title: Asymmetric Dimers of Chiral Azobenzene Dopants Exhibiting Unusual Helical Twisting Power upon Photoswitching in Cholesteric Liquid Crystals.
abstract: In this study, we synthesized asymmetric dimeric chiral molecules as photon-mode chiral switches for reversible tuning of self-assembled helical superstructures. The chiral switches bearing two mesogen units-cholesterol and azobenzene moieties connected through flexible alkylenedioxy bridges-were doped into nematic liquid crystals, resulting in a chiral nematic (cholesteric) phase. Under irradiation with UV light, photoisomerization of the azobenzene units led to unprecedented switching of the cholesteric pitch and helical twisting power (HTP, β), with a higher HTP found in the cis-rich state (bent-form) than in the trans-state (rod-form). We attribute this behavior to the elongated cybotactic smectic clusters disrupting the helical orientation of the molecules in the cholesteric liquid crystals; their reversible decay and reassembly was evidenced upon sequential irradiation with UV and visible light, respectively. In addition to the photoisomerization of the azobenzene units, the odd/even parity of the alkylenedioxy linkers of the dimeric dopants also had a dramatic effect on the transitions of the cybotactic smectic domains. On the basis of the large rotational reorganization of the cholesteric helix and HTP switching (Δβ/βini of up to 50%), we could control the macroscopic rotational motion of microsized glass rods upon irradiating the surface of a cholesteric liquid crystal film featuring a polygonal fingerprint texture using UV and visible light.
authors.affiliation: Research Institute for Electronic Science, Hokkaido University , Kita20, Nishi10, Kita-ku, Sapporo, Hokkaido 001-0020, Japan.
authors.forename: Yuna
authors.initials: Y
authors.lastname: Kim
authors.affiliation: Research Institute for Electronic Science, Hokkaido University , Kita20, Nishi10, Kita-ku, Sapporo, Hokkaido 001-0020, Japan.
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Tamaoki
mesh_terms: D001391:Azo Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D002784:Cholesterol / Q000138:chemical synthesis / Q000737:chemistry*; D019281:Dimerization; D050866:Liquid Crystals / Q000737:chemistry*; D017785:Photons; D011108:Polymers / Q000737:chemistry; D013237:Stereoisomerism; D014466:Ultraviolet Rays
chemical_list: D001391:Azo Compounds; D011108:Polymers; D002784:Cholesterol; C009850:azobenzene
keywords: azobenzene; chiral dopant; cholesteric liquid crystal; helical twisting power; photoswitching