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

• Published in: Japanese Journal of Applied Physics Vol. 38; no. 9A; p. L1042
• Main Authors: M. Blinov, Lev, Mikhail Kozlovsky, Mikhail Kozlovsky, Taisuke Nagata, Taisuke Nagata, Masanori Ozaki, Masanori Ozaki, Katsumi Yoshino, Katsumi Yoshino
• Format: Journal Article
• Language: English
• Published: 15-09-1999
• ISSN: 0021-4922; 1347-4065
• DOI: 10.1143/JJAP.38.L1042

A pyroelectric technique was used for direct measurements of the flexoelectric polarization
P
f
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
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 kinetics of
P
f
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.