Liquid Crystal Phase Transition Temperature Research Articles

Regulating Crystallinity Mismatch Between Donor and Acceptor to Improve Exciton/Charge Transport in Efficient Organic Solar Cells.

Achieving a more balanced charge transport by morphological control is crucial in reducing bimolecular and trap-assisted recombination and enhancing the critical parameters for efficient organic solar cells (OSCs). Hence, a facile strategy is proposed to reduce the crystallinity difference between donor and acceptor by incorporating a novel multifunctional liquid crystal small molecule (LCSM) BDTPF4-C6 into the binary blend. BDTPF4-C6 is the first LCSM based on a tetrafluorobenzene unit and features a low liquid crystal phase transition temperature and strong self-assembly ability, conducive to regulating the active layer morphology. When BDTPF4-C6 is introduced as a guest molecule into the PM6 : Y6 binary, it exhibits better compatibility with the donor PM6 and primarily resides within the PM6 phase because of the similarity-intermiscibility principle. Moreover, systematic studies revealed that BDTPF4-C6 could be used as a seeding agent for PM6 to enhance its crystallinity, thereby forming a more balanced and favourable charge transport with suppressed charge recombination. Intriguingly, dual Förster resonance energy transfer was observed between the guest molecule and the host donor and acceptor, resulting in an improved current density. This study demonstrates a facile approach to balance the charge mobilities and offers new insights into boosting the efficiency of single-junction OSCs beyond 20 %.

Regulating Crystallinity Mismatch Between Donor and Acceptor to Improve Exciton/Charge Transport in Efficient Organic Solar Cells

AbstractAchieving a more balanced charge transport by morphological control is crucial in reducing bimolecular and trap‐assisted recombination and enhancing the critical parameters for efficient organic solar cells (OSCs). Hence, a facile strategy is proposed to reduce the crystallinity difference between donor and acceptor by incorporating a novel multifunctional liquid crystal small molecule (LCSM) BDTPF4‐C6 into the binary blend. BDTPF4‐C6 is the first LCSM based on a tetrafluorobenzene unit and features a low liquid crystal phase transition temperature and strong self‐assembly ability, conducive to regulating the active layer morphology. When BDTPF4‐C6 is introduced as a guest molecule into the PM6 : Y6 binary, it exhibits better compatibility with the donor PM6 and primarily resides within the PM6 phase because of the similarity‐intermiscibility principle. Moreover, systematic studies revealed that BDTPF4‐C6 could be used as a seeding agent for PM6 to enhance its crystallinity, thereby forming a more balanced and favourable charge transport with suppressed charge recombination. Intriguingly, dual Förster resonance energy transfer was observed between the guest molecule and the host donor and acceptor, resulting in an improved current density. This study demonstrates a facile approach to balance the charge mobilities and offers new insights into boosting the efficiency of single‐junction OSCs beyond 20 %.

Cholesterol's inhibition effect on entering of chlorzoxazone into phosphatidylethanolamine bilayer: Relevance to cytochrome P450 drug metabolism at endoplasmic reticulum membranes

Many drugs are metabolized by cytochrome P450 (CYP) in the endoplasmic reticulum (ER) membrane. Recent studies have shown that CYP-substrate drugs reach the CYP active site after entering the lipid hydrophobic part of the ER membrane. To clarify the role of cholesterol (Chol) in the CYP-related drug metabolic process, we investigated the lipid bilayer entry of CYP-substrate drugs using a model membrane system as follows. The model membrane system comprised palmitoyl-oleoyl-phosphatidylethanolamine (POPE) and Chol. Phosphatidylethanolamine is the second major phospholipid component of ER membranes. Chlorzoxazone (CZX) was used as the CYP-substrate drug. Calorimetric measurements showed that the addition of CZX to POPE bilayers decreased the gel–liquid crystal phase transition temperature; X-ray diffraction indicated that CZX distributes into the liquid crystal phase bilayers but not practically the gel phase POPE bilayers. In the presence of Chol, dialysis and X-ray structural analyses showed that Chol inhibited CZX entry into the bilayer with an increase in Chol concentration. The Chol concentration in the ER membrane (5–10 mol%) is much lower than that in the plasma membrane (approximately 30 mol%). This fact may allow CYP-substrate drugs to enter the hydrophobic portion of the ER membrane more easily than other organelle membranes, yielding efficient drug metabolism.

Tuning three-dimensional nano-assembly in the mesoscale via bis(imino)pyridine molecular functionalization

We investigate the effect of bis(imino)pyridine (BIP) ligands in guiding self-assembly of semiconducting CdSe/ZnS quantum dots (QDs) into three-dimensional multi-layered shells with diameters spanning the entire mesoscopic range, from 200 nm to 2 μm. The assembly process is directed by guest–host interactions between the BIP ligands and a thermotropic liquid crystal (LC), with the latter’s phase transition driving the process. Characterization of the shell structures, through scanning electron microscopy and dynamic light scattering, demonstrates that the average shell diameter depends on the BIP structure, and that changing one functional group in the chemical scaffold allows systematic tuning of shell sizes across the entire range. Differential scanning calorimetry confirms a relationship between shell sizes and the thermodynamic perturbation of the BIP molecules to the LC phase transition temperature, allowing analytical modeling of shell assembly energetics. This novel mechanism to controllably tune shell sizes over the entire mesoscale via one standard protocol is a significant development for research on in situ cargo/drug delivery platforms using nano-assembled structures.

Unique two-way free-standing thermo- and photo-responsive shape memory azobenzene-containing polyurethane liquid crystal network

In this work, azobenzene-containing polyurethane liquid crystal networks (PULCN(AZO)s) were synthesized using a one-pot strategy to demonstrate excellent two-way free-standing thermo-/photo-responsive shape memory effects. Based on the step-growth nature of hydroxyls and isocyanates, the architectures of the networks were adjusted by controlling the stoichiometries of chemical materials. A uniform monodomain sample was prepared by external stress relaxation via a reversible addition reaction of a dynamic carbamate bond. Two independent transition temperatures assigned to glass transition temperature/melting temperature and liquid crystal phase transition temperature were employed to thermally trigger triple shape memory effects and two-way autonomous actuation. In addition, taking advantage of the trans-cis photoisomerization of azobenzene, the programmed network showed a reversible bending and unbending shape change upon irradiation by visible light at 450 and 550 nm, respectively. Coupling the autonomously thermo-induced contraction/extension actuation and reversible photo-induced bending/unbending behaviors of PULCN(AZO)s in one system will expand their potential applications in emerging multifunctional devices.

Reconfigurable Large‐Scale Pattern Formation Driven by Topological Defect Separation in Liquid Crystals

AbstractLarge‐scale patterning of topological defects is vital and challenging from both fundamental and technological points of view in anisotropic fluids. However, this is usually difficult because of their unfavorably high‐energy states. Here, a simple but general pathway for topology engineering is presented: processing topological defects and shape large‐scale patterns in materials with liquid crystalline nature. Dragging field is created through flowing materials at liquid–liquid crystal phase transition temperature or designing electric‐field driven temperature gradient. The dragging fields coupled to a dense colony of topological defects with random spatial distribution form nontrivial periodic ordered topological patterns that are energetically unfavorable compared to the uniform ground state, but are stable in the stationary state. Topological polymeric films based on the strategy are also fabricated. The dragging speed and surface interactions are found to be dominant factors in generating and stabilizing the patterns. This strategy endows fluids with regular and large‐scale topological patterns, paving a new way for the development of fluids and gels with spatially modulated topological nature.

Improvement in electro-optical and dielectric characteristics of ZnO nanoparticles dispersed in a nematic liquid crystal mixture

Reported herein is the effect of dispersion of a fixed amount of 0.5% (wt/wt) ZnO nanoparticles (NPs) in a nematic (E7) liquid crystal (LC) mixture. Dispersion of ZnO NPs results in a significant improvement in electro-optical and dielectric parameters of the nematic mixture. Comparative analysis of undoped and NP-dispersed samples shows a reduced threshold voltage with better transmittance without compromising dielectric permittivity characteristics. In addition, an increase in the contrast ratio, birefringence, alternating-current conductivity and band gap was observed after dispersion of ZnO NPs in LCs. A polarized optical microscopic study of the NP-dispersed sample substantiates a slight increase in nematic–isotropic phase transition temperature of LCs.

Gel–liquid crystal phase transition in dry and hydrated SOPC phospholipid studied by differential scanning calorimetry

ABSTRACTWe study the thermodynamic properties of pure and hydrated samples of SOPC (stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) membrane via Differential Scanning Calorimetry. We estimate the fundamental thermodynamic quantities, such as enthalpy and entropy, of the bilayer phosphatidylcholine. It is found that the gel-liquid crystal phase transition is driven by the van’t Hoff enthalpy, revealing the occurrence of an intermediate phase transition. We discuss the influence of the heating rate on the enthalpy and on the gel ↔ liquid crystal phase transition temperature by introducing the adequate thermodynamic Gibbs potential. The effect of hydrogen bonding of the water molecules with the polar head and polar-apolar interface on the energetics of the bilayer membrane matrix is analysed. The obtained transition temperature was found to vary between 3 and 4°C depending on the hydration level. A result corroborated by the behaviour of the heat capacity of SOPC computed via Molecular dynamics simulation.

Liquid Crystals under Confinement in Submicrometer Capsules.

Liquid crystal (LC) ordering and phase transition behavior under confined conditions have attracted extensive attention and enabled many applications. However, the ordering and phase transition behavior of LCs in submicrometer capsules have seldom been studied, primarily due to the lack of proper capsulizing and visualization approaches to such small LC microcapsules. Herein, we achieve submicrometer LC capsules with the sizes down to 100 nm by using emulsion-based interfacial sol-gel reaction. The behavior of LCs under the submicrometer confinement conditions is investigated while the sizes and chemical composition of the microcapsule shell surface are tuned in a controllable way. The phase transition temperatures of LCs in the submicrometer capsules shift from those of bulk LCs due to the surface-induced ordering of LCs under the strong confinement conditions, which causes formation of topological defects and alters the order parameter. Using nonlinear optical imaging technology, we explore the structures of director field of LCs that arise as a result of the competition between the surface boundary conditions and LC elasticity. The results show that the nanoscale encapsulation can significantly influence the structural configurations of the director and phase transitions of LCs under various confinement conditions.

Effect of an electric field applied during the injection procedure on the pretilt angle of a nematic liquid crystal

We examined the effect of an electric field applied during the injection procedure on the polar pretilt angle of a nematic liquid crystal (LC). The pretilt angle of the sample injected at 25°C gradually increased as the electric field was increased. On the other hand, the pretilt angle of a sample injected at 90°C (which is above the nematic-isotropic phase transition temperature of LC) showed a sudden increase in the presence of the electric field and also increased with a greater electric field. We think the alignment layer might be swollen with LC molecules, and the rotation of the immersed LC molecules by the electric field induces a deformation of the alignment layer. These results imply LC and the alignment layer were coupled, and their cooperation had an influence on determining the bulk pretilt angle.

Determination of the liquid crystals phase transition temperatures using optical rotation effect

Using optical rotation effect, a sensitive, simple optical analytical system is developed for determining the phase transition temperatures of liquid crystals (LCs). When a monochromatic polarized light passes through LCs sample and analyzer, the light intensity changes with temperature. Especially, during the phase transition process, the intensity varies greatly due to optical rotation effect. The variation of light intensity versus variation of temperature curve shows the phase transition temperatures of LCs clearly. The phase transition temperatures of three cholesteric liquid crystals (ChLCs) and a nematic liquid crystals (NLCs) were detected by this method, and compared with those of the differential scanning calorimetry (DSC) and polarized light microscope (PLM) methods.

全息方法制备嵌段式液晶弹性体

The preparation method of triblock Liquid Crystal(LC) elastomer films with thermal induced contraction was researched and their polarized characteristics and thermal induced contrasction rates were analyzed.By using the holographic exposure technology,a liquid crystal elastomer with a grid-like structure was prepared.Based on a polarization microscope and a He-Ne laser,grid-like structure and polarization were detected,respectively.The relationship between the LC molecular alignment and the polarization of LC elastomer was analyzed.In addition,the thermal induced contraction rate for LC elastomer was tested.The experimental results indicate that LC molecules are arranged perpendicular to grid-like structure.The deformation rate of thermal induced contraction for the liquid crystal elastomer can be achieved 12% in the LC phase transition temperature(65-80℃).

Effects of acceptor composition and mechanism of ABCG1-mediated cellular free cholesterol efflux

Among the known mechanisms of reverse cholesterol transport (RCT), ATP binding cassette transporter G1 (ABCG1)-mediated free cholesterol (FC) transport is the most recent and least studied. Here, we have characterized the efficiencies of different acceptors using baby hamster kidney (BHK) cells transfected with human ABCG1 cDNA, which is inducible upon treatment with mifepristone. When normalized on particle number and particle surface area, the acceptor efficiency for FC efflux was as follows: small unilamellar vesicles (SUV)>LDL>reconstituted HDL>HDL2 = HDL3. Based on phospholipid content, the order was reversed. ABCG1 also mediated phospholipid efflux to human serum and HDL3. ABCG1-mediated FC efflux correlated significantly with a number of HDL subfractions and components in serum collected from 25 normolipidemic individuals: apolipoprotein A-II (apoA-II) (r2 = 0.7), apolipoprotein A-I (apoA-I) (r2 = 0.5), HDL-C (r2 = 0.4), HDL-PL (r2 = 0.4), α-2 HDL (r2 = 0.4), and preβ HDL (r2 = 0.2). ABCG1 did not enhance influx of FC or cholesteryl oleyl ether (COE) when cells were incubated with radiolabeled HDL3. ABCG1 expression did not increase the association of HDL3 with cells. Compared with control cells, ABCG1 expression significantly increased the FC pool available for efflux and the rate constant for efflux. In conclusion, composition and particle size determine the acceptor efficiency for ABCG1-mediated efflux. ABCG1 increases cell membrane FC pools and changes its rate of desorption into the aqueous phase without enhancing the association with the acceptor.

Cryo-responses of two types of large unilamellar vesicles in the presence of non-permeable or permeable cryoprotecting agents

Cryo-responses of two types of large unilamellar vesicles (LUV) that were made from either egg yolk l-α-phosphatidylcholine (EPC) or 1,2-dipalmitoyl-rac-glycero-3-phosphocholine (DPPC), in the presence of non-permeable or permeable cryoprotective agents (CPA) was investigated. Partial ternary phase diagrams of CPA–salt–water with specific CPA to salt ratio ( R), were constructed to estimate the phase volume of ice and unfrozen matrix of the LUV dispersion, which could aid in understanding the mechanistic actions of CPA. Leakage of both EPC and DPPC LUV was reduced if the sugar concentrations are above 10% (w/w) for disaccharides and 5% (w/w) for monosaccharides. Above these sugar concentrations, non-permeable CPA were more effective in preventing leakage of DPPC LUV than in EPC LUV. Below these sugar concentrations, EPC and DPPC LUV with limited mobility in the remaining unfrozen matrix were more likely to approach and interact with one and another, which were not anticipated when the LUV were completely embedded in the ice matrix. In the presence of Me 2SO or EG, EPC LUV that had been subjected to freezing and thawing processes were protected from leakage. At room temperature, Me 2SO and EG were detrimental to the DPPC LUV. This study suggests that the choice of CPA for cell cryopreservation depends on the type of phospholipids in plasma membranes, which vary in their acyl chain length and gel–liquid crystal phase transition temperature.

Cholesterol/Phospholipid Interactions in Hybrid Bilayer Membranes

The interactions between cholesterol and saturated phospholipids in hybrid bilayer membranes (HBMs) were investigated using the interface-sensitive technique of vibrational sum frequency spectroscopy (VSFS). The unique sensitivity of VSFS to order/disorder transitions of the lipid acyl chains was used to determine the main gel to liquid crystal phase transition temperature, Tm, for HBMs of binary cholesterol/phospholipid mixtures on octadecanethiolate self-assembled monolayers. The phase transition temperature and the breadth of the transition were shown to increase with cholesterol content, and the phase boundaries observed in the cholesterol/phospholipid HBMs were comparable to the published phase diagrams of binary cholesterol/phospholipid vesicles. A thermodynamic assessment of the cooperative units of the HBM phase transitions revealed the presence of <10 nm diameter domains that were independent of the cholesterol composition.

Interaction of benzoic acid, aspirin and para-hydroxy benzoic acid with dipalmitoyl phosphatidic acid vesicles

The effect of benzoic acid (BA), aspirin (ASA) and para-hydroxy benzoic acid ( p-HOBA) on the thermotropic behaviour and molecular mobility of dipalmitoyl phosphatidic acid (DPPA) vesicles has been investigated using DSC and 1H NMR. These investigations were carried out with DPPA dispersion in both multilamellar vesicular (MLV) and unilamellar vesicular (ULV) forms. The DSC data indicated that the mechanism by which BA, ASA and p-HOBA interact with the DPPA is similar in MLV and ULV. In the presence of BA, ASA and p-HOBA, the gel to liquid crystal phase transition temperature ( T m) of the DPPA vesicles increases. The DPPA vesicles thus becomes more rigid in the presence of BA, ASA and p-HOBA molecules, due to increased interaction between the lipid headgroups. The increase in rigidity of the DPPA membrane was found to be maximum when doped with ASA and least when doped with salicylic acid (SA). There seems to be good correlation between the acidity of –COOH group present in the drugs and the increase in T m value (representing rigidity). The transition width ( Δ m) decreases with increasing concentration of BA, ASA and p-HOBA, implying an enhanced co-operativity of the acyl chain. DSC and NMR data indicate that BA, ASA and p-HOBA molecules, at all concentrations are located close to the interfacial region of the DPPA bilayer but not in the acyl chain region.

Effect of chain unsaturation on bilayer response to pressure

Using wide-line deuterium NMR, the effects of pressure on saturated-chain orientational order and gel-to-liquid-crystal phase transition temperature were observed in bilayers of 16:0-18:1 PC-d31 (POPC-d31) and 16:0-18:2 PC-d31 (PLPC-d31). Spectra were recorded for a range of pressures at selected temperatures and for a range of temperatures at selected pressures up to 193 MPa. For 16:0-18:1 PC-d31, the main transition temperature increased by approximately 0.18 K/MPa, a rate that is similar to what is found for bilayers of disaturated PC's. For 16:0-18:2 PC-d31, the increase in transition temperature with pressure was slightly smaller at approximately 0.13 K/MPa. To investigate the isothermal response of chain orientational order to pressure, spectra for each lipid were obtained for three pressures (ambient, 55 MPa, and 110 MPa) near room temperature (approximately 25 degrees C) and for three pressures (ambient, 110 MPa, and 193 MPa) at higher temperature (approximately 40 degrees C). These temperatures were chosen such that the difference between the higher observation temperature and the main transition of 16:0-18:1 PC-d31 would be similar to the difference between the lower observation temperature and the main transition of 16:0-18:2 PC-d31. Application of a given pressure was found to raise the orientational order for all methylene groups on the saturated chain of a particular lipid by roughly similar amounts. For comparable pressure differences, the pressure-induced ordering of the 16:0-18:1 PC-d31 saturated chain at approximately 40 degrees C was greater than that of the corresponding chain in 16:0-18:2 PC- d31 at approximately 25 degrees C. These observations suggest that increasing levels of chain unsaturation may reduce the sensitivity of bilayer order to variations in pressure at corresponding temperatures relative to their ambient pressure transitions.

Electrically tunable, optically induced dynamic and permanent gratings in dye-doped liquid crystals

We report on further experimental studies of the ac field response of transient and permanent gratings written with visible light in both planar and homeotropic, dye-doped liquid crystal cells. It is found that the diffraction efficiency of these gratings can be controlled by the applied ac field. High ac frequencies can switch off diffraction completely in both permanent and transient gratings. The response time to the applied electric field is shown to be in the range of milliseconds. Permanent gratings persist for months and, when heated above the liquid crystal phase transition temperature, they can be only partially erased. Furthermore, on cooling the diffraction efficiency can be restored, indicating a strong anchoring at the boundaries of the cell.

Surface Phenomena of Liquid Crystalline Substances

The influence of the sorbed N2 on the surface tension of the low molecular liquid crystal MBBA (p-methoxybenzylidene-p ′-n-butylanilin) has been investigated using the pendant drop method. The time- and temperature-dependent measurements of the surface tension of MBBA have been carried out under different N2 pressures (1–15 bar). The time-dependence of the surface tension is interpreted in terms of the sorption process of N2. The results depend strongly on the liquid crystal (LC) phase, the gas pressure and the gas type. All the temperature-dependence curves of the surface tension of MBBA, obtained at different N2 pressures, show a similar shape within the experimental temperature range (25–50°C), yet with a systematic decrease of the phase transition temperature isotropic (i)/nematic(n) by the increasing N2 pressure and thus the increasing concentration of the sorbed gas in the sample. The positive slope of the surface tensiontemperature characteristic near the phase transition temperature indicates the development of higher ordered molecular states in the surface layer. The correlation between the i-n phase transition temperature of LCs and the concentration of sorbed gases, the gas type and the impurities in LCs has been also discussed.

Photochemical Phase Transition in Hydrogen-Bonded Liquid Crystals

Photoisomerization of the azo component in the supramolecular hydrogen-bonded assembly between 4,4‘-azabipyridine and 4-alkyloxybenzoic acid brings about a marked increase in its crystal to liquid crystal phase transition temperature.