Layering Transitions and Schlieren Textures in Langmuir Films of Two Organic Radicals

Abstract

Two paramagnetic radicals have been investigated in terms of their film-forming properties at the air-water interface. Although the radicals failed to display any mesomorphic behavior in the bulk, they were found prone to built-up multilayer films on the Langmuir trough. The molecules seem to dimerize in the upper layers of the films that exhibit striking Schlieren textures when observed with Brewster angle microscopy. These Schlieren textures, together with the ability to form multilayers, indicate that the molecules came close to displaying smectic mesomorphism. A tentative model of the layers' structure is proposed, and a suggestion for synthesizing new molecules with actual mesomorphism is offered. The presented results show that the study of the behavior of molecules at the air-water interface can shed a new light on their behavior in the bulk and help in the design of new magnetic mesogens.

Collective Rotations of Ferroelectric Liquid Crystals at the Air/Water Interface

“…The materials characterized above, able to convert a kinetic energy of linear flow into rotational movement and to transfer the torque throughout the microscopic distance, could be very useful in nanotechnology and, especially, in nanotribology as a special medium transmitting the driving force or a compression coupling for the multiple molecular motors. Since the first announcement about this type of molecular rotations with self-organization, this phenomenon has been extensively studied in some other aspects. − Textures composed of alternate dark and white stripes and rings have also been observed for Langmuir films of organic radicals as well as for the film of organosiloxane . However, in the latter case, the birefringent textures were formed only for the topmost layer of a bilayer film.…”

“…Since the pioneering work of Tabe and Yokoyama, who undoubtedly proved that water evaporation is a driving force of the rotation, many papers reporting other reasons for creation of birefringent textures were published. − A stable texture developed during phase separation of trans- and cis- isomers of azobenzene derivatives was studied by Ignés-Mullol et al Subsequent papers of the same group from Barcelona reported dynamic self-assembly in the same system induced by illumination with polarized light and collective molecular precession induced by illumination through the rotating polarizer . The experimental results described in the papers − were confirmed by computer simulations.…”

“…Different systems creating textures in multilayered films were studied by Gallani et al , The monolayers prepared using the compounds containing groups known as organic radicals were isotropic; however, when compressed to the bilayers they developed very interesting patterns observed usually in thick layers of nematic liquid crystals. The texture composed of approximately parallel disclination lines was stable, not revealing any visible molecular motion.…”

Supramolecular Polymerization in Liquid Crystalline Media: Toward Modular Synthesis of Multifunctional Core–Shell Columnar Liquid Crystals

“…Following our previous work, we initially attempted the supramolecular polymerization of OCB Disk NH * (Figure a) in a nematic LC comprising 7TB (Figure b) that bears a thermally stable organic radical 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) unit . However, OCB Disk NH * and 7TB, upon mixing, gave a dark image in polarized optical microscopy (POM) (Figure S2), suggesting the absence of any ordered structure.…”