A quantitative assessment of chemical perturbations in thermotropic cyanobiphenyls.

Chemical programming of the temperature domains of existence of liquid crystals is greatly desired by both academic workers and industrial partners. This contribution proposes to combine empirical approaches, which rely on systematic chemical substitutions of mesogenic molecules followed by thermal characterizations, with a rational thermodynamic assessment of the effects induced by chemical perturbations. Taking into account the similarities which exist between temperature-dependent cohesive Gibbs free energy densities (CFEDs) and pressure-temperature phase diagrams modeled with the Clapeyron equation, chemical perturbations are considered as pressure increments along phase boundaries, which control the thermotropic liquid crystalline properties.

Chemical Programming of the Domain of Existence of Liquid Crystals.

This work illustrates how enthalpy and entropy changes responsible for successive phase transitions of cyanobiphenyl-based liquid crystals can be combined to give cohesive free energy densities. These new parameters are able to rationalize and quantify the demixing of the melting and clearing processes that occur in thermotropic liquid crystals. Minor structural variations at the molecular level can be understood as pressure increments that alter either the melting or clearing temperatures in a predictable way.

Enthalpy-entropy compensation combined with cohesive free-energy densities for tuning the melting temperatures of cyanobiphenyl derivatives.

This work illustrates how minor structural perturbations produced by methylation of 4'-(dodecyloxy)-4-cyanobiphenyl leads to enthalpy-entropy compensation for their melting processes, a trend which can be analyzed within the frame of a simple intermolecular cohesive model. The transformation of the melting thermodynamic parameters collected at variable temperatures into cohesive free-energy densities expressed at a common reference temperature results in a novel linear correlation, from which melting temperatures can be simply predicted from molecular volumes.

A polyaromatic terdentate binding unit with fused 5,6-membered chelates for complexing s-, p-, d-, and f-block cations.

The polyaromatic terdentate ligand 6-(azaindol-1-yl)-2,2′-bipyridine (L7) combines one 5-membered chelate ring with a fused 6-membered chelate ring. It is designed to provide complexation properties intermediate between 2,2′;6′,2″-terpyridine (L1) (two fused 5-membered chelate rings) and 2,6-bis(azaindol-1-yl)pyridine (L6) (two fused 6-membered chelate rings). In polar organic solvents, L7 displays remarkable affinities for the successive fixation of two small univalent cations M = H+ or Li+, leading to stable [M(m)(L7)]m+ (m = 1–2) complexes.