Photoinduced dichroism and optical anisotropy in a liquid-crystalline azobenzene side chain polymer caused by anisotropic angular distribution of trans and cis isomers

Abstract

Photochromism has been studied for two comb-like liquid-crystalline copolymers (I) and (II) containing azobenzene chromophores in their side chains. In a smectic glass phase of both copolymers, upon short-time irradiation by UV light, long-living cis isomers are observed. Both copolymers manifest the photoinduced anisotropy, the physical mechanisms of which seem to be quite different. In spin-coated films of polymer (II), the origin of the anisotropy is a strong stable dichroism, which is due to an enrichment and depletion of the chosen angular direction, correspondingly, with trans and cis isomers of the azobenzene chromophores. Polymer (I) manifests no dichroism at all, and its induced optical anisotropy may be accounted for by a rather slow chromophore reorientation. In copolymer (II) a considerable reorientation of the mesogenic groups also occurs as a secondary phenomenon at the stage of the cis isomer formation only. This observation shed more light on the general process of the light-induced molecular reorientation in polymers, liquid crystals, and Langmuir–Blodgett films, which is of great importance for holographic information recording.

The Photoinduced Birefringence in Thick Films of Chiral Azodye Copolymers

“…[38] The former two bands are strong enough, to make the 20-30 mm thick films of the copolymers completely non-transparent at l < 400 nm; some examples of the transmission curves are given in Figure 3.…”

“…This band is shown to be much better developed for cisisomers, as compared with trans-ones. [38] However, since film preparation and all the measurements were carried out not in darkness but at the common office illumination, all the samples contained a small steady-state portion of cis-groups.…”

Photoinduced three-dimensional orientational order in side chain liquid crystalline azopolymers

“…The stable POA was observed later on in polymers containing chemically linked azochromophores (azopolymers) [4]. It turned out that stable anisotropy can be induced in both amorphous and liquid crystalline (LC) azopolymers [4,5,6,7,8,9,10]. The efficiency of POA in LC azopolymers is generally much higher than it is for amorphous homologues.…”

Electric‐Field‐Assisted Acceleration of the Photostimulated Nematic—Isotropic Transition

“…It must be remarked here that the only mode of fast recovery known till now is by irradiating the sample with radiation of a longer wavelength (around 420 nm). [4][5][6]9,10] This process however increases the complexity of the system by the requirement of a second light source and the associated problems of having the aligning optics. In contrast, the phenomenon described here provides a convenient electrooptic (electric field and actinic light, i.e., UV) control of the optical devices.…”