsentropie begründet liegen muss. Der Vergleich mit einer 1:3-Mischung ohne spezifische Wirt-Gast-Wechselwirkung bestätigt in diesem Zusammenhang den Einfluss der Bindungsart der Gäste auf die Mesophasenstabilität. Die Klärtemperaturen der Sternmesogene lassen sich folglich über die Art der Bindung der Gastmoleküle kontrollieren. Dies ist vor allem für die Orientierung kolumnarer Phasen in dünnen Filmen großer funktionaler Mesogene, die häufig erst bei sehr hohen Temperaturen unter Zersetzung in die isotrope Phase übergehen, interessant. / This dissertation examines the structure-property relationships of star-shaped mesogens with controllable conformations in the LC phases. First it should be clarified by means of different molecular designs, how a folding of the arms can be prevented, and thus, if reliable star-shaped conformations can be realized in the columnar stacks. For this reason, a library of three-armed amide stars, semi-flexible oligoester stars with a hexasubstituted benzene core and shape-persistent hexasubstituted benzenes was successfully synthesized. The particular challenge in the preparation of the latter was the C3-symmetry of the compounds and could be accomplished by optimizing the synthetic strategy using subsequently Wittig-Horner and Suzuki reactions in a divergent route. An outstanding result is the liquid crystallinity of these shape-persistent hexasubstituted structures, if they contain at least nine or twelve peripheral chains. The detailed evaluation of the column diameters, using equatorial reflections, as well as the density and the meridional scattering patterns shows that only the shape-persistent hexasubstituted benzene derivatives can be prevented from folding. Intrinsic free spaces (cavities) between the oligo(phenylenevinylene) arms are compensated by exceptional dimer formation and helical packing of the molecules. Carboxylic acid functionalized guests can be incorporated into the cavities of the trispyridyl compounds by forming hydrogen bonds. Upon increasing the guest concentration the helical dimer phase of the host is continuously converted into a new columnar phase of monomeric supermesogens without a helical structure. Since the guests of the supermesogens are entirely enclosed by the oligo(phenylenevinylene) arms and the aliphatic chains, the host compound is the first liquid crystalline endoreceptor with three binding sites. The star mesogen with larger intrinsic free spaces facilitates the incorporation of functional building blocks such as anthracene chromophores. Investigations by solid state NMR and fluorescence spectroscopy uncovered that the mesophase with three anthracene guests slowly converts into a double nanosegregated structure in which oligo(phenylenevinylene) arms und anthracenes stack side by side and thus form segmented columns. This type of double nanosegregation reveals the potential of the molecular design used with respect to the development of mesomorphic multi cable structures. In comparison to the supermesogens the analog star compounds with covalently bound pseudo guests exhibit more than 100 °C higher clearing points. Considering the structural similarity and the similar mixing enthalpy this must be due to different values of the mixing entropy. In this context, the comparison with a 1:3 mixture without any specific host-guest interaction confirms the influence of the binding mode of the guests on the mesophase stability. As a consequence, the clearing temperatures of the star mesogens can be controlled by the binding mode of the guest molecules. This is particularly interesting for the orientation of columnar phases in thin films of large functional mesogens which frequently transform into the isotropic phase at very high temperatures under decomposition.

A new method, probability distribution of diffusivities (time scaled square displacements between succeeding video frames), was developed to analyze single molecule tracking (SMT) experiments. This method was then applied to SMT experiments on ultrathin liquid tetrakis(2-ethylhexoxy)silane (TEHOS) films on Si wafer with 100 nm thermally grown oxide, and on thin semectic liquid crystal films. Spatial maps of diffusivities from SMT experiments on 220 nm thick semectic liquid crystal films reveal structure related dynamics. The SMT experiments on ultrathin TEHOS films were complemented by fluorescence correlation spectroscopy (FCS). The observed strongly heterogeneous single molecule dynamics within those films can be explained by a three-layer model consisting of (i) dye molecules adsorbed to the substrate, (ii) slowly diffusing molecules in the laterally heterogeneous near-surface region of 1 - 2 molecular diameters, and (iii) freely diffusing dye molecules in the upper region of the film. FCS and SMT experiments reveal a strong influence of substrate heterogeneity on SM dynamics. Thereby chemisorption to substrate surface silanols plays an important role. Vertical mean first passage times (mfpt) in those films are below 1 µs. This appears as fast component in FCS autocorrelation curves, which further contain a contribution from lateral diffusion and from adsorption events. Therefore, the FCS curves are approximated by a tri-component function, which contains an exponential term related to the mfpt, the correlation function for translational diffusion and a stretched exponential term for the broad distribution of adsorption events. Lateral diffusion coefficients obtained by FCS on 10 nm thick TEHOS films, thereby, are effective diffusion coefficients from dye transients in the focal area. They strongly depend on the substrate heterogeneity. Variation of the frame times for the acquisition of SMT experiments in steps of 20 ms from 20 ms to 200 ms revealed a strong dependence of the corresponding probability distributions of diffusivities on time, in particular in the range between 20 ms and 100 ms. This points to average dwell times of the dye molecules in at least one type of the heterogeneous regions (e.g. on and above silanol clusters) in the range of few tens of milliseconds. Furthermore, time series of SM spectra from Nile Red in 25 nm thick poly-n-alkyl-methacrylate (PnAMA) films were studied. In analogy to translational diffusion, spectral diffusion (shifts in energetic positions of SM spectra) can be studied by probability distributions of spectral diffusivities, i.e. time scaled square energetic displacements. Simulations were run and analyzed to study contributions from noise and fitting uncertainty to spectral diffusion. Furthermore the effect of spectral jumps during acquisition of a SM spectrum was investigated. Probability distributions of spectral diffusivites of Nile Red probing vitreous PnAMA films reveal a two-level system. In contrast, such probability distributions obtained from Nile Red within a 25 nm thick poly-n-butylmethacrylate film around glass transition and in the melt state, display larger spectral jumps. Moreover, for longer alkyl side chains a solvent shift to higher energies is observed, which supports the idea of nanophase separation within those polymers.

A new method, probability distribution of diffusivities (time scaled square displacements between succeeding video frames), was developed to analyze single molecule tracking (SMT) experiments. This method was then applied to SMT experiments on ultrathin liquid tetrakis(2-ethylhexoxy)silane (TEHOS) films on Si wafer with 100 nm thermally grown oxide, and on thin semectic liquid crystal films. Spatial maps of diffusivities from SMT experiments on 220 nm thick semectic liquid crystal films reveal structure related dynamics. The SMT experiments on ultrathin TEHOS films were complemented by fluorescence correlation spectroscopy (FCS). The observed strongly heterogeneous single molecule dynamics within those films can be explained by a three-layer model consisting of (i) dye molecules adsorbed to the substrate, (ii) slowly diffusing molecules in the laterally heterogeneous near-surface region of 1 - 2 molecular diameters, and (iii) freely diffusing dye molecules in the upper region of the film. FCS and SMT experiments reveal a strong influence of substrate heterogeneity on SM dynamics. Thereby chemisorption to substrate surface silanols plays an important role. Vertical mean first passage times (mfpt) in those films are below 1 µs. This appears as fast component in FCS autocorrelation curves, which further contain a contribution from lateral diffusion and from adsorption events. Therefore, the FCS curves are approximated by a tri-component function, which contains an exponential term related to the mfpt, the correlation function for translational diffusion and a stretched exponential term for the broad distribution of adsorption events. Lateral diffusion coefficients obtained by FCS on 10 nm thick TEHOS films, thereby, are effective diffusion coefficients from dye transients in the focal area. They strongly depend on the substrate heterogeneity. Variation of the frame times for the acquisition of SMT experiments in steps of 20 ms from 20 ms to 200 ms revealed a strong dependence of the corresponding probability distributions of diffusivities on time, in particular in the range between 20 ms and 100 ms. This points to average dwell times of the dye molecules in at least one type of the heterogeneous regions (e.g. on and above silanol clusters) in the range of few tens of milliseconds. Furthermore, time series of SM spectra from Nile Red in 25 nm thick poly-n-alkyl-methacrylate (PnAMA) films were studied. In analogy to translational diffusion, spectral diffusion (shifts in energetic positions of SM spectra) can be studied by probability distributions of spectral diffusivities, i.e. time scaled square energetic displacements. Simulations were run and analyzed to study contributions from noise and fitting uncertainty to spectral diffusion. Furthermore the effect of spectral jumps during acquisition of a SM spectrum was investigated. Probability distributions of spectral diffusivites of Nile Red probing vitreous PnAMA films reveal a two-level system. In contrast, such probability distributions obtained from Nile Red within a 25 nm thick poly-n-butylmethacrylate film around glass transition and in the melt state, display larger spectral jumps. Moreover, for longer alkyl side chains a solvent shift to higher energies is observed, which supports the idea of nanophase separation within those polymers.