Patrick Keller

Excited-state electron-transfer quenching by a series of water photoreduction mediators

Rate constants kq for the quenching of the excited state of Ru(bipy)32+ by a series of viologen salts having different redox potential E12 have been determined in deaerated aqueous solutions at pH = 5 by laser flash photolysis. The kq values are found to decrease with increasing —E12 and to correlate with the reaction free-energy change ΔG. Such a correlation is shown to be consistent with the Rehm—Weller model for electron-transfer reactions.

Synthesis of stable and low melting ferroelectric liquid crystals of the ester family

Abstract Three different series of chiral ester compounds, 4′-((S)2-methylbutyloxy)-phenyl 4-alkyloxybenzoates, 4′-((S)4-methylhexyloxy)phenyl 4-alkyloxybenzoates, and 4′-alkyloxyphenyl 4-((S)6-methyloctyloxy)benzoates, have been synthesized and their mesomorphism studied. Low melting compounds have been obtained, all showing a chiral smectic C phase in conjunction with various other mesophases, depending on the series. These new ferroelectric liquid crystals have a remarkable chemical stability.

Optical Study of a Chiral Smectic C Under Shear

Abstract We study the distortion of a chiral smectic C, under a shear applied parallel to the plane of the layers, using an original technique based on the modification of the conoscopic image. Both the amplitude of the distortion and the phase lag between the alternating applied shear and the deformation are found to vary like the flow induced transverse polarization (described in Ref. 7). The behaviour is understood in terms of a viscoelastic-nematic-like model and of a simplified optical description which gives account of the displacement without deformation of the conoscopic image.

Ferroelectric Liquid Crystals with New Chiral Building Blocks

Abstract Esterification of 4-nitrocinnamoyl chloride by new chiral alcohols((d) and (1) 2-butanol; (d) 2-heptanol), followed by reduction of the nitro group by a new reagent(SnC12), gives new chiral 4-aminocinnamates.Condensation of these amines with 4-alkyloxybenzaldehydes or terephthaldehyde gives Schiff bases showing in some cases chiral smectic C phases. A comparative study of the mesomorphism of both these compounds and the compounds of the DOBAMBC family is made, in order to evaluate the effect of the chiral building block structure on the liquid crystalline properties.During this study, a new compound has been prepared, which exhibits the smectic O phase: ((d) 1-methylhexyl)terephthalidene Bis 4-aminocinna-mate.

Synthesis of New Mesomorphic Polyesters by Polymerization of Bifunctional Monomers

Abstract New polyesters have been synthesized by high temperature polycondensation of bifunctional monomers (4-(ω-(4-acetyloxyphenyloxy)alkyloxy)benzoic acids). Their mesomorphic properties have been studied. By analogy with the chemistry of polypeptide polymers (use of bifunctional monomers and of protective groups), a step by step polymerization has been performed in order to obtain model compounds sketching a “monomer,” a “dime” and a “trimer” Preliminary results are given on the mesomorphic properties of the model compounds.

Ruthenium dioxide: a redox catalyst for the generation of hydrogen from water

Ruthenium dioxide, already known to be a catalyst for the oxidation of water to oxygen, has been shown to mediate effectively the generation of hydrogen from water in Ru(bipy)2+3, methyl viologen and EDTA model systems, with efficiencies even higher (pH > 5) than those of previously investigated platinum catalysts. The main factor limiting the evolution of hydrogen is found to be the destruction of the organic electron-transfer relay, and this side-reaction is attributed to H˙(ads) species formed on the particles of catalyst.

Synthesis and Properties of new Liquid Crystalline Side-Chain Polysiloxanes

Abstract The synthesis of new mesomorphic side-chain polymethylsiloxanes with spacers derived from ω-alkenoic acids is described. These liquid crystalline polymers exhibit nematic and(or) smectic A and(or) smectic C phases, depending of spacer and terminal tail lengths.

New Mesogenic Polyethers Derived From Benzalazines

Abstract Four homologous series of flexible main-chain polyethers derived from benzalazines are reported. Enantiotropic nematic phases were found for three of the series. The influence of molecular structure on the mesomorphic properties has been studied. A comparative study is made between their properties and those of the analogous small molecules. The existence of OH group in positions 2,2’ of the azine central core made it difficult to synthesize the polymers using the phase transfer catalysis method and consequently a new method was employed.

Light-induced hydrogen generation from water catalysed by ruthenium dioxide

Ruthenium dioxide, which is known to be a catalyst for the oxidation of water to oxygen, has been observed to mediate hydrogen formation from water in a model system containing Ru(2,2′-bipyridyl)32+, methyl viologen, and ethylenediaminetetra-acetic acid with higher efficiency above pH = 5 than that previously observed with colloidal platinum as catalyst.