Christopher D. Jones

End-to-End Stacking and Liquid Crystal Condensation of 6– to 20–Base Pair DNA Duplexes

Short complementary B-form DNA oligomers, 6 to 20 base pairs in length, are found to exhibit nematic and columnar liquid crystal phases, even though such duplexes lack the shape anisotropy required for liquid crystal ordering. Structural study shows that these phases are produced by the end-to-end adhesion and consequent stacking of the duplex oligomers into polydisperse anisotropic rod-shaped aggregates, which can order into liquid crystals. Upon cooling mixed solutions of short DNA oligomers, in which only a small fraction of the DNA present is complementary, the duplex-forming oligomers phase-separate into liquid crystal droplets, leaving the unpaired single strands in isotropic solution. In a chemical environment where oligomer ligation is possible, such ordering and condensation would provide an autocatalytic link whereby complementarity promotes the extended polymerization of complementary oligomers.

Helical Nanofilament Phases

Packing Bananas and Boomerangs Assembling achiral molecules typically generates achiral domains. However, odd things can happen when the molecules are banana-or boomerang-shaped—their cores can twist out of plain to form left- or right-handed helices, which can then pack into chiral domains that will polarize light (see the Perspective by Amabilino). Hough et al. (p. 452) show that if you make the situation even more complex by frustrating the packing of adjacent layers, you can create a material that appears to be macroscopically isotropic with only very local positional and orientational ordering of the molecules but still shows an overall chirality. In a second paper, Hough et al. (p. 456) also show that if you change the chemistry of the molecules to allow for better overall packing, you can create a situation where helical filaments form that also tend to pack in layered structures. However, the frustration between the two types of packing leads to macroscopically chiral and mesoporous structures. Molecules lacking handedness can form layered, mesoporous helical structures. In the formation of chiral crystals, the tendency for twist in the orientation of neighboring molecules is incompatible with ordering into a lattice: Twist is expelled from planar layers at the expense of local strain. We report the ordered state of a neat material in which a local chiral structure is expressed as twisted layers, a state made possible by spatial limitation of layering to a periodic array of nanoscale filaments. Although made of achiral molecules, the layers in these filaments are twisted and rigorously homochiral—a broken symmetry. The precise structural definition achieved in filament self-assembly enables collective organization into arrays in which an additional broken symmetry—the appearance of macroscopic coherence of the filament twist—produces a liquid crystal phase of helically precessing layers.

Chiral Isotropic Liquids from Achiral Molecules

Packing Bananas and Boomerangs Assembling achiral molecules typically generates achiral domains. However, odd things can happen when the molecules are banana-or boomerang-shaped—their cores can twist out of plain to form left- or right-handed helices, which can then pack into chiral domains that will polarize light (see the Perspective by Amabilino). Hough et al. (p. 452) show that if you make the situation even more complex by frustrating the packing of adjacent layers, you can create a material that appears to be macroscopically isotropic with only very local positional and orientational ordering of the molecules but still shows an overall chirality. In a second paper, Hough et al. (p. 456) also show that if you change the chemistry of the molecules to allow for better overall packing, you can create a situation where helical filaments form that also tend to pack in layered structures. However, the frustration between the two types of packing leads to macroscopically chiral and mesoporous structures. Banana-shaped molecules lacking handedness form a macroscopically isotropic fluid that still has overall chirality. A variety of simple bent-core molecules exhibit smectic liquid crystal phases of planar fluid layers that are spontaneously both polar and chiral in the absence of crystalline order. We found that because of intralayer structural mismatch, such layers are also only marginally stable against spontaneous saddle splay deformation, which is incompatible with long-range order. This results in macroscopically isotropic fluids that possess only short-range orientational and positional order, in which the only macroscopically broken symmetry is chirality—even though the phases are formed from achiral molecules. Their conglomerate domains exhibit optical rotatory powers comparable to the highest ever found for isotropic fluids of chiral molecules.

ALUMINA ULTRAFILTRATION MEMBRANES DERIVED FROM CARBOXYLATE-ALUMOXANE NANOPARTICLES

Abstract The fabrication of asymmetric alumina ultrafiltration membranes using acetic acid surface stabilized alumina nanoparticles (A-alumoxanes) has been investigated. Contacting α-alumina supports with an aqueous solution of A-alumoxane (after firing to 1000°C) yields a defect free alumina membrane with a thickness of ca. 2xa0μm. The alumoxane-derived membranes have a molecular weight cut-off in the range of 35,000–44,000xa0gxa0mol −1 , high porosity, and a permeability that is comparable to or greater than that of commercially available alumina membranes. SEM and AFM show that the surface of the alumoxane-derived membranes is quite smooth and contact angles show that the membrane is hydrophillic. A comparison with commercial alumina and polymer membranes, as well as those derived from sol–gel methods is presented.

Method for characterizing self-assembled monolayers as antirelaxation wall coatings for alkali vapor cells

We describe a method for characterizing self-assembled monolayers (SAMs) in terms of their performance as antirelaxation wall coatings for alkali atom vapor cells. A combination of initial surface analysis and subsequent laser spectroscopy is used to provide insight into the quality of the coating, as well as its performance under the exposure to alkalis as it occurs, for example, when used in applications such as atomic magnetometers or clocks. Fused silica plates coated with octadecyltrichlorosilane SAMs were used to make cubic R87b gas cells. The surface was characterized by x-ray diffraction, contact angle measurements, and atomic force microscopy. Measurements of hyperfine resonance linewidths and frequency shifts show that the rubidium vapor atoms collide up to 40 times with the walls of the cells before their coherence relaxes and their adsorption energy is around 0.065 eV. Chemical analysis of the cell indicates some weak reactions between the coating and the rubidium atoms.

Siloxane-terminated phenylpyrimidine liquid crystal hosts

We report the synthesis and characterization of trisiloxane-terminated liquid crystals with 2-phenylpyrimidine cores that form partially bilayered SmA and SmC phases. Variable temperature measurements of smectic layer spacings by powder X-ray diffraction combined with measurements of optical tilt angles and observations of birefringence changes by polarized optical microscopy reveal that the trisiloxane end-group causes the SmA–SmC transition of these compounds to be more ‘de Vries-like’ when compared to a non-siloxane analogue. One such compound, 2-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecyloxy)phenyl)-5-(1-chlorooctyloxy)pyrimidine (3), is characterized by a maximum layer shrinkage of only 1.6% and may be considered a bona fide de Vries material.

Characterization of alumoxane-derived ceramic membranes

Abstract Atomic force microscopy, nitrogen adsorption–desorption, and measurements of intrinsic permeability are used to characterize ceramic membranes made using a newly developed alumoxane-based method conceived as an environmentally benign process. AFM, BET and permeability data indicate that the alumoxane synthesis pathway can be used to readily produce unsupported membranes with pore sizes in the range of 10–20xa0nm and with permeabilities comparable to those of anodized alumina membranes.

The first bent-core mesogens exhibiting a dimorphism B7–SmCPA

New homologous achiral five-ring bent-core mesogens with a lateral fluorine substituent at the central core and chlorine substituents at the terminal rings have been synthesized. Two mesophases could be distinguished by polarizing microscopy, calorimetry, X-ray, and electro-optical investigations. The high-temperature phase was found to be an antiferroelectric SmCP phase whereas the low-temperature phase could be identified as a ferroelectric B n 7 phase. This phase sequence was observed for the first time. NMR measurements as well as optical studies indicate that both phases have a helical superstructure.

The B2–B7 phase transition in symmetrical bent‐shaped mesogens with methoxy substitution

New mesogens composed of achiral bent molecules with thermally stable ester linkages, and laterally substituted by a methoxy group symmetrically near the central benzene ring, were synthesized. Texture, calorimetric, electro‐optical, X‐ray and dielectric measurements were performed. In most of studied compounds the antiferroelectric B2 phase was found on cooling from the isotropic phase, followed by the B7 phase at lower temperatures. Undulation of layers in the B7 phase was confirmed by precise synchrotron studies.

On the origin of the "giant" electroclinic effect in a "de Vries"-type ferroelectric liquid crystal material for chirality sensing applications.

W415 is a chiral smectic compound with a remarkably weak temperature dependence of its giant electroclinic effect in the liquid crystalline smectic A* phase. Furthermore it possesses a high spontaneous polarization in the smectic C* phase. The origin of this striking electroclinic effect is the co-occurrence of a de Vries-type ordering with a weak first-order tilting transition (see the synchroton X-ray scattering profiles).