Cholesteric mixture containing a chiral azobenzene-based dopant: material with reversible photoswitching of the pitch of the helix

Abstract

A new low molar mass chiral-photochromic dopant was synthesized. It contains a menthyl fragment as the chiral group and an azobenzene group, capable of E - Z photoisomerization, as the photochromic component. The substance obtained was used as a chiral dopant in mixtures with a comb-shaped cholesteric acrylic copolymer with menthyl-containing chiral side groups and phenyl benzoate nematogenic side groups. Such mixtures form a cholesteric mesophase. The chiral dopant led to an additional twisting of the cholesteric helix, i.e. to a shift of the selective light reflection peak to a shorter wavelength region of the spectrum. The initial copolymer gave selective light reflection in the spectral range 1200-1400 nm; the mixture containing 3.5 mol % of chiral-photochromic dopant reflects light with λ_max~ 850 nm. The action of light with λ_ir~ 440 nm results in E - Z isomerization of the azo-group of the chiral dopant and in a shift of the selective light reflection peak to the long wavelength region of the spectrum (amplitude of shift = 30 nm). This is explained by a lower helical twisting power of the Z-isomer of the chiral dopant. This process is thermally reversible: annealing of irradiated films leads to a back shift of the selective light reflection peak to the short wavelength region of the spectrum due to Z - E isomerization. Kinetic features of the direct and backward processes of isomerization were studied: it was shown, that mixtures of the chiralphotochromic azobenzene-containing dopant with cholesteric polymers give new possibilities for the creation of polymer materials with a reversibly regulated helical supramolecular structure which determines their optical properties.