Liliana Cseh

Structure–property relationships in nematic gold nanoparticles

Design rules for the investigation of nanoparticles covered with an organic monolayer are explored. The combination of small “spherical” particles in conjunction with nematogenic mesogens and hydrocarbon chains allows for the rational design of materials with nematic phase behaviour. The usefulness of the approach is demonstrated for two systems, where the particles have been characterised by NMR and TEM and mesomorphic properties have been identified using DSC and OPM.

Combinatorial synthesis and characterization of new asymmetric porphyrins as potential photosensitizers in photodynamic therapy.

Porphyrins play a major role as active photosensitizers in noninvasive optical photodynamic therapy (PDT). In a modular approach, this paper presents a short review of the recent developments of porphyrin structures and materials with improved photosensitizing properties and then presents the synthesis and characterization of a series of new second generation asymmetrical meso-tetraphenylporphyrins varied by substituent in the meta positions of the phenyl rings with either -OH or -OCH3 groups, whereas in the para positions only with -OCH3 groups. The new series of differentially functionalized porphyrins were obtained by a combinatorial multicomponent synthesis (Adler-Longo method) by simultaneously using two different aldehydes: 3,4-dimethoxybenzaldehyde and 3-hydroxybenzaldehyde. The porphyrins were isolated, purified and characterized by HPLC, TLC, UV-vis, fluorescence, MS, 1H-NMR, and 13C-NMR analysis, accompanied by DEPT 135 experiments. Because of the fact that the medium in cancerous tissues is often more acidic than in normal tissues, the capacity of these porphyrins to exist simultaneously in aggregated and protonated forms was also investigated, in tetrahydrofuran (THF) and acid THF-water systems, underlying the changes in the photophysical behaviour. The relative fluorescence quantum yields (Phif) were calculated in comparison with meso-tetraphenylporphyrin (TPP), and the values between 0.14-0.26 were found to be promising for further trials. The series of asymmetrically substituted tetraphenylporphyrins, as the new class of supramolecular materials, are suitable for further functionalization in order to improve their photophysical properties, and they could represent interesting potential PDT photosensitizers.

Control of anisotropic self-assembly of gold nanoparticles coated with mesogens

The self-assembly was investigated of a series of gold nanoparticles (GNP) that were covered with laterally attached mesogenic molecules (ligands) and different amounts of alkylthiols (co-ligands). The presence of mesogens induces long-range ordered packing of GNPs. Here we report three different ordered liquid crystalline phases built around the GNPs, two of which have not been observed previously in thermotropic liquid crystals. These are the 3D simple hexagonal packing and the face-centred cubic (FCC) packing of spheres, i.e. GNPs in this case. It has been found that the volume fraction of co-ligands in such systems is critical for the anisotropy of the self-assembly of GNPs; either too small or too large a co-ligand volume fraction causes the formation of one of the two nearly or fully isotropic packing types mentioned above. In these two structures the rod-like mesogens have no preferred orientation. At intermediate co-ligand fractions highly anisotropic strings of GNPs are obtained, packed in a 2D hexagonal lattice, with the mesogens surrounding the strings and lying parallel to their axis. By matching the experimental X-ray diffraction intensities with those calculated from models, the mean deviation of the 2 nm GNPs from their ideal lattice position was found to decrease from 0.84 nm for the material with no co-ligands to 0.44 nm for the complex with dodecylthiol; this increase in order is attributed to the soft alkyls evening out the particle non-uniformities. The results help us to understand and control the way organic molecules affect the organization of nanoparticles, and facilitate the design of new hybrid materials through self-assembly.

Testing the triple network structure of the cubic Imm (I) phase by isomorphous replacement and model refinement

This study provides further tests of the validity of the triple network structure of the body-centred Imm (I) cubic phase in polycatenar (phasmidic) liquid crystals, proposed by Zeng et al., Nat. Mater., 2005, 4, 562. New hemiphasmid compounds were synthesised, two of which (6 and 7) display the above phase, while 8 shows the hexagonal columnar phase. Comparison of 3-d electron density maps based on small-angle X-ray diffraction of two compounds with different proportions of high and low electron density regions (aliphatic and aromatic) helped with phasing the X-ray reflections of the cubic structure. This variant of the isomorphous replacement technique gave further support to the triple network structure proposed earlier. Furthermore, fitting calculated diffraction intensities of geometrical models to the observed ones confirmed that the oscillating segment cross-section of two of the three networks (the “octahedral networks”) are genuine structural features rather than artefacts. A rational explanation is proposed for the variable thickness of the aromatic network segments. The study has highlighted complexities of molecular packing in bicontinuous phases of thermotropic liquid crystals that are not encountered in related phases in lyotropics or block copolymers.

Helically Twisted Chiral Arrays of Gold Nanoparticles Coated with a Cholesterol Mesogen

Gold nanoparticles have been prepared and surface-functionalized with a 1:1 molar mixture of a hexylthiol ligand and a chiral mesogenic ligand consisting of a cholesterylbenzoate attached via an undecylthiol spacer. Grazing incidence X-ray diffraction showed that upon annealing a columnar liquid crystal (LC) structure develops with the nanoparticles forming strings on a regular oblique 2d lattice. Synchrotron radiation circular dichroism is substantially enhanced upon the isotropic-LC transition. In the proposed structural model, layers of Au columns rotate by a small angle relative to their neighbors, with the columns winding around a helical axis. The work demonstrates that it is possible to obtain chiral LC superstructures from nanoparticles coated with chiral mesogen without the addition of a separate LC or chiral dopants. The results provide direction in the development of plasmonic metamaterials interacting selectively with circularly polarized light.

Light-induced changes of the refractive indices in a colloid of gold nanoparticles in a nematic liquid crystal

It was shown that irradiation of a nematic liquid crystal doped with metal nanoparticles in the visible near the plasmon resonance band led to strong thermal changes of the refractive indices. The effect was studied by recording of dynamic optical gratings in the colloid. Nanoparticles “worked” as effective nano-heaters in a matrix causing the order parameter decrease around the particles. A large nonlinearity parameter (n2 ≈ 10-2 cm2/kW and fast response (≈ 0.7 ms), with no detectable particles’ aggregation and excellent photo- thermo-stability make these colloids potentially attractive nonlinear optical media. Application of a dynamic holography technique allowed measuring the coefficients of thermal conductivity of the liquid crystal along the director k|| = (0.4 ± 0.02) W m−1K−1 and perpendicular to the director k⊥ = (0.2 ± 0.01) W m−1K−1.

Strong Cubic Optical Nonlinearity of Gold Nanoparticles Suspension in Nematic Liquid Crystal

We report on the observations of extremely strong and fast cubic optical nonlinearity in the suspension of gold nanoparticles in nematic liquid crystal (LC). The nonlinearity was observed by recording dynamic phase sinusoidal holograms with a continuous operating laser. It is caused by changes of the refractive index of a LC due to efficient light-induced heating of the nanoparticles at plasmon resonance excitation and following thermal transport of heat to LC matrix. Large nonlinearity parameter (n 2 ∼ 10−2 cm2/kW) and fast characteristic times, together with excellent photo- and thermo-stability of the system make its extremely promising for optical processing applications.

Added Alkane Allows Thermal Thinning of Supramolecular Columns by Forming Superlattice-An X-ray and Neutron Study.

We report a columnar superlattice formed by blends of dendron-like Li 3,4,5-tris(n-alkoxy)benzoates with n-alkanes. Without the alkane, the wedge-shaped molecules form liquid crystal columns with 3 dendrons in a supramolecular disk. The same structure exists in the blend, but on heating one dendron is expelled from the disks in every third column and is replaced by the alkane. This superlattice of unequal columns is confirmed by complementary X-ray and neutron diffraction studies. Lateral thermal expansion of dendrons normally leads to the expulsion of excess molecules from the column, reducing the column diameter. However, in the already narrow columns of pure Li salt, expulsion of one of only three dendrons in a disk is not viable. The added alkane facilitates the expulsion, as it replaces the missing dendron. Replacing the alkane with a functional compound can potentially lead to active nanoarrays with relatively large periodicity by using only small molecules.

Chemistry and Photochemistry of 2,6-Bis(2-hydroxybenzilidene)cyclohexanone. An Example of a Compound Following the Anthocyanins Network of Chemical Reactions

The kinetics and thermodynamics of the 2,6-bis(2-hydroxybenzilidene)cyclohexanone chemical reactions network was studied at different pH values using NMR, UV-vis, continuous irradiation, and flash photolysis. The chemical behavior of the system partially resembles anthocyanins and their analogue compounds. 2,6-Bis(2-hydroxybenzilidene)cyclohexanone exhibits a slow color change from yellow to red styrylflavylium under extreme acidic conditions. The rate constant for this process (5 × 10(-5) s(-1)) is pH independent and controlled by the cis-trans isomerization barrier. However, the interesting feature is the appearance of the colorless compound, 7,8-dihydro-6H-chromeno[3,2-d]xanthene, isolated from solutions of acid to neutral range, characterized by (1)H NMR and single crystal X-ray diffraction. Light absorption by 2,6-bis(2-hydroxybenzilidene)cyclohexanone solutions immediately after preparation exclusively results in cis-isomer as photoproduct, which via hemiketal formation yields (i) red styrylflavylium by dehydration under extremely acidic solutions (pH < 1) and (ii) colorless 7,8-dihydro-6H-chromeno[3,2-d]xanthene by cyclization in solutions of acid to neutral range.

New Compounds with Potential Liquid Crystal Properties. Copper(II) and Nickel(II) Complexes of N,N′-bis(4-decyloxysalicyliden-N-n-propyl)-Piperazine. Synthesis and Characterization

The new Schiff base N,N′-bis(4-decyloxysalicyliden-N-n-propyl)-piperazine (ZOPP) was designed and synthesized. Structures were confirmed by elemental analysis and IR, 1H-NMR spectroscopy. ZOPP can act as ligand through the two set of donor atoms (one set: two nitrogen atoms and one oxygen) situated on each side of the piperazine bridge. The nature of the complexes depends strongly on the metal to ligand molar ratio. Thus, starting from metal to ligand of 1 to 1 ratio, where M=Cu(II), Ni(II), binuclear complexes with a polymeric structure have been obtained, whereas at metal to ligand molar ratio of 2 to 1, discrete binuclear complexes were formed.