Donald Lupo

Nanomaterials-Based Electrochromics for Paper-Quality Displays

Electrochromic displays based on nanostructured films modified with electrochromophores are capable of becoming high quality paper-like displays due to their excellent ink-on-paper optical qualities, fast response times, and low power consuming features. The nanostractured films are composed of nanoparticles of a semiconductor, e.g., TiO 3 and other doped metal oxides. The high coloration efficiencies of these devices is due to the use of organic chromophores and the umplification of the color change due to the extremely high surface area of the nanostructured film they are bound to.

BLUE POLARIZED ELECTROLUMINESCENCE FROM A LIQUID CRYSTALLINE POLYFLUORENE

demonstrated an organic EL device based on aligned con-jugated polymers which directly emitted polarized light,and realized that such devices would be particularly usefulas backlights for conventional liquid crystal displays(LCDs). However, typical supertwisted nematic LCDs re-quire a contrast ratio of 40 to 50,

Second-harmonic generation in Langmuir–Blodgett monolayers of stilbazium salt and phenylhydrazone dyes

The second-order nonlinear optical susceptibilities χ(2) of several phenylhydrazone and stilbazium salt dyes in Langmuir–Blodgett monolayers have been determined from second-harmonic-generation measurements. Three of the substances demonstrated χ(2) values greater than 10−6 electrostatic units, although two of the three did not absorb light significantly at the second-harmonic wavelength.

Polarized Electroluminescence from Highly Aligned Liquid Crystalline Polymers

Two classes of polymer materials, hexyl-dodecyl copoly(phenylene-ethynylene) and poly(2,7-(9,9-di(ethylhexyl)-fluorene)), respectively, were aligned by two different alignment techniques. For the copolymer, the crystalline and liquid crystalline state were found to differ only slightly and alignment was achieved in the crystalline state, whereas the two phases differ substantially for the polyfluorene and alignment could be obtained in the liquid crystalline state. Light emitting diodes of differing structures were developed using the two materials as emissive layers. In the case of the polyfluorene, a state-of-the-art polarization ratio of 21 was realized.

Liquid crystalline polyfluorenes for blue polarized electroluminescence

A series of 9,9-dialkyl-poly(fluorene-2,7-diyl)s containing linear and branched alkyl substituents with a Mn of up to 200000 g/mol has been synthesized. Moreover, some of the polymers were end capped with a suitable hole transport functionality, such as a triphenylamine derivative, to improve their charge transport properties and to control the molecular weight. The thermal alignment of these novel polymers on a rubbed polyimide layer led to highly anisotropic film formation with dichroic ratios (absorption parallel and perpendicular to the rubbing direction) of up to 26 in absorption and 21 in emission.

Polymeric light-emitting diodes with highly polarized blue emission

Polymer light-emitting diodes (LED) with aligned conjugated molecules and polymers as the active layer can emit polarized light as shown recently in several reports. We have synthesized several liquid-crystalline polyfluorenes with different side chain patterns. The thin polymer films were aligned at elevated temperatures on a rubbed polyimide layer. For the most suitable substitution, a dichroic ratio of twelve was observed in absorption. This polymer was used to construct light-emitting devices with ITO and Ca electrodes. Suitable hole transport molecules were added to the polyimide in order to obtain layers with good alignment properties and large hole mobility. Polyimide and poly-fluorene layer thicknesses were varied to optimize the device performance. For those optimized devices, blue light with a dichroic ratio in emission of fifteen was measured and the luminance was approx. 100 Cd/m2 at 18 V. Even though these values are still below the requirements in application, the high dichroic ratio combined with a reasonable brightness represents an important step towards the use of organic LEDs as illumination sources in LCD displays.

Langmuir-Blodgett films for optical applications

Abstract Langmuir-Blodgett films exhibit interesting properties which make them promising materials for application in second order-nonlinear optics. However, there are still some problems to solve. One of these is to maintain the good orientation of chromophores found in monolayers also in noncentrosymmetric multilayer films. Another issue is to prepare Langmuir-Blodgett films that form low optical loss waveguides. In this paper we show that these problems can be overcome by using specifically designed compounds. So for multilayer films with alternating monolayers from a nitrophenylhydrazone dye and a polymerizable amphiphile a quadratic increase of second harmonic intensity with increasing film thickness was found. This proves that the orientation of the dyes remains unchanged in the noncentrosymmetric multilayer. Langmuir-Blodgett film waveguides with low optical losses were obtained from a prepolymerized amphiphile which has flexible side chains that cannot crystallize and thus prevent domain formation within the monolayer. Multilayer films consisting of 500 monolayers from that compound exhibit a light attenuation as low as 2 dB/cm for 633 nm light.