Jong Gun Lee

Anchoring transition of bent-rod liquid crystal dimers on different surfaces

Non-symmetrical dimers have been synthesised, in which bent- and rod-shaped mesogenic molecules are linked by a flexible polymethylene spacer –(CH2) n –. It has been shown that compounds in which n = 5, 6 or 7 exhibit one (nematic), two (nematic and smectic), and no mesogenic phases, respectively. These exhibit a variety of anchoring transition behaviours depending on the mesogenic molecules (n = 5 or 6) and surfaces (polyimide surfaces for homeotropic and planar alignment, or a perfluoropolymer surface). Discontinuous and continuous planar-to-homeotropic orientational changes take place with decreasing temperature. These differences may originate from different dielectric anisotropy, and steric interactions caused by the directional relationship of bent and rod mesogens, depending on the length of the spacer. A possible explanation is suggested on the basis of surface–molecule interactions.

Synthesis and mesomorphic properties of new chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups

New chiral banana‐shaped liquid crystals with chiral 3‐(alkoxy)propoxy terminal groups (Pn‐O‐PIMB5*‐4O, n = 7, 8, 9 and 10) were synthesized and their mesomorphic properties and phase structures investigated by means of electro‐optic measurements, polarizing optical microscopy, differential scanning calorimetry, and second harmonic generation measurements. Most of these chiral bent‐core mesogens (n = 7–9) showed the antiferroelectric B2 phase, whereas P10‐O‐PIMB5*‐4O exhibited the B7 phase. Comparing with the previously reported homologue Pn‐O‐PIMB(n‐2)*, we conclude that the terminal chain structure, particularly the position of chiral centres, plays an important role in the emergence of particular phase structures.

Longer-terminal-chain-sensitive phase structures in mixtures and nonsymmetric molecules of bent-core mesogens

Odd–even behavior for the emergence of ferroelectricity and antiferroelectricity has been examined in bent-core compounds containing two different chain lengths; i.e., 1:1 mixtures of neighboring homologues of symmetric bent-core mesogens and nonsymmetric pure compounds. The odd–even behavior was found to be very strictly held for longer terminal chains: longer terminal chains play an important role in determining the polar structure. Namely, the ferroelectric state emerges when the carbon number n of the longer chain is even, whereas the antiferroelectric state emerges for odd n.