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To investigate size and capping agent-dependent alignment and electro-optic effects of quantum dots (QDs) in nematic LCs, mixtures of a nematic phenylpyrimidine LC (LC1) doped with hexadecylamine-capped CdSe and thioglycolic acid-capped CdTe QDs were prepared and investigated by polarized optical microscopy between plain, untreated glass slides and in ITO-coated glass cells with rubbed polyimide alignment layers as well as by electro-optic analysis. The five investigated CdSe QDs, varying in size from 2.5 to 5.2 nm, show fluorescence emission maxima ranging from 480 to 610 nm, and the four CdTe QDs, varying in size from 3.2 to 4.0 nm, emit light from 530 to 610 nm depending on their size. The CdSe QDs favoring homeotropic anchoring of the nematic LC induce vertical alignment at 1 and 2 wt% in LC1between plain glass slides and only at 1 wt% in the planar cells, except for CdSe QD emitting at 590 nm (CdSe590), which induces vertical alignment independent of the substrate only at 1 wt%. Concentrations above 2 wt% of the CdSe QDs in LC1show planar alignment mainly due to aggregation of the QDs in LC1at these higher concentrations. On the contrary, the CdTe QDs favoring planar anchoring of the nematic LC induce homeotropic alignment of LC1at concentrations above 2 wt% between plain glass slides. All QDs are effective in reducing the threshold voltage (Vth) and the splay elastic constant (K11). The most drastic reduction over the entire temperature range, however, was observed for the smallest CdSe480QDs, which showed the least tendency of all investigated QDs to aggregate in LC1. [ABSTRACT FROM AUTHOR]

We present the first synthesis of a series of thiol-, thioacetate-, and xanthate-terminated bent-core derivatives, as well as the preparation of gold nanoclusters using the thiol-terminated bent-core compounds. The mesomorphic properties of the bent-core compounds have been investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and electro-optic tests. From this series, the mono-xanthate derivatives form rectangular or hexagonal columnar phases, and the mono-thiol derivative with shorter alkyl chains forms a metastable, likely smectic, phase. All other bent-core compounds and intermediates exclusively form disordered or multi-layer lamellar crystalline solid modifications. High-resolution transmission electron microscopy (HR-TEM) images of the bent-core functionalized gold nanoclusters prepared from the thiol-terminated derivatives, taken after slow evaporation of the solvent, provide evidence for the formation of self-assembled nanocluster arrays. Upon dispersion of these gold nanoclusters in two structurally related bent-core LC hosts (one forming a SmCPA, and the second with a terminal double bond displaying a Colr phase), a shift in the surface plasmon resonance was observed indicating a LC host-dependent aggregation of the nanoparticles. Finally, we report initial results on the effects of different concentrations of these nanoclusters on the thermal properties as well as the applied voltage/current response of the parent SmCPA host. [ABSTRACT FROM AUTHOR]

In a recent study, we found that nematic liquid crystals (N-LCs) doped with chiral (S)-naproxen-functionalized dodecane thiolate protected gold nanoclusters (Au2, Au3) or non-chiral alkyl thiolate protected Au clusters (Au1) produce thin film textures with characteristic uniform stripe patterns separated by areas of homeotropic alignment. While these textures closely resemble textures commonly observed for chiral nematic (N*-)phases with large helical pitch, so-called cholesteric finger textures, they originate from local concentration differences of the nanoclusters in the N-LC solvent. While areas with higher particle content form linear particle aggregates (stripe domains) due to the surface anchoring of the N-LC molecules to the cluster surface, areas of lower particle concentration give homeotropic alignment as a result of particles residing at the glass–N-LC interfaces. To elucidate and confirm a chirality transfer from the chirally modified gold clusters to the non-chiral N-LC, despite the complex thin film textures, we here present detailed induced circular dichroism (ICD) studies of thin films of 5CB doped with the three different Au clusters. These experiments revealed that the chiral Au nanoclusters (Au2, Au3) successfully transfer chirality to the N-LC host producing a chiral nematic phase (N*) with the opposite helical sense in comparison to the pure, organic chiral dopant dispersed in the same N-LC host. Thus, these results provide the first experimental proof for the usefulness of gold nanoclusters as chiral dopants for N-LCs. In contrast, for the non-chiral Au cluster, at a macroscopic level, no relationship between the cholesteric finger-like textures and chirality was found. [ABSTRACT FROM AUTHOR]

Electron microscopy, optical microscopy, and X-ray diffraction studies of dispersions of hydrophilic and hydrophobic thiolate-capped gold nanoclusters in bolaamphiphilic liquid crystals (LCs) indicate random inclusion of the nanoclusters in the LC matrix with a concentration- and monolayer-type-dependent effect on the mesophase stability of the LC host. Both types of nanoclusters show no effect on the thermal properties of the smectic A phase of 1. In contrast, the rectangular columnar phase of 2 is gradually destabilized with an increasing concentration of the hydrophobic hexane thiolate gold clusters Au1, and gradually stabilized with an increasing concentration of the hydrophilic ethyleneglycol protected gold nanoclusters Au2. In addition, structural characterization using SAXS indicated no change in the lattice of the rectangular columnar phase of 2 in mixtures with both nanoclusters (c2mm), but provided evidence for a macroscopic phase separation into particle-rich and particle-poor domains with the same rectangular lattice but slightly different lattice parameters for mixtures with higher concentrations of Au2 in 2. [ABSTRACT FROM AUTHOR]

Results of STXM investigations of a binary mixture (1–TNF = 2 : 1; SmAb 140 M 180 Iso) known to form a SmAb phase [T. Hegmann, J. Kain, S. Diele, G. Pelzl and C. Tschierske, Angew. Chem. Int. Ed., 2001, 40, 887] are presented. Near edge X-ray absorption fine spectra (NEXAFS) of the 1–TNF board-like aggregates, in particular the intensity of the low energy peaks associated with aromatic ring π* orbitals (284.5–286.5 eV), show that the molecular plane of these aggregates is very sensitive to the relative orientation of electric field vector E of linearly polarized light, which is used to determine the molecular orientation in the LC phase. The observed strong in-plane dichroic signal suggests the predominant orientation of the 1–TNF aggregates to be along the smectic layer normal as well as long-range ordering of the in-plane molecular orientation (biaxiality). Orientational maps derived from series of measurements at different sample rotation angles around the specimen normal clearly show a Schlieren-type texture, and permit a detailed examination of exclusive ±½ disclination theoretically predicted for the SmAb phase. [ABSTRACT FROM AUTHOR]