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Zhang et al.
subscripts (“s” and “a”) denote syn- and anticlinicity of
tilt, while the capital subscripts (“F” and “A”), denote
ferroelectricity and antiferroelectricity (PF = FE, PA = AF),
respectively.11 Most bent-core molecules possess anti-
ferroelectric layer structures (SmCPA) that can be
switched to the corresponding FE states upon the appli-
cation of a sufficiently strong external electric field. This
AF switching process is characterized by two polarization
current peaks in each half period of the applied triangular
wave (TW) voltage and is also called tristable switching,
as the polar FE states relax to a nonpolar AF state at 0 V.
A few bent-core molecules have been reported to exhibit
FE switching mesophases.12 This bistable process
switches directly between the two polar FE states without
relaxation to an apolar AF state and, hence, it is char-
acterized by only one polarization current peak in each
half period of the applied TW voltage.
Two different switching mechanisms;rotation of the
molecules (i) on the tilt cone or (ii) around their long
axes;were observed for a variety of bent-core molecules
(see Figure S3 in the Supporting Information). The first
switching mechanism changes only the polarity of layers
while retaining their chirality, and the second changes
both the polarity and chirality.
Figure 1. Organization of bent-core molecules in polar SmCP phases,
indicating the origin of superstructural chirality. (a) Right-handed (left)
and left-handed (right) Cartesian coordinate systems are defined by the
relationship among three vectors. The geometries of molecules with
opposite chirality were drawn in different colors (red = (þ) chirality,
blue = (-) chirality). The drawing shows one possible direction of the
polar vector, parallel to the bend direction. (b) The chirality (þ) or (-) of
molecules can be determined by the right- or left-hand rule. (c) All four
possiblearrangementsfor bent-core moleculeswith macroscopic (þ) (red)
and (-) (blue) chirality are shown with opposite polarity (green). Note
that the correct choice of a set of drawings in panel c is necessary for an
accurate description of molecular orientations under a field.
Some bent-core molecules can organize into ground-
state dark conglomerate (DC) mesophases,13,14 in which
macroscopic chirality can be identified under a polarizing
microscope by slightly uncrossing the polarizer and the
analyzer. Under this condition, domains that consist of
dark and bright regions can be identified. Reversing the
directions of the polarizer and analyzer reverses the bright-
ness of these domains. This indicates that these meso-
phases are composed of a conglomerate of macroscopic
domains of opposite handedness. The optical activity is
attributed to the inherent chirality of the polar SmCP
layers, but it can be observed only if the mesophase
structure is homogeneously chiral, i.e., for a SmCaPA or
SmCsPF structure.15 The molecular organization of the
DC phase has recently been established by using freeze
fracture transmission electron microscopy (TEM) and
X-ray diffraction (XRD).7 TEM images show the local
smectic layering and saddle splay layer deformation in the
DC phase. The molecular organization resembles in
topography that of the lyotropic sponge phase.16 Because
In bent-core smectic (B2) phases, the four basic struc-
tures, SmCsPA (racemic), SmCaPA (chiral), SmCsPF
(chiral), and SmCaPF (racemic) are possible, because the
tilt and bend directions (= polar direction) in adjacent
layers could be either identical or opposite (see Figure S1
in the Supporting Information). Herein, the lowercase
(10) Depending on the molecular structure the polar direction could be
either parallel or antiparallel to the bend-direction, while the bend
direction is exclusive and coincides with the polar C2-axis. To avoid
causing confusion, we herein propose to use the bend direction instead
of the polar direction to define absolute supramolecular chirality.
(11) Keith, C.; Reddy, R. A.; Hauser, A.; Baumeister, U.; Tschierske, C.
J. Am. Chem. Soc. 2006, 16, 3051–3066.
(12) Examples of nonsilylated FE switching bent-core materials:
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