Carbohelicenes generally incorporate a helical, distorted, conjugated, polyaromatic system with ortho-fused benzenoid rings, which is a fundamental molecular characteristic of this class of compounds. They have been described as “molecules in distress” due to their distortion. The generation of a chiral helicity in helicenes was observed because of a severe intramolecular steric strain. Helicity is a molecular necessity in the higher series of carbohelicenes, when at some point, a helical pitch occurs when a second coil is formed. The most interesting properties resulting from such molecular distortion are the very high chiroptical and circular dichroism values. For instance, the resolution of some helicene racemates by “hand picking” of a few homochiral single enantiomeric crystals allowed for a measurement of their optical rotation. Due to that intrinsic chirality spanned over a large polyaromatic template, preliminary results clearly established the efficiency of carbohelicenes to induce asymmetry and chirality in organic synthesis and in supramolecular chemistry. Additionally, they have some potential uses in several fields: materials science, nanoscience, chemical biology and supramolecular chemistry. It has encouraged many attempts to develop new asymmetric syntheses of carbohelicenes, as well as some chiral separations of enantiomers and diastereoisomers. This review is thus dedicated to carbohelicene chirality. It gathered a substantial collection of data, and a comprehensive review on the preparations of enantioenriched helicenes, either from an asymmetric synthesis or from a chiral separation. Utilizations of non-racemic helicenes and their applications will be treated in the following review (Part 3), and will not be the subject of this manuscript.

One hundred years of helicene chemistry. Part 2: stereoselective syntheses and chiral separations of carbohelicenes

The bend elastic constants (k 33) and order parameters (S) derived from optical measurements in the nematic phase of pentlyl, hexyl, heptyl and octyl cyanobiphenyls (5CB, 6CB, 7CB and 8CB respectively) are reported. The odd-even effect is clearly seen in the K 33 values of all four compounds and in the S values of the first three. 5CB, 6CB and 7CB show only the nematic mesophase but their elastic constants indicate the existence of smectic-like short range order. For example, even the lowest homologue, 5CB, does not obey the law k 33 ∞ S 2 given by the mean field throry. 7CB, has about the same order parameter as 5CB but a much higher elastic constant. 8CB, which shows a smectic A phase as well, exhibits a lower k 33 than 7CB near the nematic-isotropic transition point but a pronounced pretransitional increase near the A-N transition.

Elastic and Optical Properties of Some 4′-n-Alkyl-4-Cyanobiphenyls

A hydrothermal method of synthesizing hydroxyapatite by heating a precipitate, formed by mixing Ca(NO3)24H2O and (NH4)2HPO4 with distilled water, in a hydrothermal reactor at 200 °C for 24-72 hrs is described. A treatment time of 24 hrs produced single phase (as shown by XRD) hydroxyapatite powder, however for longer treatment times XRD patterns were indicative of the presence of a secondary phase, monetite (CaHPO4). SEM examination of the treated powders displayed particles of rod-like morphology with dimensions 100-500 nm in length and 10-60 nm in diameter. Preliminary results on the use of the particles for the infiltration of dentine tubules are presented.

https://iopscience.iop.org/article/10.1088/1742-6596/26/1/064/pdf

Hydrothermal synthesis of hydroxyapatite

The classical Maier-Saupe theory of spatially uniform nernatics is here extended to the case of distorted configurations. Under the assumption that the molecules are rodlike, the mean field potential is here defined as an average over a suitable neighborhood of the test molecule. The energy density thereby calculated automatically accounts for the distortion energy, and properly reduces to the classical expres- sion for the homogeneous situation. In the linear limit, contrary to existing predictions based on Maier- Saupe potential, the three constants of Frank elasticity are found to be different from one another, and ordered as K 1, <K 2, <K 3. The main variables determining the magnitude of the elastic constants are: the order parameter S, the molecular length L, and an interaction distance ℒ A quantitative test of the theory on a standard nematic (PAA) shows good agreement with data for all three constants if ℒ is of the order of 1 nanometer.

The Elastic Constants of Maier-Saupe Rodlike Molecule Nematics

Molecular modelling techniques have been applied to compute the conformation accessible to bacterial deep rough lipopolysaccharide of Escherichia coli (Re-LPS). Analyses of the results showed that the models typically exhibit a tilt of the diglucosamine backbone with respect to the membrane normal of 53 +/- 7 degrees while both the glucosamine ring planes are oriented approximately parallel to the membrane normal. Different models were found to show compact and elongated types of acyl chain arrangements, both producing anisotropic lateral dimensions of the models of 1.0-1.1 nm and 1.7-2.0 nm for the shorter and the longer side, respectively. The conformationally allowed range of the isolated dOclA(alpha-2-4)dOclA disaccharide (dOclA = 3-deoxy-D-mannooctulosonic acid) was found to be extremely limited. It appeared that the dOclA disaccharide (dOclA)2 is centred at the top of the Re-LPS molecule preferring two orientations stabilized by hydrogen bonds involving only one phosphate group of the lipid A moiety at a time. The effect of charges on the Re-LPS conformations has been studied in separate calculations. From these calculations it was obvious that charges have no significant effects on the conformations of the isolated lipid A and (dOclA)2 moieties. However, it was found that the orientation of (dOclA)2 with respect to the lipid A part is highly sensitive to charges, i.e. in the charged models the proximity of phosphate and carboxyl groups is prevented by strong electrostatic repulsion between these negatively charged groups. In order to rationalize the acyl chain packing of the models, a simple geometrical model which correlates the tilt of the diglucosamine backbone with the energically favoured close packing of the acyl chains is proposed. Furthermore, the possibility of a chelate-like complexation of divalent cations and its contribution to head group mobility is discussed.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1991.tb15962.x

Molecular modelling of bacterial deep rough mutant lipopolysaccharide of Escherichia coli.

A relationship is derived between curvature, elasticity and orientational order in nematic liquid crystals on the basis of the molecular statistical theory developed in Part I. Calculations show that the elastic moduli of p-azoxyphenetole should be greater than those of p-azoxyanisole, in quantitative agreement with the values reported by the Orsay Group. The theoretical temperature variation of the elastic moduli is also in accord with the available data.

Molecular statistical theory of nematic liquid crystals. II. Relation between elasticity and orientational order

The constants of the potential energy function deduced from the statistical theory of orientational order in nematic liquid crystals are given for a few compounds and their significance in relation to the molecular structure is discussed. The dependence of the free energy of elastic deformation and the intensity of light scattering on the orientational order parameters is then investigated theoretically. Calculations show that the elastic moduli should be greater and the scattering cross-section less in p-azoxyphenetole than in p-azoxyanisole. The results are in quantitative agreement with the available experimental data. The theory is also able to account for the observed fact that though the order parameters and the elastic moduli decrease rapidly with rise of temperature, the light scattering remains practically constant throughout the nematic range.

K. Shubha

Papers

Molecular statistical theory of nematic liquid crystals. II. Relation between elasticity and orientational order

Relationship between elasticity and orientational order in nematic liquid crystals