Unexpected photoinduced phenomena in chiral-photochromic cholesteric copolymers with a triplet sensitizer

摘要

For the first time the pos-sibility of energy transfer from a triplet photosensitizer to chiral-photochromic fragments in photo-active cholesteric systems was dem onstrated.For this purpose we prepared mixtures containing chiral-photochromic cholesteric copolymers with a triplet sensi tizer-acrydine orange(AO).Chiral-photochromic groups in copolymers contain a C=C bond capable for undergoing E-Z isom-erization during UV irradiation.All polymer mixtures form a chiral nematic phase displaying selective light reflection with lambda_max~650-1,000 nm depending on the structure and concentration of the chiral groups.Irradiation of mixtures by visible light(lambda>450 nm)leads to a shift of the selective light reflection peak to a long-wavelength spectral region.This effect is associated with a decrease of anisometry of chiral-photochromic fragments in copolymers during their E-Z isom-erization.It is important to empha-size,that chiral-photochromic side groups of copolymers do not absorb visible light themselves;therefore,the previously mentioned changes can be explained by the energy transfer from photoexcited AO molecules in the triplet state to isomerizable fragments.The study of the kinetics of this process revealed a rather unexpected phenomenon:after the first 60-80-min irradiation,the helix pitch of the supramolecular structure of the mixtures increases,but after succes-sive irradiation the helix pitch de creases.The possible explanations of this phenomenon were suggested.It was demonstrated that these mix tures may be used for irreversible recording of optical information.