Molecular dynamics at the glass transition: One dimensional and two dimensional nuclear magnetic resonance studies of a glass‐forming discotic liquid crystal

Unique molecular dynamics of structural elements in an asymmetric Janus bisamide supramolecule characterized by solid-state NMR

An asymmetric tapered Janus bisamide supramolecule consisting of 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido]benzene bisamide (abbreviated as C22PhBAEO3) can possess three-dimensional (3D) long-range order under mild thermal treatment conditions. To understand its structural formation and unique phase-transition processes, the locally detailed structure and molecular dynamics of its structural elements in disordered and ordered phases of C22PhBAEO3 were investigated using various solid-state (SS) NMR techniques at the atomic level. On the basis of the determined conformations and packing structures of the alkyl chains in ordered and disordered crystalline phases, along with the geometry and kinetic parameters of the structural elements' dynamics, this study addresses the self-assembly, the phase-transition mechanisms, and the relationship between the structure and dynamics of these asymmetric Janus bisamide supramolecules.

Mesomorphic phthalocyanines, tetraazaporphyrins, porphyrins and triphenylenes as charge-transporting materials

The columnar mesomorphism of triphenylene-, phthalocyanine-, tetraazaporphyrin- and porphyrin-based compounds is briefly reviewed and the potential use of this particular supramolecular structure for charge-conducting devices is discussed. Suitably substituted triphenylenes exhibit ideal material properties for photoconducting devices, and well-aligned layers have been shown to give a very high charge carrier mobility of 0.1 × 10-4 m 2 V -1 s -1 (time-of-flight measurements). From a molecular point of view, porphyrin systems should be superior to triphenylenes, but the properties of their mesophases do not support the advantageous homeotropic alignment of the columnar stacks. However, pulse radiolysis time-resolved microwave conductivity measurements revealed the highest charge carrier mobility value of 0.28 × 10-4 m 2 V -1 s -1 for mesomorphic phthalocyanines.

Mesomorphic phthalocyanines, tetraazaporphyrins, porphyrins and triphenylenes as charge-transporting materials

The columnar mesomorphism of triphenylene-, phthalocyanine-, tetraazaporphyrin- and porphyrin-based compounds is briefly reviewed and the potential use of this particular supramolecular structure for charge-conducting devices is discussed. Suitably substituted triphenylenes exhibit ideal material properties for photoconducting devices, and well-aligned layers have been shown to give a very high charge carrier mobility of 0.1 × 10-4 m 2 V -1 s -1 (time-of-flight measurements). From a molecular point of view, porphyrin systems should be superior to triphenylenes, but the properties of their mesophases do not support the advantageous homeotropic alignment of the columnar stacks. However, pulse radiolysis time-resolved microwave conductivity measurements revealed the highest charge carrier mobility value of 0.28 × 10-4 m 2 V -1 s -1 for mesomorphic phthalocyanines.

Polar switching in trialkylbenzene-1,3,5-tricarboxamides

The hydrogen-bonded hexagonal columnar LC (Col(hd)) phases formed by benzene-1,3,5-tricarboxamide (BTA) derivatives can be aligned uniformly by an electric field and display switching behavior with a high remnant polarization. The polar switching in three symmetrically substituted BTAs with alkyl chains varying in length between 6 and 18 carbon atoms (C6, C10, and C18) was investigated by electro-optical switching experiments, dielectric relaxation spectroscopy (DRS), and solid-state NMR. The goal was to characterize ferroelectric properties of BTA-based columnar LCs, which display a macroscopic axial dipole moment due to the head-to-tail stacking of hydrogen-bonded amides. The Col(hd) phase of all three BTAs can be aligned uniformly by a dc field ∼30 V/μm. Moreover, C10 and C18 display extrinsic polar switching characterized by a remnant polarization and coercive field of 1-2 μC/cm(2) and 20-30 V/μm, respectively. In the absence of an external field, the polarization is lost in 1-1000 s, depending on device details and temperature. DRS revealed a columnar glass transition in the low-temperature region of the LC phase related to collective vibrations in the hydrogen-bonded columns that freeze out below 41-54 °C. At higher temperatures, a relaxation process is present originating from the collective reorientation of amide groups along the column axis (inversion of the macrodipole). Matching activation energies suggest that the molecular mechanism underlying the polar switching and the R-processes is identical. These results illustrate that LC phases based on BTAs offer the unique possibility to integrate polarization with other functionalities in a single nanostructured material.

Axial thermal rotation of slender rods

Axial rotational diffusion of rodlike polymers is important in processes such as microtubule filament sliding and flagella beating. By imaging the motion of small kinks along the backbone of chains of DNA-linked colloids, we produce a direct and systematic measurement of axial rotational diffusivity of rods both in bulk solution and near a wall. The measured diffusivities decrease linearly with the chain length, irrespective of the distance from a wall, in agreement with slender-body hydrodynamics theory. Moreover, the presence of small kinks does not affect the chain's axial diffusivity. Our system and measurements provide insights into fundamental axial diffusion processes of slender objects, which encompass a wide range of entities including biological filaments and linear polymer chains.

Origin of the complex molecular dynamics in functionalized discotic liquid crystals

The molecular dynamics of three dipole functionalized hexa-peri-hexabenzocoronenes have been studied using site-specific NMR techniques and dielectric spectroscopy as a function of temperature and pressure. These probes (i) suggest that the thermodynamic state completely controls the dynamic response, (ii) clarify the origin of two dynamic processes associated with the presence of two glass temperatures, and (iii) provide the first phase diagram for substances of this kind.

Photochemistry and Mobility of Stilbenoid Dendrimers in Their Neat Phases

Selectively deuterated, dodecyloxy substituted stilbenoid dendrimers of the first and second generation were synthesized by a convergent synthesis, using the Wittig-Horner reaction. The photochemistry and the fluorescence in the different crystalline and liquid crystalline phases were investigated. Molecules deuterated at the alpha-position of the alkoxy chains were used to study the photoreactions in the neat phases by 1H NMR. Reactions of the double bonds are exclusively observed in the liquid crystal phases. No photoreactions occur in the crystalline state. The mobility of the dendrimers was studied by means of 2H solid-state NMR spectroscopy. The onset of the photochemistry for dendrimer 1 [all-(E)-1,3,5-tris[2-(3,4,5-tridodecyloxyphenyl)ethenyl]benzene] corresponds to the increasing mobility at the Cr/LC transition. The first generation dendrimers still show large angle motion, whereas dendrimers of the second generation 2 [all-(E)-1,3,5-tris(2-[3,5-bis[2-(3,4,5-tridodecyloxyphenyl)ethenyl]phenyl]ethenyl)benzene] are restricted to librational motions. Photochemical conversion and fluorescence quenching for first and second generation dendrimers 1 and 2 increase with increasing molecular motion and reach a maximum in the isotropic phase.

Dynamics of a triphenylene discotic molecule, HAT6, in the columnar and isotropic liquid phases

Discotic molecules have planar, disklike polyaromatic cores that can self-assemble into "molecular wires". Highly anisotropic charge transfer along the wires arises when there is sufficient intermolecular overlap of the pi-orbitals of the molecular cores. Discotic materials can be applied in molecular electronics, field-effect transistors, and-recently with record quantum efficiencies-photovoltaics (Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Frien, R. H.; MacKenzie, J. D. Science 2001, 293, 1119). A combination of quasielastic neutron scattering (QENS) measurements with molecular dynamics simulations on the discotic molecule hexakis(n-hexyloxy)triphenylene (HAT6) shows that the dynamics of the cores and tails of discotic molecules are strongly correlated. Core and tail dynamics are not separated, the system being characterized by overall in-plane motion, on a time scale of 0.2 ps, and softer out-of-plane motions at 7 ps. Because charge transfer between the molecules is on similar time scales, these motions are relevant for the conducting properties of the materials. Both types of motion are dominated by van der Waals interactions. Small-amplitude in-plane motions in which the disks move over each other are almost entirely determined by tail/tail interactions, these also playing an important role in the out-of-plane motion. The QENS measurements reveal that these motions are little changed by passing from the columnar phase to the isotropic liquid phase, just above the clearing temperature. The model of four HAT6 molecules in a column reproduces the measured QENS spectrum of the liquid phase, suggesting that correlations persist within the liquid phase over about this number of disks.

Molecular self-diffusion in a columnar liquid crystalline phase determined by deuterium NMR

We report translational diffusion coefficients in a columnar phase of a discotic liquid crystal formed by a triphenylene-based compound. The experiments were performed using 2H stimulated-echo-type pulsed-field-gradient spin-echo NMR applied to a chain-deuterated sample. The diffusion coefficients were found in the range of 1x10(-14)-4x10(-14) m2/s, three orders of magnitude lower than in the isotopic phase of the same compound. This, together with the high activation energy obtained in columnar phase, indicates that the diffusion is dominated by solidlike jump processes.

High-resolution 1H NMR spectroscopy in the solid state: very fast sample rotation and multiple-quantum coherences

In the past few years, solid-state 1H NMR spectroscopy under fast magic-angle spinning (MAS) has developed into a versatile tool for elucidating structure and dynamics. Dipolar multiple-quantum (MQ), in particular double-quantum (DQ), MAS spectroscopy has been applied to a variety of materials and provided unique insight, e.g., into the structure of hydrogen-bonded systems. This review intends to present solid-state 1H DQ and MQ MAS spectroscopy in a systematic fashion with a particular emphasis on methodological aspects, followed by an overview of applications.

Solid state 2H-NMR studies of segmental dynamics in polymer blends

Recent two-dimensional (2D) 2H-NMR studies on nearly ideal mixtures are reviewed. The use of selective deuterium labeling allows the unambiguous observation of the individual segmental dynamics of each component in the blend. 2D exchange spectra provide both the mean motional rates and the motional distributions, which was necessary to discriminate among the disparate explanations of blend behavior. The components exhibit very different mean mobilities and broad mobility distributions near the glass transition. The broad macroscopic glass transition in the blends is attributed to both these two kinds of dynamic heterogeneities. The individual mean motional rates are used to define distinct glass transition temperatures, T(g,i) for each species, i. The separation between the T(g,i),s of the two species increases with the content of the high-T(g) component. The widths of their individual motional distributions also increase with the content of the high-T(g) component. These two effects combine to produce the increase in both the anomalously broad glass transition and the thermorheological complexity of these blends with fraction of the high-T(g) component.