ABSTRACT
The mesogens QL32-6, QL33-6 and QL-34-6 contain 5-phenylpyrimidine cores and terminal nanosegregating carbosilane end groups of different lengths and are known to exhibit 'deâ Vries-type' properties of varying strength. We report a systematic study of the influence of the nanosegregating sublayer on the dynamics and rotational viscosities of the collective modes in the smectic A* (SmA*) and smectic C* (SmC*) phase using dielectric spectroscopy. It was found that the dynamics of the Goldstone mode corresponding to phase angle fluctuations are almost not affected while the relaxation time and rotational viscosity of the soft mode are influenced by the degree of nanosegregation. In other words, the nanosegregating sublayer does not influence the dynamics of ferroelectric switching in the SmC* phase, but is critical in inducing 'deâ Vries-type' properties.
ABSTRACT
Structural variants of the 'de Vries-like' mesogen 5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]-2-hexyloxypyrimidine (QL16-6), including two isomers with branched iso-tricarbosilane end-groups, were synthesized and their mesomorphic and 'de Vries-like' properties were characterized by polarized optical microscopy, differential scanning calorimetry, small angle and 2D X-ray scattering techniques. A comparative analysis of isomers with linear and branched tricarbosilane end-groups shows that they exhibit comparable mesomorphic and 'de Vries-like' properties. Furthermore, the difference in effective molecular length Leff between the linear and branched isomers in the SmA and SmC phases (ca. 4-5 Å), which was derived from 2D X-ray scattering experiments, suggests that the linear tricarbosilane end-group is hemispherical in shape on the time-average, as predicted by a DFT conformational analysis at the B3LYP/6-31G* level.
ABSTRACT
The chiral carbosilane-terminated liquid crystal 2-[(2S,3S)-2,3-difluorohexyloxy]-5-[4-(12,12,14,14,16,16-hexamethyl-12,14,16-trisilaheptadecyloxy)phenyl]pyrimidine () undergoes a smectic A*-smectic C* phase transition with a maximum layer contraction of only 0.2%. It exhibits an electroclinic effect (ECE) comparable to that reported for the 'de Vries-like' liquid crystal and shows no appreciable optical stripe defects due to horizontal chevron formation.
ABSTRACT
The construction of liquid crystal compositions consisting of the dimeric liquid crystal, CB_C9_CB (cyanobiphenyl dimer = 1'',9''-bis(4-cyanobiphenyl-4'-yl)nonane), and the range of nematic systems is explored. The materials include a laterally functionalized monomer, which was used to construct a phase diagram with CB_C9_CB, as well as one laterally linked dimer liquid crystal material and two liquid crystal gold nanoparticle (LC-Au-NPs) systems. For the Au-NP-LCs, the NP diameters were varied between ~3.3 nm and 10 nm. Stable mixtures that exhibit a nematic-nematic phase transition are reported and were investigated by POM (polarizing optical microscopy), DSC (differential scanning calorimetry) and X-ray diffraction studies.
ABSTRACT
Electroclinic measurements, in which an applied electric field E induces a rotation Δθ ([proportional]E) of the liquid crystal director about the electric field axis in a chiral environment, were performed on several configurationally achiral liquid crystals in the presence of an imposed helical director profile. This imposed twist establishes a chiral symmetry environment for the liquid crystal. It was observed that a conformationally racemic mesogen possessing a flexible phenyl benzoate core exhibits a measurable electroclinic response in the nematic phase. On the other hand, when the phenyl benzoate mesogen is mixed with a mesogen containing a rigid, conformationally achiral core (fluorenone), or with a racemic dopant with an axially chiral core that mimics a mesogen having rigid right- and left-handed conformations (2,2'-spirobiindan-1,1'-dione), the magnitudes of the electroclinic responses were found to decrease sharply, apparently going to zero when extrapolated to the pure 2,2'-spirobiindan-1,1'-dione or fluorenone limit. (Note that neither of these additives possesses a nematic phase.). The results suggest that the flexibility of the core and its ability to deracemize conformationally in order to compensate the elastic energy cost of the imposed twist is the primary mechanism behind the observed electroclinic response.
ABSTRACT
The synthesis of a novel class of organosiloxane oligopodes, based on an axially chiral binaphthalene core is described and their mesogenic properties are fully characterised.
ABSTRACT
The use of the liquid-crystalline state to control the assembly of large (>5 nm) gold nanoparticles (NPs) is of considerable interest because of the promise of novel metamaterial properties of such systems. Here we report on a new approach for the preparation of large nematic gold NPs using a bifunctional capping agent that enables control over the particle size and serves as a linkage for subsequent functionalization with mesogenic groups. Properties of the NPs were characterized by HRTEM, NMR, DSC, TGA, UV/vis, OPM, and XRD studies. The results confirmed the formation of a stable nematic mesophase above 37.5 °C for NPs in the 6-11 nm size range.
Subject(s)
Amines/chemistry , Gold/chemistry , Metal Nanoparticles/chemistry , Molecular Structure , Particle Size , Surface Plasmon Resonance , Surface PropertiesABSTRACT
An intriguing class of quinones that efficiently catalyze the air oxidation (overall hydroxylation) of arylboronic acids to the corresponding phenol is reported. Autocatalysis in the parent system is particularly efficient and leads to rapid, quantitative synthesis of quinones such as 4 from boronic acid 1 at room temperature using air as stoichiometric oxidant. The efficiency results from a balance between two-stage conjugate addition and migration with each step driven by aromatization of a naphthalene fragment.
