Tuning the Thermotropic and Lyotropic Properties of Liquid-Crystalline Terpyridine Ligands

• Published in: Chemistry : a European journal 2006-05, Vol.12 (16), p.4261-4274
• Main Authors: Camerel, Franck, Donnio, Bertrand, Bourgogne, Cyril, Schmutz, Marc, Guillon, Daniel, Davidson, Patrick, Ziessel, Raymond
• Format: Article
• Language: English
• Subjects: Amides - chemistry; amphotropic; Dimethyl Sulfoxide - chemistry; Esters - chemistry; gelator; Gels - chemistry; Hot Temperature; hydrogen bonds; Ligands; liquid crystals; Liquid Crystals - chemistry; Liquid Crystals - ultrastructure; mesogenic ligand; Microscopy, Electron, Transmission; Models, Molecular; Pyridines - chemical synthesis; Pyridines - chemistry; Spectroscopy, Fourier Transform Infrared; Temperature; terpyridine
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200501431

A rational synthetic strategy is developed to provide compact and simple terpyridine (terpy) mesogens that show liquid‐crystallinity both as pure compounds and in organic solution (amphotropic compound). The use of a central 4‐methyl‐3,5‐diacylaminophenyl platform equipped with two lateral aromatic rings, each bearing three appended aliphatic chains, allows connection of a 2,2′:6′,2′′‐terpyridine fragment through a polar group such as an ester, amide, or flat conjugated alkyne linker. For the T12ester and T12amide scaffolds, the mesophase is best described as a lamellar phase, in which the molecules self‐assemble into columnar stacks held together in layers. In the T12amide case, the additional amide link results in significant stabilization of the lamellar phase. The driving forces for the appearance of columnar ordering are the hydrogen‐bonding interactions of the amide groups, which induce head‐to‐tail π‐stacking of the terpy subunits. Replacing the polar linker by a nonpolarized but linear alkyne spacer, as in the T12ethynyl compound, provides a columnar mesophase organized in a rectangular lattice of p2gg symmetry. In this arrangement, two nondiscotic molecules arranged into dimers by hydrogen bonding and π–π stacking pile up in a head‐to‐tail manner to form columns. In addition, the T12amide compound proves to be an excellent gelator of cyclohexane, linear alkanes, and DMSO. The resulting robust and transparent gels are birefringent and formed by large aggregates that are readily aligned by shear‐flow. TEM and freeze‐fracture microscopy reveal that the gels have an original layered morphology made of fibers. Ce travail décrit une synthèse rationnelle des tous premiers ligands mésogènes à base d′unité terpyridine et montre que les propriétés thermotropes et lyotropes de ces composés dépendent notoirement du choix du connecteur reliant l′unité terpyridine à l′unité structurante. L′utilisation de l′unité structurante 4‐méthyl‐3,5‐diacylaminophényle équipée de deux cycles aromatiques portant 3 chaînes aliphatiques permet en effet de connecter des sous‐unités 2,2′:6′,2′′‐terpyridine par l′intermédiaire de groupes polaires tels qu′un ester ou une fonction amide ou encore par le biais d′une connexion apolaire linéaire comme la liaison triple. La mésophase thermotrope obtenue dans le cas des composés T12ester et T12amide est une phase lamellaire dans laquelle les molécules sont organisées par un réseau de liaisons hydrogène en colonne maintenue entre‐elles p