Dynamics and local ordering of pentachloronitrobenzene: a molecular-dynamics investigation

Plastic phases are constituted by molecules whose centers of mass form a long range ordered crystalline lattice, but rotate in a more or less constrained way. Pentachloronitrobenzene (PCNB) is a quasi-planar hexa-substituted benzene formed by a benzene ring decorated with a -NO2 group and five chlorine atoms that displays below the melting point a layered structure of rhombohedral (R3̄) planes in which the molecules can rotate around a six-fold-like axis. Dielectric spectroscopy [Romanini et al., The Journal of Physical Chemistry C, 2016, 120, 10614] of this highly anisotropic phase revealed a complex relaxation dynamics with two coupled primary α processes, initially ascribed to the in-plane and out-of-plane components of the molecular dipole. In this work, we perform a series of molecular dynamics simulations together with single crystal X-ray synchrotron diffraction experiments to investigate the puzzling dynamics of PCNB. We conclude that the molecule undergoes very fast movements due to the high flexibility of the -NO2 group, and two slower movements in which only the in-plane rotation of the whole ring is involved. These two movements are related to fast attempts to perform a 60° in-plane rotation, and a diffusive motion that involves the rotation of the molecule completely decorrelating the dipole orientation. We have also investigated whether a homogeneous or a heterogeneous scenario is better suited to describe the restricted orientational disorder of this anisotropic phase both from a structural and dynamical point of view.

Self-assembled discotics as molecular semiconductors

With the advent of a new era of smart-technology, the demand for more economic optoelectronic materials that do not compromise with efficiency is gradually on the rise. Organic semiconductors provide greener alternatives to the conventional inorganic ones, but encounter the challenge of balancing charge carrier mobility with processability in devices. Discotic liquid crystals (DLCs), a class of self-assembling soft organic materials, possess the perfect degree of order and dynamics to address this challenge. Providing unidimensional charge carrier pathways through their nanoscale columnar architecture, DLCs can behave as efficient charge transport systems across a wide range of optoelectronic devices. Moreover, DLCs are solution-processable, thus reducing the fabrication cost. In this article, we have discussed the approaches towards developing DLCs as semiconductors, focusing on their molecular design concepts, supramolecular structures and electronic properties in the context of their charge carrier mobilities.

Luminescent Conductive Columnar π-Gelators for Fe(II) Sensing and Bio-Imaging Applications

The high demand and scarcity of luminescent, photoconductive, and transparent gels necessitate its finding as they are potential components in photonic devices such as solar cell concentrators where optical losses via scattering and reabsorption require to be minimized. In this direction, we have reported highly transparent, blue luminescent as well as photoconductive gels exhibiting the hole mobility of 10^-3 cm²/V s at ambient temperature as investigated by the time-of-flight technique. The π-driven self-standing supergels were formed using triazole-modified phenylene-vinylene derivatives as gelators in a nonpolar solvent. Different microscopic studies revealed its entangled network of interwoven fibrilar self-assembly and anisotropic order in the gel state. Supramolecular assembly of xerogels, studied by small- and wide-angle X-ray scattering (SAXS/WAXS) suggesting their local columnar hexagonal (Col_h) superstructure, is beneficial for conducting gels. Rheological measurements direct the stiffness and robustness of the organogels. In addition, the gelators were developed as a sensing platform for the ultrasensitive detection of Fe(II) ions at ppb level. ¹H nuclear magnetic resonance (NMR) titrimetric studies revealed that the interaction of the H-atom of triazole units with Fe(II) is responsible for quenching of blue fluorescence. Also, one of the gelators was successfully applied in bio-imaging using the pollen grains of the Hibiscus rosa-sinensis plant.

Electrical Conductivity and Multiple Glassy Dynamics of Crown Ether-Based Columnar Liquid Crystals

The phase behavior of two unsymmetrical triphenylene crown ether-based columnar liquid crystals bearing different lengths of alkyl chains, KAL465 and KAL468, was investigated using differential scanning calorimetry (DSC). A plastic crystalline (Cry), a columnar liquid crystalline (Col_h), and an isotropic phase were observed along with two glass transitions in the Cry phase. The molecular mobility of the KAL compounds was further studied by a combination of broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques. By the BDS investigations, three dielectric active relaxation processes were observed for both samples. At low temperatures, a γ-process in the Cry state was detected and is assigned to the localized fluctuations taking place in the alkyl chains. An α₂-process takes place at higher temperatures in the Cry phase. An α₃-process was found in the Col_h mesophase. The advanced calorimetric techniques consist of fast scanning calorimetry (FSC) and specific heat spectroscopy employing temperature-modulated DSC and FSC. The advanced calorimetric investigations revealed that besides the α₂-process in agreement with BDS, there is a second dynamic glass transition (α₁-process), which is not observed by dielectric spectroscopy. The results are in good agreement with the glass transitions detected by DSC for this process. The temperature dependences of the relaxation rates of the α₁-, α₂-, and α₃-processes are all different. Therefore, different molecular assignments for the relaxation processes are proposed. In addition to the relaxation processes, a conductivity contribution was explored by BDS for both KAL compounds. The conductivity contribution appears in both Cry and Col_h phases, where the conductivity increases by ca. 1 order of magnitude at phase transition from the Cry to the hexagonal phase.

Low frequency vibrational density of state of highly permeable super glassy polynorbornenes - the Boson peak

Inelastic incoherent neutron time-of-flight scattering was employed to measure the low frequency density of states for a series of addition polynorbornenes with bulky side groups. The rigid main chain in combination with the bulky side groups give rise to a microporosity of these polymers in the solid state. The microporosity characterized by the BET surfaces area varies systematically in the considered series. Such materials have some possible application as active separation layer in gas separation membranes. All investigated materials show excess contributions to the Debye type density of states characteristic for glasses known as Boson peak. The maximum position of the Boson peak shifts to lower frequency values with increasing microporosity. Data for PIM-1 and Matrimid included for comparison are in good agreement to this dependency. This result supports the sound wave interpretation of the Boson peak.

Complex molecular dynamics of a symmetric model discotic liquid crystal revealed by broadband dielectric, thermal and neutron spectroscopy

The molecular dynamics of the triphenylene-based discotic liquid crystal HAT6 is investigated by broadband dielectric spectroscopy, advanced dynamical calorimetry and neutron scattering. Differential scanning calorimetry in combination with X-ray scattering reveals that HAT6 has a plastic crystalline phase at low temperatures, a hexagonally ordered liquid crystalline phase at higher temperatures and undergoes a clearing transition at even higher temperatures. The dielectric spectra show several relaxation processes: a localized γ-relaxation at lower temperatures and a so called α2-relaxation at higher temperatures. The relaxation rates of the α2-relaxation have a complex temperature dependence and bear similarities to a dynamic glass transition. The relaxation rates estimated by Hyper DSC, Fast Scanning calorimetry and AC Chip calorimetry have a different temperature dependence than the dielectric α2-relaxation and follow the VFT-behavior characteristic for glassy dynamics. Therefore, this process is called α1-relaxation. Its relaxation rates show a similarity with that of polyethylene. For this reason, the α1-relaxation is assigned to the dynamic glass transition of the alkyl chains in the intercolumnar space. Moreover, this process is not observed by dielectric spectroscopy, which supports its assignment. The α2-relaxation is assigned to small scale translatorial and/or small angle fluctuations of the cores. The neutron scattering data reveal two relaxation processes. The process observed at shorter relaxation times is assigned to the methyl group rotation. The second relaxation process at longer time scales agree in the temperature dependence of its relaxation rates with that of the dielectric γ-relaxation.

Collective orientational order and phase behavior of a discotic liquid crystal under nanoscale confinement

The phase behavior and molecular ordering of hexakishexyloxy triphenylene (HAT6) DLCs under cylindrical nanoconfinement are studied utilizing differential scanning calorimetry (DSC) and dielectric spectroscopy (DS), where cylindrical nanoconfinement is established through embedding HAT6 into the nanopores of anodic aluminum oxide (AAO) membranes, and a silica membrane with pore diameters ranging from 161 nm down to 12 nm. Both unmodified and modified pore walls were considered. In the latter case the pore walls of AAO membranes were chemically treated with n-octadecylphosphonic acid (ODPA) resulting in the formation of a 2.2 nm thick layer of grafted alkyl chains. Phase transition enthalpies decrease with decreasing pore size, indicating that a large proportion of the HAT6 molecules within the pores has a disordered structure, which increases with decreasing pore size for both pore walls. In the case of the ODPA-modification, the amount of ordered HAT6 is increased compared to the unmodified case. The pore size dependencies of the phase transition temperatures were approximated using the Gibbs-Thomson equation, where the estimated surface tension is dependent on the molecular ordering of HAT6 molecules within the pores and upon their surface. DS was employed to investigate the molecular ordering of HAT6 within the nanopores. These investigations revealed that with a pore size of around 38 nm, for the samples with the unmodified pore walls, the molecular ordering changes from planar axial to homeotropic radial. However, the planar axial configuration, which is suitable for electronic applications, can be successfully preserved through ODPA-modification for most of the pore sizes.

Multiple glassy dynamics in dipole functionalized triphenylene-based discotic liquid crystals revealed by broadband dielectric spectroscopy and advanced calorimetry - assessment of the molecular origin

A selected series of dipole functionalized triphenylene-based discotic liquid crystals (DLCs) was synthesized and investigated in a systematic way to reveal the phase behavior and molecular dynamics. The later point is of particular importance to understand the charge transport in such systems which is the key property for their applications such as organic field-effect transistors, solar cells or as nanowires in molecular electronics, and also to tune the properties of DLCs. The mesomorphic properties were studied by polarizing optical microscopy, X-ray diffraction, and differential scanning calorimetry, which were compared to the corresponding unfunctionalized DLC. The molecular dynamics were investigated by a combination of state-of-the-art broadband dielectric spectroscopy (BDS) and advanced calorimetry such as fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). Besides localized fluctuations, surprisingly multiple glassy dynamics were detected for all materials for the first time. Glassy dynamics were proven for both processes unambiguously due to the extraordinary broad frequency range covered. The α1-process is attributed to fluctuations of the alky chains in the intercolumnar space because a polyethylene-like glassy dynamics is observed. This corresponds to a glass transition in a confined three-dimensional space. The α2-process found at temperatures lower than α1-process, is assigned to small scale rotational and/or translational in plane fluctuations of the triphenylene core inside distorted columns. This can be considered as a glass transition in a one-dimensional fluid. Therefore, obtained results are of general importance to understand the glass transition, which is an unsolved problem of condensed matter science.

Quantized Self-Assembly of Discotic Rings in a Liquid Crystal Confined in Nanopores

Disklike molecules with aromatic cores spontaneously stack up in linear columns with high, one-dimensional charge carrier mobilities along the columnar axes, making them prominent model systems for functional, self-organized matter. We show by high-resolution optical birefringence and synchrotron-based x-ray diffraction that confining a thermotropic discotic liquid crystal in cylindrical nanopores induces a quantized formation of annular layers consisting of concentric circular bent columns, unknown in the bulk state. Starting from the walls this ring self-assembly propagates layer by layer towards the pore center in the supercooled domain of the bulk isotropic-columnar transition and thus allows one to switch on and off reversibly single, nanosized rings through small temperature variations. By establishing a Gibbs free energy phase diagram we trace the phase transition quantization to the discreteness of the layers' excess bend deformation energies in comparison to the thermal energy, even for this near room-temperature system. Monte Carlo simulations yielding spatially resolved nematic order parameters, density maps, and bond-orientational order parameters corroborate the universality and robustness of the confinement-induced columnar ring formation as well as its quantized nature.

Dynamics and ionic conductivity of ionic liquid crystals forming a hexagonal columnar mesophase

For the first time, the molecular mobility of two linear-shaped tetramethylated guanidinium triflate ionic liquid crystals (ILCs) having different lengths of alkyl chains was investigated using a combination of broadband dielectric (BDS) and specific heat spectroscopy (SHS).

Anomalies in the low frequency vibrational density of states for a polymer with intrinsic microporosity – the Boson peak of PIM-1

For the first time the low frequency vibrational density of states is reported for a polymer with intrinsic microporosity.

Soft matter in hard confinement: phase transition thermodynamics, structure, texture, diffusion and flow in nanoporous media

Spatial confinement in nanoporous media affects the structure, thermodynamics and mobility of molecular soft matter often markedly. This article reviews thermodynamic equilibrium phenomena, such as physisorption, capillary condensation, crystallisation, self-diffusion, and structural phase transitions as well as selected aspects of the emerging field of spatially confined, non-equilibrium physics, i.e. the rheology of liquids, capillarity-driven flow phenomena, and imbibition front broadening in nanoporous materials. The observations in the nanoscale systems are related to the corresponding bulk phenomenologies. The complexity of the confined molecular species is varied from simple building blocks, like noble gas atoms, normal alkanes and alcohols to liquid crystals, polymers, ionic liquids, proteins and water. Mostly, experiments with mesoporous solids of alumina, gold, carbon, silica, and silicon with pore diameters ranging from a few up to 50 nm are presented. The observed peculiarities of nanopore-confined condensed matter are also discussed with regard to applications. A particular emphasis is put on texture formation upon crystallisation in nanoporous media, a topic both of high fundamental interest and of increasing nanotechnological importance, e.g. for the synthesis of organic/inorganic hybrid materials by melt infiltration, the usage of nanoporous solids in crystal nucleation or in template-assisted electrochemical deposition of nano structures.

Thermotropic orientational order of discotic liquid crystals in nanochannels: an optical polarimetry study and a Landau-de Gennes analysis

Optical polarimetry measurements of the orientational order of a discotic liquid crystal based on a pyrene derivative confined in parallelly aligned nanochannels of monolithic, mesoporous alumina, silica, and silicon as a function of temperature, channel radius (3-22 nm) and surface chemistry reveal a competition of radial and axial columnar orders. The evolution of the orientational order parameter of the confined systems is continuous, in contrast to the discontinuous transition in the bulk. For channel radii larger than 10 nm we suggest several, alternative defect structures, which are compatible both with the optical experiments on the collective molecular orientation presented here and with a translational, radial columnar order reported in previous diffraction studies. For smaller channel radii our observations can semi-quantitatively be described by a Landau-de Gennes model with a nematic shell of radially ordered columns (affected by elastic splay deformations) that coexists with an orientationally disordered, isotropic core. For these structures, the cylindrical phase boundaries are predicted to move from the channel walls to the channel centres upon cooling, and vice-versa upon heating, in accord with the pronounced cooling/heating hystereses observed and the scaling behavior of the transition temperatures with the channel diameter. The absence of experimental hints of a paranematic state is consistent with a biquadratic coupling of the splay deformations to the order parameter.