- Extraordinary magnetic field effects on the LC phases of homochiral and racemic 4-cyanoresorcinol-based diamagnetic bent-core mesogens
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4-Cyanoresorcinol based bent-core compounds combining one branched chiral with one achiral linear end-chain have been synthesized in enatiomerically pure and one compound also in racemic form. All homochiral compounds form a chiral cybotactic nematic phase at relatively low temperature with a selective reflection ranging from the near IR to near UV. For the compound with the longest chains superparaelectric and antiferroelectric switching smectic phases were observed, whereas the corresponding racemate is non-polar. This is attributed to sterically induced polarization by the denser packing of uniform enantiomers due to chirality synchronization of the helical conformers. For the racemic mixture this chirality synchronization requires additional surface stabilization. There are unprecedented effects of an applied magnetic field (1 T) on the LC phases, leading to a layer shrinkage by 6-13% for the enantiomer and a layer expansion by 5-8% for the racemate. It is proposed that the magnetic field couples with the π-systems of the almost rod-like molecules. For the racemate this increases the core order, whereas for the enantiomer the reduction of the heliconical twist is considered to provide the major effect. These magnetic field effects could lead to new applications of chiral LC materials at the cross-over between rod-like and bent shapes.
- Ocak, Hale,Bilgin Eran, Belk?z,Nuray, Sevgi,?zkonstanyan, Aykun,Poppe, Silvio,Tschierske, Carsten
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p. 1895 - 1910
(2021/03/01)
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- Bent-core compounds with two branched chains: Evidence of a new dark conglomerate mesophase
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Biphenyl-3,4′-diol derived bent-core compounds with branched side chains have been synthesized and characterized to study the influence of terminal branched groups on mesophase properties and transition temperatures of bent-core mesogens. The liquid crystalline properties of the compounds were investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray scattering. Depending on the length of the chain, the size of molecular branches and the position of branched chains, either enantiotropic or monotropic rectangular columnar phases were observed. For one of the compounds combining two different branched side chains, a new type of optically isotropic dark conglomerate phase (MIso[*]) was found. It is proposed to be a distorted version of a non-tilted or only slightly tilted B6-like intercalated smectic phase with additional in plane order.
- Ocak, Hale,Bilgin-Eran, Belkiz,Prehm, Marko,Tschierske, Carsten
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p. 4590 - 4597
(2013/07/26)
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- Effects of molecular chirality on superstructural chirality in liquid crystalline dark conglomerate phases
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The first examples of polyphilic bent-core compounds consisting of a biphenyl central core, a chiral terminal chain and an oligo(dimethylsiloxane) end group at the other terminus have been synthesized and characterized. The mesomorphic behaviour of the compounds was investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction and electro-optic methods. The olefinic precursors show monotropic or enantiotropic B1 type mesophases. The siloxane substituted analogues containing a racemic chain exhibit dark conglomerate phases (DC[*] phases) which are composed of chiral domains with opposite chirality, whereas the siloxane derivatives with a homogeneously chiral moiety show "dark enantiomer" phases (DE* phases) with uniform chirality. Under electric fields, different types of ferroelectric switching (FE) smectic and modulated smectic phases were induced. The effects of chain branching, spacer length and molecular chirality on the stability of the DCFE [*] and DEFE* phases and on the field-induced smectic and modulated smectic phases were investigated. The Royal Society of Chemistry 2012.
- Ocak, Hale,Bilgin-Eran, Belkiz,Prehm, Marko,Tschierske, Carsten
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p. 7773 - 7783
(2012/09/22)
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- Effects of chain branching and chirality on liquid crystalline phases of bent-core molecules: Blue phases, de Vries transitions and switching of diastereomeric states
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Bent-core molecules based on a resorcinol bisbenzoate core with a series of distinct substituents in different positions at the central resorcinol core have been synthesized and characterized. The focus is on the effect of branched terminal groups in the racemic and chiral forms on the mesomorphic properties. These were investigated by differential scanning calorimetry, optical polarizing microscopy, X-ray diffraction, electro-optic and dielectric methods. Only bent-core mesogens derived from 4-cyanoresorcinol exhibit liquid crystalline phases and the mesophases of these compounds are strongly influenced by the branching and enantiomeric composition of the terminal chains. Depending on the structure of the rod-like wings and the enantiomeric composition, cybotactic nematic phases (NcybC), BPIII-like isotropic mesophases (BPIII cybC*) and various polar and apolar smectic phases (SmA, SmC, SmC*, SmCsPA, SmCsPA*) are formed. For one compound, a de Vries type smectic phase is observed and it appears that with decreasing temperature, order develops in two steps. First, at the SmA to SmC transition, the tilt direction becomes long range ordered and in a second step a long range ordering in bend direction takes place. Hence, for the optically active compound a transition from chirality induced polar switching to bend induced (shape induced) antiferroelectricity takes place. In this SmCsPA* phase a homogeneous layer chirality is induced under an applied electric field which interacts with the fixed molecular chirality leading to the energetically favoured diastereomeric state and giving rise to a field direction dependent uniform tilt director orientation. Field reversal induces a flipping of the layer chirality, which first leads to the less favorable diastereomeric state, and then this slowly relaxes to the more stable one by a spontaneous reversal of the tilt direction.
- Ocak, Hale,Bilgin-Eran, Belkiz,Prehm, Marko,Schymura, Stefan,Lagerwall, Jan P. F.,Tschierske, Carsten
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p. 8266 - 8280
(2012/03/10)
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- Luminescent columnar liquid crystals generated by self-assembly of 1,3,4-oxadiazole derivatives
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Five new V-shaped acids derived from 1,3,4-oxadiazole are described. These compounds were used to prepare supramolecular complexes via hydrogen bonding interactions with 2,4-diamino-6-dodecylamino-1,3,5-triazine in a 3:1 ratio. The formation of the comple
- Vieira, Andre A.,Gallardo, Hugo,Barbera, Joaquin,Romero, Pilar,Serrano, Jose Luis,Sierra, Teresa
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p. 5916 - 5922
(2011/12/03)
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