Three chiral tolane series with twist grain boundary smectic A and C mesophases

Bouchta, A.; Nguyen, H. T.; Navailles, Laurence; Barois, P.; Destrade, C.; Bougrioua, F.; Isaert, N.

doi:10.1039/jm9950502079

Cited by 22 publications

(8 citation statements)

References 15 publications

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“…1) lies in the y-z plane normal to the TGB helix axis x. Thus the GBTGBC phases studied have the Renn͞Lubensky-proposed TGBC layer structure, in contrast to previously studied TGBCs in which N makes an angle L with respect to the y-z plane (16,17).…”

Section: Resultsmentioning

confidence: 68%

Giant-block twist grain boundary smectic phases

Fernsler

Hough

Shao

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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Study of a diverse set of chiral smectic materials, each of which has twist grain boundary (TGB) phases over a broad temperature range and exhibits grid patterns in the Grandjean textures of the TGB helix, shows that these features arise from a common structure: ''giant'' smectic blocks of planar layers of thickness l b > 200 nm terminated by GBs that are sharp, mediating large angular jumps in layer orientation between blocks (60°< ⌬ < 90°), and lubricating the thermal contraction of the smectic layers within the blocks. This phenomenology is well described by basic theoretical models applicable in the limit that the ratio of molecular tilt penetration length-to-layer coherence length is large, and featuring GBs in which smectic ordering is weak, approaching thin, melted (nematic-like) walls. In this limit the energy cost of change of the block size is small, leading to a wide variation of block dimension, depending on preparation conditions. The models also account for the temperature dependence of the TGB helix pitch.liquid crystal ͉ chirality ͉ screw dislocation ͉ helix T he nearly simultaneous prediction of the twist grain boundary (TGB) phase, the liquid crystal (LC) analog of the Abrikosov type II superconductor (1), and its discovery in the nP1M7 series of chiral smectics (2) has led to a class of soft-matter phases exhibiting particularly striking manifestations of chirality. Although fluidlayered smectics in general tend to expel twist of the layer normal, the TGB phases adopt a state of layer twist, driven by molecular chirality in a way analogous to the accommodation of magnetic field by the formation of flux vortices in a type II superconductor. In the LC case twist is enabled by formation of GBs, which behave as arrays of screw dislocations, mediating change in layer orientation between blocks of planar smectic layers, and acting as the ''flux tubes'' in deGennes' smectic͞superconductor analogy (3).The early TGBs (2, 4, 5) exhibited a set of common characteristics, including narrow TGB phase temperature (T) ranges, T R Ϸ 1°C, small angular jumps in layer orientation at the GBs (5), and Grandjean-like textures of the director rotation (TGB) helix (2). However, beginning with the 1993 report of the nitrotolane system having homologs with TGB phase ranges of up to 100°C (6), a distinct class of TGB materials has emerged (6-11) characterized by: (i) large T R values (10°C Ͻ T R Ͻ 100°C); (ii) modulated and͞or undulated Grandjean textures, first described in the ''UTGBC'' phase of the Bangalore S1014͞CE8 mixture (7) and observed in other mixtures (10, 11), as well as in neat materials (6,8,9) § § ; (iii) evidence for large angular jumps between blocks, 90°in the case of the UTGBC square lattice (7) and 60°inferred from nitrotolane x-ray data showing 6-fold symmetric block orientation (9); and (iv) electric field-induced unwinding of the TGB helix (6, 12). Here, we report detailed structural studies using freeze-fracture electron microscopy (FFEM), x-ray diffraction (XRD), and depolarized transmission li...

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Section: Resultsmentioning

confidence: 68%

Giant-block twist grain boundary smectic phases

Fernsler

Hough

Shao

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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“…Within the N L phase, we also observe an additional high frequency (MHz) mode attributable to smectic layering and consistent with recent x-ray studies [4] that reveal evolving smectic order within the N L phase. The relaxation rate of the high frequency mode abruptly drops at the transition from the N L to a lower temperature TGB phase, which was previously identified as a commensurate TGB A phase [4,10]. However, we observe no critical dynamics at the subsequent transition to the smectic-C phase, indicating that the TGB phase is in fact a weakly tilted TGB C variant.…”

supporting

confidence: 41%

“…However, we observe no critical dynamics at the subsequent transition to the smectic-C phase, indicating that the TGB phase is in fact a weakly tilted TGB C variant. Finally, in the N phase, we demonstrate a coupling of one-dimensional Brillouin zone-''edge'' fluctuations to zone-center The chiral tolane compound [10] studied in our experiments, abbreviated 10F2BTFO1M7, has the structure shown in Fig. 1 and the phase sequence N -107:5 C)-N L 102:4 C) -TGB-(99:60 C)-smectic-C .…”

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confidence: 54%

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Light Scattering Study of a Twist Grain Boundary Liquid Crystal

et al. 2003

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Dynamic light scattering is used to probe the fluctuation modes of a liquid crystal exhibiting twist grain boundary (TGB) structure. At the chiral nematic to proposed "chiral line liquid" phase transition, anomalous temperature dependence in the fluctuation spectrum and an instability in the helicoidal director structure signify developing TGB order. At lower temperatures, the behavior of the smectic layer-director fluctuations ("soft" mode) indicates that the previously identified commensurate TGB(A) phase may in fact be a TGB(C) phase with an unusually small tilt angle.

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“…Although the mean-field phase diagram derived by Renn (1992) does not show the occurrence of TGB phases away from the SmA-N Ã -SmC Ã TCP, experimentally many of the systems have shown the TGBA phase away from TCP (Kuczynski and Stegemeyer, 1995;Srivastava et al, 1996;Srivastava and Dhar, 2001). Most of the transitions involving TGB phases were predicted by Renn (1992) to be second-order, but experimentally they have been found to be weakly first order transitions, with the transition enthalpy of the order of few tens of Joule mol À1 (Bouchta et al, 1995;Young et al, 1998). In the ChC-NOBA system also, we have observed that the transitions related with the TGB phases are of the INDUCED TGB PHASES 963 first-order nature, although they are very weak.…”

Section: Phase Diagrammentioning

confidence: 96%

Induced twisted-grain-boundary phases in the binary mixtures of a cholesteric and a nematic compound

Dhar¹,

Srivastava

Agrawal

2003

Phase Transitions

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Thermodynamical, optical-texture and dielectric studies have been performed to study the phase diagram of the binary system of 5-cholesten-3-ol-octanoate and 4-n-nonyloxybenzoic acid. It is observed that low concentrations of 5-cholesten-3-ol-octanoate (2-30 mol %) in 4-n-nonyloxybenzoic acid induce a meanfield phase diagram derived by Renn within the framework of the chiral Chen-Lubenski model. Various optical textures of the twisted-grain-boundary (TGB) phases under different conditions of molecular anchoring have been observed. Weak transitions related to the TGB phases have been detected by temperature dependent dielectric spectroscopy.

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Three chiral tolane series with twist grain boundary smectic A and C mesophases

Cited by 22 publications

References 15 publications

Giant-block twist grain boundary smectic phases

Giant-block twist grain boundary smectic phases

Light Scattering Study of a Twist Grain Boundary Liquid Crystal

Induced twisted-grain-boundary phases in the binary mixtures of a cholesteric and a nematic compound

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