Ordered arrays of organometallic iridium complexes with long alkyl chains on graphite

Iridium(III) fac-tris(2-phenylpyridine) fac-[Ir(ppy)3] complexes equipped with long alkyl chains were prepared to examine their capability to form organized arrays on the surface of highly oriented pyrolytic graphite (HOPG). The molecules form lamellar arrays at the 1-phenyloctane/HOPG interface. From the analysis of the STM images, it was concluded that the molecules align with alkyl chains being interdigitated. Similar lamellar arrays were also obtained at the air/HOPG interface upon drop-casting of toluene solutions. The lamellar structure at the molecular level leads to rectangular two-dimensional crystalline domains a few hundred nanometers long (nanoslips). Infrared external reflection spectroscopy suggested that the adsorbed alkyl chains adopt the trans-zigzag conformation in the nanoslip, although the orientations of the zigzag plane of the alkyl groups are mixed. Cyclic voltammetry indicates fast electron transfer between the adsorbed molecules and the substrate and significant intermolecular electronic interactions. It was found that annealing at high temperatures is an effective method to prepare ordered assemblies more than a few micrometer scale (microslips). The orientations of the nanoslips prepared from the racemic mixture exhibited an apparent 12-fold symmetry, while its optically active enantiomer resulted in more irregular domains with a six-fold symmetry, implying an important role of chirality on packing at the molecular level and on the orientation of the domains at larger scales. When drop-cast from more concentrated solutions than a few hundreds of micromolar, multilayers were obtained, in which the alkyl chains in the molecules are more or less perpendicular to the surface. This structure can be transformed into the nanoslips upon standing.     

On the stability of Langmuir films of pyramidic mesogens

Abstract

Pyramidic mesogens forming thermotropic liquid crystal bulk phases were spread in an air-water interface. Pressure surface measurements and polarizing microscopy on the Langmuir films were used to characterize the various states of these pyramidic-like molecules. For two compounds bearing short lateral aliphatic chains, the surface pressure isotherms exhibit a large plateau region corresponding to a metastable monolayer in which the molecules may adopt an ‘edge-on’ arrangement. The coexistence of multilayered, anisotropic, slowly growing domains with the monolayer in the plateau region has been observed at long time scale. The film area relaxation kinetics at constant surface pressure show the existence of two nucleation mechanisms for the formation of these domains.     

Fundamentals and application of ordered molecular assemblies to affinity biosensing

Organization of biomolecules in two/three dimensional assemblies has recently aroused much interest in nanobiotechnology. In this context, the development of techniques for controlling spatial arrangement and orientation of the desired molecules to generate highly-ordered nanostructures in the form of a mono/multi layer is considered highly significant. The studies of monolayer films to date have focused on three distinct methods of preparation: (i) the Langmuir-Blodgett (LB) technique, involving the transfer of a monolayer assembled at the gas-liquid interface; (ii) self-assembly at the liquid-solid interface, based on spontaneous adsorption of desired molecules from a solution directly onto a solid surface; and (iii) Layer-by-layer (LBL) self-assembly at a liquid-solid interface, based on inter-layer electrostatic attractions for fabrication of multilayers. A variety of monolayers have been utilized to fabricate biomolecular electronic devices including biosensors. The composition of a monolayer based matrix has been found to influence the activity(ies) of biomolecule(s). We present comprehensive and critical analysis of ordered molecular assemblies formed by LB and self-assembly with potential applications to affinity biosensing. This critical review on fundamentals and application of ordered molecular assemblies to affinity biosensing is likely to benefit researchers working in this as well as related fields of research (401 references).     

聚苯乙烯-聚(4-乙烯基吡啶)嵌段共聚物LB膜结构形态的研究

采用氯仿作为铺展溶剂,将嵌段共聚物聚苯乙烯-聚(4-乙烯基吡啶)(PS-b-P4VP)稀溶液铺展于空气与水界面上,利用Langmuir-Blodgett(LB)膜技术转移至固体基底.研究了不同的嵌段比、表面压和小分子1-芘丁酸(PBA)的加入对嵌段共聚物气液界面聚集组装的影响.研究发现随着亲水段(P4VP)的增加,聚集组装结构由纳米片状、带状转变成纳米条状、纳米点状结构.表面压对纯PS-b-P4VP聚集组装产生影响,表面压增大,组装体排列紧密;随着表面压的继续增大,单层聚集结构遭到破坏,发生堆叠.加入PBA小分子后,PBA与PS-b-P4VP形成氢键,形态发生明显变化,原来的片状结构转变为条状或点状结构.     

Adsorption-induced asymmetric assembly from an achiral adsorbate

Symmetry breaking in the self-assembled monolayer (SAM) structure of 1-octadecanol on highly ordered pyrolytic graphite (HOPG) is observed. Due to the slight mismatch of the octadecanol molecule with the graphite lattice, the alkane chain undergoes distortion upon adsorption on the surface. The asymmetric distortion of the octadecanol SAM unit cell pair is observed by scanning tunneling microscopy at the liquid/solid interface. Asymmetric distortion is due to the requirement for planarity of the hydrogen bond connecting the two octadecanol molecules in the chevron-shaped unit cell. This very simple structure provides the first example of an adsorption-induced distortion to form a supramolecular asymmetric structure, which is formed by achiral molecules adsorbed on an achiral surface. What makes this system interesting and different from other examples of adsorption-induced chirality is that the adsorbate itself undergoes asymmetric distortion due to the existence of the substrate and the adsorbate conformation is different from the molecule in solution.     

Supramolecular ordering of tripod dyes at the air/water interface

Monolayers of 2-(3,4,5-(trisdodecyloxyl)phenyl)[1,3,4]oxadiazole based "tripod" dye, P2G, has been studied at the air/water interface with in situ X-ray reflectivity. Compression of the disordered Langmuir-Blodgett monolayer film induces a transition to a unique ordered phase, representing a supramolecular assembly with a unique spatial distribution and orientation of the molecules. At low pressure, the molecules having face-on orientation are interdigitated by the three arms. After first transition in the pi-A isotherm, the molecular conformation is turned into an edge-on orientation, where the molecules are self-assembled into supramolecular structures.     

Spontaneous formation of polypeptides in the interfacial thin films of amphiphilic amino acid esters: acceleration of the polycondensation and control of the structure of resultant polymers

For the purpose of elucidating the effects of molecular arrangements on the reaction rates and the structure of products, polycondensation of long-chain esters of alpha-amino acids in the monolayer on the water surface and the LB multilayers deposited on CaF2 plates were investigated by monitoring changes of the IR spectra. Spontaneous formation of the polypeptides occurs in the mono- and multilayers at room temperature without any catalyst. The rates of polycondensation in the monolayers are markedly influenced by the degree of molecular packing. Maximum polymerizability is obtained in the vicinity of the transition region from expanded to condensed films. The rates of polycondensation in the LB films are much higher than those in the bulk solids and the molten states. The polycondensation seems to be accelerated by regular arrangements of the monomer molecules in the LB films, where the functional groups are concentrated and situated more effectively for the reaction than in the bulk states. However, the polycondensation rates in the LB films are considerably slower when compared with those in the monolayers on the water surface kept at the optimum area or surface pressure, because the molecules in the LB films deposited under high compression are packed more closely than the optimum condition. Thus, suitably close packing of the monomer molecules, retaining a particular orientation together with some conformational freedom in the monolayer, is most favorable for the polycondensation. Two probable mechanisms for the polycondensation in the Y-type multilayers have been proposed. In the assembly of head-to-head double layers of the monomer molecules, the interlayer reaction propagates by sewing up the functional groups facing each other in the adjacent layers, and the polypeptide of a helical structure or random coil can be obtained. In contrast, for the alternating assembly of the amino acid ester and non-polymerizable octadecyl acetate, the polycondensation should proceed only in each single layer (intralayer reaction) and the polypeptide of the extended beta-form can be formed. In the case of dioctadecyl glutamate LB films, as well as the monolayer on water, the resultant polypeptide is the comb-like polymer with unreacted long-alkyl ester groups as side chains and abundant in the beta-form, indicating the dominant intralayer reaction. On the other hand, in the Y-type multilayer of the equimolar mixture of dioctadecyl glutamate (with two ester groups) and octadecyl ester of lysine (with two amino groups), both of the intra- and interlayer reactions occur effectively, resulting in a two-dimensional network structure of the polypeptide. In conclusion, not only the rate of polycondensation but also the higher-order structure of the resultant polypeptides can be controlled by organized arrangements of the monomer molecules in the interfacial thin films.     

Thermodynamic characteristics and Langmuir-Blodgett deposition behavior of mixed DPPA/DPPC monolayers at air/liquid interfaces

The role of dipalmitoylphosphatic acid (DPPA) as a transfer promoter to enhance the Langmuir-Blodgett (LB) deposition of a dipalmitoylphosphatidylcholine (DPPC) monolayer at air/liquid interfaces was investigated, and the effects of Ca2+ ions in the subphase were discussed. The miscibility of the two components at air/liquid interfaces was evaluated by surface pressure-area per molecule isotherms, thermodynamic analysis, and by the direct observation of Brewster angle microscopy (BAM). Multilayer LB deposition behavior of the mixed DPPA/DPPC monolayers was then studied by transferring the monolayers onto hydrophilic glass plates at a surface pressure of 30 mN/m. The results showed that the two components, DPPA and DPPC, were miscible in a monolayer on both subphases of pure water and 0.2 mM CaCl2 solution. However, an exception occurs between X(DPPA)=0.2 and 0.5 at air/CaCl2-solution interface, where a partially miscible monolayer with phase separation may occur. Negative deviations in the excess area analysis were found for the mixed monolayer system, indicating the existence of attractive interactions between DPPA and DPPC molecules in the monolayers. The monolayers were stable at the surface pressure of 30 mN/m for the following LB deposition as evaluated from the area relaxation behavior. It was found that the presence of Ca2+ ions had a stabilization effect for DPPA-rich monolayers, probably due to the association of negatively charged DPPA molecules with Ca2+ ions. Moreover, the Ca2+ ions may enhance the adhesion of DPPA polar groups to a glass surface and the interactions between DPPA polar groups in the multilayer LB film structure. As a result, Y-type multilayer LB films containing DPPC could be fabricated from the mixed DPPA/DPPC monolayers with the presence of Ca2+ ions.     

液晶聚合物的单层与Langmuir-Blodgett膜

系统研究了手性液晶聚硅氧烷和光致变色液晶聚硅氧烷两个毓的侧链液晶聚合物在空气/水界面的单层行为和Langmuir-Blodgett(LB)膜沉积特性,对LB膜结构与存在的聚集现象进行了系统的表征,并初步探讨了LB膜中液晶聚合物表现的功能性。     

The cis-bis(decanoate)tin phthalocyanine/DPPC film at the air/water interface

Films made of cis-bis-decanoate-tin(IV) phthalocyanine (PcSn10) and racemic dipalmitoylphosphatidylcholine (DPPC) are studied with compression isotherms and Brewster angle microscopy (BAM) at the air/water interface. Films enriched in PcSn10 present phase separation elliptical-shaped domains. These domains present optical anisotropy and molecular order. They are enriched in PcSn10, and the film outside these domains is enriched in DPPC, as shown in by high-angle annular dark-field transmission electron microscopy on Langmuir-Blodgett (LB) transferred films. Film collapse area and atomic force microscopy images of LB transferred films on mica indicate that the films are actually multilayers. A computational survey was performed to determine how the PcSn10 molecules prefer to self-assemble, in films basically made of PcSn10. The relative energetic stability for several dimeric assemblies was obtained, and a crystal model of the film was developed through packing and repeating the PcSn10 molecules, along the crystallographic directions of the unit cell. Our results contribute to understanding the strong interaction between PcSn10 and DPPC at the air/water interface, where even small quantities of DPPC (~1-2%) can modify the film in an important way.     

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.     

Self-organization of a sulfonamido-porphyrin in Langmuir monolayers and Langmuir-Blodgett films

Langmuir monolayers (LM) and Langmuir-Blodgett (LB) films of pure lipophilic meso-tetra(4-dodecylaminosulfophenyl)porphyrin (PC12) and mixed with the anionic surfactant sodium hexadecylsulfate (SHS) were studied. The molecular packing and structure of PC12 and PC12-4SHS with variable surface pressure were investigated by surface pressure-area measurements, steady-state absorption, fluorescence emission and anisotropy, as well as by fluorescence lifetime imaging microscopy (FLIM). At low surface pressure, the porphyrin molecules are organized with the rings tilted on the water surface whereas at high surface pressure the porphyrin rings achieve a more perpendicular arrangement. Using the FLIM images a gradual change of aggregates into large "islands" is observed. Different patterns are observed in the pure PC12 multilayer films (n = 3 and 5) with ordered patches superimposed which are not observed in the PC12-4SHS multilayer LB films.     

Synthesis and supramolecular self-assembly study of a novel porphyrin molecule in Langmuir and Langmuir-Blodgett films

The self-assembly and supramolecular engineering of porphyrins into ordered arrays have recently attracted much interest because of their promising application potential in molecular and electronic devices, spintronics, energy harvesting and storage, catalysis, and sensor development. We herein report the synthesis and supramolecular self-assembly study of a novel porphyrin molecule, 2Por-TAZ, in Langmuir and Langmuir-Blodgett films. The 2Por-TAZ molecule contains two porphyrin macrocycles attached to a triaminotriazine headgroup. Triaminotriazines are known to form a highly ordered linear supramolecular self-assembly through complementary hydrogen bonding with barbituric acid molecules at the air-water interface. Surface pressure-area isotherm measurements and polarized UV-vis absorption spectroscopic studies indicate that the 2Por-TAZ molecules adopted an edge-on orientation at the air-water interface. Polarized UV-vis absorption study also revealed that the 2Por-TAZ molecules formed linear supramolecular networks on pure water and barbituric acid subphase with porphyrin flat planes facing toward the compression direction. The binding of barbituric acid with 2Por-TAZ molecules was observed from the expansion of the Langmuir monolayer film. Compared to the transferred LB film from pure water subphase, both the UV-vis absorbance and fluorescence emission intensity of the LB film transferred from barbituric acid subphase increased significantly.     

Tetradecylferrocene: ordered molecular array of an organometallic amphiphile in the crystal and in a two-dimensional assembled structure on a surface

Tetradecylferrocene (4, Fc-(CH2)13CH3) was synthesized via lithiation of ferrocene by treatment with tert-butyl lithium, followed by alkylation with 1-bromotetradecane. Complex 4 forms a physisorbed ordered molecular monolayer on the surface of highly oriented pyrolytic graphite (HOPG) that was analyzed by scanning tunneling microscopy (STM). It features a lamellar structure with single rows of ferrocenyl moieties separating connecting areas formed by the long alkyl chains which are arranged parallel to each other. The ordering principle of 4 on the surface can be related to the arrangement of Fc-(CH2)13CH3 molecules in the three-dimensional crystal lattice.     

不同取代位置的萘衍生两亲分子在气/液界面组装膜中的螺旋结构与圆偏振发光

Langmuir-Blodgett(LB)技术可在二维气/液界面上精确地控制分子之间的排列及堆积方式, 构建有序超薄膜及纳米组装体. 同时, 界面这一不对称环境也可有效放大组装体的手性信息, 实现超分子体系中的手性传递和手性放大. 本文研究了萘环取代位置不同的两种手性两亲分子——N,N′-双十八烷基-α-萘-L-氨基-谷氨酸二酰胺(1NLG)和N,N′-双十八烷基-β-萘-L-氨基-谷氨酸二酰胺(2NLG)在气/液界面的铺展及组装行为, 发现同分异构效应影响了两亲分子在界面的排列, 1NLG组装形成了均一的纳米带状结构, 而2NLG则形成了左手螺旋结构, 并且2NLG薄膜表现出圆偏振发光(CPL)性质, 其不对称因子(g_lum)比三维体相组装体(超分子凝胶)大23倍, 表明界面促进了超分子手性的放大.     

β-联碳酰基类衍生物有序自组装膜的STM研究

在大气条件下, 利用扫描隧道显微镜研究了四个β-联碳酰基类衍生物在高定向裂解石墨(HOPG)表面的自组装结构. 研究分子的结构中均包含π电子共轭体系和烷基链. 实验研究了分子结构对自组装结构的影响, 并利用分子结构的变化实现了自组装膜结构的调控. 结果表明, 在甲苯溶剂中制备的这些自组装结构均长程有序, 分子间氢键和偶极相互作用是影响自组装膜结构变化的重要因素.     

Preparation and structure evolution of bowknot-like calcium carbonate particles in the presence of poly(sodium 4-styrene sulfate)

We determined how glucose or insulin interacts with a phospholipid monolayer at the air/water interface and explained these mechanisms from a physico-chemical point of view. The 1,2-dipalmitoyl-2-sn-glycero-3-phosphatidylcholine (DPPC) monolayer at an air/water interface acted as a model membrane, which allowed the effect of the molecular packing density in the monolayer on the interactions to be determined. The interaction of glucose, insulin, and a mixture of glucose and insulin to the DPPC monolayer were investigated via surface pressure-area per molecule Langmuir isotherms and fluorescence microscopy. Glucose adsorbed to the underside of the DPPC monolayer, while insulin was able to penetrate through the monolayer when the phospholipid molecules were not densely packed. The presence of a mixture of insulin and glucose affected the molecular packing in the DPPC monolayer differently than the pure insulin or glucose solutions, and the glucose-insulin mixture was seen to be able to penetrate through the monolayer. These results indicated that glucose and insulin interact with one another, giving a material that may then transported through a pore in the monolayer or through the spaces between the molecules of the monolayer.     

Fullerene nanowires: self-assembled structures of a low-molecular-weight organogelator fabricated by the Langmuir-Blodgett method

Fullerene derivative C60TT, which is substituted with the low-molecular-weight organogelator tris(dodecyloxy)benzamide, formed nanowire structures on application of the Langmuir-Blodgett (LB) method. The surface morphology of the C60TT LB film was dependent on the holding time before deposition at a surface pressure of 5 mN m(-1); it changed from a homogeneous monolayer to a bilayer fibrous structure via a fibrous monolayer structure, which was estimated to have dimensions of 1.2 nm in height, 8 nm in width, and 5-10 microm in length. From the structural and spectroscopic data, it is inferred that close packing of the fullerene moiety occurs along with intermolecular hydrogen bonding within the monolayer fibrous structure. The morphological changes in the LB film are explained kinetically by the Avrami theory, based on the decrease in the surface area of the monolayer at the air/water interface. The growth of the quasi-one-dimensional fibrous monolayer structures at holding times from 0 to 0.2 h is considered to be an interface-controlled process, whereas the growth of the quasi-one-dimensional bilayer fibrous structures from 0.2 to 18 h is thought to be a diffusion-controlled process.     

Films of novel mesogenic molecules at air-water and air-solid interfaces

Discotic molecules are known to form highly anisotropic structures at the air-water (A-W) interface. We have studied two novel ionic discotic mesogenic molecules, viz., pyridinium tethered with hexaalkoxytriphenylene with bromide counterion (Py-Tp) and imidazolium tethered with hexaalkoxytriphenylene with bromide counterion (Im-Tp) at A-W and air-solid interfaces. The monolayer phases were investigated at the A-W interface employing surface manometry and Brewster angle microscopy techniques. They indicate a uniform monolayer phase which shows negligible hysteresis on expanding and compressing. Also, in both the systems the collapsed state completely reverts to the monolayer state. These monolayer films transferred at different surface pressures by Langmuir-Blodgett technique were studied by employing atomic force microscopy. The topographies of these films transferred at the low and high surface pressure region of the isotherm indicate a transformation of the monolayer from face-on to edge-on structure.     

Aggregation and photoisomerization of amphiphilic crown-ether styryl dye in monolayers at the interface

The molecules oftrans-isomer of styryl dye1a, containing the benzo-18-crown-6 moiety and the methoxybenzothiazole residue with theN-octadecyl substituent, were shown to form stable, highly ordered monolayers at the water/air interface. Increasing the surface pressure of the monolayer results in the formation of the H-aggregated molecules of compound1a. The H-aggregates tend to dissociate to the monomerictrans-form, when the monolayer forms in the Langmuir-Blodgett film (LB film). The light irradiation of the LB films leads to the partially reversibletrans-cis-photoisomerization of compound1a, whose efficiency is much lower than that observed in an acetonitrile solution of1a. The aggregation of cis-1a isomers in the LB film is suggested to hamper the reversecis-trans-isomerization. The aggregation oftrans-1a molecules in the LB films does not occur in air, but, after dipping in an aqueous solution, molecules of the LB film slowly form J-aggregates. A model of the structural organization oftrans-1a molecules in the LB films was proposed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2490–2496, October, 1996.