Interfacial assembly of an achiral zinc phthalocyanine at the air/water interface: a surface pressure dependent aggregation and supramolecular chirality

The aggregation and supramolecular chirality of the interfacial assemblies of an achiral phthalcyanine derivative, zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29 H,31 H-phthaloxyanine (ZnPc), were investigated, and a surface pressure dependent behavior was observed. It was found that ZnPc could be spread as a Langmuir film on water surface and transferred onto solid substrates by the horizontal lifting method. The compound formed mixed J- and H-aggregates in the transferred Langmuir-Blodgett (LB) films. Deconvolution of the broaden Q-band revealed three main components of the spectra, which corresponded to H- and J-aggregates and medium transition aggregates, whose relative contents could be modulated by the surface pressure at which the films were transferred. On the other hand, the transferred LB films composed of these aggregates showed Cotton effects in circular dichroism spectra when the floating film was compressed over a certain surface pressure although the compound itself was achiral. The cooperative arrangement of the macrocylic ring in a helical manner through the interfacial organization was suggested to be responsible for such optical activity in the LB films. A possible explanation based on the cooperative arrangement of the ZnPc building blocks in a helical sense stacking in the aggregates was proposed.     

J-aggregation of a water-soluble tetracationic porphyrin in mixed LB films with a calix[8]arene carboxylic acid derivative

The molecular organization of a mixed film, containing a water-soluble tetracationic porphyrin (TMPyP) and a p-tert-butyl calix[8]arene octacarboxylic acid derivative (C8A), at the air-water interface and on a solid support (LB film), has been investigated. Although the TMPyP aggregation was not detected at the air-water interface, TMPyP J-aggregates have been found in the LB films (Y-type). Unlike tetraanionic porphyrins, for example TSPP, the TMPyP J-aggregates are not induced by a zwitterion formation. The TMPyP J-aggregation is a result of a "double comb" configuration, where porphyrins from opposite layers are interwoven in a linear infinite J-aggregate. Our results confirm that TMPyP molecules tend to self-aggregate strongly, provided the electrostatic repulsions of their peripheral groups are cancelled by the anionic groups of the C8A matrix.     

Photoreactions and lateral patterning in Langmuir and Langmuir-Blodgett films

Reversible morphological changes occur with photoisomerization of azobenzene in Langmuir-Blodgett (LB) films complexed with polycations, which contradicts an implicit assumption of the concept of free volume that two-dimensional film structures are preserved during the photoisomerization. J-aggregates of chromophores are formed by two processes. The first process is "light-induced J-aggregation" in which photoisomerized molecules form J-aggregates. The other process is "triggered J-aggregation," in which photoisomerization of one of the components triggers J-aggregation of another chemical species in the mixed films. Both processes of J-aggregation are in many cases accompanied by large morphological changes of the films. However, LB films fabricated using processes under isobaric conditions do not change their morphology during light-induced J-aggregation and are patterned with J-aggregates using ultraviolet illumination through a photomask. Phase separation in mixed LB films gives rise to two-dimensional patterns, which are used to fabricate templates by using an amphiphilic silane-coupling agent as one of the components in the mixed LB films. Nanopatterns are also fabricated.     

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.     

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.     

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.     

Molecular design of "super" hydrogelators: understanding the gelation process of azobenzene-based sugar derivatives in water

[structure: see text]. 1: R = beta-D-glucopyranoside. 2: R = alpha-D-glucopyranoside. 3: R = alpha-D-galactopyranoside. 4: R = alpha-D-mannopyranoside. As an attempt to rationally design aqueous organogelators, a bolaamphiphilic azobenzene derivative (1) bearing two sugar groups was synthesized. Compound 1 formed a gel in water even at concentrations as low as 0.05 wt % (0.65 mM). Spectroscopic studies and electron-micrographic observations have clarified the gel structure and the origin of the gelation ability for water.     

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.     

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.     

Langmuir-Blodgett Films from Schiff Base Aluminium (Ⅲ) Complex

The characteristics of the LB films of Schiff base aluminium(Ⅲ), tris(2-hydroxy-5-nitro-N-dodecyl-benzylideneaminato) aluminium(Ⅲ)[Al(TA12)_3], were studied. The surface pressure-area(π-A) isotherm of Al(TA12)_3 in the pure water subphase was investigated. The molecular area, 0.48 nm~2, is one-third of the expected value that indicates the formation of an aggregate. The Langmuir-Blodgett(LB) films of Al(TA12)_3 were transferred and characterized. The UV-Vis spectra and the AFM image both confirmed that the J-aggregates formed. The polarized UV-Vis spectra indicated that the complex plane had to be oriented with an angle of about 30° to the substrate surface. The IR spectra suggested that the complexation took place between aluminium ions and the oxygen atoms of the ligand rather than the nitrogen atom.     

J-aggregation and its characterization in Langmuir-Blodgett films of merocyanine dyes

Langmuir-Blodgett (LB) films are constructed by successively transferring monomolecular layers formed at the air-water interface onto solid substrates. One of the advantages of the LB technique in fabricating molecular aggregates lies in the fact that it can employ various kinds of molecules by mixing them at the air-water interface. The mixed system may exhibit new properties that are not observed for individual components. This method would be useful, for example, in the studies of the formation and control of the J-aggregates of functional dyes that attract attention both in science and technology. In this paper, I review this subject mainly based on our recent results in merocyanines. LB films of merocyanine dyes, mixed with arachidic acid (C(20)), exhibit J-aggregate formation and have been serving as typical systems in revealing the physical and structural aspects of nanosized molecular aggregates constructed as monolayers. In the case of LB films of a merocyanine dye having benzothiazole as donor nucleus (abbreviated as DS), electron spin resonance (ESR) spectroscopy has been successful in determining the characteristic in-plane orientation of dye molecules with respect to the dipping direction, which led to the discovery of the flow orientation effect during the dipping process of LB films as the origin of optical dichroism often observed in LB films. In this article, after an introduction of ESR spectroscopy, three major topics on the merocyanine J-aggregation and its characterization in mixed films are discussed. The first topic is the observation and control of the size of J-aggregates in the dilution limit of dyes in arachidic acid matrix for a methyl-substituted DS (6-Me-DS). Dependence of atomic force microscopy (AFM) patterns on the molar ratio allows the identification of dye domains. J-band optical peak analysis based on the Kuhn's extended dipole model, supplemented by a novel application of femtosecond pump-probe optical spectroscopy, yields the size of the J-aggregates of 10(3). The second topic is the control of the J-band peak wavelengths by mixing two different kinds of dye molecules. The first case is the mixture of a J-forming 6-Me-DS and non-J-forming merocyanine analog, DO with benzo-oxazole instead of benzothiazole of DS. The second case is the mixture of both J-forming dyes but with different J-band peak positions, 6-Me-DS and another analog of 5-Cl-DS. The optical peak shifts depending on the molar mixing ratio will be presented. The last topic is related to the elucidation of electronic states of dye molecules in the J-aggregates. Light-induced ESR (LESR) of DS films with stable isotope ((15)N or (13)C)-substituted dyes provide clear evidence for the photoinduced charge transfer by the detection of hyperfine structures. Moreover, infrared (IR) spectroscopy of (13)C-enriched dye identifies the IR absorption peak of the relevant carbon in the chromophore. The results give evidence for the enhanced intramolecular charge transfer of dyes in the J-aggregates compared with an isolated merocyanine composed of donor and acceptor moiety. Lastly, the Cl attachment in 5-Cl-DS leads to a significant enhancement of the nitrogen hyperfine coupling in the LESR spectra. These examples and others demonstrate the potential of LB films of merocyanines in the studies of the nanosized molecular aggregates in monolayer assemblies.     

Experimental and theoretical study on molecular aggregation and its effect on the photo-physical properties of the mesogenic bi-1,3,4-thiadiazole derivative

A bi-thiadiazole derivative, 5,5'-bis(4- tetradecyloxyphenyl)-2,2'-bi-1,3,4-thiadiazole (BTD-14), was revealed to exhibit an extremely stable thermotropic SmC phase and very interesting aggregation behavior in solutions. H- and J-aggregates could be formed simultaneously in chloroform solutions of BTD-14 with moderate concentration (10(-4) M), and the population of J-aggregates enlarges during further concentration increase. All monomers, H-aggregates and J-aggregates in solutions could be reserved in the drop-cast films, and both the presence of J-aggregates and the energy transfer path from H-aggregates to J-aggregates were considered to contribute to the relative high solid state fluorescence quantum yield (33%). The BTD-1 dimer potential energy surface (PES) was computed with M062x/6-31G** method, and the molecular packing pattern corresponding to the lowest minimum of the PES are in good agreement with the crystal structures. Exploring the effect of molecular packing on its electronic structure with the TD-M062x method revealed that J-aggregates could be formed by enlarging the intermolecular displacement along the molecular long axis by about 9.8 ?.     

Thin films of hydrophobically modified poly(N,N-dimethylacrylamide)

The behavior of a poly(N,N-dimethylacrylamide) hydrophobically modified by incorporating 0.33 mol % of a pyrenyl derivative, [4-(1-pyrenyl)butyl]amine hydrochloride (PY) and 3.56 mol % of dodecylamine (DO) has been studied at the air/water interface. Surface pressure-area isotherm measurements show that the film is initially anchored by the hydrophobic groups at the air-water interface with a pancake-like structure and, with increasing surface pressure, evolves to a quasi mushroom structure, finally reaching a brush configuration at high pressures. Monolayers of this polymer were transferred to silica substrates using the Langmuir-Blodgett (LB) technique at 5, 15, and 25 mN.m(-1). The properties of the LB films were studied by steady-state and time-resolved fluorescence as well as by atomic force microscopy. The results show that the aggregates formed at low pressures are disrupted by pressure increase, while the water-soluble poly(N,N-dimethylacrylamide) becomes dissolved in the water subphase.     

Structure of thermally treated oxadiazoleamide Langmuir-Blodgett films

The thermal treatment of Y-type Langmuir-Blodgett (LB) films formed from the amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole 1 results in changes of the molecular packing. These changes have been analysed by a combination of X-ray specular reflectivity data, X-ray grazing incidence diffraction data and scanning force microscopy images. On the basis of these experimental data we have simulated possible supramolecular structures. These simulations provide insight into the intermolecular interactions giving rise to the observed structural transitions. The crystalline structure induced by thermal treatment of the LB films is chracterized by a uniaxial texture, which is correlated with the dipping direction during deposition of the LB film.     

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.     

Conversion of the aggregation state of merocyanine dye, modification of the subcell packing of arachidic acid, and removal of the majority of n-octadecane by hydrothermal treatment in the liquid phase in a mixed Langmuir-Blodgett film of the ternary system

We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir-Blodgett (LB) film of merocyanine dye with an octadecyl group (MS(18))-arachidic acid (C(20))-n-octadecane (AL(18)) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS(18) hydrocarbon chain, the slight orientation change in the C(20) hydrocarbon chain, and the complete evaporation of AL(18). The dissociation of MS(18) is probably ascribed to the complete evaporation of AL(18) from the mixed LB film and the increase in thermal mobility of the long axis of the MS(18) hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS(18) aggregation state from H- to J-aggregates, the modification of the C(20) subcell packing from hexagonal to orthorhombic, and the removal of most of the AL(18) molecules. The conversion of the MS(18) aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS(18) aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL(18) molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS(18) permanent dipole interaction. Moreover, the modification of the C(20) subcell packing is possibly due to the hydrophobic effect, where the C(20) hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTL is quite effective to reorganize the chromophore alignment of MS(18), to modify the subcell packing of C(20) and to erase the majority of AL(18) molecules in the mixed LB film of the MS(18)-C(20)-AL(18) ternary system in a short time.     

Perfluorophenyl-Directed Giant Porphyrin J-Aggregates

Quadrupolar interactions of porphyrin bearing two pentafluorophenylethynyl terminals ( 1 ) drove the formation of a successive one-dimensional staircase structure, i.e., J-aggregates, to yield millimeter-length needles with a single-crystalline character in methylcyclohexane solution. In contrast, π-stacked interactions of porphyrin bearing two nonfluorinated phenyl terminals ( 2 ) formed no aggregates in solution. A spin-cast film of 1 also showed bathochromic shift of the Soret and Q bands, indicating the formation of J-aggregates. The molecular arrangement of the J-aggregates was revealed by microbeam glazing-incidence wide-angle X-ray diffraction (GIWAXD), and was in good agreement with the optimized structure generated by density functional theory (DFT) calculations.     

In-situ infrared spectroscopy of buried organic monolayers: influence of the substrate on titanium reactivity with a Langmuir-Blodgett film

The reactivity of metals vapor deposited onto organic monolayers has historically been correlated to the metal/terminal organic group chemistry. Here we demonstrate that the chemical composition of the substrate unexpectedly plays a significant role as well. In particular, the reactivity of evaporated titanium toward a cadmium stearate Langmuir-Blodgett (LB) film was found to depend on the substrate upon which the LB film was deposited. Infrared spectra taken in a modified ATR (Kretschmann) geometry with a thin Au substrate showed large changes in peak shape, peak position, and peak width in the C-H stretching region, indicating titanium penetration into the LB film and decomposition of the original well-packed monolayer structure. LB monolayers formed on a platinum oxide (PtO(x)) surface showed remarkably small changes after Ti deposition, indicating only a slight increase in disorder and no significant metal penetration into the film. Films on SiO2 substrates showed reactivity between that of Au and PtO(x). These differences in reactivity can be correlated primarily with the amount of available oxygen associated with each substrate, including surface oxide layers and water incorporated within the LB film.     

Monolayer behavior of a rare-earth bisphthalocyanine derivative and determination of its molecular orientation in an LB film

In this paper, the monolayer behavior of a new symmetrically substituted ytterbium bisphthalocyanine derivative Yb-octa-4-(2,4-di-tert-pentylphenoxyl) bis phthalocyanine was investigated, its phase change morphology at the air/water interface was directly observed by Brewster Angle Microscopy (BAM). Its molecular orientation in an LB film was determined by polarized UV–Vis spectra.     

Self-assembled lipid nanotube hosts: The dimension control for encapsulation of nanometer-scale guest substances

Supramolecular nanotube hosts with precisely controlled inner or outer diameters have been synthesized by self-assembly of unsymmetrical bolaamphiphilic monomers or glucopyranosylamide lipids, respectively. Time-resolved fluorescent measurement using 8-anilinonaphthalene-1-sulfonate (ANS) as a probe revealed that the water confined in a cardanyl-β-D -glucopyranoside lipid nanotube has relatively lower solvent polarity corresponding to that of propanol than bulk water. Extensively developed hydrogen bond networks also characterize the confined water in comparison to the case in bulk water. Encapsulation ability of the glucopyranosylamide lipid nanotube has been examined by filling the lyophilized LNTs with gold or silver nanoparticles, ferritin, or magnetic crystals. Filling the unsymmetrical bolaamphiphile nanotube possessing positively charged inner surfaces with negatively charged polymer beads or ferritin proved to be successful without depending on capillary action. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5137–5152, 2006