Supramolecular architecture in Langmuir and Langmuir-Blodgett films incorporating a chiral azobenzene

This article describes the synthesis and fabrication of Langmuir and Langmuir-Blodgett (LB) films incorporating a chiral azobenzene derivative, namely, ( S)-4- sec-butyloxy-4'-[5'-(methyloxycarbonyl)pentyl-1'-oxy]azobenzene, abbreviated as AZO-C4(S). Appropriate conditions for the fabrication of monolayers of AZO-C4(S) at the air-water interface have been established, and the resulting Langmuir films have been characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and UV-vis reflection spectroscopy. The results indicate the formation of an ordered trilayer at the air-water interface with UV-vis reflection spectroscopy showing a new supramolecular architecture for multilayered films as well as the formation of J aggregates. Films were transferred onto solid substrates, with AFM revealing well-ordered multilayered films without 3D defects. Infrared and UV-vis absorption spectroscopy indicate that the supramolecular architecture may be favored by the formation of H bonds between acid groups in neighboring layers and pi-pi intermolecular interactions. Circular dichroism spectra reveal chiro-optical activity in multilayered LB films.     

Supramolecular chirality of the hydrogen-bonded complex Langmuir-Blodgett film of achiral barbituric acid and melamine

Complex monolayers of barbituric acid and melamine were formed by spreading a chloroform solution of amphiphilic barbituric acid on the subphase of melamine solution. It was confirmed that the complex monolayer was formed through in situ complementary hydrogen bonding at the air-water interface. It was interesting to find that the complex LB films showed supramolecular chirality although both of the molecules were achiral, as verified by the circular dichroism spectral measurements. It was suggested that the pi-pi stacking of the neighboring barbituric acid and melamine group in a helical sense resulted in the chirality of the molecular assemblies. Due to the directionality of the hydrogen bonding, the BA-M film could form regular aligned nanofibers on the AFM images. Increasing the subphase temperature will lead to the decrease of CD intensity and the change of the morphologies. We suggested that the strength of the hydrogen bonding resulted in the difference.     

Equilibrium structure and dynamics of (2R, 3R)-(+)-bis(decyloxy)succinic acid at the air-water interface

A twin-tailed, twin-chiral fatty acid, (2R,3R)-(+)-bis(decyloxy)succinic acid was synthesized and its two dimensional behavior at the air-water interface was examined. The pH of the subphase had a profound effect on the monolayer formation. On acidic subphase, stable monolayers with increased area per molecule due to hydrogen bonding and bilayers at collapse pressures were observed. Highly compressible films were formed at 40 degrees C, while stable monolayers with increased area were observed at sub-room temperatures. Langmuir monolayers formed on subphases containing 1 mM ZnCl2 and CaCl2 revealed two dimensional metal complex formation with Zn2+ forming a chelate-type complex, while Ca2+ formed an ionic-type complex. Monolayers transferred from the condensed phase onto hydrophilic Si(100) and quartz substrates revealed the formation of bilayers through transfer-induced monolayer buckling. Compression induced crystallites in 2D from monolayers and vesicle-like supramolecular structures from multilayers were the noted LB film characteristics, adopting optical imaging and electron microscopy. The interfacial monolayer structure studied through molecular dynamics simulation revealed the order and packing at a molecular level; monolayers adsorbed at various simulated specific areas of the molecule corroborated the (pi-A) isotherm and the formation of a hexagonal lattice at the air-water interface.     

Controlling the Structural and Electrical Properties of Diacid Oligo(Phenylene Ethynylene) Langmuir–Blodgett Films

The preparation, characterization and electrical properties of Langmuir–Blodgett (LB) films composed of a symmetrically substituted oligomeric phenylene ethynylene derivative, namely, 4,4′‐[1,4‐phenylenebis(ethyne‐2,1‐diyl)]dibenzoic acid (OPE2A), are described. Analysis of the surface pressure versus area per molecule isotherms and Brewster angle microscopy reveal that good‐quality Langmuir (L) films can be formed both on pure water and a basic subphase. Monolayer L films were transferred onto solid substrates with a transfer ratio of unity to obtain LB films. Both L and LB films prepared on or from a pure water subphase show a red shift in the UV/Vis spectrum of about 14 nm, in contrast to L and LB films prepared from a basic subphase, which show a hypsochromic shift of 15 nm. This result, together with X‐ray photoelectron spectroscopic and quartz crystal microbalance experiments, conclusively demonstrate formation of one‐layer LB films in which OPE2A molecules are chemisorbed onto gold substrates and consequently ? COO? Au junctions are formed. In LB films prepared on a basic subphase the other terminal acid group is also deprotonated and associates with an Na⁺ counterion. In contrast, LB films prepared from a pure water subphase preserve the protonated acid group, and lateral H‐bonds with neighbouring molecules give rise to a supramolecular structure. STM‐based conductance studies revealed that films prepared from a basic subphase are more conductive than the analogous films prepared from pure water, and the electrical conductance of the deprotonated films also coincides more closely with single‐molecule conductance measurements. This result was interpreted not only in terms of better electron transmission in ? COO? Au molecular junctions, but also in terms of the presence of lateral H‐bonds in the films formed from pure water, which lead to reduced conductance of the molecular junctions.     

Langmuir monolayers of an inclusion complex formed by a new calixarene derivative and fullerene

The design of new molecules with directed interactions to functional molecules as complementary building blocks is one of the main goals of supramolecular chemistry. A new p-tert-butylcalix[6]arene monosubstituted derivative bearing only one alkyl chain with an acid group (C6A3C) has been synthesized. The C6A3C has been successfully used for building Langmuir monolayers at the air-water interface. The C6A3C molecule adopts a flatlike orientation with respect to the air-water interface. The molecular structure gives the molecule amphiphilic character, while allowing the control of both the dissociation degree and the molecular conformation at the air-water interface. The C63AC has been combined with pristine fullerene (C60) to form the supramolecular complex C6A3C:C60 in 2:1 molar ratio (CFC). The CFC complex retains the ability of C6A3C to form Langmuir monolayers at the air/water interface. The interfacial molecular arrangement of the CFC complex has been convincingly described by in situ UV-vis reflection spectroscopy and synchrotron X-ray reflectivity measurements. Computer simulations complement the experimental data, confirming a perpendicular orientation of the calixarene units of CFC with respect to the air-water interface. This orientation is stabilized by the formation of intermolecular H-bonds. The interfacial monolayer of the CFC supramolecular complex is proposed as a useful model for the well-defined self-assembly of recognition and functional building blocks.     

Langmuir-Blodgett Films and Calcium Ion Coordination of Biliverdin and Its Amphiphilic Derivatives

Up to now, the investigations of the chemistry of biliverdin and its analogs has been limited in organic solvents1-4. There is no report on organized molecular films of biliverdin and its derivatives. The structural organization and biophysical properties of biliverdin molecules in ordered molecular assemblies might be different from those in organic solvents, but similar to those in biological membranes in mammals. Therefore, biliverdin or its derivatives incorporated in ordered molecular fil…     

Spectroscopic studies on molecular recognition capabilities of a nucleolipid bearing thymine headgroup to adenosine

The Langmuir-Blodgett (LB) films of octadecanoyl ester of 1-(2-carboxyethyl) thymine deposited from pure water and aqueous adenosine subphases were investigated by ultraviolet-visible (UV-vis), Fourier transform infrared-attenuated total reflection (FTIR-ATR), and Fourier transform surface-enhanced Raman scattering (FT-SERS) spectroscopy. The obtained spectral results indicate that the adenosine molecules in the subphase can be transferred onto solid substrates by LB techniques as a result of the formation of base pairs at the air/water interface. UV-vis spectra alternations indicated that, with increasing adenosine concentration in subphase, more adenosine molecules were recognized by nucleolipid monolayer and were transferred onto the quartz substrates. The closed-packing of the constituent molecules facilitates the photodimerization of the thymine moieties in the headgroup under ultraviolet irradiation. FTIR-ATR results suggest that the hydrocarbon chains of nucleolipid in the LB films deposited from pure water and aqueous adenosine take on a close-packed all trans conformation. By analyzing the FT-SERS spectra results, it can be deduced that the orientation of nucleobase in the headgroup is different before and after the recognition effect occurred. For LB film deposited from pure water, the nucleobases are lying flat on the silver substrates; whereas for LB film deposited from aqueous adenosine, the base pairs take an end-on adsorption on silver substrate.     

The incorporation of C_(60) in cavities of 5,10,15,20-tetra-4-oxy(2-stearic acid)phenyl porphyrin in LB film

The structure and properties of mixed monolayer or LB film of 5,10,15,20-tetra-4-oxy(2-stearic acid)phenyl porphyrin/C60 were studied. A isotherms and small angle X-ray diffraction (SAXD) results show that C60 molecules were incorporated into the cavities of por-phyrin molecules in mixed monolayer and LB film . UV-vis spectra of mixed LB films show that the absorption intensity of porphyrin varied compared with pure porphyrin film, probably arising from the interactions between C60 and porphyrin ring. C60 molecules in mixed systems are well-dispersed. The TPP(CO2H)/C60 mixed LB film is a kind of two-dimensional host-guest system.     

Self-assembled spiral nanoarchitecture and supramolecular chirality in Langmuir-Blodgett films of an achiral amphiphilic barbituric acid

An amphiphilic barbituric acid derivative was found to form stable monolayers showing a clear phase transition at the air/water interface. It is interesting to find that the deposited Langmuir-Blodgett (LB) films of the compound showed circular dichroism (CD) although the molecule itself was achiral. AFM measurements on the transferred one-layer LB film revealed that spiral nanoarchitectures were formed. It was further found that the supramolecular chirality of the LB films was related to symmetry breaking at the interface. Hydrogen bonding and the pi-pi stacking between the neighboring molecules resulted in chiral fibers which formed the spiral structures. To the best of our knowledge, this is the first report on the chirality of the molecular assemblies and spiral nanostructures formed through the air/water interface by achiral molecules.     

Supramolecular self-assembly study of a flexible perylenetetracarboxylic diimide dimer in Langmuir and Langmuir–Blodgett films

A novel perylenetetracarboxylic diimide molecule (2PDI-TAZ), which contains two perylenetetracarboxylic diimide (PDI) attached to a melamine headgroup, was designed and synthesized. Supramolecular self-assemblies were studied in Langmuir and Langmuir–Blodgett films. Surface pressure–area isotherm measurements and the spectroscopic studies indicate that the 2PDI-TAZ molecules adopted a face-to-face configuration and edge-on orientation in Langmuir or the multilayer LB films. The presence of the barbituric acid in subphase change the hydrophilicity of 2PDI-TAZ due to the hydrogen bonding between melamine and barbituric acid, which has been revealed by the π–A isotherms and the FT-IR spectra. Transmission electron microscopy images of the LB films deposited from the barbituric acid solution revealed uniform nanowire morphology while the X-ray diffraction studies indicate that the molecules in the solid film packed with high order. The strong excimer emission of 2PDI-TAZ in LB films suggests enforced face-to-face configuration for the PDI unites in LB films in relative to that in solution.     

Langmuir-Blodgett films of 9-phenyl anthracene molecules incorporated into different matrices

This communication reports the surface pressure (pi) versus area per molecule (A) isotherm characteristics of the mixed films of 9-phenyl anthracene (PA) in stearic acid (SA) and polymethyl methacrylate (PMMA) matrices, at the air-water interface. The mixed Langmuir films at the air-water interface have been observed to be easily transferred onto solid substrates to form uniform Langmuir-Blodgett films. By changing various parameters, namely molefraction, surface pressure of lifting and number of layers, the mixed Langmuir-Blodgett (LB) films of various types have been fabricated successfully and their spectroscopic characteristics have been reported. From the isotherm characteristics and the area per molecule versus molefraction plot, it is evident that the PA molecules are successfully incorporated into mixed Langmuir-Blodgett films. UV-vis absorption spectroscopic study of the mixed LB films at various molefractions of PA in two different matrices reveal that formation of I-type aggregate in PMMA matrix whereas both I- and H-type aggregates are playing their dominant role in SA matrix. Moreover, fluorescence spectroscopic study reveals reabsorption effect. Molecular movement persists in the freshly prepared LB films, as is evident from the time dependent changes in both UV-vis absorption and fluorescence spectra of the mixed LB films in both matrices. From our observation it is evident that about 200 h is required to get the LB films in a stable condition. Dimers and higher order n-mers are formed at a higher surface pressure of 30 mNm(-1).     

Adsorption kinetics of a fluorescent dye in a long chain fatty acid matrix

This work reports the adsorption kinetics of a highly fluorescent laser dye rhodamine B (RhB) in a preformed stearic acid (SA) Langmuir monolayer. The reaction kinetics was studied by surface pressure-time (π-t) curve at constant area and in situ fluorescence imaging microscopy (FIM). Increase in surface pressure (at constant area) with time as well as increase in surface coverage of monolayer film at air-water interface provide direct evidence for the interaction. ATR-FTIR spectra also supported the interaction and consequent complexation in the complex films. UV-vis absorption and Fluorescence spectra of the complex Langmuir-Blodgett (LB) films confirm the presence of RhB molecules in the complex films transferred onto solid substrates. The outcome of this work clearly shows successful incorporation of RhB molecules into SA matrix without changing the photophysical characteristics of the dye, thus making the dye material as LB compatible.     

Langmuir-Blodgett films of octadecanethiol--properties and potential applications

Octadecanethiol (ODT) is known to form self-assembled monolayer on noble metal surfaces which has potential technological applications. Langmuir-Blodgett (LB) technique is another useful method of obtaining highly ordered assembly of molecules. It is of interest to find whether ODT molecules can also form a stable Langmuir monolayer which facilitates the preparation of LB films. In literature, it has been reported that ODT molecules form an unstable Langmuir monolayer. We have studied the stability of the monolayer of the ODT molecules at air-water interface using surface manometry and microscopy techniques. We find the monolayer to be stable on ultrapure water of resistivity greater than 18MOmega cm. However, the behavior changes in the presence of even small amount of additives like NaOH or CdCl2 in the subphase. Our AFM studies on the LB films of ODT deposited from ion-free ultrapure water showed streak-like bilayer domains. The LB films of ODT deposited from CdCl2 containing aqueous subphase yield dendritic domains of the complexed unit grown over ODT monolayer. These nanostructures on surfaces may have potential applications in molecular electronics.     

Peptidolipid as binding site of acetylcholinesterase: molecular recognition of paraoxon in Langmuir films

Peptidolipid C18H35O (stearoyl)-Phe-Trp-Ser-His-Glu (peptidolipid A) was synthesized and spread at the air-water interface to study the interaction with an organophosphorus compound. Paraoxon, sodium dihydrogen phosphate, or 4-nitrophenyl phosphate disodium was added to the subphase, but only paraoxon changed the surface pressure-area (pi-A) isotherm of peptidolipid A. This indicated a specific interaction between paraoxon and peptidolipid A. To clarify which amino acid residue of peptidolipid A was responsible for the interaction, peptidolipid B, namely, C18H35O-Gly-His-Ser-Glu-Glu, was synthesized and studied as a Langmuir film. The difference between the pi-A isotherms of peptidolipid B in the absence and presence of paraoxon in the subphase was minimal; consequently, the presence of amino acids phenylalanine (Phe) and tryptophan (Trp) in peptidolipid A may explain the interaction between peptidolipid A and paraoxon. The compression-decompression cycles and kinetic studies of peptidolipid A showed that the Langmuir film was stable. The in situ optical properties of the peptidolipid A Langmuir film such as UV-vis and fluorescence spectroscopies were examined to elucidate the interaction between peptidolipid A and paraoxon. UV-vis absorption of peptidolipid A was investigated in the presence and absence of paraoxon in the subphase. The emission maximum of fluorescence of Trp in peptidolipid A was observed at 351 nm on pure water, and the band intensity decreased when the concentration of paraoxon increased in the subphase. This suggested that the Trp was involved in the molecular recognition process. Epifluorescence micrographs showed domains of peptidolipid A on the pure water subphase. In the presence of paraoxon in the subphase, the Langmuir film of peptidolipid A showed a homogeneity, which was another indication of the recognition between paraoxon and peptidolipid A.     

Supramolecular assemblies and molecular recognition of amphiphilic schiff bases with barbituric acid in organized molecular films

A bolaform Schiff base, N,N'-bis(salicylidene)-1,10-decanediamine (BSC10), has been synthesized and its interfacial hydrogen bond formation or molecular recognition with barbituric acid was investigated in comparison with that of a single chain Schiff base, 2-hydroxybenzaldehyde-octadecylamine (HBOA). It has been found that while HBOA formed a monolayer at the air/water interface, the bolaform Schiff base formed a multilayer film with ordered layer structure on water surface. When the Schiff bases were spread on the subphase containing barbituric acid, both of the Schiff bases could form hydrogen bonds with barbituric acid in situ in the spreading films. As a result, an increase of the molecular areas in the isotherms was observed. The in situ H-bonded films could be transferred onto solid substrates, and the transferred multilayer films were characterized by various methods such as UV-vis and FT-IR spectrosopies. Spectral changes were observed for the films deposited from the barbituric acid subphase, which supported the hydrogen bond formation between the Schiff bases and barbituric acid. By measuring the MS-TOF of the deposited films dissolved in CHCl3 solution, it was concluded that a 2:1 complex of HBOA with barbituric acid and a 1:2 complex of BSC10 with barbituric acid were formed. On the other hand, when the multilayer films of both Schiff bases were immersed in an aqueous solution of barbituric acid, a similar molecular recognition through the hydrogen bond occurred. A clear conformational change of the alkyl spacer in the bolaform Schiff base was observed during the complex formation with the barbituric acid.     

Formation, structure, and morphology of triazole-based Langmuir-Blodgett films

The formation, morphology, and structure of two-dimensional Langmuir-Blodgett (LB) assemblies of octadecyltriazole (ODT)-based metal-containing oligomers presenting, in the case of iron, the spin-crossover phenomenon is studied with Brewster angle microscopy, IR dichroism, X-ray diffraction, and atomic force microscopy. Two processes occurring at the air-water interface are confirmed to dominate the mechanism of formation of these LB films, the instability of the coordination polymers at the air-water interface and recoordination of metal ions in the subphase at the interface during the LB deposition process. The Langmuir film allowing the LB film formation is mostly made of ODT. The films do present a lamellar structure in which the ODT molecules are tilted and incorporate coordinated isolated metal ions and oligomers of metal ions. The morphology of the LB films is globally flat but with a rather high roughness resulting from inhomogeneities related to phenomena occurring during the LB film formation. These observations are in agreement with the relative affinity of the metal ions with ODT and the relative stability of the coordination polymers at the air-water interface, which have been determined for the group Cu-Fe-Co-Ni.     

Pure and mixed films of a nitrostilbene derivative at the air-water interface, Langmuir-Blodgett multilayer fabrication, and optical characterization

This paper reports the preparation and characterization of pure Langmuir and Langmuir-Blodgett (LB) films of a stilbene derivative containing two alkyl chains, namely 4-dioctadecylamino-4'-nitrostilbene. Mixed films incorporating docosanoic acid and the stilbene derivative are also studied. Brewster angle microscopy (BAM) analysis has revealed the existence of randomly oriented three-dimensional (3D) aggregates, spontaneously formed immediately after the spreading process of the stilbene derivative onto the water surface. These 3D aggregates coexist with a Langmuir film that shows the typical gas, liquid, and solid-like phases in the surface pressure and surface potential vs area per molecule isotherms, indicative of an average preferential orientation of the stilbene compound at the air-water interface, and a gradual molecular arrangement into a defined structure upon compression. A blue shift of 55 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a chloroform solution of the nitrostilbene indicates that two-dimensional (2D) H-aggregates are formed at the air-water interface. The monolayers are transferred undisturbed onto solid substrates with atomic force microscopy (AFM) revealing that the one layer LB films are constituted by a monolayer of the stilbene derivative together with some 3D aggregates. When the nitrostilbene compound is blended with docosanoic acid, the 3D aggregation is avoided in the Langmuir and Langmuir-Blodgett films, but does not limit the formation of 2D H-aggregates, desirable for second-order nonlinear optical response in the blue domain. The AFM images of the mixed LB films show that they are formed by a docosanoic acid monolayer and, on the top of it, a bilayer of the stilbene derivative.     

Langmuir-Blodgett films and chiroptical switch of an azobenzene-containing dendron regulated by the in situ host-guest reaction at the air/water interface

An amphiphilic dendron containing an azobenzene ring at the focal point and the l-glutamate peripheral groups was designed. Its monolayer formation and host-guest reaction with cyclodextrins at the air/water interface and the properties of the transferred Langmuir-Blodgett (LB) films were investigated. The individual dendron, although without any long alkyl chains, could still form a stable monolayer at the air/water interface because of the good balance between hydrophilic and hydrophobic parts within the molecule. When cyclodextrin (CyD) was added to the subphase, a host-guest reaction occurred in situ at the air/water interface. The inclusion of the focal azobenzene moiety into the cavity of cyclodextrin decreased the packing of the aromatic ring and also led to the diminishment of the molecular area. Both the films formed at the surface of pure water and aqueous cyclodextrins were transferred onto solid substrates. Nanofiber structures were obtained for the film from the water surface as a result of the packing of the azobenzene groups, and circular domains were obtained for the film transferred from the aqueous CyD phases. The film transferred from the water surface showed an exciton couplet in the absorption band of azobenzene, whereas a negative Cotton effect was obtained for the film from CyD subphases. It was found that the supramolecular chirality in the LB film transferred from water was due to the transfer of the molecular chirality to the assemblies whereas that from the CyD subphase was due to the inclusion of azobenzene into the chiral cavity. Interestingly, the film from the water surface was photoinactive, whereas a reversible optical and chiroptical switch could be obtained for the film from the α-CyD subphase. The work provided a way to regulate the assembly and functions of organized molecular films by taking advantage of the interfacial host-guest reaction.     

Deposition and photopolymerization of phase-separated perfluorotetradecanoic acid-10,12-pentacosadiynoic acid Langmuir-Blodgett monolayer films

Mixed monolayer surfactant films of perfluorotetradecanoic acid and the photopolymerizable diacetylene molecule 10,12-pentacosadiynoic acid were prepared at the air-water interface and transferred onto solid supports via Langmuir-Blodgett (LB) deposition. The addition of the perfluoroacid to the diacetylene surfactant results in enhanced stabilization of the monolayer in comparison with the pure diacetylene alone, allowing film transfer onto a solid substrate without resorting to addition of cations in the subphase or photopolymerization prior to deposition. The resulting LB films consisted of well-defined phase-separated domains of the two film components, and the films were characterized by a combination of atomic force microscope (AFM) imaging and fluorescence emission microscopy both before and after photopolymerization into the highly emissive "red form" of the polydiacetylene. Photopolymerization of the monolayer films resulted in the formation of diacetylene bilayers, which were highly fluorescent, with the apparent rate of photopolymerization and the fluorescence emission of the films being largely unaffected by the presence of the perfluoroacid.     

Air-water interfacial behavior of linear-dendritic block copolymers containing PEG and azobenzene chromophores

Fabrication of Langmuir films at the air-water interface of four linear-dendritic block copolymers (LDBCs) is described. The LDBCs are composed of a linear hydrophilic chain of poly(ethylene glycol) (PEG) and the first four generations of hydrophobic aliphatic polyester dendrons functionalized at the periphery with cyanoazobenzene chromophores. Langmuir films of the LDBCs, coded as PEG-AZOn (n indicates the number of cyanoazobenzene units at the periphery of the dendritic block), have been characterized by a combination of surface pressure versus area per molecule isotherms, UV-vis reflection spectroscopy and Brewster angle microscopy. The observed PEG-AZOn Langmuir film behavior depends strongly on the hydrophilic/hydrophobic ratio. A typical transition, related to PEG chains desorption from the air-water interface into the water subphase is observed for all the LDBCs, except for PEG-AZO16. In addition, PEG-AZO2 and PEG-AZO4 show a second transition whose nature has been studied in detail. Azobenzene chromophore interactions have been shown to be relevant in the organization of PEG-AZOn (n=4, 8 and 16) Langmuir films. Moreover, for PEG-AZO16 the orientation of the azobenzene units has been determined, revealing the formation of a well organized structure of azobenzene moieties at the air-water interface.