Spectroscopic and Optical Characterization of a Series of Azobenzene-Containing Side-Chain Liquid Crystalline Polymers

Abstract

A series of azobenzene-containing side chain liquid crystalline polymers with various spacer lengths (pXMAN, where X, the number of methylene units, varies from 4 to 12) have been synthesized and characterized. Phases and aggregation have been studied by UV-visible spectroscopy as a function of temperature. Polarized spectra were measured before and after irradiation with a resonant pump light at 488 nm. The annealing process on polymers with short spacers (p4MAN-p8MAN) induces a hypsochromic shift while, for longer spacers (p9MAN-p12MAN), it induces a large bathochromic shift. These effects may be due to the strong dipolar interaction and interdigitation of the side chain chromophores in an antiparallel orientation. The irradiation of such organized films with either circularly or linearly polarized light induces a bathochromic shift by breaking the antiparallel organization. This is a consequence of the angular reorientation of the azobenzene moieties as a result of trans-cis photoisomerization. To estimate the orientation of the side chain azobenzene photoactive group, of the aliphatic spacer and of the main chain, linear dichroism in the infrared spectral range was measured using the polarization-modulation technique with in situ irradiation. From these experiments, a high and stable value of photoinduced linear dichroism reveals a strong interaction and self-organization of azobenzene side chain molecules during the relaxation process. The anisotropic reorientation of the polymer main chain and of the spacer is small when compared to the reorientation of the azobenzene side chains, which become perpendicular to the linearly polarized pump beam. Additional birefringence measurements were carried out with in situ irradiation followed by a relaxation cycle for the whole series of polymers in various phases. The stability and the rate of the induced birefringence depend on the initial degree of organization within the thin film and on its thermal history. It is noteworthy that annealing can lead to a significant improvement of the birefringence level (+44%). The results obtained on photoinduced orientation and phase organization are correlated with the polymer properties.

Photo-induced deformations in azobenzene-containing side-chain polymers: molecular dynamics study

“…For this reason we have chosen this particular architecture for our simulations. The smectic phases inherent to LC azo-polymers may also shed some light on the importance of the aggregation of azobenzenes [17][18][19], since the rearrangement of these aggregates is expected to be on much shorter time scale in LC azo-polymers. We empolyed the semi-atomistic modelling, in which the CH n groups of the alkyl chains are represented as united-atom LennardJones monomers and the azobenzene groups (including possible substituents) are considered to be anisometric units interacting via the Gay-Berne potential [20] with the following parametrisation, µ = 1, ν = 2, κ = 3, κ = 5.…”

The Photoinduced Birefringence in Thick Films of Chiral Azodye Copolymers

“…That would be of great importance for the supramolecularly ordered structures such as liquid crystalline (LC) polymers. [14][15][16][17][18] The application of photochromic LC polymers is limited however by thin films only, since spontaneous formation of a high scattering polydomain texture is typical for the LC materials. Therefore, monodomain samples should be prepared first, to achieve an acceptable transparency required for optical applications, and the thicker the film, the more difficult is to obtain the large size non-defect monodomain orientation.…”

New Siloxane‐Organic Polyesters with Azobenzene Side Chains. Synthesis, Thermotropic Behavior and Surface Properties

“…In such cases of slow forming LC phases, the significant role of the thermal treatment in obtaining high levels of birefringence and organization of the side-chain azobenzene mesogenic chromophores was emphasized. [30,31] The influence of the organo-siloxane backbone can be well observed on comparing the data in Table 1. For the two model compounds M1 and M2, the melting temperature and the thermotropic properties are dictated by the chemical composition.…”