Electrostatic self-assembly of a macrocyclic amphiphile for inducing the flat orientation of chromophores

In this article, a new methodological approach to the fabrication of organized molecular films with chromophores lying flat on the film surface has been investigated with the design of an azacrown-type multication, 1,4,10-[3-(4-(4'-methoxy-phenylazo)-2-nitro-phenoxy)propyl]-1,4,7,10,13,16-hexamethylhexaazacyclooctadecane, and LBL film deposition by using the electrostatic self-assembly method. The chromophore alignment was analyzed on the basis of the polarized UV-visible spectra: the tilt angle of the transition moment of trans-azobenzene with respect to the surface normal was determined to 64 +/- 2 degrees , indicating a strong trend for chromophores in the multilayered films to planar alignment.     

Self-assembled electrooptic thin films with remarkably blue-shifted optical absorption based on an X-shaped chromophore

An "X-shaped" two-dimensional electrooptic (EO) chromophore with extended orthogonal conjugation was designed and synthesized. Self-assembled thin films of this chromophore were fabricated via layer-by-layer chemisorptive siloxane-based self-assembly. The films exhibit a dramatically blue-shifted optical maximum (325 nm) while maintaining a large EO response (chi33(2) approximately 232 pm/V at 1064 nm; r33 approximately 43 pm/V at 1550 nm).     

Layer-by-layer deposited organic/inorganic hybrid multilayer films containing noncentrosymmetrically orientated azobenzene chromophores

Organic/inorganic hybrid multilayer films with noncentrosymmetrically orientated azobenzene chromophores were fabricated by the sequential deposition of ZrO2 layers by a surface sol-gel process and subsequent layer-by-layer (LbL) adsorption of the nonlinear optical (NLO)-active azobenzene-containing polyanion PAC-azoBNS and poly(diallyldimethylammonium chloride) (PDDA). Noncentrosymmetric orientation of the NLO-active azobenzene chromophores was achieved because of the strong repulsion between the negatively charged ZrO(2) and the sulfonate groups of the azobenzene chromophore in PAC-azoBNS. Regular deposition of ZrO(2)/PAC-azoBNS/PDDA multilayer films was verified by UV-vis absorption spectroscopy and quartz crystal microbalance measurements. Both UV-vis absorption spectroscopy and transmission second harmonic generation (SHG) measurements confirmed the noncentrosymmetric orientation of the azobenzene chromophores in the as-prepared ZrO2/PAC-azoBNS/PDDA multilayer films. The square root of the SHG signal (I(2omega)(1/2)) increases with the increase of the azobenzene graft ratio in PAC-azoBNS as the number of deposition cycles of the ZrO(2)/PAC-azoBNS/PDDA films remains the same, while the second-order susceptibility chi(zzz)(2) of the film decreases with the increase of the azobenzene graft ratio. Furthermore, the present method was successfully extended to realize the noncentrosymmetric orientation of azobenzene chromophores in multilayer films when small organic azobenzene compounds with carboxylic acid and/or hydroxyl groups at one end and sulfonate groups at the other end were used. The present method was characterized by its simplicity and flexibility in film preparation, and it is anticipated to be a facile way to fabricate second-order nonlinear optical film materials.     

Dye-polyoxometalate composite films: self-assembly, thermal and photochemical properties

A series of dye-polyoxometalate composite films were prepared by alternately depositing cationic dye molecules and anionic polyoxometalates such as Keggin-type [BW(12)O(40)](5-) and the sandwich complex [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-)via layer-by-layer (LbL) self-assembly method. These cationic dye molecules (MB, AA, TH, BB3, BCB and NB) are heterocyclic planar and rigid phenothiazine and phenoxazine dye molecules with different substituting groups in the side chains. The self-assembly of the films was studied by UV-vis and IR spectra. The results show that the substituting groups of dye molecules such as NH(2) and CH(2)CH(3) have influence on the self-assembly properties. The continuous and regular growth of the films was also dependent upon hydrogen bonding (NHO) formed between the amino groups of dye molecules and oxygen atoms of POMs as well as electrostatic interactions. The investigation of thermal and photochemical treatments of the composite films is also presented. The thermal stability experiments indicate that the composite films of TH with two NH(2) substitute groups and NB with more pi-conjugated system exhibit high thermal stability, whereas the sunlight irradiation results indicate that the composite films of TH have good photochemical stability.     

Study of frictional properties of a phospholipid bilayer in a liquid environment with lateral force microscopy as a function of NaCl concentration

Friction properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-supported planar bilayers deposited on mica were tested in a liquid environment by lateral force microscopy. The presence of these bilayers was detected by imaging and force measurements with atomic force microscopy. To test how the presence of NaCl affects the frictional properties of the phospholipid bilayers, four DMPC bilayers were prepared on mica in saline media ranging from 0 to 0.1 M NaCl. Changes in the lateral vs vertical force curves were recorded as a function of NaCl concentration and related to structural changes induced in the DMPC bilayer by electrolyte ions. Three friction regimes were observed as the vertical force exerted by the tip on the bilayer increased. To relate the friction response to the structure of the DMPC bilayer, topographic images were recorded at the same time as friction data. Ions in solution screened charges present in DMPC polar heads, leading to more compact bilayers. As a consequence, the vertical force at which the bilayer broke during friction experiments increased with NaCl concentration. In addition, the topographic images showed that low-NaCl-concentration bilayers recover more easily due to the low cohesion between phospholipid molecules.     

Investigation of dipole alignments by swelling in poled thin films of a side-group polymer using second harmonic generation

This paper deals with relations between solvent diffusion and changes in second harmonic generation (SHG) in a side-group polymer with nonlinear optical (NLO) chromophores. The SHG signal of poled polymer thin films was measured as a function of time during the swelling process with methanol. The results indicated that the diffusion process was Fickian. The topographic results obtained by atomic force microscopy, together with the SHG data, also demonstrated the existence of the domain layer formed during poling that gave rise to the principal SHG intensity for this sample. Also, a noncentrosymmetric chromophore ordering was induced by swelling in the absence of a poling electric field. This time-varying SHG signal upon swelling was discussed in terms of dipolar and polar alignments of the side-group NLO chromophores. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3108–3114, 1999     

Fabrication of titanium dioxide and tungstophosphate nanocomposite films and their photocatalytic degradation for methyl orange

Photocatalytic multilayer films with different numbers of bilayers were prepared via an electrostatic layer-by-layer (LbL) self-assembly method. These LbL films were characterized by UV-vis spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). Our results indicate that TiO(2) and tungstophosphate (H(3)PW(12)O(40), abbreviated as PW(12)) are successfully incorporated into the thin films. The as-prepared (TiO(2)/PW(12))(n) films show good photocatalytic performance toward methyl orange (MO) solution at pH 2.0, which is attributed to the synergistic effect between TiO(2) and PW(12). The effect of experimental parameters including number of bilayers, initial concentration, and pH value of dye solution were also studied. The multilayer films can be easily recovered and reused several times with little change of degradation, indicating that they are stable under the ultraviolet (UV) irradiation. The detection of active species displays that active holes (h(+)) play a dominant role for MO photodegradation in the TiO(2)/PW(12) system. Taking advantage of immobilization of catalysts on glass slides, the problem of recovery is solved. It is expected that photocatalytic multilayer films have substantial applications in industry.     

Layer-by-layer self-assembled ultrathin multilayer films of lanthanide polyoxometalates and poly(allylamine hydrochloride) and their photoluminescent properties

Ultrathin multilayer films of two lanthanide polyoxometalates (LPOMs), K(17)[Eu(P(2)W(17)O(61))(2)] (EPW) and K(13)[Eu(SiW(11)O(39))(2)] (ESW), and poly(allylamine hydrochloride) (PAH) have been prepared by layer-by-layer self-assembly from dilute aqueous solutions. UV-vis spectroscopy and ellipsometry respectively show that the absorbance values at characteristic wavelengths and the thicknesses of the multilayer films increase linearly with the number of LPOM/PAH bilayers, suggesting that the deposition process is linear and highly reproducible from layer to layer. Average thicknesses of ca. 3.4 and 2.4 nm were determined for the EPW/PAH and ESW/PAH bilayers by ellipsometry, respectively. In addition, scanning electron microscopy (SEM) and atomic force microscopy (AFM) images provide the surface morphology of the LPOM/PAH films, indicating that the film surface is relatively uniform and smooth. The photoluminescent properties of these films have also been investigated by fluorescence spectroscopy. The LPOM/PAH multilayer film has a good thermal stability as shown by UV-vis, X-ray photoelectron, and fluorescence spectra.     

Self-assembled structures with liquid-crystalline order in aqueous solution by patterning poly(phenylene ethynylene)s

Facially amphiphilic (FA) phenylene ethynylene (PE) polymers that self-assemble in aqueous solution were studied by small-angle X-ray diffraction (SAXD) and found to self-assemble into bilayers with a fully extended backbone. The resulting bilayers have long-range liquid-crystalline order. This self-assembly is programmed into the molecule by placing polar and nonpolar groups at precise locations so that they segregate onto opposite sides of the molecular structure. The absence of FA patterning generated an amorphous sample confirming the importance of this programmed amphiphilicity in the self-assembly process. Facially amphiphilic patterning represents a new design criterion for supramolecular chemistry, illustrated here in the observation of molecular ordering into bilayers reminiscent of self-assembled structures commonly found in biology, including amphiphilic beta-sheet polypeptides and phospholipid bilayers.     

New Ru(II) chromophores with extended excited-state lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.     

Nonlinear Optical Response of a Polymer Electret with Incorporated Organic Chromophores

An analytical approach aimed at modeling the nonlinear optical (NLO) response of a polymer electret with incorporated organic dipole chromophores is extended to the dynamic case. The approach allows the effect of the locally anisotropic, polarizable, deformable polymer matrix on the chromophore NLO response to be accounted for. The method exploits the original cavity ansatz according to which the virtual cavity occupied by the chromophore in the polymer matrix is chosen to be conformal to the characteristic ellipsoid of the generalized permittivity tensor. Analytical expressions for the dynamic electric moments of dipole chromophores and macroscopic polarization of a polymer electret at fundamental and second harmonic frequencies are obtained by using a computational scheme based on two self‐consistency procedures. On the basis of the expressions for dynamic macroscopic polarization, the analytical formulas for linear, ${\chi _j^{i(1)} (\omega )}$ , and quadratic, ${\chi _{jk}^{i(2)} (2\omega )}$ , electric susceptibilities are obtained. The presented expressions establish the relationship between the molecular polarizabilities of chromophores and the macroscopic electrical properties of the polymer electret.     

The role of surface free energy on the formation of hybrid bilayer membranes

The interaction of small phospholipid vesicles with well-characterized surfaces has been studied to assess the effect of the surface free energy of the underlying monolayer on the formation of phospholipid/alkanethiol hybrid bilayer membranes (HBMs). The surface free energy was changed in a systematic manner using single-component alkanethiol monolayers and monolayers of binary mixtures of thiols. The binary surfaces were prepared on gold by self-assembly from binary solutions of the thiols HS-(CH(2))(n)()-X (n = 11, X = CH(3) or OH) in THF. Surface plasmon resonance (SPR), electrical capacitance, and atomic force microscopy (AFM) measurements were used to characterize the interaction of palmitoyl,oleoyl-phosphatidylcholine (POPC) vesicles with the surfaces. For all surfaces examined, it appears that the polar part of surface energy influences the nature of the POPC assembly that associates with the surface. Comparison of optical, capacitance, and AFM data suggests that vesicles can remain intact or partially intact even at surfaces with a contact angle with water of close to 100 degrees. In addition, comparison of the alkanethiols of different chain lengths and the fluorinated compound HS-(CH(2))(2)-(CF(2))(8)-CF(3) that characterize with a low value of the polar part of the surface energy suggests that the quality of the underlying monolayer in terms of number of defects has a significant influence on the packing density of the resulting HBM layer.     

Electrochemical tailoring of lamellar-structured ZnO films by interfacial surfactant templating

Zinc oxide films with ordered lamellar structures can be electrochemically produced by interfacial surfactant templating. This method utilizes amphiphile assemblies at the solid-liquid interface (i.e., the surface of a working electrode) as a template to electrodeposit inorganic nanostructures. To gain the ability to precisely tailor inorganic lamellar structures, the effect of various chemical and electrochemical parameters on the repeat distances, homogeneity, orientation, and quality of the interfacial amphiphilic bilayers were investigated. Surfactants with anionic headgroups (e.g., 1-hexadecanesulfonate sodium salt, dodecylbenzenesulfonate sodium salt, dioctyl sulfosuccinate sodium salt, mono-dodecyl phosphate, and sodium dodecyl sulfate) are critical because they incorporate Zn(2+) ions into their bilayer assemblies as counterions and guide the lamellar growth of ZnO films. Unlike surfactant structures in solution, the interfacial surfactant assemblies are insensitive to the surfactant concentration in solution. The use of organic cosolvents (e.g., ethylene glycol, dimethyl sulfoxide) can increase the homogeneity of bilayer assemblies when multiple repeat distances are possible in a pure aqueous medium. In addition, organic cosolvents can make the interfacial structure responsive to the change in bulk surfactant concentrations. The presence of quaternary alkylammonium salts (e.g., cetyltrimethylammonium bromide) as cationic cosurfactants improves the ordering of anionic bilayers significantly. Consequently, it also affects the orientation of lamellar structures relative to the substrate as well as the surface texture of the films. The quality of lamellar structures incorporated in ZnO films is also dependent on the deposition potentials that determine deposition rates. A higher degree of ordering is achieved when a slower deposition rate (I < 0.15 mA/cm(2)) is used. The results described here will provide a useful foundation to design and optimize synthetic conditions for the electrochemical construction of broader types of inorganic nanostructures.     

Comparative Raman studies of molecular interactions at a dye/polymer and a dye/glass interface

Integrated optical techniques and resonance Raman spectroscopy have been combined to investigate the intermolecular interactions at dye/polymer and dye/glass interfaces. Frequency shifts and intensity changes of bands assigned to the stretching vibrations of the bridged quinoline rings of the cyanine dye chromophore have been utilized to gain insight into the relative strength of adhesive forces at the surface. Polarized Raman measurements were made to determine the orientation of the chromophores on a poly(vinyl alcohol) surface. This was done to assess the possibility of hydrogen bond formation between the ring nitrogen atoms and the polar hydroxyl groups at the surface.     

光响应性偶氮聚电解质自组装研究

将3种侧链带有不同偶氮生色团的聚电解质,用逐层浸渍的方法使其组装成多层膜.利用原子力显微镜(AFM)研究了自组装膜的表面形貌以及随着组装层数增加的变化.详细研究了在紫外光的照射下,薄膜内偶氮生色团由反式到顺式构型的光响应转变,以及在暗室中由顺式回复到反式构型的回复特性.与自组装膜的光响应性和相应的旋涂膜进行比较发现,其中的PNAPE/PDAC自组装膜没有明显的光色效应,而PEAPE/PDAC,PPAPE/PDAC具有明显的光色效应.在暗室中,随生色团的结构不同和在不同的凝集态中,由顺式回复到反式构型的速率明显不同.     

Distribution of BODIPY-labelled phosphatidylethanolamines in lipid bilayers exhibiting different curvatures

In this paper we have investigated the behaviour of newly synthesised mono-palmitoyl- and dipalmitoyl-phosphatidylethanolamine probes (abbreviated as mPE and dPE, respectively) labelled in the polar headgroup region by either the FL-BODIPY or the 564/570-BODIPY fluorophore and solubilised in lipid systems that exhibit different curvatures. Because of the bulky BODIPY-groups, the monoacyl-form derivatives have a conic-like shape, whereas that for the diacyl derivatives is rather cylindrical. A careful analysis of time-resolved resonance energy transfer experiments by means of analytical models as well as Monte Carlo simulations shows that the mPE derivatives have a comparable affinity to highly curved bilayer regions (torroidal pores formed by magainin-2 in lipid bilayers, or the rims of discoid bicelles) and to planar bilayer regions (i.e. the flat region of lipid bilayers and bicelles). Furthermore, the monoacyl-probes are as compared to the diacyl-probes effectively closer to each other in a lipid bilayer, while none of these probes seems to be randomly distributed. Self-aggregation is most efficiently induced by the larger aromatic 564/570-BODIPY chromophore, but it is suppressed when using the diacyl instead of the monoacyl-form, and/or by attaching BODIPY-groups to the acyl-chain.     

Thickness, surface morphology, and optical properties of porphyrin multilayer thin films assembled on Si(100) using copper(I)-catalyzed azide-alkyne cycloaddition

We report the structure, optical properties and surface morphology of Si(100) supported molecular multilayers resulting from a layer-by-layer (LbL) fabrication method utilizing copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), also known as "click" chemistry. Molecular based multilayer films comprised of 5,10,15,20-tetra(4-ethynylphenyl)porphyrinzinc(II) (1) and either 1,3,5-tris(azidomethyl)benzene (2) or 4,4'-diazido-2,2'-stilbenedisulfonic acid disodium salt (3) as a linker layer, displayed linear growth properties up to 19 bilayers. With a high degree of linearity, specular X-ray reflectivity (XRR) measurements yield an average thickness of 1.87 nm/bilayer for multilayers of 1 and 2 and 2.41 nm/bilayer for multilayers of 1 and 3. Surface roughnesses as determined by XRR data fitting were found to increase with the number of layers and generally were around 12% of the film thickness. Tapping mode AFM measurements confirm the continuous nature of the thin films with roughness values slightly larger than those determined from XRR. Spectroscopic ellipsometry measurements utilizing a Cauchy model mirror the XRR data for multilayer growth but with a slightly higher thickness per bilayer. Modeling of the ellipsometric data over the full visible region using an oscillator model produces an absorption profile closely resembling that of a multilayer grown on silica glass. Comparing intramolecular distances from DFT modeling with experimental film thicknesses, the average molecular growth angles were estimated between 40° and 70° with respect to the substrate surface depending on the bonding configuration.     

FT-Raman studies of langmuir-blodgett monolayer components containing absorbing chromophores

Structural studies of Langmuir-Blodgett monolayers and multilayers have revealed that both the hydrophilic and the hydrophobic parts of an amphiphilic molecule can influence the exent of 2-dimensional packing which occurs within a film. In particular, dye chromophores to which are appended long hydrocarbon tails will usually form films dominated by the interaction of the chromophores unless a small amount of hydrocarbon liquid is added. In this case the packing will be dominated by the intermolecular interaction of the hydrocarbon tails. FT-Raman studies of these amphiphilic molecules provides information about both parts of the molecule without resonance enhancement since the laser wavelength used for excitation is far removed from any absorption maxima of the dye chromophore. Specific examples of thiocyanine and merocyanine dyes will be discussed.     

聚酰亚胺多层膜的制备及其在非线性光学中的应用

报道了一种通过旋涂制备NLO聚合物多层膜的方法.紫外-可见吸收光谱及膜的厚度表征说明,在所得多层膜的结构中,聚合物单层膜的厚度可以较好地控制在100~200nm之间,所得含有5个双层的NLO多层膜(厚度1.6μm)具有良好的结构均一性,光学显微镜下没有观察到明显的结构缺陷.与单层具有较大厚度的NLO聚合物薄膜(如2~4μm)相比,所得聚合物多层膜可以允许掺杂更多的发色团而不发生相分离.     

Self‐Rolled Polymer Tubes: Novel Tools for Microfluidics,Microbiology, and Drug‐Delivery Systems

Recent work on the fabrication of tubular microstructures via self‐rolling of thin, bilayer polymer films is reviewed. A bending moment in the films arises due to the swelling of one component of the bilayer in a selective solvent. The inner diameters of the tubes vary from hundreds of nanometers to dozens of micrometers. The position of the tubes on the substrate and their length can be preset by photolithographic patterning of the bilayer. Prior to rolling, the bilayers can be exposed to different methods of surface functionalization, providing opportunities for engineering the microtube inner surfaces for use in microfluidic circuits and “microbiological” applications. The self‐rolling approach is promising for the development of novel drug‐ and cell‐delivery systems, as well as for tissue engineering.