Mixed Monolayers Research Articles

The Complete Phase Diagram of Monolayers of Enantiomeric N-Stearoyl-threonine Mixtures with Preferred Heterochiral Interactions.

Langmuir monolayers of chiral amphiphiles are well-controlled model systems for the investigation of phenomena related to stereochemistry. Here, we have investigated mixed monolayers of one pair of enantiomers (l and d) of the amino-acid-based amphiphile N-stearoyl-threonine. The monolayer characteristics were studied by pressure–area isotherm measurements and grazing incidence X-ray diffraction (GIXD) over a wide range of mixing ratios defined by the d-enantiomer mole fraction xD. While the isotherms provide insights into thermodynamical aspects, such as transition pressure, compression/decompression hysteresis, and preferential homo- and heterochiral interactions, GIXD reveals the molecular structural arrangements on the Ångström scale. Dominant heterochiral interactions in the racemic mixture lead to compound formation and the appearance of a nonchiral rectangular lattice, although the pure enantiomers form a chiral oblique lattice. Miscibility was found to be limited to mixtures with 0.27 ≲ xD ≲ 0.73, as well as to both outer edges (xD ≲ 0.08 and xD ≳ 0.92). Beyond this range, coexistence of oblique and rectangular lattices occurs, as is clearly seen in the GIXD patterns. Based on the results, a complete phase diagram with two eutectic points at xD ≈ 0.25 and xD ≈ 0.75 is proposed. Moreover, N-stearoyl-threonine was found to have a strong tendency to form a hydrogen-bonding network between the headgroups, which promotes superlattice formation.

Mixing Behavior of the Binary Monolayers of Fatty Acids Based on Their Cohesive Energy Differences.

The morphology involving the height difference and the surface roughness of the binary monolayers of saturated fatty acids were evaluated using atomic force microscopy (AFM) to investigate the mixing behavior of their monolayers. AFM observations revealed that the mixed monolayers of (palmitic acid/arachidic acid) and (arachidic acid/lignoceric acid), which had four methylene group differences between fatty acids, were in a molecularly mixed state. Further, the mixed monolayer of (stearic acid/lignoceric acid), which had six methylene group differences, was in a phase-separated state. From the results of the present and previous studies, it became clear that the difference in the cohesive energy between fatty acids, which corresponds to the enthalpy difference, was an important factor in determining whether the molecular aggregation state of a fatty acid mixed monolayer was in a molecularly mixed or phase-separated state. Moreover, the boundary value of cohesive energy difference was approximately 2.5 kJ mol-1 at a subphase temperature of 293 K.

Interaction of Lipophilic Cytarabine Derivatives with Biomembrane Model at the Air/Water Interface.

Cell membrane models are useful for obtaining molecular-level information on the interaction of biologically active molecules whose activity is believed to depend also on their effects on the membrane. Cytarabine was conjugated with fatty acids to improve the drug lipophilicity and the interaction with the biomembrane model. Cytarabine was conjugated with fatty acids of different lengths to form the trimyristoyl cytarabine and the tristearoyl cytarabine derivatives. Their interaction with biomembrane models constituted by dimyristoylphosphatidylcholine (DMPC) monolayers was studied by employing the Langmuir–Blodgett technique. DMPC/cytarabine, DMPC/trimyristoyl cytarabine and DMPC/tristearoyl cytarabine mixed monolayers at increasing molar fractions of the compound were prepared and placed on the subphase. The mean molecular area/surface pressure isotherms were recorded at 37 °C. Between the molecules of DMPC and those of cytarabine or prodrugs, repulsive forces act. However, these forces are very weak between DMPC and cytarabine and stronger between DMPC and the cytarabine derivatives, thus avoiding the expulsion of the compounds at higher surface pressure and modifying the stability of the mixed monolayer. The fatty acid moieties could then modulate the affinity of cytarabine for biomembranes.

Potential Distribution across Model Membranes.

Membrane models assembled on electrodes are widely used tools to study potential-dependent molecular processes at or in membranes. However, the relationship between the electrode potential and the potential across the membrane is not known. Here we studied lipid bilayers immobilized on mixed self-assembled monolayers (SAM) on Au electrodes. The mixed SAM was composed of thiol derivatives of different chain lengths such that between the islands of the short one, mercaptobenzonitrile (MBN), and the tethered lipid bilayer an aqueous compartment was formed. The nitrile function of MBN, which served as a reporter group for the vibrational Stark effect (VSE), was probed by surface-enhanced infrared absorption spectroscopy to determine the local electric field as a function of the electrode potential for pure MBN, mixed SAM, and the bilayer system. In parallel, we calculated electric fields at the VSE probe by molecular dynamics (MD) simulations for different charge densities on the metal, thereby mimicking electrode potential changes. The agreement with the experiments was very good for the calculations of the pure MBN SAM and only slightly worse for the mixed SAM. The comparison with the experiments also guided the design of the bilayer system in the MD setups, which were selected to calculate the electrode potential dependence of the transmembrane potential, a quantity that is not directly accessible by the experiments. The results agree very well with estimates in previous studies and thus demonstrate that the present combined experimental-theoretical approach is a promising tool for describing potential-dependent processes at biomimetic interfaces.

Time oscillating magnetic field effect on critical behavior and dynamic phase transition of monolayer nanographene structure

ABSTRACT Dynamic magnetic properties of a ferrimagnetic mixed (1/2,1) Ising spin monolayer nanographene structure with a crystal-field interaction in a time oscillating magnetic field are studied by the use of the kinetic Monte Carlo simulation based on the Metropolis algorithm. Dynamic phase transitions are obtained, the effect of the next-nearest neighbor interactions, the crystal field, the amplitude, and the period of the external dynamic magnetic field on the critical and compensation behaviors and on the thermal variations of the dynamic order parameters are investigated. We have found that in the presence of the time oscillating field, the system may exhibit a dynamic phase transition from the ferrimagnetic to the paramagnetic phase at a critical point. The system may even present two dynamic compensation temperatures for appropriate values of the hamiltonian parameters. This behavior of dynamic compensation temperature has not been observed previously in published papers with kinetic Monte Carlo simulation.

Micellization and gelation characteristics of Pluronic P123 and single ester-bonded cleavable cationic gemini surfactant: A potential system for solubilization and release of ibuprofen

We herein report the mixed micelle and monolayer formation of mixtures of a biocompatible and cleavable oxy-diester twin tailed (gemini) surfactant, 2,2′-[(oxybis(ethane-1,2-diyl))bis(oxy)]bis(N-hexadecyl-N,Ndimethyl-2-oxoethanaminium) dichloride (16-E1-16) with the triblock copolymer, P123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) using tensiometry, dynamic light scattering (DLS), and rheology. It was observed that P123 and 16-E1-16 have strong interaction and form small mixed micelles with similar micellar composition throughout the bulk mole fraction range with insignificant counterion binding. Besides, P123 and 16-E1-16 mixed surfactant system forms thermodynamically stable mixed monolayer that is rich in P123. Variation of viscosity with shear rate revealed better injectable property of the formulations of binary mixtures of P123 and 16-E1-16 compared to pure P123. Also, below α16-E1-16 = 0.3, all the mixtures were in the form of sol which transform into a weak gel at physiological temperatures making these formulations relevant as injectable delivery system. Solubilization capacity of these mixed micelles towards ibuprofen (IBU) showed that addition of 16-E1-16 to P123 micelles reduces its solubilization capacity but with the advantage of making the formulation injectable due to their favorable shear induced viscosity changes and thermogelation properties. Mixed micelles showed faster drug release rate predominantly following Fickian mechanism with negligible contribution of relaxation process in the release phenomena. There is no report showing interaction of biocompatible and cleavable gemini surfactant 16-E1-16 with P123 in mixed micelle and monolayer formation with detailed rheological, drug encapsulation and release studies even though this amphiphilic system has a potential to act as an injectable drug delivery system.

Modulated Structure and Rectification Properties of a Molecular Junction by a Mixed Self-Assembled Monolayer.

The organization of the self-assembled monolayer (SAM) determines its electronic structure and so governs the charge transport process and device performance when adopted into a molecular device. We report a systematic study on the supramolecular structure and rectification performance of the ferrocene (11-ferrocenyl-1-undecanethiol, FUT) based SAM modulated by mixed SAM with inert 1-undecanethiol (C11SH) as diluent. We compared mixed SAMs by two different post assembly strategies, i.e., post assembly of C11SH on FUT SAM and post assembly of FUT on C11SH SAM. The organization and structure of FUT in the mixed SAM were extensively studied by cyclic voltammetry (CV) using the Laviron model. Rectification properties of the mixed SAM obtained using eutectic indium gallium (EGaIn) as the top electrode revealed that the magnitude and stability of the rectification ratio (RR) strongly correlated to not only the amount but also the phase structure and orientation of the FUT in the monolayer, resulting in a tunable RR and increased stability. The mixed monolayer achieved an increased performance relative to pure FUT by post assembling FUT on C11SH SAM, which formed an optimally dense and well-packed monolayer with the FUT head resting on the top of the alkane SAM.

Marangoni effect and cell spreading.

Cells are very sensitive to the shear stress (SS). However, undesirable SS is generated during physiological process such as collective cell migration (CCM) and influences the biological processes such as morphogenesis, wound healing and cancer invasion. Despite extensive research devoted to study the SS generation caused by CCM, we still do not fully understand the main cause of SS appearance. An attempt is made here to offer some answers to these questions by considering the rearrangement of cell monolayers. The SS generation represents a consequence of natural and forced convection. While forced convection is dependent on cell speed, the natural convection is induced by the gradient of tissue surface tension. The phenomenon is known as the Marangoni effect. The gradient of tissue surface tension induces directed cell spreading from the regions of lower tissue surface tension to the regions of higher tissue surface tension and leads to the cell sorting. This directional cell migration is described by the Marangoni flux. The phenomenon has been recognized during the rearrangement of (1) epithelial cell monolayers and (2) mixed cell monolayers made by epithelial and mesenchymal cells. The consequence of the Marangoni effect is an intensive spreading of cancer cells through an epithelium. In this work, a review of existing literature about SS generation caused by CCM is given along with the assortment of published experimental findings, to invite experimentalists to test given theoretical considerations in multicellular systems.

Characterization of self-assembled Bis[2-(2-bromoisobutyryloxy) undecyl] disulphide (DTBU) on gold surfaces suitable for use in surface-initiated atom transfer radical polymerization (SI-ATRP)

• Initiator DTBU on gold electrode (SAM) enhances electrochemical mediation ATRP. • CV, EIS, CA, XPS and IRRAS methods are used for DTBU monolayer characterization. • Well organized DTBU SAM with polar head and non-polar inside. • EIS, CA and IRRAS prove preferential presence of 6MP in mixed SAM with DTBU. • Mixed monolayers with 6MP provide optimal conditions for adaptation and use in e-ATRP. Bis[2-(2-bromoisobutyryloxy) undecyl] disulphide (DTBU) is an initiator for surface-initiated atom transfer radical polymerization (SI-ATRP) able to functionalize metal surfaces through formation of self-assembled monolayers (SAM). The goal of this work is the making monolayers with the DTBU initiator on gold polycrystalline electrodes and their study by electrochemical methods as cyclic voltammetry (CV), capacity-potential curves (C-E), electrochemical impedance spectroscopy (EIS), and angle contact measurements for their characterization. Information of the integrity and permeability of the monolayers has been obtained by capacitance-potential measurements and impedance spectra. Also, the blocking effect of the physical barrier that SAM can produce to different redox probes is explored by using voltammetry and impedance. DTBU forms a compact monolayer whose main features are related with heterogeneous chain structures, having a non-polar hydrocarbon internal part and a polar head group. This is confirmed by X-ray photoelectron spectroscopy (XPS) which shows the presence of the elements of the DTBU adsorbate of SAM. Moreover, mixed monolayers of DTBU and 6-mercaptopurine (6MP) have been prepared and characterized, to check if the thiol facilitates electron exchange through monolayers. On this approach, different DTBU:6MP ratios have been employed and characterized studying IRRAS spectra, electrochemical properties, and contact angle on the modified surface. These mixed monolayers provide optimal conditions for adaptation and use in e-ATRP.

Evaluation of Emulsifying Ability of Phospholipids by Langmuir Monolayers and Stability of High Oil Ratio O/W Emulsions

High oil ratio fat emulsion injections are prone to poor emulsification, rapid creaming, and other quality problems; therefore, the selection of emulsifiers with high emulsifying ability is crucial for the production of fat emulsions. The existing methods used to evaluate the emulsifying ability of emulsifier are to evaluate the emulsifying ability from the emulsifier itself. In the study, Langmuir monolayer selected the most miscible phospholipid with oil phase from the alternative three phospholipids by studying the molecular interaction between oil phase and phospholipid at the air/water interface. The miscibility and thermodynamic stability analyses of the different oil phase/phospholipid mixed monolayers were performed, and the data from [Formula: see text] and [Formula: see text] concluded that all three oil phases had the strongest molecular interaction with E80 and the best miscibility. The emulsions were then prepared and analyzed by the results of particle size, ζ-potential, and stability of the emulsions, where the surface free energy in the stability test echoed the results reflected by the [Formula: see text] values in the thermodynamic stability test. These results indicate that Langmuir monolayers can be used to study the interaction between oil phase and emulsifier, thus providing new ideas for evaluating the emulsifying ability of phospholipids.

Effect of Ester Moiety on Structural Properties of Binary Mixed Monolayers of Alpha-Tocopherol Derivatives with DPPC.

Phospholipid membranes are ubiquitous components of cells involved in physiological processes; thus, knowledge regarding their interactions with other molecules, including tocopherol ester derivatives, is of great importance. The surface pressure–area isotherms of pure α-tocopherol (Toc) and its derivatives (oxalate (OT), malonate (MT), succinate (ST), and carbo analog (CT)) were studied in Langmuir monolayers in order to evaluate phase formation, compressibility, packing, and ordering. The isotherms and compressibility results indicate that, under pressure, the ester derivatives and CT are able to form two-dimensional liquid-condensed (LC) ordered structures with collapse pressures ranging from 27 mN/m for CT to 44 mN/m for OT. Next, the effect of length of ester moiety on the surface behavior of DPPC/Toc derivatives’ binary monolayers at air–water interface was investigated. The average molecular area, elastic modulus, compressibility, and miscibility were calculated as a function of molar fraction of derivatives. Increasing the presence of Toc derivatives in DPPC monolayer induces expansion of isotherms, increased monolayer elasticity, interrupted packing, and lowered ordering in monolayer, leading to its fluidization. Decreasing collapse pressure with increasing molar ratio of derivatives indicates on the miscibility of Toc esters in DPPC monolayer. The interactions between components were analyzed using additivity rule and thermodynamic calculations of excess and total Gibbs energy of mixing. Calculated excess area and Gibbs energy indicated repulsion between components, confirming their partial mixing. In summary, the mechanism of the observed phenomena is mainly connected with interactions of ionized carboxyl groups of ester moieties with DPPC headgroup moieties where formed conformations perturb alignment of acyl chains, resulting in increasing mean area per molecule, leading to disordering and fluidization of mixed monolayer.

Amphiphilic Gold Nanoparticles: A Biomimetic Tool to Gain Mechanistic Insights into Peptide-Lipid Interactions.

Functional peptides are now widely used in a myriad of biomedical and clinical contexts, from cancer therapy and tumor targeting to the treatment of bacterial and viral infections. Underlying this diverse range of applications are the non-specific interactions that can occur between peptides and cell membranes, which, in many contexts, result in spontaneous internalization of the peptide within cells by avoiding energy-driven endocytosis. For this to occur, the amphipathicity and surface structural flexibility of the peptides play a crucial role and can be regulated by the presence of specific molecular residues that give rise to precise molecular events. Nevertheless, most of the mechanistic details regulating the encounter between peptides and the membranes of bacterial or animal cells are still poorly understood, thus greatly limiting the biomimetic potential of these therapeutic molecules. In this arena, finely engineered nanomaterials—such as small amphiphilic gold nanoparticles (AuNPs) protected by a mixed thiol monolayer—can provide a powerful tool for mimicking and investigating the physicochemical processes underlying peptide-lipid interactions. Within this perspective, we present here a critical review of membrane effects induced by both amphiphilic AuNPs and well-known amphiphilic peptide families, such as cell-penetrating peptides and antimicrobial peptides. Our discussion is focused particularly on the effects provoked on widely studied model cell membranes, such as supported lipid bilayers and lipid vesicles. Remarkable similarities in the peptide or nanoparticle membrane behavior are critically analyzed. Overall, our work provides an overview of the use of amphiphilic AuNPs as a highly promising tailor-made model to decipher the molecular events behind non-specific peptide-lipid interactions and highlights the main affinities observed both theoretically and experimentally. The knowledge resulting from this biomimetic approach could pave the way for the design of synthetic peptides with tailored functionalities for next-generation biomedical applications, such as highly efficient intracellular delivery systems.

Patterned Carboxymethyl-Dextran Functionalized Surfaces Using Organic Mixed Monolayers for Biosensing Applications.

The deposition of biomolecules on biosensing surface platforms plays a key role in achieving the required sensitivity and selectivity for biomolecular interactions analysis. Controlling the interaction between the surface and biomolecules is increasingly becoming a crucial design tool to modulate the surface properties needed to improve the performance of the assay and the detection outcome. Carboxymethyl-dextran (CMD) coating can be exploited to promote chemical grafting of proteins, providing a hydrophilic, bioinert, nonfouling surface and a high surface density of immobilized proteins. In the present work, we developed and optimized a technique to produce a cost-effective CMD-based patterned surface for the immobilization of biomolecules to be used on standard protocols optimization. They consist of silicon or glass substrates with patterned bioactive areas able to efficiently confine the sampling solution by simply exploiting hydrophilic/hydrophobic patterning of the surface. The fabrication process involves the use of low-cost instruments and techniques, compatible with large scale production. The devices were validated through a chemiluminescence assay we recently developed for the analysis of binding of DNA nanoassemblies modified with an affinity binder to target proteins immobilized on the bioactive areas. Through this assay we were able to characterize the chemical reactivity of two target proteins toward a dextran matrix on patterned surfaces and to compare it with model CMD-based surface plasmon resonance (SPR) surfaces. We found a high reproducibility and selectivity in molecular recognition, consistent with results obtained on SPR sensor surfaces. The suggested approach is straightforward, cheap, and provides the means to assess patterned functionalized surfaces for bioanalytical platforms.

DNA patterning utilizing mixed monolayer template by phase separation between hydrocarbons and fluorocarbons and evaluation of second-order structural maintenance ability

Adsorption of DNA molecules on mixed phase-separated monolayers composed of a hydrocarbon-containing comb copolymer with an s-triazine ring and a fluorinated s-triazine derivative and their fluorescence emission behavior were investigated. DNA molecules derived from salmon testes were selectively adsorbed only on the diamino-s-triazine ring of the hydrogenated copolymer in the monolayer from the subphase. A mixed monolayer of a hydrocarbon-based comb copolymer and an s-triazine derivative with fluorocarbons exhibited phase separation behavior on water surfaces and indicated expansion behavior, reflecting interactions on the subphase containing DNA molecules. The phase-separated morphology changed depending on the mixing ratio of the hydrogenated copolymer and fluorinated s-triazine derivative. After DNA adsorption, the height of the domain formed by the hydrocarbon chain was selectively increased. In addition, because the DNA molecules adsorbed on the phase-separated template film maintain their fluorescence emission ability, the second-order structure, which is the origin of the emission ability, was evaluated using circular dichroism spectroscopy. The phase-separated, template-adsorbed DNA molecules maintained the same positive Cotton effect as in solution and were predicted to retain exquisite and delicate steric structure, leading to functional occurrence.

28.3% efficient perovskite-silicon tandem solar cells with mixed self-assembled monolayers

A certified 28.3% efficient monolithic perovskite-silicon tandem (PST) solar cell with a mixed self-assembled monolayer (SAM) containing carbazole cores with H-ligands (2PACz) and methoxy-ligands (MeO-2PACz) is reported. Our analysis revealed that there existed uncovered areas of MeO-2PACz on indium tin oxide, which would be caused by the steric effect, and they were filled with 2PACz in the mixed SAM, leading to the improvement of fill factors in the PST cells. This result was explained by the passivation qualities as hole transport layers and the local interaction between methoxy ligands and perovskite materials.

Thermodynamic Analysis of the Adsorption and Micellization Activity of the Mixtures of Rhamnolipid and Surfactin with Triton X-165.

The surface tension of aqueous solutions of Triton X-165 with rhamnolipid or surfactin mixtures was measured. The obtained results were applied for the determination of the concentration and composition of the Triton X-165 and biosurfactants mixture at the water–air interface as well as the contribution of the particular component of the mixtures to water surface tension reduction and the mutual influence of these components on the critical micelle concentration. The determination of these quantities was based on both the commonly used concepts and a new one proposed by us, which assumes that the composition of the mixed monolayer at the water–air interface depends directly on the pressure of the monolayer of the single mixture component and allows us to determine the surface concentration of each mixture component independently of surface tension isotherms shape. Taking into account the composition of the mixed monolayer at the water–air interface, the standard Gibbs adsorption free energy was considered. The obtained results allow us to state that the concentration of both mixture components corresponding to their saturated monolayer and the surface tension of their aqueous solution can be predicted using the surfactants’ single monolayer pressure and their mole fraction in the mixed monolayer determined in the proposed way.

Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering.

ConspectusControlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM–semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS2, they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces to systems such as metal–organic frameworks is currently underway, which further underlines the high potential of this approach.

Correlation of Relaxation Modeling Parameters of the Studied Polymer Blends at the Air/Water Interface

The main studied mixed monolayers consisting of polyethylene-block-polycaprolactone ( PEO-b-PCL) and its component polymers with poly( vinyl phenol). It was shown previously that most of the area relaxation process of mixed monolayers could be well represented by a model considering the nucleation and growth mechanisms. The characteristic exponent x values were found to be slightly temperature dependent. The kx values were detected to decrease with increasing temperature. When PVPh was added into the most unstable PEO, the mixed monolayer films exhibited a remarkably improved stable relaxation behavior than PCL (or PEO-b-PCL). The characteristic exponent x and kx values were plotted vesus the A∞/A0 values. Mostly increasing characteristic exponent x and kx values with increasing A∞/A0 values were observed. For the PCL and PVPh monolayers, the xkx values were detected to increase with increasing A∞/A0 values. The xkx values of previously published stereoregular and different molecular weight PMMAs were also demonstrated to become larger with the increasing A∞/A0 values.

Mechanical Tuning of Aggregated States for Conformation Control of Cyclized Binaphthyl at the Air-Water Interface.

An air-water interface enables molecular assemblies and conformations to be controlled according to their intrinsic interactions and anisotropic stimuli. The chirality and conformation of binaphthyl derivatives have been controlled by tuning molecular aggregated states in solution. In this study, we have tuned molecular aggregated states of monobinaphthyldurene (MBD) by applying different mechanical stimuli to control the conformation at the air-water interface. Density functional theory calculations indicate that MBD exists essentially in two conformations, namely, 1-MBD (most stable) and 2-MBD (less stable). MBD was mechanically dissolved in appropriate lipid matrices using the Langmuir-Blodgett (LB) method, while pure MBD was self-assembled at the dynamic air-water interface in the absence of or by applying vortex motions (vortex LB method). In MBD mixed monolayer, surface pressure-molecular area measurements and atomic force microscopy observations suggest that separate lipids and MBD phases transform to mixed phases induced by the dissolution of MBD into the lipid matrices during mechanical compression at the air-water interface. Circular dichroism measurements indicate that molecular conformation changes from 1-MBD to 2-MBD in passing from a separated phase to a mixed MBD/lipid phase. In addition, the molecular aggregated states and conformations of MBD depend on the spreading volume and vortex flow rate when applying the vortex LB method. Molecular conformations and aggregated states of MBD could be controlled continuously by applying a mechanical stimulus at the air-water interface.

Large cooperative effects in tunneling rates across van der Waals coupled binary self-assembled monolayers

This paper describes large positive cooperative effects of two orders of magnitude in the tunneling rates across molecular junctions of mixed self-assembled monolayers (SAMs) of rectifying (ferrocenyl undecanethiol HS(CH2)11Fc) and non-rectifying molecules with different terminal groups (11-undecanethiol and its derivatives, denoted as HS(CH2)11X, where X = -H, -NH2 or -NO2). By gradually diluting the surface fraction of HS(CH2)11Fc in the mixed SAM, it is found that the large positive cooperative effect is only important in the coherent tunneling regime but not in the incoherent tunneling regime. Density functional theory (DFT) shows that the measured cooperative effects in the tunneling rates in these binary systems are caused by Fc---X van der Waals interactions which increase in the order of -H< -NH2< -NO2. These strong cooperative effects dramatically alter the operation of a molecular diode, further highlighting the importance of taking cooperative effects into account, in this case driven by van der Waals interactions, in the rational design of electronic devices working at tunneling regime.