Cholesteric copolymers and mixtures containing dithienylethene photochromic fragments

Abstract

Abstract The phase behaviour and optical properties of mixtures based on cholesteric copolymers with 3 - 6 wt.-% of photochromic dithienylethene dopants as well as a cholesteric copolymer containing dithienylethene side groups were studied. It was shown that planar-oriented films of the mixtures and of the copolymer possess a selective light reflection in the visible or near IR regions of the spectrum. UV irradiation of the mixture films leads to the appearance of an intense absorbance peak in the spectral range coinciding with the selective reflection band (λmax ≈ 522 - 615 nm depending on the chemical structure of the photochromic fragment). This process is associated with the photocyclization of dopants and photochromic side groups and the ‘degeneration’ of the selective light reflection. The photoinduced coloured form of dopants and photochromic side groups is thermally stable at room temperature, but subsequent irradiation of the film by visible light (550 or 633 nm) induces a decoloration process. The influence of temperature on the rates of forward and back photoprocesses in such polymeric liquid crystalline films was studied. The potentialities of the use of such type of materials for reversible data recording and storage were demonstrated.