The role of lateral substituent size in azobenzene chromophores on photo- and chiro-optical properties of photoactive polymethacrylates

Abstract

Properties of functional liquid crystalline (LC) macromolecular materials, especially those showing self-organizing behaviour and targeted for specific purposes, strongly rely on the molecular structure of the mesogens used as side-chains. In order to get inside this relationship, new series of chiral LC polymethacrylates (PMA6Azo-R) with azobenzene groups having different lateral substituents (R), namely the methyl (-Me) and ethyl (-Et) groups on the central benzene ring, were synthesized and their comparative study was carried out. The synthesized polymers consist of photochromic azobenzene fragments that differ in the size of the lateral substituent (R = -H, -Me, -Et). All polymers exhibit the cholesteric phase, whereas the polymer without lateral alkyl group in addition forms the low-temperature SmĈ* phase. Investigations of the photoorientation process under linearly polarised visible light irradiation reveal that an increase in the size of side substituent considerably reduces the values of maximum achievable photoinduced dichroism (from 0.89 for PMA6Azo-H to 0.33 for PMA6Azo-Et that is associated with a decrease in the degree of mesogen anisometry). The effect of the lateral substituent size in azobenzene photochrome on the chiro-optical properties and mesomorphic behaviour of studied polymers was established. The presence of the lateral substituent in the photochrome/mesogen fragment leads to a significant decrease in isotropization temperature. Chiro-optical properties of the polymers and their photoswitching were studied, and it was found, that the introduction of the lateral substituent in the azobenzene fragment prevents the formation of exciton couples, that leads to a decrease in the circular dichroism (CD) values. UV irradiation and E-Z isomerisation of the azobenzene chromophores results in complete disappearance of CD signal; subsequent visible light irradiation does not lead to its full recovery. On the other hand, annealing of the films at temperatures higher than the glass transition almost fully recovers the initial CD values. The designed polymethacrylates can be considered as promising materials for various photonic applications.