Here,
we report the first redox-active chiral dopant <sup><i><b>Fc</b></i></sup><b>D</b>, which electrically
alters its helical twisting power (HTP) for a cholesteric liquid crystalline
(LC) medium and quickly changes the reflection color. <sup><i><b>Fc</b></i></sup><b>D</b> is composed of an axially
chiral binaphthyl unit in conjunction with a redox-active ferrocene
unit. A cholesteric LC phase of 4′-pentyloxy-4-cyanobiphenyl,
doped with <sup><i><b>Fc</b></i></sup><b>D</b> (3.0 mol %), developed a blue reflection color. When nitrosyl tetrafluoroborate,
a one-electron oxidant, was added to this cholesteric LC phase, <sup><i><b>Fc</b></i></sup><b>D</b> was oxidized
to decrease its original HTP value by 13%, so that a green reflection
color was developed. In the presence of a supporting electrolyte,
the reflection color was electrochemically modulated using a sandwich-type
glass cell with indium tin oxide electrodes. In quick response to
the applied voltage of +1.5 V, the reflection color changed from blue
to green within 0.4 s. When 0 V was applied, the reflection color
returned to its original blue color. The <sup><i><b>Fc</b></i></sup><b>D</b>-doped cholesteric LC is characterized
by its fastest electrochemical response and lowest operating voltage
among those reported for electrically driven cholesteric LC devices