Influence of Guest Conformation (Rod- or Banana-like Photo-Isomers) on Flexoelectric Coefficients in Nematic Liquid Crystals

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A pyroelectric technique was used for direct measurements of the flexoelectric polarization Pf in hybrid aligned nematic cells filled with a guest host mixture (4-pentyl-4'-cyanobiphenyl (5CB) doped with an azo-dye). Under UV-light excitation, the mixture manifested strong photochromism due to an almost complete conversion of the rod-shaped trans-isomers of the azobenzene moiety into long-living banana-shaped cis-isomers. The temperature dependences of Pf and, consequently, the sum of the flexoelectric coefficients (e_1+e_3) were measured in situ without and with UV illumination and the kinetics of Pf was shown to follow the kinetics of trans-cis-trans isomerization. UV light induced decrease in (e_1+e_3), that is, a negative "photo-flexoelectric effect" is accounted for by a decrease in the orientational order parameter of the mixture caused by the strongly bent cis-isomers.
社団法人応用物理学会の論文
1999-09-15