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.     

Photo-induced structural changes of azobenzene Langmuir-Blodgett films

Structural changes of the Langmuir-Blodgett (LB) films of azobenzene accompanied by photoisomerization are described. First, photoisomerization is explained in terms of 'free volume'. In the polyion complex monolayers of amphiphiles having two azobenzene units at the air-water interface, the area per molecule depends on the polycation species. The fraction of cis-azobenzene in the LB films at the photostationary state under the illumination with UV light increased with increasing area per molecule, which is consistent with the concept of free volume. Second, a counter example of the concept of free volume is presented. Three-dimensional cone-shaped structures developed with trans-to-cis photoisomerization in the polyion complex LB film of a water-soluble amphiphilic azobenzene. These structures appeared and disappeared reversibly by alternate illumination with UV and visible light. The results indicate that the two-dimensional LB film structure exerts significant modification by photoisomerization. This is against the concept of free volume because this concept does not consider the possibility that the two-dimensional LB film structures may change into three-dimensional ones. Finally, photo-induced J-aggregate formation of non-photochromic and photochromic dyes is described. Two cyanine dyes were each mixed with an amphiphilic azobenzene in the LB films. These cyanine dyes are known to form J-aggregates in single-component LB films. In the mixed LB films, the J-aggregate formation was suppressed to some extent. The alternate illumination of the films with UV and visible light caused the photoisomerization of azobenzene in the mixed LB films, which triggered the J-aggregate formation of the cyanine dyes. The J-aggregate formation was accompanied by the development of three-dimensional cone-shaped structures from the film surface. When an amphiphilic merocyanine was mixed with the azobenzene in the LB films, J-aggregate formation was also induced by the alternate illumination with UV and visible light. This J-aggregate formation was also accompanied by a large morphological change: circular domains changed into fractal-like ones. The J-aggregate formation of the dyes and the concomitant morphological change were irreversible. In these cases, the photoisomerization of azobenzene served as a trigger to induce self-organization of the dye molecules.     

In situ spectroscopic characterization of rectifying molecular monolayers self-assembled on gold.

We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.     

Spacer-controlled aggregation and surface morphology of a selenacarbocyanine dye on gemini monolayers

The adsorption and aggregation of a selenacarbocyanine dye (3,3-disulfopropyl-9-methyl-selenacarbocyanine, SeCy) onto Langmuir monolayers of a series of gemini amphiphiles with different methylene spacers were investigated. When the monolayers of the gemini amphiphiles were spread on the dye-containing subphase, the dye could be easily adsorbed and aggregated onto the monolayers through the electrostatic and pi-pi interaction. The dye formed complexes with gemini amphiphiles and stacked as J-aggregates in all the transferred multilayers, regardless of the structure of gemini amphiphiles. However, the surface morphologies of the complex monolayers showed a significant dependence on the spacer length of the gemini amphiphiles and the temperature of the subphase. Nanorods were observed for the complex films with spacer lengths ranged from 4 to 10 methylenes. With the temperature of the subphases increased from 20 degrees C to 30 degrees C, aligned longer nanofibers were formed instead. Although both the gemini amphiphiles and the dye were achiral, strong circular dichroism (CD) signals were observed for the transferred complex films. However, the CD signals could be just opposite in different places of the transferred films, suggesting that a resolved enantiomeric micro/nanostructure coexisted in the films. On the other hand, for the complex film of the dye with the gemini amphiphile of two methylenes spacer, neither CD signal nor ordered surface morphologies were detected in any place of the film although the dye itself stilled formed J-aggregate in the film. It was suggested that regular nanoarchitectures and resolved chiral domains could be observed only when the spacer of gemini amphiphiles is comparative to the distance between the two SO3- groups in the dye.     

Preparation of ordered films containing a phenylene ethynylene oligomer by the Langmuir-Blodgett technique

This paper reports the fabrication and characterization of Langmuir and Langmuir-Blodgett (LB) films incorporating an oligo(phenylene-ethynylene) (OPE) derivative, namely, 4-[4-(4-hexyloxyphenylethynyl)-phenylethynyl]-benzoic acid (HBPEB). Conditions appropriate for deposition of monolayers of HBPEB at the air-water interface have been established and the resulting Langmuir films characterized by a combination of surface pressure and surface potential versus area per molecule isotherms, Brewster angle microscopy, and ultraviolet reflection spectroscopy. The Langmuir films are readily transferred onto solid substrates, and one-layer LB films transferred at several surface pressures onto mica substrates have been analyzed by means of atomic force microscopy, from which it can be concluded that 14 mN/m is an optimum surface pressure of transference, giving well-ordered homogeneous films without three-dimensional defects and a low surface roughness. The optical and emissive properties of the LB films have been determined with significant blue-shifted absorption spectra indicating formation of two-dimensional H aggregates and a Stokes shift illustrating the effects of the solid-like environment on the molecular chromophore.     

Enhanced optical and nonlinear optical responses in a polyelectrolyte templated Langmuir-Blodgett film

Optical and nonlinear optical properties like fluorescence and second harmonic generation (SHG) of molecular materials can be strongly influenced by the mode of assembly of the molecules. The Langmuir-Blodgett (LB) technique is an elegant route to the controlled assembly of molecules in ultrathin films, and complexation of ionic amphiphiles in the Langmuir film by polyions introduced in the aqueous subphase provides a simple and efficient access to further control, stabilization, and optimization. The monolayer LB film of the hemicyanine-based amphiphile, N-n-octadecyl-4-[2-(4-(N,N-ethyloctadecylamino)phenyl)ethenyl]pyridinium possessing a "tail-head-tail" structure, shows fluorescence as well as SHG response. The concomitant enhancement of both of these linear and nonlinear optical attributes is achieved through templating with the polyanion of carboxymethylcellulose. Brewster angle and atomic force microscopy reveal the influence of polyelectrolyte templating on the morphology of the Langmuir and LB films. Polarized absorption and fluorescence spectroscopy provide insight into the impact of complexation with the polyelectrolyte on the orientation and deaggregation of the hemicyanine headgroup leading to fluorescence and SHG enhancement in the LB film.     

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.     

电荷转移配合物薄膜制备方法和结构表征的研究进展

回顾了与Langmuir-Blodgett(LB)技术有关的电荷转移配合物薄膜的各类制备方法、结构表征结果,并比较了制备方法对薄膜结构的影响.例如,将LB膜C18H37TCNQ(电子受体)插入到电子给体3,3’,5,5’-tetramethylbenzidine(四甲基联苯胺, TMB)的石油醚溶液中进行掺杂,制备了TMB•C18H37TCNQ电荷转移配合物薄膜.在这种薄膜中,给体和受体以面对面的方式堆积,两者的环平面与基片平面接近垂直.而采用硬脂酸和C18H37TCNQ的混合LB膜通过类似的掺杂路线制备的TMB•C18H37TCNQ薄膜的结构发生了一些变化,例如其长的烃链C18H37更加垂直于基片平面.通过比较以前的各种实验结果可以得出以下结论:电荷转移配合物的结构可以通过制备方法得到控制.     

Polyelectrolyte templating strategy for the fabrication of multilayer hemicyanine Langmuir-Blodgett films showing enhanced and stable second harmonic generation

Polyelectrolyte templating effectively suppresses the aggregation of cationic hemicyanine-based amphiphiles in monolayer Langmuir-Blodgett (LB) films leading to enhanced and stable optical second harmonic generation (SHG). The current study explores the impact of different polyelectrolytes (salts of poly(4-styrenesulfonic acid), deoxyribonucleic acid, and carboxymethylcellulose) on the mode of formation of multilayer LB films of the hemicyanine amphiphile and their SHG response. Pressure-area isotherms and Brewster angle microscopy reveal the impact of the polyelectrolyte complexation on the Langmuir films. Transfer ratios observed during film deposition, supported by electronic absorption spectra and atomic force microscope images of the multilayer LB films, suggest that the polyanions influence the deposition sequence, leading to significant variations in the SHG. Carboxymethylcellulose is identified as an optimal template that induces favorable z-type deposition, leading to the formation of stable multilayer films. These films exhibit the expected quadratic increase of SHG with the extent of deposition; significantly the film response is very stable under extended laser irradiation. It is proposed that structural adjustments of the sandwiched polymer layer lead to the observed deposition sequence and film stability. Polyelectrolyte templating is demonstrated to be a simple and effective strategy for the fabrication of multilayer LB films to elicit efficient quadratic nonlinear optical response.     

Mode of interaction of two fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC) at the air-water interface

Two-component Langmuir monolayers formed on 0.02M Tris buffer solution (pH 7.4) with 0.13M NaCl at 298.2K were investigated for two different fluorinated-hydrogenated hybrid amphiphiles (F6PH5PPhNa and F8PH5PPhNa or F6 and F8, respectively) with DPPC. Surface pressure (pi), surface potential (DeltaV) and dipole moment (mu( perpendicular)) as a function of molecular surface area (A) were measured by employing the Whilhelmy method and an ionizing electrode method. From the A- and DeltaV-X(F6) (or X(F8)) curves, partial molecular surface area (PMA) and apparent partial molecular surface potential (APSP) were determined as a function of surface mole fraction (X(Fn)) at discrete surface pressures. Then, the behavior of occupied surface areas and surface potentials of the respective components could be made clearer. Compressibility (C(s)), elasticity (C(s)(-1)), and excess Gibbs energy (DeltaG((ex))) as a function of X(F6) (or X(F8)) were estimated at definite pressures. These physico-chemical parameters were found to reflect the mechanical strength of monolayer films formed. The regular solution theory being applied to DeltaG((ex)), the activity coefficients (f) as well as the interaction parameter (I(p)) between DPPC and two hybrid amphiphiles in the binary monolayers were evaluated. I(p) values thus obtained indicated that F8 molecules interact more strongly with DPPC molecules than F6. Moreover, in order to better understand the morphological monolayer state, Langmuir-Blodgett (LB) films made from DPPC and fluorinated-hydrogenated hybrid amphiphiles were examined by atomic force microscopy (AFM). The miscibility of the two components in the monolayer state is evidenced by these thermodynamic quantities and AFM observations. Furthermore, AFM images demonstrated that F8 could more effectively disperse the ordered domains of DPPC than F6.     

Two-dimensional arrays of amphiphilic Zn2+-cyclens for guided molecular recognition at interfaces

The sterically guided molecular recognition of nucleobases, phosphates, adenosine, and uridine nucleotides on Langmuir monolayers and Langmuir-Blodgett monolayers of amphiphilic mono- or bis(Zn2+-cyclen)s assembled on thiolated surfaces was investigated. The stepwise selective binding of metal ions, uracil, or phosphate by dicetyl cyclen monolayers with variously tuned structures at the air/water interface was corroborated by the measurements of the corresponding LB films deposited onto quartz crystals. Two types of recognition surfaces were fabricated from Zn2+-dicetyl cyclen. The surface covered with a complex preformed in the Langmuir monolayer was capable both of imide and of phosphate binding. The similar complex formed directly in an LB film on thiolated gold was inactive with respect to imide. The surface plasmon resonance measurements evidenced the stepwise assembly of complementary nucleotides on SAM/LB templates through consecutive phosphate-Zn2+-cyclen coordination. Base pairing between nucleotides resulted in a formation of A-U bilayers comprising two complementary monolayers. Finally, we report on SAM/LB patterns designed for divalent molecular recognition of uridine phosphate by amphiphilic bis(Zn2+-cyclen).     

Self-organization of a wedge-shaped surfactant in monolayers and multilayers

The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmethylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at approximately 10 degrees C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are approximately 0.5 nm thinner than the LC regions. AFM images were also obtained of self-assembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed self-assembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles.     

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.     

Langmuir-Blodgett film of hydrophobin protein from Pleurotus ostreatus at the air-water interface

We present results concerning the formation of Langmuir-Blodgett (LB) films of a class I hydrophobin from Pleurotus ostreatus at the air-water interface, and their structure as Langmuir-Blodgett (LB) films when deposited on silicon substrates. LB films of the hydrophobin were investigated by atomic force microscopy (AFM). We observed that the compressed film at the air-water interface exhibits a molecular depletion even at low surface pressure. In order to estimate the surface molecular concentration, we fit the experimental isotherm with Volmer's equation describing the equation of state for molecular monolayers. We found that about (1)/ 10 of the molecules contribute to the surface film formation. When transferred on silicon substrates, compact and uniform monomolecular layers about 2.5 nm thick, comparable to a typical molecular size, were observed. The monolayers coexist with protein aggregates, under the typical rodlet form with a uniform thickness of about 5.0 nm. The observed rodlets appear to be a hydrophilic bilayer and can then be responsible for the surface molecular depletion.     

不含亲水基团的偶氮苯衍生物L-B膜的制备和性质研究

本文合成了具有一定结构特点的偶氮苯的衍生物, 以元素分析和^1HNMR鉴定了B产物的组成和结构。采用L-B技术制备了其多层L-B膜, 以紫外可见光谱研究了膜中分子的聚集状态和膜的纵向均匀性; 以偏振紫外确定了膜中分子偶氮基的取向; L-B膜的小角度X射线衍射结果表明膜具有层状有序的周期性结构。     

Second harmonic generation from insoluble films of a rhodamine dye at the air-water interface: effect of sodium dodecylsulfate

The second harmonic generation (SHG) from the insoluble monolayers of bis-(N-ethyl, N-octadecyl)rhodamine perchlorate (RhC18) formed on the surface of sodium dodecylsulfate (SDS) solutions of different concentrations has been studied. An enhancement of the second harmonic response was observed in the mixed films of RhC18/SDS compared to the pure-dye layer. To clarify the origin of the phenomenon, the films were characterized by surface pressure-area isotherm and reflection-absorption spectroscopy studies. The analysis of surface pressure-area isotherms of RhC18/SDS layers showed that incorporated SDS molecules essentially influence the rheological properties of the dye monolayer. The film parameters, such as the molecular surface area, maximum surface pressure, and solid-condensed phase composition, are the functions of SDS bulk concentration. A joint analysis of the SHG results and the reflection-absorption spectra revealed that the structural ordering within films was responsible for the enhancement of the nonlinear optical response, whereas the contributions from the spectral shifts and increased absorption upon aggregate formation are of less importance.     

Langmuir monolayers based on rigid wedge-shaped dendrons of benzenesulfonic acid

The structure formation of wedge-shaped monodendrons based on symmetric benzenesulfonic acid with different lengths of peripheral alkyl chains was studied in Langmuir monolayers and Langmuir–Blodgett (LB) films. A phase transition from the liquid-expanded state to the liquid-condensed state was observed on compression of the Langmuir monolayers of the dendrons containing dodecyl lateral chains. The transition is accompanied by the formation of star-shaped aggregates visualized by Brewster angle microscopy. The three-layer LB transfer results in the reorganization of the monolayer into regions of bi-, tetra-, and hexalayers on a solid substrate with a low coverage of the surface. Homogeneous liquid-condensed mono layers are formed for the dendrons with hexa- and octadecyl chains, and the film thickness achieved by the LB transfer corresponds to the monolayer alignment of the molecules with the surface coverage up to 90%. It was determined that varying the alkyl length of wedge-shaped dendrones based on symmetric benzenesulfonic acid leads to a change in phase behavior of Langmuir monolayers as well as Langmuir–Blodgett films formed by them.     

Experimental confirmation of the importance of orientation in the anomalous chiral sensitivity of second harmonic generation

Macromolecular interactions were demonstrated to yield large chiroptical effects in second harmonic generation measurements of ultrathin surface films. Second harmonic generation (SHG) has recently shown to be several orders of magnitude more sensitive to chirality in oriented systems than common linear methods, including absorbance circular dichroism (CD) and optical rotary dispersion (ORD). Numerous mechanisms have been developed to explain this anomalous sensitivity, with a general emphasis on understanding the molecular origins of the chromophore chirality. In this work, orientational effects alone are shown to be the dominant factor for generating large SHG chiral dichroic ratios in many surface systems. Three distinct uniaxial surface films of SHG-active achiral chromophores oriented at chiral templated surfaces were observed to yield chiral dichroic ratios as great as 40% in magnitude.     

Surface enhanced resonance Raman scattering imaging of Langmuir-Blodgett monolayers of bis (benzimidazo) thioperylene.

The synthesis, spectroscopic characterization and surface-enhanced spectroscopy of a new electro active organic material bis (benzimidazo) thioperylene (Monothio BZP) are reported. Langmuir monolayers of Monothio BZP were successfully formed on water subphase and characterized by the pi-A surface-pressure area isotherm. Langmuir-Blodgett (LB) monomolecular layers of Monothio BZP were fabricated onto glass substrates and onto silver island films for surface-enhanced spectroscopic studies. The results of surface-enhanced resonance Raman scattering (SERRS), SERRS imaging and surface-enhanced fluorescence (SEF) studies for Monothio BZP LB monolayers are reported. Raman imaging (global imaging and point-by-point mapping) of the SERRS signal for a single monomolecular layer on silver islands were obtained using the 514.5 nm laser line. The SERRS imaging permits a visualization of the variation of the SERRS intensity across of the rough metal surface. The SEF was recorded for the excimer emission of aggregates in the LB film. The distance dependence and the enhancement factor of SEF were determined using fatty acid spacing layers. A temperature dependence study of the LB monolayer SERRS and SEF spectra was carried out between -190 degrees and + 200 degrees C confirming the thermal stability of the LB monolayer on silver. The specificity and the sensitivity of SERRS signal on metal island films was probed using mixed LB films with 0.01% molecular ratio of Monothio BZP in Arachidic Acid (AA). The micro-Raman SERRS spectra from ca. 10(-3) attomole of the dye were recorded.     

Investigation of alignment mechanism of liquid crystal on polyimide Langmuir-Blodgett films by scanning tunnelling microscopy

The homogeneous alignment of a liquid crystal material, 4'-n-octyl-4-cyanobiphenyl (8CB), was achieved by polyimide Langmuir-Blodgett (LB) films. Scanning electron microscopy and scanning tunnelling microscopy measurements show that the alignment of the polyimide LB films with a grooveless surface occurs due to the orientation of the polyimide chains. We directly observe 8CB monolayers on oriented polyimide LB films. We find that the monolayers form a two dimensional crystalline structure and the 8CB molecules are always aligned along the chain direction of the polyimide. The results show that the alignment of 8CB molecules arises by an epitaxial growth on the oriented polyimide LB films.