Cholesteric Polymer Guest−Host Mixture with Circularly Polarized Fluorescence: Two Ways for Phototuning of Polarization and Its Intensity

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Abstract
A photosensitive fluorescent cholesteric guest−host mixture consisting of a nematic polyacrylate, a chiral, photochromic dopant sensitive to UV light, and a fluorescent dopant was prepared. The nematic polyacrylate contains 4-phenyl-4‘-methoxybenzoate nematogenic side groups and photochromic 4-cyanoazobenzene side groups. The chiral-photochromic dopant formed by isosorbide and cinnamic acid is capable of E−Z photoisomerization and [2 + 2] photocycloaddition under light irradiation. The planarly oriented films possess a selective light reflection in the visible spectral region coinciding with the emission peak of the fluorescent dopant. The fluorescence emitted by the planarly oriented films of the mixture is strongly circularly polarized and characterized by a large value of the dissymmetry factor. At temperatures below glass transition (Tg) the polarized light action of an Ar+ laser (488 nm) leads to the photoorientation of the azobenzene fragments resulting in a strong and reversible disruption of the selective reflection and a decrease of the dissymmetry factor of fluorescence. UV irradiation leads to E−Z isomerization and/or [2 + 2] cycloaddition of the chiral-photochromic dopant, causing an irreversible shift of the maximum of the dissymmetry factor to a long-wavelength spectral region under subsequent annealing at temperatures higher than Tg. Such multifunctional glass-forming guest−host mixtures combining photosensitive and fluorescent properties with the unique optical properties of cholesteric liquid crystals can be considered as promising material for optical data processing technologies and photonic applications.

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