Supramolecular Assembly and Headgroup Effect in Interfacial Organized Films (I): A Study of Some Bolaamphiphiles

Two bola form Schiff bases derivatives with different substituted head groups have been designed and their interfacial phase behaviors and coordination with Cu(II) ions were investigated. It has been found that while one molecule with benzene headgroup formed dotted aggregations at the air/water interface, another with naphthyl moiety as head group formed crystalline multilayer films on water surface. When on the sub phase containing Cu(II) ions, both of the Schiff bases can coordinate with Cu(II) in situ in the spreading films with the obvious conformational change of alkyl chains. The in situ Cu(II)-coordinated films could be transferred onto solid substrates and subsequently characterized by various spectroscopic methods such as UV-vis and Fourier transform infrared spectra as well as the morphological character with atomic force microscopy measurement. In comparison, the ex situ coordination process at the liquid/solid interface have also been investigated by continuous spectral measurement. Depending on the different head groups, these amphiphiles showed different aggregation behaviors in the Langmuir-Blodgett films. Particularly, during the coordination process of ligand with Cu(II) ions in organized molecular films, great conformational change of the alkyl chains was observed. At the same time, a rational explanation about the head group effect on regulating the aggregation behaviors was discussed.     

芳香席夫碱衍生物在有序分子膜中的相行为与二维-三维形变

研究了芳香席夫碱的设计合成、界面相行为以及与铜离子的配位过程,并与离位合成的铜复合物的界面组装行为进行了对比.利用表面压-面积等温线、紫外光谱、红外光谱、原子力显微镜等一系列实验方法来详细表征组装膜.结果表明,席夫碱配体与铜复合物均可在气液界面上形成稳定的有序组装膜,并可以转移到固体基片上构筑多层膜.进而,席夫碱配体在纯水界面上表现出奇特的相变.在相变点前后,二维的平膜变化成了三维的纤维状组装结构.然而,对于铜离子亚相上的组装过程却没有出现较大的变化.两者之间明显的差异可归因于在气液界面超分子组装过程中的分子构型以及疏水性能的不同引起的.目前的工作为制备和调控有序组装膜提供了有益探索.     

Investigation of solid surfaces modified by Langmuir-Blodgett monolayers using sum-frequency vibrational spectroscopy and X-ray photoelectron spectroscopy

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30 degrees with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N 1s lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu(2+) ions in a thin Cu(OH)(2) layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the non-resonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are approximately 10-20 degrees.     

Supramolecular assemblies of a new series of gemini-type schiff base amphiphiles at the air/water interface: in situ coordination, interfacial nanoarchitectures, and spacer effect

Great interest has been devoted to the gemini amphiphiles because of their unique properties. In this article, we report some interesting properties of the interfacial films formed by a series of newly designed gemini amphiphiles containing the Schiff base moiety. This novel series of gemini amphiphiles with their Schiff base headgroups linked by a hydrophobic alkyl spacer (BisSBC18Cn, n = 2, 4, 6, 8, 10) could be spread to form stable monolayers and coordinated with Cu(Ac)(2) in situ in the monolayer. The alkyl spacer in the amphiphiles has a great effect on the regulation of the properties of the Langmuir monolayers. A maximum limiting molecular area was observed for the monolayers of the gemini amphiphile with the spacer length of hexa- or octamethylene groups. Both the monolayers on water and on the aqueous Cu(Ac)(2) subphase were transferred onto solid substrates, and different morphologies were observed for films with different spacers. Nanonail and tapelike morphologies were observed for amphiphile films with shorter spacers (n = 2 and 4) on the water surface. Wormlike morphologies were observed for gemini films with longer spacers of C(8) and C(10) when coordinated with Cu(Ac)(2). An interdigitated layer structure was supposed to form in the multilayer films transferred from water or the aqueous Cu(Ac)(2) subphase.     

Substitution controlled molecular orientation and nanostructure in the Langmuir-Blodgett films of a series of amphiphilic naphthylidene-containing Schiff base derivatives

Three amphiphilic Schiff bases containing naphthylidene group, N-octadecyl-2'-hydroxy-naphthalenylideneamine (HNOA-1), N-(4'-octadecyloxy)-2'-hydroxy-naphthalenylideneaniline (HNOA-2), and N-(4'-N-octadecylbenzamide)-2'-hydroxy-naphthalenylideneaniline (HNOA-3), were designed and their interfacial assemblies were investigated. These amphiphiles have different substitution groups in the Schiff base moiety. It was observed that HNOA-1, being in lack of second aromatic ring, formed multilayer films at the air/water interface, while the other two compounds, with another aromatic ring and some hydrophilic groups, can be spread as monolayers on water surface. All of Schiff bases could coordinate with Cu(Ac)2 in situ in the spreading films. Both the spreading films from water and aqueous Cu(Ac)2 subphases were transferred onto solid substrates and their surface morphologies as well as molecular packing modes were investigated by a series of methods such as atomic force microscopy, Fourier transform infrared spectra and X-ray diffraction. Depending on the different substitutions, these amphiphiles showed different orientations in the Langmuir-Blodgett films. Particularly, during the process of complex formation at the air/water interface, great conformational change of the alkyl chain was observed for HNOA-2 in comparison with that of other compounds. In addition, nanofiber structures were observed for the Cu(II)-complexed HNOA-1 and HNOA-2 films.     

Structure Studies on Ion Coordination in 2,4-Dihydroxy-N-Octadecybenzylideneamine LB Films by UV-Vis Absorption and FTIR Spectroscopy

The coordination reaction of Cu(2+) ions with the Schiff base amphiphile 2,4-dihydroxy-N-octadecylbenzylideneamine in ultrathin organic LB films was probed by Fourier transform infrared (FTIR) transmission spectroscopy and UV-Vis electronic absorption spectroscopy. FTIR spectral changes owing to LB films of 2,4-dihydroxy-N-octadecylbenzylideneamine contacting aqueous Cu(OAC)(2) solution suggest the formation of ion-coordination films, which is corroborated by UV-Vis absorption spectra. The coordination reaction of the annealing film at different temperatures suggests that the microstructure of LB films had an important effect on the reaction rate. Detailed analysis shows that ion-coordination films take an isotropic unaxial orientation and exhibit phase transition behavior, which is determined by the different complex structures of headgroups and of subphase and surface film. Copyright 2000 Academic Press.     

Complexation of ferrous and cupric ions by phenanthroline and terpyridine langmuir films

The complexation of Fe(II) and Cu(II) ions by phenanthroline and terpyridine-based Langmuir films was studied by the effect the added ions in the subphase on the isotherms. We found that Cu(II) had a pronounced effect on the isotherm consisting of phenanthroline moieties forming a square-planar 1:2 (Cu(II)/phenanthroline) complex. However, Fe(II) affected the isotherm made of 1:1 phenanthroline/terpyridine amphiphiles as a result of forming a pentacoordinate complex.     

A comparative Langmuir-Blodgett study on a set of covalently linked porphyrin-based amphiphiles: a detailed atomic force microscopic study

A set of covalently linked phenyl-amidophenyl-substituted porphyrin amphiphiles with n-C15H31 tails have been synthesized and completely characterized. These amphiphiles form good Langmuir-Blodgett (LB) films at the air/water interface. Mean molecular areas for the series were measured from the isotherms and found to increase as the number of aliphatic chains increased from one to four. No influence of the subphase pH was observed on the isotherms. LB films can be transferred successfully onto different solid surfaces. The LB films were characterized using tapping mode atomic force microscopy (AFM). Bis-, tris-, and tetra-substituted porphyrins were found to be fairly good film-forming amphiphiles, whereas irregular aggregates were seen in the case of the monosubstituted porphyrin amphiphile. Multilayers were also formed with tetra-substituted amphiphiles on mica. Detailed AFM studies of tetra-substituted amphiphiles have been carried out to investigate the effect of preparation procedure and solid substrates on film formation and transfer. The absorption and fluorescence spectra for the amphiphiles in solution and LB films deposited onto mica and glass were recorded, which demonstrated the successful transfer of LB films onto the substrates and provided more information about the arrangement of porphyrin molecules within the LB films. For comparison, self-assembled monolayers (SAMs) and the cast thin films of the amphiphiles were prepared and characterized.     

Potentiometric response of lipid modified ISFET

The gate surface of an ion-sensitive field effect transistor (ISFET) was modified with Langmuir-Blodgett (LB) film composed of fatty acid or crown ether amphiphiles to examine their potentiometric response to H⁺ and K⁺ ions. The results demonstrate the possible use of the lipid films for preparing ISFET ion sensors.     

Inducing spin crossover in metallo-supramolecular polyelectrolytes through an amphiphilic phase transition

A phase transition in an amphiphilic mesophase is explored to deliberately induce mechanical strain in an assembly of tightly coupled metal ion coordination centers. Melting of the alkyl chains in the amphiphilic mesophase causes distortion of the coordination geometry around the central transition metal ion. As a result, the crystal field splitting of the d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state. The diamagnetic-paramagnetic transition is reversible. This concept is demonstrated in a metallo-supramolecular coordination polyelectrolyte-amphiphile complex self-assembled from ditopic bis-terpyridines, Fe(II) as central transition metal, and dialkyl phosphates as amphiphiles. The magnetic properties are studied in a Langmuir-Blodgett multilayer. The modularity of this concept provides extensive control of structure and function from molecular to macroscopic length scales and gives access to a wide range of new molecular magnetic architectures such as nanostructures, thin films, and liquid crystals.     

NO2 gas detection by Langmuir-Blodgett films of copper phthalocyanine multilayer structures

Copper (II) phthalocyanine derivatives with four alkyl chains were synthesized. Langmuir-Blodgett (LB) films of the following two compounds were deposited: Cu(II)[tetrakis(3,3-dimethyl-butoxycarbonyl)] phthalocyanine and Cu(II)[tetrakis(n-butoxycarbonyl)] phthalocyanine. Moreover, LB films from a 1:1 mixture of the two compounds were prepared. All isotherms and optical data were consistent with the molecules being stacked nearly edge-on to the substrate, with the molecular faces having a preferred orientation perpendicular to the dipping direction. The dynamic response characteristics of the electrical conductance of the LB films to different NO₂ concentrations in dry air are described. A relationship has been observed between the anisotropic molecular orientation and the electrical conductivity parallel to the substrate.     

Photo-induced magnetic bistability in a controlled assembly of anisotropic coordination nanoparticles

Anisotropic coordination nanoparticles of the photomagnetic network Cs(I)(2)Cu(II)(7)[Mo(IV)(CN)(8)](4) are obtained through a surfactant-free high-yield synthetic procedure in water. These particles are organised as Langmuir-Blodgett films with a preferential orientation of the nano-objects within the film that exhibit a magnetic bistability below 20 K with a very large coercivity due to an efficient photo-transformation.     

Controllable growth of straight nanorods and nanowires in the Langmuir films of a bolaamphiphilic par derivative.

A novel bolaamphiphilic compound, 1, 10-bis[3'-hydroxy-4'-(2'-pyridylazo)phenoloxy] decane [(PAR)2C10], was synthesized and its spreading film and in situ coordination with metal ions in the Langmuir monolayer at the air/water interface were investigated. It was found that (PAR)2C10 could be spread on the water surface, and the Langmuir film showed a phase transition from a flat conformation to a U-shaped conformation upon compression. Interfacial coordination between (PAR)2C10 and metal ions could occur in situ in the Langmuir monolayer. Depending on the concentration of the subphase, different coordination modes were observed. A 2:1 (ligand-PAR-to-metal-ion) complex was formed at a lower concentration of the subphase, while a 1:1 complex was obtained on the subphase with a higher concentration of the salt. Interestingly, very straight nanowires, extending to several micrometers, were observed in the AFM images of the films transferred from the subphase containing CuCl2 in higher concentrations. However, tortuous nanowires were observed under the subphase containing Cu(CH3COO)2. The formation of such kind of wire-like structures was only observed for the combination of the bolaamphiphilic PAR derivative with the Cu(II) ions. Other metal ions or the single-chain PAR derivative did not show this behavior. A possible growth mechanism of the nanowires was proposed based on the coordination of the bolaamphiphilic (PAR)2C10 with the Cu(II) ions as well as with the counteranions.     

Bis(oxazoline)-based coordination polymers: a recoverable system for enantioselective Henry reactions

An efficient release-capture strategy for the recovery and reuse of enantioselective catalysts in the Henry reaction is described. This strategy is based on the precipitation of an insoluble coordination polymer at the end of each reaction, allowing easy separation from products. The coordination polymer is formed through aggregation of Cu(II) ions with ditopic bisoxazoline-based chiral ligands. Up to 11 catalytic cycles have been conducted keeping good yields and enantioselectivities.     

Synthesis and characterization of amphiphilic fullerenes and their Langmuir-Blodgett films

We report here the synthesis and characterization of three amphiphilic fullerene derivatives and their Langmuir-Blodgett thin films. Two of the C(60) amphiphiles are mono-derivatives with a long alkyl chain terminated with either -COOH (2) or NH(2) (3) as the hydrophilic headgroup, and the third one (5) is designed to bear the same NH(2) group as 3 but with 10 additional hydrophobic alkyl chains grafted on the C(60) sphere (Scheme 1). These amphiphiles form stable, ordered monolayers at the air-water interface. The molecular packing at the air-water interface and the mean area per molecule are determined by pressure isotherms at room temperature. Hysteresis of pressure isotherms of side chain C(60) (5) shows complete reversibility upon compression and decompression, which suggests that side chains on the C(60) sphere inhibit formation of aggregates at the air-water interface. Comparative studies of all three amphiphiles allow us to better determine the interaction between C(60)'s and their self-assembly kinetics at the air-water interface. Monolayers of monoderivatized amphiphiles (2 and 3) were transferred successfully onto quartz substrates as Z-type multilayered Langmuir-Blodgett films, and monolayers of 5 were transferred as Y-type films. Detailed characterization of the multilayer films (Z-type deposition) prepared from amine-terminated C(60) (3) using X-ray and neutron reflectometry reveals staggering of C(60) spheres and a head-to-head (Y-type) structure presumably due to flipping and reattaching of C(60) amphiphiles to the previous underlying C(60) layer.     

Nature of metal binding sites in Cu(II) complexes with histidine and related N-coordinating ligands, as studied by EXAFS

Knowledge of the complexes formed by N-coordinating ligands and Cu(II) ions is of relevance in understanding the interactions of this ion with biomolecules. Within this framework, we investigated Cu(II) complexation with mono- and polydentate ligands, such as ammonia, ethylenediamine (en), and phthalocyanine (Pc). The obtained Cu-N coordination distances were 2.02 A for [Cu(NH(3))(4)](2+), 2.01 A for [Cu(en)(2)](2+), and 1.95 A for CuPc. The shorter bond distance found for CuPc is attributed to the macrocyclic effect. In addition to the structure of the first shell, information on higher coordination shells of the chelate ligands could be extracted by EXAFS, thus allowing discrimination among the different coordination modes. This was possible due to the geometry of the complexes, where the absorbing Cu atoms are coplanar with the four N atoms forming the first coordination shell of the complex. For this reason multiple scattering contributions become relevant, thus allowing determination of higher shells. This knowledge has been used to gain information about the structure of the 1:2 complexes formed by Cu(II) ions with the amino acids histidine and glycine, both showing a high affinity for Cu(II) ions. The in-solution structure of these complexes, particularly that with histidine, is not clear yet, probably due to the various possible coordination modes. In this case the square-planar arrangements glycine-histamine and histamine-histamine as well as tetrahedral coordination modes have been considered. The obtained first-shell Cu-N coordination distance for this complex is 1.99 A. The results of the higher shells EXAFS analysis point to the fact that the predominant coordination mode is the so-called histamine-histamine one in which both histidine molecules coordinate Cu(II) cations through N atoms from the amino group and from the imidazole ring.     

Characterization of Langmuir and Langmuir-Blodgett films of a thiomacrocyclic ionophore by surface pressure and AFM

The new synthesized thiomacrocyclic ionophore 4-phenyl-4-sulfide-11-(1-oxodecyl)-1,7-dithia-11-aza-4-phosphacyclotetradecane has proven to form Langmuir and Langmuir-Blodgett (LB) films. This ionophore shows a large affinity for copper(II) ions. Thus, the influence of the subphase composition on the surface pressure-area isotherms has been studied. The LB films have been observed by AFM and the effect of the subphase composition and the deposition surface pressure on the LB films is discussed. AFM image morphology has been correlated to the ionophore molecular structure. Surface pressure-area isotherms and AFM images show that the presence of copper(II) ions has an important role in the film structure.     

The Influence of Macrocyclic Tetraamine Immobilization in Langmuir Monolayers on Complex Formation Selectivity

Selective complex formation in Langmuir dicetyl cyclene monolayers on the surface of aqueous solutions of Cu(II), Ni(II), and Zn(II) salts and their mixtures was studied. The effect of selectivity “inversion” of diphilic cyclene immobilized in monolayers on the surface of solutions of a mixture of copper(II) and nickel(II) salts was observed; the inversion was induced by a change in subphase pH. An analysis of the isotherms of monolayer compression and X-ray fluorescence spectra of the corresponding Langmuir-Blodgett films showed that subphase acidification caused a gradual transition from the selective formation of copper-containing macrocyclic complexes to selective complex formation between the ligand and nickel ions. The effect observed was not characteristic of complex formation with similar unsubstituted tetraamines in bulk solution. The phenomenon was interpreted from the point of view of specific conformational transitions of the diphilic macroring in the two-dimensional system organized at the interphase boundary.     

Interactions and reactions of monolayers and Langmuir-Blodgett multilayers with compounds in the bulk phase

Studies performed on the interactions and reactions of compounds in the bidimensional state, essentially in monolayers and Langmuir-Blodgett multilayers, with substances in the aqueous subphase are reported. More precisely, the following is illustrated: (i) interactions between acid amphiphiles and prevalently bivalent ions placed in the aqueous support and between compounds capable of functioning like ion carriers in monolayers and ions in the subphase, in order to build mimetic membranes capable of selective ion transport; and the complexation of amphiphiles in monolayer with ions in the bulk liquid phase, in order to build chemical sensors to ions; (ii) the reactions of photoinduced electron transfer between a partner in mono- or multimolecular films and a partner in the subphase, which may determine the fundamental parameters and the differences with the same reactions in the bulk phase; and (iii) the reactions of enzymatic hydrolysis between the monolayer of a glyceride, which constitutes the reaction support, and the enzyme in the liquid bulk phase, which constitutes the subphase. The mechanism of the reactions and its inhibition are clarified. To conclude, possible future developments connected with the areas studied are examined.     

Structure and temperature behavior of metallo-supramolecular assemblies

A detailed structural analysis of a Langmuir-Blodgett (LB) multilayer composed of a polyelectrolyte-amphiphile complex (PAC) is presented. The PAC is self-assembled from metal ions, ditopic bis-terpyridines, and amphiphiles. The vertical structure of the LB multilayer is investigated by X-ray reflectometry. The multilayer has a periodicity of 57 A, which corresponds to an architecture of flat lying metallo-supramolecular coordination polyelectrolyte (MEPE) rods and upright-standing amphiphiles (dihexadecyl phosphate, DHP). In-plane diffraction reveals hexagonal packing of the DHP molecules. Using extended X-ray absorption fine structure (EXAFS) experiments, we prove that the central metal ion is coordinated to the terpyridine moieties in a pseudo-octahedral coordination environment. The Fe-N bond distances are 1.82 and 2.0 A, respectively. Temperature resolved measurements indicate a reversible phase transition in a temperature range up to 55 degrees C. EXAFS measurements indicate a lengthening of the average Fe-N bond distance from 1.91 to 1.95 A. The widening of the coordination cage upon heating is expected to lower the ligand field stabilization, thus giving rise to spin transitions in these composite materials.