Liquid crytals (LCs) are probably the most exciting state of matter. They attract scientists across disciplines such as chemistry, physics, materials science, and engineering—theorists and experimentalists alike. These days, you can buy quite affordable LCD TV sets around the corner, but that's just one of the many things liquid crystals can do. The selection of recent research articles presented below illustrates the broad interest in this area of soft condensed matter.
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Stars in a helix: Fluorescent tristriazolotriazines with dialkoxyaryl branches (see figure) were prepared, and their optical and thermotropic properties were studied. The discotic molecules form broad hexagonal columnar mesophases with small π-stacking distances. In the crystalline state, the columns are composed of a helical superstructure.
Riot of color: Alternate stacking of aromatic donor and acceptor building blocks by complementary and directional charge-transfer interactions produce versatile supramolecularly assembled materials including micelles, vesicles, nanotubes, fibrillar gels, folded polymers, cross-linked networks, and liquid-crystalline phases.
Helical carbon and graphite films are prepared from helical poly(3,4-ethylenedioxythiophene) films with a tunable helical sense and degree of helicity as precursors. The precursor polymer films are synthesized through asymmetric electrochemical polymerization in chiral nematic liquid-crystal (LC) fields. The spiral morphologies of the precursors are retained in the present graphite films.
I'm stuck on you: Droplet-based liquid crystal (LC) chemical sensors can be immobilized on living cells. The decorated cells can report in real time on the presence of toxins in surrounding culture media. The approach provides new principles for the design of droplet-based LC sensors as well as methods for the local detection and reporting of chemical agents that are difficult to achieve in cellular environments using free-floating LC droplets.
Driving the self-assembly: Nematic liquid crystalline environments drive the reversible self-aggregation of a β-pentapeptide into oligomers with a well-defined structure. The inter- monomer distance distributions were obtained by electron paramagnetic resonance spectroscopy and this information enabled modeling of the oligomeric structures.
A good turn: Three compounds that bear two axially chiral bridged binaphthyl units were developed as photodynamic chiral dopants for nematic liquid crystals. For compounds with suitable bridge lengths, a change in the dihedral angle induced a switch of the binaphthyl units from the cisoid to the transoid form upon UV irradiation, which led to an inversion of the handedness of the helices.
Magnetic moustaches: Inorganic surfactants (I-SURFs) with head groups containing Dy3+ undergo a hierarchical self-organization cascade controlled by magnetic interactions. The resulting aggregates are shaped like dumbbells with frayed, moustache-like ends.
Great balls of fire: C60 and Y3N@C80 were connected to the same oligo(phenyleneethynylene) unit to investigate their structural and photophysical properties. NMR investigations revealed a fulleroid structure for the Y3N@C80 derivative, and both dyads gave rise to columnar phases with core-shell cylinders. The black and gray spheres represent the fullerene core units of the Y3N@C80 derivative, which is an ideal candidate to be involved in energy and electron transfer processes.
Assembly line: Truxene derivatives self-assemble in solution and are able to gelate different solvent mixtures despite not having groups able to establish strong directional interactions (see figure). By taking advantage of the balance between molecule–molecule and molecule–solvent interactions, it is possible to control the final morphology of the resulting structures from fibrous superstructures to nanospheres.
Ionic liquid crystals from amino acids: Novel chiral amino acid derived imidazolium salts with amine or amide units were prepared through the sulfamidate. Depending on the steric bulk of the R group, the chain lengths and the type of anion X, stable SmA phases were detected (see scheme; Bn=benzyl).
Organized nanomaterials: Nanocrystalline chitin (NCh) prepared by sequential deacetylation and hydrolysis of chitin fibrils isolated from king crab shells has been used to template mesoporous silica and organosilica (see figure). The large, crack-free films accurately replicate the organization of the NCh films originating from a transcription of the layered nematic phase of NCh.
All in order: The self-assembly of nanocrystalline cellulose (NCC) with hydrogel precursors leads to nanocomposites with long-range chiral nematic order. The combination of chiral structure and hydrogel swelling behavior gives rise to iridescence that rapidly responds to various stimuli.
Colors of nature: Mimicking of the structural colors of nature was achieved by the preparation of easily accessible chiral nematic polymer composites based on phenol–formaldehyde resins templated by cellulose nanocrystals. Removal of the template led to mesoporous polymer films with unique optical and physical properties. The potential application of these materials in optical sensors was also demonstrated.
Patterns of communication: Reinitzer's discovery of liquid crystals in 1888 was followed by 30 years of scholarly dispute. One hundred years later, Pierre-Gilles de Gennes was awarded the Nobel Prize in Physics for his contribution to this scientific revolution. The commercial success of liquid crystals was achieved in display applications. Today more than 4 billion people use them in mobile communication devices. Painting: Detail from Raphael's School of Athens fresco.
Activity from order: Liquid-crystalline materials (see picture; blue) are formed from anisotropic molecules. They are used in liquid-crystal displays (LCDs), the prototype of flat-panel displays. Moreover, the combination of order and mobility in these phases allows the realization of mechanical actuators (green) or the improvement of materials for organic electronics (red).
A push in the right direction: An electron-accepting organophosphorus system has been combined with self-assembly features to create a strongly electron-accepting liquid-crystalline material (see picture). The stability and behavior of the self-assembled liquid crystal could be controlled by adjusting weak intermolecular forces, such as hydrogen bonding and π–π interactions.
Liquid crystals on the way to complexity: Recent developments in liquid-crystalline materials have lead to new structures with enhanced complexity, including honeycombs and multicompartment structures, vesicular phases, and periodic and quasiperiodic arrays. New properties emerge, such as ferroelctricity and spontaneous achiral symmetry-breaking.
On the other hand: Azoarene compounds with axially chiral binaphthyl units of the same and opposite chiral configurations were doped into achiral liquid crystals (LCs). They were found to efficiently induce self-organized helical superstructures, which could be reversibly tuned by light irradiation using trans–cis photoisomerization to change the handedness of the helix (see scheme) in LC hosts.
Discotics studied by EPR: The application of EPR spectroscopy to columnar discotic liquid crystals using a novel rigid-core nitroxide spin probe (see picture) is possible. EPR spectra measured at different temperatures across three phases of hexakis(n-hexyloxy)triphenylene show a strong sensitivity to the phase composition, molecular rotational dynamics, and columnar order.
A lyotropic analogue of the ferroelectric smectic C* phase has been found. The lyotropic smectic C* phase shows macroscopic chirality effects, such as a helical ground state and polarity-dependent electrooptic switching, thus indicating the presence of a spontaneous electric polarization. The helicity implies communication of the chiral director twist across the achiral solvent layers separating adjacent layers of the chiral mesogens.
Liquid refreshment: Over the past10 years liquid-crystal display (LCD) technology has been established as the leading display technology for televisions, PCs, and smartphones. The design of new materials to fulfill the stringent technical specifications with regard to electrooptical performance and reliability is getting more and more challenging, and the synthetic chemistry requires increasingly creative solutions.
Controlling chirality: A general method to obtain homochiral helical nanofilaments (HNFs) based on a twisted nematic (TN) configuration was developed. By mixing bent-core molecules in the B4 phase with rod-like molecules in the nematic phase, the mixtures show the phase sequence of N–Bx(B4/N). Homochiral HNFs in the Bx phase were obtained from the mixtures when TN cells were cooled. The homochiral HNFs were observed by atomic force microscopy (see picture).