Wanli He

Polymer stabilized liquid crystal films reflecting both right-and left-circularly polarized light

A single-layer polymer stabilized liquid crystal (PSLC) film reflecting both right- and left-circularly polarized light has been developed by a wash-out/refill method. The PSLC film was achieved by prefabricating the polymer network with a left-handed helical structure and then refilling a cholesteric liquid crystal with a right-handed helical structure into the network. The reflection intensity of the PSLC film is close to 100% when the pitch lengths of the two opposite helical structures are the same. It was demonstrated that the memory effect of the polymer network is an important mechanism for the resulting film properties.

Wide blue phase range and electro-optical performances of liquid crystalline composites doped with thiophene-based mesogens

Thiophene-based bent-shaped mesogens were firstly applied to extend the blue phase range. And the electro-optical performances of liquid crystalline composites doped with different concentration of these bent-shaped mesogens were also explored by incorporating in-plane electric field geometry.

Light-controllable reflection wavelength of blue phase liquid crystals doped with azobenzene-dimers

A new series of azobenzene-dimers were synthesized and doped into the blue phase liquid crystals to broaden the temperature range of BPs. It is found that not only can the reflection wavelength of BPI be reversibly controlled but BPI can also be transformed into the cholesteric phase owing to isomerization of azobenzene induced by light.

Effects of 1,3,4-oxadiazoles with different rigid cores on the thermal and electro-optical performances of liquid crystalline blue phases

A series of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles with different rigid cores were synthesised under the microwave irradiation based on green chemistry and atom economy. The induced blue phase (BP) behaviours of these bent-shaped compounds were investigated after blending them with a high twisted power chiral dopant or doping them into the chiral nematic liquid crystal (N*LC). It was found that the BP ranges varied greatly with the difference in the rigid cores of 2,5-disubstituted 1,3,4-oxadiazoles, and the bent-shaped molecules with higher miscibility and twisted molecular conformation significantly contribute to the stabilisation of BPs. In addition, the electro-optical performances of pure BPs and polymer-stabilised BPs in the systems containing 1,3,4-oxadiazoles with different rigid cores were also explored, which shows that the Kerr constant decreases after being polymerised and varies with the doping of the compounds possessing different rigid cores in the polymerised mixtures.

Fabrication of multi-pitched photonic structure in cholesteric liquid crystals based on a polymer template with helical structure

In this study, we demonstrate a novel method for fabricating a single-layer Ch-LC film with multi-pitched photonic structure as new photonic band gap (PBG) materials by utilizing a polymer template. The polymer template with helical structure was originated from polymer-stabilized cholesteric liquid crystals. Various types of Ch-LCs are subsequently imbibed into the polymer template, resulting in films with distinct optical characteristics. Simultaneous red, green and blue reflection (multiple PBGs) and wide-band reflection in a single-layer Ch-LC film can be achieved by using nematic liquid crystals with different birefringence in Ch-LCs/polymer template composites, and a H-bonded Ch-LC with a great temperature-dependence of pitch length was introduced into the polymer template, and a temperature-tunable PBG of the Ch-LC film with a double-handed circularly polarized light reflection band was obtained. Additionally, the lasing emission of a single-layer Ch-LC film and patterning of a single-layer Ch-LC film with a double-handed circularly polarized light reflection band were also realized by the above approach. These special optical properties make the novel Ch-LCs composites interesting for their great potential applications in many fields, such as information recording, optical components, flat displays, photonic crystals etc.

Effect of lateral fluoro substituents of rodlike tolane cyano mesogens on blue phase temperature ranges

Several kinds of tolane cyano derivatives with different lateral fluoro substituents are doped with chiral additives to induce the blue phase (BP). Fluoro substituents are found to exert a great effect on the BP ranges for biphenyl mesogens. The laterally difluorinated compounds exhibit wider BP ranges than their corresponding mono-fluorinated homologues when equal chiral dopant is added, while the non-fluorinated compounds show direct isotropic (Iso) to chiral nematic (N*) transitions, even in highly chiral systems. By adjusting the concentrations of the chiral dopant ISO(6OBA)2 in the fluorinated liquid crystals, the BP ranges can be broadened to more than 12 °C. Competition between the steric repulsions caused by the lateral fluoro substituents and π–π interactions of the aromatic cores is supposed to be the main factor that leads to the above results. Apart from this, a weak C–H⋯F interaction may have also been conducive to the BP stabilization.

Effects of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles on the temperature range of liquid crystalline blue phases: a systematic study

Four series of symmetrically 2,5-disubstituted 1,3,4-oxadiazoles with different lateral substituents and terminal alkoxy chain length were synthesised and characterised. Then, all these bent-shaped compounds were separately doped into the blue phase (BP) liquid crystal host and their effects on the BP range of the host were investigated, which shows that the BP ranges varied greatly with the difference in the structure of 2,5-disubstituted 1,3,4-oxadiazoles and their doped concentration. Moreover, in order to explore the influence of the inherent molecular geometry and properties of the bent-shaped molecules on the stability of BPs, the molecular structures of all the compounds were studied by using the theory of density functional theory, and the optimised structural parameters of the molecules were calculated at the B3LYP/6–31G* level.

Third-order nonlinear optical properties of a novel series of D-π-A pyrene-aldehyde derivatives

A series of mono-substituted pyrene derivatives having a pyrene mesogenic core and aldehyde group peripherals were designed and synthesized, which had typical D-π-A structures. Photophysical properties of the compounds were characterized by UV/Vis spectroscopy and the third-order nonlinearities of the compounds were studied by “open aperture” and “closed aperture” Z-scan techniques. Both of the compounds showed typical nonlinear absorptions and refractions, and the third-order nonlinear susceptibilities have been found to be very high in all cases. The significant nonlinear optical response combined with the possibility to further modify it, by changing the electron acceptor and the electron-donor groups, suggests that these systems can prove to be important candidates for a variety of photonic/optoelectronic applications.

Broadband reflection characteristic of polymer-stabilised cholesteric liquid crystal with pitch gradient induced by a hydrogen bond

A polymer-stabilised cholesteric liquid crystal (PSChLC) was fabricated by ultraviolet (UV) induced polymerisation of photopolymerisable acrylate monomers mixed in a cholesteric liquid crystal (ChLC). A polymer network with a concentration gradient, which was induced by UV light absorption of dye along the propagation direction, was formed. A hydrogen bond, arising between the polymer network with a concentration gradient containing carboxyl as proton donors and chiral dopant (CD) as proton acceptors, induced a pitch gradient in PSChLC and then, as a consequence, broadband reflection. The broadband reflection is associated with the concentration and the composition of photopolymerisable acrylate monomers, the concentration of CD and the polymerisation temperature. Examining the morphologies of the polymer network by scanning electron microscopy, the helix structure and pitch gradient were verified, confirming the pitch gradient of the PSChLC and revealing the essence of the formation of broadband reflection.

Bandwidth-controllable reflective cholesteric gels from photo- and thermally-induced processes

A bandwidth-controllable reflective gel has been investigated from photo- and thermally-induced processes. Due to the pitch of cholesteric liquid crystal (CLC) composite increasing as the temperature rises, the CLC with short pitch is frozen by UV-curing the polymer network at low temperature and the CLC with long pitch is fixed by heat curing the polymer network at high temperature. A non-uniform pitch distribution of CLCs forms in the gel when temperature becomes low. It is demonstrated that the memory effect of the polymer network is an important mechanism for the resulting gel.