Regarding the mesophasic types of banana-shaped liquid crystals, the particular mesophases, including columnar stacking, tilted smectic phases, and three dimensional structures, named as B1 to B8 phases were explored and identified (see Figure 1.4).[3,4] These can be divided into four main types (i) layer modulation, (ii) modification of layer correlation, (iii) helix formation and (iv) splay modulation. The classification of “B phases” has been collected by C. Tschierske and R. A. Reddy as shown in Table 1.1,[5] and the detail introduction of individual “B phases” is described as following.
Figure 1.1. The molecular formation of bent-core liquid crystalline designs. [2]
Figure 1.2. The dipole direction of bent-core structure.
Figure 1.3. The chirality of bent-core structures according to the polar direction and tilt direction. [3]
Figure 1.4. The mesophasic types of banana-shaped liquid crystals. [3,4]
1.2.1. B1 phase
The B1 phase, which is generally obtained in the bent-core structures with longer and shorter molecular length ratio of rigid core and flexible chain, respectively, was generalized as a columnar (Col) phase because of the two dimension columnar layer modulation. As shown in Figure 1.5,[6] dendritic-like and mosaic textures were revealable in B1 phase. Based on the traditional identification of B1 phases via XRD investigations, a broad diffuse scattering peak in wide angle region and several sharp peaks in small angle region were observed. In previous studies, the Colr of B1 phase,
which is the non-tilted columnar arrangement as the undulated wavy molecular stacking, generally exhibited no spontaneous polarization behavior. However, in contrast to the conventional Colr phase, two other kinds of polar switchable B1 phases named as Colrev and Colob (Colrev,tilt) were developed under a suitable applying of an electric field, and those different tilted modulated columnar molecular arrangements and 3D structures were shown and compared in Figure 1.5 as well.
Table 1.1. The positional ordered mesophases formed by bent-core molecules[5]
Figure 1.5. The mesophase textures, XRD models and molecular arranged models of B1 phase. [3,6]
1.2.2. B2 phase
The B2 phase (so called SmCP phase) is still the most extensively studied banana phase. The spontaneous polarization behavior is obtainable in this phase, which is explained the polar order based on close packing of molecules in a smectic layer with a unique bending along a C2V symmetry axis. The schileren, broken fan, and circular textures were possessed in this phase (see Figure 1.6).[7] The most prevailingly investigated B2 phase revealed ferroelectric (FE)/antiferroelectric (AF) properties, which possessed identical/inverse polarizations, and synclinic (S)/anticlinic (A)
arrangements with alike/opposite molecular tilted aspects between layer to layer, respectively.[8] Hence, depending on the polar directions and molecular tilted directions in neighboring layers, four kinds of different molecular architectures denoted as SmCAPA, SmCSPA, SmCAPF, and SmCSPF were categorized to homochiral (SmCAPA and SmCSPF) and racemic (SmCSPA and SmCAPF) conditions, respectively.
In the XRD investigations of B2 phase, the smallest and next-smallest angle peaks are related to the first and second order diffractions corresponding to the smectic layer with about 30° to 45° molecular tilted angles.
Figure 1.6. The mesophase textures and molecular arranged models of B2 phase. [7]
1.2.3. B3 phase
The B3 phase is a kind of higher-order lamellar arrangement, which often exhibits in lower temperature cooling from B2 phase (see Figure 1.7). Even if the spontaneous polarization behavior was received in a previous paper,[9] however the electro-optical identification was uncertainly duo to the non-confirmed second
shown in Figure 1.7, the XRD investigation of B3 phase, revealed several diffraction peaks at small and wide angle regions to describe it is similar with a crystalline structure,[9,11] and the layer spacing in B3 phase was the same as that in a crystal state, longer than that in B2 phase, and shorter than that in B4 phase.
Figure 1.7. The bent-core chemical structure and XRD patterns of B3 phase. [3,9,11]
1.2.4. B4 phase
The B4 phase, so called “smectic blue“, often appears in lower temperature cooling from B2 and B3 phases. In previous studies, the lattice distance of B4 phase was longer than that of B3 phase. Textures of B4 phase under a polarizing microscope appeared as transparent dark blue colored domains, as shown in Figure 1.8. This blue
color was first considered to be caused by selective reflection due to the helical structure like a twist-grain-boundary (TGB) phase.[11,12] In the POM observations of B4 phase, the dark and bright domains were demonstrated, which could be exchanged by the rotating one of the polarizers clockwise and counterclockwise. A CD spectrum with an opposite sense was also obtained to indicate the chirality structure.[13]
Although the B4 phase was not switchable by an electric field and its dielectric constant was low, SHG was observed even without the electric field. This indicated the existence of a spontaneous non-centrosymmetric order.[4] In the XRD investigations of B4 phase (see Figure 1.8), the number of diffraction peaks decreased in wide angle region and the peaks became wider than that of B3 phase, which suggested that the B4 phase has an in-plane order.
Figure 1.8. The POM textures, CD spectra, and molecular arranged model of B4
1.2.5. B5 phase
According to the previous articles, B5 phase was performed in the bent-core structures which bear CH3 and F substitutes at the central-ring and the F substitute at both terminal rings (see Figure 1.9).[14] In previous studies, this phase appeared below the B2 phase, and the transition enthalpy between them was small. In electro-optical studies, the FE and AF switching of spontaneous polarization behaviors in B5 phase were observable. Even if the electro-optical properties and mesophase texture of B5 phase were similar with those of B2 phase, however their XRD investigations from the surface oriented samples were distinguishable. In a small angle region, highly order reflections peaks corresponding to the smectic layer spacing could be observed.
Moreover, the model of in-plane molecular arrangement was established as well.
Figure 1.9. The bent-core chemical structures, POM texture, XRD patterns, and molecular arranged models of B5 phase. [3,14]
1.2.6. B6 phase
As shown in Figure 1.10, the B6 phase exhibited a fan-like texture as the smectic A phase but a homeotropic texture could never be obtained. In XRD investigations, only one reflection peak smaller than half of molecular length was obtained in the small angle region to indicate the intercalated molecular arrangement tilted in a layer.
In addition, four broad diffuse peaks were observed in the wide angle region, suggesting liquid-like in-plane order. [15]
Figure 1.10. The bent-core chemical structures, POM texture, XRD patterns, and molecular arranged models of B6 phase. [3,15]
1.2.7. B7 phase
Based on the previous articles, the B7 phase often revealed in the bent-core structures with a NO2 or CN substitutes at the positions of central ring.[16] Many kinds of POM textures could be observed in B7 phase such as spiral filament (coexistence of left- and right-handed spirals), chiral micro-mosaic domain, myeliniclike,
shown in Figure 1.11.[4,17] The FE and AF switching of spontaneous behaviors in B7 phase were obtainable. In XRD investigations, a liquid-like broad diffuse peak and several sharp reflection peaks were obtained in wide and small angle regions, respectively, and the sharp reflection peaks were also identified by the high resolution X-ray diffraction, suggesting the 2D ordered modulated layer structures. The immediate image of modulated layer structures was also observable by freeze fracture transmission electron microscopy (see Figure 1.12).[18] On the other hands, the B7 phase was called SmCG phase within triclinic symmetry as well. Depending on tilting of the molecular planes (called clinic) and of the layer polarization (called leaning) where the layer polarizations further exhibited in-layer and out-layer conditions, eight different general tilted structures were framed as shown in Figure 1.12.[19]
Figure 1.11. The bent-core chemical structures and several kinds of POM textures of B7 phase. [3,4,16,17]
Figure 1.12. The high resolution XRD patterns, FFTEM image, and molecular arranged model of B7 phase. [3,18,19]
1.2.8. B8 phase
Figure 1.13. The spiral and fan-like domains were examined in this phase by gradual and further cooling from isotropic phase, respectively. The AF switching of spontaneous behaviors in B7 phase was obtainable, and the bi-layer structure was also recognized by XRD investigations.[20]
In recent years, some kinds of mesophases such as nematic, smectic A, and crystalline phases, which displayed their own spontaneous switching behaviors and chirality were discovered in novel bent-core liquid crystals (those properties are not contributed form chiral-center possessed rod-like structures). It is suggested that the original “B phase” classified rules may be not satisfied for subsequently researches.
Otherwise, complete classification of the Bn-related phases is a subject for future research.
Figure 1.13. The chemical bent-core structure and POM textures of B8 phase. [3,20]
1.3. Identifications of AF/FE Spontaneous Polarizations in B2 Phase