Pierre Le Barny

Photoinduced orientation in poly(vinylcinnamate) and poly(7-methacryloyloxycoumarin) thin films and the consequences on liquid crystal alignment

Thin films of poly(vinylcinnamate) and poly(7-methacryloyloxycoumarin) have been exposed to linearly polarized UV light. The resulting anisotropic films have been characterized by using UV, conventional and polarization modulation FTIR spectroscopies. In particular, several internal vibrational modes have been used as structural probes to examine the orientation of groups in the irradiated polymers. These experiments provide new information about the photoinduced anisotropy in these photocrosslinkable polymers upon irradiation with linearly polarized UV light, and an orientation mechanism is proposed. This mechanism is confirmed by studying the liquid crystal alignment induced by PVCi and poly(7-methacryloyloxycoumarin). Finally, the stability of the photoalignment process is discussed.

Properties of side chain liquid crystal and amorphous polymers. Applications to non-linear optics

Abstract Comb-like liquid-crystalline polymers exhibit many unique properties that challenge not only basic research but also numerous technological opportunities. They combine (partly) the properties of orientation of low molecular weight liquid crystals with the rigidity of polymers. For example, they can be oriented in the mesomorphic state and the structure frozen in a glassy state. These polymers with functionalized pendent groups lead to potential applications in the field of nonlinear optics, or in the domain of electro-optical displays. Other polymers like polysilanes show interesting properties such as photo-conductivity. This paper describes the properties and applications of some new side chain liquid-crystalline polyacrylates and their amorphous copolymers. It also describes the photo-conductive properties of polysilanes and their applications in spatial light modulators with liquid crystals. In the first part of this paper, we describe the properties of liquid crystal copolymers and amorphous...

Synthesis and liquid crystal alignment properties of new cinnamate-based photocrosslinkable polymers

Because optical buffing can achieve aligning patterns with different azimuthal angles of the liquid crystal directors, photoalignment appears to be a very hopeful technique for designing complex LCD structures. Until recently, most photopolymers provided low anchoring and/or low tilt angles which are unsuitable for industrial applications. In this work, four new photocrosslinkable polymers based on biphenyl and naphthalene moieties have been prepared and characterized. These photopolymers have been irradiated with linearly polarized UV light, and liquid crystal cells have been made. The tilt angles generated by oblique irradiation have been measured as well as the anchoring energies. Finally, the stability of the liquid crystal alignment properties have been studied.

Concise Multigram-Scale Synthesis of Push-Pull Tricyanofuran-Based Hemicyanines with Giant Second-Order Nonlinearity: An Alternative for Electro-optic Materials

Highly stable and highly soluble push-pull heptamethine hemicyanines based on the tricyanofuran electron-accepting group can be prepared on a 15 g scale. The compounds display giant second-order nonlinear figure of merit, μβ of up to 31,000×10(-48) esu, and lead to a poled material with a second-order nonlinear response, r33 of 90 pm V(-1) at 1.06 μm.

Detection of nitroaromatic compounds based on photoluminescent side-chain polymers

The synthesis, spectroscopic characterization and fluorescence quenching efficiency of a polymer (PSt-NI) and a low molecular weight molecule (NI) containing the 4-(N, N disubstituted)amino-N-2,5ditertiobutylphenyl-1,8-naphthalimide chromophore are reported. Similar spectroscopic properties of thin films and solutions are observed. This is consistent with the absence of interactions between polymer side chains. The absorption and fluorescence spectra of PSt-NI studied in various solvents of different polarity are compared to the corresponding spectra of NI. The longest wavelength absorption of PSt-NI and NI is characterized by a band with a maximum wavelength around 410 nm. The peak position is sensitive to the polarity of the solvent, which is in agreement with the charge transfer character of the transition. The fluorescence spectrum of PSt-NI shows a maximum emission in chloroform at 515 nm and is red shifted compared to those of NI. Fluorescence lifetimes of PSt-NI and NI are measured in presence and absence of 2,4-dinitrotoluene (DNT) and the results are interpreted via the Stern-Volmer analysis. In solution, the fluorescence quenching of NI is purely collisional, whereas both dynamic and static quenching are observed with PSt-NI Upon 1 minute exposure to DNT vapor, it was shown that a 5 nm thick film of PSt-NI exhibited a 45% drop in its fluorescence intensity, which makes this polymer very attractive for sensing applications.

Photochromic compounds as optical limiters in the nanosecond time range: the example of mercury dithizonate complexThis paper is dedicated to Professor Jean Kossanyi on the event of his 70th birthday.

Although being an efficient photochromic compound which absorbs in the blue in its stable form and in the orange in its photoactivated form, the mercury dithizonate complex is shown to be a poor optical limiter for nanosecond laser pulses at the wavelengths where both isomers absorb. Optical limiting effect, which is a consequence of reverse saturable absorption due to the photoactivated form, is demonstrated to be weak because of the back photobleaching of this form, which is important all the more as the laser intensity is high. Numerical integration of the spatiotemporal evolution of the laser beam intensity across the solution helps the understanding of the respective roles of the laser fluence and pulse duration. Finally, we draw the conclusion that photochromic compounds can only be used as optical limiters if the time constant for the back photochemical reaction is slow compared to the pulse duration.

Synthesis of side-chain electroluminescent polymers and properties of devices that include them

A series of 4-aminonaphthalimide functionalized polymers has been synthesized. They differ by the nature of their backbone: and by the nature of the substituent at the imide nitrogen atom. The absorption and emission properties of these polymers have been investigated. Photoluminescence quantum yields in the solid state up to 35 percent were observed. Cyclic voltammetry in conjunction with UV-visible spectrometry have been performed in order to determine the HOMO and LUMO energy levels of the different materials. Electroluminescent devices were fabricated with these polymers as emitting layers, and ITO and Ca as anode and cathode, respectively. Monolayer devices showed a limited performance. Efficient green light emission was obtained with a bilayer device based on PVK as a hole transport material and a polystyrene derivative as an emitting layer. A maximum luminance of 7100cd/m2 was obtained under 16V. The device had a maximum external quantum efficiency of 1 percent and a maximum external energetic efficiency of 0.2 percent. Doping PST-NI-BuP with 20 percent DCM resulted in red-orange emission with a brightness as high as 1800 cd/m2. Doping PST-NI-BuP with 20 percent DCM resulted in red-orange emission with a brightness as high as 1800 cd/m2. Moreover, this study showed a strong influence of the chemical environment of the naphthalimide moiety on the photoluminescence and electroluminescent properties of the polymers.

Investigation of Polymer Dispersed Liquid Crystal films exhibiting nanosized liquid crystalline domains

Elaboration of Polymer Dispersed Liquid Crystals(PDLCs)was implemented via radiation curing of a selected mixture of thiolene and the liquid crystal(E7). It is made according to the process of Polymerisation Induced Phase Separation(PIPS)under the conditions where the size of the liquid crystal domains was in the nano-meter scale. Typically, the Scanning Electron Microscopy gives a distribution of nearly spherical droplets of mean diameter in the range of 90 nm. The spectral reflectivity of a Fabry Perot cell of the nanosized materials was obtained by UV-vis spectroscopy in the range of wavelengths roughly between 200 and 1000 nm. The electro-optical response of this cell was investigated in terms of the variation of the reflectivity as a function of the applied electrical field and the change of the mean refractive index was deduced from these measurements.

Some New Thermotropic Discogens

New thermotropic discogens are synthesized and characterized by microcalorimetric measurements and microscopic examinations. The mesophases are identified by the miscibility method. The rufigallol hexa-n-nonanoate exhibits a stable Dc columnar discophase. Some chemical possibilities for new side chains are explored. The hexa-p-n-heptyloxycinnamoyloxytriphenylene exhibits a fluid discophase stable from 146.7 to 242°C. The discogenic potentialities of nine aromatic cores are evaluated, good candidates are found. Five binary phase diagrams with non ideal behavior are given. One enhanced intermediate columnar discophase with mosaic texture is observed. A binary mixture exhibiting a twisted fluid discophase with small pitch is proposed.

Covalently functionalized single-walled carbon nanotubes and graphene composite electrodes for pseudocapacitor application

The use of carbon-based materials in electrochemical double-layer supercapacitors (EDLC) is currently being the focus of much research. Even though activated carbon (AC) is the state of the art electrode material, AC suffers from some drawbacks including its limited electrical conductivity, the need for a binder to ensure the expected electrode cohesion and its limited accessibility of its pores to solvated ions of the electrolyte. Owing to their unique physical properties, carbon nanotubes (CNTs) or graphene could overcome these drawbacks. It has been demonstrated that high specific capacitance could be obtained when the carbon accessible surface area of the electrode was finely tailored by using graphene combined with other carbonaceous nanoparticles such as CNTs12.In this work, to further increase the specific capacitance of the electrode, we have covalently grafted onto the surface of single-walled carbon nanotubes (SWCNTs), exfoliated graphite or graphene oxide (GO), anthraquinone (AQ) derivatives which are electrochemically active materials. The modified SWCNTs and graphene-like materials have been characterized by Raman spectroscopy, X-ray photoemission and cyclic voltammetry . Then suspensions based on mixtures of modified SWCNTs and modified graphene-like materials have been prepared and transformed into electrodes either by spray coating or by filtration. These electrodes have been characterized by SEM and by cyclic voltammetry in 0.1M H2S04 electrolyte.