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- Discotic Liquid Crystals (5) (entfernen)
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Nanoporous media exhibit structures significantly smaller than the wavelengths of visible light and can thus act as photonic metamaterials. Their optical functionality is not determined by the properties of the base materials, but rather by tailored, multiscale structures, in terms of precise pore shape, geometry, and orientation. Embedding liquid crystals in pore space provides additional opportunities to control light–matter interactions at the single-pore, meta-atomic scale. Here, we present temperature-dependent 3D reciprocal space mapping using synchrotron-based X-ray diffraction in combination with high-Resolution birefringence experiments on disk-like mesogens (HAT6) imbibed in self-ordered arrays of parallel cylindrical pores 17 to 160 nm across in monolithic anodic aluminium oxide (AAO). In agreement with Monte Carlo computer simulations we observe a remarkably rich self-assembly behaviour, unknown from the bulk state. It encompasses transitions between the isotropic liquid state and discotic stacking in linear columns as well as circular concentric ring formation perpendicular and parallel to the pore axis. These textural transitions underpin an optical birefringence functionality, tuneable in magnitude and in sign from positive to negative via pore size, pore surface-grafting and temperature. Our study demonstrates that the advent of large-scale, self-organised nanoporosity in monolithic solids along with confinement-controllable phase behaviour of liquid-crystalline matter at the single-pore scale provides a reliable and accessible tool to design materials with adjustable optical anisotropy, and thus offers versatile pathways to finetune polarisation-dependent light propagation speeds in materials. Such a tailorability is at the core of the emerging field of transformative optics, allowing, e.g., adjustable light absorbers and extremely thin metalenses.
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 a1-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 a2-process found at temperatures lower than a1-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.
The phase behavior and molecular ordering of hexakishexyloxy triphenylene (HAT6) DLC under cylindrical nanoconfinement is 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 membranes (AAO), and a silica membrane with pore diameters ranging from 161 nm down to 12 nm. Both unmodified and modified pore walls were considered, and in the latter case the pore walls of AAO membranes were chemical 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.
Discotic liquid crystals (DLC) filled into cylindrical nanopores exhibit a liquid crystalline phase with their molecules arranged in hexagonal columns. The columns orient perpendicular (radially) or parallel (axially) with respect to the pore axis depending on surface anchoring conditions and pore size. Axially oriented columns enable the fabrication of organic nanowires utilizing the high conductivity in the stacking direction due to overlapping π-electrons. This leads to interesting applications in e.g. organic semiconductorbased devices. The molecular ordering of the liquid crystalline columns can be probed by temperature dependent optical retardation measurements supplemented by X-ray diffraction sensitive to the translational order.
We investigated the DLC 2, 3, 6, 7, 10, 11 - hexakis [hexyloxy] triphenylene (HAT6) embedded in nanoporous alumina and silica membranes as function of the pore diameter (12 nm - 180 nm). Due to their hydrophilic nature porous membranes enforce face-on anchoring leading to a radial orientation. To obtain edge-on anchoring conditions, and thus favoring axial orientation, the silica membrane surface is chemically modified. The optical retardation measurements show that the columns orient radially in these membranes independent of the anchoring conditions. Interestingly, a quantized phase transition of each molecular layer is found indicated by a distinct increase of the optical orientation. Additionally, an axial orientation of HAT6 filled into alumina membranes with a pore diameter of 25 nm is achieved. A Landau-de Gennes ansatz semi-quantitatively describes the phase transition behavior observed. X-ray diffraction experiments performed at the 3rd generation synchrotron radiation source PETRA III at DESY giving detailed information about the translational order support these findings. Summarizing, this study shows the existence of a phase transition in the molecular range as well as the suitability of the membrane with 25 nm pores as a template for preparing organic nanowires.
Successful implementation of an optical polarimetry measurement setup. Due to wall anchoring interactions HAT6 embedded in an untreated alumina membrane exhibits a radial orientation for pore sizes in between 30 nm and 80 nm. Embedded in large membranes, pore size of 180 nm, HAT6 forms the favored hexagonal columnar phase along the pore axis.