Flip-flop Motion Research Articles

Pressure studies of rod-like liquid crystals – experimental facts and theoretical deficiencies

ABSTRACT In the years 1990–2015, many liquid crystalline (LC) substances were studied under elevated pressure. The main aims of studies were to determine the P vs. T phase diagrams, the equation of state V(P,T) for isotropic and LC phases, measuring the dynamical properties of molecules (mainly the flip-flop motions) and analysing them at the isobaric, isothermal and isochoric conditions. The observed phenomena were discussed using existing theoretical models of LC state or – in the absence of them – models adopted from the field of glass-forming liquids. In the present review, the attention will be paid to a comparative data analysis for two rings compounds with the strongly polar terminal group (CN or NCS) at one site and the n-alkyl or n-alkoxy group at the other. Such molecules may well mimic the rod-like molecules commonly used in theoretical models of LC state.

The Universal Scaling of Dielectric Response as a Tool in the Description of a Complex Dynamic of 4′-Butyl-4-(2-methylbutoxy)azoxybenzene (4ABO5*)

The results of dielectric relaxation spectroscopy of the chiral liquid crystal 4′-butyl-4-(2-methylbutoxy)azoxybenzene in the crystal phase are presented. The scaling procedure showed complex molecular dynamics and allows one to decompose the observed relaxation process into two closely located relaxation processes around the short molecular axis. Temperature dependences of relaxation times characterizing flip-flop motions (rotation around the short molecular axis) and rotation around the long molecular axis are of the Arrhenius type.

Flip-flop dynamics in smectic liquid-crystal organic semiconductors revealed by molecular dynamics simulations.

Asymmetric liquid-crystal (LC) organic semiconductors, such as 2-decyl-7-(p-tolyl)-[1]benzothieno[3,2-b][1]benzothiophene (pTol-BTBT-C10), exhibit high mobilities exceeding 10 cm2 V-1 s-1. The LC phases play important roles in thermal stability and self-assembly ordering during film deposition and annealing. In this study, we show molecular dynamics simulations of pTol-BTBT-C10 and reveal a unique mechanism of the molecular flip-flop motion at the smectic E/smectic B phase transition.

Dynamic behavior and alkyne insertion reactions of alkenyl- and thiolate-bridged diiron carbonyl complexes

The alkenyl- and thiolate-bridged diiron carbonyl complexes [Fe2(μ-SC6H4CR1CH)(CO)6] (R1 = H or Me), which are derived from [Fe3(CO)12] and benzothiophenes, reacted with alkynes (R2CCR3) under photoirradiation conditions to form the acyl complexes [Fe2{μ-SC6H4CR1CHCR2C(R3)CO}(CO)5]. In this reaction, an alkyne and CO are inserted into the Fe–alkenyl bond, and the bulky substituents of alkynes, including a redox active ferrocenyl group, occupy the R3 position. To elucidate the reaction pathway, the reaction of the in situ generated acetonitrile complex [Fe2(μ-SC6H4CHCH)(CO)5(NCCH3)] with ethynylbenzene was monitored by 1H NMR spectroscopy. The results demonstrated that the acetonitrile complex undergoes facile insertion of the alkyne, which is followed by CO insertion to give the corresponding acyl complex [Fe2{μ-SC6H4(CH)3C(Ph)CO}(CO)5]. The reactivity of the bridging alkenyl moiety is related to the dynamic behavior of the alkenyl-thiolate ligand (SC6H4CHCH)2- as well as the entering alkyne. Temperature dependent 31P NMR measurements of the dmpm-bridged complex [Fe2(μ-SC6H4CHCH)(μ-dmpm)(CO)4] (dmpm = bis(dimethylphosphino)methane) revealed that the activation parameters for the flip-flop motion of the alkenyl-thiolate ligand are ΔH‡ = 46.7(7) kJ mol-1 and ΔS‡ = −46(2) J mol-1 K-1.

Two mechanisms for the formation of the ferronematic phase studied by dielectric spectroscopy

A non-chiral ferroelectric nematic compound with a 1,3-dioxane unit in the mesogenic core called 2,3′,4′,5′-tetrafluoro-[1,1′-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) was studied by dielectric spectroscopy in the frequency range 0.1 Hz–10 MHz over a wide range of temperatures. The compound exhibits three nematic phases on cooling from the isotropic phase, i.e., the ordinary paraelectric nematic N; the intermediate nematic NX and the ferroelectric NF phase. The least frequency process is due to the dynamics of ions. The middle frequency relaxation process P1 is like as observed in other ferronematic compounds and this mode is a continuation of the molecular flip-flop motion in the isotropic phase to the collective dynamics of dipoles which are strongly coupled with the splay fluctuations in nematic phases. In addition to this process, DIO shows an additional collective relaxation process P2 at higher frequencies both in the N and the NX phases. This mode originates from the polar/chiral domains of the opposite chirality, these arise from the spontaneous symmetry breaking of achiral mesogens in the N phase. Both collective processes, P1 and P2, show soft mode-like characteristic behavior on cooling from the N to the NX-NF phase transition temperature and are shown to contribute independently to the formation of the ferronematic NF phase.

Artificial receptor-mediated phototransduction toward protocellular subcompartmentalization and signaling-encoded logic gates.

Engineering artificial cellular systems capable of perceiving and transmitting external signals across membranes to activate downstream targets and coordinate protocellular responses is key to build cell-cell communications and protolife. Here, we report a synthetic photoreceptor-mediated signaling pathway with the integration of light harvesting, photo-to-chemical energy conversion, signal transmission, and amplification in synthetic cells, which ultimately resulted in protocell subcompartmentalization. Key to our design is a ruthenium-bipyridine complex that acts as a membrane-anchored photoreceptor to convert visible light into chemical information and transduce signals across the lipid membrane via flip-flop motion. By coupling receptor-mediated phototransduction with biological recognition and enzymatic cascade reactions, we further develop protocell signaling-encoded Boolean logic gates. Our results illustrate a minimal cell model to mimic the photoreceptor cells that can transduce the energy of light into intracellular responses and pave the way to modular control over the flow of information for complex metabolic and signaling pathways.

Separating water isotopologues using diffusion-regulatory porous materials.

The discovery of a method to separate isotopologues, molecular entities that differ in only isotopic composition1, is fundamentally and technologically essential but remains challenging2,3. Water isotopologues, which are very important in biological processes, industry, medical care, etc. are among the most difficult isotopologue pairs to separate because of their very similar physicochemical properties and chemical exchange equilibrium. Herein, we report efficient separation of water isotopologues at room temperature by constructing two porous coordination polymers (PCPs, or metal-organic frameworks) in which flip-flop molecular motions within the frameworks provide diffusion-regulatory functionality. Guest traffic is regulated by the local motions of dynamic gates on contracted pore apertures, thereby amplifying the slight differences in the diffusion rates of water isotopologues. Significant temperature-responsive adsorption occurs on both PCPs: H2O vapour is preferentially adsorbed into the PCPs, with substantially increased uptake compared to that of D2O vapour, facilitating kinetics-based vapour separation of H2O/HDO/D2O ternary mixtures with high H2O separation factors of around 210 at room temperature.

Mixing‐Induced Orientational Ordering in Liquid‐Crystalline Organic Semiconductors

AbstractOrganic semiconductors (OSCs) often exhibit thermotropic liquid‐crystalline (LC) phases in which molecular orientational order is partially lost. The formation of a temporary LC state via a solution process is expected to promote printing‐based device production, although a method of controlling molecular ordering over LC states has not yet been developed. Here, it is demonstrated that long‐axis molecular ordering can be controlled under ambient conditions using unsymmetrically alkylated OSCs with high layered crystallinity. Introducing a small amount of OSC molecules with longer alkyl‐chain lengths leads to the conversion of the long‐axis disordered phase into a bilayer‐type ordered phase with improved hole mobility in the transistor characteristics. Analyses of the entropies of the transitions to the smectic LC phases at elevated temperatures demonstrate that the long‐axis molecular ordering can be driven by a balance between intralayer flip–flop motion and core–core interactions through LC phases. The proposed LC‐mediated solution processing paves the way toward high‐end printed electronics.

Enhancement of the physical parameters due to the dispersion of functionalised gold nanoparticles in a room temperature nematic liquid crystal

ABSTRACT A room temperature nematic liquid crystal (N-3017) mixed with surface-stabilised gold nanoparticles was examined for its thermodynamic, dielectric, and electro-optical properties. The isotropic to nematic transition temperature decreases in both the nanocomposites as compared to that of the pure nematic material, according to thermodynamic findings. In the frequency range of 1 Hz to 40 MHz, dielectric data of both the dispersed nanocomposites in homeotropic and planar aligned samples are collected. The dispersion of capped GNPs enhanced the dielectric anisotropy of the dispersed nanocomposites, according to the dielectric results. As the quantity of capped GNPs in the host matrix increases, the threshold voltage for LC reorientation decreases due to the decreased elastic constant and increased dielectric anisotropy. The dielectric study also signifies a prominent relaxation mode due to the flip-flop motion of the nematic molecules about their short axes. To analyse this mode, experimental data have been fitted with the modified Cole–Cole equation. This removes the parasitic effects in the low- and high-frequency domains.

Unraveling the Mechanism for Tuning the Fluorescence of Fluorescein Derivatives: The Role of the Conical Intersection and nπ* State.

Although a large number of fluorescein derivatives have been developed and applied in many different fields, the general mechanisms for tuning the fluorescence of fluorescein derivatives still remain uncovered. Herein, we found that the fluorescence quenching of neutral form of fluorescein derivatives in acidic medium resulted from a dark nπ* state, whereas the fluorescence of the anionic form of fluorescein derivatives in the gas phase and alkaline solutions was tuned by minimal energy conical intersection (MECI). The formation of MECI involved significant rotation of benzene ring and flip-flop motion of xanthene moiety, which would be restricted by intermolecular hydrogen bonding and lowering temperature. The energy barrier for reaching MECI depended on the substituents in the benzene moiety in accordance with experimentally observed substituent effects. These unprecedented mechanisms would lead to a recognition of fluorescein derivatives and could provide a correct and instructive design strategy for further developing new fluorescein derivatives.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar‐Ligand‐Substituent Motion

AbstractFerroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh‐terpy)2](BPh4)2⋅3ac (1⋅3 ac; FPh‐terpy=4′‐((3‐fluorophenyl)ethynyl)‐2,2′:6′,2′′‐terpyridine) and is dependent on the degree of 180° flip–flop motion of the ligand's polar fluorophenyl ring. Single crystal X‐ray structures at several temperatures confirmed the flip–flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid‐state 19F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip–flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin‐state conversion being dependent on the molecular motion of a ligand‐appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule‐based magnetoelectric materials.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion.

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)2 ](BPh4 )2 ⋅3ac (1⋅3 ac; FPh-terpy=4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 180° flip-flop motion of the ligand's polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state 19 F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

Dielectric relaxation, electric conductivity, and electro-optic properties of SWCNT-doped liquid crystal 5CB

The effect of single-walled carbon nanotubes (SWCNTs) on the dielectric, conductivity, and electro-optic properties of nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) has been studied. It was shown that the additive of SWCNTs with concentration of 0.5% leads to an increase in the order parameter of 5CB. In this case, the clearing point is raised, the longitudinal component of the dielectric permittivity increases while the transverse component decreases. The incipient percolation effect promotes to the dominance of hopping electron conductivity over ionic conductivity, leading to an increase in specific conductivity. The elastic splay constant of 5CB is enhanced and, accordingly, the threshold voltage of the Freedericksz effect increases in this situation. An increase in distance between LC molecules leads to decreasing of viscosity. As a result, the flip-flop motion of molecules become easier and the switching times are also reduced.

Improved detachable electrodynamic cleaning system for dust removal from soiled photovoltaic panels

A detachable cleaning device that utilizes electrodynamic force has been improved to clean hardly adhered dust particles owing to the moisture absorption from the surface of photovoltaic (PV) panels. The device comprises parallel screen electrodes attached in a plastic frame. When a high ac voltage is applied to the parallel screen electrodes (whose lower part is set on a soiled PV panel), the generated electrodynamic force acts on the particles under the lower electrode. A flip-flop motion of the particles arises between the electrodes owing to the electrodynamic force as well as some particles that pass through the openings of the upper screen electrode and fall downward along the inclined panel owing to gravitational force. Hence, the dust deposited on the panel is cleaned. Previously, it has been demonstrated that dust is removed efficiently from the surface of an inclined panel by applying a low-frequency high-voltage. However, the performance is low for particles that are strongly fixed on the panel. Three countermeasures are proposed herein to improve the cleaning of dust particles: by adjusting the operational scheme, improving the electrode configuration, and utilizing airflow. This improved technology is expected to realize the efficient operation of large-scale PV power generation plants constructed in deserts.

Understanding the Mechanism of Antimicrobial Peptides using Small-Angle X-ray and Neutron Scattering Techniques

It is generally believed that antimicrobial peptides, AMPs, are able to evade much of the bacterial resistance because they disturb the fundamental integrity of the entire cell by interfering with the life-defining cell membrane. However, there is no clear general consensus for the molecular basis by which AMPs act, although various structural modifications such as membrane deformation or pore formation have been suggested. [1,2] However, other factors may contribute such as changes in the lipid dynamics, changes in the lateral and transversal lipid composition and enhanced proton/ion transfer. In order to fully understand the mechanism, we embarked on a study to investigate both the structural and dynamic effects on model membranes. To this end we employed state-of-the-art Small-angle X-ray and neutron scattering (SAXS/SANS) methods which are capable to probe the structure of both lipids and peptide on nanometer length scales[3-4]. In addition, by using H/D contrast variation scheme we could determine the lipid dynamics extracting both the transversal flip flop motion as well as lipid exchange. We will show using indolicidin as a model AMP, that the peptide significantly alters the lipid dynamics without significantly altering the structure of the bilayer. The results further show that indolicidin inserts on the interface between lipid tail/head on the outer leaflet perturbing the lipid packing causing an acceleration in the dynamics. We speculate that the change in dynamics may cause effects that are detrimental to the bacterial cell. [1] H. Jenssen, P. Hamill and R. E. W. Hancock, Clinical Microbiology Reviews, 2006, 19, 491-511. [2] W. C., Wimley, ACS Chemical biology 2010, 5(10), 905-917. [3] J.E. Nielsen, et al. Soft Matter, 2018, 11, 37-14. [4] Nielsen, J. E., et al BBA - Biomembranes, 1861(7), 1355-1364.

Restriction of Flip-flop Motion as a Mechanism for Aggregation-Induced Emission.

Although the restriction of intramolecular motion (RIM) has been accepted as a general working mechanism for the aggregation-induced emission (AIE) phenomenon, some new mechanisms, such as suppression of Kasha's rule (SOKR), has also been proposed to explain the AIE of boron difluorohydrazone (BODIHY) derivatives. However, the understanding of the relation and difference between RIM and SOKR mechanisms is limited. To address this issue, we performed a theoretical study on the excited state decay of a series of BODIHY derivatives. Surprisingly, we found that the first excited state of BODIHY derivatives is a bright state and contradicts with the SOKR mechanism. Importantly, we proposed a new mechanism, termed as restriction of flip-flop motion, to explain the AIE of BODIHY derivatives. This mechanism involves the formation of an umbrella-like minimal energy conical intersection through flip-flop motion, which is easily accessible in low-viscosity solvents and will be restricted in high-viscosity solvents.

Phase behaviour and dynamics of the liquid crystal 4-ethyl-4′-octylazoxybenzene (4EOB)

ABSTRACTThe results of Broadband Dielectric Spectroscopy and Differential Scanning Calorimetry of the 4-ethyl-4′-octylazoxybenzene (4EOB) liquid crystal are presented. Numerical analysis of the dielectric spectra results points to complex dynamics of the liquid crystal molecules in isotropic liquid, nematic and conformationally disordered crystalline phases. Two well-separated maxima on the imaginary part of electric permittivity were detected and described in liquid-like and crystalline phases. A third relaxation process, hidden in the conductivity region, describing conformational disorder of alkyl chains has been found in some plastic crystal phases. Temperature dependences of mean relaxation times characterising conformational disorder, flip-flop motions and rotation around long axis are of the Arrhenius type in liquid-like phases and conformationally disordered crystalline phases, and of the Vogel–Fulcher–Tammann type in the low-temperature crystalline phases for relaxation processes around short and long axis.

Effects of Ether Linkage on Membrane Dipole Potential and Cholesterol Flip-Flop Motion in Lipid Bilayer Membranes.

In our previous work, we investigated the effect of ether linkage on the physical properties of lipid bilayers using all-atom (AA) simulations with different water models. However, the influence of ether linkage on the transportation of cholesterol in lipid bilayers is less well studied. In order to reduce computational costs in simulations at large time and length scales, we present coarse-grained (CG) simulations of diphytanyl phosphatidylcholine (ether-DPhPC) and diphytanoyl phosphatidylcholine (ester-DPhPC) bilayer membranes in this work. First, the CG and AA simulations consistently show that the substitution of ether linkage for ester linkage would prevent the penetration of water into the lipid bilayer membranes. Second, it is encouraging that the CG simulations can nicely capture the ether effect on membrane dipole potential, showing that the ether substitution for ester would significantly decrease the dipole potential. In particular, the CG results agree with the AA simulation results, revealing that the change in the dipole potential is accompanied with the alteration in the orientation of linkage group. Finally, we carried out 60 μs CG simulations of ether-DPhPC and ester-DPhPC bilayers at two cholesterol concentrations (10 and 40% mole fraction, respectively), showing that the ether substitution for ester would facilitate the cholesterol flip-flop motion in lipid bilayer membranes.

Effect of the Alkyl Chain Length of Amphiphilic Ionic Liquids on the Structure and Dynamics of Model Lipid Membranes.

We compare the biophysical and structural aspects of the interaction of amphiphilic ionic liquids containing 1-alkyl-3-methylimidazolium cation ([CnMIM]+, n = 8, 12, or 16) with membranes composed of zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or anionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol (POPG). Liposome affinity and permeabilization were determined using ζ-potential and fluorescence studies, correlated with the cytoxicity of [CnMIM]+Br- toward HeLa cell lines. Membrane affinity is strongest in the case of [C16MIM]+Br- followed by [C12MIM]+Br- and [C8MIM]+Br- for both membranes, and trends remained the same in the case of membrane permeability and cytotoxicity. Solid-state NMR spectroscopy was used to localize [CnMIM]+ inside the lipid bilayers and to study their impact on the head group and acyl chain structures and dynamics of the lipid molecules. The charged ring moiety of the [CnMIM]+ is localized in the lipid-water interface of the membranes irrespective of the chain length and membrane surface charge. While [C8MIM]+ binds the membrane most weakly, it induces the largest disorder in the lipid chain region. A lack of fast flip-flop motions of the amphiphiles in the case of long chain [C16MIM]+ is suggested to render the membrane unstable, which increases its permeability. Between the lipid molecules, the POPC membrane incurs larger disorder in lipid chain packing upon insertion of [CnMIM]+ molecules. The study provides structural details of the impact of increasing chain lengths in [CnMIM]+ on the structural properties of lipid bilayers.

Effect of Cholesterol and 6-Ketocholestanol on Membrane Dipole Potential and Sterol Flip-Flop Motion in Bilayer Membranes.

A variety of experimental and theoretical approaches have been employed to investigate the sterol flip-flop motion in lipid bilayer membranes. However, the sterol effect on the dipole potential of lipid bilayer membranes is less well studied and the influence of dipole potential on sterol flip-flop motion in lipid bilayer membranes is less well understood. In our previous works, we have demonstrated the performance of our coarse-grained (CG) model in the computation of the dipole potential. In this work, five 30 μs CG simulations of dimyristoylphosphatidylcholine (DMPC) bilayers were carried out at different sterol concentrations (in a range from 10 to 50% mole fraction). Then, a comparison was made between the effects of cholesterol (CHOL) and 6-ketocholestanol (6-KC) on the dipole potential of DMPC lipid bilayers as well as the sterol flip-flop motion. Our CG simulations show that the membrane dipole potential is impacted more significantly by 6-KC than by CHOL. This finding is consistent with recent experimental studies. Meanwhile, our work suggests that the sterol-sterol interactions (in particular, electrostatic interactions) should be critical to the formation of sterol-sterol clusters, which would hinder the sterol flip-flop motion inside the lipid bilayers. This is in support of the recent experimental study on the sterol transportation in lipid bilayer membranes.