Paramagnetic Liquid Crystals With Close π-π Packing: The Effect of Blatter Radical Planarization on Behavior of Bent-Core Mesogens.

A new, 19 π-delocalized electrons planar Blatter radical building block was developed and used to obtain paramagnetic bent-core liquid crystals. The mesogens were investigated by optical, thermal, powder XRD and DFT methods in the pure form and as binary mixtures. Comparison of their properties with those of the classical Blatter radical analogues revealed that planarization of the central angular element results in a significantly higher stability of the mesophases and increased molecular organization suitable for the formation of ordered banana and columnar mesophases with tighter π-π interactions. These results indicate access to a new, potentially rich class of functional paramagnetic soft materials.

Large Thermally Irreversible Photoinduced Shift of Selective Light Reflection in Hydrazone-Containing Cholesteric Polymer Systems.

Stimuli responsive liquid crystalline polymers are a unique class of so-called "smart" materials demonstrating various types of mesomorphic structures easily controlled by external fields, including light. In the present work we synthesized and studied a comb-shaped hydrazone-containing copolyacrylate exhibited cholesteric liquid crystalline properties with the pitch length of the helix being tuned under irradiation with light. In the cholesteric phase selective light reflection in the near IR spectral range (1650 nm) was measured and a large blue shift of the reflection peak from 1650 nm to 500 nm was found under blue light (428 or 457 nm) irradiation. This shift is related to the Z-E isomerization of photochromic hydrazone-containing groups and it is photochemically reversible. The improved and faster photo-optical response was found after copolymer doping with 10 wt % of low-molar-mass liquid crystal. It is noteworthy that both, the E and Z isomers of hydrazone photochromic group are thermally stable that enable to achieve a pure photoinduced switch without any dark relaxation at any temperatures. The large photoinduced shift of the selective light reflection, together with thermal bistability, makes such systems promising for applications in photonics.

Laser-induced formation of "craters" and "hills" in azobenzene-containing polymethacrylate films.

Functional organic polymer materials with an ability to change their surface topography in response to external contactless stimuli, like light irradiation, have attracted considerable attention. This work is devoted to the study of contactless control of the surface topography and the formation of the surface features in the amorphousized and liquid crystalline films of two azobenzene-containing polymers. The investigated polymers are side-chain polymethacrylates containing azobenzene chromophores with two lateral methyl substituents in ortho-positions and differing in the length of flexible spacer with six and ten methylene units. Two lateral methyl substituents at the azobenzene chromophore ensure high photoresponses of these polymeric samples in the whole visible spectral range. Irradiation of the polymethacrylate films by focused polarized light of green (532 nm) and red (633 nm) lasers induces a specific photodeformation of the film surface. In the case of the green light formation of circular "craters" with anisotropic borders was found, whereas for the red light highly asymmetric "hills" were observed. The possible mechanisms of the surface topography formation and their features are discussed.

Influence of photoinduced isomerization on the chiral separation of novel liquid crystalline materials with a diazene moiety.

The influence of photoinduced isomerization on the enantiomeric separation of two newly synthesized liquid crystalline materials, liquid crystals 1 and 2, was studied by high-performance liquid chromatography on a chiral stationary phase Chiralpack AD-3. Both materials have one chiral center and one diazene moiety. The compounds were separated into their E and Z isomeric forms. The conditions and time scale of the ultraviolet-induced E to Z transition were briefly evaluated. Under the optimized conditions, we were able to baseline separate the S and R enantiomers of both the studied materials in their E isomeric form. The chiral separation of liquid crystal 2 after ultraviolet irradiation was unsuccessful. In contrast, the chiral separation of liquid crystal 1 possessing a similar structure to liquid crystal 2 provided baseline separation in its Z isomeric form as well. Previously, we have shown the influence of photoinduced isomerization and its utilization in the enantioseparation on relatively simple molecules. Here, we demonstrate that (1) much more complex compounds can also be successfully separated despite the bulkiness of the achiral part of the structure and (2) photoinduced isomerization even for such complex molecules still strongly influences their chromatographic properties.

All-optical 3D blue phase photonic crystal switch with photosensitive dopants.

Blue phase (BP) liquid crystals (LC) have lately become the focus of extensive research due to their peculiar properties and structure. BPs exhibit a highly organized 3D structure with a lattice period in the hundreds of nm. Owing to such structure, BPs are regarded as 3D photonic crystals. The unique properties of this complex LC phase are achieved by the self-assembly of the LC molecules into periodic cubic structures, producing bright selective Bragg reflections. Novel applications involving 3D photonic crystals would certainly benefit from enhanced ground-breaking functionalities. However, the use of BPs as 3D has been traditionally curtailed by the BP crystals trend to grow as random polycrystals, making it difficult to develop practical BP-based photonic devices. The possibility of generating mm-sized BP monocrystals was recently demonstrated. However, besides increasing the scarce number of 3D photonic structural materials, their applications as 3D photonic crystals do not show apparent advantages over other solid materials or metamaterials. Having a tunable BP monocrystal, where crystals could be switched, modulating simultaneously some of their properties as 3D photonic crystals, they would constitute a new family of materials with superior performance to other existing materials, opening up a plethora of new applications. In this work, an all-optical switchable 3D photonic crystal based on BPs doped with tailored photoactive molecules is demonstrated. Two switching modes have been achieved, one where the BP reversibly transitions between two BP phases, BPI and BPII, (two different cubic crystal systems) while maintaining the monocrystallinity of the whole system. The second mode, again reversible, switches between BPI and isotropic state. None of these modes are related to the regular thermal transitions between LC phases; switching is triggered by light pulses of different wavelengths. This all-optical approach allows for a seamless fast remotely controlled optical switch between two 3D photonic crystals in different cubic crystal systems and between a photonic crystal and an isotropic matrix. Applications of switchable BPs for adaptive optics systems or photonic integrated circuits would make great advances using 3D photonic crystal switches. All-optical photonic systems such as these hold great promise for the development of tunable and efficient photonic devices such as dynamic optical filters and sensors, as they enable light-driven modulation and sensing applications with unprecedented versatility.

Design and Self-Assembling Behaviour of Calamitic Reactive Mesogens with Lateral Methyl and Methoxy Substituents and Vinyl Terminal Group.

Smart self-organising systems attract considerable attention in the scientific community. In order to control and stabilise the liquid crystalline behaviour, and hence the self-organisation, the polymerisation process can be effectively used. Mesogenic units incorporated into the backbones as functional side chains of weakly cross-linked macromolecules can become orientationally ordered. Several new calamitic reactive mesogens possessing the vinyl terminal group with varying flexible chain lengths and with/without lateral substitution by the methyl (methoxy) groups have been designed and studied. Depending on the molecular structure, namely, the type and position of the lateral substituents, the resulting materials form the nematic, the orthogonal SmA and the tilted SmC phases in a reasonably broad temperature range, and the structure of the mesophases was confirmed by X-ray diffraction experiments. The main objective of this work is to contribute to better understanding of the molecular structure-mesomorphic property relationship for new functional reactive mesogens, aiming at further design of smart self-assembling macromolecular materials for novel sensor systems.

Communication: Experimental proof of symmetry breaking in tilted smectics composed of molecules with axial chirality.

For the first time, domains with twisted structures have been established in planar samples of achiral compounds in tilted smectic C phase. This evidences separation of molecular conformers differing in the sense of axial chirality and confirms polar C(2) symmetry of these domains. A simple model considering polar surface anchoring energy and bulk energy of the twist can account for this finding. Conditions for coexistence of twisted and homogeneous domains are discussed.

Silver Nanoparticles with Liquid Crystalline Ligands Based on Lactic Acid Derivatives.

We have prepared and studied silver nanoparticles functionalized with ligands based on lactic acid derivatives. Several types of hybrid systems that differed in the size of silver nanoparticles as well as the length of surface ligands were analyzed. Transmission electron microscopy (TEM) observation provided information about the size and shape of nanoparticles and proved good homogeneity of studied systems. By dynamic light scattering (DLS) measurements, we have measured the size distribution of nanoparticle systems. Plasmonic resonance was detected at around 450 nm. For two hybrid systems, the mesomorphic behaviour has been demonstrated by x-ray measurements. The observed thermotropic liquid crystalline phases reveal lamellar character. We have proposed a model based on self-assembly of intercalated liquid crystalline ligands.

Organic nanotubes created from mesogenic derivatives.

A facile route to prepare nanotubes from rod-like mesogens dissolved in typical organic solvents is reported. For selected types of chiral rod-like molecules, nanotubes were formed from both enantiomers and racemic mixtures by slow evaporation from solution, regardless of the solvent, concentration or deposition type. The obtained supramolecular assemblies were studied using AFM, TEM and SEM techniques, and other experimental techniques (IR, UV-Vis spectroscopy and X-ray diffraction) were also applied. The difference in the surface tension at opposite crystallite surfaces is suggested as a possible mechanism for nanotube nucleation. We propose a quite new rolling-up mechanism related to the surface tension difference at opposite crystallite surfaces.