LC Transition Research Articles

End Group Effect of Asymmetric Benzodithiophene-Based Donor with Liquid-Crystal State for Small-Molecule Binary Solar Cell.

Liquid-crystal small molecule donor (LC-SMD) is a new type organic semiconductor, which is attractive not only for the easy synthesis and purification, well-defined chemical structures, etc., but also for the LC state that makes the crystallinity and aggregation state of molecules adjustable. Here, one new LC-SMD (a-BTR-H4) is synthesized with 1D alkoxyl and 2D thiophene-alkylthiol side-chained benzo[1,2-b:4,5-b']dithiophene core, trithiophene π-bridge, and 3-(2-ethylhexyl) rhodanine end group. a-BTR-H4 shows low LC transition temperature, 117 °C, however, counterpart material (a-BTR-H5) with the same main structure but 3-ethyl rhodanine terminal group does not show LC properties. Although a-BTR-H4/H5 show similar Ultraviolet-visible absorption spectrum and energy levels, a-BTR-H4 affords relatively high photovoltaic performances due to favorable blend morphology produced by the consistent annealing temperature of Y6-based accepters and liquid crystal temperature of donors. Preliminary results indicate that a-BTR-H4 gains a power conversion efficiency (PCE) of 11.36% for Y6-based devices, which is ascribed to better light harvest as well as balanced carrier generation and transport, while a-BTR-H5 obtains 7.57% PCE. Therefore, some materials with unique nematic LC phase have great application potential in organic electronics, and further work to utilize a-BTR-H4 for high-performance device is underway.

Spin crossover phenomena in long chain alkylated complexes.

Soft materials represented by polymers, liquid crystals, and colloids are a class of materials that exhibit a variety of unique functionalities which derive from their non-rigid structures. In particular, soft metal complexes incorporating flexible long alkyl chains in their structures have been shown to display synergy between the structural dynamics of the long alkyl chain moieties and the electron dynamics occurring at the metal centre. This review presents a discussion of such soft metal complexes with a focus on spin crossover (SCO) behaviours that are associated with a structural phase transition, including a liquid crystal (LC) transition, arising from the flexible natures of the respective complexes. We also discuss how to fabricate soft materials based on SCO complexes.

Optical and thermal characterization of thermotropic hydrogen bonded liquid crystal mixture

A novel intermolecular hydrogen-bonded liquid crystal mixture (HBLC) has been synthesized from non-mesogenic itaconic acid (IA) and mesogenic 4-undecyloxybenzoic acid (11OBA). The formation of hydrogen bond has been analyzed by using spectroscopic studies. Polarizing optical microscope (POM) confirms the existence of liquid crystal phase corresponding to the transition temperature. LC transition temperature, enthalpy values, stability factors, and thermal width have been calculated using thermal analyzer. Band gap energies are evaluated by using ultraviolet visible and photoluminescence spectroscopy studies. The resultant POM and DSC studies reveal the present mixture enclose nematic and smectic mesogenic phase transitions. Induced thermoluminescence with parachromic variations are observed and represented as novel detection in the present HBLC mixture. This synthesized mixture is more suitable for HD display devices.

Influence of meta-extended rigid-core, complementary hydrogen bonding and flexible chain on polymorphism in Schiff-base liquid crystals: (4)MeOBD(3)AmBA:nOBAs

ABSTRACTSchiff-base complementary hydrogen-bonded liquid crystals (HBLC), viz., (4)MeOBD(3)AmBA:nOBAs with flexible chain length for n = 3,4,5,6,7,8,9,10,11 and 12 are reported. 1H; 13C-NMR and Infra-Red spectroscopy used to confirm the formation of HBLCs. LC phases and transition temperatures (Tc) determined by polarised optical microscopy (POM) and differential scanning calorimetry (DSC). Tc and enthalpy (∆H) determined by DSC also. Odd-Even effect observed at clearing and melting transitions. Influence of Schiff base and Oxygen as bridging atom promote smectic phase abundance. HBLCs exhibit tetra- or penta-phase variance. Maximum (penta) phase variance is exhibited by n = 8 and 12 with long flexible chain. Prevalent abundance of quasi-two-dimensional (2D) LC phases of SmF and SmI is observed. Nematic phase is quenched. Lower (n = 4) and intermediate (n = 6) members exhibited SmBcryst phase. Predominant occurrence of enantiotropic LC phases is noticed. All members exhibited 3D tilted SmG phase. A-C transition exhibited by intermediate homologues (for n = 7, 8, 9 and 10) is found to be either second order or with very small enthalpy. Phase diagram reveals the abundance of multi-critical points with LC phasesinvolving exotic symmetries. Influence of meta-extended rigid core, complementary HB and flexibility is studied for the LC phase abundance with characteristic structural order. POM and DSC results are discussed in the wake of reports in other achiral calamitic LCs.

Stability and softening of a lipid monolayer in the presence of a pain-killer drug

The aim of this study is to investigate the interaction of a drug (Piroxicam, 4-hydroxy-2-methyl-N-(2-pyridinyl)-2H-1,2-benzothiazine-3-carboxamide 1,1 dioxide) with a lipid (DMPC) monolayer used as a membrane-mime in terms of drug-induced changes in stability and compressibility with variation in temperature, surface-pressure, drug-dose and ionic states of the monolayers. Drug-induced fluidization is noticed in the π−A isotherms through increase in phase-transition pressure at constant temperature. The long-term dynamics of the lipid-monolayer is characterized by algebraic decays in surface-energy E with time t, E∼t−p1,2,3, with an initial decay exponent p1 that changes to p2 after ∼1000s, and, at high surface pressures and/or drug-dose, to a third exponent p3 after ∼3500s, suggesting structural reorganizations in the monolayer. With increasing drug–lipid ratio (D/L), p1 shows a decrease ending at an almost constant value after 0.05, p2 shows an almost negligible lowering while p3 shows a monotonic and considerable increase. The reorganization is summarized by proposing two mechanisms: (a) ‘charging–discharging’ where drug-molecules sitting parallel to the interface increase headgroup separations and (b) ‘discharging–charging’ where drug-molecules sitting roughly perpendicular to the interface bring headgroups closer. Drug-induced softening of lipid-monolayers is characterized by the compressibilites of pure and mixed lipid monolayers. Compressibility-change (i.e., compressibility difference between drug/lipid and pure lipid monolayer) with pressure is maximum in the LE–LC transition zone and compressibility-change with drug-dose reveals an optimum dose of drug for maximum increase in compressibility. Molecular dynamics simulation shows that the ordering in the different parts of the lipid chains is changed to different extents in the presence of drugs with maximum change near the headgroups and again points to an optimum dose for maximum disorder.

Dynamic anchoring transitions at aqueous–liquid crystal interfaces induced by specific and non-specific binding of vesicles to proteins

This paper reports on the dynamics of continuous anchoring transitions at interfaces formed between nematic liquid crystals (LCs, 4′-pentyl-4-cyanobiphenyl (5CB)) and immiscible aqueous phases that are induced by either non-specific or specific interactions between phospholipid vesicles and proteins adsorbed at the LC interfaces. By analyzing the dynamic response of LCs to non-specific adsorption of lipids onto bovine serum albumin (BSA)-decorated LC interfaces, we provide evidence that the LC anchoring transitions are slower than diffusion-controlled accumulation of lipid at the interface, consistent with the hypothesis that the LC transition involves lateral reorganization of proteins and lipids at the interface. Significantly, optical measurements of the tilt angle of the LC as a function of the amount of lipid captured at the interface were found to be quantitatively consistent with theoretical predictions of LC anchoring directed by nanoscopic domains of molecules that cause planar (protein) and homeotropic (lipid) anchoring of the LC. Finally, specific binding interactions between the antibody-decorated LC interfaces and vesicles (through antibody-antigen recognition) greatly accelerated the continuous LC anchoring transitions, with dynamics that were measured to scale with the logarithm of the ligand composition of the vesicles (over four orders of magnitude). The latter dynamics were found to be strongly influenced by addition of synthetic surfactants, consistent with our proposal that the rate-limiting step underlying the response of the LC was the transfer of lipids from captured vesicles into the protein-decorated LC interface. Overall, the results presented in this paper provide quantitative insight into the origin of continuous anchoring transitions triggered by vesicles at protein-decorated LC interfaces and, more broadly, guidance for the design of stimuli-responsive LC systems.

Unusual Temperature-Dependent Photophysics of Oligofluorene-Substituted Tris-Cyclometalated Iridium Complexes

The photophysical properties of a series of tris-cyclometalated Ir(III) complexes bearing oligofluorene-substituted 2-phenylpyridine (ppy) and/or 1-phenylisoquinoline (piq) ligands were studied at both room temperature and 77 K, for delineating the oligomer-substitution effects on the photophysics in such metal-complex-containing conjugated oligomers/polymers. Unique temperature dependence was observed with the triplet excited-state lifetime of the studied oligomers. Molecules having one of the three ppy ligands substituted with an oligofluorenyl group at varied positions exhibited two distinct types of phosphorescing behaviors. When the oligoflurene group was coupled to ppy in a conjugative fashion (i.e., at 5- or 4′- position), the complexes appeared to emit from a 3MLCT-dominated state perturbed by LC transition, as evidenced by the relatively short lifetimes of phosphorescence as well as hypsochromic shift upon lowering the temperature. Surprisingly, even shorter triplet lifetimes were detected at 77 ...

Photochemical and photophysical properties of ruthenium(II) bis-bipyridine bis-nitrile complexes: Photolability

The electrochemical and photophysical properties of two bis-nitrilo ruthenium(II) complexes formulated as [Ru(bpy) 2(L) 2](PF 6) 2, where bpy is 2,2′-bipyridine and L is AN = CH 3CN and sn = NC–CH 2CH 2–CN, have been investigated. Electrochemical data are typical of Ru-bpy complexes with two reversible reduction peaks located near −1.3 and −1.6 V assigned to each bipyridine ligand and one Ru II/Ru III oxidation wave centered at approximately +1.5 V. The sn derivative is both IR and Raman active with its coordinated CN stretch appearing at 2277 cm −1 and 2273 cm −1, respectively. The UV/Vis absorption spectrum of the sn derivative is dominated by an intense ( ε max ∼ 58700 M −1 cm −1) absorption band at 287 nm assigned as a LC (π → π∗) transition. The peak observed at 418 nm ( ε ∼ 10 400 M −1 cm −1) is an MLCT band while the one at 244 nm ( ε ∼ 23 600 M −1 cm −1) is of LMLCT character. The AN derivative behaves similarly. Both complexes show low-temperature emission at around 537 nm with a lifetime near 10.0 μs. 1H and 13C assignments are consistent with the formulation of the complexes. The complexes undergo photosubstitution of solvent with quantum efficiencies near one. Calculated and experimental results support replacement of the nitrile ligands by solvent. Based on DFT calculations, the electron density of the HOMO lies on the metal center, the bipyridine ligands and the nitrile ligands and electron density of the LUMO resides primarily on the bipyridine ligands. The electronic spectra obtained from TDDFT calculations closely match the experimental ones.

Sol/gel transition and liquid crystal transition of HPC in ionic liquid

This work uses an ionic liquid (IL), 1-butyl-3-methylimidazolium acetate, as a solvent to study the sol/gel transition (SGT) and liquid crystal transition (LCT) of hydroxypropylcellulose (HPC) solution. The LCT concentration of HPC at room temperature is 7 wt%, which is slightly higher than its SGT concentration of 6 wt%. For HPC concentrations of over 7 wt%, three rheological approaches were utilized, and the parameters relaxation time, hysteresis ratio and loss modulus (G″) are measured to determine the LCT temperature. The relaxation study concludes that the LC-critical concentration of HPC is 7 wt%. When HPC exceeds 7 wt%, the LC transition temperatures, ranging from 45 to 51 °C, can be measured and are proportional to the HPC concentration. The rheological results are then confirmed by making observations under a polarized optical microscope. All results are highly mutually consistent. Most significantly, the rheological parameters adopted herein can be used as good indicators of the LC transition. Of those indicators, G″ changes distinctly at the LCT point, and can thus be suggested to be the most helpful indicator in determining the LCT temperature.

Ionic Liquid Crystals: Self-assembly of Imidazolium Salts Containing an l-Glutamic Acid Moiety

Abstract Liquid-crystalline (LC) behavior and ionic conductivities of imidazolium salts containing an l-glutamic acid moiety have been studied. The ionic conductivities of the (CF3SO2)2N− salt forming the columnar LC phase are higher than those of the Br− salt in the columnar LC state. The ionic conductivity shows a sudden decrease at the columnar–micellar cubic LC transition of the Br− salt.

Photophysical Properties of Heteroleptic Iridium Complexes Containing Carbazole‐Functionalized β‐Diketonates

Twelve iridium complexes with general formula of Ir(C;N)(2)(LX) [C;N represents the cyclometalated ligand, i.e. 2-(2,4-difluorophenyl) pyridine (dfppy), 2-phenylpyridine (ppy), dibenzo{f, h}quinoxaline (DBQ); LX stands for beta-diketonate, i.e. acetyl acetonate (acac), 1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate (CBDK), 1-(carbazol-9-yl)-5,5,6,6,7,7,7-heptafluoroheptane-2,4-diketonate (CHFDK), 1-(N-ethyl-carbazol-3-yl)-4,4,5,5,6,6,6-heptafluorohexane-1,3-diketonate (ECHFDK)] are synthesized, characterized and their photophysical properties are systemically studied. In addition, crystals of Ir(DBQ)(2)(CHFDK) and Ir(DBQ)(2)(acac) are obtained and characterized by single crystal X-ray diffraction. The choice of these iridium complexes provides an opportunity for tracing the effect of the triplet energy level of ancillary ligands on the photophysical and electrochemical behaviors. Data show that if the triplet energy level of the beta-diketonate is higher than that of the Ir(C;N)(2) fragment and there is no superposition on the state density map, strong (3)LC or (3)MLCT-based phosphorescence can be obtained. Alternatively, if the state density map of the two parts are in superposition, the (3)LC or (3)MLCT-based transition will be quenched at room temperature. Density functional theory calculations show that these complexes can be divided into two categories. The lowest excited state is mainly determined by C;N but not beta-diketonate when the difference between the triplet energy levels of the two parts is large. However, when this difference is very small, the lowest excited state will be determined by both sides. This provides a satisfactory explanation for the experimental observations.

Degradation induced variation of LC transition of poly( n-undecyl isocyanate)

The first-order thermal transitions including side-chain melting, crystal-LC, biphasic chimney-like, and isotropic transitions of poly( n-undecyl isocyanate) (PUDIC) were investigated by varying its thermal history. After annealing at 140 °C for different periods of time, it leads to different degrees of backbiting degradation into n-undecyl isocyanurate trimer which behaves as a solvent for the rest non-degraded/rigid PUDIC to change into a lyotropic LC from thermotropic transition. Based on the degree of degradation, various amounts of trimers are present and the PUDIC demonstrates different types of thermal transitions found in DSC thermograms due to the variation of the non-degraded PUDIC concentration in solvent trimers. Finally, a phase diagram includes all types of crystallines, LC, and isotropic regions can be drawn according to the DSC thermograms.

Dynamic mechanical and thermal behavior of liquid-crystalline polybutadiene-diols with mesogenic groups in side chains

Liquid-crystalline polybutadiene-diols (LCPBDs) with the comb-like architecture were synthesized by reaction of a LC thiol with the double bonds of telechelic HO-terminated polybutadiene (PBD). LCPBDs with various initial molar ratios of thiol to double bonds of PBD, R 0, in the range from 0.15 to 1, were prepared by the radical reaction at temperature 60 °C for 48 h. The experimentally obtained degree of modification, R e, after the reaction and purification, was determined from elemental analysis – from the amount of sulphur bounded in LCPBDs, GPC and from 1H NMR spectra. The physical properties were investigated by differential scanning calorimetry and dynamic mechanical spectroscopy. With increasing R e ratio the glass transition temperature of LCPBDs, T g, increases from ∼ − 45 °C (neat PBD) to ∼20 °C ( R e ∼ 0.5). LC transition starts at R e ∼ 0.27 (the transition temperature T m ∼ 27 °C). With increasing R e temperature T m increases and for R e ∼ 0.5 reaches the value T m ∼ 74 °C; at the same time also the change in enthalpy at LC transition increases. The LC transition could be detected also by the dynamic mechanical spectroscopy; especially shape and position of mechanical functions on frequency and free volume parameters strongly depend on degree of modification.

In Situ Studies of Phase Transitions in Thin Discotic Films

The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.

Effect of the polar headgroup of phospholipids on their interaction with actin

It is generally admitted that actin filaments are anchored to a membrane by membranar actin-binding-proteins. However, we found that actin may also interact directly with membrane phospholipids. The actin–phospholipid complex has been investigated at the air–water interface using a film balance technique. In order to probe the effect of the phospholipid headgroup on the actin–phospholipid interaction, we focus mainly on phospholipids that have the same acyl chain length but different headgroups. For all the phospholipids, the apparent area per molecule (the total surface divided by the number of lipid molecules) increases after the injection of the protein into the subphase, which suggests an intercalation of actin between the phospholipid molecules. This effect seems to be more important for DMPE and DMPS than for DMPG, suggesting that the headgroup plays an important role in this intercalation. The critical surface pressure associated to the liquid expanded–liquid condensed (LE-LC) phospholipid transition increases with the concentration of G-actin and thus suggests that G-actin acts as an impurity, simply competing as a surfactant at the air–water interface. On the other hand, F-actin affects the LE to LC transition of phospholipids differently. In this case, the LE to LC transition is broader and F-actin slightly decreases the critical surface pressure, which suggests that electrostatic interactions are involved.

Anisotropic aggregation and phase transition in Langmuir monolayers of methyl/ethyl esters of 2,3-dihydroxy fatty acids

We studied interfacial properties of a series of methyl and ethyl esters of enantioenriched syn-2,3-dihydroxy fatty acids with different chain lengths at the air–water interface, using a Langmuir type film balance and a Brewster angle microscope (BAM). After analyzing their surface pressure ( Π)–area ( A) isotherms, we inferred that these molecules existed as an E conformation in the liquid-expanded (LE) phase of monolayers, and the E conformation of molecules changed into a Z conformation during the LE–LC transition in a monolayer. BAM images evidenced the formation of elongated LC aggregates. This is possibly induced by the intermolecular hydrogen bonds, leading to the anisotropic growth of LC domains, on the basis of the FT-IR spectroscopy data. The enthalpy change of the LE–LC phase transition is considered to result from the three types of intermolecular interactions at the air–water interface during compression of these amphiphiles. These findings are discussed in terms of various physical factors that influenced intermolecular interactions and macroscopic aggregations of these amphiphiles.

Octakis(2-benzyloxyethylsulfanyl) Copper (II) Phthalocyanine: A New Liquid Crystalline Discotic Material with Benzyl-Terminated, Thioether-Linked Side Chains

The synthesis and materials properties of a new liquid crystalline phthalocyanine, octakis (2-benzyloxyethylsulfanyl) copper (II) phthalocyanine, 2, are reported. This phthalocyanine possesses eight benzyl-terminated side chains with thioether links to the Pc core, which promote greater interaction between the discotic mesogens than has been seen in previous Pcs reported from this group which had benzyl-terminated alkoxy-based side chains. The use of thioether links to these side chains promotes a red-shift in the Q-band absorbance spectrum, small decreases in first oxidation potential, increased solution aggregation constants, and significantly higher K→LC transition temperatures than seen in conventional alkoxy-based side-chain-modified Pcs. Powders and single crystals of this material were characterized by X-ray diffraction and found to represent two closely related polymorphs. Room-temperature XRD of powders of 2 showed a monoclinic unit cell: a = 22.81(1) Å, b = 9.780(5) Å, c = 19.314(6) Å, α = 90°, β = 106.918(14)°, γ = 90°, V = 4472 Å3, two molecules per unit cell, with a helical twist of the Pcs along the b-axis. The packing architecture in these powders showed a transition to an oblique (distorted rectangular, Colrd) phase at temperatures in the LC regime, which was reversible upon cooling. The single-crystal diffraction data showed a monoclinic unit cell: a = 23.911 (18) Å, b = 5.224(3) Å, c = 39.17(3) Å (c ≈ √3a); α = 90°, β = 96.971(14)°, γ = 90°, volume = 4903(8) Å3, and AFM characterization of the 〈001〉 plane of these single crystals clearly showed evidence for layered growth in this material. Langmuir−Blodgett films of 2, characterized by AFM, X-ray reflectivity, and X-ray diffraction, showed hexagonal close packing of parallel Pc columns in the as-deposited film, and upon annealing, showed formation of at least two different domains, with column−column spacings of ca. 21 Å and 26 Å, corresponding to the 〈10〉 and 〈01〉 faces of a 2D lattice, closely related to the unit cell found for the polycrystalline material.

Nitro and bromo derivatives of a highly fluorinated thiobenzoate

Several compounds derived from monosubstituted thiobenzoates, with substituents in the meta- or para-position, have been prepared; these contain a thioester spacer between a semifluorinated chain and the rigid core unit. The substituent is either a bromo or a nitro group. Their mesomorphic properties were evaluated using polarizing optical microscopy and differential scanning calorimetry. The influence of the different substituents on the phenyl core was considered. No mesomorphic behaviour was seen except for one nitro derivative. The introduction of the nitro group in the para-position of the thiobenzoate core leads to a LC transition on heating. As with previous studies, these results show that substitution at the 4 position on a monophenyl core can lead to liquid crystallinity, whereas substitution in the meta-position does not; this supports the view that meta-substitution is not a necessary condition for obtaining a LC phase.

Anisotropy of diffusion in a liquid crystalline system of semi-flexible polymer chains

Molecular dynamic simulations are reported for semi-flexible systems consisting of rod-like linear molecules. The molecules are composed of eight tangent isotropic soft spheres, connected by continuous elastic springs into a linear chain. Rigidity is introduced by additional springs between each sphere along the chain. The elasticity of the springs is used to tune the flexibility of the molecule. The formation of only a nematic LC phase is shown for all systems considered. Persistence length dependences of the jump of the order parameter and boundary volume fractions in isotropic and nematic phases at LC transition agree well with predictions of the Khokhlov–Semenov theory and with available simulation data. The effect of the flexibility on the translational and rotational diffusion in the nematic phase is studied. The anisotropy of translational diffusion was observed. For the estimation of the rotational diffusion coefficient for molecules in a nematic phase the wobbling-in-a-cone model was applied. Anisotropy of translational diffusion and ratio of translational and rotational diffusion coefficients show universal dependences on the order parameter.

Polymer−Organosilane Interactions Studied at the Air/Water Interface

Mixed monolayers of octadecyltrimethoxisilane, OTMS, and poly(ethylene oxide), PEO, have been studied at the air/water interface as a function of pH by means of surface pressure and surface potential isotherms. PEO had a strong influence on the isotherms at large monolayer areas, while it was squeezed out of the air/water interface at high surface pressures. The LE−LC transition pressure of unhydrolyzed OTMS was increased by several mN/m in the presence of PEO, while PEO had little influence on the isotherms of hydrolyzed OTMS at small monolayer areas. The results are discussed in terms of attractive van der Waals interactions between methoxy groups of nonhydrolyzed OTMS and PEO.