Langmuir and Langmuir-Blodgett (LB) films of tetrapyridyl metalloporphyrins

We report on the formation of Langmuir films of 5,10,15,20-tetra(4-pyridyl) 21H,23H-porphine,hereafter named tetrapyridyl porphyrins with distinct central ions (2H(+), Zn(2+), Cu(2+), Ni(2+)). The films were characterized with surface pressure and surface potential isotherms and in situ UV-vis absorbance. The measurements indicated strong aggregation of porphyrin monomers at the air-water interface, with a red shift of the Soret band in comparison with the spectrum obtained from CHCl(3) solutions. The shift was larger for the non-substituted H(2)TPyP, and depended on the metal ion. Significantly, aggregation occurred right after spreading of the Langmuir film, with on further shifts in the UV-vis spectra upon compression of the film, or even after transferring them onto solid substrates in the form of Langmuir-Blodgett (LB) films. The buildup of LB films from H(2)TPyP and ZnTPyP was monitored with UV-vis spectroscopy, indicating an equal amount of material deposited in each deposition step. Using FTIR in the transmission and reflection modes, we inferred that the H(2)TPyP molecules exhibit no preferential orientation in the LB films, while for ZnTPyP there is preferential orientation, with the porphyrin molecules anchored to the substrate by the lateral pyridyl groups. (C) 2008 Elsevier B.V. All rights reserved.

Langmuir and Langmuir-Blodgett (LB) films of poly [(2-methoxy,5-n-octadecyl)-p-phenylenevinylene] (OC1OC18-PPV)

A PPV derivative, poly(2-methoxy,5-(n-octadecyl)-p-phenylenevinylene) (OC1OC18-PPV), has been synthesized via the Gilch route and used to fabricate Langmuir and Langmuir-Blodgett (LB) films. True monomolecular films were formed at the air/water interface, which were successfully transferred onto different types of substrate. Using UV-visible absorption, FTIR, fluorescence and Raman scattering spectroscopies we observed that the polymer molecules were randomly distributed in the LB film, with no detectable anisotropy. This is in contrast to the anisotropic LB films of a previously reported PPV derivative, poly(2-methoxy-5-n-hexyloxy)-p-phenylenevinylene (OC1OC6-PPV), which is surprising because the longer chain of OC1OC18-PPV investigated here was expected to lead to more ordered films. As a consequence of the lack of order, LB films of OC1OC18-PPV exhibit lower photoconductivity and require higher operating voltage in a polymer light-emitting diode (PLED) in comparison with LB films of OC1OC6-PPV. This result confirms the importance of molecular organization in the LB film to obtain efficient PLEDs. (c) 2005 Elsevier Ltd. All rights reserved.

Polymer light emitting devices with Langmuir-Blodgett (LB) films: Enhanced performance due to an electron-injecting layer of ionomers

Polymer light-emitting devices (PLEDs) have been produced with Langmuir-Blodgett (LB) films from poly(2-methoxy-5-hexyloxy)-p-phenylenevinylene (OC1OC6-PPV) as the emissive layer and an ionomer of a copolymer of styrene and methylmethacrylate (PS/PMMA) as an electron-injection layer. The main features of such devices are the low operating voltages, obtainable firstly due to the good quality of the ultrathin LB films that allows PLEDs to be produced reproducibly and secondly due to the improved electrical and luminance properties brought by the electron-injection layer. Also demonstrated is the superior performance of an all-LB device compared to another one produced with cast films of the same materials. Published by Elsevier B.V.

Langmuir and Langmuir-Blodgett (LB) films of tetrapyridyl metalloporphyrins

We report on the formation of Langmuir films of 5,10,15,20-tetra(4-pyridyl) 21H,23H-porphine,hereafter named tetrapyridyl porphyrins with distinct central ions (2H(+), Zn(2+), Cu(2+), Ni(2+)). The films were characterized with surface pressure and surface potential isotherms and in situ UV-vis absorbance. The measurements indicated strong aggregation of porphyrin monomers at the air-water interface, with a red shift of the Soret band in comparison with the spectrum obtained from CHCl(3) solutions. The shift was larger for the non-substituted H(2)TPyP, and depended on the metal ion. Significantly, aggregation occurred right after spreading of the Langmuir film, with on further shifts in the UV-vis spectra upon compression of the film, or even after transferring them onto solid substrates in the form of Langmuir-Blodgett (LB) films. The buildup of LB films from H(2)TPyP and ZnTPyP was monitored with UV-vis spectroscopy, indicating an equal amount of material deposited in each deposition step. Using FTIR in the transmission and reflection modes, we inferred that the H(2)TPyP molecules exhibit no preferential orientation in the LB films, while for ZnTPyP there is preferential orientation, with the porphyrin molecules anchored to the substrate by the lateral pyridyl groups. (C) 2008 Elsevier B.V. All rights reserved.

Polyaniline mixed LB films exposed to X-rays

X-ray irradiation is shown to affect electronic properties of polyaniline (PANi) in composite Langmuir-Blodgett (LB) films of PANi and cadmium stearate, in a similar way to acid doping. The time it takes for the shift in the UV-vis spectra, characteristic of PANi doping, increases linearly with the film thickness, thus indicating a surface-controlled process. The humidity of the environment under which the films are irradiated is also of extreme importance. No shin is observed under vacuum or under dry atmospheres of N-2, O-2 and Ar. For humid environments the time for the shift decreases with increasing relative humidity.

A study on X-ray irradiation of composite polyaniline LB films

Composite Langmuir-Blodgett (LB) films from polyaniline and cadmium stearate have been irradiated with ionizing X-rays for various exposure times. In the initial stages of X-ray irradiation the absorption peak at 580 nm of an as-deposited film was seen to decrease with a concomitant increase in the absorption in the long wavelength region (700-1100 nm). Upon prolonging the irradiation, the absorption maximum shifted to 800 nm with the LB film color changing to green, characteristic of acid doped polyaniline. The changes in the Fourier transform infrared (FTIR) spectra upon irradiation are also similar to those observed upon acid doping of polyaniline. When compared with acid doping, two major differences were observed for the LB films exposed to X-rays. First, the packing order of the cadmium stearate domains in the composite LB films - as observed by X-ray diffraction - is not affected by the X-ray irradiation. In addition, no significant increase in the DC conductivity was noted after the X-ray exposure whereas similar LB films have their conductivity increased by an order of magnitude upon acid doping. These differences may be explained by considering that the inter-domain contribution to the conductivity is increased by the acid doping because the insulating cadmium stearate domains are destroyed, which does not occur with the X-ray irradiation. (C) 1998 Elsevier B.V. S.A. All rights reserved.

Temperature dependence of photoinduced birefringence in mixed Langmuir-Blodgett (LB) films of azobenzene-containing polymers

The temperature dependence of photoinduced birefringence was investigated for mixed Langmuir-Blodgett (LB) films from the homopolymer poly[4'-[[2-(methacryloyloxy)ethyl]ethyl-amino]-2-chloro-4-nitroazobenzene] (HPDR13) and cadmium stearate (Cdst) and from the copolymer 4-[N-ethyl-N-(2-hydroxyethyl)]amino-2'-chloro-4'-nitroazobenzene (MMA-DR13) and CdSt. Birefringence was achieved by impinging a linearly polarized light on the LB films. The maximum birefringence achieved decreased with temperature as thermal relaxation of the chromophores was facilitated. The buildup curves for birefringence were fitted with biexponential functions representing distinctly different mechanisms with time constants. The first, fast process is thermally activated and may be represented by an Arrhenius process. The decay of birefringence after switching off the laser source was described by a Kohlraush-Williams-Watts (KWW) function, consistent with a distribution of relaxation times for the polymer system. Activation energies were obtained from Arrhenius plots of the rate constant of the exponential functions and KWW function, which showed that the buildup of birefringence was very similar for the two polymer systems. The decay, however, was slower for the LB film from MMA-DR13/CdSt. (C) 2002 Published by Elsevier B.V. Ltd.

Synthesis of a functionalized europium complex and deposition of luminescent Langmuir-Blodgett (LB) films

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Polymer light emitting devices with Langmuir Blodgett (LB) films of a polyfluorene derivative

Polyfluorenes are promising materials for the emitting layer of polymer light emitting devices (PLEDs) with blue emission. In this work, we report on PLEDs fabricated with Langmuir-Blodgett (LB) films of a polyfluorene derivative, namely poly(9,9-di-hexylfluorenediyl vinylene-alt-1,4-phenylenevinylene) (PDHF-PV). Y-type LB films were transferred onto ITO substrates at a surface pressure of 35 mN m-1 and with dipping speed of 3 mm min -1. A thin aluminum layer was evaporated on top of the LB film, thus yielding a sandwich structure (ITO/PDHF-PV(LB)/Al). Current-voltage (I vs V) measurements indicate that the device displays a classical behavior of a rectifying diode. The threshold value is approximately 5 V, and the onset for visible light emission occurs at ca. 10 V. From the a.c. electrical responses we infer that the active layer has a typical behavior of PLEDs where the real component of ac conductivity obeys a power-law with the frequency. Cole-Cole plots (Im(Z) vs. Re(Z)) for the device exhibit a series of semicircles, the diameter of which decreases with increasing forward bias. This PLED structure is modeled by a parallel resistance and capacitance combination, representing the dominant mechanisms of charge transport and polarization in the organic layer, in series with a resistance representing the ITO contact. Overall, the results presented here demonstrate the feasibility of LEDs made with LB films of PDHF-PV.

Synthesis of a functionalized europium complex and deposition of luminescent Langmuir-Blodgett (LB) films

The synthesis, characterization and formation of Langmuir-Blodgett (LB)films for the luminescent [(C12H25)(2)(CH3)(2)N][Eu(tta)(4)] complex, where [(C12H25)(2)(CH3)(2)](N+) is didodecyldimethylammonium and the tta ligand is thenoyltrifluoroacetone, are reported. The coordination of tta ligands to the Eu3+ ion was confirmed by FTIR spectroscopy and the emission spectrum comprised bands corresponding to D-5(0) -> F-7(0-4) transitions. The lifetime (tau) from the emission state (D-5(0))was 0.41 ms, measured by monitoring the hypersensitive D-5(0) -> F-7(2) transition, with the curve being fitted with a first-order exponential function. The surface pressure-area isotherm indicated that the anionic complex may form condensed structures at the air-water interface due to the amphiphilic properties of the counter ion and the beta-diketone ligand. Y-type LB films of [(C12H25)(2)(CH3)(2)N][Eu(tta)(4)] were deposited on quartz substrates, with preserved luminescence and a band assigned to the D-5(0) -> F-7(2) transition. The molecular arrangement at the air/water interface and the preserved luminescence in LB films are consistent with theoretical predictions using a semi-empirical Sparkle/AM1 calculation method for the molecule in vacuum. These tools were used for the first time to predict the behavior of organized films.

Formation of thin luminescent Eu3+-LB films by in situ coordination with 2,3,5,6-tetra(2 '-pyridyl)pyrazine and 1-octadecanol in pure and mixed Langmuir monolayers

The in situ complexation between 2,3,5,6-tetra(2-pyridyl)pyrazine (tppz) molecules and europium ions at the air-liquid interface by means of mixed 1-octadecanol Langmuir films is reported. These films were transferred to solid supports by means of the Langmuir-Blodgett (LB) technique. The EDS maps attested the homogeneity of the LB films as well as the presence of the europium ions. The mixed alcohol/tppz LB film contained a larger amount of europium ions as compared to the pure octadecanol LB film. This work reports the production of a thin luminescent Eu3+ film containing europium ions using only alcohol molecules as ligands an unexpected result, since it is well known that there is an occurrence of non-radiative deactivation of excited europium by hydroxyl groups. Europium ion multiple binding sites were detected from lifetime decay measurements of these films in the presence of tppz molecules. (C) 2012 Elsevier B.V. All rights reserved.

LB multilayers and superlattice films of erbium fatty acid salts

Multilayers of Y-type bilayers of pure and mixed erbium palmitate(EP), nonadecanate(Er) and behenate(EB) on CaF2 substrates were prepared by conventional Langmuir-Blodgett (LB) method. II is demonstrated that two systems composed of alternating bilayer of different fatty acid salts are unidimensional superlattices. These LB films were characterized by means of x-ray photoelectronic Spectrometry (XPS), FTIR and x-ray diffraction (XRD) measurements.

Influence of the type of phospholipid head and of the conformation of the polyelectrolyte on the growth of calcium carbonate thin films on LB/LbL matrices

Calcium carbonate is one of the most important biominerals, and it is the main constituent of pearls, seashells, and teeth. The in vitro crystallization of calcium carbonate using different organic matrices as templates has been reported. In this work, the growth of calcium carbonate thin films on special organic matrices consisting of layer-by-layer (LbL) polyelectrolyte films deposited on a pre-formed phospholipid Langmuir-Blodgett (LB) film has been studied. Two types of randomly coiled polyelectrolytes have been used: lambda-carrageenan and poly(acrylic acid). A precoating comprised of LB films has been prepared by employing a negatively charged phospholipid, the sodium salt of dimyristoilphosphatidyl acid (DMPA), or a zwitterionic phospholipid, namely dimyristoilphosphatidylethanolamine (DMPE). This approach resulted in the formation of particulate calcium carbonate continuous films with different morphologies, particle sizes, and roughness, as revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The crystalline structure of the calcium carbonate particles was analyzed by Raman spectroscopy. The randomly coiled conformation of the polyelectrolytes seems to be the main reason for the formation of continuous films rather than CaCO3 isolated crystals. (C) 2012 Elsevier B.V. All rights reserved.

Infrared spectroscopic study on thermal behavior of Langmuir-Blodgett films of octadecyl ammonium octadecanoate and octadecylammonium octadecanoate-d(35)

Langmuir-Blodgett (LB) films of octadecylammonium octadecanoate (C(18)H(37)j7NH(3)(+)C(17)H(35)COO(-),ODASA) and octadecylammonium octadecanoate-d(35) (C18H37+NH3+C17D35COO-, ODASA-d(53)) were prepared and their thermal behaviors were investigated by variable-temperature Fourier transform infrared transmission spectroscopy. It was found that the two hydrocarbon chains of ODASA molecule in LB films are highly ordered while that protonated (H) chain in ODASA-d(35) is partially disordered with some gauche conformers introduced at room temperature.

Nucleation and growth of glycine crystals with controllable sizes and polymorphs on Langmuir-Blodgett films

Control of crystal polymorph and size is very important in many application fields. Herein we demonstrate that Langmuir-Blodgett (LB) films of stearic acid (SA) and octadecylamine (ODA) can serve as templates and generate different polymorphs of glycine crystals. In the neutral aqueous solutions, gamma-glycine crystallizes on LB films of ODA while the polymorphic outcome becomes the (x-form on LB films of SA. These observed results could be explained by the electrostatic interactions and geometric lattice matching at the LB film/crystal interfaces, respectively. By keeping the appropriate supersaturation, we have successfully controlled the number of crystals grown on LB films; for example, in some certain cases, only one piece of crystal was grown on LB films in solution. Therefore, large crystals of centimeter size could be prepared. These experimental results suggest a new approach to produce an organic crystal with bulk scale.