Photoinduced supramolecular chirality in side-chain liquid crystalline azopolymers

Abstract

A series of liquid crystalline homopolymers that have photoisomerizable cyanoazobenzene groups in the side chain has been synthesized and characterized. Thin films of these polymers have been processed in order to study the absorption spectra and circular dichroism responses after illumination with 488 nm circularly polarized light. The tendency of the azobenzene units to aggregate and the modification of the electronic spectra by illumination were studied by UV-vis spectroscopy. Illumination with circularly polarized light of the azopolymers gave rise to a photoinduced circular dichroism response, which suggests supramolecular structures with a chiral orientation of the azobenzene chromophores in H-aggregates. The photoinduced supramolecular order depends on the polymer structure, i.e., on the T g and the length of the flexible spacer, and the nature of the liquid crystal phase. Identical absorption spectra and reverse circular dichroism spectra were obtained after irradiation with circularly polarized light of opposite handedness and this is due to the photoinduction of enantiomeric supramolecular structures. The thermal stability above T g of the photoinduced supramolecular chirality is determined by the polymer structure.

Photoinduced Chiral Nematic Organization in an Achiral Glassy Nematic Azopolymer

“…[27] Moreover, the thermal stabilities of the photoinduced supramolecular chiral organizations were studied for glassy nematic and smectic polymers and it was concluded that the photoinduced structure is stable in the glassy state. [28] Chiral nematic liquid crystals that have periodic helical structures are able to selectively reflect CPL at a wavelength that depends on the helical pitch. CPL with the same handedness as the helix is Bragg reflected, whereas light with opposite handedness passes through.…”

“…[30,31] Recently, we detected a selective reflection in films of a nematic azopolymer by irradiation with CPL, but not in a homologous smectic one having the same chromophoric unit. [28] The work described here involved the characterization and properties of a glassy nematic azopolymer after irradiation with CPL by techniques used in the study of chiral liquid crystalline phases. A model of chirality transfer from chiral light to material is proposed.…”

A Supramolecular Chiroptical Switch Using an Amorphous Azobenzene Polymer

“…[16] Recently, a very large optical rotation has been reported for an amorphous azopolymer, [17] and the influence of liquid-crystalline structures has been discussed. [18] In this work, we have investigated the details of photoinduced supramolecular chirality in 'achiral' and 'amorphous' azobenzene polymer films as a function of the handedness and ellipticity of light. In addition, we demonstrate chiroptical switching mediated by the reversible conversion of the helical sense of the supramolecular structures upon changing the handedness of the incident light.…”

Optical and Physical Applications of Photocontrollable Materials: Azobenzene-Containing and Liquid Crystalline Polymers

“…Their system consists of nonmesomeric V-shaped azobenzene arms and melamine derivative core. Oriol and coworkers investigated the photoinduced supramolecular order as a function of the chemical structure of the azobenzene polymer, and discussed it from the viewpoint of the glass transition temperature, the length of the spacer and the nature of the liquid crystalline phase [183]. They reported that a selective reflection was observed in the polymer having a nematic liquid crystalline phase but not in the polymer having a smectic phase.…”

“…and circular dichroic plates [192]. Oriol and coworkers reported on the thermal stabilities of the photoinduced supramolecular chiral structures in nematic and smectic polymers, and concluded that the thermal stability above the glass transition temperature depends on the liquid crystalline nature of the polymer [183]. Sourisseau and coworkers used Jones' matrix formalism for detailed analysis of photoinduced chirality and concluded that the degree of photoinduced rotation angle can easily be controlled depending on the ellipticity of the excitation light [193].…”