Influence of guest conformation (rod- or banana-like photo-isomers) on flexoelectric coefficients in nematic liquid crystals

A pyroelectric technique was used for direct measurements of the flexoelect ric polarization P-f in hybrid aligned nematic cells filled with a guest ho st mixture (4-pentyl-4'-cyanobiphenyl (5CB) doped with an azo-dye). Under U V-Iight excitation, the mixture manifested strong photochromism due to an a lmost complete conversion of the rod-shaped trans-isomers of the azobenzene moiety into long-living banana-shaped cis-isomers. The temperature depende nces of P-f and, consequently, the sum of the flexoelectric coefficients (e (1)+e(3)) were measured in situ without and with UV illumination and the ki netics of P-f was shown to follow the kinetics of trans-cis-trans isomeriza tion. 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 o rder parameter of the mixture caused by the strongly bent cis-isomers.