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.     

A novel amphiphilic push-pull ferrocene derivative: Langmuir–Blodgett films and second-order optical nonlinearity

The Langmuir–Blodgett (LB) films built from the mixture of an amphiphilic push–pull ferrocene derivative (P) and behenic acid were investigated. Langmuir films of P diluted by behenic acid exhibit a very good cohesion and the mixed films can easily be transferred onto solid substrates. Linear dichroism UV-visible and IR spectroscopy measurements of the mixed LB multilayers confirm that the molecules (P) are oriented to the substrate. The nonlinear optical experiments on the mixed monolayer deposited on the CaF₂ slide showed that P displayed efficient optical second harmonic generation (SHG) with a molecular hyperpolarizability (β) as high as 6.0×10^-29 e.s.u.     

SHG active crystals of a remote functionalized achiral NLO-phore assembled through zinc(II) complexation

An achiral nonlinear optical chromophore with a "remote functionality" that can act as a ligand is developed on the basis of 4-nitroaniline derivatized with pyridine. The molecules are assembled through complexation with simple achiral zinc(II) salts and the H-bond network mediated by the counterions, to generate noncentrosymmetric materials exhibiting optical second harmonic generation (SHG). The crystal structures of the new complexes are determined; the counterion strongly influences the ligand orientations and lattice structure. SHG of the microcrystalline materials is investigated. Correlation between the structure and SHG is rationalized using semiempirical quantum chemical estimation of the hyperpolarizabilities of molecules and molecular clusters. The metal complexation plays a significant role in molecular assembly but affects the SHG very little, enabling simplified analysis of the bulk property in terms of molecular responses. Organization of remote functionalized molecules by metal ion complexation thus offers a convenient approach to the rational design of quadratic NLO materials.     

A Novel Amphiphilic Push-pull Ferrocene Derivative: Langmuir-Blodgett Films and Second-order Optical Nonlinearity

We investigated Langmuir-Blodgett(LB) films built from the mixture of an amphiphilic push-pull ferrocene derivative(P) and behenic acid. Langmuir films of P diluted by behenic acid exhibit a very good cohesion, and the mixed films can easily be transferred onto solid substrates. Linear dichroism UV-visible and IR spectroscopy measurements of the mixed LB multilayers confirm that the molecules(P) are oriented to the substrate. The nonlinear optical experiments on the mixed monolayer deposited on the CaF₂ slide showed that P displays efficient optical second harmonic generation(SHG) with a molecular hyperpolarizability (β)h, high as 6.0×10^-29e. s. u..     

Second harmonic generation at the interface of copper tetra-tert-butyl phthalocyanine Langmuir-Blodgett film/metal

As one of the most primitive nonlinear optical phenomena, optical second harmonic genera-tion (SHG) has been investigated for half a century and it has become a very useful spectroscopic tool in the study of surface and interface[1—3]. It was theoretically shown that the SH signal cannot be generated in a centrosymmetric system. However, the generation of SH signal from the centro-symmetric molecules such as fullerene (C60) and CuPc has been detected[4—7]. In our recent ex-periments, an …     

Adsorption kinetics of a fluorescent dye in a long chain fatty acid matrix

This work reports the adsorption kinetics of a highly fluorescent laser dye rhodamine B (RhB) in a preformed stearic acid (SA) Langmuir monolayer. The reaction kinetics was studied by surface pressure-time (π-t) curve at constant area and in situ fluorescence imaging microscopy (FIM). Increase in surface pressure (at constant area) with time as well as increase in surface coverage of monolayer film at air-water interface provide direct evidence for the interaction. ATR-FTIR spectra also supported the interaction and consequent complexation in the complex films. UV-vis absorption and Fluorescence spectra of the complex Langmuir-Blodgett (LB) films confirm the presence of RhB molecules in the complex films transferred onto solid substrates. The outcome of this work clearly shows successful incorporation of RhB molecules into SA matrix without changing the photophysical characteristics of the dye, thus making the dye material as LB compatible.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

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.     

Adsorption of poly(N‐ethyl‐4‐vinyl pyridinium bromide) onto langmuir‐blodgett films built up from amphiphilic polymers

The interaction of a strong cationic polyelectrolyte, poly(N‐ethyl‐4‐vinyl pyridinium bromide), with Langmuir‐Blodgett (LB) films built up from four monolayers of amphiphilic derivatives of the alternating copolymers of maleic acid and alkenes (one of the monolayers was formed by the amphiphilic copolymer containing pyrenyl groups as fluorescent labels) was examined. Transformations of absorbance spectra and quenching of fluorescence of the LB films were detected after their contact with aqueous solutions of the cationic polyelectrolyte. These changes were attributed to the adsorption of poly(N‐ethyl‐4‐vinyl pyridinium bromide) onto such films. The efficiency of this process was found to be rather sensitive to the variations in pH of the surrounding medium: adsorption of the cationic polyelectrolyte onto the LB films was pronounced in basic media while it became rather weak in acidic media.     

一个对称萘酞菁的LB膜及其二阶非线性光学性质的研究

用水平提拉法制备了四叔丁基萘酞菁锌的LB膜。利用表面压-面积曲线、紫外-可见吸收光谱、小角X-射线衍射等表征了LB膜的结构,结果表明该化合物可以形成较高质量的具有很好层状结构的多层LB膜,在膜中分子大环平面垂直于基片平面并且以分子平面并不完全重合的H-聚集体存在。采用透射SHG方法测量了相应LB膜的二阶非线性光学性质,结果显示在实验条件下该化合物LB膜的宏观二阶非线性系数以及分子超极化率分别为X     

表面离子法制备多组分LB膜和CeO_2-Gd_2O_3陶瓷膜

以铈、钆的β二酮螯合物Ce(tmhd)_4和Gd(tmhd)_3(tmhd = tetramethylheptanedionate)为表面离子,与花生酸（AA）在水面上共铺展,可 形成具有良好相容性、稳定性和可压缩性的混合Langmuir膜,这是由于稀土螯合物 与AA间发生了新的配合,而且Gd(tmhd)_3与AA间的相互作用更强。用垂直法将它们 的三组分混合Langmuir膜沉积,制得了具有良好周期性结构的Y型三组分混合LB膜 。以它作前驱物,经过紫外臭氧（UVO）和热处理,制得了CeO_2-Gd_2O_3(CGO)超 薄陶瓷膜。X射线光电子能谱（XPS）表明,由于两种稀土螯合物和AA间的结合强度 不同,在沉积LB膜时表面离子发生了选择性转移,而且温度对表面离子的转移选择 性有影响。这对制备组分和厚度可控的应用薄膜很有意义。     

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.     

Correlation of Molecular Assembly and Interactions in Crystals and Langmuir–Blodgett Films of N‐(2,4‐dinitrophenyl)‐n‐octadecylamine

Correlation of molecular organization in crystals and in ultrathin films is of fundamental interest in the design of molecular materials based on thin films. We have chosen as a test case, N‐(2,4‐dinitrophenyl)‐n‐octadecylamine (DNPOA), a potential candidate for the fabrication of Langmuir–Blodgett (LB) films for quadratic nonlinear optical applications. Like several other 4‐nitroaniline derivatives, DNPOA does not form stable monolayers at the air–water interface. This has precluded investigations of their organization in LB films. We have stabilized composite Langmuir films of DNPOA with the phospholipid molecule DSPC and fabricated their LB films. Successful growth of single crystals of DNPOA allowed structure determination and detailed analysis of molecular associations in the solid state. Electronic absorption spectra of DNPOA in solution, in the solid state and in the LB film are investigated. Modeling of the various spectral signatures by semiempirical computations on molecular clusters extracted from the crystal lattice provides insight into the correlation between the molecular organization in crystals and in LB films.     

Nonlinear optical properties of polymers and thin polymer films

An introduction to nonlinear optics (NLO) and the major challenges in the field of NLO-properties of polymeric materials, is given in this paper. Methods for the investigation of nonlinear optical properties of [4-(2-methacryloxyethyl)methylamino]-4′-cyanoazobenzene copolymers with methyl methacrylate (MMA) are demonstrated: Electric Field Induced Second Harmonic Generation (EFISHG), Hyper Rayleigh Scattering (HRS), Second Harmonic Generation (SHG) in floating monolayers and SHG in poled polymer films. An example of phase-transition analysis by SHG is given.     

Significant enhancement of the optical second harmonic generation in a poled azopolymer thin grating

Nonlinear optical properties of an electrically poled surface relief grating inscribed on a thin film of azopolymer have been investigated. The linear and nonlinear optical far-field diffraction patterns of the grating are compared, and they show a clear angular separation of the fundamental (lambda = 1.064 mum) and second harmonic generated beams that are diffracted at different angles. The intensity of the zero order transmitted second harmonic generation (SHG) beam from inscribed surface relief grating (SRG) areas has been recorded using the Maker fringe technique and compared to the response from flat areas that were only poled. Poled gratings exhibit a sharp second harmonic generation enhancement for coupling angles of theta(i) = +/-52 degrees due to a quasi-phase matching process. A simple phenomenological model allows one to explain both the second harmonic intensity generated from the thin polar film and the dispersion curve observed under "pp" polarization due to the mismatch of the angular coupling condition: this gives us a good indication of the intensity enhancement of the SHG beam in the forward direction.     

中心对称双酞菁铥LB膜二次谐波产生特性

利用二次谐波产生(SHG)方法研究了中心对称分子稀土夹心双酞菁铥(TmPc₂) Langmuir-Blodgett (LB)膜二阶非线性光学特性, 测量了二次谐波强度随入射基频光入射角的关系, 并对其二阶非线性产生机制进行了讨论. 实验结果表明, TmPc₂分子LB膜具有较好的二次谐波信号, 二次谐波信号强度的最大值在基频光入射角为45°的地方, 其二阶非线性极化率χ⁽²⁾和分子超极化率β分别为1.152×10^-8和1.905×10^-30 esu. 通过测量样品二次谐波信号的偏振特性, 并与理论分析相比较, 得出其二阶非线性起源于电四极子作用机制.     

强绿荧光铽－异丁酰基吡唑啉酮配合物的合成、荧光性能及LB膜

强绿荧光铽－异丁酰基吡唑啉酮配合物的合成、荧光性能及ＬＢ膜李琴，周德建，姚光庆，黄春辉Ｓ．ＵｍｅｔａｎｉＭ，Ｍａｔｓｕｉ（北京大学稀土材料化学及应用国家重点实验室，北京，１００８７１）（日本京都大学化学研究所）关键词铽，１－苯基－３－甲基－４－异丁酰...     

Molecular organization in two-dimensional films of liquid II. Langmuir and Langmuir-Blodgett films of a perylene-like dye mixed with liquid crystals having a terminal cyano group crystalline mixtures

Langmuir and Langmuir-Blodgett (LB) films of a perylene-like compound and its binary mixtures with 4-octyl-4'-cyanobiphenyl (8CB) and 4-pentyl-4"-cyano-p-terphenyl (5CT) have been studied. On the basis of the surface pressure-area isotherms, the molecular organization on the air-water interface has been estimated. Information about the miscibility or the phase separation of components in the binary mixtures has been obtained. The spectroscopic study of the LB films has allowed conclusions to be drawn about the arrangement of the molecules on the quartz slides. The fluorescence spectra of the LB films of the perylene-like compound have revealed the formation of self-aggregates.     

Molecular organization in binary mixtures of derivatives of naphthalenebicarboxylic acid and naphthoylenebenzimidazole with a liquid crystal in two-dimensional layers II. Langmuir-Blodgett films

Langmuir-Blodgett (LB) films of some dichroic dyes, namely derivatives of naphthalenebicarboxylic acid and derivatives of naphthoylenebenzimidazole, and of their mixtures with mesogens: 4-octyl-4'-cyanobiphenyl or 4-pentyl-4'-cyano-p-terphenyl were prepared. Absorption and fluorescence studies using unpolarized and linearly polarized light were carried out. Both absorption and fluorescence spectra indicated the formation of aggregates of dye molecules in monomolecular layers. Moreover, it was found that dye molecules are more tilted to the quartz surface in LB films than to the plane of the air-water interface in Langmuir films.     

Time-resolved fluorescence spectroscopic and scanning near-field optical microscopic studies of rhodamine dye adsorbed in cationic Langmuir-Blodgett films

The fluorescence lifetimes decays and picosecond time-resolved fluorescence spectra were measured to investigate the dynamics of the excited state of sulforhodamine B (SRB) molecules adsorbed in the mono- and multilayered Langmuir-Blodgett (LB) films of octadecylamine. Steady-state and time-resolved fluorescence spectroscopy reveals that the fluorescence lifetimes and contents of the monomer and dimers in the molecular organizates depend upon the concentration of the dye in the solution and the adsorption process. SRB dye molecules adsorbed in LB films have been imaged with scanning near-field optical microscopy (SNOM). This information is exploited to map the distribution with molecular spatial resolution. SNOM provide the visual evidence of the monomers and dimers of SRB in cationic LB films.